Frequency support, ramping and balancing, and voltage support are facets of power grid operations that are considered essential to reliability of the North American bulk power system. Together, these facets are referred to by the electricity industry as essential reliability services, or ERS. Until now, these services have been provided by the large and dispatchable fleet of generation resources. New inverter-based and variable resources (wind and solar) do not inherently provide essential reliability services at the same level as conventional resources. As the proportion of conventional generation resources declines, and inverter-based and variable resources grow, essential reliability services will be in shorter supply and have an increased likelihood of causing bulk power system reliability problems. Quantifying the level of essential reliability services needed to maintain system stability is unfortunately not straightforward. It is a new engineering challenge as the system enters uncharted territory with fewer conventional resources able to provide these services inherently.
Frequency Support: The inertia from the large, spinning mass of conventional generation (ex., coal, natural gas plants) aids frequency support to arrest the decline in frequency following a system disturbance – returning the frequency to the desired level. Since inverterbased resources have little to no spinning mass, frequency declines will be larger and more sudden if alternative frequency support is not acquired.
Ramping and Balancing: To maintain grid reliability, the supply and demand of electricity must always remain in balance. This is accomplished by a fleet of generators that are dispatchable and can be ramped up and down to match changes in load. The variable nature of wind and solar generation creates challenges with dispatchability and ramp up capability, while also increasing ramp requirements that strain the diminishing fleet of dispatchable resources. If there are not sufficient dispatchable resources online to meet the ramp requirements, reduction in load (voluntary or non-voluntary) is necessary to maintain system balance.
Voltage Support: The transmission system has been designed and built around centralized generation that provides the bulk of the dynamic reactive power into the grid to support voltages. New generation interconnections are being installed further from load centers, which puts additional strain on the grid to maintain voltages. The electric grid in the MRO footprint is characterized by long transmission lines connecting generation to load, making the region more susceptible to this risk. New interconnections are more distributed and smaller, which change the location and size of available reactive power that can be injected or consumed by generation. These changing supply and requirements for reactive power will require additional, non-generation resources to avoid areas of voltage instability.
There are several NERC standards that help mitigate the different aspects of this risk.
NERC has also developed an Essential Reliability Services Working Group (ERSWG) and a Distributed Energy Resources Task Force (DERTF) to address this issue. More information on the activities of these groups can be found on NERC’s website.