Active bilateral primary frequency and AC voltage control of hybrid LCC-VSC-HVDC for asynchronously interconnection systems

With the increasing penetration of renewable energy generators and the replacement of conventional synchronous generators in asynchronously interconnected systems, the frequency stability has become a major challenge for power systems, and the grid-flexibility requirements have increased. Since high voltage direct current (HVDC) systems are seen as a key solution for enhancing grid flexibility, a new coordinated control scheme for a hybrid HVDC system, featuring a line commutated converter (LCC) rectifier and a voltage source converter (VSC) inverter, is proposed. The proposed method utilizes the DC control of the LCC for bilateral primary frequency control and the DC voltage control of the VSC to suppress AC voltage fluctuations on the rectifier side without compromising the frequency support objective. The level of support and the coupling dynamics of the sending and receiving end systems are quantified by developing a frequency response model. Based on the proposed model, a control parameterization process is presented to avoid under-frequency or over-frequency conditions in the hybrid HVDC system. The effectiveness of the proposed frequency model and control method is verified using a two-area asynchronously interconnected system, modified from the IEEE 39-bus test system.