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Selection of Resistor-capacitance Absorber for Rectifier Thyristor (SCR)
Release time:
2024-03-12
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Selection and calculation of SCR parallel resistance-capacitance absorption circuit
Selection and calculation of SCR parallel resistance-capacitance absorption circuit
We know that thyristors have an important characteristic parameter-the critical rise rate of off-state voltage dlv/dlt. It shows that the thyristor at the rated junction temperature and gate open conditions, the thyristor from the off-state to the on-state of the lowest voltage rise rate. If the voltage rise rate is too large to exceed the value of the voltage rise rate of the thyristor, the thyristor is turned on without the gate signal. This may occur even when the forward voltage applied to the thyristor is lower than its anode peak voltage. Because the thyristor can be regarded as consisting of three PN junctions.
In the thyristor is in the blocking state, because the layers are very close, the J2 junction is equivalent to a capacitor C0. When the thyristor anode voltage changes, there will be a charging current through the capacitor C0, and through the J3 junction, this current plays the role of the gate trigger current. If the anode voltage rises too fast when the thyristor is turned off, the greater the charging current of C0, it is possible to cause the gate to mislead the thyristor in the absence of a trigger signal, which is often called hard on, which is not allowed. Therefore, the rate of increase of the anode voltage applied to the thyristor should be limited.
In order to limit the circuit voltage rise rate is too large, to ensure the safe operation of the thyristor, often in parallel at both ends of the thyristor RC resistance capacity absorption network, the use of capacitor voltage at both ends of the characteristics of the mutation to limit the voltage rise rate. Because there is always inductance in the circuit (transformer leakage inductance or load inductance), the resistor R in series with the capacitor C can play a damping role, which can prevent the overvoltage at both ends of the capacitor from damaging the thyristor due to oscillation in the transition process of the R, L and C circuits. At the same time, to avoid the capacitor through the thyristor discharge current is too large, resulting in overcurrent and damage to the thyristor.
Due to the poor over-current and over-voltage capability of the thyristor, it cannot work normally without reliable protection measures. RC resistance-capacitance absorption network is one of the commonly used protection methods.
Selection of Resistor-capacitance Absorber for 2. Rectifier Thyristor (SCR)
Selection of Capacitors
C =(2.5-5)× 10 to the negative 8th × If
If=0.367Id
Id-DC current value
If the rectifier side adopts 500A thyristor (thyristor)
Can calculate C =(2.5-5)× 10 to the negative 8th power × 500=1.25-2.5mF
Selection of 2.5mF,1kv capacitors
Selection of resistors:
R=((2-4) ×535)If=2.14-8.56
Choose 10 Euros.
PR =(1.5 ×(pfv × 2π fc) squared × 10 to the negative 12th power × R)2
Pfv = 2u(1.5-2.0)
u = effective value of three-phase voltage
In practical applications, the RC time constant is generally 1 to 10 milliseconds.
Small power load usually takes about 2 milliseconds, R = 220 ohm 1W,C = 0.01 microFarad 400~630V.
The high power load usually takes 10 milliseconds, R = 10 ohm 10W,C = 1 microFarad 630~1000V.
Selection of R: Low power selection of metal film or RX21 wire winding or cement resistance; High power selection of RX21 wire winding or cement resistance.
Selection of C: CBB series of non-polar capacitors with corresponding withstand voltage.
Look at the protection object to distinguish: the damping absorption of contactor coil and the damping absorption of thyristor less than 10A current are included in the category of low power; The damping absorption of contactor contact and thyristor more than 10A is included in the category of high power.
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