1. Why are the resistors and capacitors connected in parallel at both ends of the thyristor (SCR)? In the actual thyristor (SCR) circuit, an RC series network is often connected in parallel at both ends. This network is often called an RC resistance-capacitance absorption circuit.
We know that the thyristor (SCR) has an important characteristic parameter-the critical rate of rise of the off-state voltage dlv/dlt. It shows that the thyristor (SCR) is at the rated junction temperature and gate open condition, the lowest voltage rise rate of the thyristor (SCR) from the off state to the on state. If the voltage rise rate is too large and exceeds the value of the voltage rise rate of the thyristor (SCR), it will be turned on without a gate signal. This may happen even if the forward voltage applied to the thyristor (SCR) is lower than its anode peak voltage at this time. Because the thyristor (SCR) can be regarded as composed of three PN junctions.
In the blocking state of the thyristor (SCR), because the layers are very close, the J2 junction junction is equivalent to a capacitor C0. When the anode voltage of the thyristor (SCR) changes, a charging current will flow through the capacitor C0 and through the J3 junction. This current acts as a gate trigger current. If the anode voltage rises too fast when the thyristor (SCR) is turned off, the greater the charging current of C0 is, it may cause the gate to be turned on by mistake without a trigger signal. , That is, hard opening is often said, which is not allowed. Therefore, there should be a certain limit on the rate of increase of the anode voltage applied to the thyristor (SCR).
In order to limit the excessive voltage rise rate of the circuit and ensure the safe operation of the thyristor (SCR), an RC resistance-capacitance absorption network is often connected in parallel at both ends of the thyristor (SCR), and the characteristic that the voltage at both ends of the capacitor cannot be changed is used to limit the voltage rise rate. . Because the circuit always has inductance (transformer leakage inductance or load inductance), the resistance R in series with the capacitor C can play a damping effect. It can prevent the R, L, and C circuits from appearing on both ends of the capacitor due to oscillation during the transition process. Overvoltage damages the thyristor (SCR). At the same time, avoid excessive discharge current of the capacitor through the thyristor (SCR), causing overcurrent and damaging the thyristor (SCR).
Because the thyristor (SCR) has poor over-current and over-voltage capability, it will not work properly if reliable protection measures are not taken. RC resistance-capacitance absorption network is one of the commonly used protection methods.
2. Selection of rectifier thyristor (SCR) resistance-capacitance absorption element
The choice of capacitance:
C=(2.5-5)×10 minus 8 power×If
Id-DC current value
If a 500A thyristor (SCR) is used on the rectifier side
can calculate C=(2.5-5)×10 minus 8th power×500=1.25-2.5mF
choose 2.5mF, 1kv capacitor
The choice of resistance:
choose 10 Euro
PR=(1.5×(pfv×2πfc) square×10 minus twelfth power×R)/2
U=effective value of three-phase voltage