01 - fault analysis

If it rains and the air is humid, it is easy to leak electricity, which indicates that the insulation of the components, cables or live parts of the inverter in the system is damaged. The general performance is that the inverter reports low insulation impedance fault, or the leakage protection switch trips. Once the photovoltaic modules, cables, connectors, switches and other equipment have grounding fault, it will not only lead to the power station can not be connected to the grid, affect the power generation, but also lead to the equipment failure or casualties of the whole power generation system. Therefore, before grid connected operation of photovoltaic inverter, it is necessary to detect the ground insulation to prevent electric shock.

The inverter has insulation fault and leakage current detection. When the system fails and the insulation value is lower than the set value of the inverter, the inverter will start the fault alarm program until it is shut down. If there is a problem in the inverter detection system or the alarm threshold is raised, the leakage current protection switch of the AC part will act.

02 - cause of failure

Insulation impedance of DC part is too low: insulation impedance is used to detect the DC part of photovoltaic system, including modules and DC cables. When the inverter detects that the insulation impedance of positive or negative side of modules to ground is too low, it indicates that the insulation impedance of cables or modules to ground is abnormal. Low insulation impedance is a common fault of photovoltaic system. Components, DC cables and joints are damaged. Aging insulation layer will produce low insulation impedance. When DC cables pass through the bridge, because there may be barbs on the edge of the metal bridge, in the process of threading, the outer insulation of the cable may be damaged, resulting in leakage to the ground.

AC Leakage Current: leakage current is also known as residual current of square array, which is caused by the parasitic capacitance between photovoltaic system and earth. When the parasitic capacitance photovoltaic system grid forms a loop, in the photovoltaic system without transformer, the loop impedance is relatively small, and the common mode voltage will form a larger common mode current on the parasitic capacitance between photovoltaic system and earth, That is leakage current.

The threshold value of DC insulation fault alarm is 50K Ω, and the threshold value of leakage current fault is 300mA. Therefore, when the insulation layer of DC part is damaged, the insulation impedance will be reported first, and the inverter will be shut down. Unless the particularly large DC cable is damaged, the leakage current fault will not be reported. When the inverter has leakage current fault, the internal part of the inverter and the AC cable will be checked generally.

03 - cause of failure

Construction factors: this situation is generally common. In many cases, faults occur frequently in nearly half a year after the completion of grid connection, which is mainly related to the construction quality. For example, in the construction process, the DC incoming cable of inverter is unqualified, the insulation performance is low, the damage caused by human or the connection terminal is not well protected, which may cause DC grounding, The number of on-site insulation problems can also reflect the construction quality of the project to a certain extent.

Environmental factors: it has a great impact on the application environment. This kind of fault alarm only occurs in cloudy and rainy days, or the installation environment is relatively humid for a long time, such as the common complementary project of fishing and light, so corresponding measures should be taken for this situation.

04 - Troubleshooting

Check by series: if the DC side of the inverter is multi-channel access, check the components one by one. Only one series of components are reserved on the DC side of the inverter. Check whether the inverter continues to report an error after starting up. If the inverter does not continue to report an error, it means that the insulation performance of the connected components is good. If the inverter continues to report an error, It is likely that the insulation of the string component does not meet the requirements.

Use the multimeter to check each circuit: when the AC and DC of the inverter are all disconnected, unplug each incoming line of the connected string, use the positive and negative poles of the multimeter to measure the voltage of the inverter to the ground respectively, first check whether there is difference between the strings, whether the voltage to the ground is directly 0, or close to 0, and compare with the normal operation inverter for confirmation, If this method can not be determined, further investigation is needed.

On site inspection of DC cable connection of components: first of all, the abnormal insulation impedance is caused by damaged DC cables, including cables between components, cables between components and inverters, especially cables in the corner and cables laid in the open without pipes. It is necessary to carefully check whether the cables are damaged. Secondly, the PV system is not well grounded, including the module grounding hole is not connected, the module briquette is not in good contact with the support, and some tributary cable bushing water, which will lead to low insulation impedance.

Use the megger or other insulation professional equipment to test in series: in the case of no light and no power cable, use the megger to measure the insulation resistance of the positive and negative pole of the module to the ground in series, and the impedance should be greater than the threshold value of the inverter insulation impedance.