Research On Main Circuit And Control System Of P-MTDC Transmission System

The reverse distribution of China’s energy and load centers determines the need to use HVDC transmission for long-distance,large-capacity transmission of electrical energy.Compared with DC transmission at both ends,multi-terminal DC transmission has become the trend of DC transmission development due to its multiple sending ends and multiple receiving ends,good power grid structure,high power supply reliability and suitable for renewable energy network access.At present,compared with MMC-MTDC transmission and hybrid MTDC transmission,LCC-MTDC has obvious advantages in ultra-long-distance and ultra-large-capacity transmission.There are few studies on the design of the main circuit of LCC-MTDC transmission.To this end,this paper has done the following work:(1)The development direction and operating conditions of LCC-MTDC transmission are analyzed.The possible topological structure and characteristics of DC grid development are introduced.It is pointed out that MTDC transmission is one of the stages in the development of DC grid.In addition,the operating conditions of MTDC transmission are discussed,including the operating mode,control mode and the constraints of the main circuit parameters.(2)The design method of the main circuit of MTDC transmission is analyzed,including the design of the main circuit parameters and the calculation of the steady-state parameters of the main circuit.Through the design of the basic operating conditions of the system,the design of the main parameters of the converter transformer and the smoothing reactor,the design method of the main circuit parameters is analyzed.The design of the tap of the converter transformer is introduced in detail.By focusing on the adjustment method of the parameters of each converter station and the calculation of DC power flow,the calculation method of the steady-state parameters of the main circuit is introduced.Finally,taking the 3-terminal DC system as an example,the converter transformer tap design and the calculation of the main circuit steady-state parameters are carried out.(3)The calculation method of switching point of reactive power compensation of MTDC transmission system is analyzed.The switching strategy,calculation method of capacity and switching point of reactive power compensation are introduced.And the reactive power compensation switching point calculation is summarized.The 3-terminal DC system is taken as an example to calculate the switching point.(4)Briefly analyzed the MTDC transmission control system from three aspects:control system configuration,system control mode,fault analysis and protection control.Taking the current margin as an example,the hierarchical structure in the basic configuration is analyzed,and the control mode configuration is introduced.The system control method is mainly introduced with master-slave control and droop control.For fault analysis and protection,the handling of converter station faults,DC line faults,and T-zone faults is analyzed by fault area.