SCR Parallel Operation

SCR Parallel Operation Tutorial

Introduction:                        

The SCRs are connected in a parallel manner to meet the high current demand. In this post we are going to see the parallel operation of thyristors and problems associated with it and equalization circuit arrangement for them.
We recommend to read the Thyristor Basics before proceeding further. Click here to know more about the Basics of SCR. 
           

When current required by the load is more than the rated current of a single SCR, the SCRs are connected in parallel in a string. For an example, current in the circuit is 100A. But we have a SCR of current rating 60A. We can solve this problem by connecting two thyristors in parallel, so that each SCR carries 100/2 =50A of current only.
SCR_parallel_static

VI Characteristics of Parallel Connected SCRs:

  • The VI characteristics must be identical as far as possible for the SCRs to be connected in parallel. 
  • For proper operation of these parallel connected SCRs, they should get turned on at the same moment.
  • We can understand this with the help of following discussion. Consider n parallel connected SCRs.
  • For satisfactory operation of these SCRs, they should get turned on at the same time. Consider that SCR1 has large turn-on time whereas the remaining (n-1) SCRs have low turn-on time.
  • Under this assumption, (n-1) SCRs will turn on first but one SCR1 with longer turn-on time is to remain off.
  • The voltage drop across (n-1) SCRs falls to a low value and SCR1 is now subjected to this low voltage.
  • If the voltage across SCR1 goes below finger voltage, then this SCR will not turn on.
  • So the remaining (n-1) SCRs will have to share the entire load current. Consequently these SCRs may be overloaded and damaged because of heating caused by overcurrent.

What is meant by finger voltage?
For a given gate drive power, the anode to cathode must have some minimum forward voltage for a thyristor to turn-on. This particular voltage is known as finger voltage.

We should ensure that the parallel connected SCRs should operate at the same temperature.

  • This can be achieved by mounting the parallel unit on one common heat sink. 
  • The unequal current distribution in a parallel unit is also caused by the inductive effect of current carrying conductors.
  • When SCRs are arranged unsymmetrical manner, the middle conductor will have more inductance because of more flux linkages from two nearby conductors.
  • The result is less current flows through the middle SCR as compared to outer two SCRs.
  • The unequal current distribution can be avoided by mounting the SCRs symmetrically on the heat sink. 
  • In AC circuits current distribution can be made more uniform by the magnetic coupling of the parallel paths.

Switching of Parallel Connected SCRs:
With parallel connected switches, the first to turn on will momentarily carry the full current. At turn-off, the last to turn off will have the full current through it. It is obviously desirable to turn on and turn off all the switches simultaneously.
SCR_parallel_gating

The gate-cathode circuits will not be identical, and to compensate for this a series resistance can be connected in the gate circuit of each switch. This will have the effect of reducing the spread of the gate currents. A simple gate circuit for parallel switches is shown in above figure.

Read more about thyristors:
SCR Turn-On Methods
SCR Turn-Off Methods
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