Improved Communication-Free Coordinated Control of VSC-MTDC Integrated Offshore Wind Farms for Onshore System Frequency Support

Abstract

To reduce the adverse influence of communication time delay, the DC voltage of offshore converters is usually employed as input signals of existing communication-free frequency support schemes of offshore wind farms (OWFs). However, the offshore DC voltage varies with the output power of OWFs, this distortion will deteriorate the frequency support performance. To address this issue, an improved communication-free coordinated control (ICFCC) scheme of VSC-MTDC integrated OWFs for frequency support is proposed. The ICFCC scheme involves the onshore frequency variation into the DC voltage deviation once frequency events occur, and an estimator is proposed to obtain the onshore DC voltage in offshore VSC stations to provide fast frequency support and reduce the distortion influence. Meanwhile, to obtain better support performance, the frequency support power is allocated reasonably by regulating droop coefficients adaptively, more power will be injected via the station closer to the disturbing bus. After frequency support, the offshore wind turbines will recover their rotor speed with asymptotic control to reduce second frequency drop. Case studies are carried out on the 4-terminal and 5-terminal test systems, respectively, while considering the parameter uncertainties and noise disturbance. Different control methods are compared to illustrate the effectiveness of the ICFCC scheme.