Modular Multilevel Converter Based Bus Bar Controller In Flexible Substation

With the development of power electronics technology,modular Modular Multilevel Convert(MMC)is applied more and more in HVDC system because of its modular structure.In practical engineering applications,the development of primary equipment for flexible substation has great challenges.One of the key objectives of a flexible substation is to improve a device into a more flexible,versatile,and centralized high voltage intelligent device.This requires a large number of power electronic technologies to be used in flexible substations,so as to simplify the type of substation equipment and reduce the number of equipment.However,the traditional bus circuit breaker is only a conventional mechanical switch,and its function is too single to meet the development needs of flexible substation.Therefore,in order to adapt to the development of flexible substation,this paper proposes a MMC Based Bus Bar Controller.Compared with the traditional bus coupler,the controller has stronger regulating ability,faster response speed,lower action cost and less fault influence.It also has the functions of flexible control of power flow,instantaneous compensation of reactive power,precise control of voltage,balance of three-phase load and suppression of harmonics.First of all,based on the topology of the flexible bus controller,the functional characteristics and working principle are analyzed.The power analysis of the sub module of MMC and the single-phase analog circuit is carried out.The influence of the AC and DC in the bridge arm current and the circulation components on the power is analyzed,which provides a theoretical basis for the design of the follow-up control strategy;then,based on the current technology level capacitor voltage balancing control method,a comprehensive control strategy based on the direct current control of bridge arm is proposed and applied to the MMC based bus controller;Full use of the two frequency doubling current is used for capacitor voltage stability of bus circuit breaker in zero power transmission state;for the passive power supply,the inverter side fixed AC voltage control strategy is added to provide a stable AC voltage for the passive side AC bus.In order to deal with the asymmetry of the three-phase current in the AC side due to the loss of the sub module in the case of fewer sub modules,a comprehensive control strategy combining single loop and central point voltage is also presented in this paper,and the block diagram of control strategy is also given.The two control strategies are compared and analyzed in the PSCAD simulation software.It is proved that this control strategy can ensure the symmetry of the three-phase current of the AC side under the asymmetrical current condition of the bridge arm.