Voltage source converter based high voltage DC system (VSC-HVDC) has become a very promising solution to integrate offshore wind farm. However, the equivalent inertia of the modern power system with large renewable energy integration becomes small, which will arouse some frequency stability problems. To tackle this problem, this paper proposes an improved frequency regulation strategy for VSC-HVDC integrated offshore wind farm. Firstly, in the frequency decrease stage, the rotor kinetic energy of wind turbines (WTs) is used to suppress the decrease of the frequency, and the control parameters are determined to make full use of the mechanical power and rotor kinetic energy of WTs, the frequency nadir is improved. Secondly, in the rotor speed recovery stage, the DC capacitors of VSC-HVDC are used to release power to compensate the deficiency value of wind farm output power and avoid the secondary frequency drop (SFD) problem. Lastly, the simulation is conducted in PSCAD/EMTDC to validate the effectiveness of the proposed coordinated frequency control strategy.
The modular multilevel converter (MMC) has become a very promising technology for long-distance and large-capacity transmission of offshore wind power. However, both sides of the AC transmission line at the sending end are controllable electronic power devices, resulting in difficulty in fault identification and inapplicability of traditional differential protection schemes. In order to solve this problem, a wave-similarity-based protection scheme is proposed for AC transmission line faults. Firstly, the symmetrical and asymmetrical fault current characteristics of the double-fed induction generator (DFIG) and MMC are studied, indicating that the fault current characteristics are obviously different from the synchronous units. Secondly, the wave-similarity-based protection scheme is proposed based on the different wave forms of the fault currents of the MMC and DFIG. When the similarity coefficient is less than the margin coefficient, there is a fault in this phase. Moreover, the proposed wave-similarity-based protection scheme can identify all types of short-circuit faults correctly and is not affected by the transition resistance. Finally, simulations of an MMC-HVDC system with offshore wind farms are conducted to validate the effectiveness and correctness of the proposed protection scheme.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.