Reliability analysis of string and centralized inverters - SHIELDEN

Reliability analysis of string and centralized inverters

The long-term reliability of photovoltaic power plant products directly affects the investor's income, in the 25-year-long investment return period, improve power generation efficiency, reduce product failure is the eternal theme. At present, the long-term reliability of domestic PV power plant quality hidden problems, including solar inverter reliability has become a hotspot for everyone's attention.

In 2014, at the intersolar forum in Munich, Manfred Bachler, a senior PV practitioner (once the world's largest EPC manufacturer Phoenixsolar's CTO), put forward a string inverter to transform the existing centralized inverter program, and gave the conclusion that the transformation cost can be recovered in 5-6 years, the main reason is that The main reason is that centralized inverters are troublesome to maintain and have poor availability, which is 6% worse than string inverters in terms of availability alone.

Recently, the industry has been divided on the failure rate and reliability of string and centralized inverters. So who is more reliable and who is stronger? This paper analyzes in detail from the following perspectives, to attract jade.

1. Inverter failure rate: centralized higher than string type, 3 times as much

Industry-renowned string inverters are designed to operate reliably in accordance with the 25-year system, adopting IP65 protection level to realize internal and external environmental isolation, so that the internal devices are kept in a dust-free and stable operating environment, which greatly reduces the impact of temperature, wind and sand, humidity, salt spray, and other external environments on the life of the devices, and therefore greatly enhances the reliability. At the same time, the system has no wearing parts, no fuses, no fans and other devices that need to be replaced periodically, realizing a maintenance-free system; machine failure rate of less than 1%.

The centralized solution generally adopts the conventional civil engineering room or container type machine room with IP54 protection level, but because the machine room generally adopts the direct ventilated heat dissipation program, the actual can only reach IP44 or lower, and can't block the wind and sand, dust and corrosive gases from entering the inverter. The centralized inverter has only IP20 protection level, and the internal circuit devices are easily exposed to the harsh working environment.

For example, the accumulation of dust in the inverter circuit boards, terminal rows, etc. will cause a reduction in creepage distance, ultimately resulting in discharge, fire and other safety risks; wet dust in the PCB or components are easy to form a leakage effect and corrosion effect, resulting in signal anomalies or high-voltage arcing and firing, and may cause a short circuit of the grid to the PE, resulting in abnormal shutdown of the inverter or blow up. Meanwhile, the inverter fan is a fragile part with an average service life of about 5 years.

The failure rate of centralized inverters in the industry is generally more than 3%, and the failure rate is more than 3 times of string inverters. Many power stations in the north are built on the Gobi Beach, which is actually a typical saline land with a lot of salt ion content in the dust. The probability of corrosion and leakage is much higher than conventional electrical equipment in the day and night intermittent working mode.

In addition from the inverter auxiliary power supply comparison, centralized auxiliary power supply market failure rate is generally between 2~5%, the main reason for its high failure rate is also straight ventilation and heat dissipation, dust, salt spray, etc. will have an impact on the reliability. The string inverter auxiliary power supply is a self-contained circuit, working in a fully enclosed environment, the probability of failure is extremely low.

From the inverter history, SMA launched the first inverter in 1995 is string inverter, after 7 years, only in 2002 launched the first centralized inverter. Outdoor string inverters have experienced nearly 20 years of practical applications, and have been used in large-scale ground power stations in Europe for a longer period of time than centralized inverters, and the technology is also more mature.

2. Impact of heat dissipation on reliability: centralized type is larger than string type

String inverters, with hundreds of watts of power consumption, can realize natural heat dissipation design, and can meet the 60℃ ambient temperature for reliable operation. Centralized inverters, especially the 10-foot container program, 1MW loss of up to 30kW, equivalent to 30 1kW electric stove wire baking inside the 10-foot container, relying on a few fans is very difficult to achieve reliable heat dissipation, which greatly increases the risk of burner.

According to the actual measurement, the western ground power station, string inverter hanging in the component below, open ventilation is good, the ambient temperature of 21 ℃ when the internal temperature of 43.8 ℃, the temperature rise of 22.8 ℃, while the centralized room temperature of 23 ℃, the cabinet temperature up to 53 ℃, the temperature rise of 30 ℃. In the hot summer, the temperature inside the room is more than 50 degrees is often the case, centralized inverter cabinet body temperature is higher and worse. The high temperature will bring the potential danger of device life reduction, downgrade or even blowing up the machine.

In areas with serious sandstorms, the accumulation of dust in the inverter room leads to the blockage of dust nets, poor heat dissipation performance of heat sinks, a sharp rise in the temperature of large power consumption devices, causing temperature alarms, and even more serious damage to the internal IGBT devices, the equipment is constantly restarted, and even burned out. Since the first half of this year, almost every month because of heat dissipation problems, centralized solutions have blown up and other serious accidents.

3. System availability and its impact on power generation: centralized loss is 17 times that of string type

The top string inverters applied in Europe can achieve MTBF of 350,000 hours, with annual failure rate below 1%, and some manufacturers can achieve less than 0.5%, while the MTBF of centralized system can only achieve 50,000 to 100,000 hours.

Centralized inverters generally require professional personnel from manufacturers to arrive at the site to locate and repair, which takes a long time to process and affects power generation; while string inverters can be directly replaced and processed by on-site operation and maintenance personnel, which is simple and quick to ensure power generation first, and then analyze and repair the faults. When any one of the string inverters fails, other inverters can keep working without affecting other power generation. Once any part of the system equipment in the centralized scheme fails, the whole square array may stop working, and the repair time depends on the site positioning and preparation of spare parts, generally at least 2-3 days, some up to 1 week, or even a month, the loss of power generation will be far more than the expected value of the loss of failure.

Take 10MW power station as an example, according to the average daily peak sunshine hours of 6 hours, the centralized program average annual loss of power generation will reach 7154KWH, while the string type is only 420KWH, compared to the centralized loss is 17 times the string type.

4. The maintenance cost of string type is much lower than that of centralized type.

Centralized inverter maintenance must be carried out by professional maintenance personnel, high skill requirements, even if the parts are damaged, also need professional operation and maintenance personnel to the scene, fault location and replacement, in addition to the cost of component replacement, labor costs, travel costs are high maintenance costs. Moreover, if the centralized inverter is completely damaged, it is extremely troublesome to replace it. Heavy machinery and equipment have to be admitted, and it is very difficult to move the inverter out of the machine room.

Affected by the software and hardware matching, etc., centralized inverters must use the original manufacturer's spare parts, once the manufacturer closes down, future maintenance will be forced to interrupt, the owner can only replace the new product. The string-type only need to directly replace the whole machine, do not need professional maintenance personnel, 30 minutes can be replaced. 5 years of maintenance period, free replacement, warranty period outside the failure of the machine, with the help of modern logistics platform, low-cost maintenance back to the factory, only a small amount of maintenance fees, maintenance costs are far lower than the centralized large machine program.

At present, the industry inverter brands of good and bad, recently some people said that string inverters need a lot of replacement after 5 years of the statement is to cover the whole. If there is no breakthrough in the design of string inverters, and centralized inverters, as well as fan cooling and fuses, a lower level of protection, then after 5 years and centralized as much need to be replaced. But if you choose IP65, natural cooling and no fuse design brand, these problems do not exist.

The choice of string inverter should fully consider its quality. Preferred quality is reliable, superior quality, can guarantee the good operation of the power station for a long time, and have a large-scale application of the inverter manufacturers. The annual failure rate of the world's top string inverters is below 0.5%, which is much lower than that of centralized solutions. If some string inverter brands go out of business or cease to exist after a few years, they can be replaced with a string inverter of another brand, which has low replacement cost and little impact on the overall power generation of the system.

To summarize, string inverters are more advantageous than centralized ones, both in terms of failure rate and operation and maintenance cost, which can ensure the long-term reliable operation of the power plant. In the past 10 years of inverter development process, Europe's technology leads the global market for 2-3 years, is the wind vane of technology and market, China, the United States and Japan and other markets, basically follow the European route, while absorbing the European experience and lessons learned.

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