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| A Multi-Port DC Circuit Breaker with Fault-Current Limiting Capability |
| Li Hao1, Pei Xiangyu1, Li Zewen1, Zhou Wandi2, Li Pengzhi2 |
1. School of Electrical and Information Engineering Changsha University of Science & Technology Changsha 410114 China;
2. State Key Laboratory of Advanced Power Transmission Technology State Grid Smart Grid Research Institute Co. Ltd Beijing 102209 China |
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Abstract The hybrid DC circuit breaker (DCCB) is the critical equipment to realize the fast interruption of fault currents for VSC-based DC grids. However, with the continuous expansion of VSC-based DC grids, based on the local protection strategy, each DC line must be equipped with an expensive two-port hybrid DCCB at both ends. It causes enormous project investment costs and seriously restricts the large-scale application of hybrid DCCB in future complex DC grid projects. Therefore, this paper proposes a multi-port DCCB with fault current limiting capability (FCL-MHDCB) to improve the fault current limiting effect by optimizing the overall operation sequence. At the same time, after the DC bus fault isolation is completed, based on the proposed FCL-MHDCB scheme, the healthy DC lines connected to the fault bus can still carry out normal power transmission, which greatly improves the power supply reliability of the system.
Firstly, the FCL-MHDCB topology is proposed, which consists of the main branch, transfer branch, energy dissipation branch, current limiting unit, and auxiliary unit. Secondly, the fault current blocking principle of the proposed FCL-MHDCB under DC line fault conditions is analyzed. The working process can be divided into three stages: the steady-state operation stage, the fault current limiting stage, and the fault current interruption stage. Thirdly, taking the n-terminal VSC-based DC grids as an example, the action process of the proposed FCL-MHDCB is theoretically deduced. The proposed scheme is verified by the derived formula. Finally, considering the thyristor withstand voltage, fault current peak, full input time of current limiting inductor, and current limiting effect, the value range of pre-charged capacitor C and current limiting inductor LFCL in the current limiting unit is designed. Considering comprehensively, the range of capacitor C can be determined to be 10~15 μF, and the range of inductor LFCL can be determined to be 0.1~0.15 H.
In general, to highlight the advantages of the proposed FCL-MHDCB, a metal grounding fault occurs at the bus of the positive outlet of the converter station at t =3 s. The simulation results of the port current show that the proposed FCL-MHDCB can still maintain the power transmission of the sound line after DC bus fault isolation, which ensures the continuous and reliable operation of VSC-based DC grids to the greatest extent. In addition, the simulation results of each branch current of the proposed FCL-MHDCB show that it can effectively complete the fault current suppression and fast blocking. At about 3.011 s, the fault current drops to 0 under the transient breaking voltage of metal oxide varistors, and FCL-MHDCB completes the fault current interruption. Compared with the multi-port DCCB without the current limiting unit, the peak fault current is as high as 9.09 kA at 3.006 s, while the peak fault current of the proposed FCL-MHDCB is only 5.64 kA, reducing by 38.0 %. Moreover, the energy dissipation stress of the arrester is reduced by 61.21 % compared with the conventional scheme.
The following conclusions can be drawn from the simulation analysis: (1) Compared with the existing technical scheme, the proposed FCL-MHDCB can still transmit power between healthy lines after completing the DC bus fault isolation, ensuring the continuous and reliable operation of VSC-based DC grids to the greatest extent. (2) When the circuit breaker recloses in the permanent fault, the permanent fault isolation can be quickly realized only by coordinating the thyristors in the ordered conduction current limiting unit with the pre-charged capacitors. Immediately charging the capacitors of the current limiting unit is not needed. (3) The whole action sequence of DCCB is optimized, and the limiting effect of fault current is improved. The peak value of fault current decreases by 38.0 %, and the energy dissipation stress of the arrester decreases by 61.21 %.
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Received: 16 March 2022
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2023, Vol. 38 
