Analysis and Simulations of Hybrid Circuit Breaker Technologies in Multi-Terminal HVDC Networks
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- Institutt for elkraftteknikk 
It is established that voltage source converters (VSC) are required for multiterminal operation of a HVDC network. This converter technology is vulnerable to DC side faults, and it is crucial for a fast interruption to avoid its freewheeling diodes to have a breakdown. Several circuit breakers have been investigated to find the best one for multi-terminal operation. It is found that the fastest circuit breaker, with a reasonable on-state loss, is the hybrid circuit breaker. Two circuit breaker models are tested, active hybrid circuit breaker with 1 % on-state losses, and passive hybrid circuit breaker with 0.001 % on-state losses. They are compared to check how the interruption time will affect the system, and the requirements of the circuit breaker. For a multi-terminal operation, it is important with a proper detection algorithm. The algorithm allows for a proper detection of the fault and where it has occurred. This is important for a seamless operation of the multi-terminal network. It is found that the derivative of the current is a good detection algorithm, as it is quite simple and works properly. A fault that occurs close to the converter in a symmetrical system is difficult to handle for the circuit breakers detection algorithm. This is due to the similar magnitude of the derivative that may occur on the circuit breakers in the adjacent cables. When the faulty cable is isolated, the power flow will change direction through the adjacent cable. This gives a seamless operation of the multi-terminal network. It is found that the faulty cable can be isolated within 3 ms, using the active hybrid circuit breaker.