The role of the PV inverter

1. Active operation and shutdown function

After sunrise in the morning, the intensity of solar radiation gradually increases, and the output of the solar cell also increases, and when the output power required for the inverter's task is reached, the inverter will start to operate actively. After entering operation, the inverter will take care of the output of the solar cell module at all times, and the inverter will continue to operate as long as the output power of the solar cell module is greater than the output power required for the inverter task. Until sunset is shut down, the inverter can operate even on rainy days.

2. The maximum power tracking control function

The output of a solar cell module is transformed with the intensity of solar radiation and the temperature of the solar cell module itself (chip temperature). In addition, because the voltage of solar cell modules decreases with the increase of current, there is an optimal task point to obtain the maximum power. The intensity of solar radiation is changing, and so is the apparent sweet spot.

How a PV inverter works

The basic working principle of a PV inverter

Inverter refers to the process of converting direct current into alternating current, inverter circuit refers to the circuit that can complete the inverter function, and inverter refers to the device equipment that can realize the inverter function. Photovoltaic inverter (also known as power regulator) is a kind of inverter, which is often used in photovoltaic power generation systems, so it is called photovoltaic inverter. In addition to the direct AC conversion function, the photovoltaic inverter also has automatic operation, shutdown function and maximum power tracking control function.

The composition of a PV inverter

Input circuit: responsible for providing DC input voltage;

Main inverter circuit: complete the inverter program through the action of semiconductor switching devices;

Output circuit: mainly compensate and correct the frequency, phase, voltage and current amplitude of the output alternating current of the main inverter circuit to meet certain standards;

Auxiliary circuit: the DC voltage of the input circuit is replaced with the DC voltage suitable for the operation of the control circuit, and a series of detection circuits are also included;

Control circuit: provide pulse signal for the main inverter circuit to control the opening and shutdown of semiconductor devices;

Classification of PV inverters

1. Classification according to the isolation method

Including isolated and non-isolated two categories, of which the isolated grid-connected inverter is divided into power frequency transformer isolation mode and high-frequency transformer isolation mode, photovoltaic grid-connected inverter at the beginning of the development of power frequency transformer isolation mode, but due to its size, weight, cost obvious defects, in recent years, high-frequency transformer isolation mode of grid-connected inverter has developed rapidly, non-isolated grid-connected inverter with its high efficiency, simple control and other advantages have gradually been recognized, has been promoted and applied in Europe, However, key issues such as reliability and common-mode current need to be addressed.

2. Classification according to the number of output phases

It can be divided into two categories: single-phase and three-phase grid-connected inverters, with single-phase inverters generally used in small and medium-power occasions and three-phase grid-connected inverters in high-power occasions. According to the classification of power level, it can be divided into small power grid-connected inverters with a power of less than 1 kVA, medium power and grid-connected inverters with a power level of 1 kVA ~ 50 kVA, and high-power grid-connected inverters with a power level of more than 50 kVA.

3. Classification according to the direction of power flow

It is divided into two categories: unidirectional power flow grid-connected inverter and bidirectional power flow grid-connected inverter, unidirectional power flow grid-connected inverter is only used for grid-connected power generation, and bidirectional power flow grid-connected inverter can be used as a rectifier in addition to grid-connected power generation to improve grid voltage quality and load power factors.

4. Classification according to topology

At present, the topologies used include: full-bridge inverter topology, half-bridge inverter topology, multi-level inverter topology, push-pull inverter topology, forward inverter topology, flyback inverter topology, etc., among which high-voltage and high-power photovoltaic grid-connected inverters can adopt multi-level inverter topologies, medium-power photovoltaic grid-connected inverters mostly use full-bridge and half-bridge inverter topologies, and low-power photovoltaic grid-connected inverters adopt forward-excited and flyback inverter topologies.

The difference between a photovoltaic inverter and an energy storage inverter

1. The self-consumption rate of traditional photovoltaic inverters is only 20%, while the self-consumption rate of energy storage inverters is as high as 80%.

2. In the event of a mains failure, the grid-connected inverter is paralyzed, but the energy storage inverter can still work efficiently.

3. In the context of the continuous reduction of grid-connected power generation subsidies, the income of energy storage inverters is higher than that of photovoltaic inverters.

4. The photovoltaic inverter can only generate electricity during the day, and the power generation power will be affected by the weather, which has unforeseen problems, while the energy storage inverter can perfectly solve the problems of the photovoltaic inverter.

Selection of PV inverters

1. Rated output power

The rated output power indicates the ability of the PV inverter to supply power to the load. Photovoltaic inverters with high rated output power can drive more loads, and the first consideration should be to have sufficient rated power when selecting photovoltaic inverters to meet the requirements of the equipment for electrical power under the maximum load, as well as the expansion of the system and the access of some temporary loads.

2. Adjustment performance of output voltage

The adjustment performance of the output voltage indicates the ability of the output voltage of the PV inverter to be regulated. Generally, photovoltaic inverter products give the percentage of the fluctuation deviation of the output voltage of the photovoltaic inverter when the DC input voltage changes within the allowable fluctuation range, which is usually called the voltage regulation rate. The voltage regulation of a high-performance PV inverter should be less than or equal to ±3%, and the load regulation should be less than or equal to ±6%.

3. The efficiency of the whole machine

The efficiency of the whole unit indicates the power loss of the PV inverter itself. PV inverters with larger capacity should also give efficiency values under full load and low load work, and the efficiency of inverters below KW level should be 80%-85%; The efficiency of the 10KW class should be 85%-90%; The efficiency of higher power must be above 90%-95%.

4. Start-up performance

The PV inverter should be guaranteed to start reliably at the rated load. High-performance PV inverters can be started at full load for multiple times in a row without damaging the power switching devices and other circuits. For their own safety, small inverters sometimes use soft-start or current-limiting start-up measures.

5. Type

At present, the inverters on the market are basically divided into three types: centralized inverters, string inverters, and microinverters. The centralized inverter is mainly used in large-scale ground power stations, with a voltage level of 315V, which is suitable for high-voltage grid connection; String inverter, also known as distributed inverter, is mainly used in all kinds of barren mountains, industrial and commercial or household rooftops, and the scale of the power station is generally not large, and it is integrated into the national grid through full grid access or surplus power grid; The main application mode of microinverter is directly integrated on the battery panel, which is suitable for small household power stations, and is rarely used in China at present.

How to install a PV inverter

How PV inverters are wired

1. Choose a good place

The PV inverter should be installed as close to the battery as possible and in a dry, well-ventilated place. Wall-mounted PV inverters have mounting holes that can be screwed to the wall. When choosing a location, make sure that the DC terminals, outlets, switches, and fuses are still usable.

2. Connect the battery

Use the included cable to connect the inverter to the battery. The cable can be shortened, but if the cable is too short, the thicker and longer cable should be replaced.

3. Ground the photovoltaic solar inverter

Grounding is very important. If the PV inverter and your equipment are not grounded, equipment failure may leave the connected equipment and inverter live. If your PV inverter has a grounding point, use a thicker wire to connect it to a suitable grounding point.

4. Insert the test device

Observe the maximum power rating of the PV inverter, plug the device into the AC power outlet of the PV inverter, and turn on the PV inverter. The power indicator should be on, and you may hear the sound of the device running.

Precautions for PV inverter installation

1. The inverter can only be powered on after the approval of the commissioning engineer.

2. The installation of the inverter should be carried out by an engineer with operating qualifications according to the steps described in this manual.

3. The inverter can only be installed on a concrete base or other non-flammable installation surface.

4. Ensure that the ground wire in the distribution room is well grounded, and the grounding resistance in a dry environment should be less than 4Ω.

5. If the inverter control power supply is taken from the low-voltage power distribution in the computer room or the heater is optional, please ensure that the power distribution in the computer room can provide 4KW power.

How to maintain the PV inverter

1. Before using the photovoltaic controller and inverter, you should study the instruction manual.

2. Pay attention to the warning signs on the machine.

3. Before turning on the machine, check whether the input voltage is normal.

4. During operation, pay attention to whether the sequence of switching on and off is correct, and whether the indication of each meter head and indicator light is normal.

5. During operation, pay attention to the setting of the overcharge voltage and overdischarge voltage of the controller, which should meet the design requirements.

6. In the event of faults such as circuit break, overcurrent, overvoltage, and overheating, the photovoltaic controller and inverter will usually automatically shut down and enter the protection state. At this time, Sunshine Hengyu New Energy recommends that you do not turn on immediately! You should find out the cause, fix it, and then try to start it.

7. There is high voltage in the inverter cabinet, please lock the cabinet door on weekdays.

8. The ambient temperature > 30°C, and the heat should be dissipated in time to prevent the equipment from overheating and prolong the service life.

9. Check whether the temperature, sound, and smell in the machine are abnormal.

10. Check whether the cooling fans of modules, reactors and transformers in the inverter can automatically start and stop according to the temperature. When the fan is running, whether there is a large vibration or abnormal noise, once it is abnormal, the power will be cut off first, and then repaired.

11. Check the insulation resistance between the positive electrode to the ground, the negative electrode to the ground, and the positive and negative electrodes of the DC bus, and the resistance value should be > 2MΩ.

12. Check whether the wiring and terminals of each part are loose or corroded (such as fuses, fans, power modules, input and output terminals, grounding, etc.), and repair them immediately once they are found to be abnormal.

13. Regularly disconnect the circuit breaker on the output side (grid side) of the AC power grid for 1 time, and check whether the inverter can stop feeding the grid immediately.