Innovation at the Interconnection Solves Inrush Current Issue on PV Solar+Storage Site in NC

ENFIELD, N.C. -- Last month, Pine Gate Renewable's Grissom Solar (Grissom) went commercial. Grissom is a 6.9MW solar plus 10MWh battery storage site. Although Pine Gate has 37 solar facilities in operation or under construction in the state, this was their first solar plus storage site to be energized in NC. For Grissom, Pine Gate requested support from Solar Operations Solutions (now Aderis Energy) to overcome the challenging requirements that surfaced during the utility's interconnection study.

The Grissom project has a single 5MW interconnection that must be maintained when solar and battery storage are working under the supervision of a third-party SCADA system. Both the solar inverters and the battery storage system are independently AC coupled to the interconnection through two ClearSky™ units from Solar-Ops.

Each ClearSky provides revenue-grade metering and recloser protection while eliminating much of the overhead medium voltage equipment, reducing construction time and long-term operational costs.

"AC coupling battery storage to new or existing solar facilities is increasingly common as developers and owner operators look to maximize value from their existing interconnection agreements and PPAs," says Brad Micallef, Solar-Ops' President. "The ClearSky product line is ideal for these applications due to its small size and built-in ability to satisfy protection and reporting requirements from the utility and ITC."

Utilities have concerns regarding power quality issues from increased inrush current. Therefore, distribution interconnected generators, like Grissom, can face approval challenges. When AC coupled battery storage is added to a solar facility, the additional transformers increase the chances of inrush current being a problem.

A distribution grid will experience a rapid reduction in voltage because of the sudden demand for current when too many transformers are energized at the same time. This sudden rapid voltage change can lead to equipment damage for power customers and requires mitigation for the utility to operate within IEEE standards.

Normally requiring a dedicated piece of equipment, the Grissom facility was able to mitigate their inrush current by sequencing the two ClearSky units' internal medium voltage reclosers. Programming one ClearSky to energize upon grid restoration and the other to delay energization by a few seconds, the inrush event was reduced to an acceptable level. 

"Adding storage to a solar site allows for backup power to be supplied to the grid when people need it, ensuring its reliability," said Mak Nagle, SVP of Development at Pine Gate Renewables. "We're excited that Grissom Solar is now online, and Pine Gate Renewables has more than 2.5GW of storage projects currently in development elsewhere around the country."