When bushings go bad: what you don't know can hurt you

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Without bushings, key elements of the electric supply system, such as power transformers and circuit breakers, would not operate effectively or efficiently. A bushing is simply a device that allows a conductor to pass through a barrier: it has an insulating medium, which must be sustained to prevent the passage of excess current to ground. Bushings are a bit like the tires on your car: they enable the car to do what it is supposed to do, but are not the reason why you buy the car - they are enablers, and when they fail the results can be catastrophic. Ton McGrail, Doble Engineering Company, explains

 

Over the last two decades, there has been an increase in interest in monitoring the performance of bushings on critical assets and in cases where failure modes are suspected for particular bushing types, as reflected by papers at the annual Doble client conference (1, 2, 3). Traditionally, it was older bushings which were expected to deteriorate. However, over the last few years we at Doble have been encountering failures with relatively new bushings.

Bushing failures and condition
Infrared measurements and off-line measurements of the dissipation factor provide useful data points in the detection of anomalous bushing conditions. However, the infrequency of such measurements and the requirement to take a transformer off line to perform the dielectric tests are such that deterioration can occur between routine measurements without being detected except in the resulting possibly catastrophic failure. Doble has developed and spent many years refining an on-line monitoring system for bushings, the Intelligent Diagnostic Device (IDD), which is being used globally and has saved many bushings from failure, with consequent improved reliability of the associated transformer (4).

The IDD measures and records the leakage current which flows naturally from the high voltage conductor, through the bushing insulation, to ground. The magnitude, harmonic content and relative phase of the leakage currents provides information about the state of installation. For some bushing types, notably the GE type U bushings, the deterioration mode is ‘graceful’ and may take place over several weeks to several months - still a short enough period for the deterioration not to be detected through normal inspection and maintenance practices. Yet, over the last few years we have seen examples of bushing deterioration which is taken place over a few hours. We have documented examples where transformer failure has been avoided saving multimillion dollar power transformers and avoiding the consequent business interruption (5).

Case Study 1: Graceful Deterioration - Traditional
A 370 MVA, 345/129 kV autotransformer, was manufactured by GE in 1986 (6). During routine maintenance elevated dissipation factor results for the H2 bushing were found. The transformer was a critical link for an independent power producer, so a Doble IDD was installed to monitor the bushings on this critical asset; all the high voltage and neutral bushings were GE Type U, which have a known failure mode. The transformer needed to return to service, so a decision was made to rely on the IDD monitor with its incorporated expert system, until the next scheduled outage. The approach was successful; the H2 bushing was replaced in early 2008, and the transformer returned to service.

In July 2008, the bushing IDD system issued an alert, indicating that the dissipation factor of the H1 bushing had exceeded the first of a series of thresholds. In this instance, the alert being generated by the expert system was based on the monthly trend of the H1 bushing’s dissipation factor that had exceeded a 1% threshold. The owner was advised by Doble’s technical experts to schedule an outage in the near future to corroborate the on-line results with off-line tests

Figure 1 shows that the dissipation factor results exhibit a very gradual increase through the end of May when the dissipation factor was approximately 0.5%; after this, the rate increased more significantly. The overall increase in capacitance is far less significant and on its own does not justify an investigation.

Knowing there was a distressed asset allowed the owners to anticipate various scenarios; in this situation, the replacement bushings were located and brought on site prior to taking the transformer out of service.

As the paper was unwrapped during the forensic examination of the bushing, discoloration was observed on the paper at the grading foil edges.
It is the belief of the parties involved that failure of this bushing was imminent if it remained in service, which would result in damage to the transformer and its proximity or possibly a catastrophic transformer failure. By having the Doble IDD on-line monitoring device, early detection of a deteriorating bushing was possible, and replacement planned in a controlled manner.

Case Study 2: Rapid deterioration - More recent failure mode
In the last few years the industry has become aware of an increase in failure rate with particular types of bushings which are less than 10 years old. The failures have occurred in different locations around the world and a number of organizations are using the Doble IDD to support them as part of their asset management program. In the example given here, a transmission utility had deployed the Doble IDD to monitor bushings of the suspect type across its fleet of power transformers.

An IDD generated an alarm based on increased leakage current through one of the bushings being monitored. The raw leakage current data showed a significant increase for the bushing – a 30% increase in two hours –as per Figure 3. The transformer was taken off line, all bushings were tested and the suspect bushing was found to have a dissipation factor at three times its normal value. The subsequent forensic tear down showed many punctured foils and deteriorated paper; it was estimated the bushing would have failed in a few more hours had it been left in service. With the appropriate warning, a failure estimated in excess of $5,000,000  was avoided. Coincidentally, the same organization had an identical situation only a month later; again the IDD detected deterioration and a catastrophic failure was avoided (5).

Discussion
Bushings are generally very reliable, but it makes sense to monitor them in critical applications or in cases where the bushings themselves are suspect. On-line bushing monitoring is a valuable tool in the transformer engineer’s toolbox, and should be seen as a key element in the transformer asset manager’s decision-making process. The Doble IDD on-line bushing monitoring system provides data which can be used to make significant decisions regarding transformer operation including the avoidance catastrophic transformer failure.

Acknowledgements
The authors wish to thank their many colleagues, both in Doble and within the industry, who have contributed to the discussion of bushings and bushing monitoring at Doble Client Conferences for more than twenty years and, thus, contributed to the discussions within this paper.

References
1. Sokolov, V. V., Vanin B. V, "On-Line Monitoring of High-Voltage Bushings," Proceedings of the Sixty-Second Annual International Conference of Doble Clients, 1995, page 3-4.1.
2. Lachman, M. F., Walter, W. and Skinner J. S., "Experience with On-Line Diagnostics and Life Management of High-Voltage Bushings," Proceedings of the Sixty-Sixth Annual International Conference of Doble Clients, 1999, page 3-4.2.
3. Bahr P., Christensen J., Intermountain Power Service Corp.; Brusetti, R., Doble Engineering “On-Line Diagnostic Case Study Involving A General Electric Type U Bushing” Proceedings of the Seventy-Fourth Annual International Conference of Doble Clients, 2007.
4. Brusetti R., “Update On On-Line Bushing Diagnostics”, Proceedings of the Seventy-Third Annual International Conference of Doble Clients, 2006.
5. Wyper K., Mackay G., McGrail T. “Condition Monitoring in the Real World” Proceedings of the Eightieth Annual International Conference of Doble Clients, 2013.
6. Wancour R., Molter S., ITC Holdings, Brusetti R., Doble Engineering, Weatherbee E., Hubbell Power Systems/PCORE, ” Chronicling The Degradation Of A 345 Kv General Electric Type U Bushing”, Proceedings of the Seventy-Sixth Annual International Conference of Doble Clients, 2009.