Factors affecting the power generation of photovoltaic power stations
The calculation method of photovoltaic power station power generation, theoretical annual power generation = annual average total solar radiation * total battery area * photoelectric conversion efficiency. However, due to various factors, the power generation of photovoltaic power stations is not actually that much, and the actual annual power generation = theoretical annual power generation * actual power generation efficiency. So what are the factors that affect the power generation of photovoltaic power stations? The following is some basic knowledge about the power generation of distributed power stations based on my daily design and construction experience.
1. Solar radiation
Solar cell modules are devices that convert solar energy into electrical energy, and the intensity of light radiation directly affects the power generation. Solar radiation data for various regions can be obtained through NASA's meteorological data query website, or with the help of photovoltaic design software such as PV-SYS and RET Screen.
2. Tilt angle of solar cell modules
The data obtained from the meteorological station is generally the solar radiation on the horizontal plane, which is converted into the radiation on the inclined surface of the photovoltaic array to calculate the power generation of the photovoltaic system. The optimal tilt angle is related to the latitude of the project location. The approximate empirical values are as follows:
A. Latitude 0°～25°, the inclination angle is equal to the latitude
B. Latitude 26°～40°, the inclination angle is equal to the latitude plus 5°～10°
C. Latitude 41°～55°, the inclination angle is equal to the latitude plus 10°～15°
3. Solar cell module conversion efficiency: Solar cell modules are composed of high-efficiency crystalline silicon solar cells, ultra-white tempered glass, EVA, transparent TPT backplane and aluminum alloy frame, with glass transmittance loss and line loss.
4. System loss: Like all products, the efficiency of components and the performance of electrical components of photovoltaic power stations will gradually decrease during the life cycle of 25 years, and the power generation will decrease year by year. In addition to these natural aging factors, there are also quality problems of components and inverters, line layout, dust, series and parallel losses, cable losses and other factors. In the financial model of general photovoltaic power stations, the system power generation decreases by about 5% in three years, and the power generation decreases to 80% after 20 years.
A. Combination loss: Any series connection will cause current loss due to the current difference of the components; parallel connection will cause voltage loss due to the voltage difference of the components; and the combination loss can reach more than 8%, and the standard of the China Engineering Construction Standardization Association stipulates that it is less than 10%. Therefore, in order to reduce the combination loss, it should be noted that: components with consistent current should be strictly selected for series connection before the installation of the power station; the attenuation characteristics of the components should be as consistent as possible.
B. Dust shielding: Among all the factors that affect the overall power generation capacity of photovoltaic power stations, dust is the number one killer. The main effects of dust on photovoltaic power stations are: by shielding the light reaching the components, thereby affecting the power generation; affecting heat dissipation, thereby affecting the conversion efficiency; dust with acidity and alkalinity is deposited on the surface of the components for a long time, corroding the board surface and causing the board surface to be rough and uneven, which is conducive to the further accumulation of dust and increases the diffuse reflection of sunlight. Therefore, the components need to be wiped and cleaned from time to time. At present, there are three main ways to clean photovoltaic power stations: sprinkler trucks, manual cleaning, and robots.
C. Temperature characteristics: When the temperature rises by 1°C, the maximum output power of crystalline silicon solar cells decreases by 0.4%, the open circuit voltage decreases by 0.3% (-2mv/°C), and the short circuit current increases by 0.04%. In order to reduce the impact of temperature on power generation, the components should be kept in good ventilation conditions.
D. Line and transformer losses: The line losses of the DC and AC circuits of the system should be controlled within 5%. For this reason, the design should use wires with good conductivity and sufficient diameter. During system maintenance, special attention should be paid to whether the connectors and terminals are firm.
E. Inverter efficiency: Inverters have inductors, transformers, IGBTs, MOSFETs and other power devices, which will cause losses during operation. Generally, the efficiency of string inverters is 97-98%, the efficiency of centralized inverters is 98%, and the efficiency of transformers is 99%.
F. Shadows and snow cover: In distributed power stations, if there are tall buildings around, they will cast shadows on the components, so they should be avoided as much as possible during design. According to the circuit principle, when the components are connected in series, the current is determined by the least number of components. Therefore, if there is a shadow on one component, it will affect the power generation of this component.
When there is snow on the components, it will also affect power generation and must be cleared as soon as possible.
2. Common faults of distributed photovoltaic power stations
1. Fault phenomenon: The inverter screen does not display
Fault analysis: There is no DC input. The inverter LCD is powered by DC.
Possible reasons:
(1) The component voltage is insufficient. The inverter working voltage is 100V to 500V. When it is lower than 100V, the inverter does not work. The component voltage is related to the solar irradiance.
(2) The PV input terminal is connected in reverse. The PV terminal has positive and negative poles, which must correspond to each other and cannot be connected in reverse with other strings.
(3) The DC switch is not closed.
(4) When the components are connected in series, one of the connectors is not connected properly.
(5) One component is short-circuited, causing other strings to fail to work.
Solution: Use the voltage range of a multimeter to measure the DC input voltage of the inverter. When the voltage is normal, the total voltage is the sum of the voltages of each component. If there is no voltage, check the DC switch, wiring terminals, cable connectors, components, etc. in turn to see if they are normal. If there are multiple components, connect them separately for testing.
If the inverter has been used for a period of time and no cause is found, it is the inverter hardware circuit that has a fault. Please contact our after-sales service.
2. Fault phenomenon: The inverter is not connected to the grid. The screen shows that the mains is not connected
Fault analysis: The inverter is not connected to the grid.
Possible reasons:
(1) The AC switch is not closed.
(2) The AC output terminal of the inverter is not connected
(3) When wiring, the upper row of the inverter output wiring terminal is loose.
Solution: Use the voltage range of the multimeter to measure the inverter AC output voltage. Under normal circumstances, the output terminal should have a voltage of 220V or 380V. If not, check whether the wiring terminal is loose, whether the AC switch is closed, and whether the leakage protection switch is disconnected.
3. The PV module voltage is too high, and the screen shows that the PV voltage is high:
Fault analysis: DC voltage is too high alarm
Possible reasons: There are too many components in series, causing the voltage to exceed the voltage of the inverter.
Solution: Due to the temperature characteristics of the components, the lower the temperature, the higher the voltage. The input voltage range of the single-phase string inverter is 100-500V, and the recommended voltage after stringing is between 350-400V. The input voltage range of the three-phase string inverter is 250-800V, and the recommended voltage after stringing is between 600-650V. In this voltage range, the inverter has a high efficiency and can generate electricity when the irradiance is low in the morning and evening, but the voltage will not exceed the upper limit of the inverter voltage, causing an alarm and shutdown.
4. Isolation fault, the screen shows that the PV insulation impedance is too low:
Fault analysis: The insulation resistance of the photovoltaic system to the ground is less than 2 megohms.
Possible reasons: There is a short circuit or insulation damage to the wires in the solar panels, junction boxes, DC cables, inverters, AC cables, terminals, etc. The PV terminals and AC wiring casings are loose, causing water ingress.
Solution: Disconnect the power grid and inverter, check the resistance of the wires of each component to the ground in turn, find the problem point, and replace it.
5. Leakage current fault, the screen shows that the output leakage current is too high:
Fault analysis: The leakage current is too large.
Solution: Remove the PV array input terminal, and then check the peripheral AC power grid.
Disconnect all the DC and AC terminals, let the inverter power off for more than 30 minutes, if it can be restored by itself, continue to use it, if it cannot be restored, contact the after-sales technical engineer.
6. High grid voltage: The screen shows that the mains voltage is out of range
Fault analysis: The grid voltage is too high. The grid impedance is too large, and the photovoltaic power generation user side cannot digest it. When it is transmitted out, the impedance is too large, causing the inverter output side voltage to be too high, causing the inverter to shut down or reduce the rated operation.
Common solutions are:
A. Increase the output cable, because the thicker the cable, the lower the impedance.
B. The inverter is close to the grid connection point, the shorter the cable, the lower the impedance.
7. The system output power is too small: the ideal output power cannot be achieved
Possible reasons: There are many factors that affect the output power of the photovoltaic system, including solar radiation, the tilt angle of the solar cell module, dust and shadow blocking, and the temperature characteristics of the module. See Chapter 1 for details.
The system power is too low due to improper system configuration and installation. Common solutions are:
A. Before installation, check whether the power of each component is sufficient.
B. According to Chapter 1, adjust the installation angle and direction of the component;
C. Check whether the component has shadows and dust.
D. Check whether the voltage is within the voltage range after the components are connected in series. If the voltage is too low, the system efficiency will be reduced.
E. Before installing multiple strings, check the open circuit voltage of each string first. The difference should not exceed 5V. If the voltage is found to be incorrect, check the line and connector.
F. During installation, you can connect in batches. When each group is connected, record the power of each group. The power difference between the strings should not exceed 2%.
G. The installation place is not well ventilated, the heat of the inverter is not dissipated in time, or it is directly exposed to the sun, causing the inverter temperature to be too high.
H. The inverter has dual MPPT access, and the input power of each channel is only 50% of the total power. In principle, the design and installation power of each channel should be equal. If only one MPPT terminal is connected, the output power will be halved.
I. The cable connector has poor contact, the cable is too long, the wire diameter is too thin, there is voltage loss, and finally power loss.
J. The grid-connected AC switch capacity is too small and cannot meet the inverter output requirements.
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