Research On Multifunctional Converter System Control And Protection

Wind power,solar power,fuel cell are typical renewable energy,which releases low carbon emission and becomes the alternative of fossil fuels.Power electronics converters are used as the key interface circuits between renewable energy and loads,the power conversion efficiency and control characteristic are the main evaluation factors.The multifunctional converter utilizes one hardware circuit to realize multiple functions according to system demands,thus it can reduce converter cost,volumn and maintannce fee.In order to ensure high efficiency and stability operation of Microgrid,this paper focuses on the researches of control,stability and protection of multifunctional converter in Microgrid,the main research works include 1)The unified control structure of a multifunctional bidirectional DC/AC converter;2)The impedance stability criterion of a parallel multifunctional converter system;3)The anti-islanding dection method for a multifunctional grid-connected converterThis paper proposes a control structure of the multifunctional bidirectional DC/AC converter.The multifunctional bidirectional converter is the interface circuit between AC components(AC bus,AC sources and AC loads)and DC components(DC bus,DC source and DC loads).The bidirectional DC/AC converter can operate at islanding condition or grid-connected condition.This paper designs an unfied control strategy in the positive and negative sequence rotating coordinate system,which allows the multifunctional bidirectional DC/AC converter to work in birectional power mode,DC side voltage mode and AC side voltage mode with unbalanced loads.Finally,a 100kW prototype of multifuNctional bidirectional T-type three-level DC/AC converter is built,a volume optimal heatsink of the prototype is designed to improve converter power density and a low stray inductor busbar is designed.The experimental results show the developed multifunctional bidirectional T-type three-level DC/AC can meet the design demands.This paper proposes two stability criteria for a parallel multifunctional converter system:the first is based on impedance numerators and denominators,and second is based on paralleled impedance estimation.Firstly,the unified equivalent impedance models of the multifunctional Buck/Boost converter and multifunctional T-type DC/AC converter are derived.Secondly,the criteria are derived based on a dc multifunctional converters power system with a number of potential control modes rather than a system with specific control mode.Instead of checking all the stability related transfer functions,their identical denominator DA,containing impedances’numerators and denominators,is proposed as the stability criterion.Criterion one checks the right-half-plane zeros number(RHPZ)of DA,the system is stable once RHPZ(DA))equals zero.Criterion two extends criterion one to converter parameters unknown application,the system stability can be obtained by paralleled impedance measurement.Comparison between the existing criteria and the proposed criterion is given,the proposed impedance criteria have advantages of handling parallel multifunctional converter sysetem.Experimental results of a multiple multifunctional Buck/Boost converter system verify the effectiveness of the proposed criteria.This paper proposes an intermittent reactive power variation anti-islanding method based on the resonance frequency of loads.The proposed method can detect islanding phenomenon in time and eliminate the non-detected zone(NDZ).The minimum reactive power variation sequence is derived based on the relation between Point of Common Coupling(PCC)frequency and reactive power variation value.The reactive power variation sequence is set as the power loop reference of inverter.When grid is disconnected,the PCC frequency will deviate to the thresholds and trigger the protection with the proposed method.Then the application of the proposed method in unbalanced loads is discussed.Compared with the traditional bilateral reactive power variation method,the proposed method can reduce the reactive power variation period and value,thus the grid-connected power quality is improved.A three phase grid-connected inverter is built to test the proposed method.The experimental results show the proposed method can detect islanding with groups of balanced and unbalanced RLC loads.