Partial Discharge Signals Switchgear Condition

Feb 1, 2008 12:00 PM
By Paul Shiel, ESB Networks

In 2002, a post-fault forensic investigation revealed that an insulation failure was the root cause Of a catastrophic failure of switchgear that occurred shortly after a switching operation. The failure was on a cast-resin ring main unit (RMU) that had been installed on ESB Networks' (Dublin, Ireland) 10-kV distribution network for more than 20 years. A RMU is a node on the distribution feeder, usually at a distribution transformer, that has breakers on each side to allow energisation from either direction. This failure resulted in the need for the utility to revisit the processes and procedures it had developed to manage the aging switchgear population.

As a result of the failure, ESB placed an immediate operational restriction on all 250 substations equipped with the same type of switchgear. The restriction required that this type of switch only be operated after it had been de-energised by switching farther up the line. The restriction resulted in a severe disruption to customers' supplies and led ESB to experience major difficulties in managing network operations.

ESB is the owner and operator of the distribution network in the Republic of Ireland. As the licensed distribution system operator, it is responsible for the construction, maintenance and operation of the subtransmission, medium- and low-voltage electricity network infrastructure. This responsibility includes overhead line and underground cable networks and substations installed to supply energy to Ireland's 1.8 million domestic, commercial and industrial customers.

FAULT-CAUSE INVESTIGATION

To resolve the problem and assist in the development of an operational protocol to help remove the restriction and reduce the disruption, ESB engaged the services of EA Technology Ltd. (EATL; Cheshire, U.K.). EATL and ESB agreed that the cause of the fault was an insulation breakdown, which is known to occur in high-voltage plant and equipment due to partial-discharge (PD) activity. It was suggested that the use of a field-based PD detection instrument could have detected the PD activity before the failure.

PD measurement in high-voltage/medium-voltage switchgear is an internationally recognised and approved test procedure. IEEE Standard 1291 (issued in 1993), the Guide for Partial Discharge Measurement in Power Switchgear states: “Partial-discharge measurements are an ideal method for evaluating switchgear apparatus with nonself-restoring insulation. During a temporary overvoltage, during a high-voltage test or under transient-voltage conditions during operation, partial discharges may occur on insulation of this type, which includes gas, liquid and solid materials. If these partial discharges are sustained due to poor materials design and/or foreign inclusions in the insulation, degradation and possible failure of the insulation structure may occur.”

In the United Kingdom, the Health and Safety Executive (HSE), the organisation responsible to the government for all aspects of workplace safety, has mandated that PD testing be carried out on all high-voltage/medium-voltage switchgear in its publication HSE 230 Keeping Electrical Switchgear Safe (issued in 2002).

In practice, PD in high-voltage insulation can be considered to take two forms, surface and internal. When surface PD is present, tracking occurs across the surface of the insulation, which is exacerbated by airborne contamination and moisture, leading to erosion of the insulation surface. Internal PD occurs within the bulk of the insulation materials and is caused by age, poor materials or poor-quality manufacturing processes. Figures 1 and 2 show examples of surface PD activity and surface tracking.

ESB Networks awarded EATL a contract to conduct field tests on the cast-resin units in service, and agreed to pioneer the introduction and development of the EATL UltraTEV Detector.

EXAMINATION PROTOCOL

The EATL UltraTEV Detector is a handheld field-based instrument designed to detect both surface and internal PD. The instrument indicates with red, amber and green lights the presence or absence of PD at voltages up to 90 kV. The instrument functionality was developed with ESB input to ensure that threshold settings — the levels at which the LEDs change colour — accurately correspond to the presence or absence of PD.

A trial was suggested to test the protocol on a small sample of cast-resin units. Further nonintrusive checks also were proposed for the trial using two instruments developed by EATL, namely the MiniTEV for internal PD and a sensitive ultrasonic detection instrument for surface PD. Following the nonintrusive tests, the switchgear would be de-energised, racked out and stripped down for detailed internal visual inspection. The operational restriction on tested switchgear could be lifted once the internal inspection confirmed the test results.

FIELD TESTS

The trial and test protocol satisfied ESB Networks' safety representatives, and 10 substations in Cork, southwest Ireland, were tested in November 2003. No detectable PD was identified for 7 of the 10 substations. But, for the remaining three, a very low level of ultrasonic activity, below the UltraTEV Detector's threshold levels, was detected. As the results at all 10 substations were below the instrument's preset levels, the switchgear was isolated and prepared for internal inspection.

The internal inspections revealed evidence of PD in the switchgear installed in the 3 substations where the sensitive ultrasonic equipment identified PD activity. The area affected was the fixed-copper contacts that exhibited verdigris growth with some resulting acid damage contributing to early stages of insulation degradation (Fig. 3). The findings confirmed PD activity that ultimately could result in switchgear failure. Although difficult to estimate, consensus was that failure of these 3 units was unlikely to occur within 12 months. For these substations, the operational restriction remained in place until maintenance or replacement was performed.

The testing and inspection field trials confirmed that the protocol was viable, providing a basis for the practical management of the 250 cast-resin units on the network. Also, the trial proved that the threshold levels set for the UltraTEV Detector were correct, sufficiently sensitive to detect dangerous levels of PD but not oversensitive to cause an unmanageable number of substations to be subject to restrictions pending internal inspection.

MEDIUM-VOLATGE SWITCHGEAR POPULATION

ESB Networks also has 5000 of another type of 10-kV cast-resin RMUs on its distribution network, which have suffered from disruptive failures in the past. During the trial in Cork, two of these units were also subjected to PD tests, and both produced red lights on the UltraTEV Detector, indicating the need for maintenance or replacement.

These findings led ESB Networks to consider incorporating the use of the UltraTEV Detector into the standard procedure prior to operating switchgear. It was considered that this discipline would significantly improve the safety and reliability of operational activities, in addition to identifying targets for the ongoing replacement programme. ESB Networks also included some new SF6 gas-insulated switchgear, aging oil-filled RMUs and some open-cubical substations in the trial.

SWITCHGEAR OPERATIONAL STANDARDS

ESB Networks has adopted the protocol developed and tested in the Cork field trials. The utility now issues all operational switching and inspection staff an UltraTEV Detector for use prior to switching. Two models of the UltraTEV are available, the first model having a red/amber/green LED indication system and the later model having a red/green LED indication system. ESB Networks chose to deploy the later model and issued several of these instruments to staff between May 2004 and March 2005.

During 2005 and 2006, ESB Networks staff completed more than 5000 inspections, and the results have been analysed by switchgear category. Namely, the “cast resin” category accounts for 70% of the tests completed and “other” includes a range of equipment, for example, terminations on SF6 switchgear, oil-filled RMUs and open-cubicle substations. The results are included in the nearby table.

The results demonstrated to ESB Networks that the majority of switchgear on its distribution network is in good condition, and as the positive results are targeted for maintenance or replacement, the number of red indications will decrease.

SAFETY OBJECTIVE MET

ESB Networks has confirmed its satisfaction with the results obtained following the introduction of this new procedure prior to switching. ESB has achieved its main objective of ensuring operational staff is not unknowingly being exposed to potentially dangerous situations and that defective equipment can be targeted for removal or replacement. The use of the UltraTEV Detector, as shown in Fig. 4, has resulted in an increase in ESB Networks' confidence in the continued safety of its staff and the integrity of its switchgear asset management strategy.

The development and subsequent availability of the Ultra-TEV Detector was instrumental in allowing ESB Networks to resolve and safely manage the removal of an operational restriction on 250 substations that contained cast-resin switchgear. The disruption to ESB Networks' customers was minimised, and the protocol developed between ESB Networks and EATL has been adopted nationally to include all switchgear.

Currently, the instrument is used by all the distribution network operators in the United Kingdom and by many non-electricity organisations such as water companies, the U.S. Air Force and the U.K. Ministry of Defence, who all have their own medium-voltage networks. More recently, power utilities in North America have expressed an interest in the use of this instrument and a number of field trials are in progress.

ESB Networks is satisfied with the results of the first-year survey, which has produced the following benefits:

Identifies clearly which switchgear not to operate live
Improves operational staff safety
Shows the network is clear of operational restrictions
Reports that no failures of cast-resin switchgear have occurred due to PD since the protocol was adopted
Enables accurate targeting of maintenance and replacement resources
Establishes a cost-effective standard.

ACKNOWLEDGEMENTS

ESB Networks and the author wish to record their thanks and appreciation for the support and assistance they received in the preparation of this article from Darren Jones at EA Technology. Since completing this article, the UltraTEV has received the U.K. Queen's Award for Innovation, 2007.

Paul Shiel joined ESB networks in 1980 and worked on load research before progressing to system studies and to the MV substation section, where he was responsible for maintenance and design policy. It was during his period in the substation section when the UltraTEV Detector was incorporated into the standard working practices. Since then, Shiel has been appointed to the overhead line section to manage the current major refurbishment programme of ESB's network. He has a bachelor's degree in engineering and a master's degree, and is a member of Engineers Ireland. paul.shiel@esb.ie