通过改变外电路电容减小变压器剩磁的方法

Method of Reducing Transformer Remanence by Changing Capacitance of External Circuit

由于磁滞效应,变压器分闸后铁心中会有剩磁。再次合闸时,若合闸角不合适剩磁会使铁心进入饱和区,引起励磁涌流。励磁涌流会造成继电保护装置动作而使合闸失败,另外,非正弦励磁涌流可能会引起电力系统谐振而使其失去稳定性。如果要消除剩磁,可以在绕组中施加一个振荡衰减的电流进行铁心消磁,但这需要一套消磁电源和复杂的操作,实际中也鲜有应用。因此,理想的情况是让变压器分闸后直接没有剩磁或剩磁很小,不再需要额外采取方法消磁。文中提出一种基于自动消磁原理的减小变压器剩磁的方法,思路是利用变压器外接电路中的电容效应,主要是断路器的并联均压电容,使分闸后的电流在电容与变压器的等效电感的共同作用下发生一定程度的振荡。该振荡衰减电流相当于一个铁心退磁电流,使铁心内的磁场终止于较小的量值。首先给出剩磁大小随外电路中电容值的变化特性仿真结果,然后通过仿真计算给出断路器上的并联均压电容在多大时能够实现足够的铁心退磁效果。另外还分析断路器和变压器的对地电容对剩磁的影响特性。最后,给出电容的综合配置方案和此方法的适用范围。

Because of the magnetic hysteresis effect, there will be remanence in the iron core after the de-energization of the transformer. When the transformer is re-energized, if the closing angle is not appropriate, the remanence will make the iron core reach the saturation state and will cause inrush current. The inrush current will cause the action of relay protection devices and fail the re-energization. Besides, the non-sinusoidal inrush current may cause power system resonance and thus lose the stability. In order to eliminate the remanence, an oscillating attenuated current can be applied in the winding, but this requires a set of demagnetization power source and complicated operations, which are rarely used in practice. Therefore, the ideal situation is that the transformer directly has no or little remanence after the de-energization,and additional demagnetizing methods are not necessary. A method to reduce the transformer remanence based on the principle of automatic demagnetization was proposed. The idea is that utilizing the capacitance effect in the external circuit of the transformer, especially the parallel voltage equalizing capacitor of the circuit breaker, makes the current after the de-energization oscillate to a certain extent under the influence of the capacitance and the equivalent inductance of the transformer. The oscillating attenuated current is equivalent to the demagnetization current, and will make the magnetic field in the core decrease to a small value. The simulation results of the remanence variation characteristics with the capacitance in the external circuit were given, and then the value of the parallel voltage equalizing capacitor of the circuit breaker with sufficient core demagnetization effect was given. Besides, the influence of the ground capacitances of the circuit breaker and the transformer on the remanence was also analyzed. Finally,the comprehensive configuration scheme of the capacitors and the application scope of this method were given.