Thyristor Protection· January 24, 2013
- For reliable operation of SCR, it should be operated within the specific ratings.
- SCRs are very delicate devices and so they must be protected against abnormal operating conditions. Various protection of SCR are
- di/dt Protection
- dv/dt Protection
- Over voltage Protection
- Over Current Protection
It is recommended to read Thyristor- Basics to understand various Protections of Thyristor.
- di/dt is the rate of change of current in a device.
- When SCR is forward biased and is turned ON by the gate signal, the anode current flows.
- The anode current requires some time to spread inside the device. (Spreading of charge carriers)
- But if the rate of rise of anode current(di/dt) is greater than the spread velocity of charge carriers then local hot spots is created near the gate due to increased current density. This localised heating may damage the device.
- Local spot heating is avoided by ensuring that the conduction spreads to the whole area very rapidly. (OR) The di/dt value must be maintained below a threshold (limiting) value.
- This is done by means of connecting an inductor in series with the thyristor.
- The inductance L opposes the high di/dt variations.
- When the current variation is high, the inductor smooths it and protects the SCR from damage. (Though di/dt variation is high, the inductor ‘L’ smooths it because it takes some time to charge). L ≥ [Vs / (di/dt)]
- dv/dt is the rate of charge of voltage in SCR.
- We know that iC=C.dv/dt. ie, when dv/dt is high, iC is high.
- This high current(iC) may turn ON SCR even when gate current is zero. This is called as dv/dt turn ON or false turn ON of SCR.
- To protect the thyristor against false turn ON or against high dv/dt a “Snubber Circuit” is used.
- The snubber Circuit is a series combination of resistor ‘R’ and capacitor ‘C’.
- They are connected across the thyristor to be protected.
- The capacitor ‘C’ is used to limit the dv/dt across the SCR.
- The resistor ‘R’ is used to limit high discharging current through the SCR.
- When switch S is closed, the capacitor ‘C’ behaves as a short-circuit.
- Therefore voltage across SCR is zero.
- As time increases, voltage across ‘C’ increases at a slow rate.
- Therefore dv/dt across ‘C’ and SCR is less than maximum dv/dt rating of the device.
- The capacitor charges to full voltage Vs; after which the gate is triggered, and SCR is turned ON and high current flows through SCR.
- As di/dt is high, it may damage the SCR.To avoid this, the resistor R in series with ‘C’ will limit the magnitude of di/dt.
- The technique of ‘snubbing’ can apply to any switching circuit, not only to thyristor/triac circuits.
- The rate of rise of turn-off voltage is determined by the time constant
RLC. Where RL is the circuit minimum load resistance, for instance the cold resistance of a heater or lamp, the winding resistance of a motor or the primary resistance of a transformer.
- Overvoltage may result in false turn ON of the device (or) damage the device.
- SCR is subjected to internal and external over voltage.
- The reverse recovery current of the SCR decays at a very fast rate. ie, high di/dt.
- So a voltage surge is produced whose magnitude is L(di/dt).
- These are caused by the interruption of current flow in the inductive circuit and also due to lightning strokes on the lines feeding the SCR systems.
- The effect of overvoltage is reduced by using Snubber circuits and Non-Linear Resistors called Voltage Clamping Devices.
Voltage Clamping Device:
- It is a non-linear resistor called as VARISTOR (VARIable resiSTOR) connected across the SCR.
- The resistance of varistor will decrease with increase in voltage.
- During normal operation, varistor has high Resistance and draws only small leakage current.
- When high voltage appears, it operates in low resistance region and the surge energy is dissipated across the resistance by producing a virtual short-circuit across the SCR.
Over Current Protection:
- In an SCR due to over-current, the junction temperature exceeds the rated value and the device gets damaged.
- Over-current is interrupted by conventional fuses and circuit breakers.
- The fault current must be interrupted before the SCR gets damaged and only the faulty branches of the network should be isolated.
- Circuit breaker has long tripping time. So it is used for protecting SCR against continuous over loads (or) against surge currents of long duration.
- Fast acting current limiting fuse is used to protect SCR against large surge currents of very short duration.
Electronic Crowbar Protection:
- SCR has high surge current ability.
- SCR is used in electronic crowbar circuit for overcurrent protection of power converter.
- In this protection, an additional SCR is connected across the supply which is known as ‘Crowbar SCR’.
- Current sensing resistor detects the value of converter current.
- If it exceeds preset value, then gate trigger circuits turn ON the crowbar SCR.
- So the input terminals are short-circuit by SCR and thus it bypass the converter over current.
- After some time the main fuse interrupts the fault current.