0 Preface
Single-phase ground faults are the most common faults in power systems. When a single-phase earth fault occurs in a neutral point ungrounded network, it can be operated with a fault for 2 hours. However, if the capacitance to ground in the power grid is large (such as cable lines), a large arc will be formed at the grounding point, posing a threat to the safe operation of the power system. The arc suppression coil can be used to reliably extinguish the arc. The design scheme of the thyristor control circuit based on TSC/TCR arc suppression coil proposed in this paper is tested by experimental circuit, and the effect is satisfactory. The feasibility of the scheme is verified.
1 TSC/TCR arc suppression coil structure and working process
The TSC and TCR circuits compensate the capacitive current of the grid by changing the inductive reactance value of the secondary side of the arc suppression coil, thereby changing the inductance of the arc suppression coil in the system.
TSC (Thyristor Switched capacitor) is the thyristor switching capacitor.
It consists of three capacitors with a capacity ratio of 1:2:4 and a thyristor switch. By controlling the on and off of the thyristor, the capacitance value of the secondary side input changes according to a certain law. This adjustment is graded and not continuous.
TCR (Thyristor Controlled Reactor) is a thyristor control reactor consisting of a reactor and a thyristor switch. By controlling the firing angle of the thyristor, the equivalent reactance is changed. Its current varies continuously within a certain range.
The capacitance current value of the power grid is measured by the arc suppression coil controller, and the capacitance value and the inductance amount to be input are calculated. The capacitance is input by the TSC control circuit, and the reactance is input by the TCR trigger circuit. The control circuits of TSC and TCR will be analyzed separately.
2 TSC control circuit
2.1 Voltage zero crossing detection
The capacitor will have a large inrush current during the switching process, damaging the thyristor. Therefore, the input AC voltage should be equal to the residual voltage on the capacitor, that is, when the voltage across the thyristor is zero, it will be triggered for the first time. The zero-crossing detection circuit can output a zero-crossing pulse when the input signal crosses zero, as shown in Figure 2.
It can be seen that the sinusoidal signal passes through the uncontrollable rectifier bridge, and a sine wave with a period of π in the upper half of the cycle is generated at point B. After the op amp is compared with a voltage close to zero, a pulse at the zero point is generated at point C. As shown in Figure 3.
