The Control Strategy And Line Protection Scheme For Cascaded Multi-terminal UHVDC System

The construction of UHVDC transmission system is an effective way to meet the requirement of long-distance large capacity power transmission.When the application scenario can be described as one energy source providing electrical power to multiple load centers,this HVDC system can be constructed with cascaded valves from each station in different geographical locations.And the proper control strategy and line protection scheme are the premise of the system’s stable operation.Nevertheless the original control strategy and the line protection scheme are not fully adaptable to the novel cascaded multi-terminal UHVDC system due to the change of transmission structure.Therefore,starting from the adaptability of the original DC system control and protection scheme,this thesis improves,optimizes the control and protection strategy,besides that a suitable control and protection strategy is proposed for the multi-terminal UHVDC system with cascaded receiving ends.First,the three topology structure,their strength and weakness in cascaded multi-terminal UHVDC system(CMT-UHVDC)are introduced,and the cascaded connection mode is clarified as the research subject.Based on the theoretical analysis of traditional line commutated technology,the mathematical model of the cascaded multi-terminal UHVDC system is demonstrated.The research,analysis and theoretical verification of CMT-UHVDC system all have to be based on the accurate electromagnetic transient model.And in this thesis,the simulation model is established on PSCAD/EMTDC simulation platform using the actual parameters from Yazhong-Huadong project.Thus the steady and transient operation characteristics under different faults of the CMT-UHVDC system are analyzed.Due to the different AC systems receiving ends connected to,the controllers in the pole control level need to be lowered to the valve control level so that AC systems can be responded independently.At the same time,the input and output variables of voltage balance controller need to be transmitted between receiving terminals with real-time communication,which can easily lead to the time action delay for control and protection,even worse the mis-action of valve protection.To address the difficulties mentioned above,a master-slave control is proposed in this thesis to maintain the balanced operation of receiving valves.The master control valve retains the original controller configuration,while the slave valve calculates and applies the firing angle of master valve through the common variable,DC current,to follow the current operation state of master valve.In order to achieve a better following performance of slave valve,the function of VDCOL has been optimized to make sure the input and the output of it have a unique corresponding value.Thus,the control strategy has been realized in electromagnetic transient simulation model,several wording conditions have been demonstrated including various types of faults and conditions,such as the sending and receiving AC system faults,DC line faults,voltage surge condition of AC system,measurement deviation condition,etc.It is confirmed that the master-slave control strategy can maintain the voltage balance of valve group without divergence in the case of system transient and measurement deviation.Based on the improved control strategy,the influence factors of the line fault are analyzed with taken inherited characteristics of cascaded connection into consideration,in the meantime the adaptability of conventional protection scheme is discussed.It has been drawn on the conclusion that each line in the CMT-UHVDC system needs to be configured with an independent line protection.Based on this conclusion,the traveling wave protection of two segments of the transmission line in the system is calculated and set.And considering the different roles of each transmission line playing in the power transmission,a reasonable protection action scheme is set for the temporary and permanent faults of different lines.The results of the protection configuration scheme and traveling wave protection setting are verified through the established electromagnetic transient model.CMT-UHVDC system hasn’t been applied in the current UHVDC transmission projects.But the research on the control and protection scheme of this structure is prospective and provides theoretical basis for practical application.