Evaluation of Virtual Inertia Control Strategies for MMC-based HVDC Terminals by P-HiL Experiments

Abstract

This paper presents an experimental performance evaluation of different control strategies for providing virtual inertia from power electronic converters. The evaluation is based on a laboratory-scale prototype of a point-to-point HVDC transmission system with Modular Multilevel Converters (MMCs). Operation with a grid emulator connected to a realtime simulator reproduces the behavior of a power system and allows Power-Hardware-in-the-Loop (P-HiL) experiments. Control of the inverter terminal for providing virtual inertia to the simulated power system is evaluated with four different control system implementations. The four cases include a conventional control system enhanced with df/dt-based inertia emulation functionality and three different Virtual Synchronous Machine (VSM) implementations based on emulation of a synchronous machine swing equation. The dynamic response in the power flow and the frequency transients are evaluated for all cases, and the inertial energy exchanged with the isolated power system is assessed in comparison to a conventional power controller without inertia emulation. The results demonstrate that all the evaluated implementations can provide similar inertial response when operated in a small isolated grid, while clear differences in dynamic response and stability properties are revealed during operation under strong grid conditions.