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Research On Series Z-source Inverter With High Boost Gain And Its Application

Posted on:2014-04-08Degree:DoctorType:Dissertation
Country:ChinaCandidate:C W CaiFull Text:PDF
GTID:1262330392967659Subject:Power electronics and electric drive
Abstract/Summary:PDF Full Text Request
Z-Source Inverter (ZSI) has been proposed as a novel power conversiontopology in recent years. Due to the brand new circuit topology providing thefunctionality of single stage buck-boost power conversion and shoot-through statein the same leg, ZSI is especially suitable for renewable recourse. However, thetraditional ZSI applied in power generating system has the following drawbacks: therequired input voltage range and the voltage stress of the active devices is toohigher due to the limited boost inversion ability. In view of the issues above, afamily of high gain ZSIs will be proposed in this dissertation,and the improvedZSI topologies, modulation method, mathematical model, the control strategy in dcside and control strategy of the photovoltaic (PV) generating system in islandingoperation mode will be investigated.To improve the capability of DC boost inversion, a family of high gain ZSIs isproposed based on switched-inductor. The proposed series high gain ZSI topology isderived from investigating the mechanism of traditional DC/DC voltage boostcircuit, which performance is improved by the switched-inductor. The positions ofthe inverter bridge and the Z-source network are changed, as result, the capacitorvoltage stress can be reduced significantly and the inrush-current can be limited.But its input current is discontinues. To solve this problem, firstly the quasi-switched-inductor ZSI is proposed which input current ripple is half of the previousone. Subsequently, the input LC filter is transformed as a boost conversion circuit,and then a novel continues topology is proposed by cascading the boost circuit withthe series high gain ZSI. This continues input current topology can not onlydecrease the input current ripple but also further improve the boost conversioncapability. Unfortunately, the simulation results show that the dynamiccharacteristics of Z-source network components are inconsistent and this mayincrease the difficulty of the modeling and control.In order to improve the AC inversion capability of ZSI, a novel maximumconstant boost control method based on SVPWM is proposed. Based on the serieshigh boost gain topology, the novel method is the result of analyzing differentinserting effect on the current stress of active component, charging frequency of Z-source impedance components, as well as investigating shoot-through acting timeeffect on the boost inversion capability. In addition, the implementation method isgiven. The maximum ac gain is obtained without any extra switching stateintroduced, and the volume of the Z source can be lessened as a result of increasing charge-discharge frequency. What is more, this novel method avoids the Z-sourcediodes discontinuous conduction mode (DCM) by the active state and shoot-throughstate alternating. The simulation and experiment results verify the merits of theimproved module strategy.In order to control the dc-link voltage accurately, the ac small-signal model ofthe series high boost gain Z-Source inverter is set up in current continuous mode,and the dc-link voltage direct control strategy is proposed based on the small-signalmodel. Firstly, the equivalent circuit is obtained by the output filter and loadconverted, then the signal-flow-graph is utilized to set up the model taking intoaccount of shoot through state, active and null state. The effects of differentparameters on system dynamic performance are analyzed in frequency-domain andtime-domain. Results of the analysis would provide some basis to optimize the Z-source network. Secondly, the dc-link voltage is further analyzed and the small-signal model is obtained by state space averaged method, then the dc-link voltagecontrol method is derived based on this model, and the double closed loop controlleris built, including voltage loop control and current loop control. The simulation andexperimental results show that the accuracy of the dc-link voltage control and theinput disturbance rejection capability is improved, and the inrush surge issuppressed effectively.In order to decrease the voltage stress of the switch in PV power system whileimprove the conversion efficiency of the whole inversion system at the same time,the control strategy coordinating dc-side with ac-side is studied. Analysis of theoutput characteristics of PV module shows that PV module output voltage changesslowly. The PV module output voltage feedforward dc-link voltage control isproposed according to the relationship between dc-link voltage and input voltageunder the maximum constant boost control. The dc-link voltage setpoint can beadjusted real-time according to the output voltage of PV module, so the zero statetime can be utilized sufficiently and the voltage stress of the active component canbe decreased. In the ac-side, the outer voltage loop and inner current loopcontrollers are designed based on pole-assignment theory, then output steady-stateerror can be avoid by the outer loop and the load disturbance can be suppressed bythe inner loop.
Keywords/Search Tags:Z-source Inverter, ac small signal model, maximum constant boostcontrol method, dc-link voltage control strategy, high boost gain
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