Technical assessment of load commutation switch in hybrid HVDC breaker

The development of a large scale high voltage direct current (HVDC) power grid requires a reliable, fast and low-loss circuit breaker. The load commutation switch (LCS) is an essential part of ABB´s 1200 MW hybrid HVDC breaker concept which builds up a low-loss conducting path for the load current. The technical requirements for the LCS are expressed in this paper by studying the operation principle of the hybrid HVDC breaker. The voltage stress over the LCS is calculated and simulated based on a dc grid with 320kV and 2kA rated voltage and current. A system model of the hybrid HVDC breaker is developed in PSCAD/EMTDC software to study the design criteria for snubber circuit and arrester blocks. It is observed that conventional snubber circuits are not suitable for a bidirectional hybrid HVDC breaker as the current of snubber capacitors prevent the fast interruption action. A modified snubber circuit is proposed in this paper along with two more alternatives for the load commutation switch to overcome this problem. Moreover, the power loss model for a semiconductor device is discussed in this paper based on the 4.5 kV StakPak IGBT. The model is used to calculate the conduction power losses for different LCS topologies. Ultimately, a matrix of 3x3 IGBT modules is selected to provide a reliable LCS design which can handle several internal fault cases with no interruption of operation. A full-scale prototype has been constructed and tested in ABB HVDC Center, Ludvika, Sweden. The experimental test results are also included in the paper in order to verify the calculation and simulation study.