| The application of renewable energy, such as solar energy and wind energy, has emerged and gradually became one of alternatives of fossil fuel. The photovoltaic (PV) technology, one of the most efficient ways of solar energy utilization, is applied broadly in various capacities, scales and operation types. Compared to traditional generation system, the PV generation is costly, instable and needs complex control method. The distributed PV generation systems can be divided into the stand-alone systems and grid-connected systems. Stand-alone systems completely independent from grid and they are mainly configured in the area without grid or weakly covered by grid to meet the electricity demand but fail to guarantee the reliability of the PV system under different loads. On the other hand, the grid-connected systems are constructed in grid covered areas like cities or villages, which overcome their inability to output power continuously and stably, and improve the reliability of the PV systems. Recently under the great pressure of environment and fuel crisis, the grid-connected systems are now developing fast and the typical applications are PV roof or microgrid, which are usually constructed in the city.The proposal and application of distributed generation and microgrids produce an efficient solution to the problem that the solar energy is distributed sparsely, the power sources are close to the loads and they make little pollution. The cooperation among several power sources or between sources and storages has overcome the fluctuation problems of the output power.The PV generation belongs to DC generation, making traditional AC power supply less flexible and effective. DC microgrid helps to work with distributed PV and provides effective electricity to local DC loads.This paper pursues on the high efficient application of PV system and low building costs and focuses on the topologies and control methods of DC microgrid control strategies of grid-connected PV systems and AC/DC hybrid microgrids. In the studying process, following main results are obtained:(1) The operation principles and equivalent circuits of PV cells and PV arrays are analyzed. Traditional methods is subjected to interruption of sensors, causing the lost of reliability. And a new MPPT method is proposed:the method takes the combination of the current from the PV array and the duty cycle of the DC-DC converter, rather than the output voltage of the PV array, as the input variables for the MPPT algorithm. This means that the voltage sensor on the output terminal of PV array can be eliminated, avoiding the overuse of sensors in the traditional perturbation and observation method for MPPT. This elimination has improved the reliability of the system, significantly simplifying the configuration and decreasing the construction cost.(2) Topologies and operation principles of stand-alone PV systems are summerized and analyzed. What’s more, a method with a unified power management strategy which concerns the DC-bus voltage variation, PV array output, charge and discharge of the battery and power supplied for the loads, is proposed. The method is a unified control method, which avoids the work mode switch, and consequently the PV source power can be controlled smoothly. In addition, within the unified power management strategy, the working point optimization method for the DC converter on the PV cell side is proposed.(3) The paper proposes the topology and control method of a microgrid which can be applied in the DC-load dominated business buildings. In this system, the PV array can be constructed flexibly. A low voltage DC-bus is applied for PV array to plug-in and supplying digital equipments with low DC voltage and a high voltage DC-bus is employed to integrate separate units and batteries with the benefits of reducing circuit losses. The microgrid controls the DC-bus voltage to balance the power in the whole system and guarantees the stability of the system.(4) A control strategy of Two-stage three-phase grid-connected inverters without dc-link voltage sensor is proposed, which can be used in distributed PV system, or grid-connected PV generation in microgrid. Under the premise that overall performance is kept, this strategy eliminates the dc-link voltage sensor and its relevant circuit.(5) Based on the strategy of Two-stage three-phase grid-connected inverters without dc-link voltage sensor stated above and incorporating with perturbation and observation MPPT method, a two-stage grid-connected inverters control strategy without dc side voltage sensors is proposed, which can be used in distributed PV generation system.(6) In the distributed PV generation AC/DC hybrid microgrid topology constructed with single DC microgrid and AC microgrid, an off-grid paralleled AC inverter control strategy is proposed based on the characteristics of AC/DC microgrid, the instability of distributed PV generation and its P-U curve.(7) After the research of distributed PV generation system and PV generation DC microgrid and in order to dig into the rules of microgrid in electric generation in the future and obtain actual operation data of microgrid, an experimental platform of PV generation microgrid is constructed.Analysis and simulations and experiments show the methods proposed in the paper make improvements include simplifications of the system and the increase of reliability. In addition, the characteristics of urban power supply and electricity consumption are analyzed, based on which a PV generation microgrid used under specific condition and its control method are proposed. At last, the paper introduces the microgrid experimental platform. |
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