The Research Of Techniques On Single-phase PV Grid-connected Generation System

Nowadays, global energy shortage and environment pollution could be effectively solved by the large-scale exploitation and efficient utilization of solar energy. As the key point to achieve high-efficient and clean utilization of solar energy, the photovoltaic (PV) grid-connected generation system has been paid more and more attention to by researchers at home and abroad by virtue of its high efficiency, favorable reliability and excellent control characteristics. Based on summarization and analysis about the status quo of researches on photovoltaic (PV) grid-connected generation system, an experimental platform involving single phase photovoltaic (PV) grid-connected generation system is built and several crucial techniques of it are researched. Maximum power point tracking (MPPT), islanding detection, unity power factor output, high-efficient high gain LLC resonant converter and so forth are realized successfully. The effects of attenuating high frequency harmonics of grid-connected current and reducing the total harmonics distortion (THD) of that are compared, emphasized and accomplished between L output filter and LCL output filter.First, the market prospect and status quo of researches on photovoltaic (PV) grid-connected generation system are summarized and at the same time, some relative fields involving that are analyzed briefly. Then, the structural schematic diagram and control block diagram of single phase photovoltaic (PV) grid-connected generation system are posed respectively. Of course, several critical techniques about it are investigated in details. The rapid and effective tracking of maximum power point (MPP) in photovoltaic (PV) arrays can be achieved through perturbation & observation (P&O) algorithm with variable size. The lower output voltage from photovoltaic (PV) arrays will be boosted up to higher voltage level in DC bus for grid-connected application by full-bridge LLC resonant converter. With optimized designs of resonant parameters and operating point other than resonant converter's characteristics itself, zero-voltage switching (ZVS) of main power devices in primary side of transformer and zero-current switching (ZCS) of rectifier diodes in secondary side of transformer will be obstained. The exact and rapid detection aiming at islanding phenomenon will be realized by under-voltage/over-voltage, high-frequency/low frequency method which is one kind of passive detection techniques. With the help of digital synchronized phase detection which is to detect positive zero-crossing of grid voltage, the same frequency and phase between grid voltage and grid-connected current can be ensured. That is unity power factor output. Last but not least, system modeling and stability analysis of L output filter and LCL output filter are presented and according to these, the effectiveness of restraining high frequency harmonics of grid-conneceted current is researched. Compared with traditional LC output filter, better performances in reduction on values of filter parameters and attenuation of high frequency harmonics in close-loop control system with grid-connected current of LCL output filter are demonstrated through experiments. However, LCL output filter is a third order system without damping and easy to resonate. Thus an additional resistance in series with capacitor is needed to increase damping, eliminate resonance and raise stability of the system mentioned above. Meanwhile, the damping of control system could also be increased through capacitor current close-loop. In dual close-loop control system with grid-connected current and capacitor current in LCL ouput filter, not only restrained resonance and increased stability could be achieved without any dispensible losses but also larger attenuation towards high frequency harmonics will be realized which are verified by Matlab/SIMULINK.Finally, the practical characteristics and effectiveness of maximum power point tracking (MPPT), high-efficient LLC resonant converter, islanding detection, digital synchronized phase detection and LCL output filter are tested by single phase photovoltaic (PV) grid-connected generation system which illustrates the feasibility of it.