1, device life
The inverter belongs to the switching power supply from the broad sense, so its components can basically be divided into resistors, capacitors, diodes, power devices (IGBT or MOS), inductors and transformers, current sensors, ICs, optocouplers, and relays.
The service life of resistors, chip capacitors and ceramic capacitors can generally reach 20 years or more. In the design of inductors and transformers, as long as the material temperature is not exceeded, it is theoretically considered that it can work for a long time without failure; low-power diodes The triode can work for more than 100,000 hours; the mechanical life of the relay is generally more than 1 million times, and the electrical life is more than 10,000 times; the power device IGBT or MOS does not generally consider the service life as long as it satisfies the design specifications. Fans and fuses are fragile components, which will not affect the life of the inverter. They only need to be replaced in time. The life of the film capacitor is more than 100,000 hours. The life of the electrolytic capacitor is generally 2000- at 105°C. 3000H, long life is only 5000-6000H, but as the temperature decreases by 10 °C, life expectancy doubled. So in the inverter, the largest short board of the device is in the life of the electrolytic capacitor.
2, design factors
The lifetime of the device is the basic factor that guarantees the life of the inverter, but the excellent design is the core factor that guarantees the life of the inverter. Which design factors will seriously affect the life of the inverter and be difficult to detect in a short time?
Temperature is one of the important factors affecting the life of the inverter, especially electrolytic capacitors and optocouplers. For every 10°C increase in temperature, the life of the electrolytic capacitor is reduced by half. If the temperature is too high, the photo-degradation of the optocoupler will also be accelerated. However, the IGBT driver is generally used optocoupler, so the failure of the optocoupler will cause damage to the IGBT.
When the zero current is switched, the relay has a lifetime of 1 million times. However, as the current increases during switching, the lifetime decays almost exponentially. Accurate software control makes the relay switch at zero current angle is the key factor to ensure the life of the relay.
The working environment of the inverter is generally rather harsh, and there are many interference factors, such as poor quality of the power grid, large local inductive loads, etc. If the inverter protection function and EMC design are not good, it is easily subject to external interference, once the IGBT If the driver is disturbed, it can easily lead to misfire and cause explosion.
3, comprehensive factors
Although the string inverters are IP65 rated and can be installed indoors or outdoors, the installation environment is also very influential to the life of the inverter. If the inverter is installed in the environment with direct sunlight, humidity, and high pH, ​​the life of the inverter will be reduced, and the inverter will easily cause the over-temperature and load reduction of the inverter and affect the power generation. Therefore, choosing the proper installation environment is also a key factor to ensure the life of the inverter.
20kw inverters were about 20,000 five years ago. At present, 20kw inverters only need less than 10,000, and as the inverters are used for a long time, the aging of components and the loss increase, the efficiency will be It has been reduced. At the same time, as technology continues to innovate, the function of the inverter will become more and more powerful, the price will become cheaper, and the conversion efficiency will become higher and higher. Therefore, it is not as good as a direct channel change.
Why does the inverter need to dissipate heat?As a power electronics device, inverters face the challenge of temperature as well as all electronic products.
Of all electronic product failure cases, up to 55% is due to temperature.
The electronic components inside the inverter are also very sensitive to the temperature. According to the reliability theory of 10 degrees, from the room temperature, the temperature decreases by half every 10 degrees, so the thermal design of the inverter is very important.
Several ways of heat dissipation in the inverterThe inverter heat dissipation system mainly includes radiators, cooling fans, thermal grease, and other materials.
At present, there are two main types of heat dissipation in inverters: one is natural cooling, and the other is forced air cooling.
1) Natural cooling
Natural cooling refers to the purpose of realizing the temperature control by radiating the local heating device to the surrounding environment without using any external auxiliary energy. This usually includes the three main heat conduction methods: heat conduction, convection and radiation, in which the convection is natural. The main method of convection.
Natural heat dissipation or cooling is often applied to low-power devices and components with low heat-flux density and low heat-flux requirements for devices, and sealed or densely populated devices that are not (or need not) use other cooling technologies.
Currently, mainstream single-phase inverters and three-phase inverters below 20kW are used in the market, and most manufacturers use natural cooling methods.
2) Forced air cooling
Forced air cooling is mainly a method of forcing heat around the device by means of a fan or the like to force air flow around the device.
This method is an easy-to-use, effective cooling method.
If the space between the components in the component is suitable for air flow or suitable for installing a local heat sink, this cooling method can be used as much as possible.
Increasing the method of forced convection heat transfer increases the area of ​​heat dissipation and produces a relatively large forced convection heat transfer coefficient at the heat dissipation surface. Increasing the heat dissipation surface of the heat sink to enhance the heat dissipation of electronic components has been widely used in practical projects.
In the project, fins are mainly used to expand the heat dissipation area of ​​the radiator surface in order to enhance heat transfer. The choice of heat sink material itself has a direct relationship with its heat dissipation performance.
At present, the material of the heat sink is mainly made of copper or aluminum, and its expansion heat exchange surface is made by a process of folding fins/punching thin fins.
3) Comparison of two heat dissipation methods
There is no natural heat dissipation fan, low noise, but slow cooling, generally used for low-power inverters;
Forced air cooling is required to configure the fan, which is noisy but has high heat dissipation speed. It is generally used in high-power inverters.
In the medium-power string inverter, both methods have.
According to the comparative experiment of the heat dissipation capability of the string inverter, it is found that string inverters with a power level of 50 kW or more are better than the natural cooling method in forced air cooling, and the internal components of the inverter, IGBTs and other key components are warm. Rise is reduced by about 20°C, which ensures long-life and high-efficiency operation of the inverter.
With the use of natural cooling, the temperature of the inverter rises and the life of the components decreases.
Forced air cooling also uses high speed fans and medium speed fans.
Using a high-speed fan can reduce the size and weight of the heat sink, but it will increase noise, and the fan life will be shorter.
The use of medium-speed fans, the radiator is slightly larger, but at low power, the fan does not turn;
The fan runs at low speed at medium power.
Actually, the full-power inverter running time is not so much, so the life of the fan can be very long.
The latest cooling technologyWith the continuous development of electronic technology, inverters have achieved great development in heat dissipation:
(1)Cavity management: The devices most affected by temperature in the inverter are op amps, sensors, electrolytic capacitors, etc. Inductors, cables, power switch tubes, etc. are relatively high-temperature resistant and can be separated by cavity separation Components, the power of the heating device, such as the inductor on the outside of the inverter, lower the temperature inside the chassis. At the same time, an integral housing structure can be adopted, and the heat sink and the housing are directly and tightly combined, so that the aluminum alloy housing is cooled through two paths, so as to achieve the effect of reducing the temperature of the components and the internal temperature of the inverter, and ensuring the components and the inverse thereof. Longer service life of the transformer.
(2) Heat dissipation simulation technology: Simulation software can simulate the thermal status of the system more realistically, and the temperature value of each component can be predicted during the design process, so that the unreasonable inverter structure layout can be corrected. This shortens the design's development cycle, lowers costs, and increases the product's success rate.
(3) The application of new heat-dissipating materials: such as steel radiators, aluminum alloy radiators, copper radiators, copper-aluminum composite radiators, steel-aluminum composite radiators, stainless steel radiators, etc.
(4) New heat pipe heat dissipation technology: The heat pipe is a new type of heat transfer element with extremely high thermal conductivity. It transfers heat through evaporation and condensation of liquid in a totally enclosed vacuum tube. It uses the principle of fluid absorption and other fluids. To a good cooling effect. With high thermal conductivity, good isothermal, heat and heat on both sides of the heat transfer area can be arbitrarily changed, long-distance heat transfer, temperature control and so on.
Pay attention to heat dissipation when installing the systemThe inverter itself is a heat source, all the heat must be emitted in a timely manner, can not be placed in a closed space, otherwise the temperature will rise higher and higher.
The inverter should be placed in an air-circulated space and direct sunlight should be avoided.
When multiple inverters are installed together, sufficient distance must be maintained between the inverter and the inverter in order to avoid mutual influence.
Original title: How to increase the lifetime of PV inverters?
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