Active Bilateral Frequency Support using LCC-HVDC systems for asynchronous interconnected systems

The involvement of Line-Commutated Converter-based High-Voltage Direct Current (LCC-HVDC) systems in grid frequency support has been studied in previous research, with a number of practical applications. Existing methods, however, fail to achieve active bidirectional frequency support and are mainly geared towards large frequency events. Specifically, previous studies have identified the inability of LCC-HVDC to achieve dynamic regulation of the frequency of asynchronous interconnected systems. This is due to DC power variations causing fluctuations in the reactive power of LCC-HVDC, leading to fluctuation in AC voltage. In this study, it is shown that LCC-HVDC can achieve reactive power control by flexibly adjusting the DC current and DC voltage in a coordinated manner. Based on this, an active bilateral frequency support (ABFS) control is proposed, involving a flexible allocation of DC current and DC voltage, that aims to achieve dynamic regulation of the frequency of asynchronous interconnected systems while maintaining a constant reactive power consumption of the inverter. This enables the minimization of AC voltage fluctuation caused by DC power variations. Furthermore, the concept and stability of the proposed control are presented using the V-I trajectory, and the extinction angle limitation of the inverter has been taken into account. Therefore, the risk of commutation failure in the proposed method can be effectively avoided as well. The effectiveness of the proposed method is demonstrated using a modified four-generator and two-area (4G2A) system under a variety of load changes.