How does low insulation impedance affect power generation?
After the installation of the photovoltaic system, users are most concerned about power generation, as it is directly related to the user’s return on investment. There are many factors that affect power generation. This article mainly discusses the impact of low insulation impedance on the photovoltaic system.
Detecting the insulation impedance of the array is a mandatory standard and requirement for inverters. When the insulation impedance of the photovoltaic array is detected to be less than the specified value, the inverter must display a fault. For non-isolated inverters, it must be shut down and cannot be connected to the grid. If an inverter shows “insulation impedance is too low”, it means that the inverter has detected that the insulation impedance of the positive or negative pole on the component side to the ground is too low, indicating that there is an abnormal situation in the insulation impedance to the ground of the DC side cable or component.
1. Requirements for insulation resistance detection
Inverters connected to ungrounded photovoltaic arrays:
Inverters connected to ungrounded photovoltaic arrays should measure the DC insulation resistance between the input end of the photovoltaic array and the ground before the system is started. If the impedance is less than U…/30mA (U… is the maximum output voltage of the photovoltaic array), then:
a): For inverters with isolation, a fault should be indicated, but other actions and operations can still be carried out during the fault. It is allowed to stop the alarm when the insulation resistance meets the above requirements;
b): For non-isolated inverters or inverters with isolation but whose leakage current does not meet the requirements, a fault should be indicated and its connection to the grid should be restricted.
2. The principle of inverter insulation impedance detection
3. Causes of low insulation impedance
When components, DC cables, or joints are damaged or the insulation layer is aged, the problem of low insulation impedance is likely to occur. When the DC cable passes through the cable tray, because there may be barbs on the edge of the metal cable tray, the outer insulation layer of the cable may be damaged during the threading process, resulting in leakage to the ground.
4. Solutions for low insulation impedance
For “PV insulation impedance is too low”, the following treatment methods are generally used:
4.1. Check the DC cables and grounding conditions of the components
First of all, the reason for the abnormal insulation impedance in some parts is caused by the damage of the DC cables, including the cables between components and the cables from components to the inverter. Especially the cables in the corners and the cables laid outdoors without threading through pipes need to be carefully checked for damage. Secondly, the photovoltaic system is not well grounded, including the unconnected grounding holes of components, the poor contact between component pressing blocks and brackets, and the water ingress in some branch cable sleeves, which will all lead to low insulation impedance.
4.2. Rely on the inverter to check string by string
If the DC side of the inverter has multiple accesses, the method of checking one by one can be used to test the components. Only one string of components is reserved on the DC side of the inverter. After starting up, check whether the inverter continues to report errors. If it does not continue to report errors, it means that the insulation performance of the connected components is good. If it continues to report errors, it means that it is very likely that the insulation of this string of components does not meet the requirements. For example, the Growatt MAC 60KTL3 – X LV inverter is connected to 8 strings. If a certain string is pulled out and the fault alarm disappears, it means that this string has a fault.
4.3. Use a megger or other professional equipment to test string by string
During on-site inspection, use a megger to measure the insulation resistance of PV+/PV- on the component side to the ground string by string. The impedance needs to be greater than the threshold requirement of the inverter’s insulation impedance. In some projects, special insulation measuring equipment can also be borrowed.
5. Summary
In order to ensure the long-term and reliable operation of the photovoltaic power station, the installation and construction process is very critical. Before the photovoltaic components are connected to the inverter, it is very necessary and an indispensable step to first test the insulation resistance of the string to the ground.