Switching off gigawatts of solar inverters

At the annual Otti symposium on photovoltaics held in Bad Staffelstein, Germany, experts came to an agreement on how inverters could disconnect from the grid in cascades to avoid a blackout.

With peak demand generally ranging from 70-80 gigawatts during the work week and often dropping below 60 gigawatts on the weekend, Germany has quite a lot of photovoltaics installed. Although the official figures have not yet been published, it is assumed that some 17-18 gigawatts of photovoltaics was already connected to the grid at the end of 2010.

Up to now, solar inverters have been designed to switch off if the hertz frequency on the grid rises above 50.2 as required by German law. But with potentially more than 20 percent of power coming from photovoltaics in the afternoon, having all photovoltaic arrays disconnect from the grid more or less at once is not an option – the result would almost certainly be a blackout that could possibly affect a large part of Europe.

In 2010, Germany's Network Agency therefore called on inverter manufacturers and grid operators to work together on a solution, and one was proposed at the beginning of the month at the Otti Symposium. The goal is to reduce the amount of solar power injected to the grid when the hertz frequency increases, but to do so gradually rather than at once.

Obviously, it only makes sense for all of the existing systems to be revamped, and SMA, the leading global manufacturer of inverters, says that some reprogramming will do the trick. Nonetheless, the cost will probably exceed 100 euros per inverter, and it is not clear who will have to foot that bill.

The German government will probably be adopting a new regulation for solar inverters, which could go into effect at the beginning of 2012. A similar regulation has already been adopted for wind turbines, which have been expected to provide more reactive power to the grid in order to stabilize it. There is, of course, one major difference between wind turbines and most solar arrays: while the former are generally connected to the medium-voltage grid, the latter are connected to low-voltage lines.

While the changes initially only affect Germany, other countries will face similar problems as they ramp up solar to the level Germany has; the Germans currently get around two percent of their electricity from photovoltaics. (cm)