Thyristors are usually used to switch reactive power compensation capacitor at low voltage distribution system.In this paper

the mathematical model for switching on and off process of thyristors is analyzed and derivated in detail.It is proved by the theoretically derivated result that the residual voltage on 3 units of capacitors is 1.366

1 and 0.366 times of the peak of line voltage respectively

regardless of switching sequence of the thyristors;and the voltage on both sides of V1 and V2 thyristors are always within a definite variation.The result provides a theoretical basis for restraining overvoltage in switching process and for rating parameter selection of thyristors in TSC device.