Research On Regulation Technology For Safe And Efficient Operation Of High-frequency-link AC-DC Matrix Converter

With the rapid development of new energy technology,new energy vehicles represented by electric vehicles show good application prospects,but the modern power grid would have higher requirements for the performance of power conversion equipment in the new energy system.High-frequency-link AC-DC matrix converter is a novel AC-DC converter evolved from the conventional matrix converter.Since it does not require an energy storage element with large capacity and volume in the circuit,it has the characteristics of compact structure,high power density and electrical isolation.It is very suitable for AC-DC power conversion of new energy.Hence,this dissertation selects the high-frequency-link AC-DC matrix converter as the research subject,and makes an in-depth study on its key regulation technologies around the converter topology,modulation strategy,modeling analysis,control strategy,system protection,etc.It is expected to realize the safe and efficient operation of high-frequency-link AC-DC matrix converter,and can provide the technical support for the development and application of matrix converter and the promotion of new energy technology.Firstly,the current space vector modulation strategy for high-frequency-link AC-DC matrix converter is introduced,and the volt-second balance scheme of transformer is analyzed in detail.It is pointed out that multiple redundant vectors in symmetrical operation mode are the main reason for the higher switching actions of conventional schemes.In order to reduce redundant vectors and switching losses,a general volt-second balance model is established.On this basis,a modulation strategy with quasi-symmetrical switching period is proposed,which can effectively reduce the number of switching actions.At the same time,a two-period ampere-second balance scheme is designed to realize the ampere-second self-balance of the transformer.Then the soft switching characteristics of the converter with different strategies in continuous mode and discontinuous mode are analyzed in detail,and the operation range,duty cycle loss,narrow pulse and high-frequency current of the converter are further compared and analyzed.Theoretical analysis and experimental results show that the proposed strategy can not only reduce the switching losses,but also have lower conduction losses and input current harmonics,and it can obtain better system performance in normal operation.Aiming at the problems that the conventional modulation strategy has low control freedom and can not make the high-frequency-link AC-DC matrix converter fault-tolerant operation after switch failure,a general modulation model with more control freedom is established by adding additional active vectors to synthesize the reference current vector.Based on the general modulation model,type-I,type-II and type-III modulation strategies with asymmetric vector distribution are designed.At the same time,the vector duration formulas of the three strategies in the switching period are deduced and calculated in detail,and the operation range,dc output voltage,input current and switching actions of the converter with different strategies are compared and analyzed.Then by selectively combining the proposed strategies with the conventional strategy,a fault-tolerant scheme is proposed to make the high-frequency-link AC-DC matrix converter operate safely after switch failure,and the fault-tolerant performance of the converter is compared and analyzed.Theoretical analysis and experimental results show that the proposed strategy has good system performance in normal operation,and compared to the conventional phase-loss fault-tolerance strategy,the proposed strategy during fault-tolerant operation also has superior performance in output voltage ripple,current stress,current harmonic,maximum output power,etc.In order to further improve the system performance of the converter after switch failure,a novel three-phase four-wire high-frequency-link AC-DC matrix converter with neutral structure is constructed by connecting the neutral line to the circuit through the transformer.Then,a fault-tolerant topology decoupling scheme of the novel converter is proposed.By decomposing it into two neutral matrix converters,the switch fault-tolerant strategy of high-frequency-link AC-DC matrix converter is then designed.The operation mode of the fault-tolerant converter after topology decomposition is analyzed in detail,and a zero vector switching state with discontinuous mode is designed.At the same time,based on the general modulation model in three-phase four-wire system,the modulation strategy of fault-tolerant converter is deduced and evolved,and the performance of the converter during fault-tolerant operation is compared.Theoretical analysis and experimental results show that the proposed fault-tolerant strategy with topology reconstruction can enable the high-frequency-link AC-DC matrix converter to continue fault-tolerant operation after switching failure with less hardware cost.And also,it could obtain constant output voltage and grid side sinusoidal input current in phase with the input voltage,which has better fault-tolerant performance.In addition,the operation strategy of high-frequency-link AC-DC matrix converter with three-phase input imbalance is studied.First,by changing the solution process of modulation matrix in conventional modulation strategy,a novel modulation strategy with unbalanced suppression capability is derived and designed.In the case of input voltage disturbance or power factor angle variation,a constant dc voltage output can be obtained.At the same time,according to the unbalanced input current model,a method to improve the quality of the input current waveform is analyzed and designed.By adding an additional unbalance compensation angle,the input current harmonic can be significantly reduced without affecting the output voltage.In addition,the system dq model and small signal model based on the novel modulation strategy are established.The dynamic and static characteristics of the converter are analyzed in detail and compared with the conventional modulation strategy.With the established system model,the sliding mode control strategy is then designed for the output voltage and grid side input power factor of the converter,and the sliding mode chattering is studied in depth.Based on the above novel modulation strategy and sliding mode control strategy,the unbalanced operation strategy of high-frequency-link AC-DC matrix converter is composed.Theoretical analysis and experimental results show that the proposed unbalanced operation strategy is completely robust to external disturbances,and can obtain better system performance when the input voltage is balanced or unbalanced.