Experimental assessment and validation of inertial behaviour of virtual synchronous machines

Increasing integration of converter-interfaced renewable generation has led to significant operational challenges for power systems. Such challenges are mainly caused by the different capabilities and dynamic responses of the converters compared with synchronous machines, for example, converters do not naturally provide inertia to the system. Virtual Synchronous Machine (VSM) is considered as a promising solution to address the challenges associated with reduced system inertia via the provision of emulated inertial response to support the operation of converter-dominated power systems. However, it has been observed that the dynamic behaviour of the VSM could differ significantly from that of a Synchronous Condenser (SC) and a Synchronous Generator (SG) in terms of inertial response provision, even when the VSM is configured with the same inertia constant. Furthermore, effective practical methods for evaluating the damping performance of VSMs are not presently available. To gain a better understanding and achieve a more accurate assessment of the dynamic inertial and damping performance of VSMs, this paper presents an experimental methodology for systematic evaluation of the dynamic response of the VSM in the frequency domain using the Network Frequency Perturbation (NFP) method. Experimental design and implementation of the NFP method are presented to assess VSM system's equivalent inertia and damping constants, where the VSM system under test can be treated as a black box without any knowledge of internal settings and control design. Case studies are conducted, where the proposed experimental design has been applied for testing and assessing the inertial and damping constants of a physical 246 kVA VSM prototype driven by a Battery Energy Storage System with comparison of the SC and SG with equivalent inertia constant. Power-Hardware-in-the-Loop (PHiL) testing is also conducted to demonstrate the VSM's inertia performance. The studies demonstrate that the developed experimental approach based on NFP method provides a valuable tool for network operators and manufacturers for evaluating the inertial and damping performance.