The inverter has the following main functions:
(1) Automatic operation and shutdown. After sunrise in the morning, the solar radiation intensity gradually increases, and the output of the photovoltaic cell also increases. When the output power required by the inverter is reached, the inverter will run automatically. After the inverter is running, the output of the photovoltaic array will be monitored at all times. As long as the output power of the photovoltaic array is greater than the input power required for the inverter to work, the inverter will continue to run until it stops at sunset.
(2) Maximum power tracking limit. The output power of the photovoltaic array has non-linear characteristics and is affected by factors such as load status, ambient temperature, and sunshine intensity. The maximum power point of its output changes all the time. If the load operating point deviates from the maximum power point of the photovoltaic cell, the output power of the photovoltaic cell will be reduced.
Maximum power tracking control is to make the photovoltaic array work at the maximum power point under a certain control strategy to improve its energy conversion efficiency.
(3) Island detection. When the grid power supply fails or is repaired due to a power outage, the photovoltaic grid-connected power generation system on the user side fails to detect the power outage in time, a self-sufficient power supply island composed of the photovoltaic power generation system and the surrounding loads.
The islanding effect may adversely affect the entire power distribution system and the equipment on the user side, such as: endangers the safety of maintenance personnel of transmission lines, affects the action procedures of protective switches on the power distribution system, and the power supply voltage and frequency in the power island area will be unstable, and when the power supply is restored, the phase is not synchronized, and the photovoltaic power supply system causes the lack of phase operation of the system’s three-phase load due to the single-phase power supply. Therefore, the photovoltaic grid-connector should have the ability to detect the islanding state and immediately disconnect from the grid.
(4) Automatic voltage adjustment. When the photovoltaic power generation system is connected to the grid, if there is a reverse current operation, the voltage at the receiving point will rise due to the reverse transmission of electric energy, which will exceed the operation range specified by the grid. In order to avoid this situation, it is necessary to set up an automatic voltage adjustment function to prevent the voltage from rising.
(5) DC detection. Check the DC input voltage. When the DC voltage is too high or too low, the inverter will stop working.
In the inverter, because the high-frequency switch is used to control the semiconductor device, the output of the inverter has a slight DC superimposition due to the unbalanced operation of the components. In the inverter with built-in power frequency insulation transformer, the DC component is isolated by the insulation transformer, and there is no DC outflow on the system side; in the high-frequency transformer insulation method and the transformerless method, because the output of the inverter is connected to the system, there is a DC component, which causes the magnetic saturation of the pole transformer on the system side. In order to avoid this situation, high-frequency transformer insulation or transformerless inverters require the DC component superimposed on the output current to be controlled below 1% of the rated AC current. In addition, it is necessary to set up a DC control function that suppresses the DC component. Once this function fails, the inverter’s protection function will stop operating.
(6) DC grounding detection function. In a transformerless inverter, because the photovoltaic cell and the system are not insulated, safety measures for the grounding of the photovoltaic cell are required. A leakage circuit breaker is usually installed at the power receiving point (distribution board) to monitor the grounding of indoor power distribution lines and load equipment. Photovoltaic cells are grounded, and the ground current is superimposed with DC components, which cannot be protected by ordinary leakage circuit breakers. Therefore, a DC grounding detector must be installed inside the inverter to realize the detection of DC grounding protection function, and the detection current is mostly set to 100 mA.
(7) Other protection functions. The inverter shall have protection or alarm functions such as overheating, lightning strikes, abnormal output, and internal faults.
(8) Protection automatic recovery function. After the inverter has a protective shutdown in various abnormal conditions, it can automatically resume operation after the fault is eliminated.