NREL reports examine effects of solar penetration on grid stability

by Peter Maloney
APPA News
February 16, 2021

Rising contributions from variable generation sources, particularly photovoltaic (PV) solar, present challenges but, overall, the Western Interconnection has enough inherent flexibility and ramping ability to manage fluctuations in net load, a new report from the National Renewable Energy Laboratory (NREL) found.

The report’s authors also noted, however, that flexibility in the Western Interconnection is “heavily dependent” on transmission between regions and the dominant sources of ramping vary by region, and each region’s flexibility is dependent on one another’s ramping capability.

System flexibility can be provided from conventional generation, from less conventional generation – such as storage, demand response, concentrating solar power with thermal energy storage – as well as from imports and exports between neighboring regions, the report said.

The report, Power System Flexibility Requirements and Supply, is one in a series that examines the challenges related to planning power systems with higher solar photovoltaic penetrations.

The report, along with several others, was commissioned by the Western Interstate Energy Board (WIEB) as part of the Enhanced Distributed Solar Photovoltaic Deployment via Barrier Mitigation or Removal in the Western Interconnection project funded by the Department of Energy in collaboration with Lawrence Berkeley National Laboratory.

For its power system flexibility report, NREL created an open-source tool to analyze the flexibility of the results of a specific commercial unit commitment and economic dispatch tool (PLEXOS). The tool assesses the demand of a system through a net load analysis. The constraints and limitations of each generator are then considered to determine the available supply. Supply and demand are then compared to paint a more complete picture of potential flexibility concerns.

NREL’s reports for the project focused on four broad categories: bulk dispatch, system planning, dynamic stability, and the distribution grid.

NREL’s Resource Adequacy Considerations report examined the availability of resources to meet requirements during periods of peak load. The report found the Western Interconnection is overbuilt in general, with “extremely high levels of capacity adequacy,” and that the future buildouts computed by NREL’s models appear to have sufficient resources to serve peak net load with PV penetrations of up to 33%.

The authors said that planning reserve constraints used in resource planning may be conservative and could lead to higher-than-necessary investments in capacity resources. They cautioned, however, that a more accurate representation of the grid’s planning reserve needs could be achieved through studies that test NREL’s results under a wider range of system operating conditions and use more detailed representations of transmission and energy-limited resources.

Another NREL report, Simulating Distributed Energy Resource Responses to Transmission System-Level Faults Considering IEEE 1547 Performance Categories on Three Major WECC Transmission Paths, studied transmission issues related to wider solar power penetration. The report found that distributed energy resources could ride through transmission-level faults associated with prolonged voltage events, thereby limiting the impact of faults on power reliability by keeping more distributed generation online both during and after a disturbance.

NREL’s report on Behind-the-Meter Solar Accounting in Renewable Portfolio Standards, found that a common accounting practice among states has the potential to decrease the total amount of renewable generation in a state.

If a renewable portfolio standard (RPS) is designed so that behind-the-meter solar renewable energy credits (RECs) can be used for compliance and the load served by behind-the-meter solar generation is not covered by the RPS, the presence of behind-the-meter solar resources and the transfer of those RECs for compliance can decrease the total amount of renewable generation in a state, compared with a situation in which there is no behind-the-meter solar power, NREL said.

On the other hand, if load served by behind-the-meter solar generation counts toward the RPS load and behind-the-meter solar RECs cannot be used for compliance, the presence of behind-the-meter resources does not change the amount of RECs a utility is required to retire, but additional RECs will be retired by the behind-the-meter solar resource owner, increasing the total amount of renewable generation on a one-to-one basis as behind-the-meter solar generation rises.

Finally, NREL’s report on how power inverters used by distributed energy resources can influence grid stability. The report, Stability and Control of Power Systems with High Penetrations of Inverter-Based Resources: An Accessible Review of Current Knowledge and Open Questions, reviewed literature describing what may be required to transition from spinning inertia associated with traditional generation, to mostly inverter-based power systems and serves as a reference for grid operators and planners.