DETECTION AND LOCATION OF INTERNAL DEFECTS IN THE INSULATION OF POWER TRANSFORMERS

Reliable methods and suitable equipment for sensitive detection and for precise location of internal defects in the insulating systems of power transformers during commissioning tests are still very much a subject of discussion among testing engineers and researchers. Currently, the widely applied electrical and acoustical partial discharge (PD) measurement techniques fulfill only partly the requirements to assess the risk and the location of detected internal defects during commissioning tests. With the development of both fast and low cost analog-digital converters (ADC) and new mathematical procedures for waveform description, e.g. time encoded signal processing and recognition (TESPAR), novel approaches to detect and locate internal defects in the insulation of power transformers become feasible if the following requirements are fulfilled: the equivalent circuit of the insulating system suitable for fast PD signals with rise times similar to 10 ns is known, the response of the insulating system to PD signals injected at least at three different sites of each winding (top, center, bottom) are calculated or measured at the signal tap-off points (i.e. bushings), and the pattern matrix of all calculated or measured signals exists e.g. encoded and signal processed with TESPAR as a reference. In this contribution, the above mentioned requirements for quasi-simultaneous detection and location of internal defects in power transformers will be examined and this method applied to PD measurements on power transformers.