Virtual Synchronous Generator for VSC-HVDC Stations With DC Voltage Control

This paper presents a two-degree-of-freedom virtual synchronous generator (VSG) for converter stations that control the dc voltage. VSGs are currently being considered in applications such as high voltage dc (HVDC) links and multiterminal dc (MTDC) systems to improve the frequency stability of power systems. In these applications, converter stations participate in the dc voltage regulation; therefore, the VSG has to accurately track the power reference of the outer dc voltage control loop. However, in the VSG design, there is a trade-off between fast reference tracking and high virtual inertia. In the present work, modifications to the VSG control are introduced to overcome this constraint by allowing reference-tracking speed and virtual inertia to be set independently. This characteristic is relevant to improve the VSG performance in voltage-source converters (VSCs) involved in the dc voltage regulation. The proposed two-degree-of-freedom VSG is applied to VSC-HVDC stations based on modular multilevel converters and validated in a practical multimachine power system with an embedded MTDC system. Results show that the frequency support provided by VSC-HVDC stations with dc voltage control can be significantly improved.