The long-term reliability of photovoltaic power plant products directly affects the income of investors. During a period of up to 25 years of investment return, it is an eternal theme to increase power generation efficiency and reduce product failure. At present, the long-term reliability of the quality of photovoltaic power plants in China is of great potential. The reliability of inverters has become a hot topic for everyone.
At the Intersolar Forum in Munich in 2014, senior photovoltaic man, Manfred Bachler (formerly the chief technology officer of Phoenixsol, the world’s largest EPC company) proposed a solution for retrofitting existing centralized inverters with string inverters. The conclusion is that the cost of retrofitting can be recovered in 5 to 6 years. The main reason is that the centralized inverter is inconvenient to maintain and has poor usability. It is only 6% worse than the string inverter in terms of availability.
Recently, there are many opinions about the failure rate and reliability of string inverters and centralized inverters in the industry. So who is more reliable and who is stronger? This article analyzes in detail from the following angles, and initiates jade.
1. Inverter failure rate: Centralized type is higher than the string type, which is 3 times
The industry-famous String Inverter is designed according to 25 years of reliable system operation and adopts IP65 protection class to achieve internal and external environmental isolation, so that the internal devices are kept in a dust-free and stable operating environment, greatly reducing the temperature and sand The influence of the external environment such as moisture, salt spray, etc. on the lifetime of the device is greatly enhanced. At the same time, the system has no consumable parts, no fuses, no fans, etc., need to replace the devices regularly, and the system is maintenance-free; the failure rate of the whole machine is less than 1%.
The centralized solution generally uses conventional civil or containerized equipment, and the protection class is IP54. However, because the computer room generally adopts a direct-ventilation cooling scheme, it can only achieve IP44 or lower, and it cannot prevent the ingress of sand, dust, and corrosive gases. Changer. The centralized inverter protection class is only IP20, and internal circuit devices are easily exposed to harsh working conditions.
For example, the accumulation of dust in the inverter circuit board, terminal block, etc. will cause a decrease in the creepage distance, eventually causing safety risks such as discharge, fire; wet dust in the PCB or components easily form a leakage effect and corrosion effects, causing the signal Abnormal or high voltage arcing may cause a short circuit in the power grid, causing the inverter to abnormally shut down or blow up. At the same time, the inverter fan is a consumable part and the average service life is about 5 years.
The failure rate of centralized inverters in the industry generally exceeds 3%, and the failure rate is more than 3 times that of string inverters. Many power stations in the north are built on the Gobi Desert. Actually, they are typical saline-alkali lands. The dust contains a large amount of salt ions. In day and night intermittent operation mode, the probability of corrosion and leakage occurs much greater than that of conventional electrical equipment.
Figure 1 Dust in the inverter room Figure 2 Dust inside the inverter device
In addition, compared to the auxiliary power supply of the inverter, the failure efficiency in the centralized auxiliary power supply market is generally between 2% and 5%. The main reason for the high failure rate is direct ventilation and heat dissipation. Dust and salt fog will affect reliability. influential. The auxiliary power supply of the string inverter is a self-contained circuit and operates in a completely enclosed environment, and the failure probability is extremely low.
From the history of inverters, the first inverter introduced by SMA in 1995 was a string inverter. It was only after 7 years that the first centralized inverter was introduced in 2002. Outdoor string inverters have undergone practical application tests in the past 20 years. In Europe, the history of large-scale ground power stations is longer than that of centralized inverters, and the technology is more mature.
2. The effect of heat dissipation on reliability: The centralized type is larger than the string type
The string inverter consumes several hundred watts of power, can realize natural heat dissipation design, and can meet the reliable operation at 60°C ambient temperature. The centralized inverter, especially the 10 foot container solution, has a 1MW loss of up to 30 kilowatts. Equivalent to having 30 1 kilowatt electric wire baked in a 10 foot container. It is difficult to achieve reliable heat dissipation by only a few fans, which greatly increases the risk of burning.
According to actual measurement, in the western ground power station, the string inverters are hung under the components, and the openings are well ventilated. The internal temperature is 43.8°C when the ambient temperature is 21°C, the temperature rises 22.8°C, and the centralized temperature is 23°C. Temperature up to 53 °C, temperature rise 30 °C. In the hot summer, it is often the case that the temperature in the equipment room exceeds 50 degrees, and the temperature of the centralized inverter cabinet is higher and worse. The high temperature will bring about a reduction in the life of the device, derating, and even hidden dangers.
In areas with severe wind and sand in northwestern China, dust accumulation in the inverter room leads to blockage of the air filter and radiator heat dissipation performance deteriorates. The temperature of high power devices rises sharply, causing temperature alarms, and more serious damage to the internal IGBT devices. The equipment is continuously restarted. Even burned. Since the first half of this year, there have been serious accidents such as explosions in centralized solutions due to heat problems almost every month.
3. System availability and its impact on power generation: Centralized losses are 17 times larger
The top string inverters used in Europe can achieve an MTBF of 350,000 hours and an annual failure rate of less than 1%. Some manufacturers can achieve less than 0.5%, while centralized conventional MTBF can only do To 50,000 to 100,000 hours.
The centralized type generally requires the professionals of the factory to locate and maintain the site, and it takes a long time to process, affecting the power generation. The string inverter can be directly replaced by the on-site operation and maintenance personnel. It is simple and quick to ensure the power generation, and then the Fault analysis and maintenance. When any inverter in the string fails, the other inverters can work without affecting other power generation. If any system equipment in a centralized solution fails, the entire array may stop working. The repair time depends on the location of the site and the availability of spare parts. It usually takes at least 2-3 days, some as long as 1 week, or even In one month, the loss of power generation will far exceed the expected value of failure losses.
Taking 10MW power plant as an example, according to the average daily peak hours of sunshine of 6 hours, the average annual power loss of the centralized scheme will reach 7154KWH, while the string type only has 420KWH. In contrast, the centralized loss is a string of 17 times.
4. String maintenance costs are much lower than centralized
Maintenance of centralized inverters must be carried out by professional maintenance personnel. High technical requirements are required. Even if the components are damaged, professional operation and maintenance personnel are required to go to the site to locate and replace the faults. In addition to component replacement costs, labor costs and travel expenses are all It is a high maintenance cost. In addition, if the centralized inverter is completely damaged, the replacement is extremely troublesome. Heavy machinery and equipment must be introduced into the market, and it is very difficult to remove the inverter from the engine room.
Affected by the software and hardware matching, the centralized inverter must use spare parts from the original manufacturer. Once the manufacturer closes, future maintenance will be interrupted and the owner will only replace the new product. The string-type only need to replace the entire machine, no professional maintenance personnel, 30 minutes can be replaced. During the five-year maintenance period, free replacement machines with out-of-warranty failures can use modern logistics platforms to return to the factory for maintenance at a low cost, requiring only a small amount of maintenance costs. The maintenance cost is far lower than the centralized large-scale solution.
At present, inverter brands in the industry are mixed. Recently, it has been said that the string inverters need to be replaced after 5 years. If there is no breakthrough in the design of the string inverter, as with the centralized inverter, fan cooling and fuses are also required, and the protection level is low. After 5 years, as with the centralized type, a large number of replacements are required. But if you choose IP65, natural heat dissipation and no fuse design brand, these problems will not exist.
The string inverter selection must fully consider its quality. The preferred quality is reliable, the quality is superior, the power plant can be well maintained for a long period of time, and there are large-scale inverter manufacturers. The world's top string inverters have an annual failure rate of less than 0.5%, which is far below the centralized scheme. Some serial brand brands have closed down or have not existed in a few years, and it is also possible to replace another string inverter of other brands. The replacement cost is low, and the impact on the overall power generation of the system is minimal.
In summary, the string inverter is more dominant than the centralized one, both in terms of failure rate and operation and maintenance cost, and can ensure long-term and reliable operation of the power plant. During the development of inverters over the past 10 years, Europe’s technology has been leading the global market for 2-3 years. It is a benchmark for technology and the market. Markets such as China, the United States, and Japan are basically following the European route while absorbing Europe’s experiences and lessons.
Nowadays, technology manufacturers represented by SMA are pushing for string solutions in the United States and Europe. More and more photovoltaic power plants in China are also adopting string-type solutions. It can be seen that the string type is becoming the mainstream scheme for the construction of photovoltaic power plants. Of course, when selecting a string of equipment manufacturers, we should also choose a product with reliable quality and good reputation so that our owners can truly rest assured for 25 years!
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