Causes and Countermeasures of Low Insulation Resistance of Solar Grid-connected System

When the solar panels or DC cables and joints are damaged or the insulation layer is aging, the problem of low insulation resistance is easy to occur. When the DC cable passes through the bridge, since the edge of the metal bridge may have barbs, the outer insulation of the cable may be damaged during the threading process, resulting in leakage to the ground.

The hazards of low insulation resistance:
Low insulation resistance will cause system leakage. If the inverter is still connected to the grid at this time, it will cause the casing of the electrical equipment to be charged, which will bring hidden dangers of electric shock to people; the discharge of the fault point to the ground will cause local heating or electric sparks , causing fire hazards and other safety hazards.

Solutions for low insulation resistance:
For "PV insulation resistance is too low", the following treatment methods are generally adopted:

1.Check DC cables and grounding of components
First of all, the abnormal insulation resistance is caused by damaged DC cables, including cables between solar panels, cables between solar panels and inverters, especially cables in corners and cables laid in the open air without passing through pipes cables, carefully check whether the cables are damaged. Secondly, the photovoltaic system is not well grounded, including the grounding hole of the module is not connected, the module pressing block is not in good contact with the bracket, and some branch cable casings have water ingress, which will lead to low insulation resistance.

2.Rely on the inverter to check string by string
If the DC side of the inverter is connected by multiple channels, you can check the solar panels one by one. Only one string of solar panels is reserved on the DC side of the inverter. After starting up, check whether the inverter continues to report errors. If not, then It means that the insulation performance of the connected components is good. If the error continues to be reported, it is likely that the insulation of the string of solar panels does not meet the requirements.

3.Use a shaker or other professional equipment to detect string by string
During on-site inspection, use a megger to measure the insulation resistance of PV+/PV- on the module side to ground one by one. The impedance needs to be greater than the threshold requirement of the insulation resistance of the inverter. In some projects, special insulation measurement equipment can also be used.