If you work in the electric industry, you have probably already experienced the unprecedented change transforming the bulk power system across North America. The grid as we know it is in the midst of an evolutionary shift that will shape our energy resources for years to come. The rules of the road are evolving to accommodate the expected resource mix change, and swift action by state, federal and provincial regulators is needed so the electric industry can build and reliably operate the grid of the future. There has never been a more critical time for understanding the key elements of electric reliability.
As stewards for electricity reliability, it is NERC’s responsibility to not only assure reliability through mandatory standards, but to voice our concerns and put a spotlight on potential reliability risks as we identify them. Key to articulating these concerns to stakeholders is conveying the complexities of the bulk power system, the fundamentals of how the grid works, the differences or potential deficiencies in resource adequacy for various future scenarios, and what we may need to understand and adjust to maintain reliability.
The interconnected bulk power system was designed to rely on conventional generating plants that provide essential reliability services, such as frequency and voltage support, to continuously keep the grid at 60 Hertz. In the current power system, the physical interactions between the spinning mass of generators and the transmission system are key to balancing electricity generation and demand. Deviations from this 60-Hz are measured in tenths and hundredths because even a small change to system frequency can disrupt service, activate protection systems, damage equipment and lead to outages.
NERC, which was designated as the Electric Reliability Organization by the Federal Energy Regulatory Commission under the Federal Power Act, identified essential reliability services as necessary in a 2014 paper<http://www.nerc.com/comm/Other/essntlrlbltysrvcstskfrcDL/ERSTF%20Concept%20Paper.pdf>.
The two building blocks of Essential Reliability Services are:
- Frequency support, which is the capability of a power system to adjust the power consumption or output of devices connected to the grid automatically to ensure that the grid frequency stays close to 60-Hz. It includes real-power ramping capability to address changes in system conditions, like loss of electric load or generation.
- Voltage support, which provides or absorbs reactive power, while operating within a specific voltage band under steady-state and post-contingency operating conditions.
Large power plants that use coal, hydro, natural gas and nuclear fuels are connected to the bulk power system synchronously and can provide frequency and voltage support through the actions of their governor and voltage regulator. These synchronous machines generate electricity using electromagnetic induction (i.e., rotating a magnet around copper wire) to transform kinetic energy into electricity. This is the most used form for generating electricity and is based on Faraday’s law. Conventional generators have a large spinning mass (e.g., steam turbine) that also provides a source of inertia.
Renewable sources, such as wind and solar, are connected to the bulk power system asynchronously via electronically coupled equipment or power inverters. Because of this fundamental difference, inverter-based resources behave in a manner that is different from their synchronous generator counterparts. This does not make inverter-based renewable resources “bad” for reliability. However, they do require additional control, coordination, and rule adjustments. Because the transmission system was not designed to support high levels of resources which do not naturally provide frequency and voltage support, extra steps are required to support a more harmonious system that works together to support bulk power system reliability.
The good news is that making this transition is completely possible and transmission planners across the world are engineering solutions that make it feasible for renewable generation to provide some level of essential reliability services. For example, synthesized or emulated inertia and fast frequency response can be supplied by both inverter-based resources—if properly equipped and coordinated—and loads. Older wind plants are rarely retrofitted with this equipment because of cost; however, all new utility wind generation has this capability and features control upgrades that can be coordinated with grid requirements. Additionally, either incentives or robust interconnection requirements that call for essential reliability services are needed. FERC has recognized the need for essential reliability services in its recent inquiries and proposed rulemakings.
Over the past decade, North America has added wind and solar generation to the power grid in relatively small amounts to meet environmental policy objectives. While the transition thus far has not resulted in any significant reliability issues, the increasing penetration of inverter-based resources is transitioning to a system that can lead to grid operation challenges if not engineered and planned correctly. With the decreasing amount of mass spinning to support grid frequency, other mechanisms that achieve the same level of system performance must be employed. Changes to traditional planning approaches and operations are also needed. For example, peak capacity planning is a key mechanism that supports grid expansion and solutions for meeting resource adequacy requirements. While peak hour conditions will remain an important consideration to study, they do not reflect the conditions on the system when it has the highest simultaneous output of inverter-based resources. Instead of a hot summer day as the design criteria, planners will also need to evaluate and design a system that will operate reliably during the hours with low demand and high wind and solar output.
NERC has taken a series of actions in the last two years to accommodate renewable integration and to assure reliability. NERC’s Essential Reliability Services Task Force issued a framework in January 2015 for measuring and assessing levels of essential reliability services. NERC also developed three multimedia animations<https://vimeopro.com/nerclearning/erstf-1> that help explain the basics of frequency response, voltage support, and ramping. Currently, NERC is developing a white paper on sufficiency guideline methodologies to better understand the expectations of the system with the current and expected pace of renewables integration.
Enhancing reliability is NERC’s top priority and NERC will continue its work with industry to pave the path for maintaining reliability under any resource mix. Maintaining an adequate level of essential reliability services is key to that effort as North America continues to integrate more renewables.
John Moura
Director of Reliability Assessment and System Analysis
NERC
This is a great blog. The combined efforts of UVIG and reliability organizations to make this clear should be applauded.