[Photovoltaic Science] "Photovoltaic connector damage and burnout" ranks 2nd in the list of TOP20 technical failures in power plant operation and maintenance

According to statistics from the National Energy Administration, as of the first nine months of 2018, China's new grid-connected photovoltaic installed capacity was 34.5GW, and the cumulative grid-connected installed capacity was 165GW. The "531" PV New Deal has made China's PV, which has been running for several years, press the deceleration button, and the scale of incremental growth has slowed down. The operation and maintenance of 150 million kilowatts of stock power plants has attracted much attention. For the holders of photovoltaic power plants, pursuing the maximum power generation efficiency of the power station and achieving the expected income of the power station is the most core goal. Under the realistic background of limited space for cost reduction of photovoltaic equipment and increasing difficulty in efficiency improvement, through refined operation and maintenance Demanding revenue from power stations has become the fastest way for photovoltaic systems to continuously reduce costs and increase efficiency. The prerequisite for achieving the expected benefits is the safe and stable operation of photovoltaic assets. In recent years, due to construction quality defects, improper equipment selection, and irregular operation and maintenance management, there have been frequent cases of casualties and major property losses. Therefore, safety assurance is the basis and key of operation and maintenance work, and it needs to be preventive and periodic. To ensure the safe, stable, and efficient operation of the entire system by scientifically and rationally managing the power station in its operating life.

1. Fire hazards and fire prevention measures

1.1 Hot spots on components

The hot spot phenomenon is a typical fault of the photovoltaic power generation system. As shown in Figure 1, it will not only reduce the power generation efficiency of the system, but also easily cause safety problems. Causes resistance to increase, causing components to heat up. Severe hot spots at high temperatures can cause permanent damage to the battery, such as partial burnout of the battery, melting of the solder joints, destruction of the grid line, and aging of the packaging material. Local overheating may also cause the glass to break, the back plate burns through, and even the components ignite spontaneously and cause fire. Spotting has always been the most concerned issue in the operation and maintenance of power stations.

The leakage of the battery slice will affect the normal operation of the battery, which acts as a load and consumes the power of other normal batteries; the reliability of the diode is poor and cannot be bypassed in time. In addition, other factors of the module, such as external shielding and poor flame retardancy of the module material, may cause a fire in the photovoltaic power station.

Preventive measures: a handheld infrared camera or drone can be used to scan the modules in the photovoltaic area regularly, and the quality of the modules can be evaluated through the test results. As there is no uniform standard for the temperature range of the hot spot, some companies use summer and Non-summer is defined. For example, in summer, when the ambient temperature is greater than 35°C, the temperature of the glass surface of the module is greater than 90°C. In non-summer periods, the temperature range of the glass surface is greater than or equal to 20°C as the criterion for judging hot spots. Since the normal operating temperature range of the components is generally -40～+85℃, it is recommended to replace the components when the temperature continues to exceed 85℃.

1.2 Failure of component junction box

The junction box accepts the output energy from the components and is the carrier and intermediate. The diode breakdown in the junction box, the short circuit or failure of the diode, the virtual connection of the positive and negative poles of the components, and the cracking of the junction box are common problems. If the junction box The air tightness is relatively poor, air and water vapor are easy to invade, the resistance of internal components increases, accelerates corrosion, and eventually burns. The heat of the junction box may be due to poor contact or failure of the diode, and it also lays down a safety hazard. There is a reverse current inside the diode. When the temperature rises by 10°C, the reverse current will double. The reverse current will reduce the current generated by the component and affect the power of the component. Therefore, the junction box must have excellent heat dissipation or a special heat dissipation design. In the daily operation and maintenance work, it is recommended to use an infrared thermal imaging camera to check the heating problem of the junction box on a regular basis, and deal with it as soon as it is found.

1.3 Photovoltaic connector failure

Photovoltaic connectors account for less than 0.5% of the initial investment cost of the system, but they are a key component of the photovoltaic system. It ensures that the power generated by the system can be stably transmitted from the components to the inverter and the user side. The EU Horizon 2020 "Solar Bankability" project team gave a list of TOP20 technical failures in the operation and maintenance of power stations based on the on-site operation and maintenance data of 746 power stations. "Burn" is ranked 2nd in the failure list.

The failure of the connector is mainly due to its own quality, such as the contact resistance after the male and female connectors are mated. A high-quality connector must have a very low contact resistance and be able to maintain a low contact resistance for a long time. According to the international standard EN50521 for photovoltaic connectors, the contact resistance must be less than or equal to 5mΩ. The second reason for the failure is that connectors between different brands are plugged into each other. Due to differences in technology and product materials, differences in production processes and quality standards, inconsistent tolerances, and different raw materials, 100% matching cannot be guaranteed. If they are forcibly inserted into each other, it will cause problems such as temperature rise, increased contact resistance, and the inability to guarantee the IP rating, which will seriously affect the power generation efficiency and safety of the power station. Both TÜV and UL have issued written statements that they do not support the application of connectors from different manufacturers. If you must use it, it is best to do a matching test in advance.

Figure 2 shows the use of an infrared thermal imaging camera in a power station to perform thermal imaging of the temperature of good and poor-quality photovoltaic connectors. Obviously, the temperature of poor-quality photovoltaic connectors is higher when they work [1]. Therefore, when selecting connectors, choose high-quality connectors.

1.4 DC arc

According to the investigation of a third-party agency, more than 40% of the fire accidents of photovoltaic power plants are caused by DC arc faults. International standards such as NEC 690.11 require that the voltage between any two live parts in the photovoltaic system exceeds 80V, and DC arc detection and Protection ability.

An electric arc is generated between the high voltage between the positive and negative polarities of the string. When the string is not generating electricity, the voltage appears as an open circuit voltage, which can generally be as high as 700V or more. The photovoltaic cell system is a DC power generation device, and the arc fault caused by it is called "DC fault arc". The biggest difference between this and the general AC fault arc is that there is no idle cycle phenomenon caused by phase change. In other words, once a DC fault arc occurs, the high heat phenomenon will continue until the power source disappears. Because the high heat generated by the fault arc can reach 1,000 degrees Celsius, it will not only cause the surrounding insulating materials to decompose or carbonize and lose the insulation effect, but also easily cause the neighboring materials to reach the ignition point and be ignited, as shown in Figure 4 on the left. The power station combiner box caught fire and burned. Observed from the burned photos, it is mainly concentrated on the input and output terminals of the circuit breaker. The temperature on the terminals is the highest, while the fuse and photovoltaic cable have no burning traces. This can eliminate the fuse caused by poor contact or short circuit of the cable. The phenomenon of arcing on the device. The right side of Figure 4 shows that the wiring between the string and the combiner box is not strong, and there is a virtual connection phenomenon. During operation, poor contact causes current to draw arcs. The high temperature melts the fuse holder and causes a short circuit, which burns the combiner box.

1.5 Other reasons

Generally speaking: DC arc, natural wildfire, man-made open flame sources may all cause fires.

Figure 5 shows a fire in a photovoltaic power station in a mountainous area. There is a certain amount of vegetation on the ground, and there are too many weeds in the photovoltaic area. Due to the noise of dry weather in winter, the vegetation is flammable, and a fire can occur if there is an open flame. In addition, there was no effective protection and isolation in and outside the area, which caused the fire to spread to the photovoltaic area.

Precautions:

1) Fully monitor every corner of the photovoltaic plant.

2) According to the requirements in "DL 5027-2015 Typical Fire Protection Regulations for Electric Power Equipment", check the equipment and safety conditions of the fire protection facilities in the station to ensure that they are complete and available.

3) It is necessary to do a good job of emergency response measures for sudden fires, organize employees to conduct a practical drill, and improve their ability to respond to fires and fire rescue capabilities.

4) Strengthen the fire-proof sealing of cable trenches, shafts, and bridges through walls. It is forbidden to store flammable, combustible, and combustion-supporting materials in the cable trenches, and the debris must be cleaned up.

5) Customs such as winter wind and heavy grass drying, mountain burning, sacrificial offerings, etc. are very easy to cause fires, requiring plant vegetation to be cleaned up. Power stations with the conditions should be separated from the outside area by fire protection; key fire prevention parts should be suspended at obvious locations for smoking and banning fires. warning sign. Do a good job in safety education, supervision and management of outsiders.

6) The power station shall carry out special patrols and special patrols during special seasons and time periods to maintain fire alert status at all times, and promptly handle and deal with fires found.

2. Hidden dangers of natural disasters and preventive measures

2.1 Flood

The source of the impact of floods on photovoltaic power plants is that they were not designed in accordance with the correct specifications and standards at the beginning of the design of the power plant. In the design and construction of photovoltaic power stations, structural flood control and artificial flood control are usually used to deal with flood disasters. Structural flood control is usually embodied in the design stage, and the design refers to the flood assessment impact report.

For photovoltaic power plants in different regions, flood control standards should be treated differently. There are large differences in flood control levels and standards in different geographical locations. For areas adjacent to rivers and seas, a safety superelevation of +0.5 m is usually used, and for mountainous areas, additional safety is required. Consider the impact of mountain torrents to add pile foundation elevation.

At present, for most projects, the design unit usually considers the highest waterlogging level once in 50 years as a reference standard to increase the super height. The actual situation will be adjusted according to the geographical location and flood control standard design.

【Precautions】

Careful site selection: In terms of hydrological conditions, the maximum short-term precipitation, depth of water accumulation, flood level, drainage conditions, etc. should be considered in many ways. The above factors will directly affect the photovoltaic system's support system, the design of the support foundation and the installation height of electrical equipment. If the depth of stagnant water is high, the installation height of components and other electrical equipment will be high. The flood level affects the safety of the support foundation. Poor drainage conditions will cause the foundation and even the metal support to be immersed for a long time.

Add protection systems such as drainage systems: The main impact of heavy rainfall on the power station is rainwater immersion. Surface power stations, fishing and light complementary power stations, and surface power stations should be equipped with corresponding drainage facilities according to the local meteorological and hydrological conditions, or add them before the arrival of heavy rainfall. Temporary drainage facilities.

In the places where facilities and equipment such as booster stations, power distribution rooms, water pump rooms, box transformers, and cable trenches are prone to flooding, measures should be taken in advance to remove blockages.

For small area or within the power station's processing capacity, the power station should actively organize backfilling to prevent further expansion of hidden dangers after the power station is discovered.

Formulate emergency plans: The first person responsible for the safety production of the power station shall, based on the characteristics of the unit's flood prevention work, formulate reasonable flood prevention measures and emergency plans, and conduct appropriate emergency handling and safety assessments for matters that may cause the loss of power station equipment and affect the safety of the power station; Effectively prevent and reduce the adverse effects of natural disasters such as landslides, mudslides, landslides, etc. caused by heavy rainfall, as well as the collapse of power station equipment foundations, road washouts, etc., pay close attention to weather forecasts and change processes, and be adequate for rain, water, and flood conditions The sensitivity of the equipment should be grasped in time, and emergency supplies (such as sandbags, drainage pumps, power supply panels, etc.) should be prepared in advance. Once the equipment is out of power, the cause should be found as soon as possible and the equipment operation should be restored as soon as possible.

2.2 Storm

In 2017, a photovoltaic power station was hit by strong winds and sandstorms, and the strong winds did not stop until 16:00. During the period, the average wind speed was 16m/s, the instantaneous wind speed reached 30m/s, and the instantaneous wind reached level 9, and there was a sandstorm for about half an hour. The minimum visibility was 299 meters. The strong wind lasted for nearly 6 hours. After the storm, the photovoltaic modules of multiple mounting units Was blown off.

Precautions:

1) Improve the strength design requirements of photovoltaic supports, module compacts, etc., and reasonably select module inclination with better wind resistance. For places with high wind resistance, the strength design of the bracket should be strengthened, and a high-strength galvanized steel bracket should be used as much as possible.

2) In order to minimize the losses caused by storms, it is necessary to "prevent first and focus on management", establish emergency plans for typhoon prevention during construction and operation, and establish a procedural, standardized, and institutionalized response mechanism for typhoon prevention. take effective action.

3) Carry out sampling inspections on the photovoltaic area module press blocks and their bolts and bracket column fasteners, especially before the arrival of spring and autumn strong winds, formulate a special sampling inspection plan, and determine the bolts according to the implementation of bolt rolling maintenance and the actual loosening during daily inspections. Sampling inspection of the tightening ratio, judge and evaluate the tightening degree of the bolts, to ensure that the components are not damaged during the high wind. Since manual operation differs depending on individual strength and tightness, professional installation tools, such as electric torque wrenches, must be used during construction.

4) In daily operation and maintenance, strengthen the array inspection and tighten loose clamps in time.

3. Photovoltaic insurance

PICC Property & Casualty Insurance has launched various forms of photovoltaic insurance according to the distribution of the industry chain, including 25-year power protection insurance for module manufacturers, professional liability insurance for power station builders, property insurance and machinery damage insurance for completed power stations , Profit loss insurance, public liability insurance, and compensation insurance for loss of power generation revenue after the power station is connected to the grid.

The scope of protection for property all risks: underwriting direct property losses of photovoltaic power plants caused by natural disasters, accidents, etc.

1) Natural disasters: refers to lightning strikes, rainstorms, floods, storms, tornadoes, hail, typhoons, hurricanes, sandstorms, blizzards, ice, earthquakes, sudden landslides, avalanches, mudslides, sudden ground subsidence and other irresistible manpower Destructive natural phenomenon.

2) Accidents: refer to unexpected events that are unpredictable, beyond the control of the insured, and cause material losses, such as fires and explosions.

Indirect loss insurance: Operational interruption and additional cost insurance, covering indirect losses caused by natural disasters, accidents, etc. that cause the interruption of photovoltaic power plant operation, including the reduction of expected power generation and subsidies, and the increase in fixed costs.

The power station can choose appropriate insurance according to the actual situation to reduce property losses as much as possible.

4. Summary

Safety assurance is an important task in the operation and maintenance of distributed photovoltaic systems. This article explains in detail the safety risks that may occur in the operation and maintenance of photovoltaic systems, including hidden fire hazards, hidden hazards of natural disasters, etc., photovoltaic power generation as a rapidly developing In the industry, loopholes are prone to occur in various links such as design, construction, and equipment quality control. Operation and maintenance should be multi-pronged, strengthen management, and prevent problems before they occur, so as to reduce safety risks at work.