Parallel-capacitor commutation is a type of forced commutation technique used in power electronics circuits to turn off the thyristors (SCRs or GTOs) at a desired instant. In this technique, a parallel-connected capacitor is used to provide the required commutation voltage to turn off the thyristor.

How does parallel-capacitor commutation work?
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When a thyristor is conducting and a reverse voltage is applied across it, it remains in the conducting state until the current through it becomes zero. In order to turn off the thyristor faster, a commutation voltage higher than the reverse voltage is required. Parallel-capacitor commutation achieves this by providing a high initial reverse voltage across the thyristor.

Working principle of parallel-capacitor commutation
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1. Charging phase: During the charging phase, the capacitor is connected across the thyristor in parallel. The capacitor starts to charge through a charging resistor, which limits the charging current. This charging current flows through the thyristor, maintaining its conduction.

2. Commutation phase: When it is required to turn off the thyristor, a switch is opened or triggered, disconnecting the capacitor from the thyristor. The charged capacitor now discharges through the thyristor, providing a high reverse voltage. This reverse voltage aids in turning off the thyristor by forcing the current through it to become zero.

Advantages of parallel-capacitor commutation
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- Faster turn-off time: The parallel-capacitor commutation technique provides a higher commutation voltage, which helps in turning off the thyristor faster.

- Simple and reliable: The circuitry required for parallel-capacitor commutation is relatively simple and reliable compared to other forced commutation techniques.

- Reduced commutation losses: By using a higher commutation voltage, the parallel-capacitor technique helps in reducing the commutation losses.

- Suitable for high voltage applications: Parallel-capacitor commutation is particularly useful in high voltage applications where fast turn-off is required.

Disadvantages of parallel-capacitor commutation
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- Additional components: This technique requires an additional capacitor and a charging resistor, which adds to the complexity and cost of the circuit.

- Limited frequency range: Parallel-capacitor commutation is not suitable for high-frequency applications due to the limitations of the capacitor and charging resistor.

Conclusion
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Parallel-capacitor commutation is a forced commutation technique that provides a higher commutation voltage to turn off the thyristor faster. It offers advantages such as faster turn-off time, simplicity, and reduced commutation losses. However, it requires additional components and is not suitable for high-frequency applications.