TÜV Rheinland present results of Research on Safe Switching Operations in PV Systems

Together with industry partners E-T-A Elektrotechnische Apparate, Eaton Industries, and SMA, TÜV Rheinland had announced that it has embarked on a two-year research project in the field of photovoltaics in mid-2013. The research project is sponsored by the German Federal Ministry for Economic Affairs and Energy and aims at developing requirements which ensure that switch devices in photovoltaic systems do not malfunction in the course of their life cycle. In order to ensure that the switch devices in photovoltaic systems are safe, the TÜV Rheinland experts and the partners mentioned above identified and analyzed various faults and risks and assessed them in terms of their impact. "The safe function of photovoltaic components plays a decisive role in terms of acceptance and distribution, but also the returns of photovoltaic systems. So far, there are no sufficiently specific technical requirements for switch devices, especially for what are known as modular switches that are installed in such systems," Ralf-Martin Müller, business field manager at TÜV Rheinland and project leader, explains at the Intersolar 2015 trade fair in Munich. "In the context of this research project, we are deriving appropriate testing requirements from the results in order to enable switch device manufacturers to design and market safe and reliable switches and disconnectors."

Laboratory experiments as a basis for results
The effects of identified relevant faults were assessed. These include short circuits of the entire string, installation errors due to reverse polarity of the connectors, induced impulse voltage or impulse voltage caused by a lightning strike, or asynchronous switching behavior of modular switches. Then, the possible stresses for isolating devices, such as reverse currents, voltage doubling at the contacts, or transient overcurrents, were derived. "The technical standards that are currently available do not offer adequate solutions to many of the identified fault risks. The reason for this is that, in comparison to the traditional direct-current systems, particular faults can occur in photovoltaic systems," TÜV Rheinland expert Ralf-Martin Müller elaborates. "We developed laboratory testing programs to recreate the faults. In the individual programs, the switch devices are exposed to a sufficient amount of stress that is appropriate for the various test criteria. The gathered data is currently being evaluated and analyzed. In the next step, we will be working on defining a testing method on the basis of the laboratory and measurement results." The project partners' components are used for the laboratory experiments. In order to examine long-term stress in the field, continuous load tests were defined and conducted under various environmental and temperature influences. To ensure that the laboratory experiments can be performed later in a way that is reasonable from an economic perspective, comparative measurements were performed to examine the switching behavior during activation/deactivation operation with both a traditional B6 bridge rectifier and a photovoltaic simulator source. The first results reveal that the current and voltage characteristics are similar for both DC sources.

Preparation of test specifications
Once all the measurement results from the various laboratory experiment programs are available, they will be collected and defined in the form of a specification. The resulting technical groundwork contains both electrical and mechanical tests for determining the possible applications and qualification that allow the switch devices to be operated in a reliable way. The final results of the research project are expected at the end of 2015 and will then be available for transfer to national and international standardization committees.