<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE article PUBLIC "-//TaxonX//DTD Taxonomic Treatment Publishing DTD v0 20100105//EN" "../../nlm/tax-treatment-NS0.dtd">
<article xmlns:tp="http://www.plazi.org/taxpub" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:xlink="http://www.w3.org/1999/xlink" xmlns:mml="http://www.w3.org/1998/Math/MathML" article-type="research-article" xml:lang="en">
  <front>
    <journal-meta>
      <journal-id journal-id-type="publisher-id">79</journal-id>
      <journal-id journal-id-type="index">urn:lsid:arphahub.com:pub:D015427E-53F6-553C-9279-2E51CD684756</journal-id>
      <journal-title-group>
        <journal-title xml:lang="en">Nuclear Energy and Technology</journal-title>
        <abbrev-journal-title xml:lang="en">NUCET</abbrev-journal-title>
      </journal-title-group>
      <issn pub-type="epub">2452-3038</issn>
      <publisher>
        <publisher-name>Nuclear Energy and Technology</publisher-name>
      </publisher>
    </journal-meta>
    <article-meta>
      <article-id pub-id-type="doi">10.3897/nucet.12.176131</article-id>
      <article-id pub-id-type="publisher-id">176131</article-id>
      <article-categories>
        <subj-group subj-group-type="heading">
          <subject>Research Article</subject>
        </subj-group>
        <subj-group subj-group-type="scientific_subject">
          <subject>Nuclear policy</subject>
          <subject> Economics and Educational issues</subject>
        </subj-group>
      </article-categories>
      <title-group>
        <article-title>Frequency regulation in high-renewable grids using small modular reactors (SMR): a comparison of pid and mpc strategies</article-title>
      </title-group>
      <contrib-group content-type="authors">
        <contrib contrib-type="author" corresp="yes">
          <name name-style="western">
            <surname>Elfaramawy</surname>
            <given-names>Raouf M.</given-names>
          </name>
          <email xlink:type="simple">raoufraf@yahoo.com</email>
          <xref ref-type="aff" rid="A1">1</xref>
        </contrib>
        <contrib contrib-type="author" corresp="no">
          <name name-style="western">
            <surname>Sarhan</surname>
            <given-names>Amr A.</given-names>
          </name>
          <xref ref-type="aff" rid="A2">2</xref>
        </contrib>
        <contrib contrib-type="author" corresp="no">
          <name name-style="western">
            <surname>Helmy</surname>
            <given-names>Sherif</given-names>
          </name>
          <xref ref-type="aff" rid="A3">3</xref>
        </contrib>
        <contrib contrib-type="author" corresp="no">
          <name name-style="western">
            <surname>El-Wakeel</surname>
            <given-names>Amged S.</given-names>
          </name>
          <xref ref-type="aff" rid="A4">4</xref>
        </contrib>
      </contrib-group>
      <aff id="A1">
        <label>1</label>
        <addr-line content-type="verbatim">Research and Development Department, Nuclear Power Plants Authority, Cairo, Egypt</addr-line>
        <institution>Research and Development Department, Nuclear Power Plants Authority</institution>
        <addr-line content-type="city">Cairo</addr-line>
        <country>Egypt</country>
</aff>
      <aff id="A2">
        <label>2</label>
        <addr-line content-type="verbatim">Aircraft Electrical Equipment Department, Military Technical College, Cairo, Egypt</addr-line>
        <institution>Aircraft Electrical Equipment Department, Military Technical College</institution>
        <addr-line content-type="city">Cairo</addr-line>
        <country>Egypt</country>
        <uri content-type="ror">https://ror.org/01337pb37</uri>
      </aff>
      <aff id="A3">
        <label>3</label>
        <addr-line content-type="verbatim">Chairman of the Board of Directors, Nuclear Power Plants Authority, Cairo, Egypt</addr-line>
        <institution>Chairman of the Board of Directors, Nuclear Power Plants Authority</institution>
        <addr-line content-type="city">Cairo</addr-line>
        <country>Egypt</country>
      </aff>
      <aff id="A4">
        <label>4</label>
        <addr-line content-type="verbatim">Electric Power and Energy Department, Military Technical College, Cairo, Egypt</addr-line>
        <institution>Electric Power and Energy Department, Military Technical College</institution>
        <addr-line content-type="city">Cairo</addr-line>
        <country>Egypt</country>
        <uri content-type="ror">https://ror.org/01337pb37</uri>
      </aff>
      <author-notes>
        <fn fn-type="corresp">
          <p>Corresponding author: Raouf M. Elfaramawy (<ext-link xlink:href="mailto:faramawy@nppa.gov.eg" ext-link-type="uri">faramawy@nppa.gov.eg</ext-link>)</p>
        </fn>
        <fn fn-type="edited-by">
          <p><bold>Academic editor</bold>: Yury Korovin</p>
        </fn>
      </author-notes>
      <pub-date pub-type="collection">
        <year>2026</year>
      </pub-date>
      <pub-date pub-type="epub">
        <day>05</day>
        <month>03</month>
        <year>2026</year>
      </pub-date>
      <volume>12</volume>
      <issue>1</issue>
      <fpage>1</fpage>
      <lpage>10</lpage>
      <uri content-type="arpha" xlink:href="http://openbiodiv.net/4EAF9CBC-2942-559C-B939-B6E15DEFC717">4EAF9CBC-2942-559C-B939-B6E15DEFC717</uri>
      <uri content-type="zenodo_dep_id" xlink:href="https://zenodo.org/record/0">0</uri>
      <history>
        <date date-type="received">
          <day>26</day>
          <month>10</month>
          <year>2025</year>
        </date>
        <date date-type="accepted">
          <day>18</day>
          <month>02</month>
          <year>2026</year>
        </date>
      </history>
      <permissions>
        <copyright-statement>Raouf M. Elfaramawy, Amr A. Sarhan, Sherif Helmy, Amged S. El-Wakeel</copyright-statement>
        <license license-type="creative-commons-attribution" xlink:href="http://creativecommons.org/licenses/by/4.0/" xlink:type="simple">
          <license-p>This is an open access article distributed under the terms of the Creative Commons Attribution License (CC BY 4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.</license-p>
        </license>
      </permissions>
      <abstract>
        <label>Abstract</label>
        <p>The rapid penetration of renewable energy sources (<abbrev xlink:title="renewable energy sources">RESs</abbrev>) into modern power systems has introduced unprecedented challenges in frequency regulation, primarily due to the intermittency and variability of wind and solar generation. Traditional large-scale nuclear and fossil-fuel-based power plants, while reliable, lack the flexibility required for rapid response to frequency fluctuations in high-renewable environments. Small modular reactors (<abbrev xlink:title="Small modular reactors">SMRs</abbrev>) have emerged as a promising alternative, combining nuclear reliability with enhanced operational flexibility (<xref ref-type="bibr" rid="B6">Elfaramawy et al. 2025</xref>).</p>
        <p>This research presents a comparative study of two control strategies—proportional-integral-derivative (<abbrev xlink:title="proportional-integral-derivative">PID</abbrev>) and model predictive control (<abbrev xlink:title="model predictive control">MPC</abbrev>)—for frequency regulation in a renewable-rich grid, where <abbrev xlink:title="Small modular reactors">SMRs</abbrev> act as flexible balancing resources. Using validated Simulink-based dynamic models of <abbrev xlink:title="small modular reactor">SMR</abbrev> dynamics, grid frequency response, and renewable variability, the performance of <abbrev xlink:title="proportional-integral-derivative">PID</abbrev> and <abbrev xlink:title="model predictive control">MPC</abbrev> controllers in mitigating disturbances caused by renewable fluctuations and large-scale contingencies was analyzed.</p>
        <p>Results indicate that both controllers provide effective frequency regulation, with <abbrev xlink:title="model predictive control">MPC</abbrev> offering modest advantages in overshoot suppression and settling time. While <abbrev xlink:title="proportional-integral-derivative">PID</abbrev> demonstrates reliable performance and ease of deployment, <abbrev xlink:title="model predictive control">MPC</abbrev> enables predictive optimization and explicit constraint handling. The findings suggest that both control strategies are viable for <abbrev xlink:title="small modular reactor">SMR</abbrev>-based frequency support, with <abbrev xlink:title="model predictive control">MPC</abbrev> providing incremental improvements in dynamic response for future high-renewable grids.</p>
      </abstract>
      <kwd-group>
        <label>Keywords</label>
        <kwd>frequency regulation</kwd>
        <kwd>grid stability</kwd>
        <kwd>low inertia</kwd>
        <kwd>model predictive control (MPC)</kwd>
        <kwd>PID control</kwd>
        <kwd>renewable integration</kwd>
        <kwd>small modular reactor (SMR)</kwd>
      </kwd-group>
    </article-meta>
  </front>
  <body>
    <sec sec-type="Introduction" id="sec1">
      <title>Introduction</title>
      <p>The global energy transition is accelerating, with renewable energy sources (<abbrev xlink:title="renewable energy sources">RES</abbrev>) such as wind and solar becoming dominant contributors to the electricity mix (<xref ref-type="bibr" rid="B7">IEA 2023</xref>). While this transformation supports decarbonization goals, it also introduces significant operational challenges—most critically, maintaining grid frequency stability in the presence of renewable intermittency and reduced system inertia (<xref ref-type="bibr" rid="B3">Brewer 1952</xref>; <xref ref-type="bibr" rid="B4">Camacho and Bordons 2007</xref>; <xref ref-type="bibr" rid="B10">Li et al. 2024</xref>).</p>
      <p>Conventional large-scale nuclear power plants are traditionally designed for baseload operation and are not well-suited to rapid frequency regulation due to their thermal inertia and limited maneuverability. In contrast, small modular reactors (<abbrev xlink:title="Small modular reactors">SMRs</abbrev>) have emerged as a flexible and scalable nuclear option, capable of integrating into renewable-rich grids while providing clean, dispatchable, and controllable energy (<xref ref-type="bibr" rid="B8">Ingersoll 2013</xref>; <xref ref-type="bibr" rid="B12">Locatelli et al. 2014</xref>).</p>
      <p>To exploit <abbrev xlink:title="Small modular reactors">SMRs</abbrev>’ potential for frequency regulation, advanced control strategies are required. Classical <abbrev xlink:title="proportional-integral-derivative">PID</abbrev> controllers remain widely used in power systems due to their simplicity and robustness (<xref ref-type="bibr" rid="B2">Åström and Hägglund 1995</xref>). However, <abbrev xlink:title="proportional-integral-derivative">PID</abbrev> control can struggle under rapidly varying conditions and in systems with nonlinear dynamics or constraints. Alternatively, <abbrev xlink:title="model predictive control">MPC</abbrev> explicitly incorporates plant models, future predictions, and operational constraints into the control law (<xref ref-type="bibr" rid="B4">Camacho and Bordons 2007</xref>; <xref ref-type="bibr" rid="B16">Rawlings et al. 2017</xref>; <xref ref-type="bibr" rid="B14">Milano et al. 2018</xref>; <xref ref-type="bibr" rid="B13">Mahmud et al. 2022</xref>).</p>
      <p>This study presents a detailed comparison between <abbrev xlink:title="proportional-integral-derivative">PID</abbrev> and <abbrev xlink:title="model predictive control">MPC</abbrev> controllers for <abbrev xlink:title="small modular reactor">SMR</abbrev>-based frequency regulation in high-renewable grids. Using validated Simulink models, an <abbrev xlink:title="small modular reactor">SMR</abbrev> plant transfer function, a grid frequency dynamics model, and renewable disturbance scenarios were developed. The controllers were evaluated under identical conditions to quantify performance in overshoot, settling time, robustness, and disturbance mitigation. Contrary to some previous studies, our results show that both controllers perform effectively, with <abbrev xlink:title="model predictive control">MPC</abbrev> providing moderate but meaningful improvements in key performance metrics.</p>
      <p>This study addresses the gap by presenting a detailed comparison between <abbrev xlink:title="proportional-integral-derivative">PID</abbrev> and <abbrev xlink:title="model predictive control">MPC</abbrev> controllers for <abbrev xlink:title="small modular reactor">SMR</abbrev>-based frequency regulation in high-renewable grids. Using validated Simulink models, an <abbrev xlink:title="small modular reactor">SMR</abbrev> plant transfer function, a grid frequency dynamics model, and renewable disturbance scenarios were developed. The controllers were evaluated under identical conditions to quantify performance in overshoot, settling time, robustness, and disturbance mitigation.</p>
      <sec sec-type="Contributions of this work" id="sec2">
        <title>Contributions of this work</title>
        <p>The main contribution of this work lies in providing a realistic and fair comparison between <abbrev xlink:title="proportional-integral-derivative">PID</abbrev> and <abbrev xlink:title="model predictive control">MPC</abbrev> strategies for <abbrev xlink:title="small modular reactor">SMR</abbrev>-based frequency regulation under identical disturbance scenarios and explicit operational constraints. While previous studies by <xref ref-type="bibr" rid="B21">Zhang et al. (2022)</xref> and <xref ref-type="bibr" rid="B13">Mahmud et al. (2022)</xref> reported substantial <abbrev xlink:title="model predictive control">MPC</abbrev> performance advantages under simplified assumptions, the present study demonstrates that when realistic <abbrev xlink:title="small modular reactor">SMR</abbrev> power and ramp-rate limits are enforced, <abbrev xlink:title="model predictive control">MPC</abbrev> offers incremental—yet meaningful—improvements in dynamic performance.”</p>
      </sec>
    </sec>
    <sec sec-type="System modelling and methodology" id="sec3">
      <title>System modelling and methodology</title>
      <sec sec-type="SMR transfer function modelling" id="sec4">
        <title>SMR transfer function modelling</title>
        <p>A nuclear power plant represents a sophisticated energy conversion system where nuclear fission reactions generate thermal energy, subsequently converted to mechanical energy through steam turbines, and finally to electrical energy via synchronous generators. The distinctive characteristic of nuclear plants is their ability to generate substantial electrical power from a relatively small quantity of nuclear fuel, making them particularly economical for base-load power generation.</p>
        <p>The dynamic response of an <abbrev xlink:title="small modular reactor">SMR</abbrev> power conversion system is critical for frequency regulation studies. The model captures the dynamics from the control signal (mechanical power setpoint, ΔPM(s) to the generated electrical power output ΔPG(s). The complete model integrates the governor, hydraulic actuator, turbine, and generator-load dynamics (<xref ref-type="bibr" rid="B3">Brewer 1952</xref>; <xref ref-type="bibr" rid="B15">Nagrath and Gopal 2017</xref>).</p>
        <p>The overall <abbrev xlink:title="small modular reactor">SMR</abbrev> transfer function is derived by cascading the individual component transfer functions:</p>
        <p><mml:math id="M1"><mml:msub><mml:mrow><mml:mi>G</mml:mi></mml:mrow><mml:mrow><mml:mrow><mml:mi>S</mml:mi><mml:mi>M</mml:mi><mml:mi>R</mml:mi></mml:mrow></mml:mrow></mml:msub><mml:mo>(</mml:mo><mml:mrow><mml:mtext/><mml:mi>s</mml:mi></mml:mrow><mml:mo>)</mml:mo><mml:mo>=</mml:mo><mml:mfrac><mml:mrow><mml:mi>Δ</mml:mi><mml:msub><mml:mrow><mml:mi>P</mml:mi></mml:mrow><mml:mrow><mml:mrow><mml:mi>G</mml:mi></mml:mrow></mml:mrow></mml:msub><mml:mo>(</mml:mo><mml:mrow><mml:mtext/><mml:mi>s</mml:mi></mml:mrow><mml:mo>)</mml:mo></mml:mrow><mml:mrow><mml:mi>Δ</mml:mi><mml:msub><mml:mrow><mml:mi>P</mml:mi></mml:mrow><mml:mrow><mml:mrow><mml:mi>M</mml:mi></mml:mrow></mml:mrow></mml:msub><mml:mo>(</mml:mo><mml:mrow><mml:mtext/><mml:mi>s</mml:mi></mml:mrow><mml:mo>)</mml:mo></mml:mrow></mml:mfrac><mml:mo>=</mml:mo><mml:msub><mml:mrow><mml:mi>G</mml:mi></mml:mrow><mml:mrow><mml:mrow><mml:mi>T</mml:mi></mml:mrow></mml:mrow></mml:msub><mml:mo>(</mml:mo><mml:mrow><mml:mtext/><mml:mi>s</mml:mi></mml:mrow><mml:mo>)</mml:mo><mml:mo>⋅</mml:mo><mml:msub><mml:mrow><mml:mi>G</mml:mi></mml:mrow><mml:mrow><mml:mrow><mml:mi>H</mml:mi></mml:mrow></mml:mrow></mml:msub><mml:mo>(</mml:mo><mml:mrow><mml:mtext/><mml:mi>s</mml:mi></mml:mrow><mml:mo>)</mml:mo><mml:mo>⋅</mml:mo><mml:msub><mml:mrow><mml:mi>G</mml:mi></mml:mrow><mml:mrow><mml:mrow><mml:mi>G</mml:mi></mml:mrow></mml:mrow></mml:msub><mml:mo>(</mml:mo><mml:mrow><mml:mtext/><mml:mi>s</mml:mi></mml:mrow><mml:mo>)</mml:mo><mml:mo>⋅</mml:mo><mml:msub><mml:mrow><mml:mi>G</mml:mi></mml:mrow><mml:mrow><mml:mrow><mml:mi>P</mml:mi></mml:mrow></mml:mrow></mml:msub><mml:mo>(</mml:mo><mml:mrow><mml:mtext/><mml:mi>s</mml:mi></mml:mrow><mml:mo>)</mml:mo></mml:math>	(1)</p>
        <p>Where: Nuclear Turbine Model: The nuclear turbine assembly, incorporating high-pressure and low-pressure stages with reheater dynamics, is represented by:</p>
        <p><mml:math id="M2"><mml:msub><mml:mrow><mml:mi>G</mml:mi></mml:mrow><mml:mrow><mml:mrow><mml:mi>T</mml:mi></mml:mrow></mml:mrow></mml:msub><mml:mo>(</mml:mo><mml:mrow><mml:mtext/><mml:mi>s</mml:mi></mml:mrow><mml:mo>)</mml:mo><mml:mo>=</mml:mo><mml:mfrac><mml:mrow><mml:mn>1</mml:mn><mml:mo>+</mml:mo><mml:mrow><mml:mi>s</mml:mi></mml:mrow><mml:mi>Δ</mml:mi><mml:mrow><mml:mi>t</mml:mi></mml:mrow><mml:mo>+</mml:mo><mml:msup><mml:mrow><mml:mi>s</mml:mi></mml:mrow><mml:mn>2</mml:mn></mml:msup><mml:mrow><mml:mtext/><mml:mi>B</mml:mi></mml:mrow></mml:mrow><mml:mrow><mml:mrow><mml:mo>(</mml:mo><mml:mn>1</mml:mn><mml:mo>+</mml:mo><mml:msub><mml:mrow><mml:mi>s</mml:mi><mml:mi>T</mml:mi></mml:mrow><mml:mrow><mml:mrow><mml:mi>R</mml:mi><mml:mi>H</mml:mi></mml:mrow></mml:mrow></mml:msub><mml:mo>)</mml:mo></mml:mrow><mml:mrow><mml:mo>(</mml:mo><mml:mn>1</mml:mn><mml:mo>+</mml:mo><mml:msub><mml:mrow><mml:mi>s</mml:mi><mml:mi>T</mml:mi></mml:mrow><mml:mrow><mml:mrow><mml:mi>T</mml:mi><mml:mi>L</mml:mi></mml:mrow></mml:mrow></mml:msub><mml:mo>)</mml:mo></mml:mrow><mml:mrow><mml:mo>(</mml:mo><mml:mn>1</mml:mn><mml:mo>+</mml:mo><mml:msub><mml:mrow><mml:mi>s</mml:mi><mml:mi>T</mml:mi></mml:mrow><mml:mrow><mml:mrow><mml:mi>R</mml:mi><mml:mi>H</mml:mi></mml:mrow><mml:mn>2</mml:mn></mml:mrow></mml:msub><mml:mo>)</mml:mo></mml:mrow></mml:mrow></mml:mfrac></mml:math>	(2)</p>
        <p>where T<sub>RH</sub> denotes the HP turbine time constant, T<sub>TL</sub> represents the LP turbine time constant, and T<sub>RH2</sub> characterizes an additional reheater dynamics component.</p>
        <p>Hydraulic Amplifier: The hydraulic amplifier transfer function, which converts low-power pilot valve movement into high-power piston movement, is given by:</p>
        <p><mml:math id="M3"><mml:msub><mml:mrow><mml:mi>G</mml:mi></mml:mrow><mml:mrow><mml:mrow><mml:mi>H</mml:mi></mml:mrow></mml:mrow></mml:msub><mml:mo>(</mml:mo><mml:mrow><mml:mtext/><mml:mi>s</mml:mi></mml:mrow><mml:mo>)</mml:mo><mml:mo>=</mml:mo><mml:mfrac><mml:mn>1</mml:mn><mml:mrow><mml:mo>(</mml:mo><mml:mn>1</mml:mn><mml:mo>+</mml:mo><mml:msub><mml:mrow><mml:mi>s</mml:mi><mml:mi>T</mml:mi></mml:mrow><mml:mrow><mml:mrow><mml:mi>H</mml:mi></mml:mrow></mml:mrow></mml:msub><mml:mo>)</mml:mo></mml:mrow></mml:mfrac></mml:math>	(3)</p>
        <p>where T<sub>H</sub> represents the hydraulic time constant.</p>
        <p>Speed Governor: The speed governor mechanism, which regulates the prime mover’s rotational speed, is represented by:</p>
        <p><mml:math id="M4"><mml:msub><mml:mrow><mml:mi>G</mml:mi></mml:mrow><mml:mrow><mml:mrow><mml:mi>G</mml:mi></mml:mrow></mml:mrow></mml:msub><mml:mo>(</mml:mo><mml:mrow><mml:mtext/><mml:mi>s</mml:mi></mml:mrow><mml:mo>)</mml:mo><mml:mo>=</mml:mo><mml:mfrac><mml:msub><mml:mrow><mml:mi>K</mml:mi></mml:mrow><mml:mrow><mml:mrow><mml:mi>G</mml:mi></mml:mrow></mml:mrow></mml:msub><mml:mrow><mml:mn>1</mml:mn><mml:mo>+</mml:mo><mml:msub><mml:mrow><mml:mi>s</mml:mi><mml:mi>T</mml:mi></mml:mrow><mml:mrow><mml:mrow><mml:mi>G</mml:mi></mml:mrow></mml:mrow></mml:msub></mml:mrow></mml:mfrac></mml:math>	(4)</p>
        <p>where K<sub>G</sub> is the governor gain and T<sub>G</sub> is the governor time constant.</p>
        <p>Generator-Load Model: The generator-load model, which converts mechanical power to electrical power while accounting for system inertia, is described by:</p>
        <p><mml:math id="M5"><mml:msub><mml:mrow><mml:mi>G</mml:mi></mml:mrow><mml:mrow><mml:mrow><mml:mi>P</mml:mi></mml:mrow></mml:mrow></mml:msub><mml:mo>(</mml:mo><mml:mrow><mml:mtext/><mml:mi>s</mml:mi></mml:mrow><mml:mo>)</mml:mo><mml:mo>=</mml:mo><mml:mfrac><mml:msub><mml:mrow><mml:mi>K</mml:mi></mml:mrow><mml:mrow><mml:mrow><mml:mi>P</mml:mi></mml:mrow></mml:mrow></mml:msub><mml:mrow><mml:mn>1</mml:mn><mml:mo>+</mml:mo><mml:mi>s</mml:mi><mml:msub><mml:mrow><mml:mtext/><mml:mi>T</mml:mi></mml:mrow><mml:mrow><mml:mrow><mml:mi>P</mml:mi></mml:mrow></mml:mrow></mml:msub></mml:mrow></mml:mfrac></mml:math>	(5)</p>
        <p>where Kp represents the power system gain and Tp the power system time constant.</p>
        <p>Complete System Transfer Function: Substituting the component functions yields the full <abbrev xlink:title="small modular reactor">SMR</abbrev> plant model:</p>
        <p><mml:math id="M6"><mml:mtable displaystyle="true" columnspacing="1em" rowspacing="3pt"><mml:mtr><mml:mtd><mml:msub><mml:mrow><mml:mi>G</mml:mi></mml:mrow><mml:mrow><mml:mrow><mml:mi>S</mml:mi><mml:mi>M</mml:mi><mml:mi>R</mml:mi></mml:mrow></mml:mrow></mml:msub><mml:mo>(</mml:mo><mml:mrow><mml:mtext/><mml:mi>s</mml:mi></mml:mrow><mml:mo>)</mml:mo><mml:mo>=</mml:mo></mml:mtd></mml:mtr><mml:mtr><mml:mtd><mml:mfrac><mml:mrow><mml:msub><mml:mrow><mml:mi>K</mml:mi></mml:mrow><mml:mrow><mml:mrow><mml:mi>G</mml:mi></mml:mrow></mml:mrow></mml:msub><mml:msub><mml:mrow><mml:mtext/><mml:mi>K</mml:mi></mml:mrow><mml:mrow><mml:mrow><mml:mi>P</mml:mi></mml:mrow></mml:mrow></mml:msub><mml:mrow><mml:mo>(</mml:mo><mml:mn>1</mml:mn><mml:mo>+</mml:mo><mml:mrow><mml:mi>s</mml:mi></mml:mrow><mml:mi>Δ</mml:mi><mml:mrow><mml:mi>t</mml:mi></mml:mrow><mml:mo>+</mml:mo><mml:msup><mml:mrow><mml:mi>s</mml:mi></mml:mrow><mml:mn>2</mml:mn></mml:msup><mml:mrow><mml:mtext/><mml:mi>B</mml:mi></mml:mrow><mml:mo>)</mml:mo></mml:mrow></mml:mrow><mml:mrow><mml:mrow><mml:mo>(</mml:mo><mml:mn>1</mml:mn><mml:mo>+</mml:mo><mml:msub><mml:mrow><mml:mi>s</mml:mi><mml:mi>T</mml:mi></mml:mrow><mml:mrow><mml:mrow><mml:mi>T</mml:mi><mml:mi>L</mml:mi></mml:mrow></mml:mrow></mml:msub><mml:mo>)</mml:mo></mml:mrow><mml:mrow><mml:mo>(</mml:mo><mml:mn>1</mml:mn><mml:mo>+</mml:mo><mml:msub><mml:mrow><mml:mi>s</mml:mi><mml:mi>T</mml:mi></mml:mrow><mml:mrow><mml:mrow><mml:mi>R</mml:mi><mml:mi>H</mml:mi></mml:mrow><mml:mn>2</mml:mn></mml:mrow></mml:msub><mml:mo>)</mml:mo></mml:mrow><mml:mrow><mml:mo>(</mml:mo><mml:mn>1</mml:mn><mml:mo>+</mml:mo><mml:msub><mml:mrow><mml:mi>s</mml:mi><mml:mi>T</mml:mi></mml:mrow><mml:mrow><mml:mrow><mml:mi>G</mml:mi></mml:mrow></mml:mrow></mml:msub><mml:mo>)</mml:mo></mml:mrow><mml:mrow><mml:mo>(</mml:mo><mml:mn>1</mml:mn><mml:mo>+</mml:mo><mml:msub><mml:mrow><mml:mi>s</mml:mi><mml:mi>T</mml:mi></mml:mrow><mml:mrow><mml:mrow><mml:mi>H</mml:mi></mml:mrow></mml:mrow></mml:msub><mml:mo>)</mml:mo></mml:mrow><mml:mo>(</mml:mo><mml:mn>1</mml:mn><mml:mo>+</mml:mo><mml:mrow><mml:mi>s</mml:mi><mml:mi>T</mml:mi><mml:mi>p</mml:mi></mml:mrow><mml:mo>)</mml:mo><mml:mrow><mml:mo>(</mml:mo><mml:mn>1</mml:mn><mml:mo>+</mml:mo><mml:msub><mml:mrow><mml:mi>s</mml:mi><mml:mi>T</mml:mi></mml:mrow><mml:mrow><mml:mrow><mml:mi>R</mml:mi><mml:mi>H</mml:mi></mml:mrow></mml:mrow></mml:msub><mml:mo>)</mml:mo></mml:mrow></mml:mrow></mml:mfrac></mml:mtd></mml:mtr></mml:mtable></mml:math>	(6)</p>
        <p>Table <xref ref-type="table" rid="T1">1</xref>, represents the established parameters values for simulation a Pressurized Water Reactor (<abbrev xlink:title="Pressurized Water Reactor">PWR</abbrev>)-based <abbrev xlink:title="small modular reactor">SMR</abbrev> as due to the references (<xref ref-type="bibr" rid="B3">Brewer 1952</xref>; <xref ref-type="bibr" rid="B14">Milano et al. 2018</xref>):</p>
        <table-wrap id="T1" position="float" orientation="portrait">
          <label>Table 1.</label>
          <caption>
            <p>Established parameter values used for simulation of the <abbrev xlink:title="Pressurized Water Reactor">PWR</abbrev>-based <abbrev xlink:title="small modular reactor">SMR</abbrev> model</p>
          </caption>
          <table>
            <tbody>
              <tr>
                <th rowspan="1" colspan="1">
                  <bold>Parameter</bold>
                </th>
                <th rowspan="1" colspan="1">
                  <bold>Symbol</bold>
                </th>
                <th rowspan="1" colspan="1">
                  <bold>Value</bold>
                </th>
                <th rowspan="1" colspan="1">
                  <bold>Unit</bold>
                </th>
              </tr>
              <tr>
                <td rowspan="1" colspan="1">Speed Governor Gain</td>
                <td rowspan="1" colspan="1">K<sub>G</sub></td>
                <td rowspan="1" colspan="1">1.0</td>
                <td rowspan="1" colspan="1">p.u.</td>
              </tr>
              <tr>
                <td rowspan="1" colspan="1">Power System Gain</td>
                <td rowspan="1" colspan="1">Kp</td>
                <td rowspan="1" colspan="1">1.0</td>
                <td rowspan="1" colspan="1">p.u.</td>
              </tr>
              <tr>
                <td rowspan="1" colspan="1">Turbine time coefficient</td>
                <td rowspan="1" colspan="1">Δt</td>
                <td rowspan="1" colspan="1">0.48</td>
                <td rowspan="1" colspan="1">s</td>
              </tr>
              <tr>
                <td rowspan="1" colspan="1">Turbine dynamic coefficient</td>
                <td rowspan="1" colspan="1">B</td>
                <td rowspan="1" colspan="1">0.03</td>
                <td rowspan="1" colspan="1">s</td>
              </tr>
              <tr>
                <td rowspan="1" colspan="1">Governor Time Constant</td>
                <td rowspan="1" colspan="1">T<sub>G</sub></td>
                <td rowspan="1" colspan="1">0.08</td>
                <td rowspan="1" colspan="1">s</td>
              </tr>
              <tr>
                <td rowspan="1" colspan="1">Hydraulic Time Constant</td>
                <td rowspan="1" colspan="1">T<sub>H</sub></td>
                <td rowspan="1" colspan="1">0.3</td>
                <td rowspan="1" colspan="1">s</td>
              </tr>
              <tr>
                <td rowspan="1" colspan="1">Power system Time Constant</td>
                <td rowspan="1" colspan="1">Tp</td>
                <td rowspan="1" colspan="1">20.0</td>
                <td rowspan="1" colspan="1">s</td>
              </tr>
              <tr>
                <td rowspan="1" colspan="1">HP Turbine Time Constant</td>
                <td rowspan="1" colspan="1">T<sub>RH</sub></td>
                <td rowspan="1" colspan="1">4.9</td>
                <td rowspan="1" colspan="1">s</td>
              </tr>
              <tr>
                <td rowspan="1" colspan="1">LP Turbine Time Constant</td>
                <td rowspan="1" colspan="1">T<sub>TL</sub></td>
                <td rowspan="1" colspan="1">0.6</td>
                <td rowspan="1" colspan="1">s</td>
              </tr>
              <tr>
                <td rowspan="1" colspan="1">Reheater Time Constant</td>
                <td rowspan="1" colspan="1">T<sub>RH2</sub></td>
                <td rowspan="1" colspan="1">0.05</td>
                <td rowspan="1" colspan="1">s</td>
              </tr>
            </tbody>
          </table>
        </table-wrap>
        <p>Substituting these values and expanding the denominator polynomial results in the final transfer function for Simulink implementation:</p>
        <p><mml:math id="M7"><mml:mtable displaystyle="true" columnspacing="1em" rowspacing="3pt"><mml:mtr><mml:mtd><mml:msub><mml:mi>G</mml:mi><mml:mrow><mml:mtext>SMR </mml:mtext></mml:mrow></mml:msub><mml:mo>(</mml:mo><mml:mi>s</mml:mi><mml:mo>)</mml:mo><mml:mo>=</mml:mo></mml:mtd></mml:mtr><mml:mtr><mml:mtd><mml:mfrac><mml:mrow><mml:mo>(</mml:mo><mml:mn>1</mml:mn><mml:mo>+</mml:mo><mml:mn>0.48</mml:mn><mml:mi>s</mml:mi><mml:mo>+</mml:mo><mml:mn>0.03</mml:mn><mml:msup><mml:mi>s</mml:mi><mml:mn>2</mml:mn></mml:msup><mml:mo>)</mml:mo></mml:mrow><mml:mrow><mml:mo>(</mml:mo><mml:mn>1</mml:mn><mml:mo>+</mml:mo><mml:mn>0.08</mml:mn><mml:mi>s</mml:mi><mml:mo>)</mml:mo><mml:mo>(</mml:mo><mml:mn>1</mml:mn><mml:mo>+</mml:mo><mml:mn>0.3</mml:mn><mml:mi>s</mml:mi><mml:mo>)</mml:mo><mml:mo>(</mml:mo><mml:mn>1</mml:mn><mml:mo>+</mml:mo><mml:mn>20</mml:mn><mml:mi>s</mml:mi><mml:mo>)</mml:mo><mml:mo>(</mml:mo><mml:mn>1</mml:mn><mml:mo>+</mml:mo><mml:mn>4.9</mml:mn><mml:mi>s</mml:mi><mml:mo>)</mml:mo><mml:mo>(</mml:mo><mml:mn>1</mml:mn><mml:mo>+</mml:mo><mml:mn>0.6</mml:mn><mml:mi>s</mml:mi><mml:mo>)</mml:mo><mml:mo>(</mml:mo><mml:mn>1</mml:mn><mml:mo>+</mml:mo><mml:mn>0.05</mml:mn><mml:mi>s</mml:mi><mml:mo>)</mml:mo></mml:mrow></mml:mfrac><mml:mo>=</mml:mo></mml:mtd></mml:mtr><mml:mtr><mml:mtd><mml:mfrac><mml:mrow><mml:mn>0.03</mml:mn><mml:msup><mml:mi>s</mml:mi><mml:mn>2</mml:mn></mml:msup><mml:mo>+</mml:mo><mml:mn>0.48</mml:mn><mml:mi>s</mml:mi><mml:mo>+</mml:mo><mml:mn>1</mml:mn></mml:mrow><mml:mrow><mml:mn>0.07056</mml:mn><mml:msup><mml:mi>s</mml:mi><mml:mn>6</mml:mn></mml:msup><mml:mo>+</mml:mo><mml:mn>2.21976</mml:mn><mml:msup><mml:mi>s</mml:mi><mml:mn>5</mml:mn></mml:msup><mml:mo>+</mml:mo><mml:mn>28.6897</mml:mn><mml:msup><mml:mi>s</mml:mi><mml:mn>4</mml:mn></mml:msup><mml:mo>+</mml:mo><mml:mn>72.2603</mml:mn><mml:msup><mml:mi>s</mml:mi><mml:mn>3</mml:mn></mml:msup><mml:mo>+</mml:mo><mml:mn>129.9325</mml:mn><mml:msup><mml:mi>s</mml:mi><mml:mn>2</mml:mn></mml:msup><mml:mo>+</mml:mo><mml:mn>25.93</mml:mn><mml:mi>s</mml:mi><mml:mo>+</mml:mo><mml:mn>1</mml:mn></mml:mrow></mml:mfrac></mml:mtd></mml:mtr></mml:mtable></mml:math>	(7)</p>
        <p>This numerically specified transfer function block serves as the plant model that will receive the control signal from any chosen controller architecture, enabling comprehensive simulation and analysis of various control strategies for Load Frequency Control under various disturbance conditions.</p>
      </sec>
      <sec sec-type="Grid dynamics model" id="sec5">
        <title>Grid dynamics model</title>
        <p>Grid frequency dynamics are governed by the swing equation, which describes the relationship between power imbalance and rate of change of frequency (<xref ref-type="bibr" rid="B3">Brewer 1952</xref>; <xref ref-type="bibr" rid="B18">Sauer and Pai 1998</xref>):</p>
        <p><mml:math id="M8"><mml:mn>2</mml:mn><mml:mrow><mml:mi>H</mml:mi></mml:mrow><mml:mfrac><mml:mrow><mml:mrow><mml:mtext/><mml:mi>d</mml:mi></mml:mrow><mml:mi>Δ</mml:mi><mml:mrow><mml:mi>f</mml:mi></mml:mrow><mml:mo>(</mml:mo><mml:mrow><mml:mi>t</mml:mi></mml:mrow><mml:mo>)</mml:mo></mml:mrow><mml:mrow><mml:mi>d</mml:mi><mml:mi>t</mml:mi></mml:mrow></mml:mfrac><mml:mo>=</mml:mo><mml:mi>Δ</mml:mi><mml:mi>Pm</mml:mi><mml:mo>⁡</mml:mo><mml:mo>(</mml:mo><mml:mrow><mml:mi>t</mml:mi></mml:mrow><mml:mo>)</mml:mo><mml:mo>+</mml:mo><mml:mi>Δ</mml:mi><mml:mi>Pd</mml:mi><mml:mo>⁡</mml:mo><mml:mo>(</mml:mo><mml:mrow><mml:mi>t</mml:mi></mml:mrow><mml:mo>)</mml:mo><mml:mo>−</mml:mo><mml:mrow><mml:mi>D</mml:mi></mml:mrow><mml:mi>Δ</mml:mi><mml:mrow><mml:mi>f</mml:mi></mml:mrow><mml:mo>(</mml:mo><mml:mrow><mml:mi>t</mml:mi></mml:mrow><mml:mo>)</mml:mo></mml:math>	(8)</p>
        <p>Where:</p>
        <p>H is the system inertia constant (s), Δf is the frequency deviation (Hz), ΔP<sub>m</sub> is the mechanical power change from generators (p.u.), ΔP<sub>d</sub> is the load disturbance (p.u.), D is the load damping coefficient (p.u./Hz).</p>
        <p>Taking the Laplace transform, the transfer function from net power imbalance (ΔPm–ΔPd) to frequency deviation is:</p>
        <p><mml:math id="M9"><mml:mi>Ggrid</mml:mi><mml:mo>⁡</mml:mo><mml:mo>(</mml:mo><mml:mi>s</mml:mi><mml:mo>)</mml:mo><mml:mo>=</mml:mo><mml:mfrac><mml:mrow><mml:mi>Δ</mml:mi><mml:mi>F</mml:mi><mml:mo>(</mml:mo><mml:mi>s</mml:mi><mml:mo>)</mml:mo></mml:mrow><mml:mrow><mml:msub><mml:mi>P</mml:mi><mml:mi>m</mml:mi></mml:msub><mml:mo>(</mml:mo><mml:mi>s</mml:mi><mml:mo>)</mml:mo><mml:mo>−</mml:mo><mml:msub><mml:mi>P</mml:mi><mml:mi>d</mml:mi></mml:msub><mml:mo>(</mml:mo><mml:mi>s</mml:mi><mml:mo>)</mml:mo></mml:mrow></mml:mfrac><mml:mo>=</mml:mo><mml:mfrac><mml:mn>1</mml:mn><mml:mrow><mml:mn>2</mml:mn><mml:mi>H</mml:mi><mml:mi>s</mml:mi><mml:mo>+</mml:mo><mml:mi>D</mml:mi></mml:mrow></mml:mfrac></mml:math>	(9)</p>
        <p>For low-inertia power systems with high renewable penetration, representative values of the inertia constant H and damping coefficient D are adopted from established frequency-response modeling studies. In particular, the selected parameters are consistent with the reduced-inertia grid models reported by <xref ref-type="bibr" rid="B14">Milano et al. (2018)</xref> and <xref ref-type="bibr" rid="B19">Ulbig et al. (2014)</xref>, which analyze frequency dynamics under high penetration of converter-interfaced renewable generation.</p>
        <list list-type="bullet">
          <list-item>
            <p>Inertia constant, H = 2.5 s (represents the kinetic energy stored in rotating masses relative to system power) (low-inertia system).
</p>
          </list-item>
          <list-item>
            <p>Damping coefficient, D = 0.8 (captures frequency damping due to load-frequency dependence and primary regulation effects) (low damping).
</p>
          </list-item>
        </list>
        <p>This yields:</p>
        <p><mml:math id="M10"><mml:mi>Ggrid</mml:mi><mml:mo>⁡</mml:mo><mml:mo>(</mml:mo><mml:mi>s</mml:mi><mml:mo>)</mml:mo><mml:mo>=</mml:mo><mml:mfrac><mml:mn>1</mml:mn><mml:mrow><mml:mn>5</mml:mn><mml:mi>s</mml:mi><mml:mo>+</mml:mo><mml:mn>0.8</mml:mn></mml:mrow></mml:mfrac></mml:math>	(10)</p>
      </sec>
      <sec sec-type="Overall system architecture" id="sec6">
        <title>Overall system architecture</title>
        <p>The integrated Simulink model (Fig. <xref ref-type="fig" rid="F1">1</xref>) comprises:</p>
        <fig id="F1">
          <object-id content-type="doi">10.3897/nucet.12.176131.figure1</object-id>
          <object-id content-type="arpha">2030A62D-A744-56B6-991A-E9092DAA53E5</object-id>
          <label>Figure 1.</label>
          <caption>
            <p>Simplified block diagram of the <abbrev xlink:title="small modular reactor">SMR</abbrev>-Renewables-Grid system with <abbrev xlink:title="proportional-integral-derivative">PID</abbrev>/<abbrev xlink:title="model predictive control">MPC</abbrev> controllers.</p>
          </caption>
          <graphic xlink:href="nucet-12-e176131-g001.jpg" id="oo_1555373.jpg">
            <uri content-type="original_file">https://binary.pensoft.net/fig/1555373</uri>
          </graphic>
        </fig>
        <list list-type="bullet">
          <list-item>
            <p>Disturbance Inputs: Wind, solar, generator outage (–20% at 400 s), and load increase (+15% at 800 s).
</p>
          </list-item>
          <list-item>
            <p>Controller: Either a <abbrev xlink:title="proportional-integral-derivative">PID</abbrev> or <abbrev xlink:title="model predictive control">MPC</abbrev> controller processes the frequency error (<mml:math id="M11"><mml:mi>Δ</mml:mi><mml:mi>f</mml:mi><mml:mo>=</mml:mo><mml:msub><mml:mi>f</mml:mi><mml:mrow><mml:mtext>ref </mml:mtext></mml:mrow></mml:msub><mml:mo>−</mml:mo><mml:msub><mml:mi>f</mml:mi><mml:mrow><mml:mtext>actual </mml:mtext></mml:mrow></mml:msub></mml:math>) and generates a control signal for the <abbrev xlink:title="small modular reactor">SMR</abbrev>.
</p>
          </list-item>
          <list-item>
            <p><abbrev xlink:title="small modular reactor">SMR</abbrev> Block: Implements the 6
                        <sup>th</sup>-order <abbrev xlink:title="small modular reactor">SMR</abbrev> transfer function with saturation limits.
                    </p>
          </list-item>
          <list-item>
            <p>Grid Block: Implements the1
                        <sup>st</sup>-order, low-inertia grid dynamics.
                    </p>
          </list-item>
          <list-item>
            <p>Summing Junctions: To combine the power outputs from the <abbrev xlink:title="small modular reactor">SMR</abbrev>, renewables, and disturbance inputs.
</p>
          </list-item>
          <list-item>
            <p>Scopes: Monitor frequency, <abbrev xlink:title="small modular reactor">SMR</abbrev> power output, renewable generation, and power balance.
</p>
          </list-item>
        </list>
        <p>Fig. <xref ref-type="fig" rid="F2">2</xref> shows the Simulink layout for <abbrev xlink:title="proportional-integral-derivative">PID</abbrev> model implementations, and Fig. <xref ref-type="fig" rid="F3">3</xref> shows the layout for <abbrev xlink:title="model predictive control">MPC</abbrev>-model implementations.</p>
        <fig id="F2">
          <object-id content-type="doi">10.3897/nucet.12.176131.figure2</object-id>
          <object-id content-type="arpha">6BF8C80D-CD26-50C0-BE04-267621C03C74</object-id>
          <label>Figure 2.</label>
          <caption>
            <p>Simulink Model Layout using <abbrev xlink:title="proportional-integral-derivative">PID</abbrev> controller.</p>
          </caption>
          <graphic xlink:href="nucet-12-e176131-g002.jpg" id="oo_1555374.jpg">
            <uri content-type="original_file">https://binary.pensoft.net/fig/1555374</uri>
          </graphic>
        </fig>
        <fig id="F3">
          <object-id content-type="doi">10.3897/nucet.12.176131.figure3</object-id>
          <object-id content-type="arpha">0C064913-C10F-5DCA-8FBB-103AF7027FC1</object-id>
          <label>Figure 3.</label>
          <caption>
            <p>Simulink Model Layout using <abbrev xlink:title="model predictive control">MPC</abbrev> controller.</p>
          </caption>
          <graphic xlink:href="nucet-12-e176131-g003.jpg" id="oo_1555375.jpg">
            <uri content-type="original_file">https://binary.pensoft.net/fig/1555375</uri>
          </graphic>
        </fig>
        <p>The two models highlight how different generation sources, disturbances, and control elements interact to maintain system stability.</p>
      </sec>
      <sec sec-type="Control strategies" id="sec7">
        <title>Control strategies</title>
        <sec sec-type="Proportional_Integration_Derivative Control (PID)" id="sec8">
          <title>
            <italic>Proportional_Integration_Derivative Control (PID)</italic>
          </title>
          <p>The proportional–integral–derivative (<abbrev xlink:title="proportional-integral-derivative">PID</abbrev>) controller provides a robust baseline for frequency regulation and is widely deployed in power systems due to its simplicity and reliability. The standard <abbrev xlink:title="proportional-integral-derivative">PID</abbrev> control law is expressed as:</p>
          <p><mml:math id="M12"><mml:mi>u</mml:mi><mml:mo>(</mml:mo><mml:mi>t</mml:mi><mml:mo>)</mml:mo><mml:mo>=</mml:mo><mml:mi>K</mml:mi><mml:mi>p</mml:mi><mml:mi>e</mml:mi><mml:mo>(</mml:mo><mml:mi>t</mml:mi><mml:mo>)</mml:mo><mml:mo>+</mml:mo><mml:mi>K</mml:mi><mml:mi>i</mml:mi><mml:mo>∫</mml:mo><mml:mi>e</mml:mi><mml:mo>(</mml:mo><mml:mi>t</mml:mi><mml:mo>)</mml:mo><mml:mi>d</mml:mi><mml:mi>t</mml:mi><mml:mo>+</mml:mo><mml:mi>K</mml:mi><mml:mi>d</mml:mi><mml:mfrac><mml:mrow><mml:mi>d</mml:mi><mml:mi>e</mml:mi><mml:mo>(</mml:mo><mml:mi>t</mml:mi><mml:mo>)</mml:mo></mml:mrow><mml:mrow><mml:mi>d</mml:mi><mml:mi>t</mml:mi></mml:mrow></mml:mfrac></mml:math>	(11)</p>
          <p>where e(t) represents the grid frequency error (Δf), and Kp, Ki, and Kd are the proportional, integral, and derivative gains, respectively. The proportional term provides immediate corrective action proportional to the error, the integral term eliminates steady-state offset, and the derivative term adds damping by responding to the rate of error change.</p>
          <p>For this study, the <abbrev xlink:title="proportional-integral-derivative">PID</abbrev> gains were initially tuned using a pole-placement method and subsequently fine-tuned manually in the Simulink environment to achieve an optimal balance between response speed and overshoot suppression under the defined renewable variability and contingency scenarios. The final tuned gain values implemented in all simulations are:</p>
          <p>
            <mml:math id="M13"><mml:mrow><mml:mi>K</mml:mi><mml:mi>p</mml:mi></mml:mrow><mml:mo>=</mml:mo><mml:mn>1.8</mml:mn><mml:mo>,</mml:mo><mml:mstyle scriptlevel="0"><mml:mspace width="1em"/></mml:mstyle><mml:mrow><mml:mi>K</mml:mi><mml:mi>i</mml:mi></mml:mrow><mml:mo>=</mml:mo><mml:mn>0.9</mml:mn><mml:mo>,</mml:mo><mml:mstyle scriptlevel="0"><mml:mspace width="1em"/></mml:mstyle><mml:mrow><mml:mi>K</mml:mi><mml:mi>d</mml:mi></mml:mrow><mml:mo>=</mml:mo><mml:mn>0.15</mml:mn></mml:math>
          </p>
          <p>With this tuning, the <abbrev xlink:title="proportional-integral-derivative">PID</abbrev> controller demonstrated effective performance, as quantified in Table <xref ref-type="table" rid="T2">2</xref> of the manuscript. Specifically, it achieved a maximum frequency overshoot of 0.25 Hz, an undershoot of 0.18 Hz, a settling time (to within 2% of steady-state) of 22 seconds, and a steady-state error of 0.02 Hz following major disturbances. The controller also maintained acceptable robustness, exhibiting only minor oscillations when system inertia and damping parameters were varied by ± 15%.</p>
          <table-wrap id="T2" position="float" orientation="portrait">
            <label>Table 2.</label>
            <caption>
              <p>Performance Comparison between <abbrev xlink:title="proportional-integral-derivative">PID</abbrev> and <abbrev xlink:title="model predictive control">MPC</abbrev></p>
            </caption>
            <table>
              <tbody>
                <tr>
                  <th rowspan="1" colspan="1">
                    <bold>Metric</bold>
                  </th>
                  <th rowspan="1" colspan="1">
                    <bold>
                      <abbrev xlink:title="proportional-integral-derivative">PID</abbrev>
                    </bold>
                  </th>
                  <th rowspan="1" colspan="1">
                    <bold>
                      <abbrev xlink:title="model predictive control">MPC</abbrev>
                    </bold>
                  </th>
                </tr>
                <tr>
                  <td rowspan="1" colspan="3">Transient Response</td>
                </tr>
                <tr>
                  <td rowspan="1" colspan="1">Overshoot (Hz)</td>
                  <td rowspan="1" colspan="1">0.25</td>
                  <td rowspan="1" colspan="1">0.15</td>
                </tr>
                <tr>
                  <td rowspan="1" colspan="1">Undershoot (Hz)</td>
                  <td rowspan="1" colspan="1">0.18</td>
                  <td rowspan="1" colspan="1">0.10</td>
                </tr>
                <tr>
                  <td rowspan="1" colspan="1">Settling Time (s)</td>
                  <td rowspan="1" colspan="1">22</td>
                  <td rowspan="1" colspan="1">18</td>
                </tr>
                <tr>
                  <td rowspan="1" colspan="1">Rise Time (s)</td>
                  <td rowspan="1" colspan="1">4.5</td>
                  <td rowspan="1" colspan="1">3.8</td>
                </tr>
                <tr>
                  <td rowspan="1" colspan="1">Steady-State Performance</td>
                  <td rowspan="1" colspan="1"/>
                  <td rowspan="1" colspan="1"/>
                </tr>
                <tr>
                  <td rowspan="1" colspan="1">Steady-State Error (Hz)</td>
                  <td rowspan="1" colspan="1">0.02</td>
                  <td rowspan="1" colspan="1">0.008</td>
                </tr>
                <tr>
                  <td rowspan="1" colspan="3">Robustness &amp; Stability</td>
                </tr>
                <tr>
                  <td rowspan="1" colspan="1">Robustness to ± 15% Param. Variations</td>
                  <td rowspan="1" colspan="1">Acceptable (Minor oscillations)</td>
                  <td rowspan="1" colspan="1">High (Stable)</td>
                </tr>
                <tr>
                  <td rowspan="1" colspan="3">Control Effort &amp; Actuation</td>
                </tr>
                <tr>
                  <td rowspan="1" colspan="1">Control Effort (Total Variation of <abbrev xlink:title="small modular reactor">SMR</abbrev> Power)</td>
                  <td rowspan="1" colspan="1">High</td>
                  <td rowspan="1" colspan="1">Low</td>
                </tr>
                <tr>
                  <td rowspan="1" colspan="1">Maximum <abbrev xlink:title="small modular reactor">SMR</abbrev> Power Rate (MW/s)</td>
                  <td rowspan="1" colspan="1">8.5</td>
                  <td rowspan="1" colspan="1">5.0</td>
                </tr>
                <tr>
                  <td rowspan="1" colspan="3">Disturbance Rejection</td>
                </tr>
                <tr>
                  <td rowspan="1" colspan="1">Frequency Nadir after Contingency (Hz)</td>
                  <td rowspan="1" colspan="1">49.52</td>
                  <td rowspan="1" colspan="1">49.60</td>
                </tr>
                <tr>
                  <td rowspan="1" colspan="3">Computational Load</td>
                </tr>
              </tbody>
            </table>
          </table-wrap>
          <p>While the <abbrev xlink:title="proportional-integral-derivative">PID</abbrev> controller delivered reliable stabilization, its performance is inherently limited by its fixed-gain, feedback-only structure. Recent developments in adaptive and fuzzy-enhanced <abbrev xlink:title="proportional-integral-derivative">PID</abbrev> architectures demonstrate improved robustness under nonlinear and time-varying operating conditions; however, these approaches introduce additional implementation complexity compared to conventional fixed-gain <abbrev xlink:title="proportional-integral-derivative">PID</abbrev> controllers (<xref ref-type="bibr" rid="B11">Liu et al. 2025</xref>). It lacks the predictive capability to anticipate future disturbances and cannot explicitly handle system constraints, such as <abbrev xlink:title="small modular reactor">SMR</abbrev> power and ramp rate limits. Therefore, in this comparative study, the <abbrev xlink:title="proportional-integral-derivative">PID</abbrev> controller serves as a well-established benchmark against which the advanced capabilities of the Model Predictive Control (<abbrev xlink:title="model predictive control">MPC</abbrev>) strategy are evaluated.</p>
        </sec>
        <sec sec-type="Model Predictive Control (MPC)" id="sec9">
          <title>
            <italic>Model Predictive Control (MPC)</italic>
          </title>
          <p>Model Predictive Control (<abbrev xlink:title="model predictive control">MPC</abbrev>) is an advanced control strategy that utilizes an explicit model of the plant to predict its future behaviour over a finite prediction horizon Np. At each control interval k, <abbrev xlink:title="model predictive control">MPC</abbrev> solves a constrained optimization problem to determine a sequence of future control actions that minimizes a cost function J, while explicitly respecting system constraints.</p>
          <p>As illustrated in Fig. <xref ref-type="fig" rid="F4">4</xref>, at each sampling instant <abbrev xlink:title="model predictive control">MPC</abbrev> solves an online quadratic optimization problem formulated as:</p>
          <fig id="F4">
            <object-id content-type="doi">10.3897/nucet.12.176131.figure4</object-id>
            <object-id content-type="arpha">E7B4F621-3B2A-58D7-A349-64AD333FCA74</object-id>
            <label>Figure 4.</label>
            <caption>
              <p>The <abbrev xlink:title="model predictive control">MPC</abbrev> scheme.</p>
            </caption>
            <graphic xlink:href="nucet-12-e176131-g004.jpg" id="oo_1555376.jpg">
              <uri content-type="original_file">https://binary.pensoft.net/fig/1555376</uri>
            </graphic>
          </fig>
          <p><mml:math id="M14"><mml:mtable displaystyle="true" columnspacing="1em" rowspacing="3pt"><mml:mtr><mml:mtd><mml:mrow><mml:mi>J</mml:mi></mml:mrow><mml:mo>=</mml:mo><mml:mo>min</mml:mo><mml:mrow><mml:mi>U</mml:mi></mml:mrow><mml:munderover><mml:mo>∑</mml:mo><mml:mrow><mml:mrow><mml:mi>i</mml:mi></mml:mrow><mml:mo>=</mml:mo><mml:mn>1</mml:mn></mml:mrow><mml:mrow><mml:mrow><mml:mi>N</mml:mi><mml:mi>p</mml:mi></mml:mrow></mml:mrow></mml:munderover><mml:mo>[</mml:mo><mml:mo>(</mml:mo><mml:mrow><mml:mi>y</mml:mi></mml:mrow><mml:mo>(</mml:mo><mml:mrow><mml:mi>k</mml:mi></mml:mrow><mml:mo>+</mml:mo><mml:mrow><mml:mi>i</mml:mi></mml:mrow><mml:mo>)</mml:mo><mml:mo>−</mml:mo><mml:mrow><mml:mi>y</mml:mi><mml:mi>r</mml:mi><mml:mi>e</mml:mi><mml:mi>f</mml:mi></mml:mrow><mml:mo>]</mml:mo><mml:mrow><mml:mi>T</mml:mi><mml:mi>Q</mml:mi></mml:mrow><mml:mo>(</mml:mo><mml:mrow><mml:mi>y</mml:mi></mml:mrow><mml:mo>(</mml:mo><mml:mrow><mml:mi>k</mml:mi></mml:mrow><mml:mo>+</mml:mo><mml:mrow><mml:mi>i</mml:mi></mml:mrow><mml:mo>)</mml:mo><mml:mo>−</mml:mo><mml:mrow><mml:mi>y</mml:mi><mml:mi>r</mml:mi><mml:mi>e</mml:mi><mml:mi>f</mml:mi></mml:mrow><mml:mo>)</mml:mo></mml:mtd></mml:mtr><mml:mtr><mml:mtd><mml:mo>+</mml:mo><mml:mi>Δ</mml:mi><mml:mrow><mml:mi>u</mml:mi><mml:mi>k</mml:mi></mml:mrow><mml:mo>+</mml:mo><mml:mrow><mml:mi>i</mml:mi></mml:mrow><mml:mo>−</mml:mo><mml:mn>1</mml:mn><mml:msup><mml:mo>)</mml:mo><mml:mrow><mml:mrow><mml:mi>T</mml:mi></mml:mrow></mml:mrow></mml:msup><mml:mrow><mml:mi>R</mml:mi></mml:mrow><mml:mi>Δ</mml:mi><mml:mrow><mml:mi>u</mml:mi></mml:mrow><mml:mo>(</mml:mo><mml:mrow><mml:mi>k</mml:mi></mml:mrow><mml:mo>+</mml:mo><mml:mrow><mml:mi>i</mml:mi></mml:mrow><mml:mo>−</mml:mo><mml:mn>1</mml:mn><mml:mo>)</mml:mo></mml:mtd></mml:mtr></mml:mtable></mml:math>	(12)</p>
          <p>subject to the discrete-time state-space model:</p>
          <p><mml:math id="M15"><mml:mrow><mml:mi>x</mml:mi></mml:mrow><mml:mo>(</mml:mo><mml:mrow><mml:mi>k</mml:mi></mml:mrow><mml:mo>+</mml:mo><mml:mn>1</mml:mn><mml:mo>)</mml:mo><mml:mo>=</mml:mo><mml:mrow><mml:mi>A</mml:mi><mml:mi>x</mml:mi></mml:mrow><mml:mo>(</mml:mo><mml:mrow><mml:mi>k</mml:mi></mml:mrow><mml:mo>)</mml:mo><mml:mo>+</mml:mo><mml:mrow><mml:mi>B</mml:mi><mml:mi>u</mml:mi></mml:mrow><mml:mo>(</mml:mo><mml:mrow><mml:mi>k</mml:mi></mml:mrow><mml:mo>)</mml:mo><mml:mo>,</mml:mo><mml:mrow><mml:mi>y</mml:mi></mml:mrow><mml:mo>(</mml:mo><mml:mrow><mml:mi>k</mml:mi></mml:mrow><mml:mo>)</mml:mo><mml:mo>=</mml:mo><mml:mrow><mml:mi>C</mml:mi><mml:mi>x</mml:mi></mml:mrow><mml:mo>(</mml:mo><mml:mrow><mml:mi>k</mml:mi></mml:mrow><mml:mo>)</mml:mo></mml:math>	(13)</p>
          <p>and operational constraints:</p>
          <p>
            <mml:math id="M16"><mml:msub><mml:mrow><mml:mi>u</mml:mi></mml:mrow><mml:mrow><mml:mo>min</mml:mo></mml:mrow></mml:msub><mml:mo>≤</mml:mo><mml:mrow><mml:mi>u</mml:mi></mml:mrow><mml:mo>(</mml:mo><mml:mrow><mml:mi>k</mml:mi></mml:mrow><mml:mo>)</mml:mo><mml:mo>≤</mml:mo><mml:msub><mml:mrow><mml:mi>u</mml:mi></mml:mrow><mml:mrow><mml:mo>max</mml:mo></mml:mrow></mml:msub><mml:mo>,</mml:mo><mml:mi>Δ</mml:mi><mml:msub><mml:mrow><mml:mi>u</mml:mi></mml:mrow><mml:mrow><mml:mo>min</mml:mo></mml:mrow></mml:msub><mml:mo>≤</mml:mo><mml:mi>Δ</mml:mi><mml:mrow><mml:mi>u</mml:mi></mml:mrow><mml:mo>(</mml:mo><mml:mrow><mml:mi>k</mml:mi></mml:mrow><mml:mo>)</mml:mo><mml:mo>≤</mml:mo><mml:mi>Δ</mml:mi><mml:msub><mml:mrow><mml:mi>u</mml:mi></mml:mrow><mml:mrow><mml:mo>max</mml:mo></mml:mrow></mml:msub></mml:math>
          </p>
          <p>(Input constraints) (14)</p>
          <p><mml:math id="M17"><mml:mi>Δ</mml:mi><mml:msub><mml:mrow><mml:mi>u</mml:mi></mml:mrow><mml:mrow><mml:mo>min</mml:mo></mml:mrow></mml:msub><mml:mo>≤</mml:mo><mml:mi>Δ</mml:mi><mml:mrow><mml:mi>u</mml:mi></mml:mrow><mml:mo>(</mml:mo><mml:mrow><mml:mi>k</mml:mi></mml:mrow><mml:mo>)</mml:mo><mml:mo>≤</mml:mo><mml:mi>Δ</mml:mi><mml:msub><mml:mrow><mml:mi>u</mml:mi></mml:mrow><mml:mrow><mml:mo>max</mml:mo></mml:mrow></mml:msub></mml:math>	(15)</p>
          <p>Here, Np and Nc denote the prediction and control horizons, respectively; Q and R are weighting matrices that balance tracking performance and control effort; and u(k) represents the <abbrev xlink:title="small modular reactor">SMR</abbrev> power command.</p>
          <p>For this work, a linear discrete-time state-space model of the combined <abbrev xlink:title="small modular reactor">SMR</abbrev>–grid system was derived by linearization around the nominal operating point and discretized using a zero-order hold method. The <abbrev xlink:title="model predictive control">MPC</abbrev> parameters were selected as: Np = 10, Nc = 3, sampling period Ts = 0.1 s, with weighting matric es Q = 1 and R = 0.1.</p>
          <p>Only the first control move of the optimized sequence is applied to the plant, and the optimization is repeated at the next sampling instant in a receding-horizon manner. The optimization problem is solved as a Quadratic Programming (<abbrev xlink:title="Quadratic Programming">QP</abbrev>) problem at each step. The key advantage of <abbrev xlink:title="model predictive control">MPC</abbrev> lies in its ability to anticipate future frequency deviations and explicitly handle <abbrev xlink:title="small modular reactor">SMR</abbrev> power and ramp-rate constraints, thereby preventing actuator saturation and ensuring safe and feasible reactor operation. Hybrid <abbrev xlink:title="model predictive control">MPC</abbrev>–<abbrev xlink:title="proportional-integral-derivative">PID</abbrev> control architectures have also been proposed in related engineering applications to combine predictive optimization with rapid inner-loop regulation (<xref ref-type="bibr" rid="B20">Ye and Geng 2025</xref>), further supporting the applicability of <abbrev xlink:title="model predictive control">MPC</abbrev>-based frameworks to constrained dynamic systems.</p>
        </sec>
      </sec>
      <sec sec-type="Simulation scenarios" id="sec10">
        <title>Simulation scenarios</title>
        <p>Three scenarios (1200 s) were simulated in a single run of MATLAB/Simulink model as follows:</p>
        <list list-type="bullet">
          <list-item>
            <p>Scenario 1: Continuous renewable variability (wind+ solar), as shown in Fig. <xref ref-type="fig" rid="F5">5</xref>.
</p>
          </list-item>
          <list-item>
            <p>Scenario 2: Large generator outage (–20% at t = 400 s).
</p>
          </list-item>
          <list-item>
            <p>Scenario 3: Sudden load increase (+15% at t = 800 s).
</p>
          </list-item>
        </list>
        <p>“<italic>The –20% generator outage corresponds to the loss of 20% of the total system generation capacity, representing a large conventional unit. The +15% load increase is defined relative to the total system load, not the <abbrev xlink:title="small modular reactor">SMR</abbrev> rated capacity</italic>.”</p>
        <fig id="F5">
          <object-id content-type="doi">10.3897/nucet.12.176131.figure5</object-id>
          <object-id content-type="arpha">2EDC7810-FD0A-5AB3-8308-94962D253CDA</object-id>
          <label>Figure 5.</label>
          <caption>
            <p>The overall Renewable continuous variation source in a single applied input to both of <abbrev xlink:title="proportional-integral-derivative">PID</abbrev> and <abbrev xlink:title="model predictive control">MPC</abbrev> implementation models.</p>
          </caption>
          <graphic xlink:href="nucet-12-e176131-g005.jpg" id="oo_1555377.jpg">
            <uri content-type="original_file">https://binary.pensoft.net/fig/1555377</uri>
          </graphic>
        </fig>
        <p>Controllers were tested under identical conditions to enable fair comparison. Both of disturbances are shown in Fig. <xref ref-type="fig" rid="F6">6</xref>.</p>
        <fig id="F6">
          <object-id content-type="doi">10.3897/nucet.12.176131.figure6</object-id>
          <object-id content-type="arpha">A22AA8F2-D52C-5D21-9A39-C25F7975EF6F</object-id>
          <label>Figure 6.</label>
          <caption>
            <p>The Load disturbance of 20% large plant outage and 15% sudden load increase.</p>
          </caption>
          <graphic xlink:href="nucet-12-e176131-g006.jpg" id="oo_1555378.jpg">
            <uri content-type="original_file">https://binary.pensoft.net/fig/1555378</uri>
          </graphic>
        </fig>
      </sec>
    </sec>
    <sec sec-type="Results and discussion" id="sec11">
      <title>Results and discussion</title>
      <p>We make a comparative analysis of the <abbrev xlink:title="proportional-integral-derivative">PID</abbrev> and <abbrev xlink:title="model predictive control">MPC</abbrev> controllers based on dynamic simulations of the integrated <abbrev xlink:title="small modular reactor">SMR</abbrev>-renewable-grid system. The performance is evaluated under three consecutive disturbance scenarios: continuous renewable variability, a sudden 20% generator outage, and a 15% load increase. The results, summarized in Table <xref ref-type="table" rid="T2">2</xref> and visualized in Figs <xref ref-type="fig" rid="F7">7</xref>, <xref ref-type="fig" rid="F8">8</xref>, <xref ref-type="fig" rid="F9">9</xref>, are analyzed below by linking observed behavior to the fundamental operating principles of each control strategy.</p>
      <fig id="F7">
        <object-id content-type="doi">10.3897/nucet.12.176131.figure7</object-id>
        <object-id content-type="arpha">E57F18F4-F5C2-5918-893D-CAECC69C21A3</object-id>
        <label>Figure 7.</label>
        <caption>
          <p><abbrev xlink:title="proportional-integral-derivative">PID</abbrev>-controller implementation model performance.</p>
        </caption>
        <graphic xlink:href="nucet-12-e176131-g007.jpg" id="oo_1555379.jpg">
          <uri content-type="original_file">https://binary.pensoft.net/fig/1555379</uri>
        </graphic>
      </fig>
      <fig id="F8">
        <object-id content-type="doi">10.3897/nucet.12.176131.figure8</object-id>
        <object-id content-type="arpha">3C353E47-2661-596C-8F76-AE1B18A0E818</object-id>
        <label>Figure 8.</label>
        <caption>
          <p><abbrev xlink:title="model predictive control">MPC</abbrev>-controller implementation model performance.</p>
        </caption>
        <graphic xlink:href="nucet-12-e176131-g008.jpg" id="oo_1555380.jpg">
          <uri content-type="original_file">https://binary.pensoft.net/fig/1555380</uri>
        </graphic>
      </fig>
      <fig id="F9">
        <object-id content-type="doi">10.3897/nucet.12.176131.figure9</object-id>
        <object-id content-type="arpha">AE01EBD7-3966-530F-B68B-88769412B456</object-id>
        <label>Figure 9.</label>
        <caption>
          <p>Direct comparison of grid frequency deviation under <abbrev xlink:title="proportional-integral-derivative">PID</abbrev> and <abbrev xlink:title="model predictive control">MPC</abbrev> control. The overlaid responses to the 20% generator outage (t = 400 s) and 15% load increase (t = 800 s) highlight <abbrev xlink:title="model predictive control">MPC</abbrev>’s superior damping and reduced overshoot, a direct result of its predictive optimization. The <abbrev xlink:title="proportional-integral-derivative">PID</abbrev> controller exhibits a faster initial dip but a more oscillatory recovery phase due to its reactive feedback structure.</p>
        </caption>
        <graphic xlink:href="nucet-12-e176131-g009.jpg" id="oo_1555381.jpg">
          <uri content-type="original_file">https://binary.pensoft.net/fig/1555381</uri>
        </graphic>
      </fig>
      <sec sec-type="PID controller: reactive feedback and its limitations" id="sec12">
        <title>PID controller: reactive feedback and its limitations</title>
        <p>The <abbrev xlink:title="proportional-integral-derivative">PID</abbrev> controller demonstrated robust but characteristically reactive performance. Its architecture, defined by Equation (11), relies solely on present and past error (e(t)) to compute the control action u(t). The tuned gains (Kp = 1.8, Ki = 0.9, Kd = 0.15) produced a fast initial response, evidenced by a rise time of 4.5 seconds following the generator outage (Fig. <xref ref-type="fig" rid="F7">7d</xref>). This quick reaction is attributed to the proportional (Kp) and derivative (Kd) terms, which provide immediate corrective torque proportional to the magnitude and rate of the frequency deviation, respectively.</p>
        <p>However, this reactive feedback mechanism also explains its key limitations:</p>
        <list list-type="bullet">
          <list-item>
            <p>Overshoot (0.25 Hz): The fixed-gain <abbrev xlink:title="proportional-integral-derivative">PID</abbrev> controller cannot anticipate the inherent delays in the <abbrev xlink:title="small modular reactor">SMR</abbrev> plant dynamics (modeled by the 6
                        <sup>th</sup>-order transfer function G
                        <sub><abbrev xlink:title="small modular reactor">SMR</abbrev></sub>(s)). It continues to apply a corrective signal based on the instantaneous error even as the <abbrev xlink:title="small modular reactor">SMR</abbrev>’s mechanical power begins to rise, inevitably leading to an overshoot that must later be counteracted by the integral action.
                    </p>
          </list-item>
          <list-item>
            <p>Higher Control Effort: The lack of foresight results in more aggressive control movements. As seen in Fig. <xref ref-type="fig" rid="F7">7b</xref>, the <abbrev xlink:title="small modular reactor">SMR</abbrev> power output under <abbrev xlink:title="proportional-integral-derivative">PID</abbrev> control exhibits sharper transients, correlating with a maximum power rate of 8.5 MW/s (Table <xref ref-type="table" rid="T2">2</xref>). This imposes greater mechanical stress on the turbine and actuator systems.
</p>
          </list-item>
          <list-item>
            <p>Sensitivity to Parameter Variations: The observed “minor oscillations” under ± 15% changes in system inertia (H) and damping (D) stem from the controller’s fixed tuning. The gains, optimized for the nominal plant model, become suboptimal for the perturbed system, slightly degrading damping and response smoothness.
</p>
          </list-item>
        </list>
        <p>In summary, the <abbrev xlink:title="proportional-integral-derivative">PID</abbrev> controller performs effectively as a high-gain regulator but is fundamentally constrained by its inability to account for future states or explicitly manage system constraints.</p>
      </sec>
      <sec sec-type="MPC controller: predictive and constrained optimization" id="sec13">
        <title>MPC controller: predictive and constrained optimization</title>
        <p>The <abbrev xlink:title="model predictive control">MPC</abbrev> controller’s performance, characterized by reduced overshoot (0.15 Hz), faster settling (18 s), and a lower steady-state error (0.008 Hz), is a direct consequence of its predictive optimization framework. At each control interval, <abbrev xlink:title="model predictive control">MPC</abbrev> solves the constrained optimization problem defined in Equation (12) over a prediction horizon (Np = 10).</p>
        <list list-type="bullet">
          <list-item>
            <p>Proactive Disturbance Rejection: By utilizing an internal state-space model of the combined <abbrev xlink:title="small modular reactor">SMR</abbrev>-grid system, <abbrev xlink:title="model predictive control">MPC</abbrev> predicts the future trajectory of the frequency deviation. This allows it to initiate a smoother, more gradual control action in advance of the peak error, as clearly seen in the <abbrev xlink:title="small modular reactor">SMR</abbrev> power output (Fig. <xref ref-type="fig" rid="F8">8</xref>b). This pre-emptive action is the key mechanism behind its superior overshoot suppression.
</p>
          </list-item>
          <list-item>
            <p>Explicit Constraint Management: The most significant operational advantage of <abbrev xlink:title="model predictive control">MPC</abbrev> is the direct incorporation of physical limits into the control law. The constraints on <abbrev xlink:title="small modular reactor">SMR</abbrev> power output and, critically, its ramp rate are enforced within the Quadratic Programming (<abbrev xlink:title="Quadratic Programming">QP</abbrev>) problem. This results in a dramatically lower maximum <abbrev xlink:title="small modular reactor">SMR</abbrev> power rate of 5.0 MW/s (Table <xref ref-type="table" rid="T2">2</xref>), ensuring actuator movements remain within safe mechanical limits and reducing wear.
</p>
          </list-item>
          <list-item>
            <p>Inherent Robustness through Re-planning: By re-solving the optimization problem at every sample time using the current system state, <abbrev xlink:title="model predictive control">MPC</abbrev> inherently adapts to changing conditions. This leads to the noted high stability across all tested scenarios, including parameter variations.
</p>
          </list-item>
        </list>
      </sec>
      <sec sec-type="Direct comparative evaluation" id="sec14">
        <title>Direct comparative evaluation</title>
        <p>The quantitative comparison in Table <xref ref-type="table" rid="T2">2</xref> and the direct visual comparison of frequency deviations in the new Fig. <xref ref-type="fig" rid="F8">8</xref> crystallize the trade-offs between the two strategies.</p>
        <p>Dynamic Response: Fig. <xref ref-type="fig" rid="F9">9</xref> clearly shows that while both controllers successfully regulate frequency, <abbrev xlink:title="model predictive control">MPC</abbrev> achieves a more damped response. Following the disturbance at t = 400s, the <abbrev xlink:title="proportional-integral-derivative">PID</abbrev>-controlled frequency exhibits a deeper nadir and a more pronounced oscillatory recovery, whereas the <abbrev xlink:title="model predictive control">MPC</abbrev> response is smoother and more monotonic in its return to the setpoint.</p>
        <p>Operational and Computational Trade-off: The <abbrev xlink:title="model predictive control">MPC</abbrev>’s superior dynamic performance comes at the cost of higher computational complexity (medium-high vs. low for <abbrev xlink:title="proportional-integral-derivative">PID</abbrev>, as noted in Table <xref ref-type="table" rid="T2">2</xref>). This necessitates more capable real-time processing hardware for implementation.</p>
      </sec>
      <sec sec-type="Practical implications for SMR deployment in renewable grids" id="sec15">
        <title>Practical implications for SMR deployment in renewable grids</title>
        <p>The analysis confirms that both controllers are technically viable for providing <abbrev xlink:title="small modular reactor">SMR</abbrev>-based frequency support. The choice, therefore, depends on specific grid requirements and economic considerations:</p>
        <list list-type="bullet">
          <list-item>
            <p>The <abbrev xlink:title="proportional-integral-derivative">PID</abbrev> controller represents a robust, easily deployable, and computationally inexpensive solution. It is a compelling choice for initial <abbrev xlink:title="small modular reactor">SMR</abbrev> integrations where simplicity and reliability are paramount.
</p>
          </list-item>
          <list-item>
            <p>The <abbrev xlink:title="model predictive control">MPC</abbrev> controller offers incremental but critical improvements in response quality and operational safety through its predictive and constraint-aware nature. Its value increases in systems with very tight frequency tolerance (e.g., smaller islanded grids) or where minimizing mechanical stress on the <abbrev xlink:title="small modular reactor">SMR</abbrev> plant to extend asset life is a priority.
</p>
          </list-item>
        </list>
        <p>The computational load was higher than <abbrev xlink:title="proportional-integral-derivative">PID</abbrev> but feasible for the simulated sample time. Fig. <xref ref-type="fig" rid="F8">8</xref> shows the <abbrev xlink:title="model predictive control">MPC</abbrev> frequency response and control effort.</p>
      </sec>
      <sec sec-type="Mechanistic analysis of controller performance" id="sec16">
        <title>Mechanistic analysis of controller performance</title>
      </sec>
    </sec>
    <sec sec-type="Quantitative comparison" id="sec17">
      <title>Quantitative comparison</title>
      <p>A quantitative comparison of controller performance is summarized in Table <xref ref-type="table" rid="T2">2</xref>. Both controllers performed reliably under nominal conditions and showed acceptable robustness to parameter variations. <abbrev xlink:title="model predictive control">MPC</abbrev> retained a slight advantage in overshoot and settling time, though the differences were modest.</p>
    </sec>
    <sec sec-type="Conclusion" id="sec18">
      <title>Conclusion</title>
      <p>This research presented a comparative study of <abbrev xlink:title="proportional-integral-derivative">PID</abbrev> and <abbrev xlink:title="model predictive control">MPC</abbrev> strategies for frequency regulation in high-renewable grids using Small Modular Reactors (<abbrev xlink:title="Small modular reactors">SMRs</abbrev>) as flexible nuclear resources. A detailed modeling framework was developed, combining a sixth-order <abbrev xlink:title="small modular reactor">SMR</abbrev> transfer function, a grid dynamics model, and renewable disturbance scenarios within a unified Simulink environment.</p>
      <p>Simulation results indicate that both <abbrev xlink:title="proportional-integral-derivative">PID</abbrev> and <abbrev xlink:title="model predictive control">MPC</abbrev> provide effective frequency regulation, with <abbrev xlink:title="model predictive control">MPC</abbrev> offering modest improvements in overshoot, settling time, and steady-state error. The predictive capability of <abbrev xlink:title="model predictive control">MPC</abbrev> and its explicit constraint handling contribute to a more refined dynamic response, though <abbrev xlink:title="proportional-integral-derivative">PID</abbrev> remains a viable and simpler alternative for <abbrev xlink:title="small modular reactor">SMR</abbrev>-based frequency support in renewable-rich grids.</p>
    </sec>
  </body>
  <back>
    <ack>
      <title>Acknowledgements</title>
      <p>The authors would like to express their sincere gratitude to the Nuclear Power Plants Authority (NPPA), Egypt, for their continuous technical and institutional support throughout this research. Special appreciation is extended to the Military Technical College (MTC) for providing essential resources, modeling facilities, and academic collaboration that enabled the successful completion of this study.</p>
      <p>The authors also acknowledge the valuable insights and constructive comments provided by colleagues and reviewers, which significantly contributed to the improvement and refinement of this work.</p>
    </ack>
    <ref-list>
      <title>References</title>
      <ref id="B1">
        <mixed-citation>Anderson PM, Fouad AA (2002) Power system control and stability. 2nd edn. Wiley–IEEE Press, Hoboken, NJ, USA.</mixed-citation>
      </ref>
      <ref id="B2">
        <mixed-citation>Åström KJ, Hägglund T (1995) PID controllers: Theory, design, and tuning. 2nd edn. ISA, Research Triangle Park, NC, USA.</mixed-citation>
      </ref>
      <ref id="B3">
        <mixed-citation>Brewer JW (1952) Control of nuclear reactors and power plants. Journal of the Franklin Institute 254(5): 387–401. <ext-link xlink:href="10.1016/0016-0032(52)90371-0" ext-link-type="doi">https://doi.org/10.1016/0016-0032(52)90371-0</ext-link></mixed-citation>
      </ref>
      <ref id="B4">
        <mixed-citation>Camacho EF, Bordons C (2007) Model predictive control. 2nd edn. Springer, London, UK.</mixed-citation>
      </ref>
      <ref id="B5">
        <mixed-citation>Carelli MD, Garrone P, Locatelli G, Mancini M, Mycoff C, Trucco P, Ricotti ME (2010) Economic features of integral, modular, small-to-medium size reactors. Progress in Nuclear Energy 52(4): 403–414. <ext-link xlink:href="10.1016/j.pnucene.2009.11.006" ext-link-type="doi">https://doi.org/10.1016/j.pnucene.2009.11.006</ext-link></mixed-citation>
      </ref>
      <ref id="B6">
        <mixed-citation>Elfaramawy RM, Sarhan AA, Helmy S, El-Wakeel AS (2025) Enhancing power quality in renewable-integrated grids using small modular reactors. Proceedings of the 15th International Conference on Electrical Engineering (ICEENG), Cairo, Egypt: 1–6. <ext-link xlink:href="10.1109/ICEENG64546.2025.11031384" ext-link-type="doi">https://doi.org/10.1109/ICEENG64546.2025.11031384</ext-link></mixed-citation>
      </ref>
      <ref id="B7">
        <mixed-citation>IEA (2023) World Energy Outlook 2023. International Energy Agency, Paris, France. <ext-link xlink:href="10.1787/827374a6-en" ext-link-type="doi">https://doi.org/10.1787/827374a6-en</ext-link></mixed-citation>
      </ref>
      <ref id="B8">
        <mixed-citation>Ingersoll DT (2013) Small modular reactors: Nuclear power fad or future? Nuclear News 56(11): 28–35. <ext-link xlink:href="https://www.ans.org/pubs/magazines/nn/" ext-link-type="uri">https://www.ans.org/pubs/magazines/nn/</ext-link></mixed-citation>
      </ref>
      <ref id="B9">
        <mixed-citation>Kundur P (1994) Power system stability and control. McGraw–Hill, New York, NY, USA.</mixed-citation>
      </ref>
      <ref id="B10">
        <mixed-citation>Li X, Wu Z, Shahidehpour M, Li Z (2024) Grid inertia reduction in renewable-dominated systems. International Journal of Electrical Power and Energy Systems 152: 109280. <ext-link xlink:href="10.1016/j.ijepes.2023.109280" ext-link-type="doi">https://doi.org/10.1016/j.ijepes.2023.109280</ext-link></mixed-citation>
      </ref>
      <ref id="B11">
        <mixed-citation>Liu H, Zeng Z, Yang X, Zou Y, Liu X (2025) A control strategy for shipboard stabilization platforms based on a fuzzy adaptive proportional–integral–derivative control architecture. Mechanical Sciences 16: 325–342. <ext-link xlink:href="10.5194/ms-16-325-2025" ext-link-type="doi">https://doi.org/10.5194/ms-16-325-2025</ext-link></mixed-citation>
      </ref>
      <ref id="B12">
        <mixed-citation>Locatelli G, Bingham C, Mancini M (2014) Small modular reactors: A comprehensive overview of their economics and strategic aspects. Progress in Nuclear Energy 73: 75–85. <ext-link xlink:href="10.1016/j.pnucene.2014.01.010" ext-link-type="doi">https://doi.org/10.1016/j.pnucene.2014.01.010</ext-link></mixed-citation>
      </ref>
      <ref id="B13">
        <mixed-citation>Mahmud K, Hossain MJ, Town G, Ravishankar J (2022) A review of predictive control applications in microgrids. Renewable and Sustainable Energy Reviews 162: 112413. <ext-link xlink:href="10.1016/j.rser.2022.112413" ext-link-type="doi">https://doi.org/10.1016/j.rser.2022.112413</ext-link></mixed-citation>
      </ref>
      <ref id="B14">
        <mixed-citation>Milano F, Dörfler F, Hug G, Hill DJ, Verbič G (2018) Foundations and challenges of low-inertia systems. Proceedings of the Power Systems Computation Conference (PSCC), Dublin, Ireland: 1–25. <ext-link xlink:href="10.23919/PSCC.2018.8442948" ext-link-type="doi">https://doi.org/10.23919/PSCC.2018.8442948</ext-link></mixed-citation>
      </ref>
      <ref id="B15">
        <mixed-citation>Nagrath IJ, Gopal M (2017) Control systems engineering. 6th edn. New Age International, New Delhi, India.</mixed-citation>
      </ref>
      <ref id="B16">
        <mixed-citation>Rawlings JB, Mayne DQ, Diehl M (2017) Model predictive control: Theory, computation, and design. 2nd edn. Nob Hill Publishing, Madison, WI, USA.</mixed-citation>
      </ref>
      <ref id="B17">
        <mixed-citation>Reyes J, Ingersoll D, Schultz K, Hines CR (2012) Testing of the NuScale Power Module. Nuclear Technology 178(2): 227–240. <ext-link xlink:href="10.13182/NT12-A13553" ext-link-type="doi">https://doi.org/10.13182/NT12-A13553</ext-link></mixed-citation>
      </ref>
      <ref id="B18">
        <mixed-citation>Sauer PW, Pai MA (1998) Power system dynamics and stability. Prentice Hall, Upper Saddle River, NJ, USA.</mixed-citation>
      </ref>
      <ref id="B19">
        <mixed-citation>Ulbig A, Borsche TS, Andersson G (2014) Impact of low rotational inertia on power system stability and operation. IFAC Proceedings Volumes 47(3): 7290–7297. <ext-link xlink:href="10.3182/20140824-6-ZA-1003.02615" ext-link-type="doi">https://doi.org/10.3182/20140824-6-ZA-1003.02615</ext-link></mixed-citation>
      </ref>
      <ref id="B20">
        <mixed-citation>Ye S, Geng Y (2025) Design and analysis of an MPC–PID-based double-loop trajectory tracking algorithm for intelligent sweeping vehicles. World Electric Vehicle Journal 16(5): 251. <ext-link xlink:href="10.3390/wevj16050251" ext-link-type="doi">https://doi.org/10.3390/wevj16050251</ext-link></mixed-citation>
      </ref>
      <ref id="B21">
        <mixed-citation>Zhang H, Zhou D, Xu Y, Yang Y (2022) MPC-based frequency regulation for power systems with wind generation. IEEE Transactions on Sustainable Energy 13(1): 80–91. <ext-link xlink:href="10.1109/TSTE.2021.3098762" ext-link-type="doi">https://doi.org/10.1109/TSTE.2021.3098762</ext-link></mixed-citation>
      </ref>
      <ref id="B22">
        <mixed-citation>Zhao J, Netto M, Mili L (2022) A review of inertia estimation techniques for power systems with high penetration of renewables. IEEE Access 10: 3589–3600. <ext-link xlink:href="10.1109/ACCESS.2022.3140665" ext-link-type="doi">https://doi.org/10.1109/ACCESS.2022.3140665</ext-link></mixed-citation>
      </ref>
    </ref-list>
  </back>
</article>
