The Research On Hierarchical Distributed Control Strategy And Protection Adaptability Of VSC-MTDC Systems

Voltage source converter(VSC)based DC technology,due to its advantages of high controllability,easy network formation,and the ability to power passive networks,has broad application prospects in renewable/distributed energy integration,cross-regional power transmission,and future urban power supply.In recent years,with the continuous development of power electronics technology,the cost of voltage source converters has been reduced,and the availability has been enhanced.Multiterminal direct current(MTDC)systems based on voltage source converters(VSC)have become an application hot spot in engineering and hot topic in academic research.Control and protection are the two major research priorities in VSC-MTDC systems.In terms of control,voltage regulation and power sharing are two conflicting control objectives in MTDC systems,and a tradeoff between the two must be achieved.And in terms of protection,the principle of the line protection in the MTDC system is mostly borrowed from the traditional DC and the two-terminal DC systems besed on VSC technology,thus and its adaptability needs to be studied.In this paper,the VSC-MTDC system is the research object,and the control and protection are studied separately.The main contributions are as follows:1)In terms of control,this paper proposes a distributed hierarchical voltage control method for VSC-MTDC systems based on average consensus algorithm,and the communication parameter design method for the fastest convergence of distributed controllers.The proposed control method combines the idea of secondary integral voltage control and regulator synchronization problem.It realizes adjustable trade-off between two conflicting control objectives,voltage regulation and power distribution.The communication parameter design method is obtained by analyzing the communication parameters design of the fastest convergence speed of the average consensus algorithm under discrete-time and considering the response of the electrical part of the system.A detailed electromagnetic transient simulation model was bulit in PSCAD to verify the control performance and dynamic performance boost of the communication parameter design method.2)In terms of control,by constructing the small-signal model of the VSC-MTDC system with the proposed controller,the small-signal stability and the dynamic performance of the system is analyzed.The dominant oscillation modes of the system are obtained based on the small-signal model.And the influence of each controller on each dominant oscillation mode is analyzed to obtain the influencing factors of the system dynamic characteristics.Small signal analysis was also carried out to study the influence of the control parameters on the system small-signal stability.And the range of the local voltage controller control parameter is obtained through small signal analysis.3)In terms of protection,the adaptability of single-ended traveling wave protection in VSC-MTDC system is analyzed.According to the topological characteristic and component configuration of VSC-MTDC system,combined with its fault characteristic and traveling wave protection theory,the influence of line reactor on the adaptability of traveling wave protection is analyzed.The conclusion is verified by simulation.And this work is seen as the future foundation of the VSC-MTDC system line protection.