Online Measurement of Offshore Wind Farm Impedance for Adaptive Control of HVDC Transmission Systems

This work presents online impedance measurement of offshore wind farm impedance using an MMC-based HVDC converter as an active injection and measurement device. The objective is to identify the wind farm impedance in real time to enable adaptive control of the HVDC converter that will ensure the offshore system stability under dynamic operation conditions. Due to speed requirements imposed by the adaptive control application, broadband binary sequence signals are selected as the perturbation. The proposed method takes advantage of the regularly sampled and multilevel PWM operation of the MMC to achieve optimal spectral control of the perturbation signal by synchronizing it with the PWM sampling and combining multiple binary sequences to form multilevel injection. Algorithms and their implementation to process voltage and current measurements online also play an important role in the accuracy and speed of identification and are studied in detail. System electromagnetic transient simulations using detailed turbine, cable network and HVDC converter models are used to test and evaluate the proposed methods.