Thyristor Protection

Thyristor Protection:-

  • 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
    1. di/dt Protection
    2. dv/dt Protection
    3. Over voltage Protection
    4. Over Current Protection

It is recommended to read Thyristor- Basics to understand various Protections of Thyristor. 

di/dt Protection:-

  • 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.

di/dt protection

  • 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 Protection:-

  • 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.

SNUBBER CIRCUIT:-

Snubber_Circuit_SCR

  • 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 Protection:-

  • Overvoltage may result in false turn ON of the device (or) damage the device.
  • SCR is subjected to internal and external over voltage.

Internal Overvoltage:

  • 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).

External Overvoltage:

  • 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:

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.

You may also like:-
SCR Turn ON ( Triggering) Methods
SCR Turn OFF (Commutation) Methods

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