Research On Control Method For Grid-connected Distributed Photovoltaic Generation System

Nowadays, energy shortage and environmental pollution have become the problems for the world. Countries around the world have reached a consensus that we can improve energy structure and reduce the environmental pollution through developing renewable energy. On the other hand, the solar photovoltaic(PV) power generation, which is reliable, cleanand inexhaustible, is developed into the important part of renewable energy and the world’s energy. Distributed PV grid generation is one main form of PV power generation and has the huge potential improvement. Further development of distributed PV grid power generation technology is of great significance,to solve the energy crisis, reduce environmental pollution, and promote the healthy development of world economy. Now, many researchers engaged in this field of research and made a lot of research results. However, with the rapid development of distributed energy, some new phenomena and issue gradually appear. Therefore,distributed PV grid power generation technology is facing new requirements.In this paper, single phase distributed PV grid power generation system is used as the research object. Photovoltaic maximum power point tracking(MPPT), grid current control, and islanding detection methods are studied in detail. The main contents are as follows:In this paper, output characteristic and equivalent model of the PV arrays are introduced. Meanwhile, some traditional MPPT algorithms are analyzed. However,when there are multiple local maximum power points, from partially shading or from installation on a curved surface, conventional MPPT techniques do not perform well. In this paper, a new method is presented to track the global GMPP of PV with comparison of the voltages of every PV module. Compared with the past-proposed global MPPT techniques, the method proposed in this paper has the advantages of determining whether partial shading is present, calculating the number of peaks on P-V curves, and predicting the locations of GMPP and LMPP. The new method can quickly find GMPP,and avoid much energy loss due to blind scan. The experimental results verify that the proposed method guarantees convergence to the global MPP under partial shading conditions.On the basis of the analysis of the mathematical model of inverter, the grid voltagefeed-forward is introduced to effectively inhibit grid voltage harmonic.Proportional-Integral(PI) control technology cannot eliminate steady-state error when tracking the sinusoidal signal from the grid, which results in a very high total harmonic distortion in the grid-connected current. The repetitive control theory is a theory which is based on internal model principle. What’s more, the repetitive control(RC) theory can achieve no static error tracking with the periodic interference signal. However, the periodic delay links in repetitive controller lead to the poor dynamic performance. And parameter design of RC is dependent on accurate modeling system. In this paper, a novel RC controller has been developed, which increases the expected current feedforward control and FIR digital filter. The proposed RC controller can get high stability and dynamic at the same time. Therefore, the current tracking accuracy and current harmonic content can also be improved. Finally, simulation and experiment results verify the effectiveness and advantages of the proposed controller.The islanding condition occurs when the utility is disconnected while the distributed generation(DG) continues to supply power to local load. This undesirable condition brings about the instability of the DG’s voltage and frequency as well as the power quality degradation, and presenting a great threat to maintenance personnel.Considering equipment integrity and personnel safety, unintentional islanding for DG operation will not be tolerated here. With the development of distributed generation,more inverters have been connected to the local electrical power system(EPS). Hence,more and more researchers have attached importance to these works about multiple inverters’ islanding detection. In this paper, the basic problems of islanding effect are expounded. Islanding detection standards and traditional detection methods are introduced. More, the interaction and degradation of two active and positive feedback islanding detection methods, including Current-Disturbing Positive Feedback(CDPF)and Sandia Frequency Shift(SFS) islanding detection metholds, are discusses. Then a new islanding detection method, parametric adaptive SFS detection method, has been developed. Finally, simulation and experiment results validate the improvement of SFS islanding detection methold can weaken the dilution effect in multi grid connected inverters.