Frequency enhancement of grid-forming inverters under low-SCR weak grid

Investigating frequency issues of voltage source inverters (VSIs) integrating into weak grid, a quasi-virtual synchronous generator (VSG) droop controller is proposed in this paper. The review of traditional active power droop control and VSG given in the paper reveals drawbacks of these methodologies, especially with respect to microgrid application. Motivated by the limitations, the proposed controller is designed to be capable of providing virtual inertia in support of frequency response, buffering frequency violations upon disturbances and synchronising to the grid without engagement of phase locked loop. Furthermore, considering about system strength, as this has been a critical barrier preventing renewable energy from further penetration, characteristics of weak grid are analysed as preliminary in this paper. And then criterion of the controller connecting to weak grid will be introduced based multi-input multiple-output (MIMO) Nyquist theorem. Complex transfer functions for the proposed controller are developed to understand frequency-coupling issues in inverter-weak grid integrated system, which will reinforce feasibility of the controller in frequency domain. A single machine infinite bus (SMIB) model built in PSCAD/EMTDC software is adopted as a specific case to validate the control strategy in time domain. © 2019 Institution of Engineering and Technology. All rights reserved.