IEEE C57.140-2006 pdf free download.IEEE Guide for Evaluation and Reconditioning of Liquid Immersed Power Transformers.
5.7 Partial discharge detection Partial discharge (PD) occurs in an insulation system when a localized breakdown of the insulation medium causes a redistribution of charge within the system. PD generates low-amplitude voltage and current pulses that are within radio range of frequency. Several techniques are available to detect and measure these signals in transformers. Two techniques consist of direct electrical measurements, and results are measured in micro- volts of radio frequency energy or in pico-coulombs. The other method consists of acoustical measurements with an ultrasonic transducer. It is possible to locate an active discharge in the transformer by comparing the signals from acoustic and from electrical detection. NOTE—Additional information can be found in IEEE Std C57.127 and IEEE Std C57.113. 5.7.1 Electrical measurements Because PD is an electrical phenomenon, electrical measurements allow for the most direct and quantifable data. PD measurements in the feld can be accomplished using at least two diferent methods. The frst method is the feld-induced test, which is similar to factory testing, where a portable high-frequency generator system is used to excite the transformer. The second method involves exciting the transformer at the power frequency either from the utility grid or from an isolated generator. The feld-induced test for older transformers is typically done at voltages somewhat less than the full-induced voltage test levels as specifed in IEEE Std C57.12.00. Typical tests are done at 75% to 85% of the IEEE test levels for a duration of anywhere from 30 min to 60 min. The voltage level and test duration are based on as- sessment of the age and condition of the bulk insulation and bushings or other components and the capacity of the test generator. Utilizing power frequency for this test may saturate the core at excitation higher than the rated transformer voltage.
PD activity may be measured using either the radio infuence voltage (RIV) method or the apparent charge method. Each method has its own relative advantages and disadvantages. The RIV method is less afected by external noise from the power system, but may be afected by radio stations. It also is generally less sensitive to discharges deep within the transformer windings. (The RIV of equipment was historically measured to deter- mine the infuence of energized equipment on radio broadcasting; hence the name RIV.) Typically at 100% test voltage, if the PD magnitudes are less than either 100 µV or 500 pC, the transformer is considered acceptable. If the levels are above 500 µV or 1000 pC, then the transformer may be suspect. For values in between, the results are questionable, and further testing may be needed to more precisely characterize the risk. Advanced PD measurement methods are available that can efectively flter out external noise infuences to selectively measure the PD activity in the windings. These methods involve narrow band measurements at certain resonant frequencies of the transformer to amplify the PDs and reduce the background noise. With this type of measurement, a transformer may be tested with excitation from the power grid. In other instances, the natural attenuation of PD between windings can be used to isolate the high-voltage winding while exciting a low-voltage or tertiary winding. Another advanced measuring method involves the measurement of a PD pattern based on a three-dimensional plot of the PD magnitude, phase angle of the pulses, and the number of pulses. Diferent types of insulation defects produce diferent but recognizable patterns, and the PD test result can be compared to a library of test results to make a judgment about the cause of the PD. In addition, the test can often establish a relative loca- tion of the PD within the transformer by PD pulse shape characteristic and time displacement between the bushings.