1. Characteristics of distributed and centralized photovoltaic systems

The distributed photovoltaic grid-connected power generation system is located at the user side and generates electricity to supply the local electricity load. It has the advantages of small footprint and flexible operation mode. Mainly used in the house roof, buildings, greenhouse, fish pond pump and street lamp and other occasions. Centralized photovoltaic grid-connected power generation system mainly refers to large photovoltaic power stations, as a large capacity of power supply directly to the high voltage transmission system. Generally built in the desert, it has the advantages of flexible site selection, short construction cycle, stable output, flexible operation mode, easy to participate in the power grid voltage regulation and frequency modulation, and low operation cost.

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2. Distributed and centralized photovoltaic systems have common problems

At present, distributed and centralized photovoltaic systems have common problems as follows:

(1) Optimal configuration of photovoltaic arrays. Before installing PV arrays, the selection of modules, the inclination Angle of modules and the topology structure of arrays should be optimized according to the design requirements and the surrounding environment, so as to improve the power generation efficiency of the PV system.

(2) Temperature rise, mismatch and hot spot phenomenon of photovoltaic array. The photovoltaic system is in a complex working environment. As time goes by, the surface of the components will accumulate dust, and even leaves and bird excrement. Sometimes the components are shielded by the surrounding buildings and trees, and the temperature of the components under the shield will increase significantly. All these conditions will lead to the mismatched operation of photovoltaic arrays, and in serious cases, the hot spot effect will occur, reducing the service life of the modules

(3) The output characteristics of photovoltaic array are multi-peak characteristics. A photovoltaic power station has a large area of photovoltaic arrays, and sometimes different models of photovoltaic modules may be used for combination, or even if the module models are the same, the modules do not match due to cloud, dust, aging and other reasons, so that the output characteristics show multi-peak characteristics, reducing the power generation efficiency of the photovoltaic array.

(4) The grid-connected photovoltaic system may cause power quality problems. For example, the direction of power flow in the power grid will change, resulting in increased line losses and the need to reset relay protection; The photovoltaic power generation system has randomness and fluctuation, which will cause the voltage fluctuation of the grid. Photovoltaic system uses a large number of power electronic devices, which will cause harmonic pollution to the power grid.

3. Centralized photovoltaic systems have unique problems

Currently, centralized photovoltaic systems have unique problems as follows:

(1) Cleaning of large area photovoltaic array. If the photovoltaic array works in the open field for a long time, the surface of the photovoltaic module will be covered by impurities such as bird droppings, which will seriously affect the output power of the photovoltaic module. Therefore, the attachments on the surface of the photovoltaic array should be cleaned in time. For large photovoltaic power stations with large areas of photovoltaic arrays, the efficiency and safety of components cleaning is low by manpower.

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(2) The non-ideal characteristics of the inverter. A large centralized photovoltaic power station requires multiple inverters to run in parallel. However, due to the irrational characteristics of the inverters, such phenomena as circulation and harmonic amplification are generated, which reduces the conversion efficiency of the 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 shared by the two technologies are as follows:

(1) Maximum power point tracking technology. The output characteristics of photovoltaic cells are nonlinear, and the P-U output characteristic curve has a specific maximum power point under any working conditions. 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. In order to improve the efficiency of photovoltaic system, the global maximum power point tracking control technology is needed to realize global optimization.