Parallel Operation of Thyristors


For some industrial applications, the demand for current ratings is so high that a single SCR cannot fulfill such requirements. Hence SCRs are connected in parallel as given in Fig. 1a, in order to meet the  high current (h.c.) demand.
String efficiency = Actual current rating of the whole string/(Individual current rating of one SCR x Number of SCR in the string)
Derating factor (DRF) = 1-String efficiency
String efficiency is the degree of utilization of SCR and Derating factor is the measure of the reliability of string.

I-V Characteristics

For equal sharing of currents, I-V characteristics of SCR during forward conduction must be identical as far as possible.
  • For parallel connected SCRs, voltage drop VT across them must be equal.
  • I-V characteristics shows that for the same voltage drop VT, SCR1 shares a rated current I1, whereas SCR2 carries current I2.
  • String efficiency = (I1+I2)/(I1*2) = (1/2)*(1+I2/I1)
The unequal current distribution and other operating problems concerning the parallel operation of SCR are discussed below:

Simultaneous Turn-ON

Consider n parallel connected SCRs. For satisfactory operation of these SCRs, they should get turned on at same moment. 
  • Example: Consider that SCR1 has large turn-ON time whereas the remaining (n-1) SCRs have low turn-ON time.
  • Under this assumption, (n-1) SCR will turn on first but one SCR1 with longer turn ON time is likely to remain OFF.
  • The voltage drop across (n-1) SCRs falls to a low value and SCR1 is therefore subjected to this low voltage.
  • As a consequence, the remaining (n-1) SCR will have to share the entire load current.

Same Temperature Rise

  • If one SCR1 in a parallel unit carries more current than other SCRs then this SCR1 will have greater junction temperature rise. 
  • As a result, its dynamic resistance (dVT/dIa) during forward conduction decreases and this further increases the current shared by this SCR.
  • RD at I'=oa/ab and RD at I"=oa/ac
  • Current changes from I' to I" because of junction temperature rise.
  • Therefore, when SCRs are to be operated in parallel they should operate at the same temperature.
  • This  can be achieved by mounting the parallel unit on one common heat sink.

Uniform Current Sharing

Current sharing in parallel connected SCRs can be made uniform by connecting a suitable external resistance in series with each SCR.
RT1+R1=RT2+R2=RTn+Rn  (Where: RTn is external resitance and Rn is dynamic resistance)

Symmetrical Arrangement

Unequal current distribution in a parallel unit is also caused by the inductive effect of current carrying conductors.
  • When SCRs are arranged asymmetrically, the middle conductor will have more inductance because of more flux linkages from two nearby conductors.
  • As a consequence, less current flows through the middle SCR as compared to outer two SCRs.
  • To avoid this unequal current distribution, symmetrical arrangement is used.

Mid Point Reactor

In ac circuits, current distribution can be made more uniform by the magnetic coupling of the parallel paths.
  • The tapped point A is the mid point of the reactor.
  • When I1>I2, flux linkage induce emfs in L1 and L2.
  • emf across reactor L1 opposes the flow of I1, whereas that across L2 aids the flow of I2.
  • Thus there is a tendency to buck I1 and boost I2 so as to minimize the unbalance of currents in the parallel unit.

Reference

Dr. P. S. Bimbhra, "Power Electronics", Khanna Publishers, Fifth Edition, 2013

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