Coordinated emergency control strategy of high-voltage direct current transmission and energy storage system based on Pontryagin minimum principle for enhancing power system frequency stability

With the increase of the proportion of renewable energy sources, the rotational inertia of the power system decreases, which results in the risk of frequency instability increasing. Based on Pontryagin minimum principle, this paper presents a systematic emergency control strategy by coordinating the active power of voltage source converter based high-voltage direct current transmission (VSC-HVDC) and energy storage system (ESS) to improve the system frequency stability, reduce the operation cost of VSC-HVDC and ESS and implement energy management of ESS. In the stage of frequency fall, a hierarchical control strategy is proposed to prevent the frequency from dropping below the preset threshold value and achieve the optimal distribution of support power between VSC-HVDC and ESS. For the system level, the total optimal power support trajectory of VSC-HVDC and ESS is optimized by the Gaussian pseudospectral method with the minimum total control energy, considering the constraints of frequency. For the station level, the power references of VSC-HVDC/ESS are determined by the model predictive controller (MPC), with the minimum frequency deviation of the non-disturbed system connected to VSC-HVDC, control cost of VSC-HVDC and loss cost of ESS. In the stage of frequency recovery, an energy recovery control strategy is designed to aim at quickly recovering energy of ESS while considering the minimum frequency limit. Finally, the correctness and effectiveness of the proposed strategy are verified in the improved IEEE 39 bus system and a real-power system.