The photovoltaic power generation system can be divided into three types of photovoltaic power generation system: independent type, grid-connected type and hybrid type according to the operation mode. Compared with the stand-alone photovoltaic power generation system, the grid-connected photovoltaic power generation system can use the transmission line in the power system to realize the long-distance transmission of electric energy. It is supported by the grid power and basically does not need to consider the advantages of load characteristics. At present, China's photovoltaic grid-connected power generation system presents two development methods: "large-scale development, medium and high voltage access" and "decentralized development, low-voltage local access", so China's photovoltaic grid-connected power generation system can be divided into centralized photovoltaic There are two types of grid-connected power generation system and distributed photovoltaic power generation system.
1. Characteristics of distributed and centralized photovoltaic system
The distributed photovoltaic grid-connected power generation system is located on the user side, and the power generation supplies the local power load. It has the advantages of small footprint and flexible operation mode. It is mainly used in house roofs, buildings, greenhouses, fish pond pumps and street lamps. The centralized photovoltaic grid-connected power generation system mainly refers to a large-scale photovoltaic power station, which directly transmits power to the high-voltage transmission system as a large-capacity power source. Generally built in the desert, it has the advantages of flexible site selection, short construction period, stable output, flexible operation mode, easy to participate in voltage regulation and frequency regulation of the power grid, and low operating cost.
2. There are common problems in distributed and centralized photovoltaic systems
At present, the common problems of distributed and centralized photovoltaic systems are as follows:
(1) Optimized configuration of photovoltaic array. Before installing the photovoltaic array, the component selection, component installation inclination angle, array topology and other aspects should be optimized according to design requirements and surrounding environment, so as to improve the power generation efficiency of the photovoltaic system.
(2) Temperature rise, mismatch and hot spots of photovoltaic arrays. The working environment of the photovoltaic system is more complicated. Over time, the surface of the module will accumulate dust, and even the leaves and birds' excrement. Sometimes the module is blocked by the surrounding buildings, trees, etc., and the temperature of the module will rise significantly under the shield. High, as the temperature of the component increases, its output voltage and power will decrease. These conditions will cause the photovoltaic array to be in a mismatched operating state, and in a severe case, a hot spot effect occurs, which reduces the service life of the component.
(3) Multi-peak characteristics of the output characteristics of the photovoltaic array. Photovoltaic power plants have large-scale photovoltaic arrays, and sometimes different types of photovoltaic modules may be used for combination, or even if the module types are the same, due to cloud cover, dust, and aging, the components will not match, resulting in multi-peak characteristics. , Which reduces the efficiency of photovoltaic array power generation.
(4) The problem of power quality is caused after the photovoltaic system is connected to the grid. For example, the direction of power flow in the power grid will change, resulting in increased line losses and relay protection needs to be reset; photovoltaic power generation systems have randomness and fluctuations, which will cause grid voltage fluctuations; photovoltaic systems use a large number of power electronic devices, which will Harmonic pollution caused by power grid.
3. There are unique problems with centralized photovoltaic systems
At present, the unique problems of centralized photovoltaic systems are as follows:
(1) Cleaning of large-area photovoltaic arrays. Photovoltaic arrays work in open fields for a long time. The surface of photovoltaic modules will be covered with impurities such as bird droppings, which will seriously affect the output power of photovoltaic modules. Therefore, the attachments on the surface of photovoltaic arrays should be cleaned in time. For large-scale photovoltaic power plants with large-area photovoltaic arrays, relying on manpower to complete the cleaning task of components, the efficiency is too low and the safety is poor.
(2) Non-ideal characteristics of the inverter. Centralized large-scale photovoltaic power plants require multiple inverters to operate in parallel, but due to the irrational characteristics of inverters, such phenomena as circulating current and harmonic amplification reduce the conversion efficiency of inverters.
4. Key technologies shared by distributed and centralized photovoltaic systems
In order to ensure the safe, reliable and stable operation of distributed and centralized photovoltaic systems, some key technologies are required. The key technologies that both have are as follows:
(1) Maximum power point tracking technology. The output characteristics of photovoltaic cells have non-linear characteristics. Under any operating conditions, the PU output characteristic curve has a specific maximum power point. In order to improve the power generation efficiency of the photovoltaic system, the maximum power point tracking control technology is used to make the photovoltaic system work at the maximum power. Point.
(2) Global maximum power point tracking control technology. Under the local shadow, the power output characteristics of the photovoltaic array presents a multi-peak characteristic, which improves the power generation efficiency of the photovoltaic system. It requires global maximum power point tracking control technology to achieve global optimization.
(3) Photovoltaic array hot spot detection technology. As the hot spot effect will cause serious damage to photovoltaic cells, hot spot detection technology is needed to achieve component hot spot detection and accurate positioning.
(4) Photovoltaic conversion control technology. It mainly includes inverter, grid-connected control and safety protection technologies. For photovoltaic systems with small capacity, how to improve the efficiency of the inverter and reduce energy loss is also a development direction of photovoltaic conversion control technology; for large-capacity system photovoltaics In addition to the basic inverter, grid connection and protection functions, the inverter of the system also requires the inverter to have the characteristics of large single capacity, high voltage level, good output power quality, and strong anti-interference ability.
(5) Island detection technology. The island protection function is a key condition for whether the photovoltaic system can be connected to the grid, and the island detection technology is required to have a small detection dead zone and strong anti-interference ability.
5. Key technologies unique to centralized photovoltaic systems
The key technologies unique to the centralized photovoltaic system are:
(1) Low voltage ride-through technology. For large-scale photovoltaic substations, when the system short-circuit fault causes a voltage drop, the photovoltaic power plant does not immediately quit operation, but continues to be connected to the grid, and provides certain support to the grid to help the grid voltage recovery, with low voltage ride-through capability will become The core technology of grid-connected inverter. For large-scale photovoltaic power plants, there must be island detection and low-voltage ride-through functions, but these two functions have certain contradictions with each other. How to realize these two functions together is worth studying.
(2) Unified control technology for inverter clusters. Through the unified control and cooperation of multiple inverters, the adverse effects between the inverters are reduced, and the functions of island detection, low voltage ride through, and communication are completed.
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