Weekly Update

EPRI’s Solar Power Phrasebook — a Guide for Future Communications

Last week’s Solar Power International tradeshow gave the world a view of the latest advances in solar panels and solar inverters, and examined the growing impact solar power is having on the world’s power grids. But how will utilities manage all this new solar power to keep it from crashing the grid? That task will require some standard communications and control protocols — and the Electric Power Research Institute (EPRI) has some thoughts on how to accomplish this that the industry should pay attention to.

Last year, EPRI hooked up with the Department of Energy, Sandia National Laboratory and the Solar Electric Power Association to define how inverters could help utilities manage grid stability. Most inverters are capable of communications via proprietary technologies, but without standards to link diverse systems, utilities won’t be able to tie them together in a cost-effective way.

EPRI and partners have asked two key standards groups — DNP3 for utility-scale systems and the ZigBee Alliance’s Smart Energy Profile 2.0 — to convert this preliminary work into a set of real-world standards, EPRI’s Brian Seal told me in an interview last week. EPRI’s report lays out seven categories of functions it would like “smart” inverters to provide and communications standards to support:

  1. Connecting and disconnecting from the grid
  2. Adjusting power output
  3. Managing VAR, or reactive power
  4. Managing energy storage devices
  5. Logging event history
  6. Reporting status
  7. Adjusting for time variables

EPRI’s report goes into great detail on each of those items, but here are some key takeaways to consider:

Know When to Hang Up. The ability to connect and disconnect remotely is critical for distributed solar power systems. Today’s utility rules require all rooftop solar to disconnect when the grid goes down to avoid electrocuting workers trying to fix downed power lines. Using solar to actually help the grid in times of stress will require reliable communications links between systems and utilities.

Be Autonomously Intelligent. Some of inverters’ more advanced functions, such as power factor and VAR support, are stabilizing grid fluctuations that happen faster than communications can handle. That means embedded intelligence will have to manage a host of responses; EPRI’s list of four “autonomous VAR modes,” including Normal Energy Conservation Mode, Maximum Var Support Mode, Static Var Mode and Passive Var Mode, gives an example of some.

Know When To Hold ‘Em. Intermittent solar power is more reliable when linked to energy storage, but that requires devices to manage a host of charge and discharge settings and schedules, all alterable via remote control. Solar arrays also have to make sure that they don’t shut on and off all at once when they switch from supplying the grid to recharging storage systems, lest they unbalance the grid.

Know When to Cash In. Solar power systems should know when to store owner and when to sell it back to the grid to maximize profits to the owner. That requires price setting and scheduling across a wide array of potential scenarios, and letting each inverter know which price schemes might be in place in close to real time.

What’s out in the market today? Companies such as SunEdison, Fat Spaniel and Echelon can report status and manage and calculate solar power and price data from inverters, but tend not to manage complex control functions like VAR support. Some inverter manufacturers have been working on a Modbus specification called SunSpec for cross-platform interoperability.

Contenders like General Electric, Siemens and Schneider Electric are certainly capable of systems that could meet the functions EPRI has laid out as well. The challenge for the solar industry and its partners is to come up with a phrasebook that will unlock solar power’s potential to make the grid more efficient and reliable, not less, as solar comes to play an increasingly important role.

Question of the week

What are the key functions solar power systems need to maintain grid stability?