Series and Parallel Connection of SCR or Thyristor
Nowadays, SCRs are available of ratings up to 10 KV and 3 KA. But sometimes we face demand, more than these ratings. In this case combination of more than one SCRs is used. Series connection of SCRs meets high voltage demand and parallel connection of SCRs meets high current demand. These series and parallel connection of SCR or Thyristor will work efficiently if all SCRs are fully utilized. Although all SCRs in a string are of same rating, their V-I characteristics differ from one another. This leads to unequal voltage or current division among them. Hence every SCR is not fully utilized. So the efficiency of string is always less than 100% according to the given expression
With increase in the numbers of SCRs in a string voltage or current handled by each SCR is minimized. This phenomenon increases the reliability of the string, but reduces the utilization of each SCR. Thus string efficiency decreases. Reliability of string is measured by derating factor (DRF) which is given by the expression
Series Operation of SCRWhen the operating voltage is more than the rating of one SCR the multiple SCRs of same ratings are used in series. As we know SCR’s having same rating, may have different I-V characteristic, so unequal voltage division is bound to take place. For example if two SCRs in series that is capable of blocking 5 KV individually, then the string should block 10 KV. But practically this does not happen. This can be verified with the help of an example. Let the characteristics of two SCRs are as shown in fig. 1.
So we can see from the diagram, for same leakage current, unequal voltage division takes place. Voltage across SCR1 is V1 but that across SCR2 is V2. V2 is much less than V1. So, SCR2 is not fully utilized. Hence the string can block V1 + V2 = 8 KV, rather than 10 KV and the string efficiency is given by = 80%. To improve the efficiency a resistor in parallel with every SCR is used. The value of these resistances are such that the equivalent resistance of each SCR and resistor pair will be same. Hence this will ensure equal voltage division across each SCR. But in practical different rating of resistor is very difficult to use. So we chose one value of resistance to get optimum result which is given by
Where, n = no. of SCR in the string Vbm = Voltage blocked by the SCR having minimum leakage current. ΔIb = Difference between maximum and minimum leakage current flowing through SCRs. Vs = Voltage across the string. This resistance b is called static equalizing circuit. But this resistance is not enough to equalize the voltage division during turn on and turn off. In these transient conditions, to maintain the equal volume across each device a capacitor is used along with resistor in parallel with every SCR. This is nothing but snubber ckt which also known as dynamic equalizing circuit. An additional diodes can also be used to improve the performance of dynamic equalizing circuit.