German Solar Manufacturers and Testing Institutes declare war on leakage currents in photovoltaic systems

SCHOTT Solar AG has developed a test for measuring the so-called PID effect immediately after its modules have been manufactured in collaboration with the solar manufacturers Q-Cells SE and Solon SE and the independent test laboratories Fraunhofer-Institute for Solar Energy Systems (ISE), Photovoltaic Institute Berlin AG (PI-Berlin), TÜV Rheinland Energie und Umwelt GmbH (TÜV) and VDE Prüf- und Zertifizierungsinstitut (VDE). Potential Induced Degradation (PID) refers to a drop in performance in a PV system that can be caused by high negative voltage. The test series performed by several independent test laboratories revealed that the drop in performance of the glass-foil modules from SCHOTT Solar is less than 5%. As expected, these modules can therefore be considered to be PID resistant. The modules from the other two companies who participated also passed the test. Several of the modules from other brand name manufacturers experienced drops in performance well in excess of 50 percent, however.

The test conditions call for modules from the current production series to be connected to voltage of -1,000 V for seven days. In order to be able to obtain exact comparison values, all of the modules are covered with aluminum foil and tested at a constant temperature of 25 degrees Celsius. A module is considered to be PID resistant if it loses less than five percent of its nominal capacity during this period.

This means a draft of a test that is capable of assessing and clearly distinguishing the stability of crystalline modules with respect to PID quickly and easily is now available. A further development of the test conditions that is planned for the near future in cooperation with the IEC TC82 standardization body will make it possible to make a statement on the behavior of these modules in the field while taking the exact connections and climatic conditions into consideration. "This is the focus of current research work on the PID effect," the researcher involved, Dr. Michael Köhl from the Fraunhofer-Institute for Solar Energy Systems, notes.

"The PID test that has now been developed is designed to ensure that our customers have tested quality installed on top of their roofs. As a leader in the area of quality, we guarantee this with our good name," explains Klaus Wangemann, head of development at SCHOTT Solar AG.

Basically, the PID effect can occur with all crystalline silicon solar cells that are embedded in glass-foil modules. Leakage currents can result under unfavorable conditions (accelerated by high humidity and temperatures) where the module joins with the frame and causes short circuits that lower the overall performance of the system. With the so-called double glass modules, this effect manifests itself to a significantly lower extent from the very beginning. It can be avoided at the system level if the system is grounded or inverters that do not generate negative voltages are used. However, it is even more economical to prevent PID at the module or cell level. And this can be done quite successfully, as the test results show.