A few days ago, the “Wechat on photovoltaic power station inverter in Shanxi Province caught fire†in the WeChat circle, which made the industry awkward. Some people pointed out that it was the use of inferior inverters that caused DC arcing and eventually caused internal devices to self-ignite and cause fire. Later, after investigation, this was a false news. Some unscrupulous media had been lavished to cover their influence. The direct cause of the fire was actually a "spherical thunder," and there was no direct relationship with the photovoltaic system.
However, the main part of the photovoltaic power generation system is installed in the open air and has a large area of ​​distribution. Both the components and the brackets are conductors, which are quite attractive for lightning. Therefore, there is a danger of direct and indirect lightning strikes. At the same time, photovoltaic power generation systems have direct connections with related electrical equipment and buildings. Therefore, lightning strikes on photovoltaic systems will also involve related equipment, buildings, and electrical loads. In order to avoid lightning damage to the photovoltaic power generation system, lightning protection and grounding systems need to be provided for protection.
1. About lightning and switching surge knowledge
Lightning is a discharge phenomenon in the atmosphere. In the formation of cloud and rain, some parts of it accumulate positive charges, and the other part accumulates negative charges. When these charges accumulate to a certain degree, discharge phenomena will occur and lightning strikes will form.
Thunder is divided into direct lightning and induced lightning. Direct lightning strikes directly into photovoltaic arrays, DC distribution systems, electrical equipment and wiring, and nearby lightning strikes. There are two intrusive approaches to direct lightning: one is the above-mentioned direct discharge of photovoltaic squares, so that most of the high-energy lightning currents are introduced into buildings or equipment and lines; the other is that lightning can be directly passed through lightning rods, etc. The device that transmits the lightning current into the ground discharges, causing the ground potential to increase instantaneously. A large part of the lightning current is reversed through the protection grounding line into the equipment and the line.
Inductive lightning refers to lightning strikes that occur near or farther than the relevant buildings, equipment, and lines, causing overvoltages in related buildings, equipment, and lines. This surge voltage is induced by electrostatic induction or electromagnetic induction. To related electronic equipment and lines, causing harm to equipment and lines.
In addition to the surge voltage and current that can be generated by thunder and lightning, moments such as the closing and opening of high-power circuits, the moments when inductive loads and capacitive loads are turned on or off, and the disconnection of large-scale power systems or transformers are also generated. Larger switching surge voltages and currents can also cause damage to related equipment and lines.
For large-scale or photovoltaic power generation systems installed in open fields and high mountains, especially lightning-prone areas, lightning protection grounding devices must be provided.
2. Lightning strikes on photovoltaic power generation system
(1) Damage to solar cell modules. Solar panels are the core part of solar power systems and the most valuable part of solar power systems. Its role is to convert the sun's radiant energy into electrical energy, or send it to a battery to store it, or push a load to work. However, its location is extremely vulnerable to the impact of direct lightning strikes with strong pulse currents, hot high temperatures, and violent electric power, causing the entire system to paralyze.
(2) Hazards to Photovoltaic Controllers. The function of the photovoltaic controller is to control the working state of the entire system and play the role of overcharge protection and over-discharge protection of the battery. When the system is subjected to lightning or overvoltage damage, the following occurs.
1 Charging system has been charging, discharge system has no discharge, resulting in the battery has been in a state of charge, charging too light will shorten the battery life, capacity reduction, and then cause the battery to explode, resulting in damage to the entire system and casualties.
2 The charging system is not charged, and the discharge system is always in the discharge state. The battery cannot store the electric energy, which causes the user to work normally when there is sunlight. When the sun is not bright or the light is not strong, the device cannot work.
(3) The hazard to the battery. Solar photovoltaic power generation systems generally use lead-acid batteries. In small micro-systems, nickel-metal hydride batteries, nickel-cadmium batteries, or lithium batteries can also be used. Its role is to store the electric energy emitted by the solar panel when there is light, and release it when needed. When the system is subjected to a lightning strike, an overvoltage incurs damage to the battery, which damages the battery and shortens the service life of the battery, and causes the battery to explode, resulting in serious system failure and casualties.
(4) Harm to the inverter. The direct output voltage of solar energy should be converted into AC 220V/AC 380V to provide electrical energy for the electrical appliances. The DC electrical energy generated by the solar photovoltaic power generation system needs to be converted into AC electrical energy. Therefore, DC-AC inverters are needed. The following will happen if the inverter is damaged.
1 The user's load has no voltage input, and the power equipment cannot work.
2 The inverter cannot reverse the voltage, causing the DC voltage on the solar panel to be directly used by the load. If the solar panel voltage is too high, the power equipment will be directly burned.
3, lightning invade the photovoltaic power generation system
(1) The ground potential counterattack voltage is invaded by the grounding body. When a lightning strikes a lightning rod, a potential distribution in the vicinity of the grounding body of the lightning rod will be generated, and the ground potential of the electronic equipment near it will be counter-attacked, and the intrusion voltage may be as high as tens of thousands of volts.
(2) Invaded by the DC input circuit of the solar array. This invasion is divided into the following two situations.
1 When the solar cell array is struck by direct lightning strikes, the strong lightning voltage will breakdown near the soil or the DC input circuit cable skin, so that the lightning pulse invades the photovoltaic system.
2 When a charged cloud discharges to the ground, the entire photovoltaic array induces a kilovolt of overvoltage as a large number of loop antennas, and is introduced via a DC input line to destroy the photovoltaic system equipment connected to the line.
(3) Invasion from the output power supply line of the photovoltaic system. When a lightning strike is applied to the power supply equipment and the power supply line, the lightning overvoltage appearing on the power line can reach an average of tens of thousands of volts, and the output line is still a major factor in introducing remote lightning. Lightning impulses invade photovoltaic microelectronic devices and systems along power lines, which can cause devastating blows to system equipment.
4. Lightning protection measures and design requirements for photovoltaic power generation systems
(1) The selection of solar photovoltaic power generation system or power station construction address should be avoided as far as possible in places and occasions that are vulnerable to lightning strikes.
(2) Try to avoid the projection of lightning rods falling on the solar array.
(3) According to the site conditions, different protective measures such as lightning rods, lightning protection strips, and lightning protection nets can be used to protect direct lightning strikes, reduce the probability of lightning strikes, and multiple lightning conductors with evenly arranged down conductors should be used to introduce lightning current into the ground. The diversion effect of the multiple down conductors can reduce the lead voltage drop of the down conductor, reduce the risk of side impact, and reduce the strength of the magnetic field generated by the down stream discharge.
(4) To prevent lightning induction, all metal objects of the entire photovoltaic power generation system must be equipotentially connected to the joint grounding body, including the outer frame of the battery assembly, the equipment, the enclosure/cabinet enclosure, and the metal wire tube, and be independently grounded. . Photovoltaic power system equipotential bonding.
(5) Lightning protection devices are installed step by step on the system circuit to implement multi-level protection, so that lightning or switching surge currents are discharged through multi-stage lightning protection devices. In general, the DC power line arrester is used in the DC line portion of the photovoltaic power generation system, and the AC power surge arrester is used in the reversed AC line portion. Arrester applications in solar photovoltaic systems.
(6) The types and requirements of grounding for photovoltaic power generation systems mainly include the following aspects.
1 Lightning grounding. Including the lightning rod (belt), down conductor, grounding body, etc., require the grounding resistance is less than 10 ohms, and it is better to consider setting up the grounding body separately.
2 safety protection grounding, work grounding, shield grounding. Including photovoltaic battery module frame, bracket, controller, inverter, power distribution cabinet enclosure, battery bracket, metal threaded pipe skin and battery, inverter neutral, etc., require grounding resistance is less than or equal to 4 ohms.
3 When the safety grounding, working grounding, shield grounding and lightning protection grounding share a group of grounding devices, the grounding resistance is determined according to the minimum value; if the grounding device is separately provided for lightning protection, the remaining three groundings should be shared A group of grounding devices, the grounding resistance should not exceed the minimum value.
4 When the conditions permit, the lightning protection grounding system should be set up as much as possible, not shared with other grounding systems, and ensure that the distance between the grounding body of the lightning protection grounding system and the utility grounding body is maintained at 3m or more.
5, grounding system material selection
(1) Lightning rods.
Lightning rods generally use round steel with a diameter of 12 to 16 mm. If lightning protection strips are used, round steel with a diameter of 8 mm or flat steel with a thickness of 4 mm is used. The height of the lightning rod above the protected object should be greater than or equal to the horizontal distance between the lightning rod and the object to be protected. The higher the lightning rod is, the higher the protection scope is.
(2) Grounding body.
The grounding body should adopt hot-dip galvanized steel. Its specifications are generally: 50mm diameter steel pipe, wall thickness no less than 3.5mm; 50mm×50mm×5mm angle steel or 40mmx4mm flat steel, the length is generally 1.5～2.5m. The buried depth of the grounding body is more than 0.7m from the ground on the upper end, and the welded parts must be re-prepared for anti-corrosion and rust prevention.
In order to improve the grounding effect, a special non-metallic graphite grounding body module can also be used. This kind of module is a non-metallic material-based grounding body consisting of non-metallic minerals and electrolytic substances with good conductivity and stability. The grounding body overcomes the poor affinity of the metal grounding body in acidic and alkaline soils and prone to rust on the surface of the metal body to change the grounding resistance. When there is too much organic matter in the soil, the surface of the metal body is easily formed to be covered with ink. Causes reduced conductivity and discharge capacity. The grounding body increases the flow area of ​​the grounding body, reduces the contact resistance between the grounding body and the soil, and has a strong moisture-absorbing and moisture-retaining ability, so that the resistivity of the surrounding soil is reduced, the dielectric constant is increased, and the interlayer contact is increased. The reduced resistance and enhanced corrosion resistance result in smaller grounding resistance and longer life. The shape of the grounding body module is square, the size is generally 500mm×400mm×60mm, the lead electrode adopts galvanized flat steel of 90mm×40mm×4mm, and the weight is about 20kg. The grounding body may be buried in 1 to 5 pieces according to the geological soil condition and grounding resistance.
(3) deflected.
Downstream is generally round or flat steel, round steel is preferred, diameter is not less than 8mm; if flat steel, cross-sectional area should not be less than 4mm2; higher requirements to use double-insulated multi-stranded cross-sectional area of ​​35mm2 Copper wire.
(4) special resistance reducing agent.
The special resistance reducing agent for grounding system is a physical long-term anti-corrosion and environmental resistance reducing agent. It is a resin-based symbiont composed of high-molecular water-absorbing material, electronic conductive material, and carbon-based composite material. It is non-toxic, odorless, and non-corrosive. , no pollution, etc., in line with the requirements of the national high-quality soil environmental standards. Its conductivity is not affected by acid, alkali, salt, temperature and other changes, has good moisture absorption, moisturizing, antifreeze ability, will not be lost due to the presence of groundwater, a long-term improvement of the soil resistivity. Using a special resistance-reducing agent in the grounding system can save engineering costs, reduce the soil resistivity, stabilize the grounding resistance, and prolong the service life of the grounding system.
(5) Calculate the amount of grounding module and resistance reducing agent.
According to the soil resistivity of the soil layer, the following formula is used to calculate the amount of the grounding module, the grounding module is buried horizontally, the individual module grounding resistance R=0.068Ï/, and the total grounding resistance Rn=r/(nη) after the parallel connection.
Among them, Ï is the soil resistivity, the unit is ω · m; with a, b as the length and width of the grounding module, the unit is m; r is the grounding resistance of a single module, the unit is ω; rn is the total grounding resistance, the unit is ω; n is the number of grounding modules; η is the module adjustment factor, generally taken from 0.6 to 0.9.
The amount of resistance-reducing agent varies depending on the soil. The thickness to be laid on the grounding body should be between 5 and 15cm. The grounding body should be placed horizontally and used at a rate of about 6kg per 0.5m.
6, arrester selection
Arrester is also called surge protection device (spd). Photovoltaic power generation system commonly used arrester shape. The interior of the arrester is mainly composed of a thermal circuit breaker and a metal oxide varistor, and can also be used in conjunction with a spark discharge gap module as required.
The following is a detailed description of the main technical parameters of commonly used lightning arresters in photovoltaic power generation systems.
(1) Maximum continuous operating voltage (uc): This voltage value indicates the maximum rms AC voltage available at both ends of the arrester. At this voltage, the arrester must be able to work properly without failure. At the same time, the voltage is continuously loaded on the lightning arrester and does not change the operating characteristics of the arrester.
(2) Rated voltage (un): The voltage under normal operation of the arrester. This voltage can be expressed either as a dc voltage or as an rms sinusoidal ac voltage.
(3) Maximum impact flow (umax): The maximum number of current peaks of the specified waveform when the arrester passes through the specified number of times per line or single module without causing substantial damage to the arrester. The maximum impact flow rate is generally greater than 2.5 times the rated discharge current.
(4) Rated discharge current (in): Also called the nominal discharge current, it refers to the current peak value of the 8/20μs lightning current waveform that the arrester can withstand.
(5) Impulse impulse current (iimp): In the waveform current (normal 10/350μs lightning current analog waveform) that simulates direct lightning strikes in nature, the surge arrester can withstand multiple impacts of lightning current without damage.
(6) Residual pressure (ures): When the lightning discharge current passes through the arrester, the voltage value appears between the terminals.
(7) Rated frequency (fn): The normal operating frequency of the arrester.
In the specific selection of lightning arresters, in addition to the technical parameters to meet the design requirements, but also to consider the following several parameters and the choice of features.
(1) Selection of maximum continuous operating voltage (uc).
The maximum continuous operating voltage (uc) of the zinc oxide varistor arrester is a key parameter related to the stability of the arrester operation. When selecting the maximum continuous operating voltage value of the arrester, in addition to meeting the requirements of the relevant standards, the normal fluctuations that may occur in the installation network and the highest possible continuous failure voltage may also be taken into consideration. For example, in a three-phase AC power system, the highest continuous fault voltage of the phase-to-earth line may reach 1.5 times the rated AC working voltage 220V, that is, it may reach 330V. Therefore, in places where the current is unstable, it is recommended to select the module whose maximum continuous operating voltage of the power surge arrester is greater than 330V.
In a DC power system, the ratio of the maximum continuous operating voltage to the normal operating voltage is generally taken from 1.5 to 2 according to experience.
(2) Selection of ures.
When determining the residual voltage of the arrester, the lower the residual pressure, the better the overall pressure is, and the more likely it is misleading. First of all, the residual pressure value marked by different products must indicate the size and waveform of the test current in order to have a common basis for comparison. Generally, the residual voltage recorded under the test current of 20ka (8/20μs) is used as the marked value of the arrester and compared. Secondly, the lower the residual voltage of the varistor arrester, the lower the maximum continuous operating voltage. Therefore, overemphasizing the low residual voltage requires paying the price of reducing the maximum continuous operating voltage. The consequence of this is that in areas with unstable voltage, the arrester is prone to frequent damage due to prolonged overvoltage.
In a varistor type arrester, the most suitable maximum continuous operating voltage value and the most appropriate residual voltage value are selected, just like the two sides of the balance, and neither side can be inclined. According to experience, residual voltages below 2kv (20ka, 8/20μs) can provide sufficient protection for user equipment.
(3) Selection of alarm function.
In order to monitor the operating status of the arrester, when the arrester is damaged, it can notify the user to replace the damaged arrester module in time. Arresters are generally accompanied by various types of damage indication and alarm functions to suit different requirements of different environments.
1 Window color block indication function: This function is suitable for places where people are on duty and inspect every day. The so-called window color block indication function is that each arrester has an indicating window. When the arrester is normal, the window is green; when the arrester is damaged, the window turns red, prompting the user to replace it in time.
2 sound and light signal alarm function: This feature is suitable for use in an attended environment. The acousto-optic signal alarm device is used to check the working status of the lightning protection module and display the status through the acousto-optic signal. The arrester equipped with the acousto-optical alarm device is always in the self-testing state. Once the arrester module is damaged, the control module immediately sends out a high-pitched high-frequency alarm sound, and the status display light on the monitoring module changes from green to flashing red. When the damaged module is replaced, the status indicator lights up in green to indicate that the arrester module is operating normally and the alarm sound is turned off.
3 Remote alarm function: The remote alarm device is mainly used for centralized monitoring of lightning arresters installed in unattended or difficult to check positions. The lightning arrester with remote signaling function is equipped with a monitoring module, and continuously checks the working status of all connected arrester modules. If a lightning arrester module fails, the mechanical device will send an instruction to the monitoring module to make the monitoring module open normally. The normally closed contacts are converted to normally closed and normally open, respectively, and the fault switch information is transmitted to a corresponding display or sound device remotely to trigger the operation of these devices.
4 Remote signaling and voltage monitoring alarm function: In addition to the above functions, the remote signaling and voltage monitoring alarm device can also monitor the voltage applied to the arrester during the operation of the arrester. When the system has any power supply voltage drop or backup protection of the arrester When the circuit breaker (or fuse) action and the arrester module are damaged, the long-distance signal system will immediately record and report. The device is mainly used for three-phase power supply system.
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