Modelling and analysis of grid frequency dynamics with equivalent inertia characteristic of grid-forming voltage source converters

Abstract

Grid frequency dynamic is dominant by synchronous generators (SGs), but it is more and more obviously affected the voltage-source-converter-based (VSC-based) devices. This paper presents a modelling method of the real equivalent inertia of the VSynC-based VSC and an analysis approach for grid frequency dynamics. In this paper, the virtual synchronous control (VSynC)-based VSC is taken as an example. Firstly, grid frequency dynamic is introduced with and without considering energy constraints and cascaded control of the VSynC-based VSC, respectively. Then the swing equation model is established to study the real inertial characteristic of the VSynC-based VSC, which poses main effect on grid frequency dynamics. Based on the swing equation model, the real equivalent inertia characteristic is deduced under the consideration of the energy constraints and cascaded control dynamics of DC voltage control, and it is recognized as a transfer function, rather than the constant value like traditional SGs. Moreover, under the effect of the transfer-function-based inertia of the VSynC-based VSC, the bode-diagram-based analysis method is proposed in this paper to discuss the frequency dynamics of the power system with both SGs and VSynC-based VSCs. The zero-crossing frequency and phase margin in bode diagrams are used to evaluate the major characteristics of grid frequency, i.e. the rate of change of frequency (ROCOF) and nadir (the max deviation). Finally, case studies are implemented based on New England 39 Bus System to validate the correctness and effectiveness of the proposed modelling and analysis method.

Keywords

• Grid frequency dynamics
• inertia characteristic
• grid-forming
• voltage source converter (VSC)