Hazard of low insulation impedance
Low insulation impedance may cause system leakage. If the inverter is still connected to the grid, it will cause the electrical equipment housing to be charged, which will bring the safety hazard of electric shock; Discharging to the ground at the fault point can lead to local heating or sparks, which will bring safety risks such as fire.
Reasons for low insulation impedance
Low insulation impedance is more likely to occur when components, DC cables, and connectors are damaged, and the insulation layer is aging. When a DC cable passes through a bridge, there may be barbs on the edges of the metal bridge, which may damage the outer insulation of the cable, resulting in leakage of electricity to the ground.
The principle of inverter insulation impedance detection is: Before connecting to the grid, the inverter calculates the resistance of PV+ and PV- to the ground by detecting their respective voltage levels to the ground. If the resistance value of either side is lower than the threshold value, the inverter will not be able to be connected to the grid, and the alarm display will show “F24 DC Insulation Failure”.
The insulation impedance protection parameter threshold is set in the firmware of DEYE inverters. When the actual insulation impedance value of the PV module is less than the set value, the insulation impedance alarm module contained in the inverter will trigger an alarm. In the event of an insulation fault alarm, you can safely turn off the inverter and disconnect it from the connected string for further investigation.
Solutions for low insulation impedance
- Check inverters one by one
If the DC side of the inverter is connected in multiple channels, you can check the components individually. Only one string of components is retained on the DC side of the inverter, and after powering on the inverter, check whether the inverter continues to alarm. If it continues to report errors, it means that it is very likely that the insulation of the string of components needs to comply with the requirements. if not, then it means that the connected components are well-insulated.
- On-site inspection of DC cables and grounding of components
First of all, the abnormal insulation impedance is partly due to the DC cable damage, including the cables between components, the cables between components and the inverter, especially the cables in the corner and the cables laid in the open without pipes, you need to carefully check whether the cables are damaged. Secondly, the insulation impedance of the photovoltaic system is low if the grounding hole of the component is not connected, the component compression block is not in good contact with the support, and some tributary cable bushings are filled with water.
- String-by-string inspection with a megger or other professional equipment
During the on-site inspection, the insulation resistance of PV+/PV- to ground on the module side should be measured string by string with a megger to ensure that the cables are not electrified, and the impedance needs to be greater than the threshold value of the inverter’s insulation impedance, and special insulation measuring equipment can be borrowed for some of the projects.
To ensure the long-term reliable operation of PV power plants, the installation and construction process is very critical. Testing the insulation resistance of the strings to the ground is a necessary step before connecting the PV modules to the inverter.
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