Research On The Control Strategy Of The Circulation Current Suppression And The Voltage Balancing Of The Modular Multilevel Converter

HVDC have the advantages of flexible operation,environment friendly,economical and reliable,so the become an important development direction and a research hotspot in the field of electrical engineering in our country.The MMC-HVDC has many preponderances,such as low switching losses,small harmonic content,modularization and easy to expand,etc.So the widely applied in the practical engineering.In this paper,we mainly study on the key problem of steady system control strategy of MMC-HVDC system,circulation current suppression strategy and the sub-module voltages balance control strategy,etc.First of all,this paper analyzes the circuit structure and its working principle of sub-modules and MMC-HVDC system.And the mathematical model of MMC-HVDC system is deduced.The sending end converter and the receiving end converter adopts the double closed-loop control structure,outer voltage or power loop with PI control and the optimization method of PI parameter design is given.The current inner loop using passive control,and the realization structure and parameter design of passive controller are given.Model predictive control as a new multi-objective optimization control strategy.Gradually been applied in the power electronic converter.As with contrast of passive control,the model predictive control is introduced in the current loop,and it to be improved by combined,with the sub-module voltages sorting.The first step of the improved model predictive control method can realize the inner loop current tracking control.Then second step can achieved circulation current suppression.MMC-HVDC simulation model system based on two ways of passive control and MPC control is established.The simulation results show that MPC control has the advantages such as good tracking effect of inner loop current,small AC side current harmonics and so on compared to passive control.In view of the problem of three-phase circulation current,using the second step of the improve SMPC,we can get a circulation current suppression method based on MPC control.Good circulation current suppression effect is verified in simulation.A control method for calculating the modulated wave directly by using the AC side current and the circulation current is proposed.Simulation results show that the proposed method can achieve both the inner loop current tracking control and the circulation current effectively suppressed at the same time.Using the principle of circulation current suppression based on model predictive control,and doing reducution of switch frequency pointedly in farther.Then the switching frequency of the sub-modules is reduced effectively.In order to reduce the computation of traditional sub-module voltages sorting,the principle of grouping-sorting is illustrated.Then the improved sort trigger method is put forward for fully grouping and the simulation is completed finally.Established a set of 20 kVA MMC-HVDC experiment device.Completed the basic function of debugging,created the corresponding experimental procedure.Start with single phase inverter experimental proof that the correctness of the software program design and the good running characteristic of sub-module.Then using passive control to achieve the function that MMC single-ended three phase rectifier's dc voltage control,reactive power control,dynamic voltage balancing control.