Overvoltage protection
The overvoltage capability of the thyristor is extremely poor. When the reverse voltage of the component exceeds its reverse breakdown voltage, even if the time is short, the reverse breakdown of the component will be damaged. If the forward voltage exceeds the forward turning voltage of the thyristor, it will cause the thyristor to turn on hard. It not only makes the circuit work abnormally, but also reduces the forward turning voltage of the component after multiple hard turns on, or even loses the forward blocking ability and damages it. Therefore, overvoltage protection measures must be adopted to suppress the overvoltage that may appear on the thyristor. The overvoltage protection of the module is recommended to adopt the protective measures of two ways of resistance-capacitance absorption and varistor.
(1) Resistance-capacitance absorption circuit
When the thyristor is turned on to off, just like the switching circuit, overvoltage will be generated due to the energy released by the line inductance (mainly the transformer leakage inductance LB). Since the thyristor is conducting, carriers fill the inside of the component, so when the component is turned off, when the forward voltage drops to zero, carriers still remain inside. Under the action of the reverse voltage, these accumulated carriers instantly appear a large reverse current. The accumulated carriers disappear quickly, and then the reverse current disappears extremely fast. That is, the di/dt is extremely large. Therefore, even if the inductance L of the line connected in series with the element is small, the induced potential L (di/dt) value generated by the inductance is still very large. This potential is connected in series with the power supply voltage and applied in reverse to the element that has been restored and blocked, which may cause Reverse breakdown of thyristor. This kind of overvoltage caused by the turning off of the thyristor is called the turning off overvoltage, and its value can reach 5-6 times the peak value of the working voltage, so suppression measures must be taken.
The capacitor in the resistance-capacitance absorption circuit converts the electromagnetic energy of the overvoltage into electrostatic energy storage. The resistance prevents the resonance between the capacitor and the inductance, and limits the turn-on loss of the thyristor and the current rise rate. This kind of absorption loop can suppress the overvoltage generated when the thyristor is turned on to off, and effectively avoid the breakdown of the thyristor. The installation position of the resistance-capacitance absorption circuit should be as close as possible to the main terminal of the module. That is, the lead should be short. It is best to use non-inductive resistors to achieve better protection.
The wiring method is shown in the figure:
Three-phase rectifier module Single-phase rectifier module
Three-phase AC module Single-phase AC module
(2) Varistor absorbs overvoltage
Varistors can absorb overvoltages with large energy and long duration due to lightning strikes and other reasons. The nominal voltage of a varistor (V1mA) refers to the voltage across it when a current of 1mA flows through the varistor. The choice of varistor mainly considers rated voltage and current capacity. The lower limit of the rated voltage V1mA is the peak value of the line operating voltage. Taking into account the fluctuation of the grid voltage and the V1mA value may decrease after repeated inrush currents, therefore, the value of the rated voltage should be appropriately increased. Currently, a 30% margin is usually used for calculation.
V1mA≥1.3√2·U
where U——the effective value of the normal working voltage at both ends of the varistor. The wiring method is as follows:
Three-phase module Single-phase module
