Frequency Stability Analysis of Grid-Forming Control in Voltage Source Converters

Currently most of the existing voltage-source converters (VSCs) in large scale power transmission networks are in grid-following (GFL) control. However, the rapid growth of renewable energy in electricity generation motivates the development of a new control method of VSCs, which is known as grid-forming (GFM) control, and the UK Grid Code has included a section for GFM plant compliance testing. In this paper, a droop controlled GFM VSC model and a classic GFL VSC model are developed in Simulink, and their ability to withstand frequency variation is evaluated under various scenarios when connected to AC grids with different short-circuit ratios (SCRs). The impact of the GFM converter control parameters on the system stability ranges is also studied. Moreover, small-signal analysis method is used to derive the system stability limits theoretically to verify simulation results and explain the root cause of oscillations when frequency variation is beyond the limits. Lyapunov stability criterion and La Salle's Invariance Principle are used to demonstrate the system frequency variation process in a more comprehensive way. This paper can provide guidance for GFM converter control system parameter selection and for practical applications of GFM converters in large transmission systems on frequency stability performance.