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光伏并网发电系统存在的问题及其关键技术

2019-07-24 点击:927

Photovoltaic power generation systems can be divided into three types of photovoltaic power generation systems: independent, grid-connected and hybrid. Compared with the independent photovoltaic power generation system, the grid-connected photovoltaic power generation system can utilize the transmission line in the power system to realize the long-distance transmission of electric energy, and the power supply of the grid has no need to consider the influence of the load characteristics. At present, China's photovoltaic grid-connected power generation system presents two development modes of “large-scale development, medium and high voltage access” and “distributed development and low voltage local access”. Therefore, China's photovoltaic grid-connected power generation system can be divided into centralized photovoltaic system. There are two types of grid-connected power generation systems and distributed photovoltaic power generation systems.

1. Characteristics of distributed, centralized photovoltaic systems

The distributed photovoltaic grid-connected power generation system is located on the user side, and the power generation is supplied to the local power load, which has the advantages of small footprint and flexible operation mode. Mainly used in housing 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, and directly transmits power to the high-voltage power 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 power grid voltage regulation, frequency modulation, and low operating cost.

2. There are common problems in distributed and centralized PV systems

At present, the common problems of distributed and centralized PV systems are as follows:

(1) Photovoltaic array optimization configuration problem. Before installing the PV array, the component selection, component mounting tilt angle, and array topology should be optimized according to design requirements and surrounding environment to improve the power generation efficiency of the PV system.

xx(2) Temperature rise, mismatch and hot spot phenomenon of photovoltaic arrays. The working environment of the photovoltaic system is complicated. Over time, the surface of the component will accumulate dust, even the excrement of leaves and birds. Sometimes the components are blocked by surrounding buildings and trees, and the temperature of the components under the cover will rise significantly. High, as the component temperature increases, its output voltage decreases and power decreases. These conditions will cause the PV array to be in a mismatched operating state, causing a hot spot effect in severe cases, reducing the life of the component.

(3) Multi-peak characteristics of the output characteristics of the photovoltaic array. Photovoltaic power plants have large-area photovoltaic arrays, sometimes combined with different types of PV modules, or even if the components are of the same type, the components do not match due to cloud, dust and aging, etc. reducing the efficiency of photovoltaic array power generation.

(4) The problem of power quality caused by the photovoltaic system connected to the grid. For example, the direction of the power flow in the power grid will change, resulting in increased line loss and relay protection needs to be re-set; photovoltaic power generation system has randomness and volatility, which will cause grid voltage fluctuations; a large number of power electronic devices used in photovoltaic systems will The power grid causes harmonic pollution.

3. Unique problems in centralized photovoltaic systems

At present, the unique problems of centralized photovoltaic systems are as follows:

(1) Cleaning of large-area photovoltaic arrays. PV arrays work in open air for a long time, and the surface of PV modules will be covered by impurities such as bird droppings, which will seriously affect the output power of PV modules. Therefore, the attachments on the surface of PV arrays should be cleaned in time. For large-scale photovoltaic power plants, there is a large-area photovoltaic array, and the task of cleaning components is completed by manpower, which is too low in efficiency and poor in safety.

(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 the inverter, looping, harmonic amplification and other phenomena are generated, which reduces the conversion efficiency of the inverter.

xx4. Key technologies shared by distributed and centralized PV systems

In order to ensure the safe, reliable and stable operation of distributed and centralized photovoltaic systems, some key technologies are needed. The key technologies of the two are as follows:

(1) Maximum power point tracking technology. The output characteristics of photovoltaic cells have nonlinear characteristics. Under any operating conditions, their PU output characteristic curves have a specific maximum power point. In order to improve the power generation efficiency of photovoltaic systems, the maximum power point tracking control technology is used to make the photovoltaic system work at maximum power. Point.

(2) Global maximum power point tracking control technology. In the local shadow, the power output characteristics of the PV array exhibit multi-peak characteristics, and the power generation efficiency of the photovoltaic system is improved. The global maximum power point tracking control technology is needed to achieve global optimization.

(3) Photovoltaic array hot spot detection technology. The hot spot effect will cause serious damage to the photovoltaic cell, and the hot spot detection technology is needed to realize the hot spot detection and accurate positioning of the component.

(4) Photovoltaic conversion control technology. It mainly includes technologies such as inverter, grid-connected control and safety protection. For small-capacity photovoltaic systems, how to improve the working efficiency of inverters and reduce energy loss is also a development direction of photovoltaic conversion control technology; for large-capacity system photovoltaics In addition to the functions of basic inverter, grid connection and protection, 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.

It requires the island detection technology 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 centralized PV systems are:

xx(1)低电压穿越技术。对于大型光伏变电站,当系统发生短路故障且电压下降时,光伏电站不会立即退出运行,而是继续与电网连接,并为电网提供一定的支持电网电压恢复,具有低电压穿越能力将成为并网逆变器的核心技术。对于大型光伏电站,必须有岛检测和低电压穿越功能,但这两个功能之间存在一定的矛盾。如何共同实现这两个功能值得研究。

(2)逆变器集群统一控制技术。通过多个逆变器的统一控制和协作,减少了逆变器之间的不利影响,并完成了岛检测,低电压穿越和通信等功能。

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