Condition-based monitoring of power transformer insulation should center on the prediction of a substantial drop in the dielectric safety margin under the impact of moisture, oil by-products, contaminating particles, paper insulation aging and partial discharge activity. A functional failure model of power transformer insulation and possible effective methods of the insulation condition assessment are discussed based on practical experience.

EHV open air substations experience flashovers produced by very high frequency overvoltages (VHF OV) during disconnect switch operations. It was found during field tests that the highest measured value of sparkover voltage across the EHV switch gap between contacts during its operation was 1.9 p.u. The most unfavourable scheme, theoretically, occurs when a bus is fed from a single transformer, without outgoing lines. Calculations showed that in 787 and 525 kV substations with branched buses VHF OV could result in flashovers on a bus open end with unfavourable ratio of bus lengths. For transformer insulation protected with an arrester VHF OV are not dangerous. Some protective measures are recommended.Kev words: Overvoltage-Very High Frequency-Extra High Voltage. INlXODUCTIONVery high frequency overvoltages (VHF OV) are traveling waves, similar to the lightning ones. These OV occur on the substation busbars switched by a disconnect switch (DS) at the moment of sparkover across the switch contacts. Dozens, hundreds or even thousands sparkovers are possible per pole switching. Because of this, DS switching which is a routine service operation could produce numerous OV, some of which could reach high values.The problem was studied intensively only in application to GLS where internal flashovers occurred during switch operations. This subject was presented in CIGRE Reports of 1988 Session and, particularly, in summarizing CIGRE Working Group 33/13-09But VHF OV phenomenon is of practical importance for EHV air-insulated substations as well. It is interesting to note that about 30 years ago explosions of gapped silicon-carbide arresters in 126-252 kV substations were quite Gequent during DS operations. They were caused by arrester multi-operations due to OV repetitive shots, so non-linear resistors became overheated by repetitive 50-Hi current half-waves. Replacement of gapped arresters by gapless metal-oxide ones eliminated these explosions, and it seemed initially that there were no more problems related to VHF OV. However, the investigation of some failures in the Russian 525-787 kV substations showed that they were contributed by VHF OV. For 96 W M 072-9 PWRD A paper recommended and approved by the IEEE Transmission and Distribution Russia example, during switch operations of EHV buses, phase-to-ground flashovers were observed. Another example is that of a switched-off 525 kV air-blast circuit-breaker. It was flashovered across its contacts at the moment of DS opening (the dielectric strength of this gap at lightning surge is about 3.6 p.u.). Another case was found in which a 787 kV transformer experienced an internal flashover at the time of DS operation.These cases led us to undertake a study of VHF OV in outdoor EHV substations. The results of this study are given in the Paper. METHOD OF RESEARCHVHF OV depends upon a lot of factors: the switchyard configuration; the lengths of bus sections; the location of equipment with large stray capacitances and capacitance values; -the location of overhead transmission lines...

No abstract

No abstract

No abstract