Development of a solid-state fault current limiting and interrupting device suitable for power distribution networks
In recent years there has been an increased interest in developing fault current limiters for power distribution networks. This arises from the need to cope with the ever increasing short-circuit levels and to reduce the stress on system equipment, e.g. transformers, circuit-breakers and cables. It is also due to the increased concern about power quality, where fault current limiters are expected to play an important role in mitigating voltage sags during faults. Various devices to limit the fault current have been proposed, such as controlled fuses, tuned LC circuits, solid-state and superconducting fault current limiters. This research investigate the use of a novel technique to develop a solid-state Fault Current Limiting and Interrupting device (FCLID) suitable for low voltage distribution networks. The FCLID mainly consists of a high-speed bi-directional semiconductor switch, a varistor (non-linear resistor) and a snubber circuit; all connected in parallel. The semiconductor switch and the varistor share the fault current during the period of FCLID operation. To protect the semiconductor switch and the varistor from damage due to overheating, their temperatures are indirectly monitored in order to define the maximum operating time of the FCLID. A new method for estimating the junction temperature of the switching device and the varistor under transient condition has been developed and experimental tests were carried out to validate the proposed method. The energy handling capability of varistors and associated problems due to their non-linear characteristics have also been investigated. Experimental tests were carried out to measure the energy handling capability of the varistor using thermal imaging system. A new method for improving the current sharing between parallel varistors has been implemented. A computer model of the FCLID has been developed and implemented into a typical distribution network using MATLAB/ SIMULINK. The network performance under different conditions has been analysed. An experimental single-phase 230 V prototype FCLID was developed and tested under different operating conditions. Finally, the outcome of the theoretical, simulation and experimental phases of the research was used to establish the outline design specifications of a FCLID suitable for 11 kV distribution networks.