Sharing of Primary Frequency Response Using LCC-HVDC

Power systems worldwide are transforming into renewable-dominated ones that require significant grid flexibility. While extensive research has been done on flexibility from the generation or demand side, there is a significant lack of research on studying flexibility from the grid side. Therefore, this paper proposes a primary frequency response sharing strategy using LCC-HVDC for inverter frequency control. Under an inverter frequency event, the proposed method imposes a controlled load change that is proportional to the inverter AC frequency deviation on the rectifier AC network. This in impact activates the primary frequency response from the rectifier side to support the inverter AC frequency (i.e., sharing of primary frequency response). The level of support and the coupled dynamics of rectifier and inverter frequencies are quantified by the developed mathematical model. The model reveals that the inverter load change is affecting the rectifier AC frequency through two cascaded 2(nd) order transfer functions. With the developed model and the desired frequency nadirs at both AC systems, the controller parameters of the proposed method are calculated. This ensures that appropriate level of primary frequency response is shared without jeopardizing the frequency performance of rectifier AC system. The effectiveness of the proposed method is demonstrated using a modified four-generator two-area system. Estimation errors of network inertia and droop constants are discussed.