In the first part of a two-part article for Electrical Review, Paul Owen from Substation Expertise explains how a standard, low cost, laptop computer can be installed and set up with ‘off the shelf' graphics software and used to gather information from remote devices, connected along a ‘data bus' cable, (twisted copper pair, and / or optic fibre) from remote locations around a large building, or building complex
This feature covers small scale projects, using low cost products, and is not relevant to large scale Scada schemes (supervisory, control and data acquisition) such as power network transmission and distribution monitoring over a large geographical area, for example a city or county. Such projects are large scale and require much more expensive hardware and software - to near military standards - in order to achieve the speed and reliability necessary.
The definition of monitoring is usually regarded as look but don't touch and the computer displays data collected from input devices connected along the data bus. There are no outpu' devices in this case.
This function involves signals being sent either manually, via the mouse, or automatically via a ladder diagram to outputs connected along the data bus which then, in turn, initiates:
- motors to start or stop
- circuit breakers to close or open
- valves to close or open
- solenoids to close or open
- dampers to close or open
Intelligent electronic devices
Devices connected along the data bus are known as IEDs (intelligent electronic devices) and these can be simple products such as low end protection relays, power meters and digital I/O modules (input & output modules) etc which are both low-cost and easy to set up and prove. More complex IEDs such as disturbance recorders, fault analysers and protection relays where the parameters are remotely set, have very large data bases, and are considerably more expensive, and more time consuming to apply, set up and prove. Again, this article is focusing on the low end simple applications and in the writers view, for simple, small scale applications, any accessing of data from complex devices is best done by loading the specific data in question direct to specialist software within a laptop, down a short lead. Attempting to transmit this over the data bus, unnecessarily slows down communications and increases the volume of traffic and with it the increased risk of signal collision errors occurring. Each IED has a unique bus address, set either physically by small switches, or codes within the software.
To illustrate the difference between simple and complex IEDs one only has to look at the thickness of the applications & set up documents, which can vary from a few sheets of text to very thick manuals / CDs, which are very demanding both in time to read, and understanding.
Modern graphics software can seamlessly integrate digital and analogue signals from a wide range of distantly sited modules. Colour screens can show a considerable amount of information simultaneously, with real time animation, and in many different formats. These screens can be simple or complex in nature and the appearance is limited only by the imagination of the designer. An example of innovative graphics can show the single line power distribution diagram overlaid against the physical shape and structure of the building, or plot of land, which makes for a clearer understanding by the operator.
For small scale substation monitoring, the most suitable Scada software comes from a process control background and big names include In Touch by Wonderware Invensys and iFIX by Intellution. These products are well proven, well supported and modular in nature, which means the bigger the I/O (input / output) count, the more you pay.
There are many reasons for installing a laptop based substation monitoring system, which offers a range of benefits to the end user or facilities management company operating on their behalf.
Typical applications are listed below, and these can be standalone, or bundled together.
It should be noted that engineering time and integration costs will rise as more applications are added, and that the more complex you make it, the harder it will be to prove after installation.
Probably the most common application, where alarms can be very wide ranging, and include:-
- protection trips on 11KV & 415V circuit breakers
- 415V Main Switchboard Busbar volts lost
- computer hall temperature too high
- PDU (power distribution unit) busbar voltage too high (or too low)
- transformer high temperature & bucholtz gas detection relay operation
- watchdogs on protection relays, programmable controllers and uninterruptible power supplies
- fault passage indicators on 11KV ring main units, for identifying fault location
- substation alarms from tripping batteries, intruder detectors, smoke detectors
- overload tripping of pump and fan motor starters
- pre overload alert at say 95% of circuit current rating
Volt free contacts from protection relays and thermostats are hard wired to input modules connected along the data bus. The status of these alarm inputs is monitored by the graphics and the time and date of any alarm is recorded and logged.
All graphics pages will include an area to alert the operator to any fresh alarms, and when the alarm page is subsequently accessed, any active alarms will be listed and flashing / audible. When acknowledged, by clicking the mouse on a screen button, the flashing and audible alert stops. When cleared they are deleted from the active alarm list and transfer permanently to the historical alarm list. Critical alarms can be given priority and differentiated over those which are of less importance.
An important difference between expensive high end systems and low cost low end systems is the accuracy of time stamping. For example with a low cost system, the delay in the communications may result in an alarm time as being logged say 0.3 seconds after it occurred.
High end systems will access the correct time, from the data base within the protection relay concerned, and log the exact time of the alarm to within a millisecond. This sort of accuracy requires specialist hardware and software and setting up, and is consequently very expensive to achieve.
For 132KV transmission protection such accuracy is essential, however for monitoring 11KV & 415V distribution protection, within a building complex, it is probably not.
Most microprocessor based protection relays include watchdog contacts for monitoring failure of the internal electronics, which could otherwise, and dangerously, remain undetected for many years. Watchdog circuits always usually use a volt free contact which is open when no auxiliary supplies are present and remains closed once the auxiliary supplies are connected. The only time this contact opens is when the auxiliary supply is removed, or in the case of an internal electronics failure.
In practice the vast majority of these watchdog circuits are not used and a classic example of this is the temperature monitoring devices for cast resin distribution transformers, which are usually connected to operate in a non failsafe mode such that removal of the ac auxiliary supply (or a short term loss of 11KV mains supply) will not inadvertently trip the feeding 11KV circuit breaker. (i.e. behaving like an undesirable no volt protection relay)
There is a watchdog contact provided within these temperature monitoring devices to alert attention to an internal electronics failure, however these are very rarely connected.
Status monitoring is done in the same way as alarm monitoring, but without the urgency.
The date and time of all changes of state are logged and permanently recorded on a historical list, which can analysed later if required, should there be an incident.
- circuit breaker position, for example open / closed / in service / withdrawn
- earth switch position, for example earth switch open / earth switch closed
- motor starter status, for example available / not available / running / stopped
- valve / damper position, for example open / closed
- solenoid position, for example energised / de-energised
- substation doors, for example open / closed
Power distribution network overview
Graphics can show a helicopter view of an entire electrical distribution network, quickly diagnosing what has happened during a total or partial blackout, so that supplies can be reconfigured and restored as fast as possible.
In such situations, any fault must be identified, and located with confidence, before any switching can be done, otherwise an alternative supply circuit breaker may be closed onto the same fault, and so spread the blackout. This application can avoid the time wasted travelling from one substation to another, assessing and working out what has failed and where.
The health and status of the entire distribution system can be viewed from one point, with remote control of critical circuit breakers from the PC, if required. The single line power diagram can be shown very simply and clearly, with semaphores indicating circuit breaker status. Also, if deemed a good idea, line colours can show the conductor status, such as red for energised, blue for deenergised, flashing black / yellow for fault and solid green for an earthed section, possibly under maintenance. (Note, this does add complexity, so is best limited to the main circuits.)
The clearest symbol for a circuit breaker is probably a black and white semaphore, driven by two inputs to confirm open and closed conditions. Loss of both signals results in a default symbol of an empty circle (ie. condition unknown) and as such a broken wire, or loose connection, to an input module, will not indicate a false breaker position.
A similar situation arises in the event of lost communications, where you have a choice of freezing the symbols, or declaring position unknown.
Using two inputs to confirm one of two positions is known as double binary confirmation and ensures positive contact indication. Using only one input and defaulting to an assumed position saves on inputs but is not a reliable method.
Power consumption metering for load optimisation
Power consumption can be measured, at various points in the distribution network, then trended and the resulting profiles can be analysed and optimised, in an attempt to minimise overall consumption. This sort of analysis also focuses the mind on equipment left running for periods, when it could switched off. Also some very large loads may only be operated when other loads have been turned off, to minimise the maximum demand.
Multi function digital power meters can show a wide range of parameters such as Amps / Volts / KW / KVA / KVAR / power factor / frequency / max demand / harmonics etc, but the size of the display is relatively small and all values cannot be seen simultaneously, when viewing the instrument, they must be browsed. At the computer, the display is much larger and most values can be viewed simultaneously. Analogue values can be displayed in the graphics as either a digital number, or an analogue instrument, or both, whichever is preferred.
Circuits with power meters can also have pre overload alarms set in the graphics at say 95% of full load current, such that an alarm can be raised to initiate manual (or automatic) load shedding.
Also, a lost volts condition can be detected and alerted using a power meter, providing the auxiliary supply is separate and secure, otherwise communications will be lost during the lost volt period.
There are many multi function digital power meters available on the market, and the accuracy of the low cost meters is usually around 2%, which is sufficient for general analysis but possibly not for tariff purposes. High accuracy digital power meters are available for tariff purposes at around 0.2% accuracy, and for use in the UK they may need to be Ofgem approved.
When metering at 11KV, Utility companies often set the meters up to compensate for errors in the CT's & VT's which feed them, to achieve the highest possible accuracy.
To read the second part of this article, visit
www.electrical review.co.uk or see the October 2008 issue of Electrical Review.