Module protection-overvoltage protection
Thyristors have poor overvoltage capability. When the reverse voltage of an element exceeds its reverse breakdown voltage, the element will be damaged by reverse breakdown even for a short time. If the DC voltage exceeds the forward conduction voltage of the thyristor, it will lead to the hard conduction of the thyristor, which will not only make the circuit work abnormally, but also reduce the forward conduction voltage of the component after several hard conduction times, and even lose the forward blocking ability and damage it. Therefore, overvoltage protection measures must be taken to suppress the possible overvoltage on the thyristor. The overvoltage protection of the module is recommended to adopt resistance-capacitance absorption and varistor protection measures.
(1) resistance-capacitance absorption circuit
When the thyristor is turned on and off, just like the switching circuit, overvoltage will be generated due to the energy released by the line inductance (mainly the leakage inductance LB of the transformer). During the turn-on of the thyristor, the carriers fill the inside of the element, so during the turn-off of the element, the DC voltage drops to zero and the carriers remain inside. Under the action of reverse voltage, these accumulated carriers have a large reverse current instantly, which makes the accumulated carriers disappear quickly, and then the reverse current disappears very quickly, that is, 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) generated by the inductance is still large. This potential is connected in series with the power supply voltage, and is reversely applied to the blocked components, which may lead to reverse breakdown of the thyristor. This overvoltage caused by the turn-off of thyristor is called turn-off overvoltage, and its value can reach 5-6 times of the peak value of working voltage, so it is necessary to take suppression measures.
In RC absorption circuit, the capacitor converts the electromagnetic energy of overvoltage into electrostatic energy storage, and the resistor prevents the resonance between the capacitor and the inductor, and limits the conduction loss and current rising rate of the thyristor. This kind of absorption circuit can suppress the overvoltage generated when the thyristor is turned on and off, and effectively avoid the breakdown of the thyristor. The installation position of RC absorption circuit should be as close as possible to the main terminal of the module. That is, the leads should be short. It's better to use non-inductive resistor to obtain better protection effect.
(2) The varistor absorbs overvoltage.
Varistor can absorb overvoltage with large energy and long duration caused by lightning strikes. The nominal voltage of varistor (V1mA) refers to the voltage across the varistor when 1mA current flows through it. The selection of varistor mainly considers rated voltage and current capacity. The lower limit of the rated voltage V1mA is the peak value of the line working voltage. Considering the fluctuation of the grid voltage and the possible decrease of the V1mA value after multiple times of impact current, the value of the rated voltage should be appropriately increased. At present, the allowance of 30% is usually used. V1mA≥1.3√2· U。 Where u is the effective value of the normal working voltage at both ends of the varistor.
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