Photovoltaic grid connected inverter

Inverter is used for converting DC into AC in photovoltaic power station. Grid connected system puts forward higher requirements for inverter.

① The inverter output is sine wave, and the high-order harmonic and DC components are small enough to avoid harmonic pollution to the power grid.

② The inverter can operate efficiently when the load and sunshine change greatly, that is, the inverter is required to have the maximum power tracking (MPT) function, which can be automatically adjusted to achieve the maximum power output no matter how the sunshine and temperature change.

③ It has advanced anti islanding operation protection function, that is, when the power grid loses power, the system will be automatically cut off from the power grid to prevent damage to operation, maintenance and maintenance personnel caused by separate power supply

④ It has the functions of automatic grid connection and disconnection. When the sun rises in the morning and the sunshine meets the power output requirements, it will be automatically put into power generation operation of the power grid. When the sunset output power is insufficient, it will be automatically disconnected from the power grid.

⑤ It has the function of automatic regulation of output voltage. During grid connected reverse power flow up transmission, the voltage and up transmission power shall be adjusted at any time with the change of parallel node voltage.

⑥ It has complete grid connection protection function. In case of abnormality at the system side or inverter side, the power generation system shall be cut off quickly, i.e. overvoltage and undervoltage protection, over-frequency and under frequency protection, etc., so as to meet the requirements of unattended remote monitoring. The circuit structure of the grid connected inverter is shown in Figure 3-7. Through the three-phase full bridge inverter, the DC voltage of the photovoltaic array is transformed into a high-frequency three-phase AC voltage, filtered into a sine wave voltage, isolated and boosted by the three-phase transformer, and then incorporated into the power grid for power generation.

The photovoltaic grid connected inverter can adopt DSP control chip and current controlled PWM active inverter technology, with a wide DC input voltage range of 220 ~ 450V; The grid connected inverter in the system continuously detects whether the photovoltaic array has enough energy for grid connected power generation. When the grid connected power generation conditions are met, that is, the array voltage is greater than 240V and maintained for 1min, the inverter power supply changes from standby mode to grid connected power generation mode to convert the DC power of photovoltaic array into AC power and connect it to the power grid. At the same time, in this mode, the inverter always maximizes the output energy of photovoltaic array in MPPT mode, which effectively improves the utilization rate of solar energy in the system. When the solar radiation is very weak, that is, the array voltage is less than 200V or at night, the photovoltaic array does not have enough energy to generate power, and the inverter is automatically disconnected from the power grid

Lightning protection design of photovoltaic system

(1) Causes and hazards of lightning

Lightning is a common phenomenon of atmospheric discharge. A large number of positive or negative charges are accumulated in different parts of the cloud. When the positive or negative charges in the cloud accumulate more and more and reach a certain intensity, it will break down the air and open a narrow channel for forced discharge. Because the energy released by lightning is quite large, its strong current, hot high temperature, strong shock wave, drastic electrostatic field and strong electromagnetic radiation have brought a lot of harm to people. When lightning strikes the building directly, the strong current makes the building heated, vaporized and expanded, resulting in building combustion or explosion. When the lightning strikes the lightning arrester and the current is discharged to the earth along the down lead, it will cause fire or personal casualties. Induced lightning damage is also called secondary damage. Induction mine is divided into electrostatic induction mine and electromagnetic induction mine. Due to the large variation gradient of lightning current, it will produce a strong alternating magnetic field, causing the induced current of the surrounding metal components. This current may discharge to the surrounding objects. If there are combustibles nearby, it will cause fire and explosion, and if it is induced on the on-line conductor, it will cause strong damage to the equipment.

(2) Lightning protection and design requirements of solar photovoltaic power generation system

① When selecting the construction location of solar photovoltaic power generation system or power station, try to avoid locations and places vulnerable to lightning.

② Try to avoid the projection of lightning rod falling on the solar cell module.

③ According to the site conditions, different protective measures such as lightning rod, lightning strip and lightning network can be adopted to protect the direct lightning, reduce the probability of lightning stroke, and try to use multiple uniformly arranged downleads to lead underground. The shunting effect of multiple downleads can reduce the lead voltage reduction of downleads, reduce the risk of side strike, and reduce the magnetic field intensity generated by downlead discharge.

④ In order to prevent lightning induction, all metal objects of the whole photovoltaic system, including battery module outer frame, equipment, cabinet shell, metal pipeline, etc., shall be equipotentially connected with the joint grounding body, and can be grounded independently.

⑤ Lightning protection devices shall be installed on the system circuit level by level to implement multi-level protection, so that the lightning strike or switch surge current can be discharged through multi-level lightning protection devices. Generally, DC power lightning arrester is used in the DC line of photovoltaic power generation system, and AC lightning arrester is used in the AC line after inverter.