Power system reliability

<h2 id="economics">Economics</h2>
<p>Electric grid is an extremely important piece of <a href="/facts/Infrastructure/QJvbsSpl">infrastructure</a>; a single daylong nationwide <a href="/facts/Power_outage/CBp7xY9w">power outage</a> can shave off 0.5% of the country's <a href="/facts/GDP/zQRgyFKc">GDP</a>. The cost of improvements is also high, so in practice a balance is sought to reach an "adequate level of reliability" at an acceptable cost.<a class="footnote-ref" id="fnref:7" href="#fn:7"><sup>7</sup></a>
</p>
<h2 id="adequacy">Adequacy</h2>
<p class="note">Main article: <a href="/facts/Resource_adequacy/up8eXVkH">Resource adequacy</a></p>
<p><a href="/facts/Resource_adequacy/up8eXVkH">Resource adequacy</a> (RA, also supply adequacy) is the ability of the electric grid to satisfy the end-user power demand at any time (typically this is an issue at the <a href="/facts/Peak_demand/rt93v6UJ">peak demand</a>).<a class="footnote-ref" id="fnref:8" href="#fn:8"><sup>8</sup></a> For example, a sufficient unused <a href="/facts/Dispatchable_generation/CkT25V8c">dispatchable generation</a> capacity and <a href="/facts/Demand_response/py7m5XeC">demand response</a> resources shall be available to the electrical grid at any time so that major equipment failures (e.g., a disconnection of a nuclear power unit or a <a href="/facts/High-voltage_power_line/I9w3lRmM">high-voltage power line</a>) and fluctuations of power from <a href="/facts/Variable_renewable_energy/kynTl1t2">variable renewable energy</a> sources (e.g., due to wind dying down) can be accommodated.<a class="footnote-ref" id="fnref:9" href="#fn:9"><sup>9</sup></a> 
</p><p>A typical <a href="/facts/Reliability_index/YpjGMoYI">reliability index</a> for the adequacy is the <a href="/facts/Loss_of_load_expectation/4yJYp6p5">loss of load expectation</a> (LOLE) of one event in 10 years (<a href="/facts/One-day-in-ten-years_criterion/4yJYp6p5">one-day-in-ten-years criterion</a>).<a class="footnote-ref" id="fnref:10" href="#fn:10"><sup>10</sup></a> Due to the possible need for the actual addition of physical capacity, adequacy planning is long term<a class="footnote-ref" id="fnref:11" href="#fn:11"><sup>11</sup></a> (for example, <a href="/facts/PJM_Interconnection/evdhS4eU">PJM Interconnection</a> requires capacity purchases to be 4 years in advance of delivery).<a class="footnote-ref" id="fnref:12" href="#fn:12"><sup>12</sup></a>
</p>
<h2 id="security">Security</h2>
<p>Security is the ability of the system to keep the real-time balance of the supply and demand, in particular immediately after a <a href="/facts/Contingency_(electrical_grid)/GI2Ap0HI">contingency</a> by automatically ramping up generation and shedding the <a href="/facts/Interruptible_load/MXCBAz5m">interruptible loads</a>. Security relies on the <a href="/facts/Operating_reserve/qlRQeL52">operating reserve</a>. Historically, the <a href="/facts/Ancillary_services/xcudV9uA">ancillary services</a> (e.g., the <a href="/facts/Inertial_response/1P9fI8kL">inertial response</a>) were provided by the spinning machinery of the <a href="/facts/Synchronous_generator/NY4NsL9n">synchronous generators</a>, provisioning of these services got more complicated with proliferation of the <a href="/facts/Inverter-based_resource/asTJDPGs">inverter-based resources</a> (e.g., <a href="/facts/Solar_photovoltaic/rD4rcJB8">solar photovoltaics</a> and <a href="/facts/Grid_batteries/mGVRuDGc">grid batteries</a>).<a class="footnote-ref" id="fnref:13" href="#fn:13"><sup>13</sup></a> The typical requirement is "<a href="/facts/N-1_security/GI2Ap0HI">N-1 security</a>" meaning that a sudden loss of one out of N major resources (a large generator or transmission line) should be pre-built into the system configuration at any time. The N-2 and N-3 contingencies refer to preparing for a simultaneous loss of, respectively, 2 or 3 major units; this is sometimes done for the critical area (e.g. <a href="/facts/Downtown/WvmTHvTE">downtown</a>).<a class="footnote-ref" id="fnref:14" href="#fn:14"><sup>14</sup></a>
</p>
<h2 id="essential-reliability-services">Essential reliability services</h2>
<p><a href="/facts/North_American_Electric_Reliability_Corporation/AtSDgddP">North American Electric Reliability Corporation</a> recognizes three services that have to be provided by the generation equipment in order for the grid to be reliable:
</p>
<ul><li><a href="/facts/Voltage_control/kjmn9HsY">voltage control</a>;</li>
<li><a href="/facts/Frequency_support/R9mKeJL2">frequency support</a>;</li>
<li><a href="/facts/Ramping_capability/1P9fI8kL">ramping capability</a>.</li></ul>
<p>These capabilities are called essential reliability services (ERSs). If these are lacking, the grid cannot be secured. The contribution of synchronous generators toward these services is well understood.<a class="footnote-ref" id="fnref:15" href="#fn:15"><sup>15</sup></a>
</p>
<h2 id="seel-also">Seel also</h2>
<ul><li><a href="/facts/Power_system_protection/gX2g5TNH">Power system protection</a></li></ul>

<h2 id="sources">Sources</h2>
<ul><li>Heylen, Evelyn; De Boeck, Steven; Ovaere, Marten; Ergun, Hakan; Van Hertem, Dirk (26 January 2018). <a href="https://books.google.com/books?id=YbVIDwAAQBAJ&pg=PA21">"Steady-State Security"</a>. <i>Dynamic Vulnerability Assessment and Intelligent Control for Sustainable Power Systems</i>. John Wiley & Sons, Ltd. pp. 21–40. <a href="/facts/Doi_(identifier)/muM9Etpq">doi</a>:<a href="https://doi.org/10.1002%2F9781119214984.ch2">10.1002/9781119214984.ch2</a>. <a href="/facts/ISBN_(identifier)/15AdSPa9">ISBN</a> 9781119214984.</li>
<li>Geocaris, Madeline (August 10, 2022). <a href="https://www.nrel.gov/news/program/2022/assessing-power-system-reliability-in-a-changing-grid-environment.html">"Assessing Power System Reliability in a Changing Grid, Environment"</a>. <i>NREL.gov</i>. <a href="/facts/National_Renewable_Energy_Laboratory/UMhnJKEC">National Renewable Energy Laboratory</a>. Retrieved 10 May 2023.</li>
<li>NERC (2015). <a href="https://www.nerc.com/pa/RAPA/ra/Reliability%20Assessments%20DL/ERS%20Abstract%20Report%20Final.pdf">"Essential Reliability Services"</a> (PDF). Retrieved 2025-05-11.</li>
<li>Prada, Jose Fernando (2017). <a href="https://kilthub.cmu.edu/ndownloader/files/12249647"><i>Ensuring the Reliable Operation of the Power Grid: State-Based and Distributed Approaches to Scheduling Energy and Contingency Reserves</i></a> (PhD). <a href="/facts/Carnegie_Mellon_University/XKd7L2KA">Carnegie Mellon University</a>.</li>
<li>Tezak, Christine (June 24, 2005). <a href="https://hepg.hks.harvard.edu/files/hepg/files/stanford.washington.resource.adequacy.pdf"><i>Resource Adequacy - Alphabet Soup!</i></a> (PDF). Stanford Washington Research Group.</li>
<li>Willis, H. Lee (1 March 2004). <a href="https://books.google.com/books?id=9EShPwTRnoUC&pg=PA499">"Contingency-based planning criteria"</a>. <i>Power Distribution Planning Reference Book, Second Edition</i> (2 ed.). CRC Press. pp. 499–500. <a href="/facts/ISBN_(identifier)/15AdSPa9">ISBN</a> 978-1-4200-3031-0.</li></ul>
<h2 id="external-links">External links</h2>
<ul><li>NERC. <a href="https://www.nerc.com/comm/RISC/Related%20Files%20DL/RISC_ERO_Priorities_Report_2023_Board_Approved_Aug_17_2023.pdf">"2023 ERO Reliability Risk Priorities Report"</a> (PDF). <i>nerc.com</i>. <a href="/facts/North_American_Electric_Reliability_Corporation/AtSDgddP">North American Electric Reliability Corporation</a>. Retrieved 18 September 2023.</li></ul>

<h2 id="references">References</h2>

<ol>
<li id="fn:1"><p>Heylen et al. 2018, p. 22. - Heylen, Evelyn; De Boeck, Steven; Ovaere, Marten; Ergun, Hakan; Van Hertem, Dirk (26 January 2018). "Steady-State Security". Dynamic Vulnerability Assessment and Intelligent Control for Sustainable Power Systems. John Wiley & Sons, Ltd. pp. 21–40. doi:10.1002/9781119214984.ch2. ISBN 9781119214984. <a href="https://books.google.com/books?id=YbVIDwAAQBAJ&pg=PA21" target="_blank">https://books.google.com/books?id=YbVIDwAAQBAJ&pg=PA21</a> <a href="#fnref:1" class="footnote-back-ref">↩</a></p></li>
<li id="fn:2"><p>Heylen et al. 2018, p. 21. - Heylen, Evelyn; De Boeck, Steven; Ovaere, Marten; Ergun, Hakan; Van Hertem, Dirk (26 January 2018). "Steady-State Security". Dynamic Vulnerability Assessment and Intelligent Control for Sustainable Power Systems. John Wiley & Sons, Ltd. pp. 21–40. doi:10.1002/9781119214984.ch2. ISBN 9781119214984. <a href="https://books.google.com/books?id=YbVIDwAAQBAJ&pg=PA21" target="_blank">https://books.google.com/books?id=YbVIDwAAQBAJ&pg=PA21</a> <a href="#fnref:2" class="footnote-back-ref">↩</a></p></li>
<li id="fn:3"><p>Heylen et al. 2018, p. 22. - Heylen, Evelyn; De Boeck, Steven; Ovaere, Marten; Ergun, Hakan; Van Hertem, Dirk (26 January 2018). "Steady-State Security". Dynamic Vulnerability Assessment and Intelligent Control for Sustainable Power Systems. John Wiley & Sons, Ltd. pp. 21–40. doi:10.1002/9781119214984.ch2. ISBN 9781119214984. <a href="https://books.google.com/books?id=YbVIDwAAQBAJ&pg=PA21" target="_blank">https://books.google.com/books?id=YbVIDwAAQBAJ&pg=PA21</a> <a href="#fnref:3" class="footnote-back-ref">↩</a></p></li>
<li id="fn:4"><p>Prada 2017, p. 5. - Prada, Jose Fernando (2017). Ensuring the Reliable Operation of the Power Grid: State-Based and Distributed Approaches to Scheduling Energy and Contingency Reserves (PhD). Carnegie Mellon University. <a href="https://kilthub.cmu.edu/ndownloader/files/12249647" target="_blank">https://kilthub.cmu.edu/ndownloader/files/12249647</a> <a href="#fnref:4" class="footnote-back-ref">↩</a></p></li>
<li id="fn:5"><p>Geocaris 2022. - Geocaris, Madeline (August 10, 2022). "Assessing Power System Reliability in a Changing Grid, Environment". NREL.gov. National Renewable Energy Laboratory. Retrieved 10 May 2023. <a href="https://www.nrel.gov/news/program/2022/assessing-power-system-reliability-in-a-changing-grid-environment.html" target="_blank">https://www.nrel.gov/news/program/2022/assessing-power-system-reliability-in-a-changing-grid-environment.html</a> <a href="#fnref:5" class="footnote-back-ref">↩</a></p></li>
<li id="fn:6"><p>Geocaris 2022. - Geocaris, Madeline (August 10, 2022). "Assessing Power System Reliability in a Changing Grid, Environment". NREL.gov. National Renewable Energy Laboratory. Retrieved 10 May 2023. <a href="https://www.nrel.gov/news/program/2022/assessing-power-system-reliability-in-a-changing-grid-environment.html" target="_blank">https://www.nrel.gov/news/program/2022/assessing-power-system-reliability-in-a-changing-grid-environment.html</a> <a href="#fnref:6" class="footnote-back-ref">↩</a></p></li>
<li id="fn:7"><p>Heylen et al. 2018, p. 21. - Heylen, Evelyn; De Boeck, Steven; Ovaere, Marten; Ergun, Hakan; Van Hertem, Dirk (26 January 2018). "Steady-State Security". Dynamic Vulnerability Assessment and Intelligent Control for Sustainable Power Systems. John Wiley & Sons, Ltd. pp. 21–40. doi:10.1002/9781119214984.ch2. ISBN 9781119214984. <a href="https://books.google.com/books?id=YbVIDwAAQBAJ&pg=PA21" target="_blank">https://books.google.com/books?id=YbVIDwAAQBAJ&pg=PA21</a> <a href="#fnref:7" class="footnote-back-ref">↩</a></p></li>
<li id="fn:8"><p>Tezak 2005, p. 2. - Tezak, Christine (June 24, 2005). Resource Adequacy - Alphabet Soup! (PDF). Stanford Washington Research Group. <a href="https://hepg.hks.harvard.edu/files/hepg/files/stanford.washington.resource.adequacy.pdf" target="_blank">https://hepg.hks.harvard.edu/files/hepg/files/stanford.washington.resource.adequacy.pdf</a> <a href="#fnref:8" class="footnote-back-ref">↩</a></p></li>
<li id="fn:9"><p>Geocaris 2022. - Geocaris, Madeline (August 10, 2022). "Assessing Power System Reliability in a Changing Grid, Environment". NREL.gov. National Renewable Energy Laboratory. Retrieved 10 May 2023. <a href="https://www.nrel.gov/news/program/2022/assessing-power-system-reliability-in-a-changing-grid-environment.html" target="_blank">https://www.nrel.gov/news/program/2022/assessing-power-system-reliability-in-a-changing-grid-environment.html</a> <a href="#fnref:9" class="footnote-back-ref">↩</a></p></li>
<li id="fn:10"><p>Tezak 2005, p. 2. - Tezak, Christine (June 24, 2005). Resource Adequacy - Alphabet Soup! (PDF). Stanford Washington Research Group. <a href="https://hepg.hks.harvard.edu/files/hepg/files/stanford.washington.resource.adequacy.pdf" target="_blank">https://hepg.hks.harvard.edu/files/hepg/files/stanford.washington.resource.adequacy.pdf</a> <a href="#fnref:10" class="footnote-back-ref">↩</a></p></li>
<li id="fn:11"><p>Tezak 2005, p. 2. - Tezak, Christine (June 24, 2005). Resource Adequacy - Alphabet Soup! (PDF). Stanford Washington Research Group. <a href="https://hepg.hks.harvard.edu/files/hepg/files/stanford.washington.resource.adequacy.pdf" target="_blank">https://hepg.hks.harvard.edu/files/hepg/files/stanford.washington.resource.adequacy.pdf</a> <a href="#fnref:11" class="footnote-back-ref">↩</a></p></li>
<li id="fn:12"><p>Tezak 2005, p. 16. - Tezak, Christine (June 24, 2005). Resource Adequacy - Alphabet Soup! (PDF). Stanford Washington Research Group. <a href="https://hepg.hks.harvard.edu/files/hepg/files/stanford.washington.resource.adequacy.pdf" target="_blank">https://hepg.hks.harvard.edu/files/hepg/files/stanford.washington.resource.adequacy.pdf</a> <a href="#fnref:12" class="footnote-back-ref">↩</a></p></li>
<li id="fn:13"><p>Geocaris 2022. - Geocaris, Madeline (August 10, 2022). "Assessing Power System Reliability in a Changing Grid, Environment". NREL.gov. National Renewable Energy Laboratory. Retrieved 10 May 2023. <a href="https://www.nrel.gov/news/program/2022/assessing-power-system-reliability-in-a-changing-grid-environment.html" target="_blank">https://www.nrel.gov/news/program/2022/assessing-power-system-reliability-in-a-changing-grid-environment.html</a> <a href="#fnref:13" class="footnote-back-ref">↩</a></p></li>
<li id="fn:14"><p>Willis 2004, p. 499. - Willis, H. Lee (1 March 2004). "Contingency-based planning criteria". Power Distribution Planning Reference Book, Second Edition (2 ed.). CRC Press. pp. 499–500. ISBN 978-1-4200-3031-0. <a href="https://books.google.com/books?id=9EShPwTRnoUC&pg=PA499" target="_blank">https://books.google.com/books?id=9EShPwTRnoUC&pg=PA499</a> <a href="#fnref:14" class="footnote-back-ref">↩</a></p></li>
<li id="fn:15"><p>NERC 2015. - NERC (2015). "Essential Reliability Services" (PDF). Retrieved 2025-05-11. <a href="https://www.nerc.com/pa/RAPA/ra/Reliability%20Assessments%20DL/ERS%20Abstract%20Report%20Final.pdf" target="_blank">https://www.nerc.com/pa/RAPA/ra/Reliability%20Assessments%20DL/ERS%20Abstract%20Report%20Final.pdf</a> <a href="#fnref:15" class="footnote-back-ref">↩</a></p></li>
</ol>

Power system reliability open-in-new

Power system reliability