# Parallel Operation of Thyristors

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.

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