Test & measurement - Substation surveillance Part 2

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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 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.

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.

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 one of this article can be found at www.electricalreview.co.uk

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