| The interfacing of Photovoltaic(PV)cells into the downstream system is usually done by utilizing a DC-DC converter in photovoltaic power generation systems,which is to realize the maximum power point tracking(MPPT).PV cells have both constant-current and constant-voltage dual nature depending on the operating point,which enable the use of either input-voltage or input-current control.Since input-current control is prone to saturation of the controller,input-voltage control is recommended to be used for MPPT.Because of the current-source nature of PV generator,input-voltage control forces the interfacing MPPT converter a current-fed converter,which can be implemented by adding a capacitor at the input terminal of a conventional voltage-fed converter,showing quite different steady-state and dynamic properties.The main objective is to study dynamic and terminal impedance properties of the current-fed MPPT converter,and the system stability is analyzed based on the terminal impedance.Firstly,taking the Boost converter adding an input capacitor as the application examples,the system small-signal models of both voltage-type and current-type load are established based on the circuit averaging method.The influence of different load-type and PV cells on the system in frequency response is revealed.The input voltage controller is designed,which is different from the traditional voltage control,and the implementation method is given based on digital control.The difference in MPPT efficiency and dynamic response performance between the direct duty control and input voltage cont rol is analyzed by experimental results.Secondly,the input impedance and output impedance model of the Boost converter adding an input capacitor are derived on the basis of the small signal model of the system.The influence of the control methods and load type on the input impedance and the control methods and working mode of PV cells on the output impedance are analyzed.The correctness of the model and the theoretical analysis are verified by the test of frequency domain and time domain.Finally,based on the stability analysis methodology for cascade system,the interface stability between PV cells and the Boost converter adding an input capacitor is analyzed.Then a small DC distributed power system is built based on PV cells,the Boost converter adding an input capacitor,bidirectional Buck converter and DC load,in which the stability of DC bus voltage is analyzed based on system impedance and the experimental verification is carried out.The experimental results show that the closed-loop input impedance of the Boost converter adding an input capacitor has a high resonance peak at the resonant frequency of the input capacitor and filter inductor,which can lead to the instability under input-voltage control.The output impedance of current-fed MPPT converter is very small in the medium and high frequency range.As a load subsystem cascaded by a voltage source converter,it is easy to cause system instability,which means that the output impedance Amplitude of the source converter should be reduced for stability design. |
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