A Hybrid Arm MMC Research And Physical Realization

With the development of Power Electronics technology,Voltage Source Converter based High Voltage Direct Current Transmission(VSC-HVDC)has been developed rapidly in the world.In the field of long distance and large capacity DC transmission,taking into account the economic factors,the overhead line transmission mode should be adopted.Thus the method of DC short-circuit protection becomes quite important.There are three ways to isolate DC short-circuit faults: using AC circuit breaker,DC circuit breaker or DC-fault-clamped Converter.In the three methods,the converter which can insulate DC fault is very suitable for two or multi terminal HVDC system because of its advantages such as fast response,less investment,no additional equipment and so on.In a variety of new topologies,the hybrid arm Modular Multilevel Converter(MMC)use a combination of two most mature sub-modules,which are half bridge and full bridge.It makes it reduce the loss,and also ensure the reliability of DC fault clamping.With the analysis of working principle of general MMC topology,half bridge,and full bridge sub-module topology,the Hybrid Arm MMC topology has been derived.According to the mathematical model of its equivalent circuit,the control strategy,for example,the decoupling control strategy of Pole Control and Protection system,the Modulation and Equalization strategy of Valve Based Control system,has been analyzed.The minimum number of full bridge for DC fault clamping has also been calculated.A two terminal Hybrid Arm MMC model has been built in PSCAD/EMTDC to verify its rationality of control strategy under steady-state and transient conditions.In order to validate the performance of Hybrid Arm MMC better,a basic low voltage experimental prototype has been designed and built.Firstly,according to hierarchical control principle of flexible direct current transmission system,the control system could be divided into four parts: the data acquisition system,the pole control and protection system,the valve based control system and sub-module control system.Based on the functions of each layer,a proper core control chip and its peripheral auxiliary circuits has been selected to design and make the controller board.Then,the program flow chart has been drawn according to the mathematical model of each controller;The algorithm has been programmed by hardware description language,and each subprogram validated by simulation tool in all kinds of operating conditions.Afterwards,the sequential coordination of controller of each layer has been analyzed.The requirement for transmission rate has been putting forward,estimated by the main parameter named calculation depth.At last,the main circuit parameters and main equipment type has been provided,and a experimental prototype built.To test the behavior of Hybrid Arm MMC,the experiments in steady state and transient state has been made.In steady state,the DC voltage,the Voltage of submodule capacity,and the converter valve output voltage performance well.In power step state,the controller response rapidly.In DC bipolar short-circuit test,the Hybrid Arm MMC can block the fault current path.All the test laid the foundation of engineering application for Hybrid Arm MMC.