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| Offshore Wind Power Transmission System Based on Phase-Shifting Control with High-Low Valves for Current Source Converters |
| Feng Dingteng1, Xiong Xiaoling1, Li Ming2, Wang Shengwei1, Ding Zixun1, Zhao Chengyong1 |
1. State Key Laboratory of Alternate Electrical Power System with Renewable Energy Sources North China Electric Power University Beijing 102206 China;
2. State Grid Economic and Technological Research Institute Co. Ltd Beijing 102209 China |
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Abstract With the development of offshore wind power going to the deep and distant sea, HVDC transmission is a competitive transmission technology for offshore wind power delivery. Modular multilevel converter (MMC) is currently the mainstream scheme for offshore wind power DC transmission projects. However, MMC consists of power sub-modules, which causes the problem of high construction costs for offshore platforms. The diode rectifier (DR) scheme, on the other hand, effectively reduces the construction cost. Nevertheless, DR does not have control capability and black start capability. The actively commutated current source converter (CSC) is a new type of topology with the advantages of small size, light weight and flexible control, which is suitable for remote offshore wind power delivery. Currently, CSC adopts fundamental frequency modulation (FFM) method, which uses a lower switching frequency. But there is only one control degree of freedom. Moreover, the reactive power and characteristic harmonics of FFM-CSC need to be compensated, which will limit the corresponding effect of lightning. Therefore, how to reduce the harmonic content of CSC, optimize the filter design method, and realize the stable operation of offshore AC system become the core issues of offshore wind CSC delivery system.
In order to further enhance the lightweight effect of offshore CSC and to realize the stable control of offshore AC system without relying on communication and grid-forming wind turbines. In this paper, an offshore wind power HVDC delivery system based on high-low-valve phase-shifting CSC is proposed and investigated in the following aspects. First, the topology of the offshore wind power transmission system is discussed in detail, and the modulation principle of the enhanced fundamental frequency modulation CSC (EFFM-CSC) is analyzed. Then, the harmonic equivalent model of the EFFM-CSC system is established, and the harmonic characteristics and content are analyzed. It can be found that the 11th and 13th harmonics can be drastically filtered out by shifting the trigger angle by 15° without adding a phase-shifting transformer. Then, this paper constructs a multi-objective optimization LC filter optimization method with the volume weight of the filter and the filter capacitor voltage as the objective function, which can improve the power density of the offshore CSC. Additionally, the mathematical model of offshore AC voltage magnitude and frequency is established, and the control strategy of offshore AC voltage magnitude and frequency is designed through the monotonicity analysis, which is conducive to the stable operation of offshore AC system.
Finally, the working principle and harmonic transfer characteristics of the phase-shifted EFFM-CSC can be verified as well as the effectiveness of the control strategy for the offshore AC system via the analysis of the PSCAD simulation model and physical experiments. From the analysis, the following conclusions can be drawn: (1) This paper proposes a control strategy using the high and low valve group trigger angle shifted by 15°, which can significantly reduce the 11th and 13th harmonic content at the offshore AC bus and achieve 87% of the filtering effect. (2) the optimization method is proposed for the design of the parameters of the LC filter by comprehensively taking into account the filtering effect, capacitance-voltage, and the volume and weight of the filter, which can greatly reduce the parameters of the LC filter, further realizing the lightweight of offshore platforms. (3) this paper analyzes in detail the monotonic law between the control targets and control quantities by constructing the intermediate variables, and designs the offshore AC voltage and frequency control strategy accordingly to realize the stable control of the voltage amplitude and frequency of the offshore AC system. (4) based on the constructed high and low valve phase-shifting EFFM-CSC experimental platform, the correctness of the theoretical analysis is verified, and it is proved that the proposed method can significantly reduce the 11th and 13th harmonic currents.
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Received: 18 December 2024
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2026, Vol. 41 
