How Rooftop Solar Can Stabilize the Grid

Rooftop solar power systems are picking up a second job on the distribution grids that deliver electricity to California homes and businesses.  Right now, their photovoltaic panels just generate electricity (meeting about 1 percent of the state’s consumption), but within a few months some systems will also start moonlighting as junior grid regulators—a role that could keep them busy even after the sun goes down.

While the development in California is the result of a state-specific standard, approved by the California Public Utilities Commission (CPUC) in December, it is also part of a global movement:  Germany, Japan, and other countries where solar is booming are implementing a similar change to empower rooftop solar installations to regulate voltage levels and perform other grid-support tasks.

Solar’s expanding role is the result of upgraded inverters—the power electronics that link distributed generators such as rooftop photovoltaics to the grid.  The inverters convert direct current from PV panels into a wave of alternating current that is synchronized with the AC grid. Inverters can also synthesize reactive power—AC whose current wave leads or lags the voltage wave—which grid operators worldwide use to control line voltage. Adding reactive power with leading current boosts AC line voltage.  Subtracting reactive power (by adding power with lagging current) pulls AC voltage down.

The new inverter standards mark a big change for equipment that utilities have viewed largely as a nuisance.  Standards have hitherto required solar inverters to shut down at the first hint of line trouble, in order to protect workers from unanticipated currents. “Traditionally, we’ve been looking for these devices to trip off with any hiccup on the distribution system.  Now they are becoming a resource,” says Robert Sherick, principal manager for advanced technology at utility Southern California Edison.

Utilities must adapt because maintaining power quality gets harder with rising levels of distributed generation.  German grid operators were among the first to experience this when solar started booming there a decade ago.  At times of low power demand, high solar output drove up voltage levels, explains Bernhard Ernst, grid integration director for inverter manufacturer SMA Solar Technology, based in Niestetal, Germany.  Such situations prompted utilities to freeze PV installations on certain lines.

Germany’s smart-inverter requirements, established three years ago, solved the problem by requiring inverters to start subtracting reactive power when output from their PV arrays exceeds 50 percent of capacity.  This counterbalances the voltage-boosting impact of the solar power generation.

Germany also mandated improved performance during grid emergencies.  In the event that excess supply causes AC frequency to exceed Europe’s 50-hertz standard by more than 0.2 Hz, inverters must now electronically trim output from PV panels instead of shutting down.  Frequency deviations are rare on Europe’s “electrically stiff” continental grid, but German regulators—worried about the destabilizing effect if PV systems turn off en masse—mandated an inverter upgrade costing owners and installers approximately US $300 million.

California’s standard, developed through a collaborative process that began in 2013, pushes the envelope for smart inverters.  Though solar causes few problems for California utilities today, rooftop PV is growing fast—by more than 40 percent per year in San Diego Gas & Electric’s territory.  The state’s smart-inverter standard starts with Germany’s requirements and then asks inverters to be smarter still.

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