Research On The Key Technology Of Multi-level Reinjection Current Source Converter

Large capacity power electronic equipments are the core units of the flexible AC transmission(FACTS)system of smart grid,the high voltage DC transmission(HVDC)system,renewable energy power generation,and so on.It also is the hotpot topic in the research field of power electronics for time being.In order to effectively solve the contradiction between power processing capacity and switching frequency of power electronic devices,multi-level converters emerge as the times require.This present paper puts its predominant attention on the research around high power multi-level reinjection current source converter(MLR-CSC),which can accomplish zero current switching(ZSC)in the application of main bridge switches control so that applying the half control switching device-SCR as the power device of the main bridge become practical,with original meaning.Aiming at the key technologies of MLR-CSC,such as working characteristics,pulse width modulation,closed loop control and engineering application,detailed studies have been conducted from the following aspects:In this paper,the general formula of the AC currents,DC output voltages and the equivalent capacity of the m-level reinjection CSC topology are derived.In view of the shortcomings of the larger capacity and volume of the isolated transformer,a MLR-CSC topology based on autotransformer is proposed,which is compared with the isolated transformer system from the perspective of system equivalent capacity.The advantage of the MLR-CSC based on the autotransformer in reducing the equivalent capacity of the system is proved,and the application of the two topologies is discussed.The general principle of the reinjection circuit pulse sequence is analyzed,and the method that reinjection circuit pulse is composed of the basic pulse units is proposed.Study on the effect of different reinjection circuit pulse schemes on the current quality and system reliability and propose reinjection circuit pulse scheme optimization targets.Aiming at these optimization targets,the reinjection circuit pulse schemes are classified,discussed and analyzed,and optimization design and improvement method of reinjection circuit pulse is given,which provides a basis for the selection of reinjection circuit pulse schemes in practical engineering.The working principle of the multilevel reinjection circuit is analyzed.The research shows that the injection circuit can effectively suppress the influence of the output voltage imbalance of the two main converters on the current quality of the system under the specific reinjection circuit pulses.The four quadrant working principle of MLR-CSC is analyzed and the closed-loop control strategy of the DC side current is proposed.Studies show that the system can not control the active power and reactive power dependently.Through connecting two groups of converter in parallel on the AC side and in series on the DC side,another set of control variables is added and the dual-bridge serial congfiguration is formed.The power decoupling control principle of dual-bridge serial system is studied,and a predictive direct power control strategy is proposed.Through connecting two groups of converter in parallel on the AC and DC side,dual-bridge parallel configuration is formed.The research shows that the dual-bridge parallel structure is more suitable for high current applications operating under unit power factor and a current sharing control method is proposed.Simulation results verify the effectiveness of the control strategy.A small capacity prototype experimental platform of dual-bridge MLR-CSC is built.The theoretical analysis and simulation results of single bridge,dual-bridge serial and dual-bridge parallel system are verified respectively.The relationship between the value of reinjection inductance and ripple coefficient of reinjection branch currents is analyzed,and a reinjection inductance parameter design method is proposed,so as to ensure the normal work of the system under the premise of reducing the system volume and cost as far as possible.The traditional DC inductor is replaced by the equilibrium reactor,which further reduce the size and cost of the flat wa've element.Finally,aiming at the practical application of MLR-CSC,the research work is carried out on the back-to-back HVDC transmission system and the six-phase permanent magnet synchronous motor drive system.The technology of multilevel reinjection type current source converter in specific applications is studied,including topology combination,working characteristics and system control strategy,which provides reference for practical application of MLR-CSC.