Abstract
This paper presents a simulation-based investigation of compatibility between grid forming converters with different control system implementations when operating in the islanded ac-grid of an HVDC-connected offshore wind farm. A system configuration with two HVDC-terminals connected to separate clusters of a large-scale offshore wind farm with an internal ac-connection is assumed. Both HVDC converters operate in grid forming mode and share the responsibility for voltage and frequency control within the islanded offshore ac grid. Grid forming functionality is assumed by applying the general framework of Virtual Synchronous Machine (VSM)-based control. Three different VSM-implementations are considered and all their combinations are investigated. The intention is to evaluate the compatibility of the different implementations for parallel operation in an islanded system and to reveal if any specific combinations of grid forming control schemes have advantages or disadvantages in terms of interoperability-friendliness. Simulation studies are conducted in Matlab/Simulink, and the results demonstrate that all the investigated implementations can operate safely in the same system. However, there are differences in the sensitivity to key parameters, which implies that different parameter values might be required to ensure good dynamic response for different combinations of VSM implementations.