First, the working principle and characteristics
Working principle
The core of the inverter device is the inverter switch circuit, referred to as the inverter circuit. The circuit completes the inverter function by turning on and off the power electronic switch.
Characteristics
(1) High efficiency is required.
Due to the high price of solar cells at present, in order to maximize the utilization of solar cells and improve the efficiency of the system, we must try to improve the efficiency of the inverter.
(2) High reliability is required.
At present, the photovoltaic power station system is mainly used in remote areas, and many power stations are unattended and maintained, which requires the inverter to have a reasonable circuit structure, strict component selection, and requires the inverter to have various protection functions, such as: input DC polarity reverse protection, AC output short circuit protection, overheating, overload protection, etc.
(3) The input voltage is required to have a wider range of adaptation.
Since the terminal voltage of the solar cell changes with the load and the intensity of the sun. Especially when the battery is aging, its terminal voltage varies widely. For example, for a 12V battery, its terminal voltage may vary between 10V and 16V, which requires the inverter to ensure normal operation within a large DC input voltage range.
Second, the classification of photovoltaic inverters
There are many methods for the classification of inverters, for example: according to the number of phases of the AC voltage output by the inverter, it can be divided into single-phase inverters and three-phase inverters; According to the different types of semiconductor devices used in the inverter, it can be divided into transistor inverters, thyristor invertersand shutdown thyristor inverters. According to the different principles of inverter circuits, it can also be divided into self-oscillating inverters, stepped wave superposition inverters and pulse width modulated inverters. According to the application in the grid-connected system or off-grid system, it can be divided into grid-connected inverter and off-grid inverter.
Third, the function of photovoltaic inverter
The inverter not only has the function of DC-AC conversion, but also has the function of maximizing the performance of the solar cell and the function of system fault protection. To sum up, there are automatic operation and shutdown functions, maximum power tracking control function, anti-independent operation function (for grid-connected system), automatic voltage adjustment function (for grid-connected system), DC detection function (for grid-connected system), DC grounding detection Function (for grid-connected systems).
(1) Automatic operation and stop function
After sunrise, the solar radiation intensity gradually increases, and the output of the solar cell also increases. When the output power required by the inverter is reached, the inverter starts to run automatically. After entering into operation, the inverter will monitor the output of the solar cell module all the time. As long as the output power of the solar cell module is greater than the output power required for the inverter to work, the inverter will continue to run; it will stop at sunset, even if it is cloudy and rainy. The inverter can also operate. When the output of the solar cell module becomes smaller and the output of the inverter is close to 0, the inverter will form a standby state.
(2) Maximum power tracking control function
The output of a solar cell module varies with the intensity of solar radiation and the temperature of the solar cell module itself (chip temperature). In addition, since the solar cell module has the characteristic that the voltage decreases with the increase of the current, there is an optimum operating point where the maximum power can be obtained. The intensity of solar radiation is changing, and obviously the optimal working point is also changing. Relative to these changes, the operating point of the solar cell module is always at the maximum power point, and the system always obtains the maximum power output from the solar cell module. This control is the maximum power tracking control. The biggest feature of inverters for solar power systems is that they include the function of maximum power point tracking (MPPT).
