PHOTON International The Photovoltaic Magazine

Reverse current overload tests for modules in Europe

If you compare US module specs with those of European companies, you'll find that specs on the other side of the Atlantic contain information nobody on the Continent has requested so far – for instance, the reverse current overload parameter. With the new European EN 50380 norm, things are about to change. And maybe before this year is out, the new international standard IEC 61730 will include reverse current overload tests as well.

	© photon-pictures.com
Against diodes: Werner Knaupp, co-author of the EN norm 50380 says that diode defects are the major reason why reverse current measurements should be required.

To test reverse current overload durability, modules will be subjected to reverse current for 8 hours in complete darkness and at a temperature of 60 °C. Afterwards, the panel will be operated under light at standard test conditions (STC) to check if it still can produce its rated power. It may sound like a lot of effort to measure a single value that up until now was deemed unimportant – and what's more, the test sounds rather expensive.

In the US, the use of data for reverse current overload is quite common – the reason being that, in the past, Americans preferred central inverters with parallel connected strings in their PV systems, each having an electrical fuse with its value recorded in the module's specs. Depending on the module, this fuse rating can be anywhere between 1.5 to 4 times the short-circuit current.

The more parallel module strings connected to an inverter, the greater the danger of reverse current overload. For instance, if voltage greater than the open circuit voltage is forced into a string, the current will begin to flow in reverse. This reverse current, however, is not necessarily a danger to solar cells. But if it increases beyond a certain point, the module will begin to warm, which can damage cells and contacts. In any case, reverse current will reduce yield. In Europe, diodes are the method of choice to prevent this from happening.

The reverse current overload test is part of the hotspot test in the safety standard UL1703. Its description bears a very close resemblance to the demands of IEC 61730, with one key exception: since it is a safety standard, UL does not care whether the module still works properly afterwards.

Dieter Winkler, sales manager at Solarwatt, a German module manufacturer with plans for 20 MW in 2004, can't recall a single instance in which a customer requested information on reverse current overload capacity. His colleague and director of development Markus Münch considers reverse current capacity tests simply superfluous. Likewise, Ewan Dunlop from the European Commission's Joint Research Centre does not see the need for the tests, as the aim of EN 50380 is to »provide minimum information required to configure a safe and optimum system with photovoltaic modules.« Others complain about the expense the tests will entail, though they requested that their names go unmentioned.

By contrast, Jörn Jürgens – who in the past has worked for module manufacturer AstroPower, and is now employed with US cell manufacturer SunPower – has a more positive view: »It makes sense to standardize a process in order to guarantee that diodes function safely,« says Jürgens. Werner Knaupp, co-author of the European norm in the German Commission for Electrical, Electronic, and Information Technologies (DKE) working group K373, foresees manufacturers moving away from the use of traditional string diodes. Knaupp, however, believes that testing reverse current overload is an important way to evaluate safeguarding devices.

Supposedly, the new IEC 61730 safety norm will be passed in 2004. Then reverse current overload test, commonplace in the US, will become a reality in Europe as well. But in contrast to Europe's EN 50380 norm (see PI 3/2004, p. 10), the IEC norm won't call for the measurement of reverse current overload, but rather for checking values provided by manufacturers. Only if a module, in darkened laboratory conditions, fails to leave burn marks on a cloth underlay after two hours of 1.35 times the normal reverse current, and afterward can still reach its STC power, will the reverse current overload values be considered safe. The determination of the value itself is the manufacturer's business, explains Werner Herrmann from the German testing organization TÜV.

As the co-author of a lecture last year at the German Staffelstein PV conference, Werner Herrmann warned the audience against using string diodes as protective devices – primarily for energy-related reasons. Because of shadowing, one doesn't need to anticipate reverse current overload near the operating point, which only could occur in case of connection malfunctions or damaged bypass diodes. But every diode connected in series to a string of solar modules creates a loss – and the lower the string voltage, the more significant the losses become.

Furthermore, a diode can constitute a risk factor: if a defective device gives way permanently, a simultaneous accidental grounding in the string could cause the reverse current to destroy the module. The greater the number of parallel strings, the more dangerous it becomes. But opinions differ on how likely it is to have a defective diode. Whereas Werner Knaupp claims diode defects are the major reason why reverse current measurements should be required, Markus Münch only has encountered defective diodes in the event of over voltages caused by a lighting strike, adding, »And you should have insurance for those types of disasters anyway.«