Abstract
Offshore wind power has great development potential, for which the key factors are reliable and economical wind farms and integration systems. This paper proposes a medium-frequency wind farm and MMC-HVDC integration system. In the proposed scheme, the operating frequency of the offshore wind farm and its power collection system is increased from the conventional 50/60 Hz rate to the medium-frequency range, i.e., 100–400 Hz; the offshore wind power is transmitted to the onshore grid via the modular multilevel converter-based high-voltage direct current transmission (MMC-HVDC). First, this paper explains the principles of the proposed scheme in terms of the system topology and control strategy aspects. Then, the impacts of increasing the offshore system operating frequency on the main parameters of the offshore station are discussed. As the frequency increases, it is shown that the actual value of the electrical equipment, such as the transformers, the arm inductors, and the SM capacitors of the rectifier MMC, can be reduced, which means smaller platforms are required for the step-up transformer station and the converter station. Then, the system operation characteristics are analyzed, with the results showing that the power losses in the system increase slightly with the increase of the offshore AC system frequency. Based on time domain simulation results from power systems computer aided design/electromagnetic transients including DC (PSCAD/EMTDC), it is noted that the dynamic behavior of the system is not significantly affected with the increase of the offshore AC system frequency in most scenarios. In this way, the technical feasibility of the proposed offshore platform miniaturization technology is proven.
Highlights
Offshore wind energy resources are stable and rich, and offshore wind power exploration has become the focus of wind power development worldwide in recent years [1,2].The design and implementation of transmission systems for offshore wind farm are some of the key topics in the field of offshore wind power generation [3]
It should be noted that the position of the control system reference frame for the inverter-side MMC is provided by a phase locker loop (PLL), while that for the rectifier-side MMC comes from an inside phase angle generator (PAG) at a fixed frequency signal ω0
This section involves a comparative the offshore wind farm integration system based on the MMC-high-voltage direct current (HVDC) scheme with the conventional 50 Hz scheme and the proValue scheme (f0 is set as 150 Hz) in terms of economic and operational
Summary
Offshore wind energy resources are stable and rich, and offshore wind power exploration has become the focus of wind power development worldwide in recent years [1,2]. In order to improve the economic efficiency, the diode rectifier unit-based HVDC (DRU-HVDC) scheme [9,10,11] for offshore wind farm integration was proposed, which can greatly reduce the costs for offshore converter stations [12]. This paper proposes a medium-frequency wind farm and MMC-HVDC integration system In this scheme, the MMC is adopted in the rectifier station and the inverter station due to its high scalability and low switching frequencies with satisfactory output harmonics characteristics. Based on the European SuperGrid concept, [20] analyzed the impact of the offshore AC system operating frequency on the investment costs of a large-scale wind power HVDC transmission scheme.
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