Equivalence of primary control strategies for AC and DC microgrids

Microgrid transient stability is a challenging topic that is being widely discussed in the literature. One of the main characteristics of most microgrids is their lack of inertial loads or generators, as most of these elements are converter-interfaced devices. The reduction of the total inertia of microgrids reduces their transient stability under power variations. Primary control strategies of regulating devices define the transient response and hence the dynamic behaviour of the microgrid. The aim of this paper is to study and compare autonomous primary control techniques that contribute to the improvement of this transient behaviour both for ac and dc microgrids. In this context, virtual synchronous machine (VSM) techniques are analysed for ac microgrids and their behaviour for different values of emulated inertia and droop slopes is tested. Regarding dc microgrids, a virtual impedance-based algorithm is proposed and its equivalence to VSM techniques in ac grids is demonstrated. It is confirmed that, by modifying different control parameters in the proposed technique, the transient as well as steady-state response of regulating converters can be adapted. Therefore, as it is shown in the results, the transient stability of dc microgrids can be significantly improved by the proposed technique, which mimics the behaviour of the classical ac grid and VSM algorithms. Furthermore, in the paper it is shown that by varying the control parameters both at VSM and virtual-impedance strategies, the flexibility to integrate devices with different dynamic responses is increased.