Test and measurement or T&M

  • Extended spectrum of IEC 61850 testing solutions

    The IEC 61850 international standard for power utility communications defines two types of communication to be used for substation protection, control and automation: real time communication with Goose and sampled values on the one hand and client/server (C/S) on the other. For all these cases, Omicron as a market leader for IEC 61850 testing solutions, offers internationally well accepted testing tools, such as IEDScout or Test Universe with its configuration tools for Goose and sampled values.

  • New London facility offers customers real choice

    Electro Rent, one of the world’s largest providers of electronic test equipment for rent, lease or used sales, is opening a new office in London.

  • Sophisticated test and measurement

     

    TJ|H2b is proud to announce its recent partnership with DV Power, bringing you the newest and best products DV Power has to offer.

    Founded in 2002, DV Power is internationally known to develop, manufacture and supply the most sophisticated test and measurement equipment for the power industry.

  • Test & measurement - Substation surveillance Part 1

    In service high-voltage (HV) substation equipment is exposed to many stresses, from the electrical, mechanical and thermal to the environmental. These stresses can act to accelerate the deterioration of the insulation and the electrical integrity of the HV equipment eventually leading to failure. Partial discharge (PD) is both a symptom and a cause of insulation deterioration, so the detection and measurement of PD phenomena can provide early warning signs of insulation failure.

    Critical to this detection is the availability of accurate and cost effective surveillance tools, which, if non-invasive, can provide early recognition and location of possible sites of electrical degradation while components are still in service. Gathering and trending PD activity over time is essential to monitor the rate and severity of degradation. Maintenance can then be planned in an effort to avoid unplanned outages, interruptions and inevitable loss of revenue.

    The use of radio frequency interference (RFI) measurement is an efficient, non-invasive surveillance technique to detect and locate partial discharges in individual HV apparatus. This article will look at the benefits of combining the assessment of RFI emissions with the targeted deployment of complementary, non-invasive electromagnetic interference (EMI) detection techniques. Specifically, frequency sweep data and time-resolved traces can be compared with follow up assessments using complementary EMI couplers such as high frequency CTs (HFCT) and transient earth voltage (TEV) couplers. This combination of tests provides an increased level of confidence in the location, identification and assessment of the severity of degradation and is beneficial when dealing with complex HV apparatus.

    The detection and measurement of RFI emissions from PD phenomena involves the measurement of complex waveforms varying considerably and often erratically in amplitude and time. RFI signals from such phenomena are considered to be broadband and impulsive in nature with low repetition rates.


    Measurements carried out on PD activity within oil-insulated HV equipment demonstrate that the discharges produce current pulses with rise times less than a nanosecond and therefore capable of exciting broadband signals in the VHF (30 to 300MHz) and UHF (300MHz to 3GHz) bands. Other investigations in open-air insulation substations show that signals from PD and flashover occupy a frequency range up to 300MHz.

    When PD occurs inside a metal enclosure, such as in a transformer tank, the signal propagates within the structure, suffering frequency attenuation, reflection, etc. Detection of RFI emission relies on the placement of apertures in the tank walls and penetrating conductors to allow the RFI emissions to propagate and radiate externally.

    In the following examples, the instrument used (Doble PDS100) has two different detection modes: spectrum analyser and time-resolved mode. Within spectrum analyser there are three separate detection techniques: peak detection, average detection and separated peak and average detection.

    Case Study 1
    Dissolved Gas Analysis (DGA) carried out on a South African, 275/88/11KV, 250MVA transformer showed signs of a discharge type fault. RFI measurements (using the Doble PDS100) and conducted EMI measurements (using a HFCT) were performed to establish correlation between the measurements. RFI measurements were taken around the periphery of the transformer. The frequency traces (FIGURE 2) exhibit a discrete appearance as pulses are accumulated. Short bursts of pulse accumulation were interspersed with long intervals of no or low energy activity. Triangulation based on signal intensity of the higher frequencies locates the source of propagation in the vicinity of the HV B-phase. Observing the RFI at 900MHz for a period of time in spot frequency mode sees the measured peak amplitude reaching -45dBm at that location. This mode also confirms the burst nature of the pulse sequence.

    The conducted EMI was measured using a 300MHz split-core HFCT at the HV neutral connection to earth. The measured conducted EMI is subjected to significant attenuation through the HV neutral connection and requires an extended observation time. In time-resolved mode both the RFI and conducted EMI measurement confirm the measured pulse behaviour. However, the pulsed activity is more easily captured and more of the lower energy pulses are detected.


    Partial discharge activity is indicated by both the RFI and EMI techniques. In each, the dynamic behaviour of the activity is characterised by very short burst activity interspersed by intervals of no or low energy activity. The sequence exhibits the characteristics of a floating type discharge. A secondary source of discharge is evident in the time-resolved traces. The results confirm the conclusions drawn from the DGA analysis. This study proves the use of RFI as an assessment tool while the use of an HFCT coupler provides increased sensitivity to internal PD activity, offering an increased level of confidence in the identification and assessment of PD activity.



    Case Study 2
    At a distribution substation, RFI measurements were undertaken to survey the condition of each of the oil-filled circuit breakers making up a typical 11kV distribution switchboard configuration commonly found in the UK. A high percentage of 11/33kV switchboards have an installed age of over 25 years. They are subjected to various types of duty plus a varied level of maintenance. The trend is to extend the maintenance period for medium-voltage (MV) switchgear, which in turn creates the need for interim non-intrusive condition monitoring techniques to offer confidence in the equipment’s safety and reliability.

    A baseline RFI scan was captured in an adjacent room away from the surveyed switchboard. Measurements at the rear of each circuit breaker were captured and compared with the baseline. The observed uplift of frequencies indicated a nearby discharge source, which was eventually triangulated to one particular circuit breaker by comparing the uplift in higher frequencies while moving the receiving antenna along the rear of the switchboard. Further RFI measurements were captured at the front of the switchboard. A comparison of the front and rear RFI measurements shows that the uplift in the lower frequencies was strongest to the front of the unit. These tests were followed by complementary EMI measurements to gather more information. 

    RFI Peak Measurements: Front and Rear of Circuit Breaker
    Legend: Front, Rear


    The HFCT uses inductive coupling to detect PD pulses flowing to earth and is capable of picking up both local PD in the cable end box and also the lower frequency PD pulses coming from down the cable. The results of this method confirmed the observations from the RFI survey, with uplifts of up to approximately 50dB at 75MHz and 40dB at 200MHz. Time-resolved measurements also showed pulse behaviour is similar to those obtained from RFI measurements (FIGURE 6).

    Lower Frequency ( circa 50MHz)
    Legend: RFI, HFCT, TEV

    Mid Frequency (circa 150MHz)
    Legend: RFI, HFCT, TEV

    Higher Frequency (circa 200MHz)
    Legend: RFI, HFCT, TEV


    The placement of HFCTs provides a means to trace the likely source of the signals by comparing the uplift in frequencies. The uplift reduces significantly as the location of the HFCT is moved away from the suspect circuit breaker. Repeated measurements on earth straps placed on adjacent circuit breakers indicate the circuit breaker identified is the source of the measured discharge activity.

    The most advantageous setup for metal-clad switchgear is to use an HFCT sensor in conjunction with a TEV sensor. Transient Earth Voltage (TEV) measurements work on the principle that if a PD occurs within metal clad switchgear, electromagnetic waves escape through openings in the metal casing. The electromagnetic wave propagates along the outside of the casing generating a transient earth voltage on the metal surface. TEV sensors are “capacitive couplers”, which when placed on the surface of metal cladding can detect TEV pulses as a result of PD internal to the switchgear.

    Observed peak TEV measurements on the main circuit breaker tank reach 0dBm at a frequency of 100MHz. Comparative measurements taken with the TEV sensor located on the cable end box show a reduction in uplift of approximately 20dB. The main circuit breaker tank is identified as the likely source of the discharge. Time-resolved measurements show pulse behaviour confirming the results obtained from both RFI and HFCT measurements.
    The utility opened up the circuit breaker and found signs of carbon at the cable end in the main tank of the switchgear. Results of this study confirm that deploying frequency spectra measurements and time-resolved patterns from RFI, HFCT and TEV probes can be used to pinpoint PD issues within switchgear. Using TEV sensors in conjunction with RFI surveillance on metal clad switch gear offers an additional capability in confirming and localising the partial discharge source.

    Conclusion
    RFI monitoring offers, and has proven to be, a routine non-invasive and cost-effective surveillance technique that complements and provides added value to other well established HV asset monitoring techniques such as thermal imaging and DGA analysis. As the practical examples illustrate, measurements logged with an RFI instrument platform specifically designed for substation surveillance can assist in effective discrimination and recognition of the RFI emissions radiated from potential sites of insulation deterioration.

    There are great benefits of combining the assessment of RFI emissions with targeted deployment of other complementary non-invasive electromagnetic interference (EMI) detection techniques using the same RFI instrument platform. The deployment of both ‘far field’ and ‘near field’ probes provide a diversity of sensors, which is of particular importance with complex HV apparatus such as transformers where the propagation paths for RFI are less well defined.

    This article is based on the paper Substation Surveillance Using RFI and Complementary EMI Detection Techniques, which was recently presented at the 78th International Conference of Doble Clients in Boston, Massachusetts USA. The paper was written by Alan Nesbitt, Brian Stewart and Scott McMeekin of Glasgow Caledonian University and Kjetil Liebech-Lien and Hans Ove Kristiansen of Doble Engineering Company.

    Part 2 of this article will appear in the June 2011 issue of Electrical Review.

  • Test & measurement - Substation surveillance Part 2

    At a distribution substation, RFI measurements were undertaken to survey the condition of each of the oil-filled circuit breakers making up a typical 11kV distribution switchboard configuration commonly found in the UK. A high percentage of 11/33kV switchboards have an installed age of over 25 years. They are subjected to various types of duty plus a varied level of maintenance. The trend is to extend the maintenance period for medium-voltage (MV) switchgear, which in turn creates the need for interim non-intrusive condition monitoring techniques to offer confidence in the equipment’s safety and reliability.

  • LEM publishes 2005 Test and Measurement Instruments catalogue

    LEM publishes 2005 Test and Measurement Instruments catalogue

    LEM’s 2005 Test and Measurement Instruments catalogue includes individual product descriptions and the most important technical data on the company’s ranges of installation testers, machine and device testers, network analysers, isolation and earth testers, probes, multimeters etc. LEM’s wide ranges of supporting accessories and software are also described.

    The technical tables at the beginning of each product family offer a direct comparison between the instruments. The explanation of important technical expressions at the end of each section helps in the selection process.

    The catalogue is available in eight different languages, from LEM NORMA at: This email address is being protected from spambots. You need JavaScript enabled to view it., phone +43 2236 691-0.

  • Test & measurement - Power quality measurement...What does Class A mean to me?

    Power quality measurement is still a relatively new and quickly evolving field. Whereas basic electrical measurements like RMS voltage and current were defined long ago, many power quality parameters have not been previously defined, forcing manufacturers to develop their own algorithms.

    There are now hundreds of manufacturers around the world with unique measurement methodologies. With so much variability between instruments, technicians must often spend time trying to understand the instrument’s capabilities and measurement algorithms instead of concentrating on the quality of the power itself.
    The IEC 61000-4-30 CLASS A standard defines the measurement methods for each power quality parameter to obtain reliable, repeatable and comparable results. It also defines the accuracy, bandwidth, and minimum set of parameters. Going forward, manufacturers can begin designing to Class A standards, giving technicians a level playing field to choose from and increasing their measurement accuracy, reliability, and efficiency on the job.
    IEC 6100-4-30 Class A standardises measurements of:
    • Power frequency
    • Supply voltage magnitude
    • Flicker, harmonics, and inter-harmonics (by reference)
    • Dips/sags and swells
    • Interruptions
    • Supply voltage unbalance
    • Mains signalling
    • Rapid voltage changes.
    Examples of Class A requirements:
    • Measurement uncertainty is set at 0.1% of declared input voltage. Low cost power quality measurement systems with uncertainties greater than 1% can erroneously detect dips at -9% when the threshold is set at -10%. With a Class A certified instrument, a technician can confidently classify events with internationally accepted uncertainty. This is important when verifying compliance to regulations or comparing results between instruments or parties.
    Dips, swells and interruptions must be measured on a full cycle and updated every half cycle, enabling the instrument to combine the high resolution of half-cycle sampled data points with the accuracy of full-cycle RMS calculations.
    • Aggregation windows – A power quality instrument compresses acquired data at specified periods which are called aggregation windows. A Class A instrument must provide data in the following aggregation windows:
    - 10/12 cycle (200ms) at 50/60Hz, the interval time varies with actual frequency
    - 150/180 cycles (3s) at 50/60Hz, the interval time varies with actual frequency
    Harmonics must be measured with 200ms intervals according to the new standard, IEC 61000-4-7 / 2002. The old standard allowed 320ms intervals which cannot be synchronised with the 200ms aggregation windows of other Class A measurements.
    Using 200ms intervals allows harmonic calculations to be synchronous to all the other values like RMS, THD, and unbalance.
    The Harmonics FFT algorithm is specified exactly such that all Class A instruments will arrive at the same harmonic magnitudes. The FFT methodology allows for infinite algorithms that can result in vastly different harmonic magnitudes. By standardising on 5Hz bins and summing the harmonics and inter-harmonics according to specific rules, Class A instruments will be consistent and comparable.
    • External time synchronisation is required to achieve accurate timestamps, enabling accurate correlation of data between different instruments. Accuracy is specified with ±20 ms for 50Hz and ± 16.7ms for 60Hz instruments.
    • 10 min interval sync to clock
    • 2 h interval sync to clock.
    Latest product developments
    There have been a number of significant introductions to the market in the past 12 months of power quality analysers offering compliance with IEC 61000-4-30 CLASS A. These new products include both handheld devices and those designed for leaving in a fixed location for a time period set by the user. They will log a large number of parameters at user chosen time intervals for later analysis by a PC. Thus there is a choice of products, offering different capabilities, from which a technician can choose the most appropriate tool for the job.
    These new tools are designed for ease-of-use to uncover intermittent and hard-to-find power quality issues. Suitable handheld analysers will provide on-screen display of trends and captured events even while background recording continues. Some can be used to analyse disturbances, to validate incoming power compliance, for capacity verification before adding loads, and for energy and power quality assessment before and after improvements. The best tools provide powerful reporting software to enable rapid assessment of the quality of power at the service entrance, a substation or at the load according to EN50160 standards. The software can quickly analyse trends, create statistical summaries and generate detailed graphs and tables.

  • Test & Measurement - Restoring safety after the Storm

    The recent widespread flooding in the UK has undoubtedly damaged electrical equipment worth millions of pounds. In some cases, however, it may be possible to restore this equipment to safe and reliable operation, thereby avoiding the need for costly replacements. Jeff Jowett of Megger looks at what can be done

    The key to salvaging flood damaged electrical equipment is to find ways of drying it out effectively, without risking further damage. A number of options are available for this. Probably the most satisfactory is to use a temperature controlled oven with efficient air circulation but, in many cases, this is not possible either because the equipment is too large to be moved to an oven, or because no oven is available.

    In these cases, infrared lamps can be used, or a housing can be built around the equipment, with steam coils or electric elements used as the heat source. It is important to make provision for free circulation of air, so that moisture is allowed to escape, and the use of blowers can be helpful.
    Another method of heating sometimes used with items like motors and transformers is to pass a current at low voltage through the windings. To avoid the risk of further damage, however, this should not be done until the insulation resistance has been raised to at least 100,000 ohms by other methods. Insulation testers that have kilohm ranges are invaluable in this type of work.

    On occasion, welding sets are used as a current source for drying out windings. It’s important to note that these are not intended to supply high currents continuously and they must, therefore, be used at only a fraction of their rated current.

    Whichever method of heating is used to dry out the equipment, it’s vital to monitor the insulation resistance for a long enough period to ensure that it has reached a stable value. It is very common, during the drying process, for the insulation resistance to rise to a comparatively high value then dip again. In fact, this rise and fall is often repeated several times as moisture works its way out of the equipment.

    While the comments above give general information on salvaging flood-damaged equipment, it is worth looking in more detail at what can be done with various specific types of equipment.

    Switchboards and Electrical Controls
    - Thoroughly clean and dry out all equipment, dismantling where necessary. After drying, re-varnish all coils. Check contacts for corrosion and oxidation, and make sure that all moving parts operate freely.
    - Drain all oil-filled devices, clean them and re-fill with fresh oil of the correct dielectric strength. The oil can be tested for conformance with British Standards using a test set. Dry all insulating barriers, or replace them if they have warped.
    - Meters and protection relays will usually have to be reconditioned by the manufacturer. To ensure fast return to service, it may be preferable to fit replacements.
    - Clean and dry thoroughly all busbar insulators and control wiring. A minimum of two megohm insulation resistance must be achieved before the equipment is energised, and can readily be confirmed by any good quality insulation tester.
    - Check standby batteries for functionality using a battery impedance tester, and check battery straps for corrosion or excessive resistance using a purpose-designed low-resistance ohmmeter.

    Electrical Tool and Portable Appliances
    - Many of the techniques outlined in the introductory section of this article are suitable for salvaging wet tools and appliances. Before these items are returned to service, however, it is essential that final proof testing is carried out with a portable appliance tester (PAT) in line with IEE code of practice for in-service inspection and testing. As a further precaution, it may also be desirable to flash test Class 2 assets

    Rotating Electrical Machines
    - Completely dismantle all parts and, except for ball and roller bearings, and either wash them with clean water or steam clean them. Follow this with a thorough cleaning using a grease solvent.
    - Thoroughly clean all bearings and housings paying particular attention to oil grooves and reservoirs. Disconnect and swab oil lines, or steam clean them.
    - Dismantle the brush rigging and clean the insulators. Some types retain water and must be dried very thoroughly.
    - Monitor the insulation resistance of the machine with a modern tester that uses a low applied voltage for the kilohm ranges. Once a value of at least 100,000 ohm is reached, the megohm ranges of the instrument can be used for further monitoring.
    - Commutators can be hard to dry out, and it may be necessary to loosen or even remove the clamps to let water out of the inside of the commutator. On large commutators, it may be necessary to use drying temperatures as high as 130°C to achieve effective results.
    - Check the bands on armatures or rotors for tightness, as the drying out of the underlying insulation may loosen them. If this happens, they will need to be replaced.
    - Some slot wedge materials may be affected by moisture. If this has happened, new wedges must be installed.
    - Field coils from DC motors, generators and synchronous machines can present particular problems, and it may be necessary to remove them from the machine for drying in an oven and re-varnishing. After this, the coils should be checked for shorted turns with a digital low-resistance ohmmeter.
    - After cleaning and drying, most windings will need re-varnishing. Dip-and-bake varnish is recommended but, it the original varnish is in good condition, an air-drying varnish may be used.
    - Before starting the machine, check the entire installation, paying particular attention to lubrication and electrical connections. For three-phase machines, check the phase rotation.

    Transformers
    - Remove inspection cover plates and check the condition of the windings, looking particularly for signs of failure. Check all connections for looseness and signs of heating. With oil insulated transformers, draw oil samples from top and bottom, and check them with an oil test set. Breakdown should be at least 22kV, or 25kV if an askarel is used. If it is lower, the oil/askarel will need to be replaced.
    - Check the insulation resistance. This should be at least one megohm for each 1,000 volts rating, with a minimum of two megohms. Ideally, the resistance should be comparable with the pre-flood values, which may be available from maintenance records. This is best confirmed by an insulation tester with an extended range, such as Megger’s MIT400 and MIT510 products.
    - Note the condition of the bushings, external connections, operating switches and protective devices, and take remedial action where needed. If necessary, clean the transformer externally and paint the tank.
    - If water has entered the tank, flush the windings with clean insulating oil. If the transformer is small, remove the coil and core, and dry in an oven at up to 90°C. If necessary, dip and bake the windings. Windings for larger transformers can be dried in the tank by forcing hot, dry air (not above 90°C) around the windings after the tank has been drained; by short-circuiting one winding and energising the other with a low voltage; or by using a combination of these methods.
    - During the drying process, plot a curve of insulation resistance against time, initially measuring with a low-voltage tester and subsequently, if the process proceeds successfully, changing to a high-voltage insulation tester. If the process is not successful, and the curve shows no sustained increase in insulation resistance, the transformer will need to be re-wound.
    - When the insulation resistance has reached an acceptable value, a final test should be made with a transformer turns ratio tester to confirm the transformer has been returned to full performance.

    Cables and Wiring
    - All open wiring, including non-metallic sheathed cable, can usually be retained after thoroughly cleaning and drying the cable and the junction boxes, and remaking connections.
    - Armoured cable will usually have to be replaced, as will lead cable if the ends have been under water.
    - Rubber-covered cable in rigid conduit can sometimes be reused, but it must be pulled out of the conduit so that the conduit can be cleaned. The conduit must be thoroughly cleaned to remove all silt and moisture before being used again.
    - Check and clean potheads and other insulators, and inspect them for cracks or other damage
    - Perform a comprehensive insulation resistance test before returning the installation to service.
    Hopefully, this article will have given a useful indication of the measures that can be taken to salvage electrical equipment after it has been subjected to flooding. It is essential, however, to remember, in every case, safety is of paramount importance. This can only be assured by careful testing of the salvaged equipment, during and after the repair process, using appropriate test equipment.

  • Test & Measurement - Checking the Earth

    Test & Measurement - Checking the Earth

    Most electrical installations depend on earthing via earth electrodes to protect people and equipment. In these installations, regular testing of the earth resistance is essential, but most testing methods are either time consuming and inconvenient, or prone to giving inaccurate results. As Paul Swinerd of Megger explains, however, there is now a better alternative

    - It’s tempting to think checking the resistance of an earth electrode should be no more complicated than finding a second earth connection, such as a nearby water pipe, and measuring the resistance between this and the electrode under test with an ordinary ohmmeter. Unfortunately, life isn’t quite that simple.
    Noise currents flowing in the earth will almost certainly produce large errors in the results obtained, and there’s no way of knowing how much of the resistance is due to the secondary earth connection and how much to the earth electrode itself. In other words, some sort of result will be obtained but, for all practical purposes, it is meaningless.
    For this reason, a number of alternative methods have been devised for accurately measuring earth resistance. The simplest is to carry out a direct measurement, as described earlier, but with a purpose-designed earth tester that uses an ac test current. By choosing the frequency of this current so it is not an integer multiple of the mains supply frequency, it is possible to arrange for such an instrument to provide a high degree of noise rejection.
    The results are far more meaningful than those which might be obtained with an ohmmeter, but there is still no way of confirming that they are accurate or, indeed, of separating out the contribution of the secondary earth.
    A much better method, and one which is very widely used, is usually known as the three-terminal or fall-of-potential method. This uses a connection to the electrode under test and two test spikes that must be driven into the ground before the test is carried out.
    One of the spikes – the current spike – injects the test current, and should be placed as far away as possible from the electrode under test. The other spike – the voltage spike – is then driven into the ground at a number of locations, preferably in a straight line, between the current spike and the electrode. At each location, a voltage measurement is taken. Since the current injected by the instrument is known, each of these voltage measurements can be converted, using Ohm’s law, to a resistance value. In practice, this conversion is performed by the instrument itself.
    If a graph is plotted of resistance versus the distance the voltage spike from the electrode under test, it should have a definite plateau region where the resistance hardly varies as the rod is moved. This value of resistance is the required earth resistance for the electrode under test.
    This method is accurate, and any problems with the measurements are readily apparent, as the resistance graph will depart markedly from the expected shape. The only shortcomings are that the test is time consuming to carry out, it requires a reasonable amount of space, and that the earth electrode under test must be disconnected from all other circuits while the test is underway. These are rather significant shortcomings.
    To provide a more convenient way of measuring earth resistance, the clamp-on or stakeless method was introduced. This uses a tester adapted to inject a test current into the earth electrode system via a clamp arrangement, and uses the same clamphead to measure the resulting current flowing in the electrode under test. No direct connections are required, and the earth electrode does not need to be disconnected from other circuits – indeed, for successful testing, it cannot be.
    While this method is quick and easy, it has several limitations. It only works in applications where there are multiple parallel earth connections so that there is a return path for the test current, and it cannot, therefore, be used to test isolated electrodes. Since there’s no way of verifying the result, it is also unsuitable for checks on new installations where no previous test results are available for comparison, but it is good for trending of earth system condition.
    A new solution, which is more versatile than the stakeless method and more convenient than the traditional fall-of-potential method, is provided by the Attached Rod Technique (ART). In many ways, ART is similar to fall-of-potential testing, and all of the same connections are required. There is, however, one crucial difference – there is no need to disconnect the earth electrode from other circuits while the test is being carried out.
    That may seem a relatively small advantage but, apart from the physical difficulty of breaking earth connections, it's important to remember that earthing is a safety function.
    There are dangers in disconnecting an earth electrode as a fault current may be flowing and disconnection could give rise to a potentially lethal situation. Furthermore, if equipment is disconnected from the earth electrode to enable a test to be carried out, that equipment may no longer be safe, and dangerous situations may result.
    While it may be possible to provide a temporary earth connection, or to switch off the electricity supply during the test, such measures are likely to be both inconvenient and costly.
    So how does ART testing work? The key is in a current measuring clamp (ICLAMP) that is put round the earth electrode under test. The tester is designed to ignore any system leakage and noise currents that may be flowing through the earth electrode. This means that it can accurately measure the test current, in spite of extraneous influences.
    With the equipment set up, the ART test proceeds in exactly the same way as an ordinary fall-of-potential test. It is still, therefore, time consuming, but there are a number of shortcuts that can be used in appropriate circumstances.
    For example, instead of taking readings with the voltage spike at various distances between the electrode and the current spike, it is sometimes sufficient to take a few readings, with the voltage spike around 62% of the distance between them. This means that ART testing provides a very good balance between convenience and accuracy of results.
    For earth resistance testing, it is important to have the right equipment and to understand the limitations of the various test methods – even ART testing isn't suitable in every case, although it is very versatile.
    Earth resistance testing isn’t particularly complicated, but interpreting the results from the various test methods can be, which is why Megger has developed software that will do the earth testing calculations automatically, and supply a report.
    Megger also offers an 80-page publication "Getting Down to Earth" which provides detailed practical advice on all commonly used methods of earth resistance testing. Printed copies are available free of charge from the company, or the publication can be downloaded from the Megger website (www.megger.com).

  • Test & Measurement - Complete health check for power transformers

    Test & Measurement - Complete health check for power transformers

    Liam Warren of ABB’s UK power service operation explains how the latest state-of-the-art diagnostic techniques can help to predict potential transformer faults well before they become a problem


    Power transformers are mission-critical elements in many industrial, utility and power generation installations. Should an unexpected failure occur, it can result in a lengthy downtime, with consequent loss of operating revenue, and expensive repairs. Planned maintenance is the best insurance against transformer failure and that’s where advanced diagnostic techniques come in. They offer an efficient, cost-effective way of assessing the overall condition of a transformer fleet so areas of potential concern can be flagged and action taken well before a potential failure develops into a serious fault.

    Furthermore, if an operator has a transformer that is already causing concern, then diagnostic tests can establish the severity of the problem, locate the fault and help the service team to provide expert advice on what action to take. For example, with regular testing it might be possible for the transformer to continue in service, while operating under a safe, reduced load, until a planned service interval is reached.

    ABB’s transformer diagnostic service utilizes four main techniques – SFRA (Sweep Frequency Response Analysis), FDS (Frequency Domain Spectroscopy, winding resistance measurement and oil sampling.

    SFRA
    The SFRA (Sweep Frequency Response Analysis) test, carried out by a Pax FRAX-101 system, is an important tool for identifying potential winding geometry changes. It consists of a low-voltage, off-line, measurement of the impedance of the transformer windings as a function of frequency. The test is performed by injecting a variable frequency AC voltage into each individual transformer winding and plotting the responding current as a curve.
    We recommend SFRA reference curves should be captured in the factory to provide a baseline ‘finger print’ of the windings in an as-new condition. However, for installed transformers, a field test can provide the baseline curves. SFRA testing should be performed periodically during the service life of the transformer, or after a specific incident that has caused significant fault currents. An alternative approach is to utilise a type-based comparison between sister transformers with the same design. Under certain conditions, a construction based comparison can be used when comparing measurements between windings in the same transformer.
    When interpreted by an expert, comparison of the SFRA test with the transformer’s original baseline curves is an excellent method to check for movement or displacement of windings or winding circuits that could affect its ability to withstand faults. It is much more definitive than low-voltage impedance tests routinely performed on transformers, it helps avoid catastrophic failures and can even locate the exact position of a fault.
    Figure 1 shows a typical SFRA analysis in which the pronounced dip in the frequency response curve of one of the transformer phases indicates a potential fault – most probably due either to a winding failure or core movement.

    FDS
    FDS (Frequency Domain Spectroscopy), carried out by a Pax IDAX-206 system, is used to assess the integrity of a transformer’s insulation system. The test determines the volume of moisture and presence of contaminants in the solid insulation, as well as the conductivity and power factor of the oil. This is an extremely useful tool in an overall condition assessment programme as standard power factor tests alone do not yield this type of information.

    The FDS test measures the dielectric properties (capacitance, loss and power factor) of the transformer’s insulation as a function of frequency, This off-line test utilizes the same type of connections as a standard (Doble) mains frequency insulation power factor test . However, by covering a much wider frequency range – typically 1 mHz to 1000 Hz – the test offers increased sensitivity to insulation issues.

    An important primary use of the FDS test is to determine the moisture content of the cellulose insulation structure of power transformers. It is difficult to obtain a reliable assessment of moisture content by oil sample tests, as the water is transferred between the solid insulation and the oil as the temperature changes. An oil sample has to be taken at relatively high temperatures, when the transformer is in equilibrium. But this is a relatively rare state for a transformer and can result in unreliable assessments.

    An illustration of the advantages of FDS is provided by an exercise in which a customer provided ABB with a list of seven suspect transformers. In each case, moisture in oil test results had indicated the need for oil processing and drying. By carrying out FDS tests we were able to show that only two units actually needed drying. So our recommendation was to dry these two, while keeping the other five under careful surveillance. The customer not only made a very significant saving in operational and maintenance costs, preventing unnecessary drying operations on five transformers also reduced the risk of over-drying and loosening of windings.

    Winding resistance measurement
    Winding resistance measurement tests are carried out by an Omicron CPC 100 system. This is used to inject a DC current of up to 2kV through the transformer windings and it then measures the voltage drop across that winding - enabling the resistance to be calculated.

    The main purpose of this test is to check for significant differences between the windings, which could indicate field damage or deterioration, and also to ensure that the transformer connections are correct and that there are no severe mismatches or open circuits.

    Oil sampling
    Just as a blood test can provide a doctor with a wealth of information about their patient, a sample of transformer oil can tell an engineer a great deal about the condition of a transformer, enabling them to effectively manage the asset for extended life and enhanced reliability.
    The role of the oil in the transformer is to both cool it and insulate the internal components, and in doing so it bathes every internal component. As a result, the oil contains around 70 per cent of the available diagnostic information for the transformer and laboratory analysis can provide an early indication of a developing condition such as tap changer arcing.
    The data generated from an oil sample is only as good as the sample itself. It is vital to obtain a clean uncontaminated sample to BS 5263. This includes taking the sample while warm, and measuring the temperature so that the laboratory can then adjust the results for moisture content, preflushing the sample leg and running the sample quietly into a clean glass vessel to minimise degassing and sealing the sample securely.
    We recommend that the best information can be obtained from oil sampling by viewing trends. So it is useful to take a bench-mark sample when a transformer has been energized or an oil treatment performed and to then take further samples at regular intervals so that any variation in quality can be measured in order to monitor developing faults.
    Typical tests carried out in the laboratory analysis of the oil sample include:
    - Breakdown voltage (dielectric strength)
    - Moisture content
    - Dissolved gas analysis (DGA)
    - Oxidation
    Each of these parameters impacts on the other parameters, and they all work together to affect the condition of the transformer.

    Summary
    In general, power transformers are very reliable devices and will provide excellent service for many years if maintained and serviced regularly. Failures, when they occur, are usually very serious and require costly repairs and inconvenient downtime. The best insurance against failure is a planned monitoring and testing regime. The new generation of high-technology, non-invasive, diagnostic techniques can play a vital role in this regime.

  • Test & Measurement - Changes to PAT Code

    Jim Wallace of Seaward highlights the main changes to the new IEE Code of Practice for portable appliance testing

    What is the IEE Code of Practice for PAT testing?

    The IEE Code of Practice for In-Service Inspection and Testing of Electrical Equipment provides a guide to those with a responsibility for maintaining the safety of portable electrical appliances under the Electricity At Work Regulations 1989, Health and Safety at Work Act, Management of Health and Safety at Work Regulations and Provision and Use of Work Equipment Regulations.

    By providing comprehensive guidance on periodic inspection and testing it ensures that organisations, administrators and those carrying out the testing fully understand the requirements of the EAWR 1989 and can demonstrate compliance with it.

    So why the changes?

    Recently the IEE has reviewed the Code of Practice. The new 3rd edition takes into account technology advances and the implications of other market changes in relation to in-service electrical safety testing. By expanding the Code of Practice by over 50 pages, the revised publication provides much clearer guidance on all aspects of portable appliance testing with the addition of a number of useful illustrations.  As an example, whilst the previous document provided detailed advice on checking mains plugs and cables, the revised version is supplemented by the inclusion of multiple illustrations showing typical faults that might be encountered. Similar clarification and added details are provided for all aspects of the inspection and test process.

    Has the new Code changed the scope of equipment to be tested?

    No - but it has clarified some earlier points. For example, in the past certain types of electrical equipment, hand dryers for example, may have been regarded as a appliance by anyone testing the electrical installation, or as a fixed installation by anyone carrying out in-service testing. As a result these items of electrical equipment may have remained untested. To overcome such confusion the new IEE Code makes it clear that appliances which are connected to the electrical supply by a flex should be tested, even if they are permanently installed.

    What are the new recommendations for RCD testing?

    One of the main changes in the updated IEE Code concern new requirements in relation to testing RCDs. In particular the revised version stipulates that when an extension lead or multiway adaptor is fitted with an RCD, the operation of the RCD should be checked using an RCD test instrument to determine that the trip time is within specified limits. For those responsible for carrying out portable appliance testing this may require some changes to be made to the type of test instruments used. However, Seaward has anticipated these changes and many of the company's testers are now equipped with an RCD trip time test.

    How has guidance on insulation testing changed?

    Testing insulation resistance at 500V d.c. can be problematic when the equipment under test is fitted with transient suppressors or mains filtering and until now the only alternative was to perform a protective conductor/touch current measurement. The revised Code of Practice introduces two new test methods which can be used as an alternative to the 500V insulation test. The first method is to reduce the insulation test voltage to 250V dc and the second is to perform an alternative/substitute leakage measurement.

    Alternative or substitute leakage is measured using a technique similar to that used when measuring insulation resistance. A test voltage is applied between both live conductors (phase and neutral) and the protective conductor (earth) during a Class I test or a test probe connected to the equipment enclosure during a Class II test. The resultant current is measured and then scaled to indicate the current that would flow at the nominal supply voltage.

    The test voltage is 50Hz AC and normally in the range of 40V to 250V. The test voltage is current limited and so there is no hazard to the test operative. As the test voltage has the same nominal frequency as the mains supply the leakage paths are similar to those found when the equipment is in operation. Similarly, because the test voltage is not greater than the nominal supply voltage of the equipment under test, measurements are not affected by transient suppressors, MOVs or other voltage limiting devices.

    Who should carry out the testing?

    The EAWR already require that testing should be carried by a competent person and the new IEE Code provides further clarification on the competency required. Specifically, the IEE Code advises that a competent person should possess sufficient technical knowledge or experience to be capable of ensuring that injury is prevented. The new Code continues with further explanation on what that technical knowledge or experience may comprise, including such factors as an adequate knowledge of electricity, an adequate understanding and practical experience of the system to be worked on and an understanding of the hazards that may arise and the precautions which need to be taken.

    What other changes should be highlighted?

    On a general note, it has always been recognised the PAT equipment used for testing should be calibrated annually or in accordance with manufacturers' instructions. A calibration certificate is issued which states that the test instrument is within specification at the time the calibration is performed.  However, the certificate does not guarantee the performance of the test equipment at any time after that and the revised IEE Code of Practice now recommends that test equipment is checked at regular intervals using a verification device such as a PAT Checkbox.  In addition, a record of the performance checks taken should also be kept and the revised document includes a specimen test instrument record form.

    Where can I obtain the new Code of Practice?

    Further details of the updated IEE Code of Practice for In-Service Inspection and Testing of Electrical Equipment are available by calling tel: 01438 313 311.

  • Insulation testing - Measurement in medical environments

    Bill Earlie, of test instrument and precision measurement company Cropico, explains the implications of new guidance on electrical installations in medical environments

    New Medical Electrical Installation Guidance Notes (MEIGaN) have recently been issued on the safety requirements of electrical systems installed in diagnostic imaging, patient treatment and radiotherapy rooms.

    This guidance has been issued for new buildings, refurbished rooms and transportable diagnostic or treatment rooms in medical or health premises. The notes stipulate a range of measures to be taken in relation to the electrical mains supply and include new instructions regarding the earthing and equipotential bonding connection of permanently installed medical devices and associated equipment.

    Developed for healthcare organisations and medical device suppliers, the new MEIGaN guidelines embody the basic principles of the BS7671 IEE Wiring Regulations, but refine this with specific requirements for medical environments.

    Earthing & equipotential bonding 
    Many electrically operated medical devices and equipment including sterilisation baths, heaters, treatment tables, drug cabinets and some lighting fittings can have exposed metalwork that could become live if a fault occurred. Anyone touching these surfaces could then receive a shock or even be killed depending on the current flowing through them to earth.

    Equipotential bonding is therefore necessary as part of the safety measures associated with electrical installations to prevent significant touch voltages being generated within the patient environment.

    MEIGaN requires earthing and equipotential bonding conductors shall not intentionally carry load or control circuits. In addition, it is stipulated that equipotential bonding conductor continuity between equipment and the associated mains supply isolator(s) shall not depend solely on the continuity of conduits, cable braiding, ducts or trunking and should be achieved with a dedicated copper earth cable connected with brass or copper fittings.
    An earth reference bar (ERB) is required in medical a room that comprises one or more copper connection bars installed in an enclosure as part of the room's protective earth system. The ERB is designated as a reference or datum point for the purpose of defining and measuring resistance values.

    All installed equipment needs to be earthed to the ERB if there are any conductive surfaces that are accessible to either patients or staff. This could include warning lights, injectors, water baths, contrast media warning equipment, viewing boxes and powered drug cabinets.
    All non-powered equipment with metal surfaces must also be similarly bonded to the ERB. This includes protective screens, metal sinks and work surfaces, heating pipes and radiators, water pipes, drug cupboards, ceiling mounted hardware and other steel or wire cable trays, steel floor ducts and similar hardware. In all cases, such items must be returned to the ERB by means of a cable.

    Each equipotential conductor connected to the ERB must be individually labelled and permanently identified. A typical ERB layout is as shown below (with black alligator clips of leads from test equipment).

    Testing and verification
    MEIGaN requires earthing and equipotential bonding connections are inspected and tested to verify compliance with the new guidance.
    It advises resistance should be measured between each protective earth terminal, socket outlet or every accessible metal part and the ERB.  In terms of bonding resistance, the maximum acceptable resistance is 0.1Ω (or 100mΩ). Anything above this level would represent a failure of the earth continuity provided.

    As a point of caution MEIGaN points out with the testing of socket outlets, care must be taken to ensure that the contact resistance of an inserted earth pin of a 13A plug should also be low. This is because there can be some variation in the resistance of plug pins, which could be manufacturer dependent. For this reason it is suggested the socket resistance is tested to the back of an inserted pin and not by probing the socket itself.

    There has also been much debate about the current and voltage levels that should be used for this test. Traditionally the standard instrument used for equipotential bonding was a mains powered unit capable of supplying up to 35A at up to 25V for a 5 second test.
    However, some modern electronic medical equipment could be susceptible to damage by large earth current pulses, especially on PCBs. In addition, the 35A, 5 sec test does not necessarily fuse frayed connections as easily as one might think.

    Tests have shown a single strand of earth flex can withstand the test, so it is not in itself a reliable measure of ‘robustness'. Hence, testing at lower currents is not necessarily a weaker test that would miss potential failures.

    The new MEIGaN guidelines appear to have taken these sort of considerations into account and now stipulate that a minimum test current of 1A can be used to test the equipotential bonding.

    In doing so the new guidance means highly effective testing of the earthing and equipotential bonding of medical devices and fixed equipment in a treatment room or diagnostic suite can now be undertaken with lightweight, hand held digital microhmmeters.
    One new example, the Cropico DO4002, has been specially developed for this application.
    This instrument utilises forward and reverse current measurement with auto averaging to maintain highly accurate measurement of very low resistance. It also has the advantage of battery power which means equipotential bonding measurements can be taken without the risk of earth leakage currents from the main power supply interfering with resistance values.
    In addition, the tester can be used with extended test leads of up to 20 metres without any loss of measurement accuracy, making room socket tests easily accessible and enabling individual plug tests can be undertaken fast and effectively.

    Maintaining the integrity of electrical integrity in medical environments is critical to prevent patients being put at risk. The availability of new hand held test instrumentation enables electromedical service and installation engineers to meet this need by helping them undertake earthing and equipotential bonding testing quickly and effectively - without compromising the accuracy of test results.

  • Test & Measurement - Finding fault with cables

    Costly disruption and disconnection of consumers are typical consequ-ences of faults in power cables. Yet locating these faults is often difficult and time consuming. Fortunately, there are a number of test techniques available to make this task much easier. Damon Mount of Megger looks at the most useful of these

    Power cable faults come in many guises. The easiest to locate by far are permanent faults on simple networks where the cable run is known, such as the supply system for a street lamp installation. That doesn't mean, however, that finding a fault is a trivial job. In fact, it can be enormously costly, especially for buried cables. Nowadays, digging a single hole in a street in a large city, for example, can cost well in excess of £100,000 and excavating a cable typically costs around £4m per mile. A better, more cost effective technique is to adopt a structured approach to diagnosing and locating cable faults, based on the use of modern test equipment.

    The preliminary stage is straightforward - simply carry out continuity and low-voltage resistance checks to confirm the presence of a fault. Do not, however, succumb to the temptation of subjecting the cable to a high voltage insulation test at this stage. Doing so might alter the characteristics of the fault, and make it harder to locate with subsequent tests.

    The next step is to attempt to localise the fault using a time domain reflectometer (TDR) and standard pulse echo techniques. This instrument applies a brief low voltage pulse to the cable under test and looks for voltages reflected back along the cable. Clear reflections are, in most cases, obtained from open- and short-circuit faults. By measuring the time it takes for the reflection to return to the instrument, it is possible to provide a good indication of the distance to the fault. It is always a good idea to store a reference trace before any further tests are done on the cable as any change in condition of the fault can then be seen by comparing live with recorded traces.

    Dual-channel TDRs are particularly versatile, since they allow tests to be made simultaneously on two phases. The benefit of this is that a good circuit can be compared with a faulty one, which makes the results easier to interpret as joints and cable ends will also contribute their reflections to the trace. Some models, such as the Megger TDR2000/2P, can even test live circuits without the inconvenience of having to use loose, separate blocking filters

    Basic TDRs are compact, inexpensive and very easy to use. They do have some limitations, but these low-cost instruments can find a high percentage of faults. They are, therefore, an excellent investment where the purchase of more sophisticated equipment cannot be justified.

    It sometimes happens though, particularly in the case of high resistance faults, that the TDR cannot see the fault. Conditioning (burning) of the fault is one way to change the fault condition so that it can be seen with a TDR. This is sometimes necessary but requires another instrument, and is dependent on the cable type but can cause problems later in the fault-finding process.

    A more sophisticated option is to move on to the arc-reflection method of fault location. This involves sending a high voltage pulse down the cable, which causes a temporary arc at the site of the fault. The arc is momentarily sustained by a filter built into the arc reflection test set.

    Because of its low impedance, the arc looks like a short-circuit fault that can be localised with a TDR. The time interval between the high voltage pulse and the TDR pulse is critical if good results are to be obtained. For this reason, a modified arc reflection technique, known as arc reflection plus, has been pioneered by Megger.

    With this technique, not one but fourteen TDR pulses are automatically sent along the cable at varying time intervals after the high-voltage pulses. The resulting TDR traces are recorded separately and, in almost every case, at least one of the traces will clearly show the distance to the fault.

    An alternative way of localising faults that can't easily be seen with a TDR alone is the impulse current method. For this, the test set sends out a high-voltage pulse to establish a flashover at the fault, and the transient memory function of the test set is used to record the transients created by the flashover.

    These transients travel back and forth along the cable with peaks that can be used to indicate the distance to the fault. In practice, the first reflected peak must be ignored due to the re-ionisation period, but the time interval between the second and third peaks gives a good indication of the cable length between the test set and the fault.

    The techniques described so far all have one thing in common - they provide a measurement of the fault distance to the cable fault from the point of connection of the test set. Even if details of the cable run are known, this is sufficient information to determine the fault distance but not to locate the fault, as the cable rarely sits straight and horizontal in a trench or duct. In many cases accurate information about the cable run is not available. So a little further work is needed to locate the fault.

    To precisely locate the fault position, a technique called pinpointing is used. This method of pinpointing faults in cables uses a surge generator - often known as a thumper in this application - to apply high voltage pulses to the cable. These pulses result in flashover at the fault location, which generates an audible noise - the thump. It also generates an electromagnetic field that can be detected by a suitable receiver.

    Sometimes the thump from the fault is loud enough to be heard without any additional equipment but more commonly, especially with buried cables, a pinpointer is used. This is basically a sensitive ground microphone connected to an amplifier and headphones. The user simply moves the pinpointer along the cable run until the thumping is most clearly heard and the magnetic field is strongest. This should be the fault location. 

    Faults with cables in ducts can be difficult to find, however, as the sound can travel down the duct making the listener less able to pinpoint the exact fault location. It is at least easier and less costly to replace a section of cable in a duct than dig up a direct buried cable. Although many faults in power cables are high resistance faults where the thumping technique is very useful, it's worth mentioning that not all cable faults will thump. Short-circuit faults for example do not flashover, so no electromagnetic field is formed and because the energy of the pulse is not dissipated in the form of sound, there is no thump to locate.
    In this instance a TDR and a cable route tracer can be used to find the distance to fault, but locating the exact site of the fault is more difficult. This is why the low voltage tests are applied first, before conditioning causes a resistive fault that may flashover to become a short circuit that won't thump.

    No one would claim that locating faults on power cables is easy but there are many types of test instruments now available that, when used in conjunction with a structured approach to fault location, will provide assistance in locating even the most intransigent of faults. The days of the black art of cable fault locating are past, because it is now too costly and too time consuming to go down this route. Since the faults themselves often lead to downtime and the associated consequential losses, money invested in the latest cable fault location equipment is money very well spent indeed!

  • Test and Measurement - Power transformers – are you covered?

    It's easy to assume the substation on your site belongs to the power utility, but are you absolutely sure? If you get it wrong, says Damon Mount of Megger, and you're unlucky enough to suffer a transformer fault, you could find yourself landed with a bill for tens or even hundreds of thousands of pounds

    In the substation, the power transformers are probably the most expensive items. And that's not the worst of it - the delivery time for a replacement transformer is typically months - or even years for the largest types. The direct and indirect costs associated with a transformer failure can, therefore, be enormous.

    But there's surely no need for concern. All of the power transformers on your site are the responsibility of your energy supplier, aren't they? It may be a very good idea to check again. In a surprisingly large percentage of installations, the power transformers belong to the owner of the premises, and not to the power utility.

    Of course, there's still no reason to worry, because transformer failures will certainly be covered by insurance, won't they? The answer is possibly not. Because of the huge costs involved, insurers are understandably cautious about making payouts relating to transformer faults and failures. If there is a claim, they will certainly ask for evidence to show the transformer has been regularly tested and maintained.

    Since many companies are not even aware they are responsible for the power transformers on their sites, it's not too much of a surprise there are a lot of transformers that most certainly don't get the regular attention they need.

    This is a special concern with the many transformers currently in use that have long exceeded their design lives. Although they may apparently still be working well, it is inevitable some of the materials used in their construction - in particular the insulating materials - will have started to deteriorate.

    If an unmaintained transformer fails, whether it is old or new, it's perfectly possible that the insurers will contest the claim or refuse to pay. Let's take a look at what needs to be done to avoid this potentially devastating situation.

    The first and most obvious step is for maintenance departments to check which of the transformers on their site are their responsibility. The next step is to implement a regular testing programme for these transformers.

    But what form should the testing take? There are, of course, many types of conventional tests that can be applied to power transformers to check, for instance, the performance of the tap changers or the windings.

    This means to build up a reasonably complete picture of the transformer's condition, a whole battery of tests is needed, which will take a considerable time to perform. During this time, the transformer will be out of service, which can be very inconvenient.

    There are, however, two tests that between them can provide a wealth of information, not only about the presence of faults but also, in many cases, their type and location. These tests are sweep frequency response analysis (SFRA) and frequency domain spectroscopy (FDS).

    Electrically, a transformer is made up of multiple capacitances, inductances and resistances. It is, in effect, a very complex circuit that produces a unique ‘fingerprint' when test signals are injected over a range of frequencies and the results plotted as a curve. In particular, the capacitances in the transformer are affected by the distance between conductors.
    Movement of the windings, which can be caused by electrical overloads, mechanical shocks or simply by ageing will, therefore, alter the capacitances and change the shape of the frequency response curve.

    The SFRA test technique for transformers is based on comparisons between measured curves, which allow variations to be detected. An SFRA test involves multiple sweeps and reveals whether the mechanical or electrical integrity of the transformer has been compromised.

    SFRA tests are used to capture a ‘fingerprint' reference curve for each winding when the transformer is new or when it is known to be in good condition. These curves are subsequently used as the basis for comparisons during maintenance or when problems are suspected.

    The best way to use SFRA testing is to take regular measurements on the same transformer over a period time, and to compare the results. However, it is also possible to use type-based comparisons between transformers with the same design. Finally, a construction-based comparison can be used in some circumstances, when comparing measurements between windings in the same transformer.

    A single SFRA test can detect winding problems that would otherwise require multiple tests with various kinds of test equipment, as well as problems that cannot be detected at all by tests of other kinds.

    As a general guide, magnetisation and other problems relating to the core alter the shape of the SFRA curve at the lowest frequencies, up to around 10 kHz. Medium frequencies, from 10 kHz to 100 kHz represent axial or radial movements in the windings, and high frequencies above 100 kHz correspond to problems involving the cables from the windings to bushings and tap changers. In modern SFRA test sets, built-in analytical tools simplify comparisons between curves.

    While SFRA tests provide a lot of information about the condition of a transformer, they do not give an accurate indication of the presence of contaminants - in particular water - in the transformer insulation. Standard tests, such as the widely used Karl Fischer test, are, of course, available for accurately assessing the moisture content of transformer oil, but this is not the whole story.

    In fact, it is usual for a much greater percentage of the moisture in a transformer to be held in solid insulation such as paper than is held in the oil. To further complicate matters, the moisture moves between the solid insulants and the oil in a way that is influenced by many factors including, in particular, temperature. 

    Measuring the moisture content of the oil may not, therefore, provide dependable information about the moisture content of the transformer's solid insulation. This is a serious concern, as moisture in the insulation significantly accelerates the ageing process in transformers and, in addition, it can cause bubbles between windings that lead to sudden catastrophic failures.

    To establish the moisture content in the transformer, the second of the tests mentioned earlier - frequency domain spectroscopy (FDS) - can be used. Initially, this may sound a lot like SFRA, as it involves measuring transformer characteristics at over a range of frequencies. This time, however, it's the dielectric properties of the insulation (capacitance, loss and power factor) that are measured over a range of frequencies, typically from one millihertz to one kilohertz.

    These are, in essence, the same dielectric tests that are often carried out at power frequency, but testing at a single frequency provides far less information than is revealed by FDS testing. Unlike spot-frequency testing, FDS can, for example, reliably distinguish between a transformer that is dry but has bad oil, and one that is wet but has good oil. In the first case, the oil needs refurbishing or replacing; in the second the transformer only needs drying out.

    FDS testing also has other benefits - it can be performed at any temperature, and the test can be completed quickly. Software can be used to calculate the water content in percentage terms, and modern FDS test sets typically provide accurate and detailed results in less than 20 minutes.

    As we have seen, regular testing using the SFRA and FDS test techniques provides a reliable insight into the condition of power transformers, but how can this information best be used by the transformer owner?

    A short-circuit fault on the transformer may cause unseen damage inside, and a damaged transformer put back into service could fail catastrophically. An SFRA test can be done before re-energising and compared to a reference trace taken while the transformer was in good working order. If the two traces match, nothing has changed and the transformer can be safely returned to service. Carrying out this test takes less than an hour, reducing outage time and saving money.

    Ageing, mechanical damage and moisture content can be seen as a change in the frequency response of the transformer over time and may indicate that remedial action, such as drying out the transformer, is needed to guard against future failures. In other cases, it may show that the transformer is inevitably coming to the end of its useful life, but even then the information is invaluable.

    In this situation, it may be possible, for example, to minimise the load on the transformer so it can continue in service until a replacement is obtained. And even in the worst case, there is at least a warning that failure is imminent, which can allow time for contingency plans to be made and put into place.

    There is also another very valuable aspect of regular testing, which we touched on earlier. Insurance companies are more likely to honour a claim for failure of a power transformer that's been regularly tested and properly maintained so as to remedy any issues identified by the tests. Such a transformer is, of course, less likely to fail, but if it does there is at least the consolation that the insurers will foot the bill!

    Even for those who are aware of their responsibilities in looking after power transformers, regular testing may appear as something of a burden. However, tests with modern instruments can be performed quickly and easily, and they yield dependable informative results. And, if the test regime eliminates just one unforeseen transformer failure that would otherwise have occurred, the effort involved in testing and the cost of the instruments used will have been repaid many times over.

  • Test & Measurement - Production line testing: What price safety?

    Ben Croucher, applications and sales engineer, Clare instruments, looks at the  paramount importance of electrical safety testing in the manufacturing environment

    With verification of the safe operation and functionality of electrical products being vital to ensure compliance with established industry standards and maintain customer confidence, the focus has switched to the extent of testing required.

    This requirement to ensure conformance through manufacture is clear from both generic product safety standards and European Directives, but the common reaction still seems to be ‘does this mean I have to do 100% testing?' followed by a rapid retreat into discussions that aim to reduce an erroneously perceived time/cost burden, often quoting ISO9000 procedures and focusing on sample testing as a suitable solution.

    Batch sampling and or product verification tests are essentially designed to determine type test and build instructions are being maintained via a set of ‘working standards' and rely upon there being a traceable scientific relationship between the ‘sample' and the rest of the batch.  The assumption being if the sample shows conformance, then the rest of the batch also complies. However when customer safety is paramount can anyone take this risk?
    In order to maintain a proper scientific relationship, back to the ‘type approved product' testing of the batch sample should really involve a repeat of the ‘type test' which could involve the use of external test house or the transfer of the sample to a dedicated, in-house  test laboratory. In either case the test will require the use of skilled and expensive labour, specialised (and usually high cost) test equipment, complex, time consuming, test routines and/or possible destruction of test sample.

    Taking a typical batch sampling routine as an example, the following scenario can be envisaged: Risk analysis determines a procedure for testing one sample product for every 100 that come off the assembly line.  The sample is sent to the laboratory where it undergoes rigorous testing and fails. Strictly speaking, production should now be halted until the cause and extent of the fault is identified. 

    This should include recalling and testing not only the remaining 99 items of the particular batch, but any items produced / packed and shipped out since the sample was taken.

    The cost of this exercise can be worked out in terms of re-call costs (time, labour, discard packaging etc) - even greater if products have left the factory - testing costs (which will now include skilled labour), rework costs (time, labour, parts if any), lost production (highly unlikely that all items are salvageable) and late delivery penalties.

    To review the real on-cost to a business a useful investigation would be an ISO9000 ‘re-call' procedural review and cost them accordingly. It might be argued that this worst case scenario only applies if the sample fails - but would anyone feel comfortable knowing that the electric drill used in a workshop has only a one in 100 chance of NOT causing electrocution.

    Similarly, it is clearly in the interests of manufacturers of finished products that the safety critical components used to assemble a product are satisfactory - preferably before being incorporated into the product. Many manufacturers now request ‘certificates of conformity' (CofC's) from their suppliers of safety critical components. However the question always has to be ‘how sure are you of their test regime?'

    Against this background it is clear there are increasing numbers of manufacturers of electrical products who wish to check supplied components before or during their own product assembly.

    Among such companies, there is recognition of the advantages which can result in the pro-active identification of problems and defects before assembly, increases confidence in finished products, reduces the likelihood of product re-work and permits the cost of failures to be recovered from the supplier more easily.

    By completing the cycle with 100% product testing, significant information can be gathered and used to improve and refine manufacturing processing and techniques. Identifiable reasons for product failures can be highlighted and quickly acted upon. Even simple fault counters can indicate particular areas of the build phase that may require further investigation.

    Another major plus for 100% testing is the development of a competitive advantage, in that a company's ability to offer full testing during their own production processes reduces the need for the customer to carry out their own testing, thus offering a level of added value that can be translated into increased profitability, plus customer confidence and loyalty. But what is meant by 100% testing?

    Firstly it should be noted we are talking about electrical safety requirements. Manufacturers will review their own processes for Class I and Class II products and accordingly introduce the three main tests for ensuring product safety: High current earth bond measurement, insulation resistance measurement and high voltage flash (or dielectric strength) test. In addition many manufacturers will be driven by standards/customer requirements or even their own in-house guidelines to complete functional tests (also known as run or load leakage testing).

    A number of criticisms have been made against 100% testing, again usually on the basis of time and cost.

    On the time factor, concerns normally arise from misconceptions between type testing requirements and the established practices for 100% routine production line testing. A typical regime of electrical safety testing to meet these routine test requirements can be completed in less than five seconds. Referring back to the earlier example, all 100 products could have been tested in less than nine minutes.

    In terms of cost, equipment can be expensive if the type test requirement is to be employed. However for routine production line testing, there are a number of  systems available that can cost from as little as £1500. With simple to use set-up and control features, they can be readily incorporated into the production environment without the need for highly skilled labour.

    For type testing, a flash test can require high current levels (sometimes in excess of 100mA) and extended test times (several minutes for some standards) and consequently this type of test can require application under closely controlled conditions, involving the use of highly skilled and experienced test personnel. 

    However for routine production line testing, electrical safety standards define not only a lower safer trip level, but also the setting up of the test area is well defined to keep safe the operator. Experience has shown that routine test parameters provide a realistic evaluation of electrical safety and does not harm equipment that is designed to comply with the relevant standards for creepage, clearance and insulation properties. 

    Where delicate electronic components are involved, far from omitting the flash test, various techniques can be incorporated to soft-start (ramp) the test voltage, apply DC voltages with discharge circuits etc...thus removing any likelihood of damage occurring - a procedure recognised by EN60950 for information technology (IT) equipment. 

    Far from costing time and money, 100% electrical safety testing on the production line makes sound economic and business sense creating a competitive advantage and peace of mind. After all only 100% testing can categorically show 100% conformance.

  • Test & measurement - Is your resistance low or are you getting hot?

    Low resistance measurement is a well-established technique that can be used almost anywhere electrical conductivity is important - its applications range from checking the quality of earth bonds to verifying the density of graphite electrodes in aluminium refineries. Recently, however, thermal imaging has been proposed as a simple and effective solution in many of the same applications. But is it?

    Low resistance measurement is a well-established technique that can be used almost anywhere electrical conductivity is important - its applications range from checking the quality of earth bonds to verifying the density of graphite electrodes in aluminium refineries. Recently, however, thermal imaging has been proposed as a simple and effective solution in many of the same applications. But is it?

    The real answer is that both low resistance testing and thermal imaging have their place so, in order to decide which to use where, let's take a look at the strengths and weaknesses of each.

    A big benefit of low resistance testing is it can detect problems even when there is no current (other than the test current) flowing in the object under test. This makes it very suitable for applications such as checking weld quality, verifying the performance of lightning protection bonds, confirming the integrity of aircraft structures and testing earth systems.
    Low-resistance testing is also invaluable in manufacturing applications, particularly where it is necessary to test subassemblies rather than complete systems, and for checking new or modified electrical installations prior to energisation. Thermal imaging is unlikely to be suitable for any of these applications.

    A further benefit of low-resistance testing is that it provides straightforward numerical results, which can easily be recorded and, even more useful, trended as part of a predictive maintenance programme.

    Having said that, low-resistance testing does, of course, have its limitations. It can't, for example, be used on live equipment. For equipment that's in service, therefore, it's necessary to arrange for the supply to be isolated before carrying out the test, which is not always convenient. In addition, if there are many connections to test, low-resistance testing can be time consuming.

    Turning now to thermal imaging, it is a good way of checking for overloads and unbalanced loads, which can't be done with a low-resistance tester. Thermal imagers also have non-electrical applications, such as finding the locations of heat loss from buildings, and detecting mechanical faults such as worn bearings in a motor, which heat up because of excessive friction.

    Thermal imaging also has the reputation of being easy to use, but that's not always the case - the operator needs to understand what they are seeing and to be able to interpret the results. For example, is a transformer overheating, or is it at its normal operating temperature? What is the load on the equipment while the test is being carried out? At what point does the temperature rise become a problem?

    In high-voltage environments, such as an electrical substation, a further complication is that is often not safe to get close enough to the equipment to image it clearly. In addition, items such as fuses and circuit breakers are usually mounted in metal enclosures, and thermal imaging will not work through metal.

    It is often unsafe to remove covers or open doors with the supply switched on but, by the time the supply is isolated and the covers removed, the equipment will have cooled significantly, making the thermal imaging data of dubious value.

    It can also be difficult to accurately relate the thermal image to the equipment being evaluated, and it is sometimes necessary to take a normal digital photograph and then use a PC to overlay this with the thermal data. Finally, trending thermal images to identify changes over time is not particularly straightforward.

    Thermal imaging is, as we have seen, a very useful technique but it complements rather than replaces low-resistance testing. And there are many applications where nothing but a low-resistance test will do. It does, however, pay to take a little care in selecting a low-resistance test set if it is to offer maximum versatility and convenience.

    For example, it is all too easy to make an accidental connection to a live supply when attempting to carry out low-resistance tests, particularly when testing busbar bonds and battery straps in UPS installations. It is important, therefore, the instrument is suitably protected.

    In many test sets, this protection is provided by a fuse, but this is not particularly convenient as, if a suitable replacement is not to hand, the instrument is not useable until a replacement can be obtained. Better low-resistance testers, such as those in the Megger DLRO10 family, are intrinsically protected against connection to live supplies. With these instruments, it's possible to carry on testing normally as soon as the errant supply has been properly isolated.

    It is also important to select an instrument that can supply a test current appropriate to the application - ideally, it should offer a choice of test currents covering a wide range. This is because high test currents can, in some cases, cause unwanted heating of the test piece, while in other cases the heating caused by high currents is actually desirable, as it can help to reveal weaknesses such as broken strands in a multi-core cable.

    Similarly, the usefulness of low test currents is also dependent on the application. Low currents may be a problem in some circumstances, as they make not break through the contamination in bonds. In other circumstances, however, this may be a benefit, because the same situation can provide a useful indication that contamination is present!

    In addition, a low test current combined with test current reversal may eliminate the need for temperature compensation of the results, and it also has the benefit of extending battery life in portable instruments.

    Finally, ease of use is a crucial factor. For maximum convenience in day-to-day use, the test set should have an intuitive user interface, and it should perform tests quickly and efficiently, otherwise it will rapidly become a constant source of irritation rather than a useful tool.
    In conclusion, it's clear that both thermal imaging and low-resistance testing are invaluable techniques and the ideal situation is to have access to test equipment for both. Only then can you be absolutely certain of providing a definitive answer to the question that we've all, at one time or another, asked - is your resistance low, or are you getting hot?

  • Test and measurement - Increasing the power of test data management

    Advances in electrical safety testing data collection and management systems have significant benefits, says Jim Wallace of Seaward

    With advances in technology making everything faster, smarter and smaller, and computer programs streamlining data management for even greater efficiency, the needs of those contractors and engineers involved in test and measurement work have never been better catered for.

    Advances in test instrumentation mean new lightweight, Bluetooth enabled hand held instruments complete safety test sequences very quickly and with the minimum of fuss - no matter whether installation testing or portable appliance testing is involved.

    Over time the introduction of advanced microprocessor based testers, powerful software-based record keeping systems and PDAs, mobile phones or specialist modems to transmit test results, have succeeded in helping the test engineer or contractor to provide a faster, more efficient electrical safety testing service.

    All these advances have been made in recognition of the need to improve and also to enable them to add value to the test process. The result has been not only better operating efficiencies, but also an enhanced relationship with customers and end users, generating important new business opportunities in the process.

    With the ever present need to undertake inspection and testing quickly without compromising quality, there is increasing importance on the linkage between test instrumentation used in the field and central test records systems that produce test certification and other test reports.

    Rather than simply ‘electrical testers', in broad terms the most advanced 17th edition testers and portable appliance (PAT) test instruments might now be regarded more as test data collection tools - gathering the important measurements and checks carried out on electrical systems and equipment.

    On a practical level there is now greater emphasis on the ability to enhance the transfer of this data between the tester and the database - and particularly on how the collected data is acquired, interrogated, managed and presented for more effective control of safety testing programmes.

    In this respect new innovations in both test instrumentation and record keeping software programs have not only brought electrical engineers and PAT service companies opportunities to provide more efficient test services, but have also created a means of real differentiation between the services offered by different electrical contractors.

    17th edition testing

    For example, in 17th edition testing the latest specialist instrumentation incorporates an electronic copy of the inspection and test certificate within the hand held tester - and in the process becomes a combined multi-function electrical tester and data logger.

    Onboard electronic certificate software enables electrical installation test and inspection data to be recorded directly by the tester using a replica of the inspection and test certificate which is displayed on the instrument.

    During inspection and testing, the user can navigate around the onboard certificate and when measured values are required, the results are automatically placed in the correct certificate fields.

    Once all inspection and test data has been collected, onboard software scans the certificate and warns the user if any fields appear to be incomplete or invalid. The integral ‘certificate assistant' also holds many of the commonly used tabulated values, such as earth loop impedance tables, avoiding the need to take bulky reference material onsite.

    When inspection and testing is complete, the certificate held inside the tester can be transferred to main PC records for the completion of certificates which can be printed or supplied in electronic format to customers in line with the 17th edition wiring regulations.
    The latest version of this program enables test results obtained from larger individual sites, such as shopping malls or commercial office complexes, can be merged onto one certificate.
    Another new feature is the ability to ‘clone' certificates from an existing master document. This allows the user to select an existing certificate and use this as a template to create multiple certificates for identical or similar electrical installations - for example of the type required for a housing development of the same type of properties and electrical systems.
    As a result, not only does the combined inspection, testing and certification system eliminate the need to record results on a dummy certificate while inspection and testing is being carried out, it also means that the often cumbersome and problematic use of PDAs, smart phones and laptops for test data transfer is avoided. 

    The new instrument incorporates Bluetooth download and upload of data to and from PC certification and record keeping systems and a wide variety of certificate templates can be loaded into the tester using the accompanying PC software.

    The specially developed software program includes all required 17th edition certification and can print onto ECA, NAPIT and NICEIC stationery.

    The result is a highly efficient and effective 17th edition inspection, testing and certification system with full traceability and reduced likelihood of human error in the recording and transfer of test data.

    Portable Appliance Testing
    In the PAT testing sector powerful test data management packages are available to facilitate the two-way transfer of data between the tester and the test records software.

    In this way engineers can pre-program or upload their testers directly from the PC with the necessary equipment details and testing information required before the day's work begins and then download updated results directly into the records programme at the end of the shift.

    The same software programs can also be used to create asset registers for customers, print test certificates and output test reports in different formats.

    The use of Bluetooth enabled testers further enhances this flexibility and means that for large PAT service and contracting organisations the effective use of data management software can greatly improve the margins associated with operating efficiencies gained for remote or off site working.

    For more specific monitoring of PAT productivity in the field special software is also available that works alongside the PAT results database to provide a clear picture of tester usage.

    Special time manager software provides clear information on the test activity of individual users and engineers - providing such details as time of test, number of tests undertaken and time between jobs.

    Analysis of such information enables service or contract managers to understand how often testers are being used, identify improvements in staff training and help field staff to test faster and work more efficiently.

    In terms of customer service improvements, another innovation is the use of PAT management software to identify and plan re-testing schedules quickly and effectively.

    This feature works through specialist software that constantly monitors the test records stored in a PAT results database, automatically triggering re-test notices for those that are approaching the next test date.

    The pre-trigger feature enables re-test schedules to be highlighted prior to appliances becoming overdue for test, with special e mail alerts being sent to customer contact personnel to give advance warning of the presence of any potentially unsafe electrical equipment in the workplace.

    The system can also be configured to submit formal re-test price quotations with the alerts for a complete test scheduling and costing proposal, boosting repeat business and enhancing the levels of customer support provided.

    For all types of electrical test and measurement activity, the combination of innovative test instrumentation with sophisticated record keeping programs provide real practical benefits to contractors - reducing costs, increasing revenue and improving productivity.

    In addition, integrated test systems can also play a significant role in enabling a contractor to provide a truly professional approach and this can only help in the long-term development of their business.

  • Test & measurement - Smarter choices in electrical testing

    Jim Wallace of Seaward, explains how advances in test technology have increased the range of test instrument options available to contractors

    For contractors involved in electrical testing there has never been a wider choice of test instruments available.

    In recent years the instrumentation industry has been at the forefront of innovation and technical advances. These changes have been made in recognition of the situation for electrical companies, particularly during difficult economic conditions, the challenge is to balance the provision of efficient, high quality test services with a competitive price tag and value for money offering.

    The test companies that flourish will be those that combine a fast and effective service that does not compromise the quality of testing undertaken - and who can build on existing customer relationships.
    In fact customer service and satisfaction levels have become a crucial area for electrical test companies. With less work around existing relationships become even more important. It follows that an ability to enhance existing customer services through the provision of a cost-effective and added-value test service can do much to both reinforce a company's reputation and maintain a positive profile with influential prospects.

    In addition, as well as a wide variety of testers available, the ability to provide a seamless link between test instruments used in the field and central test record systems that produce test certification and other reports also takes on even greater importance.

    The good news for large and small contractors is advances in test instrumentation mean a range of options are available to meet specific test needs - and budgets - for periodic electrical installation testing or portable appliance testing.

    In 17th edition testing
    The HSE's Guidance Note GS38 provides guidance to electrically competent people involved in electrical testing, diagnosis and repair. The note identifies three main test instrument categories - those that detect voltage, those that measure voltages and those that measure current, resistance and (occasionally) inductance and capacitance.

    The first named forms an essential part of the procedure for proving a system dead before starting work, whilst the other categories are more concerned with commissioning and testing procedures and fault finding.

    Guidance note GS38 provides details of the risks associated with the use of unsatisfactory test equipment and includes a list of safety precautions and requirements all professional electricians should be aware of.

    However, in terms of selecting appropriate 17th Edition test instruments, electrical contractors are broadly faced with a choice between ‘multifunction/combination' testers or single application specific testers.

    As the name implies the latter are designed to carry out one specific function - RCD testing, insulation, earth resistance etc - and the ‘all in one' type testers are single units designed to carry out a wide range of tests including earth loop, insulation resistance, continuity, RCDs etc.

    Choice invariably depends on the scope of work to be carried out, but increasingly it is the multifunction testers that have become the preferred tools of the trade for those involved in 17th Edition testing. This is for both practical reasons, in terms of using one meter constantly rather than swapping and changing between testers, and also for budget considerations - buying, maintaining and calibrating one combination tester is invariably cheaper than buying three separate ones.

    Multifunction 17th Edition testers carry out the required circuit tests and display the test reading for transfer onto the test certificate manually or alternatively, readings can be recorded on a PDA and transferred to a desktop application for certificate printing. Some testers are also linked with smart phone and portable laptop applications which work in the same way by gathering test data collected in the field for subsequent transfer onto a master certificate.

    The latest generation 17th Edition testers eliminate the use of intermediary devices by storing a replica of the test certificate within the tester so test data can be automatically incorporated onto the certificate as testing is undertaken.

    In this way the instrument combines the functions of a multifunction test instrument and data logger. When inspection and testing is complete, the certificate held inside the tester can be transferred to accompanying PC software for the completion and print out of formal certificates.

    As a result the time consuming (and therefore costly) practice of recording results on paper, a dummy certificate or a PDA is avoided. In addition, because the tester warns the user if any certificate fields appear  incomplete or invalid, verification of data can be carried out on site immediately and without return visits.

    Recently the concept on ‘on board certification' in testers has been extended with additional features aimed at large testing organisations or the testing of large premises.

    For example, the moist advanced 17th Edition testers now have the ability to upload certificates generated on a PC into multiple testers. This is particularly useful in situations where a number of test personnel might be working on the same large installation, such as a hospital development or shopping mall and enables specific test work to be allocated to a number of engineers very easily.
    Once testing has been undertaken, the software enables test results downloaded separately from different testers to be merged into a single certificate for the premises concerned.

    Another new feature is the ability to ‘clone' certificates from an existing master document. This allows the user to select an existing certificate and use this as a template to create multiple certificates for identical or similar electrical installations.

    The cloned certificates will contain all of the distribution boards and circuit details held in the original and therefore represents an easy way of generating certificates for, say, 20 or more  houses on a street which all have the same electrical configuration.

    In these combined testing and certification testers, all data transfer between the PC and the instrument can be achieved easily using Bluetooth connectivity. This means a certificate can be uploaded to the tester, the required test and inspection carried out and the information downloaded to a PC and the final certificate printed directly onto pre-printed NICEIC, ECA, ECA Select or Napit stationery.

    With such a wide range of test instruments and accessories to choose from, electricians and contractors involved in 17th Edition electrical testing can be sure the right test package solution is available to meet their specific needs and budget.

    In recent years substantial technical development has gone into the development of new test instrumentation so the ‘tester' can now be used in a much more effective manner - improving operational efficiencies, adding value to the test process and enhancing customer relationships.

  • Test & measurement - Putting engineers back in control

    IEC 61850, the new standard for substation data networks, is creating a lot of interest and  excitement. It's also creating more than a few challenges, says Romain Douib of Megger, not least for substation control engineers who spend their lives creating and working on interlocking schemes

    One of the biggest challenges substation control engineers face, is not how to implement interlocking schemes based on IEC 61850, but how to test them. The problem is particularly acute, because at present IEC 61850 is being more widely used for interlocking than it is in protection applications.
    Of course, options do exist for testing IEC 61850 interlocking schemes. However, these almost always involve the use of protective relay test set that supports IEC 61850. This approach, however, is far from ideal. The first concern is that, in most cases, control engineers are not protection engineers. They are unlikely, therefore, to be familiar with the operating a protective relay test set. They could, of course, learn, but that's a pretty steep learning curve for something that is not central to their work.
    Another issue is protective relay test sets are necessarily costly, since they incorporate high-performance precision amplifiers and other elements that are expensive to develop and produce. Yet these are not needed for testing interlocking schemes, so using a relay test set in this application is not only overkill, it also needlessly ties up expensive capital equipment.

    It's clear there is a pressing need for a reasonably priced instrument that is simple to use and provides all of the facilities needed for testing IEC 61850 interlocking schemes, but does not incorporate the expensive extras needed for protective relay testing.

    It's not difficult, in principle at least, to imagine how such a test set would work. First of all, it would monitor the Goose messages IEC 61850 installations use to communicate and it would convert them to the ordinary type of on/off binary signal that control engineers are used to working with in non-networked installations.

    The test set would also be capable of working in the opposite direction. That is, it should take signals from ordinary contacts and convert them into appropriate Goose messages. In effect, a test set of this kind is simply an interface between the Goose messages on the bus and the electromechanical world of the control engineer.

    Of course, there's rather more to be considered than this very basic overview initially suggests. For example, the conversion between Goose messages and binary signals must be fast enough so as not to materially affect the timing of the interlocking system. In practice, a conversion time of less than a millisecond, which is achievable with careful design, will be fast enough to satisfy the most demanding of requirements.

    Next, it is clearly necessary to be able to associate particular Goose messages with specific inputs and outputs on the test set. This is best accomplished with software but, if it is to be intuitive and easy to work with, the software needs to be carefully designed. Further refinements can also be envisaged. For example, LEDs that provide instant visual confirmation of the state of the instruments binary inputs and outputs would be an important benefit for users.

    The ideas mentioned in this article have driven the development of Megger's new Goose Message Interface.. This embodies a number of unique technical features for which patents are pending, and offers the most efficient and cost-effective solution currently available to the challenge of testing IEC 61850-based substation interlocking schemes.

    That is, however, by no means the limit of the capabilities of the Goose Message Interface. While it may not be particularly interesting to control engineers, the unit can also be used to adapt a conventional protection relay test set so that it can be used to test IEC 61850 protection schemes. This is a big benefit for users that already have protection relay test sets - whether they are units supplied by Megger or by others - as it is offers a very straightforward and cost-effective upgrade path.
    It also creates an attractive option for consultants and smaller organisations who can now purchase a Goose Message Interface and a modestly priced relay test set, to cover all their relay and interlocking test requirements for both conventional and IEC 61850 schemes.

    Equipment that allows convenient and dependable testing of IEC 61850 interlocking schemes has, until now, been difficult or even impossible to find. This situation has now been addressed by Megger's Goose Message Interface, an instrument that provides the added bonus of facilitating the testing of IEC 61850 protection schemes.

  • Test & Measurement - Top tips for cabling and test fixture safety

    In general, test cabling and test connections must all be designed to minimise resistance (R), capacitance (C), and inductance (L) between the device under test (DUT) and the used source-measure unit (SMU) explains the applications engineering team at Keithley Instruments

    To minimise resistance, use heavy gauge wire wherever possible, and definitely within the test fixture itself. The gauge required will depend on the level of current being carried; for example, for cabling that must carry 40A, a 12 gauge cable is probably necessary. For guidance on choosing cabling for higher current levels, refer to construction industry wire gauge tables, such as the one available at: www.powerstream.com/Wire_Size.htm. Check the ‘Maximum amps for chassis wiring" column to find the wire gauge needed to carry the level of current involved.

    Low-resistance cabling is critical to preventing instrument damage. Choose cables with resistances of less than 30 milliohms/meter or lower for 10A pulses. Keep cable lengths as short as possible and always use low-inductance cables (such as twisted-pair or low-impedance coax types), heavy gauge cable in order to limit the voltage drop across the leads. Ensure the voltage drop won't be excessive by checking the SMU's Voltage Output Headroom spec. For example, if you were using a Keithley Model 2602A (pictured above) SMU to output 20V, the test leads should have no more than 3V of voltage drop across them to avoid inaccurate results or instrument damage. It is specified for a maximum voltage of 3V between the HI and SENSE HI terminals and a maximum voltage of 3V between LO and SENSE LO. 

    Although many believe guarding can minimise the effects of cable charging, this is typically more of a concern for high voltage testing than for high current testing. Four-wire Kelvin connections must be kept as close to the DUT as possible; every millimetre makes a difference.

    Also, it should be noted 0voltage readback should be done with the SMU that's forcing voltage, because the current-sourcing SMU's voltage readings will all vary quite a bit due to the connections, and will differ from what is actually seen at the DUT.

    The jacks used on the test fixture should be of known high quality. For example, some red jacks use high amounts of ferrous content to produce the red colouring, which can lead to unacceptably high levels of leakage due to conduction. The resistance between the plugs to the case should be as high as possible and in all cases >1010 ohms.

    Many published test setups recommend adding a resistor between the SMU and the device's gate when testing a FET or IGBT. When pulsing large amounts of current through these kinds of devices, they tend to oscillate. Inserting a resistor on the gate will dampen these oscillations, thereby stabilising the measurements; because the gate does not draw much current, the resistor does not cause a significant voltage drop.

    If voltages in excess of 40V will be used during the test sequence, the test fixture and SMUs must have the proper interlock installed and be operated in accordance with normal safety procedures.

    Many electrical test systems or instruments are capable of measuring or sourcing hazardous voltage and power levels. It's also possible, under single fault conditions (e.g., a programming error or an instrument failure), to output hazardous levels even when the system indicates no hazard is present. These high levels make it essential to protect operators from any of these hazards at all times. Protection methods include:
    - Verify the operation of the test setup carefully before it is put into service.
    - Design test fixtures to prevent operator contact with any hazardous circuit.
    - Make sure the device under test is fully enclosed to protect the operator from any flying debris.
    - Double insulate all electrical connections that an operator could touch. Double insulation ensures the operator is still protected, even if one insulation layer fails.
    - Use high reliability, fail-safe interlock switches to disconnect power sources when a test fixture cover is opened.
    - Where possible, use automated handlers so operators do not require access to the inside of the test fixture or have a need to open guards.
    - Provide proper training to all users of the system so they understand all potential hazards and know how to protect themselves from injury. It's the responsibility of the test system designers, integrators, and installer to make sure operator and maintenance personnel protection is in place and effective.

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