Every PV operator knows the following error situation: one or more inverters start late, especially on rainy days. The message is “Fault – Insulation Fault”.
The background is that each inverter in a PV system carries out an insulation measurement before synchronizing with the grid. If the measured resistance value is below a preset limit, the inverter must not be connected to the grid.
Because the individual PV modules are connected in series to form strings, very high system voltages, in the range of approx. 300 – 1,000 V, can occur in the case of PV systems. To ensure that there is no earth potential equalization, all system components must have sufficient insulation. The ideal current to earth, which is very small, is called residual or earth leakage current. The residual current consists of the partial fault currents from the individual components (PV modules, DC cabling and inverters). The insulation resistance of the components can be considered as a parallel connection to earth.
A PV module must have an insulation resistance of at least 40 Mohm/m² (DIN IEC 61215, DIN EN 61646). The insulation resistance of a module can be calculated as follows (minimum value):
For a module array with identical module types:
Inverters measure and monitor the total insulation resistance value of all connected components. The threshold value is at Riso> 1 kOhm/V, but must be at least 500 kOhm (according to DIN VDE 0126-1-1). Exceptions are PV systems with galvanic isolation from the mains (using transformers). Due to the conflict of standards in plants with large generator areas, determining the threshold value was recently developed by SMA Solar Technology AG together with the German professional association (Berufsgenossenschaft).
The minimum value was reduced to 200 kOhm.
As the formula for the RisoModule array shows, the insulation resistance becomes smaller and smaller as the module array gets larger. This can lead to the result that although the modules still meet the required 40 MOhm/m², the threshold value stored in the inverter is not reached.
Rated inverter power: 9,000 W
Module area: 1.58 m x 0.808 m
Number of modules: 52
Thus the value is still just above the required minimum value of 200 kOhm (before 2010: 500 kOhm), but now also the insulation values of the other components must be added. In addition, the stored threshold values are higher in older inverters. For the example system the threshold value is approx. 1,000 kOhm; this can be read from the graph.
The insulation value moved to about 500 kOhm at about 9:00 o’clock. As the calculation has shown, the insulation value of the module array can even drop to 602.5 kOhm without the modules having a defect. In this case, probably no fault will be found on the system during an on-site call-out.
It is very important that insulation errors are carefully analysed before a service call-out is planned. For this, it is essential that the data logger records the insulation value measured by the inverter and, ideally, the errors and event messages.
Rated inverter power: 12,000 W
Module area: 1.58 m x 0.808 m
Number of modules: 64
In this example, the insulation value is significantly below 100 kOhm until about 11:00 am; the lowest permissible insulation value of the modules is 489.57 kOhm. So it can be assumed that there really will be on-site insulation problems. In this case the device which causes the fault must be checked on site.
The fact that inverters start late on rainy days is not a rare occurrence in photovoltaics, but the cause does not always have to be a defect. Since it is very difficult to find an insulation fault on site, checks should be carried out to determine if a defect is the cause before a service call-out. Note that the threshold value stored in inverters is not standardized. The value is generally between 200 kOhm and 1,000 kOhm (for questions about the threshold value, simply contact the relevant inverter manufacturer).
It is also clear how important it is that data loggers record and send the value of the insulation measurement. The same also applies to the inverter’s event log. This knowledge can also be applied to other faults in a PV system: the more detailed the operator’s analysis, the better is the preparation that can be made prior to on-site service interventions, possibly even avoiding them.