SolarCity's Hawaiian study reveals grid regulation potential of inverters

A groundbreaking study by solar leasing company SolarCity, the U.S. Department of Energy’s National Renewable Energy Laboratory (NREL) and Hawaiian Electric has explored the potential of solar inverters to act as junior grid regulators, and found that the technology holds the key to overcoming transient load rejection overvoltage (LRO) concerns.

Concerns over LRO limitations have stymied solar growth in regions where grid capacity restraints have been paramount, and the issue has long been a limiting factor governing Hawaiian Electric’s current integration policies on high penetration of distributed generation (DG).

However, in tackling these limitations, inverter testing carried out at NREL’s Energy Systems Integration Facility (ESIF) revealed that most inverter technology possesses the capability of safely managing distributed energy on to the grid, finding that a typical inverter could act as a proxy “junior grid”, thus overcoming the technical barriers that have limited DG penetration in Hawaii and elsewhere in the U.S.

The report, titled Inverter Load Rejection Over-Voltage Testing: SolarCity CRADA Task 1a Final Report, sought to provide an insight into how to overcome such interconnection barriers. According to SolarCity, as a direct result of this testing Hawaiian Electric is now prepared to increase circuit thresholds on its grid from current levels of 120% daytime minimum load (DML) of DG, to 250% DML – more than doubling the amount of solar rooftop PV that can be safely fed to the grid.

Once this new threshold is implemented it will represent the highest threshold in North America, and could shatter the previously unyielding connection ceiling that had worked to suppress solar penetration in many parts of the country.

For Hawaii, this heightened threshold will help assuage the bottleneck of net energy-metered solar customers that have been unable to connect their rooftop arrays to the grid.

The role of inverters

The testing collaboration between NREL, SolarCity and Hawaiian Electric will continue to analyze the capability of inverters to mitigate ground fault overvoltage concerns and other connection and safety problems.

IHS senior analyst for solar Cormac Gilligan told pv magazine that the potential of inverter and microinverter technology is only now being fully realized.

"The majority of inverters can meet the requirements [of managing grid feed] via a simple software or firmware update," he said. "The overall lesson learned from this research is that inverters are going to be one of the key ways of providing grid stabilization features in the future.

"Also, safety is a key parameter that utilities require, particularly in the U.S. Fronius, for example, has enabled some of its inverters with rapid shutdown or arc fault isolation features."

Gilligan also referred to the recent remote retrofit of 800,000 microinverters on the Hawaiian Island of Oahu – carried out by Enphase in a single day. The upgrade saw the U.S. microinverter specialist access remotely their installed microinverters’ two-way data-over-powerline link via the cloud, and simply upgrade the software installed in each to the new parameters.

The upgrade was in response to a call from Hawaiian Electric for the state’s installed inverters to help stabilize the grid. "Quite a lot of microinverters were installed in the Hawaiian market due to its booming residential sector and the limited land space for utility-scale deployment," said Gilligan. "They also had first-mover advantage, which meant that the solar lease suppliers used them quite heavily."

In Oahu alone, microinverters manage 60% of the island’s distributed solar capacity, which amounts to 140 MW. The upgraded software installed by Enphase (and HECO), allows these microinverters to ride through any fluctuating power surges or glitches, rather than shutting down in the face of substandard AC voltage and frequency, as has been the norm for solar systems in the past.

NREL and SolarCity also confirmed that it will examine the capability of inverters to "support distribution voltage regulation, mitigate utility concerns related to bi-directional power flow, and assess the effectiveness of multiple inverter islanding during faults". This testing will be carried out later this year.