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The function of grid-connected PV inverter
- Aug 10, 2018 -

The PV inverter can convert the variable DC voltage generated by the photovoltaic solar panel into a mains frequency alternating current (AC) inverter, which can be fed back to the commercial transmission system or used by the off-grid grid. Photovoltaic inverters are the core components in photovoltaic systems and can be used with general AC-powered equipment.

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Types and main functions of photovoltaic inverters

Photovoltaic inverters are classified into grid-connected inverters, off-grid inverters, and micro-grid energy-storing inverters according to their purposes. The grid-connected inverters can be divided into micro-inverters and strings according to power and usage. Inverter, centralized inverter, distributed inverter, four types, micro inverter, also known as component inverter, power class is 180W to 1000W, suitable for small power generation system; string inverter Single-phase inverter with power from 1kW to 10kW, suitable for household power generation system, grid voltage of 220V, 4kW to 80kW three-phase inverter, suitable for industrial and commercial power generation system, grid voltage is three-phase 380V. Centralized inverters and distributed inverters, with powers ranging from 500 kW to 1500 kW, are commonly used in large ground power plants.

The grid-connected inverter is a key component for connecting the PV array and the grid. In addition to turning the DC power generated by the component into AC power that the grid can receive, the following special functions are also available:

1) MPPT function of maximum power tracking

The output power of pv modules is affected by sunshine intensity, environmental temperature and other factors. When the light intensity decreases, the open circuit voltage of pv modules decreases, the short circuit current decreases, and the maximum output power decreases. When the pv module temperature drops, the short circuit current of the module decreases, but the open-circuit voltage of the module increases and the maximum output power increases. In the case of a certain component temperature and sunshine intensity, there is only one maximum power output point for the same component, and the function of MPPT is the maximum power tracking function. By adjusting dc voltage and output current, the solar module can always work at the maximum working point and output the maximum power under the current temperature and sunshine.

Common maximum power tracking control methods include: constant voltage tracking (CVT), which fixes the terminal voltage of pv modules at a fixed value. The characteristics are simple control and good stability. Classic control algorithms such as power calculation method, current optimization method, disturbance observation method, incremental conductance method, optimal gradient method, fuzzy logic control method, neural network control method and other modern control algorithms.

2) Detection and control of islanding effect

In normal power generation, the photovoltaic grid-connected power generation system is connected to the large power grid to deliver active power to the grid. However, when the grid is de-energized, the PV grid-connected power generation system may continue to work and be in independent operation with the local load. This phenomenon is called the island effect. When the inverter has an islanding effect, it will cause great safety hazard to the personal safety, power grid operation and the inverter itself. Therefore, the inverter grid entry standard stipulates that the PV grid-connected inverter must have the islanding effect detection and control function. .

The detection method of the islanding effect includes passive detection and active detection. The passive detection method detects the amplitude of the voltage and current at the output of the grid-connected inverter. The inverter does not add an interference signal to the grid, and detects the current phase offset and frequency. Whether the parameter exceeds the specified value to determine whether the power grid is out of power; this method will not cause grid pollution and no energy loss; and active detection means that the grid-connected inverter actively and regularly applies some interference signals to the grid. For example, frequency shifting and phase shifting, since the power grid can be regarded as an infinite voltage source, when there is a power grid, these interference signals will be absorbed by the power grid. If the power grid is out of power, these interference signals will form positive feedback, which will eventually form a frequency or The voltage is exceeded, and it can be judged whether the islanding effect has occurred.