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To control power flow and manage fault level in meshed MV networks, back-to-back voltage source converters (B2B-VSCs) are being used. However, their high cost and relatively low efficiency are of concerns. Partially rated series compensators, such as SSSCs or UPFCs, are desired but come with the challenge of protecting the device during grid faults. Their potential of use has been limited in comparison with the fully rated back-to-back converters. This paper proposes a new system topology including thyristor crowbars and a varistor to protect the SSSC in an MV network and improve the reliability and flexibility of the network operation. Using the proposed method, the time required for isolating the series compensator from the grid is reduced from at least 20 ms, corresponding to the interruption time of conventional circuit breakers, down to 3 μs in the worst case in addition to the grid fault detection delay. The performance is evaluated by simulation. A small-scale single-phase prototype operating at 230 V/16 A is tested in order to demonstrate the concept.

A new packaging structure for clamping the temperature of a thyristor die is analysed in this paper. The proposed structure aims to absorb the heat released from the thyristor during a short circuit protection operation, to maintain the temperature within a defined range even when the current through the thyristor is temporarily very high. The defined range is below the maximum tolerable temperature and is controlled by using phase-change material (PCM). This has the potential to reduce both space and cost when compared to traditional packaging structures which utilise heat sinks. In the course of this investigation, a thermal model of a thyristor is used for comparison. COMSOL Multiphysics was used to simulate the thyristor temperature during a fault and the effect that the PCM has on the thyristor temperature. Experiments have been performed to verify the models and validate the feasibility of the proposed concept.