Analysis and Control Of Grid Connected Multi Terminal HVDC System

Abstract

When connecting a wind park (WP) to the main grid (MG) by way of an HVDC Light transmission system, the WPbecomes decoupled from the MG, which results in several technical and economical benefits, on a variety of aspects, for transmission system operators (TSOs), WP developers and WTG manufacturers. Perhaps most important for TSOs is that an HVDC-Light connected WP becomes comparable to a normal power plant; the MG-side HVDC Light converter can be directly connected to a control or power dispatch center. A desirable consequence is also that AC faults appearing in the WP or MG grid will not be propagated by the HVDC Light transmission system, which has several desirable consequences such as possibly reduced mechanical stresses on the WTGs.This paper presents operation and control strategies of multi-terminal HVDC transmission system (MTDC) using voltage source converters (VSCs) for integrating large offshore wind farms. The framework and operation principles of the proposed system are described and control strategies for coordinating various VSCs are proposed. DC voltage control based on the DC voltage-current (V-I) droop characteristic of grid side converters is implemented, to ensure stable system operation and flexible power dispatch between various onshore AC grids. To validate the performance of the proposed control strategies, a typical four terminal MTDC networks, which connecting two offshore wind farms with two onshore AC grids, is established in PSCAD/EMTEC. Simulation results under normal and abnormal operation conditions verify the satisfactory performance of the proposed control strategy and accuracy of the theoretical analysis.