Power Flow Calculation Of The VSC-MTDC And AC Hybrid Power System

HVDC transmission technology has already been developed significantly. So far, more and more HVDC projects have been put into service. With the development of technology and increasing of demand, multi-terminal HVDC (MTDC) systems have become imcreasing attractive. The emergence of voltage source converter (VSC), a fully controlled power electronic device, gives rise to a new way of forming MTDC systems.This paper has analyzed the steady-state operation characteristics and the control modes of the traditional HVDC (PCC-HVDC) and the VSC-HVDC systems. By comparison, VSC-HVDC has more technical advantages over the PCC-HVDC, and facilitates the construction of MTDC systems.This paper researchs the power flow calculation of MTDC systems based on the analysis of HVDC systems. The power flow algorithm for HVDC systems can be divided into two categories: the sequential method and the simultaneous method. Simultaneous method has good convergence, but difficult to combine with the existing AC power flow program; Sequential method can easily be combined with the existing AC power flow algorithm, but the convergence is worse. In this paper, both the principles of the two methods and their implemention process have been studied in detail.A sequential algorithm for VSC-MTDC systems is presented in this paper. The steady-state model of VSC has been established. On the basis of the steady-state model, a mathematical model for power flow calculation has been derived from the power deviation equations and the DC network equations. Four constrol schemes are specially designed for different combinations of pahse angle delta and modulations of VSC controls. For each of the control schemes, the modified model for load flow calculation is proposed in this paper. Based on the analysis of load flow solution process, a flow chart is designed for the load flow calculation. This paper also presents a simultaneous method and gives the mathematical model for the method, which takes into account the control scheme. Finally, a 5-bus system with three VSCs is used as an example. Simulations are carried out based on the sequential method and simultaneous method with consideration of different control schemes. The simulation results show that the proposed power flow model and algorithm are correct and effective. The presented study lays a good foundation for further investigation of power flow control in AC-DC power systems with the VSCs.