| The power transformer has become to be one of the core apparatus for the power grid, and its healthy status has an immediatet relationship to the safety for the grid. A great need is therefore in existence to search for an effective way in assessing the internal and inceptive fault for power transforms. But at the same time, it is a research gap to use partial discharge (PD) to study the evolution process for the oil-paper insulation and to evaluate the severity for discharges in power transformers. The present research is thereby designed to study the evolution processes and its characteristics of typical PD in oil-paper insulated transformers.An experiment platform was set up and five typical partial discharge models were established in simulation of the operation conditions for power transformers. Long-term tests were undertaken on the established experiment platform to observe the entire evolution process of PD in transformers during its inceptive initiation, gradual development and final flashover or breakdown, with application of constant voltage for the PD models between interturn windings and gradually-stepped voltage for the other four PD models. During the tests, the pressboard models were taken photos to present the trace of discharges. The trend curve of the conventional impulse current (CIC) signals, ultra frequency (UHF) signals, and ultra wide-band current (UWB) signals as a result of test time were presented and analyzed. So were the scatter plot, histogram, grey-scale map and the time-frequency (T-F) map of these outputs and signals. It is suggested that both the phase distribution characteristics and spectrum changes can be used as criterion to infer the evolution stage and severity grade of PD in oil-paper insulated transformer.According to the observed discharge phenomenon and the corresponding statistical features, the present research concludes that partial discharges in transformer undertakes three evolution stages, i.e. initiating stage, developing stage and threatening stage. The initiating stage observes weak discharge of low frequency. Phase distribution of the discharge at this stage is rather narrow with little discharges observed around 360°. The developing stage witnesses increasing discharge in higher frequency. The amplitude and energy of discharges strengthen notable and the discharge traces becomes more obvious. A lot of discharges appear around 360°. At the threatening stage, discharges almost appear at 0°～360°, and discharge energy augments clearly.With reference to different discharge features and phenomenon of the evolution stages, the present research summarizes that in power transformers, the discharges follow three different patterns, namely, corona discharge in oil, discharge along paper surface and internal discharge in paper. These mentioned discharges incarnate in different discharge models and different discharge evolution stage.The present research also studied discharge evolution process under abnormal oil temperature and different moisture, and carried out comparative studies under different temperatures for column-plate discharge model. Based on these studies, it is summarized that the discharge evolution process is more greatly affected by these two factors mentioned above. As the oil temperature and moisture increase, the inception voltage declines, the evolution speedups, and the duration from its inception till final breakdown shortens. Therefore, it can be concluded that the increasing moisture could reduce the insulation strength, the increasing temperature could reinforce the electron's natural motion energy and enhance its heat activity. Both of these factors would accelerate the evolution process for oil-paper insulation.
|
PDF Full Text Request