Although solar modules are normally far more reliable than inverters, individual modules may over time exhibit anomalies that are attributable to installation errors or substandard module quality. Figures 4.100-4.103 illustrate this type of anomaly in Siemens M55 installations, most of which had been in operation for well over 10 years when these pictures were taken. These anomalies were observed by PV Lab technicians during an inspection. Thermographs are extremely useful for detecting solar generator anomalies, including incipient problems.
The anomalies illustrated in Figures 4.100 and 4.101 are attributable to installation errors that can be avoided through the use of high-quality special PV plug-in connectors.
Figures 4.102 and 4.103 display a manufacturing-induced module anomaly.
Figures 4.102 and 4.103 show this type of hot-spot anomaly, which damaged the module insulation. These hot spots were induced by substandard contact strip transitions, where resistance increased steadily and the temperature rose so high that the backsheet was damaged and the insulation was compromised.
Anomalies can occur over time not only in individual modules, but also in generator junction boxes. Figures 4.104-4.106 display damage induced by a smouldering fire in a generator junction box at an 11- year-old installation in Burgdorf.
In this PV installation (15 kWp, Vmpp ~ 500 V) there is more than a 100 V difference between open – circuit voltage Voc and MPP voltage Vmpp. This is more than enough to allow for the development of stable arcing, for which the combustion voltage is 30Vor higher.
An anomaly was also observed in the generator junction box of an older PV installation (3.2kWp, Vmpp ~ 100 V) resulting from a fused jumper that connected the two terminal blocks for five strings.
But no arcing occurred in this case as there was only about a 20 V difference between the open-circuit voltage VOC and the MPP voltage VMPP. In fact, following a brief fusing phase between the two terminal blocks induced by the burned material, the anomalous process came to a halt. Figure 4.107 displays the fused bridge (left) and the thermographic image of the generator junction box in go-live operation. This picture shows that the temperature rose substantially solely on the left, active side of the box, but hardly rose at all on the right, inactive side.
The long-term damage shown here was detected by the PV Lab as part of a long-term monitoring project that began in 1992 and now encompasses more than 60 PV installations. This installation mainly used (since discontinued) M55 modules from Siemens Solar, which was the market leader at the time. Whereas the damage shown in Figures 4.100 and 4.101 would be very unlikely to occur today owing to the predominant use of special PV plug connectors (although substandard or partly plugged-in jacks can overheat), modern modules are prone to other problems, one example being the ageing of internal connections that are on the way to failure in Figures 4.102 and 4.103. Two synchronous defects have been observed in two modules to date, with the result that the current in the module string affected flowed
Figure 4.105 Close-up of the negative terminals which are seared and fused
through only one bypass diode that became severely overheated and will ultimately fail. Bypass diodes are also prone to faults, particularly if they have been damaged by a power surge induced by a nearby lightning strike (see Section 126.96.36.199). Although solar generators have proven to be more reliable than inverters, the former are nonetheless prone to ageing-induced problems and thus a certain number of spare modules should be kept on hand, particularly for integrated installations.