Control of multiple parallel HVDC systems for frequency response sharing: A study based on synchronous frequency operation of the asynchronously interconnected systems in China

To enhance frequency stability, the integration of High Voltage Direct Current (HVDC) systems for frequency control has been widely employed. In 2023, a frequency control strategy termed co-frequency control was deployed in the LUXI Back-to-Back (BTB) VSC-HVDC system of the China Southern Power Gird (CSG) to mitigate frequency deviations between interconnected asynchronous grids. Nevertheless, reliance on a single HVDC system implementing co-frequency control presents three major challenges: limited frequency regulation headroom, the absence of backup control capability during HVDC maintenance, and the occurrence of low-frequency DC power oscillations (LFPO). Consequently, the control falls short of fully satisfying the operational expectations of CSG. Since incorporating additional HVDC systems into co-frequency control is regarded as an effective measure to address the first two challenges, this paper proposes a coordinated scheme for multiple parallel HVDC systems participating in co-frequency control. The proposed scheme is formulated as an optimization problem that calculates and updates the frequency control coefficients of the HVDC systems. These coefficients are obtained by solving the developed optimization problem, which accounts for the power headroom of each HVDC system, the N-1 HVDC blocking fault security criterion, and the stability requirements of the HVDC system. As a result, the scheme ensures the secure operation of multiple parallel HVDC systems in co-frequency control during both load variations and HVDC outages. The effectiveness of the proposed method is validated through Real-Time Digital Simulator (RTDS).