- SCR Definition: An SCR (Silicon Controlled Rectifier) is a type of thyristor used to control high voltage and current in electrical circuits.
- Series Connection: Connecting SCRs in series is used to handle higher voltage demands, but requires resistors and capacitors for equal voltage division.
- Parallel Connection: SCRs are connected in parallel to meet higher current demands, with equal temperature maintenance crucial to avoid thermal run away.
- Equalizing Circuits: Static and dynamic equalizing circuits, including resistors and capacitors, help manage voltage and current distribution in series and parallel connections.
- Thermal Run Away Prevention: Mounting SCRs on the same heat sink and using magnetic coupling can prevent unequal current division and thermal run away.
SCRs are available with ratings up to 10 KV and 3 KA. However, sometimes higher ratings are needed. In such cases, multiple SCRs are used together. Series connections meet high voltage demands, while parallel connections meet high current demands. For efficient operation, all SCR should be fully utilized, but due to different V-I characteristics, unequal voltage or current division occurs. This makes the string’s efficiency always less than 100%.
As the number of SCRs in a string increases, the voltage or current each SCR handles is reduced. This increases the string’s reliability but decreases the utilization of each SCR, leading to lower efficiency. The reliability of the string is measured by the derating factor (DRF).
Series Operation of SCR
When the operating voltage exceeds the rating of a single SCR, multiple SCRs with the same rating are connected in series. Even with the same rating, SCRs may have different I-V characteristics, leading to unequal voltage division. For example, if two SCRs in series can block 5 KV each, the string should block 10 KV, but this is not always the case.
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 efficiency, a resistor is placed in parallel with each SCR. These resistors are chosen so that the equivalent resistances of each SCR-resistor pair is the same, ensuring equal voltage division. In practice, using different resistor ratings can be difficult, so one optimal resistance value is often selected.
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.
Parallel Operation of SCR
When the operating current is more than the individual current ratings of SCRs then we use more than one SCRs in parallel. Due to different V-I characteristics SCRs of same rating shares unequal current in a string. Let a string consists of two transistors in parallel as shown in fig. 1 and their current rating by 1 KA. From the V-I characteristics of the devices it can be seen that for operating volume V, current through SCR1 is 1 KA and that through SCR2 is 0.8 KA. Hence, SCR2 is not fully utilized here. Though the string should withstand R KA theoretically it is only capable of handling 1.8 KA. So, the string efficiency is = 90%.
Due to unequal current division when current through SCR increases, its temperature also increases which in turn decreases the resistance. Hence further increase in current takes place and this is a cumulative process. This is known as thermal ‘run away’ which can damage the device. To overcome this problem SCRs would be maintained at the same temperature. This is possible by mounting them on same heat sink. They should be mounted in symmetrical position as flux.
Linkages by the devices will be same. So, the mutual inductance of devices will be same. This will offer same reactance through every device. Thus reducing the difference in current level through the devices. Another way of equalizing the current division in ac circuit can be achieved by using magnetic coupled reactance as shown in Fig – 2.
When I1 = I2 then resultant flux is zero as two coils are connected in anti-parallel. So, the inductance of the both path will be same. If I1 > I2 then there will be a resultant flux. This flux induces emfs in cols. 1 and 2 as shown in fig. Hence current in path 1 is opposed and in path 2 it is aided by the induced emfs. Thus reducing the current difference in the paths.
