Design and Simulation of Hybrid DC Circuit Breaker Based on Capacitor Natural Charging Commutation

Hybrid DC circuit breakers based on IGBT as the main breaking device are expensive, which limits the large-scale application of hybrid DC circuit breakers. Thyristor is one of the means to reduce the cost and technical difficulty of hybrid DC circuit breaker as the main breaking device. This type of hybrid DC circuit breaker generally combines pre-charged capacitor to complete the breaking task, but the pre-charge mode of the existing structure capacitor and the maintenance of the capacitor voltage are more complicated. Therefore, this paper proposes a hybrid DC circuit breaker based on the natural charging commutation of the capacitor. The initial energy storage of the capacitor and the automatic maintenance of the capacitor voltage are realized by using the voltage difference between the DC bus and the capacitor, so as to ensure the breaking ability of the circuit breaker. After the interruption, the capacitor voltage can be recharged again whether the line fault is restored or not, which can meet the demand of secondary interruption in a short time. Firstly, the topology and working process of the proposed topology are introduced and analyzed. According to the function of each branch, the proposed branch is named as the main branch, the transfer branch composed of thyristors, the capacitor natural charging branch, the capacitor commutation branch, the energy absorption branch. There is no charging switch in the capacitor natural charging branch, which is the key to realize the initial energy storage of the capacitor and the automatic maintenance of the capacitor voltage, also determines whether the proposed circuit breaker can successfully break the DC fault. Then the parameters of the main components in the above branches are designed, and the general principles of parameter design and the recommended range of values are given. The following two points determine whether the proposed scheme can successfully and reliably break the DC fault: (1) The peak value of the oscillating current generated by the capacitor commutation branch needs to be greater than the peak value of the fault current. (2) The diode conduction time tD in the transfer branch needs to be greater than the thyristor turn-off time tq. For the benefit of verify the applicability of the proposed structure, a single-terminal and a four-terminal DC grid model are established based on PSCAD/EMTDC to simulate the unipolar metal grounding fault at the circuit breaker outlet. The simulation results show that the proposed structure can isolate the DC side fault quickly, and the number of devices is given according to the stress of the device during the breaking process. If the thyristor with a parameter of 4.5 kV/3.5 kA is selected, 3 110 thyristors are needed. Finally, by comparing with several schemes that have been proposed, the following conclusions are drawn: (1) The proposed structure replaces the more expensive IGBT with a thyristor, and the economy is improved to a certain extent, and the breaking speed is faster. (2) The initial storage of capacitor and the maintenance of capacitor voltage can be achieved without additional control and equipment. (3) After fault isolation, the capacitor can be charged again to meet the needs of repeated breaking in a short time. © 2024 China Machine Press. All rights reserved.