Darkened skies during North America's solar eclipse on August 21 will illuminate challenges of managing California's electric grid which, at times, draws up to 40 percent of its power from solar generation.

While the 70-mile-wide shadow of total eclipse cuts through Oregon, its effects on solar power generation will be felt as far away as the Mexican border. During eclipse totality, California will lose about 4,194 MW of solar power.

To compensate for lost solar generation, the California Independent System Operator (CAISO) will likely schedule more output from traditional generation resources during the eclipse. Then to precisely regulate any deviations during the event, CAISO will issue Automatic Generation Control (AGC) commands to traditional resources further regulate generation. As the eclipse shadow passes and solar production returns to normal, CAISO must issue further commands to traditional resources to decrease generation - all without compromising the voltage or frequency on the electric grid.

"Several anxious customers have contacted us to confirm that their SCADA system can meet CAISO and utility requirements for control," said Mark Morosky, Trimark's President and CEO. "At dozens of sites every day Trimark's T1-S SCADA system operate Photovoltaic (PV) by maintaining pre-determined ramp rates, integrating with CAISO's Automatic Dispatch System (ADS), following generation schedules, and executing curtailment orders."

"In addition, our Remote Intelligent Gateways (RIG) manage secure communications for about 300 resources in California including many of the hydroelectric and natural gas resources that will provide regulation during the eclipse," said Morosky.These controls and secure communications will help CAISO maintain a precise balance between generation and load during the transition from PV to traditional generation and back again.

Between 9:02 AM and 10:22 AM, CAISO must ramp up traditional natural gas and hydroelectric generation (Figure 1) to fill the shortfall in solar power generation. Then from 10:22 AM to 11:54 AM, as the eclipse wanes, PV resources will return to normal mid-day output of over 9,000 MW. From the height of the eclipse, CAISO expects PV resources to ramp up generation at a rate that is three times faster than normal morning increases in PV. To avoid over-generation conditions, CAISO must decrease traditional generation, matching the exact increase in output that will occur in PV arrays across California.

While the effects of the eclipse can be projected and modeled, specific conditions during the eclipse may throw a curve ball at the CAISO. "It's all about predictable control," said Stephen Yee, Trimark's Director of SCADA and Security. "For example, if there is cloud cover over the PV arrays, increase in irradiation could be delayed which could change the ramp rate from a manageable 10 percent to something much faster."

T1-S SCADA can follow a predetermined schedule, accept authorized commands from a third party, or allow operators to adjust generation through the T1-S Vantage interface. "Trimark's T1-S SCADA will govern the ramp rate so the changes never exceed a 10 percent increase," said Yee.

Regardless of the amount of planning, anomalies can occur. "Most operations centers will have all-hands-on-deck for the three-hours of the eclipse," said Yee. "We expect fully staffed op centers to monitor their sites and confirm that all controls are functioning as scheduled."

Trimark Associates | www.trimarkassoc.com