Research On The Power Coordinated Control Of VSC-MTDC System Connected With Wind Farms

With the continuous increasing permeability of green renewable energy in modern power system, many advantages have appeared in modern grid, along with challenges to its safe and stable operation. Large-scale renewable energy resources in China, especially wind power, are mostly distributed in inland northwest area, but main power consumption groups located in southeast and east coastal area. Thus the development strategy of “transmission of the electricity from the western areas to East China, the superiority of the southern power to the northern power, and nationwide connectivity” is objective requirement to the present energy distribution in China and the large-capacity and the Multi-Terminal Direct Current based on VSC transmission will be the main development trend. This paper mainly studies VSC-MTDC system with wind farms and its power coordinated control.This paper discusses the mathematical model of VSC-MTDC and doubly-fed induction generator. On this basis, voltage droop control of VSC from active system side and constant alternating voltage control of VSC from the passive systems side is established. A complex VSC-MTDC system with multi-machine AC system and wind farms is proposed. Take four-terminal VSC-MTDC system with three machine nine nodes system as an example in PSCAD/EMTDC, this paper discusses the dynamic characteristic of the VSC-MTDC system. This is used to provide the basis for designing the coordinated power control of each terminal converter and wind turbines.Merits and demerits of the traditional control strategies are discussed, including a single master station, multiple master stations and compound master station control. Improved compound master station control strategy for VSC-MTDC system is proposed. The voltage compound master converter takes voltage droop control with variable droop factor. When the system active power fluctuation is less than the allowed values, the voltage compound master converter make up all the power shortage while keeping good voltage. When the system active power fluctuation is more than the allowed values, this converter will focus on the voltage control and the other power compound master converter compensate the active power shortage. If the voltage compound master converter is failed, the control of power compound master converter will convert to the DC voltage control mode. These converters constitute the compound master converters, and share the control task of a traditional master station. The proposed control strategy can improve the DC voltage’s quality of the whole system and keep the VSC-MTDC system stable.This paper proposed a power coordinated control strategy of VSC-MTDC involved in the regulation of wind turbines. Auxiliary frequency control of converter from the wind side and the wind turbines control are designed. When the wind farm is incorporated in the VSC-MTDC system, the system is divided into three operating state. The state I is the state when the power compound station can compensate all the power fluctuation. The state II is the state when the power and voltage compound station compensate the power fluctuation together. The state III is the state when the power fluctuation is beyond the power control ability of the power and voltage compound station and the wind turbines involve in the regulation of active power. Finally, the coordinate control strategy is designed unde the three states. The PSCAD/EMTDC simulation results show the effectiveness of the proposed coordination control strategy.