Research On Start-up And Voltage Balancing Control Strategies Of Hybrid MMC Based HVDC System

Modular multilevel converter based high voltage direct current transmission system has got extensive application.The hybrid MMC topology contains cascaded full-bridge and half-bridge modules,which not only has DC fault blocking capability,high operating flexibility but also costs less than full-bridge submodule based MMC,and has become a research hotspot.When this kind of hybrid MMC is used in high voltage DC systems,there are still two major problems.During the AC-side start-up process,the precharge voltages of the two sub-modules in the hybrid MMC are unbalanced and low,resulting in serious over-modulation and inrush current,which threatens the safe and stable operation of the system.During the steady-state operation,the problems of high switching frequency and large switching loss of the hybrid MMC threatens the safe operation of equipments and deteriorate the operating efficiency of the system.In this paper,the above two problems are studied,and an AC-side start-up control strategy and a steady-state voltage balancing control strategy are respectively proposed to achieve the safe start-up and low switching frequency operation of the hybrid MMC based high voltage DC system.The main research contents of the paper are as follows:Based on the topology of the hybrid MMC,the precharge voltage characteristics and the equivalent circuits of half-bridge and full-bridge submodules are analyzed respectively.A method to calculate the precharge voltages of the two kinds of sub-modules is presented.Under current ac-side start-up control,low and unbalanced pre-charge voltage of sub-modules would result in impulse current and over modulation.Then,based on the capability of producing negative level of full-bridge sub-modules,an ac-side start-up control strategy is proposed in this paper,to suppress the inrush current and over modulation,thereby achieving the safe start-up of the hybrid MMC.The theory analysis and the proposed strategy are demonstrated by the simulation.The current voltage balancing control strategy is studied,and it is pointed out that in the process of achieving the capacitors energy balance,the traditional voltage balancing method brings a large amount of invalid switching losses Based on the operation principle of MMC,the mathematical relationship between module switching and transient increment of capacitor voltage is derived,and a prediction method for the convergence of capacitor voltage sequences is proposed.Then,an improved voltage balancing control strategy is presented.According to the prediction result of the sub-module voltage convergence of each arm,the adjusting switching number can be modified dynamically to eliminate unnecessary adjusting switching.Compared with the current voltage balancing strategy,the switching frequency is effectively reduced without degrading the voltage balancing effect.The proposed voltage balancing control strategy is validated by simulation.In summary,the problems in the application of hybrid MMC in high voltage direct current transmission systems are studied in this paper,and the corresponding control strategies are presented to achieve smooth start-up and efficient operation of hybrid MMC based high voltage DC system.