Research On Key Techniques Of Photovoltaic Interfacing Converters With High Performance

The photovoltaic(PV)interface converter is the essential part of typical PV generation systems.To improve the efficiency and reliability of PV generation system,the PV interface converter should have at least two operation modes: maximum power point tracking(MPPT)and output voltage regulation(OVR).While solar cell can act as different power sources depending on its operation point,and its output power as well as the small signal output impedance will nonlinearly vary with irradiation,temperature and terminal voltage.Hence,to reasonably design the robust PI controller under variable parameters conditions,this thesis studies systematically the dynamic of PV interface converter with two operation modes from multi-aspects in terms of dynamic model of PV cells,controller structure,control strategy,system stability and parameters design.Understanding correctly the dynamic model and characteristics of solar cell is the premise to deeply explore the dynamic of the PV interface system.Based on the ideal physical model of silicon solar cell,this thesis deduces the steady-state and dynamic characteristics analytic expressions and obtain the dynamics circuit model of solar cell.After that,the expressions of diode dynamic resistance,junction capacitance and dynamic output impedance are derived,and the impact of illumination,temperature,terminal voltage and minority carrier lifetime on the above-mentioned dynamic parameters is analyzed.Since the minority carrier lifetime is an important parameter influencing the characteristic of solar cell,its new measurement method is proposed.Finally,the theory correctness is validated by experiment on the solar dynamic experiment platform which is implemented with light-emitting diode(LED)lamps and electric double layer capacitor(EDLC).The input voltage robust stability is crucial for reliable implementation of MPPT algorithm under the various practical working conditions.According to the dynamic circuit model of solar cell and the Routh stability criterion,this paper derives the input voltage stability condition together with the design guidelines-the controller parameter should be design under the maximum dynamic output resistance to ensure the input voltage stability under the entire working conditions.Furthermore,a robust PI controller design method and its simplification are proposed to solve the problem of lack of robust and practicability existing in conventional design method and actual-model design method.In addition,the PV interface system in MPPT mode can be regarded as a current source-current fed converter cascade system.Hence,the impendence-match stability criterion for this case is deduced and then the PV interface voltage(i.e.input voltage)stability is analyzed by it.The investigation shows that if a converter fed by an ideal current source can operate stably,it will also keep stablewhen interfacing with solar panels under the same working condition,which consist with the simplified design method.To accurately design the parameters and reasonably select the operation region and control strategy,it is necessary to study the stable operation region of solar cells in OVR mode.In this thesis,the small signal model including the single-diode dynamic equivalent circuit of solar cells is deduced.According to the Hurwitz criterion,the essential condition for output voltage control stability is analyzed and the key factor influenced the stable operation region is revealed.The investigation shows that: the stable operation region of solar cell is unique if the control loop of PV converter in OVR mode is determined.The type of operation region entirely depends on the control loop formation.Furthermore,no matter what kind of control loop formation is adopted,the PV cells can not keep stable at MPP in OVR mode.Hence,if MPPT is implemented by output voltage control,the MPP cannot be reached and thus the MPPT efficiency is reduced.For the purpose of reliability of PV generation system,it is essential to improve the robust stability of the interface converter in OVR mode.In this thesis,the output voltage stability conditions of PV interface converter are derived and the design criterion for output voltage robust PI controller is proposed,which indicates the parameters should be designed under the condition that ro gets its minimum.Also,the practical and simplified design methods are proposed.In addition,the impedance matching stability criterion for PV interface system in OVR mode is obtained and is used to prove again the previously obtained conclusions on the stable operation area of solar cell.To improve the life expectancy and long-term reliability,single-phase two-stage PV inverter should be designed without using electrolytic capacitors.However,this will increase the double-linefrequency(DLF)ripple input voltage oscillation,wasting the available energy and confusing MPPT algorithm.This paper investigates the suppression strategies for the DLF input voltage ripple.The analytic model of low frequency input voltage ripple component is presented.To ensure both the robust stability and DLF ripple robust suppression,the input PI controller design criterion and practical design method are given.The virtual impedance suppression strategies and their detailed design methods are also proposed.Furthermore,an input voltage controller with PI cascaded Quasi-proportion resonant regulator is proposed to maintain zero steady-state error and provide high rejection capability of DLF oscillation of input voltage.In addition,the output voltage feed-forward control is studied.Finally,the performances in terms of suppression effects,rapidity,cost and ease of implementation of all the proposed suppression strategies are compared.