Photovoltaic power plants are arranged on the open and flat desert Gobi or tidal flats. Due to the lack of tall buildings and trees, lightning strikes are extremely prone to occur, especially in areas with frequent lightning strikes. On the other hand, the lightning protection grounding of photovoltaic power plants seems simple but crucial. It is directly related to the operation of solar cell modules, photovoltaic arrays, AC and DC cables, inverters, transformers and other equipment and the safety of power station maintenance personnel.
1. Status of lightning protection and grounding of photovoltaic power plants
Solar photovoltaic power generation is an emerging environmentally friendly power generation technology and industry.
Solar photovoltaic power generation is widely used in Japan. Due to its geographical characteristics, there are not many large open spaces that can be used to arrange solar cell arrays. The scale of photovoltaic power generation construction is small. Solar cell arrays are usually connected with buildings and small farms. It is arranged in a unified and coordinated manner, and occupies a small area. The protection against direct lightning strikes is mostly realized by the protection range of lightning rods of nearby buildings.
For the 1MW solar photovoltaic power generation project on Chongming Island, Shanghai, China, some lightning rods with a length of about 40mm are installed around the aluminum alloy frame of the solar panels for the direct lightning protection of each solar panel. The lightning rods are very short, but densely arranged. .
The solar photovoltaic power generation project on the roof of the building in Lingang New Town Heavy Equipment Industrial Zone, Nanhui New District, Shanghai, uses a fixed advance discharge lightning rod to achieve direct lightning protection.
2. The reference effect of relevant policies on lightning protection and grounding of photovoltaic power generation projects
As a natural phenomenon, lightning has strong uncertainty. Clearly take different lightning protection measures for buildings of different levels. What specification is used to define the lightning protection level of large-scale solar photovoltaic power plants.
The solar panel generates direct current under sunlight, and the direct current output of many solar cell components is applied through the cable.
After parallel connection, it is sent to the inverter along the cable slot box, cable tray, etc., and the inverter converts the DC to AC and boosts the voltage and then sends it to the power grid. In the entire solar photovoltaic power generation system, the DC output part accounts for a large proportion. It can be said that in a large-area solar cell array, a large number of electrical equipment such as DC cables, cable trays, and DC combiner boxes are interspersed and arranged. If the photovoltaic power station is used as a power generation system and designed according to the relevant specifications of the power system, the core of attention will be the lightning protection and grounding of the AC electrical equipment of the power system. Photovoltaic power generation projects are different from conventional power stations and power transmission and transformation systems in the power system. Even compared with small power transmission and transformation projects, their importance and the degree of loss after disasters are also different. Simply adopting the relevant specifications of the power system for lightning protection and grounding design of AC electrical equipment does not meet the characteristic requirements of photovoltaic power generation projects.
Since the height of the solar cell array and the inverter booster device is usually not more than 5m, measured by the height index, the direct lightning protection may not be considered according to the “Lightning Protection Design Code for Buildings”; however, the solar cell array covers a large area, and the battery The frame of the modules is made of aluminum alloy, and the battery plates are fixed with angle steel, channel steel, etc., all of which are metal materials with good electrical conductivity. It is easy to be damaged by direct lightning strikes when it is arranged in the open air in large and accumulated areas. At the same time, the thundercloud charge is easy to form induced overvoltage on the internal circuit of the solar cell, the frame of the solar cell module and its supporting structure.
Therefore, it can be considered to objectively analyze the probability of direct lightning strikes on the solar cell array according to the annual average number of thunderstorm days at the location where the photovoltaic power generation project is installed and the area and layout of the panels, and refer to the “Lightning Protection Design Code for Buildings” Design.
3. The necessity of lightning protection design of solar cell phalanx array
The solar cell array of a large photovoltaic power station belongs to a non-building open-air place, and the site is open and open, and the probability of being directly struck by lightning is higher than that of an ordinary non-building open-air place.
Although large-scale solar cell arrays are not flammable, the investment in photovoltaic power plants is huge. At the same time, the power generation costs of large-scale photovoltaic power plants are high, and the power loss caused by lightning strikes is also large. In terms of safety, there is no fire hazard in the battery phalanx. However, from the perspective of reducing economic losses, it is very necessary for the solar cell array to carry out lightning protection design.
4. Lightning protection grounding design
(1) Direct lightning protection
Based on the statistical data for many years, the monthly average thunderstorm days at the location where the photovoltaic power station is constructed are listed, as shown in Table 1.
That is to say, the average annual thunderstorm days in a certain area are several days, which belong to the more minefield (or less minefield).
Taking into account factors such as the annual average thunderstorm day and the area of the solar panel, the direct lightning protection of the solar cell array in the photovoltaic power station can be realized by the need-avoiding needle method.
①Ordinary lightning rod The ordinary lightning rod is a passive type of lightning protection, and its installation height is generally 20~40m, and an independent steel structure or ring-shaped rod and needle tower needs to be set up. Ordinary lightning protection is severely shielded against the sunlight of the solar cell phalanx. Therefore, it is not suitable to use ordinary lightning rods for solar cells.
②The pre-discharge sweet lightning rod is an active lightning strike system with a chain reaction device. It adds an active trigger system on the basis of the traditional lightning rod, and generates an upward oncoming pilot ahead of the ordinary lightning rod to attract lightning, thereby increasing the lightning rod. The protection range can be compared with ordinary lightning rods to reduce the installation height, and the use of early discharge lightning rods can greatly reduce the number of lightning rods, reduce the installation height of lightning rods, and reduce the shading effect on other solar arrays.
Pre-discharge lightning rods have been adopted by tens of thousands of engineering projects around the world.
Therefore, in the design stage of the project, it is possible to consider the use of lightning rods in advance discharge. It is proposed that in order to adapt to the characteristics of photovoltaic power generation, it is hoped that manufacturers can produce a reliable and controllable lifting and shrinking type of lightning rods in advance to meet the characteristics of photovoltaic power generation. This kind of lightning rod is raised in rainy weather to prepare for the direct lightning protection of the solar cell phalanx. In fine weather, the lightning rod body shrinks and does not form shadows on the solar panel.
According to the undulating terrain of the photovoltaic power station site and the arrangement of the square array of solar panels, it is determined how many sets of controllable lifting and retracting advance discharge lightning rods with a height of 10m need to be installed. The photovoltaic power station project can calculate the protection range of the lightning rod in advance of discharge according to the French NFC17-102 standard.
(2) Direct lightning protection in other areas
In the inverter booster room, partitioned high and low voltage power distribution room, Chaipu generator room, 10kV power distribution control room, and the roof of the complex building, lightning protection belts are installed for direct lightning protection.
The direct lightning protection on the AC side is carried out in accordance with the power system industry standards. Several ordinary independent lightning rods with a height of 30m are set in the 110kV booster station to achieve direct lightning protection for 110kV power distribution devices, main transformers, outdoor SVC devices, etc. The 110kV line is equipped with lightning protection line.
(3) Induction lightning protection
Photovoltaic power plants can take effective protection against induction by methods such as grounding, shunt, shielding, voltage equalization, etc. To ensure the safety of people and equipment.
① Grounding The lightning protection grounding, protective grounding, and working grounding can be unified into a common grounding device in the photovoltaic power station. In order to ensure personal safety of the entire photovoltaic system, all electrical equipment shells should be connected to a specially designed grounding trunk line. The design principle of the grounding grid of the whole station is to use the horizontal grounding body as the main body, supplemented by the artificial composite grounding grid of the vertical grounding body. The grounding resistance value shall be considered as not more than 10Ω; the allowable value of the grounding resistance in the 10kV booster station is 0.5Ω.
The layout of the power station in the entire site is both decentralized and centralized. Therefore, a common grounding system is set up for each relatively concentrated square matrix area. The AC output of each inverter is collected into the 10kV power distribution room of the booster station through the cable tray between the areas. Therefore, the actual connection between the areas through the cable tray and each grounding grid forms a large, unified grounding system. The soil resistivity varies depending on the site where the photovoltaic power station is located.
The grounding of the solar cell array includes two aspects: one is the connection between the solar panel and the mounting bracket, and the other is the connection between the bracket and the ground. There are bolt holes on the aluminum alloy frame of the solar panel for installing the grounding wire, and a special insulated grounding wire is used to reliably conduct the battery aluminum alloy frame and the battery panel bracket during construction. Measures are taken between the installation of the rigid brackets of the solar panels to form an overall horizontal grounding belt, and physical grounding is adopted separately or jointly according to factors such as soil resistivity, soil area, soil layer thickness, and paint layer continuity at the site where each array is located. Modules, copper-clad steel vertical grounding electrodes, anti-corrosion ion grounding bodies and other different grounding materials, when using the basic excavation conditions of the solar panel installation bracket, set the vertical grounding electrode required by the foot at the same time.
②Shunting In order to complete the shunting of lightning-induced overcurrent, surge protectors are installed in DC combiner boxes and inverters at all levels, zinc oxide arresters are installed on 10kV busbars and 110kV busbars, and a surge arrester is installed at the neutral point of the main transformer. A set of zinc oxide mines and a parallel discharge pulse.
③Shielding In photovoltaic power plants, solar panels are placed in the open air. Therefore, shielding is mainly a basic measure to reduce the damage of electromagnetic waves for electrical equipment such as inverter booster room, high and low voltage power distribution room, control room, and cables. For example, it is planned to connect the main bars of beams and columns in buildings such as the inverter booster room, 10kV power distribution control room, and the down-conductors of the roof lightning protection belt, etc., and reliably connect with the main grounding grid of the power station at no less than two points to form a closed A good Faraday cage; and good connection of the metal casing of all electrical equipment with the Faraday cage to complete the external shielding measures.
④ Equipotential In the photovoltaic power station, according to the principle of lightning partition, make equipotential connection at the junction of the same lightning protection partition and partition.
Of course, the design content of large-scale centralized grid-connected photovoltaic power plants should also include related civil engineering design, water supply and drainage, fire protection, environmental protection, labor safety and industrial hygiene, etc. related literature.