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| Research on Emergency Power Control of AC-DC Hybrid Power System with Flexible DC |
| Shu Hongchun1,2, Shao Zongxue1,3, Zhao Wei2, Bao Guangjiao2 |
1. State Key Laboratory of Collaborative Innovation Center for Smart Grid Fault Detection Protection and Control Jointly Kunming University of Science and Technology Kunming 650051 China;
2. Faculty of Electric Power Engineering Kunming University of Science and Technology Kunming 650051 China;
3. Faculty of Mechanical and Electrical Engineering Kunming University of Science and Technology Kunming 650051 China |
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Abstract The emergency power control of the DC system is used to balance the power shortage in the system, reduce the frequency deviation of the generator set, and improve the transient frequency stability of the system. In the AC/DC hybrid power system with flexible DC, using the flexible DC to improve the stability of the AC/DC hybrid power system becomes a pressing technical challenge. Traditional DC systems (line commuted converter, LCC) face difficulties in independently controlling active and reactive power during emergency power support, often necessitating the addition of numerous reactive power compensation devices. Therefore, the conventional DC inverter station LCC in the existing AC/DC hybrid system is replaced with a flexible DC converter station (voltage source converter, VSC). Based on the respective advantages of LCC-LCC and LCC-VSC, a prioritized coordination control strategy for emergency power support is proposed. When the AC/DC hybrid system with flexible DC needs DC emergency power support, the LCC-VSC is preferentially used to provide emergency power support to the system. If the power shortage exceeds the power regulation margin of LCC-VSC, the remaining power is transmitted by LCC-LCC.
The fast power regulation characteristics of the DC system are used to rebalance the system power shortage. Firstly, the theory of DC participation in emergency frequency support is analyzed. Then, a priority hierarchical coordinated control strategy is designed. Finally, simulation results verify the effectiveness of the proposed scheme in realizing emergency frequency support.
For the AC system N-1, N-2 fault, and LCC bipolar blocking three fault conditions in the AC/DC hybrid power system with flexible DC, the flexible DC is preferentially selected for emergency power support control. When an N-1 fault occurs, compared to situations without DC emergency power support, LCC-VSC is preferred for emergency power support. The transmission power burden of non-fault pole AC line 2 is greatly alleviated, the AC frequency fluctuation rate is reduced by 40%, and the system frequency is quickly restored and stabilized to 50 Hz. When an N-2 fault occurs, the system frequency offset exceeds 0.5 Hz without DC emergency power support. However, using the LCC-VSC as the primary emergency power support and LCC-LCC for additional support, the system frequency fluctuation rate is reduced by 65.9%, and the LCC-LCC and LCC-VSC transmission power values are within the allowable range. When LCC-LCC bipolar blocking, LCC-LCC VSC is preferred for emergency power support, and the remaining power is transmitted by AC lines 1 and 2. The system frequency fluctuation rate is reduced by 20%. Through the emergency power coordinated control of LCC-VSC and LCC-LCC, the system frequency is quickly restored to stability. After the emergency power coordinated control starts, the system frequency is basically restored to 50 Hz after 60 ms. When relying solely on LCC-LCC for emergency power control, the system frequency recovery time is significantly longer than the coordinated control of LCC-VSC and LCC-LCC.
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Received: 30 September 2022
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2023, Vol. 38 
