3. Design of component support part

(1) Selection of support foundation

The main consideration is to meet the calculation requirements of foundation bearing capacity, foundation overturning resistance, pullout resistance, slip resistance, etc., and ensure the stability of the upper structure.

At present, reinforced concrete independent foundations, reinforced concrete strip foundations, and prestressed cement pipe pile foundations are mainly used in China. Reinforced concrete foundations are mainly used in places with relatively good site conditions, such as "complementary agriculture and solar", "complementary livestock and solar", etc. The main advantage of the reinforced concrete cast-in-place foundation is that the construction is less difficult, the foundation plane positioning and the elevation of the top floor of the foundation are easy to control, and the anti-overturning and sliding resistance are better, the overall effect is good, and the overall visual sense of the power station is better after the completion of the power station. Accuracy for optimum inclination.

The disadvantage is that the construction period is long, the damage to the ground is relatively large, and the amount of earthwork excavation, backfilling, formwork configuration, rolling steel bars, etc. is large; the prestressed cement pipe pile foundation is mainly used in places with relatively harsh geological conditions, such as "fishing and light complementary ", coastal beaches, etc. The main feature of the prestressed cement pipe pile foundation is the prefabricated finished product, the construction speed is fast, the damage to the ground is less, and the engineering volume is relatively small. The disadvantages are high technical and experience requirements for piling operators, relatively high difficulty in construction, difficult control of foundation plane positioning and foundation top floor elevation, and additional work of secondary transportation after hoisting and unloading, which increases the workload and adjustment of support installation and construction in the later stage. Difficulty, in the gravel stratum, it is difficult to enter the pile, and it is easy to eccentric or break the pile, so it should not be used. The two schemes have obvious advantages and disadvantages of interchangeability, which should be comprehensively judged in combination with local geological conditions and engineering characteristics.

According to the local geological conditions, the degree of corrosion of the groundwater to the reinforced concrete structure is judged. For areas with weak corrosion, anti-corrosion coatings shall be applied to the surface below the groundwater level; for areas with high corrosion, sulfate-resistant Portland cement shall be used below the groundwater level, mixed with sulfate-resistant admixtures, and steel bars shall be added. Rust inhibitors, mixing mineral admixtures, anti-corrosion coatings on the surface and other measures.

(2) Selection of support system

At present, domestic photovoltaic power plants mainly adopt support systems such as optimal inclination fixed type, horizontal single-axis tracking type, oblique single-axis tracking type and dual-axis tracking type. The cost of the fixed mounting bracket is relatively low, the manufacturing process is simple, the production cycle is short, the installation difficulty is small, and the bracket system is basically maintenance-free. Fixed mounting systems have a relatively small footprint and are essentially maintenance-free. The fixed support system occupies a relatively small area; the automatic tracking type has high cost and high manufacturing process, the tracking motor is easily damaged, and the operation is unstable, especially in places with high humidity, which requires a large amount of maintenance and repair.

In order to avoid occlusion, the distance between the front, rear, left, and right sides of the tracking support system array is relatively large, which increases the occupied area by about 50% and increases the investment cost, but the power generation is greatly improved compared with the optimal inclination angle fixed type. , Theoretical calculation is about 20%~30%. At present, the logic operation of the tracking support system that has been put into operation in a certain place is simpler and more reliable, which is worth learning. Therefore, it should be comprehensively analyzed from the aspects of terrain conditions, land occupation, operation reliability, equipment price, maintenance cost after completion, failure rate and power generation benefit. It is not recommended to use the automatic tracking system for places with high humidity such as "fishing and light complementarity" and coastal beaches, because the support foundation of the automatic tracking system is mainly reinforced concrete strip foundation, which is not easy to install in fish ponds, lotus ponds and beaches. construction, and the humidity is high, the motor is easily damaged by moisture and burned, and the maintenance is inconvenient.

4. Combiner box design and installation

For large and medium-sized grid-connected photovoltaic power plants, two types of combiner boxes are usually selected according to the arrangement of the array, that is, 12 inputs and 1 output and 16 inputs and 1 output, or a combination of two specifications. When designing, the one with more loops should be preferred. The combiner box should have the function of cutting off the fault current. The incoming side is protected by a photovoltaic-specific DC fuse, and the outgoing side is generally protected by a DC low-voltage molded case switch. It is not recommended to use a fuse for the outgoing side. The combiner box should be equipped with photovoltaic surge protectors, and the positive and negative poles should have lightning protection functions. The combiner box should be equipped with a monitoring device with a communication interface, which can monitor and upload the DC current, total output current, bus voltage and total output power of each incoming line branch in real time, the status of each branch fuse and DC low-voltage molded case switch, and Abnormal alarm lights for each incoming line branch.

The combiner box should be convenient for fixed installation. Generally, it is installed on the system support by hanging. The installation height of the bottom of the box should meet the requirements of various restrictive conditions. There should be sufficient installation space between the installation position of the inlet and outlet lines of the combiner box and the bottom of the box to facilitate construction and ensure installation quality.

For the incoming circuit of each branch of the combiner box, anti-reverse diodes are installed to improve the operating safety factor, but a certain amount of power generation will be lost. The design should comprehensively consider whether to install anti-reverse diodes according to the construction environment and method of the power station. If the power station is built in a place with high humidity and strong corrosion or when the DC cable is directly buried, it is recommended to install it in order to ensure safe operation; if the power station is built in a good environment and the DC cable is laid along the bridge, it is recommended not to pursue higher power generation. Installation; installing anti-reverse diodes will increase its own failure points, and it is not recommended to install them in places with high ambient temperature.

The combiner box is installed in various positions of the power station, and the protection level should be designed according to the local climate conditions. For example, in places with high humidity (such as fishing and solar complementary), the moisture-proof level should be increased accordingly; in places with high temperature (such as agricultural and solar complementary, in agricultural greenhouses), the heat dissipation function should be strengthened; in places with strong corrosion (such as coastal beaches) the shell

Materials such as stainless steel or alloy should be used.

5. Inverter design, selection and installation

The inverter is a converter device that converts DC power into AC power, and is an important component in the photovoltaic power station system. For large and medium-sized grid-connected photovoltaic power station projects, large-capacity centralized grid-connected inverters are generally selected. Generally, the larger the capacity of a single inverter, the lower the unit manufacturing price and the higher the conversion efficiency. Selecting a single inverter with large capacity can reduce investment in a certain area and improve system reliability. The higher the conversion efficiency of the inverter, the higher the efficiency of the photovoltaic power generation system, and the smaller the loss of the total power generation of the system. Therefore, when the rated capacity is the same, the inverter with high conversion efficiency should be selected.

The DC input range of the inverter should be wide, and it should have certain anti-interference ability, environmental adaptability, and instantaneous overload ability when the solar radiation is low in the morning and evening. For example, in the case of a certain degree of overvoltage, the photovoltaic power generation system should be able to operate normally; in the event of a fault, the inverter must be automatically disconnected from the main grid. After the system is disturbed, the inverter is not allowed to connect to the grid before the grid voltage and frequency return to normal, and can automatically re-connect to the grid after a delay after the system voltage and frequency return to normal. According to the requirements of the power grid for the operation mode of photovoltaic power plants, the inverter should have functions such as AC overvoltage, undervoltage protection, overfrequency, underfrequency protection, anti-islanding protection, AC and DC overcurrent protection, overload protection, and high temperature protection. The inverter should have multiple communication interfaces to collect data and send it to the control room.

In order to reduce the usage of DC cables and DC loss of centralized grid-connected inverters, the inverters should be arranged in the middle of each sub-array as much as possible. However, the sub-arrays of the "fishing-solar complementary" photovoltaic power station are built in fish ponds or lotus ponds, and the installation and operation and maintenance of the inverter are extremely inconvenient. Even so, the two sides of the road in the station should be as close as possible to each sub-array. Therefore, the organic combination of roads, inverters and combiner boxes should be considered before the overall layout of the power station. Roof-mounted photovoltaic power station inverters are generally designed to be installed on the ground, or directly installed in the underground space of the building.

For the photovoltaic array using the automatic tracking system, due to the large footprint and the long distance between the support systems, the amount of DC cables and DC loss for installing the centralized inverter will be relatively large, and the string type small-capacity inverter can be selected. Transformer.

There should be sufficient installation space between the installation position of the inverter inlet and outlet and the bottom of the box. At present, the installation of the inverter inlet and outlet in many domestic countries is quite inconvenient, which brings great difficulty to the installation and leaves certain safety and quality risks. . It is generally stipulated that there should be an installation space of &250mm between the installation position of the incoming and outgoing lines and the bottom of the box.

Six, step-up transformer design and selection

Large and medium-sized grid-connected photovoltaic power stations basically choose 2*500kW centralized inverters, and the matching designed transformers are 1000kVA low-voltage double-split transformers. It mainly adopts box-type substations with the characteristics of outdoor type, small size, convenient installation, and less maintenance. Currently, the commonly used box-type substations include American-style oil substations and European-style dry substations. The American oil transformer has compact structure, small size, relatively low cost, strong overload capacity and easy installation. The main disadvantage is that the transformer body and load switch are enclosed in the mailbox, which is inconvenient to replace when a fault occurs, easy to seep and leak oil, and an accident oil pool needs to be built.

There are common quality defects in the fuse and the internal structure of the fuel tank. After the fuse is blown, there is no three-phase jumper device, resulting in a lack of phase operation. The heavy gas trip of the oil transformer can only trip the low-voltage side of the circuit, and the high-voltage incoming power supply cannot be cut off; the European-style dry transformer has relatively large space, more convenient installation and maintenance. The high and low voltage, and transformer room are separated independently, so the operation safety factor is high. High and low voltage can be configured according to different cabinet types. The main disadvantages are large floor area, relatively high cost, general overload capacity, insulation support, and tap changer position in a humid environment where flashover and creepage are likely to occur. Failure to deal with it in time may cause the fault to expand.

Generally, a transformer comprehensive protection device is installed inside the box-type transformer, and there should be multiple communication interfaces for data collection and sending to the control room.

7. Selection of high voltage switch

At present, photovoltaic power plants mainly use metal armored central switchgear, circuit breakers are equipped with relay protection, and standard complete sets of equipment are mature in technology. Brands and cost are mainly considered for comprehensive selection. The comprehensive protection device should have multiple communication interfaces to collect data and send it to the control room.

The arrangement of the step-up transformer is generally installed close to the centralized inverter and designed on a basic platform.

Eight, lightning protection grounding project

Galvanized flat steel is the preferred grounding material for photovoltaic power stations. The average annual corrosion rate of hot-dip galvanized flat steel is 0.1mm/year. There is pitting corrosion in the steel, and the pitting corrosion rate is several times higher than the annual average corrosion rate. The actual service life is about 15 to 20 years. However, when the construction site is a strong corrosion area, it is necessary to choose copper-plated steel materials. There is no pitting corrosion in steel, which belongs to slow uniform corrosion. The corrosion rate of copper in soil is about that of steel. The annual corrosion rate of copper is 0.02mm/year. The life of pure copper grounding device can reach 50 years. The actual life of the grounding device can reach 25-30 years.

Since the photovoltaic power station occupies a large area, the photovoltaic area is generally not equipped with lightning rods. It is mainly used as grounding protection through the connection between the component support and the field ground grid, and the investment ratio is relatively small. Fully-enclosed management cannot be achieved in comprehensively utilized photovoltaic power stations, and grounding protection cannot be sloppy. A good ground grid is an important guarantee for equipment and personal safety.

9. Integrated automation system

Photovoltaic power plants should be designed according to the principle of "unattended". The switch station shall be equipped with a central control room, and through the centralized monitoring system based on the computer monitoring system, the monitoring, control and scheduling management of the photovoltaic power generation unit and the electromechanical equipment of the switch station shall be completed. The design of the integrated automation system should be safe to use, advanced in technology, and economical and reasonable. The structure, technical performance and indicators of the system should be compatible with the scale of the photovoltaic power station, its position in the power system and the development level of the current monitoring system.

At present, the photovoltaic power station monitoring system can monitor each photovoltaic incoming line branch through the monitoring device of the combiner box, but it cannot monitor each battery module.

10. Conclusion

The construction site of the photovoltaic power station is selected in an area with good solar energy resources and good site conditions, and can successfully pass the review of various departments. The overall layout should be economical, easy to maintain and avoid extensive re-planning. Choose photovoltaic modules with high efficiency, high power, and stable performance, and choose a reasonable arrangement of modules. Select the support system and foundation according to the characteristics of the project. The protection level of the combiner box and the inverter should be adapted to the local environment, the layout should minimize the amount of collector lines, and the appropriate cable cross-section should be selected through the calculation of cable length and capacity, so as to reduce line loss. The grounding system of the whole station is reliable, and the automatic monitoring system is complete. All parameters must meet the 25-year design operation period.