Practical analysis of the impact of the ratio of components and inverters on power generation

After the PV system is installed, the user is most concerned about the power generation, because it is directly related to the user's return on investment. There are many factors that affect the power generation, including the quality of components, inverters, cables, installation orientation, tilt angle, dust, shadows, component and inverter ratio system plan, line design, construction, grid voltage, etc. This article mainly discusses the impact of the ratio of components and inverters on the system power generation.

Case site 1

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.

A customer in Wenzhou, Zhejiang, has 20 260W modules with a total power of 5200W. In order to save inverter costs, the system was designed according to this technical parameter and a 4.2kW ​​inverter was selected. When the weather is good, the result is shown in the figure below. The inverter runs at the limit, which will cause power generation loss to the user.

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, adapt to local conditions, and analyze specific problems, rather than blindly using the over-matching capacity of the inverter.

Case site 2

This is an installation case in Tangshan, Hebei. The project uses 72 280 modules, the inverter uses 20kW, and the modules are connected to the inverter in 18 4-parallel mode. The customer reported that after the grid connection, the power generation has not been high. The same installation capacity is not much different when the weather is bad, but when the weather is good, the power generation will differ by about 10%, and the maximum difference is 10 degrees on one day. Through the monitoring system, the operating parameters of the inverter are observed, and it is found that the current difference between the two MPPTs is relatively large. It is estimated that the problem lies here. After confirming with the customer, this inverter has 6 sets of input interfaces. The customer connected 3 lines to one MPPT, and the other MPPT only has one string, so the current of PV2 is 3 times larger than that of PV1.

The inverter has multiple MPPTs, each with independent tracking, so different strings can be connected. The inclination angles of the strings can also be different, but each MPPT branch has a current limit. For example, a 20kW inverter has two MPPTs, and each MPPT circuit can output up to 11kW at the rated operating voltage. If more than one is connected, the power will be limited. The customer connected 3 lines, with an input power of 15.12kW, which is 4.12kW more. When the weather is good and the sun is strong, the power will be limited. It is not surprising that 10 degrees of electricity are lost a day.

Case site 3

This is a customer in Xiamen, Fujian. He used a 6kW inverter. Soon after installation, the inverter stopped working and displayed "BUS overvoltage". It was learned that the customer used 17 275W components, all connected in one line, which caused the voltage in this line to be high. Long-term operation caused great damage to the components, and in severe cases, it could cause the boost circuit to fail.

The working voltage is around the rated working voltage of the inverter, and the efficiency is the highest. For a single-phase 220V output inverter, the rated input voltage is 360V; for a three-phase 380V output inverter, the rated input voltage is 650V. For example, a 3kW inverter is equipped with a 260W component, 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 a 260W component, connected to 126 components, 21 strings in each way, the voltage is 640.5V, and the total power is 32.76kW, which is the best. The greater the difference between the string MPPT voltage and the optimal voltage of the inverter (whether it is low or high), the lower the efficiency will be. For example, if the single-phase inverter string is equipped with 260V or 460V, the efficiency is only about 90% of the optimal voltage. In addition, it should be noted that the highest open-circuit voltage of the string cannot exceed the maximum voltage of the inverter, otherwise the inverter will be damaged.

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Summarize

When configuring components and inverters, pay attention to the following four points:

(1) We cannot simply emphasize over-matching components to reduce inverter costs. We must analyze specific issues based on actual conditions.

(2) Each MPPT circuit of the inverter is also current-limited and cannot be over-allocated.

(3) The string voltage should be within the optimal voltage range of the inverter as much as possible. Both low and high voltages will have a great impact on power generation.

(4) The maximum open-circuit voltage of the string cannot exceed the maximum voltage of the inverter.