Research Of The Control Strategy For VSC-MTDC System

Since from the 1990 s,the VSC-HVDC has been more and more popular which is based on high power full-controlled semi-conductor valves,because it can transmit power to passive system,has more stable operation characteristics and so on.But the traditional VSC-HVDC has defects in grid connection between different cities and interconnection between new energy transmission system and traditional grid lead to the demand for more flexible and stable transmission system is increasing.Therefore,Multi-terminal HVDC System Based On Voltage Source Converter has become a focus in the study of many scientific research institutes.The VSC-MTDC system not only has the characteristic of VSC-HVDC system,but also could adjust the power flow between different convertor stations and solve the problem of multiple power supply or multiple power consumption.Compared with the two-terminal VSC-HVDC system,the coordination control of the VSC-MTDC system is more complex and flexible.Therefore it's necessary to do research on the control strategy of the VSC-MTDC system.At first,this paper gives the basic principle of hierarchical control about the VSC-HVDC system,and introduces the control objectives of each part.Because of using vector control in the converter station level control,this paper set up the mathematical model of three-phase two level VSC based on d-q synchronous rotating coordinate system by increasing the decoupling,it can realize decoupling to realize the target of controlling the active power and reactive power independently which makes the controller of the system more simple and intuitive.Then it gives the relevant design of inner ring current controller and outer voltage controller and the calculation method of PI parameters in the control links.In the view of the traditional of master-slave control strategy of VSC-MTDC system,this article put forward a constant-power control method through to inprove the traditional master-slave control method.This method will put the emphasis on the stability of the DC voltage,which can keep the power of the main VSC constant,when the power of the system is fluctuating.At the same time,it could make use of the other VSCs to have a share of the power fluctuation,which can make the main VSC to avoid changing the running state frequently,and also could improve the quality of the system.At last,this article analysis the simulation model which is set up in the simulation software PSCAD.The results shows that the new method has a stronger ability to deal with the fluctuating power,by the other VSCs to share the fluctuating power;When the communication system fails,the control method of the system will turn from the constant-power control to the master-slaver control,and it's visible that the system can still run in master-slave control in the worst cases.