A Coordinated Marginal Current Control Method for LCC-HVDC

Abstract

Line-commutated-converter-based HVdc (LCC-HVdc) is extensively used in modern power systems. Most of the LCC-HVdcs are implemented with the control strategy termed marginal current control method (MCCM), while their operation is sensitive to disturbance from the ac part of the system. This paper originally reveals the mechanism of two types of oscillations induced by the MCCM after ac disturbances, i.e., the current boundary induced oscillation and the current error induced oscillation. A coordinated marginal current control method (CMCCM) is proposed, aiming at suppressing the aforementioned oscillations and enhancing HVdc recovery performance. Compared to the MCCM that responds only to dc voltage, the CMCCM additionally considers ac voltage, ac current, and power factor of the converter. AC grid's parameters are also taken into account. With a more considerate design, the empirical current boundary determined by the MCCM is replaced with an analytical one to coordinately control dc current. Therefore, better performance of HVdc is achieved. Simulations in RTDS reveal that the proposed CMCCM can suppress the oscillations, speed up HVdc recovery process, and moderate transient over voltage of HVdc.