Power Synchronization Control of Modular Multilevel Converter for a 2 GW/525 kV HVDC System

In this paper, a power synchronization control (PSC) of modular multilevel converter (MMC)-based inverter is investigated to tackle stability problems that may arise while interconnection of offshore windfarms (OWFs) with weak grids. The instabilities caused by weak grids may travel through phase lock loop (PLL) via an MMC-based grid-side converter (GSC) and cause the DC-link voltage to oscillate. The system under study is 2 GW/525 kV in North Sea of Netherlands where 2 GW of power from an OWF is transferred to an onshore grid via a 525 kV HVDC system. The control system consists of outer voltage, inner current loops, DC-synchronization loop in addition to circulating current control and nearest level control for modulation of MMC. The key point of the proposed PSC is to eliminate the impact of the grid varying conditions on output of MMC inverter. This paper provides a practical overview of PSC of 2GW MMC-HVDC. Simulations are performed under varying grid conditions such as voltage, phase and frequency and results confirm that PSC can help stabilize the voltage at output of MMC inverter.