Technical requirements for grid-connected inverters

The grid-connected operation of the photovoltaic power generation system puts forward higher technical requirements for the inverter. These requirements are as follows.

(1) It is required that the system can automatically start and shut down the system according to the sunshine conditions and the specified sunshine intensity, and under the condition that the electricity generated by the photovoltaic array can be effectively utilized.

(2) It is required that the inverter must output a sine wave current. The power fed into the public grid by the photovoltaic system must meet the specified indicators of the grid. For example, the output current of the inverter must not contain DC components, and the high-order wave must be minimized, so as not to cause harmonic pollution to the grid.

(3) The inverter is required to operate efficiently when the load and sunshine vary widely. The energy of the photovoltaic system comes from solar energy, and the sunlight intensity changes with the climate, so the input DC voltage changes greatly during operation, which requires the inverter to operate efficiently under different sunlight conditions. At the same time, the inverter itself is required to have higher inverter efficiency. Generally, the inverter efficiency requirements of medium and small power inverters at full load are 88%~93%, and the inverter efficiency requirements of high power inverters under full load are 95%~99%.

(4) The inverter is required to make the photovoltaic array always work at the maximum power point state. The output power of the battery module is related to the changes of the sunlight intensity and the ambient temperature, that is, its output characteristics have a nonlinear relationship. This requires the inverter to have the maximum power point tracking control function (MPPT control), that is, no matter how the sunshine, temperature, etc. change, the maximum power output of the battery module square array can be achieved through the automatic adjustment of the inverter. This is an important link to ensure the high-efficiency work of the solar photovoltaic power generation system.

(5) High reliability is required. Many photovoltaic power generation systems are in remote areas and unattended and maintained states, requiring the inverter to have a reasonable circuit structure and design, with certain anti-interference ability, environmental adaptability, instantaneous overload protection ability and various protection functions. Such as input DC polarity reverse protection, AC output short circuit protection, overheat protection, overload protection, etc.

(6) It is required to have a wide range of DC voltage input adaptation, and the output voltage of battery components and square arrays will change with changes in sunlight intensity and climatic conditions. For grid-connected photovoltaic systems connected to batteries, although the battery has a certain clamping effect on the output voltage of the battery module, the voltage of the battery itself fluctuates with the change of the remaining power and internal resistance of the battery, especially the photovoltaic system without the battery or the photovoltaic system when the battery is aging, its terminal voltage has a wide variation range. For example, for a photovoltaic system connected to a 12V battery, its terminal voltage will vary from 11V to 17V. This requires that the inverter must be able to work normally within a wide DC voltage input range and ensure the stability of the AC output voltage.

(7) The inverter is required to have the function of grid detection and automatic grid connection. Before the grid-connected inverter is connected to the grid to generate electricity, it needs to take power from the grid and detect the parameters such as the voltage, frequency, and phase sequence of the grid, and then adjust the parameters of its own power generation to keep synchronized and consistent with the parameters of the grid, and then enter the grid-connected power generation state.

(8) It is required that in the event of a power failure in the power system, the grid-connected photovoltaic system can not only operate independently, but also prevent the islanding effect, and can quickly detect and cut off the power supply to the public power grid to prevent the occurrence of electric shock accidents. After the utility grid restores power supply, the inverter can automatically restore the grid-connected power supply.

(9) Zero (low) voltage ride-through function is required. When an accident or disturbance occurs in the power grid system, resulting in a voltage sag at the grid-connected point of the photovoltaic power generation system, the inverter must be able to ensure continuous operation without being disconnected from the grid within a certain voltage drop range and time interval.