Series and Parallel Connected Power Electronic Devices

Maximum voltage and current of a power semiconductor device limit its power-handling capability for power system applications. Hence, several devices of the same type can be connected

1. In series to increase the overall voltage rating or
2. In parallel to increase the overall current rating or
3. In series and parallel to increase overall power capability.

IGBTs require special care to match the characteristics due to the variations of the temperature coefficients with the collector current. IGBT modules (up to six in parallel) are commercially available.

However, the characteristics (i-v, β, Vth, etc) for the same type of devices must be similar and turn-on and turn-off must be simultaneously.

Voltage and current sharing elements similar to those for thyristors may be used (discussed in the next slide)

Normally, each sharing device is derated by almost 10% from its rated voltage or current for safety purpose.

Thyristors in Series: The problem of unequal voltage sharing ( during reverse and off conditions) between the thyristors in series is overcome through connecting balancing resistors (R) in parallel with the devices so that they equally share the reverse voltage. The current through the resistors is >> diode current

The RSCS networks have two functions: transient voltage sharing and dv/dt protection

Thyristors in Parallel: The problem of unequal load current sharing ( during on condition) between the thyristors in parallel series is overcome through connecting small resistors or magnetically coupled inductor in series with the device to force equal share of current.

 

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