Multi-pulse Rectifier Based Solid State Transformer

The traditional power transformer plays a crucial role in the power network such as voltage level conversion and energy transfer.However,the traditional power-frequency transformer,which is a passive component,can not make the active response control to grid voltage or load changes.In the future smart grid,solid-state transformer(SST),which has the characteristics of flexible current and voltage control,is more suitable for the development of the power grid compared to power-frequency transformer.A novel SST topology is proposed in this dissertation,the harmonic current is eliminated by multi-pulse rectifier,so this topology is called multi pulse rectifier based solid state transformer(MPR-SST).In this dissertation,the major innovative technologies of MPR-SST are studied in detail.1.Aiming at the problem that the traditional SST topology control method is complex and has a large number of controlled power devices,the proposed MPR-SST topology is based on multi-pulse rectifier technology and has the following two advantages.First,from a hardware perspective,the input stage of MPR-SST and the secondary side rectifier part are purely composed of diodes.Compared with the other three-stage input series and output parallel SST,the converter circuits in MPR-SST the number of controllable power devices,as well as drive circuits and sensor signal detection circuits.In addition,high cost and large capacity capacitor banks are avoided in the primary side DC bus of MPR-SST.Compared with SST with the same capacity.MPR-SST effectively reduces the cost of hardware circuit.Second,from a software perspective,the control algorithm of the primary side cascaded converter is simplified in MPR-SST.The grid current harmonics of MPR-SST are eliminated by the multi-pulse phase shifting transformer and the rectifier circuits.On the other hand,the automatic voltage balancing of the multiple DC buses in the primary side converters is realized by the windings with public iron core of the transformer,and the effect of voltage balancing is enhanced by the characteristic that DC buses on the primary side have no capacitor.Compared with other SST circuits,the control system isn't needed in MPR-SST for the control of the primary grid current and the DC buses voltage balancing.In addition,the characteristic is convenient to realize the flexible grid connected control of MPR-SST with no charging resistance and with no bypass switch.2.In the design process of MPR-SST,an adverse effect is given on the elimination of harmonic current in the multi pulse rectifier circuit by the high-frequency winding.Aiming at this problem,the currents of the secondary side windings are changed by parallel converter system(PCS),so the harmonic components of the secondary winding flux can be counteracted by each other,ensuring the effect of eliminating harmonic current.3.The number of converter in PCS on the secondary side increased with the number of rectifier pulses in MPR-SST.Therefore,this dissertation presents a distributed master slave control method based on CAN bus.The number of parallel converters is not limited by the computing resources.In the current sharing control method,the instantaneous value of the current command signal is transmitted by CAN bus,and the reliability of PCS is improved.4.According to the MPR-SST based on the 24 pulse rectifier circuit,a test prototype is designed.In the prototype,there are 6 sets of converter units on the primary side in each phase.and the voltage instantaneous values of DC buses are equivalent in the experiment.The stable op.eration of MPR-SST is realized under the condition of 6 kV.The harmonic current of the prototype is eliminated effectively by the high frequency 24 pulse rectifier circuit.In this dissertation,a novel SST topology named as MPR-SST,which is based on multi pulse rectifier technology.According to the "one side converter,high-frequency transformer and two side converter" sequence,the various parts and control method of MPR-SST topology are discussed.The grid-connected characteristics of MPR-SST topology are discussed compared with the traditional SST topology,and its application value is proposed in the future distribution network.The working principle of the MPR-SST.its grid-connected characteristics and its voltage balancing of the bus are verified by the test results of the prototype.