Research On Power Loss Analysis Of HVDC System Based On Modular Multi-level Converter And DC-DC Converter

Due to the merits of modular design,low switching frequency and low harmonic distortion,the modular multilevel converter(MMC)has been widely used in academy and industrial field.For the MMC-HVDC project,the transmission system of power loss analysis is of great significance.Since a MMC contains a large number of sub-modules and the power loss characteristic couples with the control strategies,it brings difficulty and challenge to evaluate the power loss of MMC-HVDC system.Based on the research of MMC topology,modulation and control methods,this dissertation puts forward a power loss calculation method using fast simulation model.The power loss calculation of key equipments in AC field and DC field is analyzed.And the generic loss calculation method of MMC-HVDC transmission system is disclosed.Firstly,the topology and power loss distribution of MMC-HVDC system is sumarized.The design scheme and power loss calculation method of interface trancformer is introduced.An accurate evaluation method of copper loss using the winding simulation is proposed.Regarding the arm reactance,the structure and design method of is presented.Refering to the winding structure,the power loss calculation method of the air core reactance is deduced.Aiming at minimizing the power loss,a method of demensioning the reactor size is proposed based on the optimal iteration function.Secondly,the limitations of using the pure analytical method to calculate the power loss of the converter valve are described.It is found that the discretized pulse signal is difficult to be analyzed using the nearest level modulation and considering the sub-module capacitance voltage constraint.A comprehensive calculation method based on the off-line simulation of fast transient MMC model is proposed.Using the basic parameters and the simulation results,an accurately evaluation of the valve power loss and operating temperature of each device in the converter can be obtained.Considering the engineering requirement,a fast calculation method of valve loss is presented.It can make an initial assessment of the entire system at the design stage.In addition,for some newly proposed topologies of sub-modules,the calculation method of valve power loss is also given.Finally,power loss calculation of the multi-terminal MMC-HVDC,unidirectional DC/DC converter with high stepping ratio and DC-DC autotransformer is presented.The power loss characteristics of each system are studied using the method proposed in this dissertation.The results provide the guidance for power loss calculation in pratical MMC-HVDC projects.