The Calculation On Transient Processes Of Large Transformer With On-load Tap-changer

Large transformer with on-load tap-changer has the advantages of widevoltage regulation range, low investment and well effect. Recently, with the wideapplication of on-load-tap-changing transformer in power system, the problem ofregulating winding insulation has aroused more and more attention.The model of a220kV three-phase transformer with on-load tap-changerwas taken for instance, transient process of winding was calculated andequivalent circuit of power transformer windings was established, a calculationmethod for the parameters of inductance, capacity and resistance in theequivalent circuit was given at the beginning of this paper. After that,a programfor calculating transient process was programmed, which was applied to calculateinitial voltage distribution and potential gradient of high voltage winding in twodifferent forms of inner shield-continuous winding and continuous winding forthis transformer. And the transient process of the two winding forms wassimulated by Simulink software, the simulation results were compared by theprogram calculation，the comparation shows that the potential gradient of innershield-continuous form is higher than the continuous one,and the correspondinginsulate margin of inner shield-continuous winding is bigger. Compared with theexperimental data, it can be discovered that high voltage winding adopt innershield-continuous form is more security and reliable.The Simulink software is applied to simulate and calculate initial voltagedistribution and oscillating process of high voltage winding with regulatingwinding in three tapping conditions， and it is derived that initial voltagedistribution and potential gradient of the high voltage winding in differenttapping conditions are same as the result which was calculated separately in highvoltage winding. The value of initial voltage distribution and potential gradient of the regulating winding are low, so it will take no effect on winding insulation.Through the simulating, oscillating process diagram of the high voltagewinding and regulating winding were obtained. The oscillation maximumpotential of the high voltage winding in rated tapping is approach to inputvoltage, which was mildly higher than amplitude of input voltage, when the highvoltage winding is in maximum tapping or minimum tapping. The oscillates inpositive and negative directions was appeared in rated tapping of regulatingwinding, the amplitude of which is lower,and the two oscillating processes ofregulating winding in maximum tapping and minimum tapping are similar, butthe oscillating amplitude is bigger than the condition in rated tapping. The initialvoltage distribution, ultimate voltage distribution and oscillation maximumpotential (Oscillation envelope line) of high voltage winding with regulatingwinding were compared and analyzed. The major insulation betweem high andmid voltage，and the insulation of high voltage end winding were calculated inoscillation maximum potential, all of which satisfiesed the demands of insulationmargin.