Research On The Control Strategies For The Residential Photovoltaic Grid-Connected Generation System

All over the world focus on the photovoltaic (PV) generation system seriously due to the worldwide energy crisis and worse and worse environment pollution. Especially, residential PV generation system will be one of the most important fields in the PV application in the near future and its foreground will be very wide, therefore, it becomes the research highlight for many scholars.In this thesis, the control strategies including the maximum power point tracking (MPPT), digital phase-locked loop (DPLL), grid-connected current control and islanding detection strategy have been analyzed in detail for the residential single-phase PV grid-connected generation system.Firstly, as for theⅠ-Ⅴnonlinearity characteristics of PV cells and the influence of irradiation intensity and temperature on PV cells, the outsideⅠ-Ⅴcharacteristics and physical characteristics are developed and their simulation models are built respectively on the basis of the working principle of PV cells. Compared the simulation results with experimental data of PV cells, the above models are proved to be right and it provides an excellent simulation platform to realize MPPT method and study the control strategy for the grid-connected generation system in depth.Secondly, the MPPT method of PV array is studied in detail. Based on the introduction of regular MPPT methods, the novel MPPT method based on intermittent step-varied search is proposed, it is composed of two control processes including step-varied timing search and constant duty ratio timing control. When the PV system starts up or the environment changes strongly, step-varied search is implemented at intervals of the shorter timing to track the maximum power point (MPP) quickly. When the environment changes slightly, the optimal constant duty ratio is utilized for a longer timing. The simulation and experimental results prove that this method can reduce the tracking power loss caused by the continuous perturbation, and track the MPP fast even under the quick environment change.Thirdly, a digital phase-locked loop (DPLL) method is developed. On the basis of description of the PLL method for the three-phase grid-connected system, a single-phase DPLL method based on the linear phase-shifted control of the finite impulse response (FIR) filter is proposed. The FIR filter is designed to construct two-phase quarter voltage vector signals. According to the DPLL principle by means of the transformation fromαβstationary reference to dq synchronously rotating reference for a three-phase system, and the synchronously sampling technique used to ensure the constant filter order, a DPLL based on a fictitious two-phase quarter coordinate transformation for a single-phase grid-connected power conversion system is implemented. The simulation and experimental results validate that this method can realize PLL quickly and it is not influenced by such conditions as voltage harmonics and the fluctuations of the voltage frequency and amplitude.Fourthly, an on-line estimated method of the filter inductance for the predictive current control is analyzed. The equivalent circuit for a single-phase PV grid-connected generation system is simplified, on-line estimated equation of the average filter inductance is given. Some conditions such as filter inductance change, magnitude change of output current and injected harmonic current, magnitude change of AC grid voltage and injected harmonic voltage are studied by the simulation and experiments, the results verify the method solves the problems which are the steady-state phase error and system instability in the conventional predictive current control system when the filter inductance changes.Lastly, two novel islanding detection methods are studied in depth. Owing to the islanding non-detection zone (NDZ) problem for over-/under-voltage protection (OVP/UVP) and over-/under-frequency protection (OFP/UFP) in passive detection methods and active frequency drift (AFD) in active detection methods, islanding detection methods based on the alternative current disturbance and the 2N period current disturbance are proposed, respectively. The proposed two methods depend on the voltage period error between the 2Nth and (2N+1)th period at the point of common coupling (PCC) changes alternately in the form of the positive and negative signs to identify the NDZ islanding phenomenon, they do not simply judge whether the voltage frequency at the PCC is beyond the presetting frequency range. In order to prevent the counteraction of current disturbance generated by some grid-connected inverters, the synchronous control strategy of islanding detection for the grid-connected inverters is proposed. The simulation and experimental results verify that two methods can not only identify the islanding very well, but also solve the NDZ problem, and some grid-connected inverters may operate synchronously.