Long-term behaviour of PV connectors

PV systems have a long lifespan and require practically no maintenance except for operational monitoring, which is usually automated. The cabling in the module field is often used for decades without requiring any intervention. However, its performance also depends on the quality of the connectors installed and the correct handling during installation. The Original MC4 connector – made by the world market leader Stäubli Electrical Connectors AG, previously called Multi Contact – is the most widely installed connector and has set the standard worldwide.

Client
 

This research project seeks to characterise the ageing behaviour of PV connectors in operation. A wide range of PV connectors used in various PV systems will be analysed at BFH’s Laboratory for Photovoltaic Systems. The ageing behaviour will be evaluated based on the following factors:

• Age of the connectors
• Situation where connectors are installed (protection from rain/sun/extreme temperatures)
• Type of connector (original connector, cross connection)
• Type of crimping (carried out by the manufacturer or in the field)

It is hoped the results will provide manufacturers with input on improving product quality while also giving the PV industry a better understanding of the ageing of a critical component of PV systems.

Firstly, a test procedure to analyse the connectors will be defined, combining requirements from three IEC, one UL and three EN standards. In particular, the test criteria of IEC 62852 “Connectors for DC-application in photovoltaic systems – Safety requirements and tests” will be applied. In addition to precisely measuring connector resistance under different current ratings, humidity and high-voltage testing will also be carried out.

The connectors will be tested on the widest possible range of PV systems. The following procedure will be used for testing the connectors:

1. System operators and installation companies interested in participating can contact project leader Matthias Burri. Detailed instructions on supplying the connector samples will be provided.
2. For each PV system, at least three connector samples must be provided between two PV modules and three samples between a PV module and cabling. When removing and transporting the connectors, it is important that the connector and cable are moved as little as possible. Under no circumstances must they be separated. Detailed requirements concerning the removal and transportation of connectors will be provided by the Laboratory for Photovoltaic Systems.
3. The connectors will be tested at BFH’s Laboratory for Photovoltaic Systems in Burgdorf.
4. The companies supplying the connectors will receive feedback on the condition of their connectors.

Stäubli will provide any replacement connectors and extension cable required free of charge. The cost of replacing the connectors will be borne by the parties wishing to test them. In return, they will receive information about the state of their components.

The findings were as follows:

• MC3 PV connectors demonstrate the lowest contact resistance of those tested.
• PV connectors from different manufacturers and of various types were examined, as well as so-called cross-connections.
• MC4 PV connectors also demonstrate low values without a large number of outliers (values that are outside 1.5 times the interquartile range).
• Cross-connections show higher contact resistances.
• Contact resistances of more than 5 mΩ were only measured for the cross-connections (normative limit value for new PV connectors according to IEC 62852).
• One cross-connection showed massively higher resistances in the >45 mΩ range. It was also severely deformed so that it could no longer be separated

With regard to the various PV systems, the following observations were made:

• Large differences are apparent between different installation sites.
• One PV plant shows a high degree of scatter and larger outliers. This system was reinstalled and the PV connectors were stored unprotected on the construction site for a long time.
• Another PV plant also demonstrates increased values and has a high outlier. This installation is located near a sewage treatment plant and the connections also fail the insulation strength test. A clear connection between contact resistance and heating was determined at the power load of the connections with current ratings.

The high voltage and leakage current tests showed the following results:

• All test specimens passed the first high-voltage test at the beginning of the measurement process.
• After the connections had also passed through the heating test, the insulation resistance test and the IP permeability test, there were various examples that did not pass the second high-voltage test.

However, based on the investigations carried out, no clear correlation can be established between observations (e.g. humidity in the plug-in connector) and failure of the insulation test, high-voltage test and IP test.

The investigations could only be carried out on a small sample size, and certain variables that were applied for the interpretations are interdependent.

For example, a system showed high contact resistances where it is known that the PV connectors were left open on the construction site without protective caps during a system reinstallation.

Despite these limitations, the tests show that, leaving PV connectors exposed may have negative effects. The manufacturer’s warning is also justified. This states that PV connectors from different manufacturers should not be cross-connected.

It is not yet possible to make more precise assertions about the causes of increased contact resistance, because the PV connectors were not opened or examined using other imaging methods.

The PV laboratory is continuing the measurement campaign and is looking for more PV connectors for this purpose. Anyone who has a PV system, especially one that requires reinstallation or repair due to hail or lightning damage, can contact the project manager Matthias Burri: matthias.burri@bfh.ch.