Interpretation of UHF signals produced by partial discharges in oil-filled power transformers
Oil-insulated power transformers are key components of the electrical transmission network. Knowing the state of their insulation is therefore of considerable interest to utilities. Failure in transformers often occurs as a result of electrical breakdown in the insulation materials. Such breakdowns may occur following insulation damage caused over a substantial period of time by the cumulative adverse effects of partial discharge (PD) activity. Recognition of PD in power transformers is important because early detection can allow utilities to take appropriate preventative measures in order that costly failures do not occur. In this work, a range of on-line partial discharge (PD) measurement techniques are reviewed. Ultra high frequency (UHF) and measurements based on the International Electro-technical Commission (IEC) standard 60270 are considered to be the most suitable. UBF measurements have the advantage that PD can be located using time flight measurements, while IEC 60270-based measurements can be quantified in terms pC. A test cell is designed, and PD current pulses are measured when the insulating oil at the tip of a sharp, conducting protrusion breaks down. Current pulses are recorded using two different measurement circuits, and different pulse shapes are observed. The experiments are of fundamental significance because UBF signals are only excited when discharges occur on timescales of around 1 ns or less. Results demonstrate that the durations of current pulses in oil can be sufficiently short to cause the excitation of UHF signals. Characteristics of PD generated in air, new transformer oil and used transformer oil are compared. PD current pulses are measured simultaneously with both EEC 60270-based and UHF measurements systems. Since EEC 60270-based and UBF measurement systems operate on very different principles their responses to PD current pulses of known magnitude provides a useful basis for comparing the performance of both systems. A range of insulation defects that would cause concern in an operational power transformer are then investigated. Radiated UHF signals are measured simultaneously using two broadband electric field sensors. The spectral content and energy of rating measured using each sensor during both half cycles are shown to be useful methods to assist with the understanding of discharge behaviour. UHF PD signals are also measured in a phase-resolved form, and these are analysed in ways that provide evidence of the physical differences between insulation defects. First tests are carried out on a 1000 MVA, 400 kV/ 275 kV power transformer, and results demonstrate that the UBF measurement technique is a very promising approach for PD monitoring. The research contributes to the knowledge base required for the development of continuous monitoring systems for partial discharge and arcing in oil-filled power transformers.