Enhanced power quality in HVDC system integrated offshore DFIG wind power plant under onshore grid voltage imbalances

Ensuring stable and efficient power transfer from an offshore doubly fed induction generator based wind power plant to the onshore grid through a high voltage direct current (HVDC) link presents a complex control challenge due to dynamic grid conditions, system nonlinearities, and the need for precise coordination between converter stations. The offshore converter of the test HVDC system employs a line-commutated converter for efficient bulk power delivery, while a voltage source converter (VSC) at the onshore terminal enables decoupled real and reactive power exchange. Perturbations in onshore grid voltage directly impact the receiving end VSC station, inducing fluctuations in its operating parameters and compromising overall system performance due to grid imperfections. Under imbalanced grid voltage conditions, the conventional vector control approach results in significant power oscillations and harmonic distortions. To address this challenge, this paper proposes a reinforced current control (RCC) strategy, which injects dynamically computed compensating currents into the VSC control loop, effectively suppressing double frequency oscillations and maintaining power quality. The proposed scheme enables independent regulation of real and reactive power, mitigating power oscillations and minimizing the grid current total harmonic distortion (THD) induced by onshore grid voltage deviations. The system's performance is validated through simulations in power systems computer aided design/electromagnetic transients, including direct current and hardware-in-the-loop testing on the OP4510 real-time digital simulator developed by optimal algorithm for real-time. The proposed RCC strategy effectively suppresses active power oscillations to below 5% and maintains grid current THD within 3.5%, even under onshore voltage imbalances. The proposed strategy enhances system stability, fault ride-through capability, and compliance with Institute of Electrical and Electronics Engineers (IEEE) grid interconnection standards.