Partial Discharge Monitoring In MV Substations

In 2005, a European utility adopted best practice in condition assessment of substations with a particular emphasis on insulation condition and the detection of partial discharge [PD] in distribution substations. A transportable PD alarm system was supplied by EA Technology, and found immediate application in the detection of intermittent PD activity. The results were corroborated by comparing it to an established PD monitor.

This article was based on a paper presented at CIRED 2009 in Prague.


The importance of continuous monitoring of Partial Discharge (PD) activity in switchgear and other assets has long been recognised at transmission voltages. The consequence of asset failure within a transmission network, in terms of interruptions and customer minutes lost, can be high. Transmission assets also have a high capital cost, therefore the addition of continuous PD monitoring can be of great value with a relatively small additional capital cost. In distribution networks, the viability equation changes. Assets are of lower relative value, and the consequences of failure of a single asset are generally lower. Portable instruments, therefore, tend to prevail as their cost can be amortised over the whole network of assets. The first stage of the utility’s investment programme was to deploy hand-held PD detectors, primarily for staff safety in entering substations or operating switches. UltraTEV Detectors detect PD activity using ultrasonic and Transient Earth Voltage (TEV) sensors. They are simple to use, with dangerous levels of PD indicated by a red light, triggering further investigation using more sophisticated instruments.




Since the introduction of a regular programme of PD surveys, discharges have been detected in 34 cable terminations (mostly heat shrinkable type), 18 switchgear busbar accessories and 11 ring main unit switches or circuit breakers. All showed high PD activity and immediate action was taken to repair or replace the defective components to prevent insulation breakdown and catastrophic failure of the substation. The use of portable instruments for regular surveys has proven to be a costeffective way forward.




However, three substations required ongoing monitoring because:

• they were critical to the security of the network, supplying important customers;
• they had a history of problems; and
• intermittent PD activity had already been identified in PD surveys.
  
  

An UltraTEV Alarm was acquired as a relocatable continuous partial discharge alarm system, to provide further information about the state of the switchgear in these substations. Where previously only a “snapshot” of readings was available, now these substations could be monitored continuously and intermittent partial discharges detected. Using the UltraTEV Alarm, maintenance engineers were able to plan an outage to repair the affected switchgear before any partial discharge became continuous or, worse still, the equipment failed.


UltraTEV Alarm
The UltraTEV Alarm system consists of a series of nodes attached to switchgear panels in the substation. Each node contains a TEV sensor, with an input from an ultrasonic microphone. Sensors and nodes are fixed to the panels with magnets. Each node indicates a “Current” TEV Alarm and a “Current” Ultrasonic Alarm. To prevent spurious triggering, the nodes indicate separate “Historic” TEV and ultrasonic alarms when a current alarm persists for at least 75% of a 2-minute rolling time period. The nodes are daisy-chained together from a Hub module with a single cable carrying both alarm signals and power. The hub registers a historic TEV or ultrasonic alarm from any node, which triggers a local visual or audible alarm. Alternatively, the alarm can be fed into a SCADA system, or to the GPRS/iHost system, allowing the alarm state to be remotely monitored by the asset manager through a web browser, which also enables EA Technology to easily view the results as part of its ongoing customer support.




Monitoring the Monitor
Almost immediately after installation of the UltraTEV Alarm, one substation indicated abnormal intermittent PD activity. To provide assurance that the new UltraTEV Alarm was operating correctly, EA Technology installed a PD Monitor in parallel on the switchboard. The PD Monitor is a 12 channel TEV-only monitor with advanced data collection and analysis. It records its data to a local FLASH memory card. Rather than just simple alarm states, it gives great detail on any PD activity [discharge amplitude, pulse counts, interval time stamping] and, by the use of its precedence circuitry between probes, it can locate PD sources in the substation. It is a well-established field service instrument used by EA Technology for many years. Short Term Severity (STS) is used to express PD activity over time, and is the product of the TEV magnitude in mV and the number of TEV pulses per cycle. The STS calculated from the PDM data was compared with the status of the UltraTEV Alarm over the same time interval.


Figure 4 shows a clear correlation between the UltraTEV Alarm status and the PD Monitor severity levels. When PD inception caused a rise in severity, the UltraTEV Alarm triggered. This gives very good confidence in the reliable operation of the UltraTEV Alarm system.




Conclusion
Two UltraTEV Alarm systems have now picked up critical partial discharge activity in both internal and external components including cast resin bushes and voltage transformers, which prevented faults which could lead to failures. The substations continue to be monitored by UltraTEV Alarms, with further investigations carried out using portable PD detection equipment on any alarm raised. Faulty components are replaced as soon as a full outage is possible.


The UltraTEV Alarm has proven itself to be a cost-effective method of low-cost PD monitoring, and the company is now considering extending its use to numerous secondary substations (11kV to 400V).


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