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Series / Parallel Operation of SCR
* (Number of SCRs in string)
When η = 1 each SCR in string has 100% identical characteristic or utilization of thyristor will be maximum. But it is not possible practically.
Series Operation
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Voltage across each of remaining (n – 1) SCRs = (n – 1) I2R
For a string voltage Vs, the voltage equation for the series circuit will be
Vs = I1 R + (n –1)I2 R
= Vbm + (n – 1) R [I1 – (Ibmx – Ibmn)]
let (Ibmx – Ibmn) = ΔIb
RI1 = Vbm
Parallel Operation
When current required by the load is more than the rated current of a single thyristor, SCRs are connected in parallel in string. For equal sharing of currents V-I characteristics of each SCRs during forward conduction must be identical as far as possible.
For parallel-connected SCRs, voltage VT across them must be equal. Figure (b) show that for a same voltage drop VT, SCR1 shares a rated current I1 whereas SCR2 carries current I2 much less than rated current I1. The total current carried by the unit is I1 + I2 and not the rated current 2I1 as required. Therefore string efficiency is given by:
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