Common Faults and Solutions of Distributed PV Power Plants

Chapter 1 Factors Affecting the Power Generation of Photovoltaic Plants

Photovoltaic plant power generation calculation method, the theoretical annual power generation = annual average total solar radiation * total battery area * photoelectric conversion efficiency. However, due to the influence of various factors, the actual amount of electricity generated by photovoltaic power plants is not so much. The actual annual power generation is equal to the theoretical annual power generation* actual power generation efficiency. What are the factors that affect the power generation of photovoltaic power plants? The following is my basic design knowledge and construction experience, give you some basic knowledge of distributed power generation.

1.1, solar radiation

Solar modules are devices that convert solar energy into electrical energy. The intensity of light radiation directly affects the amount of power generated. Solar radiation data from various regions can be obtained through NASA's meteorological data query website, or through PV design software such as PV-SYS and RETScreen.

1.2. The data obtained from the meteorological station at the tilt angle of the solar cell module is generally the amount of solar radiation on the horizontal plane, which is converted into the radiation amount of the tilted surface of the photovoltaic array, so that the photovoltaic system power generation amount can be calculated. The best inclination is related to the latitude of the location of the project. The approximate experience is as follows:

A, latitude 0° to 25°, tilt angle equal to latitude

B, latitude 26° to 40°, dip equal to latitude plus 5° to 10°

C, latitude 41° to 55°, inclination equal to latitude plus 10° to 15°

1.3. Conversion efficiency of solar modules

1.4. System losses Like all products, during the 25-year life cycle of photovoltaic power plants, the efficiency of components and the performance of electrical components will gradually decrease, and the amount of power generation will gradually decrease year by year. In addition to these natural aging factors, there are many factors such as the quality of components and inverters, circuit layout, dust, serial and parallel losses, and cable losses.

In the financial model of a typical photovoltaic power plant, the system power generation is decremented by about 5% for three years, and the power generation capacity is reduced to 80% after 20 years.

1.4.1 Portfolio loss

Any series connection will cause current loss due to the current difference of the components; the parallel connection will cause voltage loss due to the voltage difference of the components; and the combined loss can reach more than 8%. The standard of China Engineering Construction Standards Association is less than 10%.

Therefore, in order to reduce the combination loss, it should be noted:

1) The components with the same current should be selected strictly before the installation of the power station.

2) The attenuation characteristics of the components are as consistent as possible.

1.4.2 Dust Blocking

Of all the factors that affect the overall power generation capability of photovoltaic power plants, dust is the number one killer. The effects of dust photovoltaic power plants mainly include: obstructing the light of components to affect power generation; affecting heat dissipation and affecting conversion efficiency; dust with acidity and alkalinity is deposited on the surface of components for a long time, and the board surface is rough due to corrosion. It is conducive to the further accumulation of dust, while increasing the diffuse reflection of sunlight. So the components need to be cleaned occasionally.

At present, the cleanliness of photovoltaic power plants includes sprinklers, manual cleaning, and robots.

1.4.3 Temperature Characteristics

When the temperature rises by 1°C, the crystalline silicon solar cell: the maximum output power drops by 0.04%, the open circuit voltage drops by 0.04% (-2mv/°C), and the short-circuit current increases by 0.04%. In order to reduce the impact of temperature on the power generation, good ventilation of the components should be maintained.

1.4.4 Line and Transformer Losses

The line loss of the system's DC and AC circuits must be controlled within 5%. For this reason, it is designed to use conductive wires with good conductivity. The wires need to have a sufficient diameter. During system maintenance, special attention should be paid to whether the connectors and terminals are firm.

1.4.5 Inverter Efficiency

Inverters have losses due to inductance, transformers, IGBTs, MOSFETs, and other power devices. The general string inverter efficiency is 97-98%, the centralized inverter efficiency is 98%, and the transformer efficiency is 99%.

4.6 Shading and snow cover

In a distributed power plant, if there are tall buildings around, it will cause shadows to components and should be avoided when designing. According to the circuit principle, when the components are connected in series, the current is determined by the smallest one, so if there is a shadow, it will affect the power generated by this component.

When there is snow on the components, it also affects power generation and must be cleared as soon as possible.

Chapter 2 Common Faults of Distributed Photovoltaic Plants

2.1. Symptoms: The inverter screen is not displayed

Failure analysis: There is no DC input, inverter LCD is powered by DC.

Possible Causes:

(1) The voltage of the component is not enough. The working voltage of the inverter is 100V to 500V. Below 100V, the inverter does not work. Component voltage is related to solar irradiance,

(2) The PV input terminal is reversed, and the PV terminal has two poles, positive and negative. Corresponding to each other, it cannot be connected in reverse with other groups.

(3) The DC switch is not closed.

(4) When the components are connected in series, one of the connectors is not connected.

(5) A component is short-circuited, causing other strings to not work

Solution: Measure the DC input voltage of the inverter with the multimeter's voltage range. When the voltage is normal, the total voltage is the sum of the voltages of the components. If there is no voltage, check if the DC switch, connection terminal, cable connector, and components are normal. If there are multiple components, separate access to the test.

If the inverter is used for a period of time and no reason is found, the inverter hardware circuit is faulty. Please contact our company after sales.

2. Failure phenomenon: The inverter is not connected to the network.

Fault analysis: There is no connection between the inverter and the grid.

Possible Causes:

(1) The AC switch is not closed.

(2) The AC output terminal of the inverter is not connected

(3) When wiring, loose the upper output terminal of the inverter.

Solution: Measure the AC output voltage of the inverter with the multimeter's voltage file. Under normal conditions, the output terminal should have a voltage of 220V or 380V. If not, check if the connection terminal is loose, if the AC switch is closed, and if the leakage protection switch is off. open.

3, PV over pressure:

Failure analysis: DC voltage over alarm

Possible causes: The number of components in series is too large, causing the voltage to exceed the voltage of the inverter.

Solution: Because of the temperature characteristics of the components, the lower the temperature, the higher the voltage. Single-phase string inverter input voltage range is 100-500V, the proposed string voltage is between 350-400V, three-phase string inverter input voltage range is 250-800V, the proposed string voltage is Between 600-650V. In this voltage range, the efficiency of the inverter is relatively high. When the irradiance is low in the morning and in the evening, it can generate electricity, but it does not cause the voltage to exceed the upper limit of the inverter voltage, causing an alarm and stopping.

4, isolation failure:

Fault analysis: The insulation resistance of the photovoltaic system to ground is less than 2 megohms.

Possible reasons: Solar modules, junction boxes, DC cables, inverters, AC cables, wiring terminals, etc. are short-circuited to the ground or the insulation layer is damaged. The PV terminals and the AC wiring housing are loose, resulting in water ingress.

Solution: Disconnect the power grid, inverter, check the resistance of each part of the wire to ground in turn, find out the problem points, and replace it.

5, leakage current failure:

Fault analysis: too much leakage current.

Solution: Remove the PV array input and check the external AC grid.

The DC and AC terminals are all disconnected and the inverter is powered off for more than 30 minutes. If it can recover, continue using it. If it cannot recover, contact the after-sale technical engineer.

6, grid error:

Fault analysis: The grid voltage and frequency are too low or too high.

Solution: Use a multimeter to measure the voltage and frequency of the grid. If it is exceeded, wait for the grid to return to normal. If the power grid is normal, the inverter detects the power failure of the circuit board. Please disconnect the DC and AC terminals and allow the inverter to power off for more than 30 minutes. If you can restore it, continue using it. If it cannot be restored, contact the after-sales service. Technical Engineer.

7, inverter hardware failure: divided into recoverable faults and non-recoverable faults

Failure analysis: The inverter circuit board, detection circuit, power loop, communication loop and other circuits are faulty.

Solution: The above hardware fault occurs in the inverter. Please disconnect all the DC and AC terminals and allow the inverter to power off for more than 30 minutes. If you can restore it, continue using it. If it cannot recover, contact the after-sale technical engineer.

8, the system output power is too small: can not reach the ideal output power

Possible causes: There are many factors affecting the PV system output power, including the amount of solar radiation, the tilt angle of the solar cell module, the blocking of dust and shadow, and the temperature characteristics of the module. For details, see Chapter 1.

The system power is too small due to improper installation of the system configuration. Common solutions include:

(1) Before installation, check if the power of each component is sufficient.

(2) According to the first chapter, adjust the installation angle and orientation of the components;

(3) Check the components for shadows and dust.

(4) Check whether the voltage of the component in series is within the voltage range. If the voltage is too low, the system efficiency will be reduced.

(5) Before installing multiple strings, check the open circuit voltage of each string. The difference should not exceed 5V. If the voltage is incorrect, check the wiring and connectors.

(6) During installation, it can be accessed in batches. When each group accesses, the power of each group is recorded. The power difference between the strings is not more than 2%.

(7) The ventilation in the installation area is not smooth, and the inverter heat is not disseminated in time or directly exposed to sunlight, causing the inverter to overheat.

(8) The inverter has dual MPPT access, and each input power is only 50% of the total power. In principle, the power of each design and installation should be equal. If only one MPPT terminal is connected, the output power will be halved.

(9) The cable connector is in poor contact, the cable is too long, the wire diameter is too thin, there is voltage loss, and finally the power is lost.

(10) The capacity of the grid-connected AC switch is too small to meet the inverter output requirements.

9. AC side overvoltage

If the grid impedance is too large, the photovoltaic power generation users cannot digest it, and the impedance is too large due to the transmission, causing the inverter output voltage to be too high, causing the inverter to shut down, or derating the inverter.

Common solutions include:

(1) Increase the output cable because the cable is thicker and the impedance is lower.

(2) The inverter is close to the grid point, the shorter the cable, the lower the impedance

Appendix: Selection table of grid-connected inverter AC output cables and AC output breakers

Take Shenzhen Jingfuyuan Inverter as an example

AC cable length is greater than 50 meters, choose freshman model. Such as JSI-5000TL, AC length shorter than 50 meters, you can use 2.5 square cable, between 50-100 meters, use 4 square, length greater than 100 meters, use 6 square.

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