Overvoltage Protection in Thyristors

Thyristors are very sensitive to overvoltages just as other semi-conductor devices are. Overvoltage transients are perhaps the main cause of thyristor failure. Transient overvoltages cause either maloperation of the circuit by unwanted turn-on of a thyristor or permanent damage to the device due to reverse breakdown. A thyristor may be subjected to internal or external overvoltages ; the former is caused by the thyristor operation whereas the latter comes from the supply lines or the load circuit.

(i) Internal overvoltages. Large voltages may be generated internally during the commutation of a thyristor. After thyristor anode current reduces to zero, anode current reverses due to stored charges. This reverse recovery current rises to a peak value at which time the SCR begins to block. After.this peak, reverse recovery current decays abruptly with large di/dt. Because of the series ifiductance L of the SCR circuit, large transient voltage

L di/dt is produced. As this internal overvoltage may be several times the breakover voltage of the device, the thyristor may be destroyed permanently.

{ii) External overvoltages. External overvoltages are caused due to the interruption of current flow in an inductive circuit and also due to lightning strokes on the lines feeding the thyristor systems. When a thyristor converter is fed through a transformer, voltage transients are likely to occur when the transformer primary is energised or de-energised. Such overvoltages may cause random turn on of a thyristor. As a result, the overvoltages may appear across the load causing the flow of large fault currents. Overvoltages may also damage the thyristor by an inverse breakdown. For reliable operation, the overvoltages must be suppressed by adopting suitable techniques.

Suppression of overvoltages. In order to keep the protective components to a minimum, thyristors are chosen with their peak voltage ratings of 2.5 to 3 times their normal peak working voltage. The effect of overvoltages is usually minimised, by using RC circuits and non-linear resistors called voltage clamping devices.

The RC circuit, called snubber circuit, is connected across the device to be protected, see Fig. 4.29. It provides a local path for internal overvoltages caused by reverse recovery current. Snubber circuit is also helpful in damping overvoltage transient spikes and for limiting dv/dt across the thyristor. The capacitor charges at a slow rate and thus the rate of rise of forward voltage (dv/dt) across SCR is also reduced. The resistance R_s damps out the ringing oscillations between the snubber circuit and the stray circuit inductance. Snubber circuits are also connected across transformer secondary terminals to suppress overvoltage transients caused by switching on or switching off of the primary winding. As snubber circuits provide only partial protection to SCR against transient overvoltages, thyristor protection against such overvoltages must be upgraded. This is done with the help of voltage-clamping devices.

A voltage-clamping (V.C.) device is a non-linear resistor connected across SCR as shown in Fig. 4.29. The V.C. device has falling resistance characteristic with increasing voltage, Fig. 4.27 (a). Under normal working conditions of voltage below the clamping level, the device has a high resistance and draws only a small leakage current, When a voltage surge appears, the V.C. device operates in the low resistance region and produces a virtual short circuit across the SCR. The increased current associated with virtual short circuit produces an increased voltage drop in the source and line impedances and as a result, voltage across SCR is clamped to a safe value. After the surge energy is dissipated in the non-linear resistor, the operation of the V.C. device returns to its high resistance region. Selenium thyrector diodes, metal oxide varistors or avalanche diode suppressors are commonly employed for protecting the thyristor circuit against overvoltages. As the voltage clamping ability of a thyrector is inferior to those of metal oxide varistor and avalanche-diode suppressor, use of thyrector is on the decline.

It has already been stated that RC snubber is not enough for overvoltage protection of SCR. In practic, therefore, a combined protection consisting of RC snubber and V.C. device is provided to thyristors as shown in Fig. 4.29.

Written by John on April 17th, 2009 with 3 comments.
Read more articles on Power Electronics and Thyristor.

Related articles

• A comparison of Mosfet and Bipolar Transistor amplifiers (March 16th, 2011)
• Gate Protection in Thyristors (April 17th, 2009)
• Electronic crowbar protection (April 17th, 2009)
• Overcurrent Protection in Thyristors (April 17th, 2009)
• Design of Snubber Circuits for Thyristor Protection (April 17th, 2009)