Comprehensive analysis of the ratio of components and inverters, a one-month investigation summary by a photovoltaic veteran

In the technical parameters of the inverter, there is a maximum allowable access power. Most manufacturers mark it as 1.1 to 1.2 times the rated power, but some manufacturers mark it as 1.4 times the rated power in order to show their over-capacity capabilities, and some even as high as 1.7 times.

Users design their systems based on this technical parameter. As a result, when the weather is good, the inverter will operate at the limit as shown in the figure below, causing power generation losses to users.

Case of failed matching

Since the inverter only accounts for about 5% of the cost of the photovoltaic system, reducing the inverter investment by over-matching components is not only uneconomical, but also brings other problems. It is necessary to comprehensively consider factors such as light conditions, installation site factors, component factors and inverter factors, etc., scientifically design, and analyze specific problems specifically.

The author spent nearly a month investigating the factors that affect the ratio of photovoltaic modules and inverters, and collected a total of 12 influencing factors in four categories, as follows:

A factor of light conditions

(1) Different irradiance in different areas

According to the classification standards of the Wind and Solar Energy Assessment Center of the National Meteorological Administration, my country's solar energy resource areas are divided into four categories, and the irradiance in different regions varies greatly. The larger the solar altitude angle, the shorter the path through the atmosphere, the smaller the weakening effect of the atmosphere on solar radiation, and the stronger the solar radiation reaching the ground; the larger the solar altitude angle, the smaller the area of ​​equal solar radiation distribution, and the stronger the solar radiation. In areas with good solar energy resources, due to fewer clouds on sunny days, good air quality, and high atmospheric transparency, the radiation from the sun reaching the surface of the components is much higher than in areas with poor resources.

(2) Installation altitude

The higher the altitude, the thinner the air is, and the less the atmosphere weakens the solar radiation, so the stronger the solar radiation reaching the ground is. For example, the Qinghai-Tibet Plateau is the area with the strongest solar radiation in my country. The thinner the air is, the worse the inverter heat dissipation will be. When the altitude exceeds a certain height, the inverter will have to operate at a reduced rating. In high altitude places, the inverter should generally consider using fewer components.

Installation site factors

(1) DC side system efficiency

In a photovoltaic system, energy is radiated from the sun to the photovoltaic modules, and then passes through the DC cables, combiner boxes, and DC power distribution to the inverter. There are losses in each link. Different installation methods have different DC side losses. For centralized ground power stations, the DC side system efficiency is usually around 90% because the DC cables are long, and there are also combiner boxes, DC power distribution cabinets, anti-reverse diodes, DC switches and other equipment and devices. In a distributed photovoltaic system, since the modules are connected to the inverter without other accessories, if the inverter is installed nearby, the DC cable can be very short, and the DC side system efficiency can reach 98%.

(2) Inverter heat dissipation conditions

The inverter should generally be installed in a well-ventilated place away from direct sunlight, which is conducive to heat dissipation. If the inverter has to be installed in a closed place that is not conducive to heat dissipation due to site restrictions, the inverter derating problem should be considered and fewer components should be used.

(3) Grid voltage changes

The rated output power of the inverter is the maximum power at the rated output voltage, which is not fixed. If the grid voltage drops during peak hours, the inverter will not reach the rated power. For example, the 33KTL inverter of a certain HW company has a maximum output current of 48A and a nominal rated power of 33KW, which is the power at the rated output voltage of 400V, 48A*400V*1.732=33.KW. If the grid voltage drops to 360V, the inverter output power is 48A*360V*1.732=30.KW.

Three component factors

(1) Power positive tolerance

In order to ensure that the attenuation of photovoltaic modules does not exceed 20% in 25 years, many module manufacturers have a positive tolerance of 0-5% for modules that have just left the factory. For example, the actual power of a 265W module may be 275W when it leaves the factory.

(2) Negative temperature coefficient

The power-temperature system of the module is about -0.41%/℃. When the module temperature drops, the module power will increase. Without considering the equipment loss, a 250W module may have a maximum output power of 300W in the areas with the best sunshine in my country, such as northern Ningxia, northern Gansu, southern Xinjiang, etc.

(3) Bifacial modules

Bifacial modules can not only receive the radiation power of sunlight on the front side, but also the reflected radiation power of sunlight on the back side. Different objects have different reflectivity to sunlight in different spectral bands. Snow, wetlands, wheat, deserts, different objects have different reflectivity in the same band, and the same object also has different reflectivity in different bands.

Four inverter factors

(1) Inverter efficiency

The efficiency of the inverter is not a constant value. There are power switching device losses and magnetic losses. At low power, the efficiency is relatively low. At 40% to 60% power, the efficiency is the highest. When it exceeds 60%, the efficiency gradually decreases. Therefore, the total power of the photovoltaic power should be controlled between 40% and 60% of the inverter power to obtain the best efficiency.

(2) Inverter life

Photovoltaic inverters are electronic products, and their reliability is closely related to the operating temperature of the inverter. If the temperature of components such as capacitors, fans, and relays increases by 10°C, the failure rate will increase by more than 50%. The operating temperature is related to power. According to statistics, the life of an inverter that works at 80-100% power for a long time is about 20% lower than that of an inverter that works at 40-60% power.

(3) Optimal operating voltage range of the inverter

The working voltage is around the rated working voltage of the inverter, and the efficiency is the highest. For a single-phase 220V inverter, the rated input voltage of the inverter is 360V, and for a three-phase 380V inverter, the rated input voltage of the inverter is 650V. For example, a 3kW inverter is equipped with 260W components, the working voltage is 30.5V, and 12 components with a working voltage of 366V are equipped. The power is 3.12kW, which is the best. A 30KW inverter is equipped with 260W components, connected to 126 components, 21 strings in each channel, the voltage is 640.5V, and the total power is 32.76kW, which is the best.

(4) Inverter output power and overload capacity

Inverters of the same power range from different brands have different output powers. For example, the rated output power of a 40KTL inverter from Company A is only 36KW. It not only has no overload capability, but also has to be reduced to 90%. The 40KW inverter from Company B can output a maximum power of 44.4KW at a power factor of 1, and can be overloaded by 1.1 times. Therefore, for inverters of the same size, a 40KW inverter from Company B has 20% more components than a 40KTL inverter from Company A.

Company A 40KTL inverter

Company B's 40KW inverter

in conclusion

Taking all the factors into consideration, the matching of inverters and components is not a simple matter. Various factors should be considered comprehensively. For example, components are like precious goods, and inverters are like horses. We should listen to the opinions of experienced drivers and think that it is better to use a big horse to pull a small cart than a small horse to pull a big cart.

Pay special attention to the following points:

1. Areas with an altitude of more than 2,000 meters;

2. The inverter heat dissipation at the installation site is not good;

3. The grid voltage at the installation site is low;

4. The output power of the inverter is low.