introduction
Solar energy is a renewable energy source, with clean and pollution-free, large reserves, renewable and other characteristics, is currently known to mankind as one of the available new energy. The report of the Nineteenth National Congress of the Communist Party of China proposed to promote energy transformation, reduce coal consumption, and unswervingly promote clean development. In 2018, according to the prediction of relevant authoritative institutions, the global new photovoltaic installed capacity is about 106GW, and the new installed capacity in China is about 40GW.
Lightning protection is an indispensable part of the entire photovoltaic power plant, which is related to whether the power station can operate safely and normally and the safety of the power station personnel. As the main choice of new energy in the future, there is no detailed design specification in China to clarify how to carry out the lightning protection design of photovoltaic power plants, so this article mainly summarizes and summarizes the lightning protection methods of photovoltaic power plants combined with work experience and related specifications, and states personal views.
1. The composition of the grid-connected photovoltaic system and the main lightning strike facilities that need to be protected
Photovoltaic power generation systems are mainly divided into 3 categories: independent, distributed and grid-connected photovoltaic power generation systems. This paper mainly discusses the grid-connected power generation system. The working principle of the grid-connected photovoltaic power generation system is to convert solar energy into DC power by the ground light volt square array, which is transported to the DC distribution cabinet after the confluence box is combined, and then transported to the grid-connected inverter, and the inverter converts the DC into ALTERNating current, transmits it to the AC distribution cabinet, and after being boosted, it is sent to the power grid.
Grid-connected photovoltaic power station is mainly composed of photovoltaic array, bus box, DC transmission cable, DC distribution cabinet, grid-connected inverter, AC distribution cabinet, boost transformer, transmission line and metering, control, communication and other equipment. Battery packs, battery controllers, and solar tracking control systems are not considered in this article.
The harm of lightning to grid-connected photovoltaic power plants is mainly divided into three types, namely direct lightning, lightning surge intrusion and lightning strike electromagnetic pulse in the equipment room.
Direct lightning: When the lightning strikes on the ground light volt square, the equipment room or the sending line, through the electric shock generated by its discharge to the ground, due to the high temperature thermal effect of the lightning current, on the channel through which the lightning current flows, the object moisture is heated and vaporized and violently expanded, producing a strong impact mechanical force, which can make the human tissue, building structure, equipment components, etc. fractured and broken, resulting in casualties, building damage, and equipment destruction.
Lightning surge intrusion: when lightning strikes on the output line, DC cable line or metal pipe, the lightning current invades the room along the cable line or metal pipe, or when the lightning strike is near the cable line or metal pipe, due to the large gradient of the lightning current change, a strong alternating magnetic field is generated, so that the metal conductor near the lightning strike point generates an induced current, and the induced current enters the equipment room along the metal pipeline, damaging the equipment or endangering personal safety.
Lightning electromagnetic pulse in the equipment room: When lightning strikes a building or nearby, the rapidly changing electromagnetic field generated when the lightning current enters the building’s anti-direct lightning system, and it will act on all the electronic equipment around it. The change of the electric field caused by this magnetic field change may lead to the failure of electronic and electrical equipment of the internal communication and monitoring system of the computer room.
In summary, the components of the lightning protection measures required for grid-connected photovoltaic power stations are: ground light volt square array, DC transmission lines, metal pipelines, transmission lines, building machine rooms and equipment cabinets (including DC distribution cabinets, grid-connected inverters, AC distribution cabinets, step-up transformers and metering, control, communication equipment, etc.).
2. Lightning protection scheme of photovoltaic power station of grid-connected system
2.1 Lightning protection category determination
The photovoltaic square array of the photovoltaic power station belongs to the open field, which is generally distributed in the open field, and the branch area is large, and the probability of being hit by direct lightning strikes will naturally increase correspondingly. Lightning protection category reference to the “Building Lightning Protection Design Code” on grain, cotton and flammables a large number of concentrated open yards in accordance with the judgment of three types of lightning protection buildings, although the photovoltaic phalanx is not a flammable material, but the investment is huge, if the lightning strike causes personnel injury or equipment damage, its loss is also huge. Although there is a low fire hazard in the photovoltaic phalanx, it is very necessary to design the lightning protection of the light volt phalanx from the perspective of reducing economic losses. Considering various factors, it is recommended to consider three types of lightning protection buildings.
2.2 Direct lightning protection
Direct lightning protection is mainly divided into 3 parts, equipment room, ground light volt square and transmission line.
1) Equipment room direct lightning protection measures
The equipment machine room direct lightning protection should be considered in accordance with the three types of lightning protection buildings, using φ12mm galvanized round steel as a flash connector, laid in the roof corners, eaves and eaves and other vulnerable parts of the lightning, and in the entire roof to form a flash connection network of not less than 20m × 20m. The use of 2 φ≥ 16mm main ribs in the concrete column of the building as the lead line, the spacing is not more than 25m, the upper end of the lead line is connected with the flash belt, and the lower end is connected to the foundation grounding net.
2) Ground photovoltaic phalanx direct lightning protection measures
Photovoltaic components of the photovoltaic array can use its metal frame as a flasher, metal bracket as the grounding wire, the field station should be used by vertical direct ground electrode and horizontal ground electrode together composed of artificial grounding network, the use of ∟ 50mm×50mm×5mm angle steel as a vertical direct pole, the burial depth in the soil should not be less than 0.5m, the use of 40mm ×4mm flat steel as the horizontal grounding electrode, trenching and burial, the spacing of the vertical direct body should be greater than twice the burial depth and uniformly arranged. Photovoltaic modules fixed on the same metal frame should have a metal frame junction greater than 2 points, and the distance between each contact point should not be greater than 25m. The lightning protection grounding of the photovoltaic array should share the same grounding device with its protective grounding, system grounding and grounding system of the equipment room; The grounding resistance of the common grounding device should meet the minimum requirements of each grounding system, generally not more than 1Ω.
3) Send out the line direct lightning protection measures
The sending line should be equipped with lightning protection wires to reduce the harm caused by direct lightning.
2.3 Requirements for lightning surge protection
Lightning surge intrusion is mainly for the protection of outgoing and incoming metal pipelines, including DC input cables, metal pipes, combiner boxes, and transmission lines.
1) Measures to prevent lightning surge intrusion of HVDC transmission lines and metal pipelines
In order to reduce the lightning current or inrush current through the DC transmission line cable into the machine room, the DC transmission cable should be shielded cable, and its shielding layer should be set. Metal pipes are at least at the beginning, end, branch, and junction of the mine protection zone to do equipotential connections, so as to avoid lightning current or inrush current entering the machine room through the DC transmission line, causing damage to the equipment cabinet.
2) Combiner box anti-lightning surge intrusion measures
A DC surge protector should be installed in the combiner box, that is, installed between the positive pole to ground, the negative pole to the ground, and the positive pole to the negative pole at the output of the combiner box, and the main technical parameters of the DC surge protector should be satisfied: Rated inrush current Iimp≥12. 5kA(10/350μs)。 When the photovoltaic phalanx in the station or nearby ground is struck by lightning, the lightning current or inrush current formed in the transmission line will be discharged through the surge protector in the bus box. In addition, according to the actual number of HVDC transmission lines, if it is necessary to further reduce the inrush current, a surge protector with matching energy can be added to the multi-stage busbox to achieve the purpose.
It is worth noting that the DC power surge protector installed at the combiner box should meet the application characteristics of the optical volt system, and the maximum continuous working voltage should be greater than or equal to the highest open circuit voltage of the photovoltaic module.
3) Send out the line to prevent lightning surge intrusion measures
The anti-lightning surge intrusion measures of the sending line are mainly to install zinc oxide lightning arresters on the high-voltage side of the transformer, and use the good nonlinear volt-ampere characteristics of zinc oxide lightning arresters to direct the inrush current on the line into the earth.
2.4 Lightning strike electromagnetic pulse protection measures
The protection of lightning strike electromagnetic pulse is mainly aimed at the equipment in the machine room, which is divided into shielding, isopotential and integrated wiring: the equipotential connection end should be set up at the line exit and into the home, and all the in and out metal pipes or line shielding layers in the machine room should be equal potential connection at the junction of the mine protection zone. At the same time, the equipment cabinet, metal bridge, metal pipe, shielded wire and cable metal outer layer, equipment safety protection grounding, surge protector grounding terminal, etc. in the machine room should be connected to the grounding reference point of the S-type structure or the grid of the M-type structure in the shortest path. The equipotential connection network of the computer room and the grounding body of the photovoltaic square array should maintain the same potential to prevent the counterattack caused by the potential difference.
According to the working voltage requirements of the DC distribution cabinet, an appropriate DC surge protector should be installed, and the input terminal of each DC of the inverter can choose whether to install a surge protector according to the actual situation, so as to reduce the residual voltage generated by the lightning current and make it smaller than the voltage tolerated by the equipment. The low-voltage side of the booster transformer should be equipped with a Class I test (rated discharge inrush current Iimp≥12. 5kA) surge protector, high pressure side mounted zinc oxide lightning arrester. In addition, when arranging the direction of the signal cable in the machine room, the area of the induction loop formed by the cable itself should be minimized as much as possible (Figure 1). The selection of the surge protector of the signal line should be based on the operating frequency of the line, the characteristic impedance, the transmission medium, the working voltage, the interface form and other parameters, and the surge protector with small voltage insertion loss, small partial capacitance, and adapted to the longitudinal balance and proximal crosstalk index, and set at the interface of the mine protection zone.
3. ACCRI APV lightning protection photovoltaic combiner box
In a large photovoltaic power generation system, a large number of photovoltaic cell modules are required to string and combine to achieve the required voltage and current value to maximize the power generation efficiency. APV series intelligent photovoltaic combiner box in the provision of combined lightning protection function at the same time, but also to provide photocell current measurement, monitoring of the operation status of the panel in the photocell array, battery voltage, total power of the combiner, branch power, combiner box lightning protector status acquisition, DC circuit breaker state acquisition, relay contact output, with wind speed, temperature, irradiator and other sensor interface functions for customers to choose, the device is equipped with RS485 interface, can measure and collect the power data and equipment status uploaded to the monitoring system.
