Disable switch without removing variables

src/main/java/com/powsybl/openloadflow/ac/equations/AcEquationSystem.java

@@ -331,14 +331,21 @@ private static void createNonImpedantBranch(LfBranch branch, LfBus bus1, LfBus b
             bus1.setCalculatedV(vTerm);
             // add a dummy reactive power variable to both sides of the non impedant branch and with an opposite sign
             // to ensure we have the same number of equation and variables
+            var dummyQ = equationSystem.getVariable(branch.getNum(), AcVariableType.DUMMY_Q);
             equationSystem.getEquation(bus1.getNum(), AcEquationType.BUS_TARGET_Q)
                     .orElseThrow()
-                    .addTerm(equationSystem.getVariable(branch.getNum(), AcVariableType.DUMMY_Q).createTerm());
+                    .addTerm(dummyQ.createTerm());
 
             equationSystem.getEquation(bus2.getNum(), AcEquationType.BUS_TARGET_Q)
                     .orElseThrow()
-                    .addTerm(equationSystem.getVariable(branch.getNum(), AcVariableType.DUMMY_Q).<AcEquationType>createTerm()
+                    .addTerm(dummyQ.<AcEquationType>createTerm()
                                     .minus());
+
+            // create an inactive dummy reactive power target equation set to zero that could be activated
+            // on case of switch opening
+            equationSystem.createEquation(branch.getNum(), AcEquationType.DUMMY_TARGET_Q)
+                    .addTerm(dummyQ.createTerm())
+                    .setActive(false);
         } else {
             // nothing to do in case of v1 and v2 are found, we just have to ensure
             // target v are equals.
@@ -356,14 +363,21 @@ private static void createNonImpedantBranch(LfBranch branch, LfBus bus1, LfBus b
 
             // add a dummy active power variable to both sides of the non impedant branch and with an opposite sign
             // to ensure we have the same number of equation and variables
+            var dummyP = equationSystem.getVariable(branch.getNum(), AcVariableType.DUMMY_P);
             equationSystem.getEquation(bus1.getNum(), AcEquationType.BUS_TARGET_P)
                     .orElseThrow()
-                    .addTerm(equationSystem.getVariable(branch.getNum(), AcVariableType.DUMMY_P).createTerm());
+                    .addTerm(dummyP.createTerm());
 
             equationSystem.getEquation(bus2.getNum(), AcEquationType.BUS_TARGET_P)
                     .orElseThrow()
-                    .addTerm(equationSystem.getVariable(branch.getNum(), AcVariableType.DUMMY_P).<AcEquationType>createTerm()
+                    .addTerm(dummyP.<AcEquationType>createTerm()
                                     .minus());
+
+            // create an inactive dummy active power target equation set to zero that could be activated
+            // on case of switch opening
+            equationSystem.createEquation(branch.getNum(), AcEquationType.DUMMY_TARGET_P)
+                    .addTerm(dummyP.createTerm())
+                    .setActive(false);
         } else {
             throw new IllegalStateException("Cannot happen because only there is one slack bus per model");
         }

src/main/java/com/powsybl/openloadflow/ac/equations/AcEquationSystemUpdater.java

@@ -45,17 +45,38 @@ public void onShuntVoltageControlChange(LfShunt controllerShunt, boolean newVolt
     }
 
     private void updateElementEquations(LfElement element, boolean enable) {
-        // update all equations related to the element
-        for (var equation : equationSystem.getEquations(element.getType(), element.getNum())) {
-            if (equation.isActive() != enable) {
-                equation.setActive(enable);
+        if (element instanceof LfBranch && ((LfBranch) element).isZeroImpedanceBranch(false)) {
+            LfBranch branch = (LfBranch) element;
+            if (branch.isSpanningTreeEdge()) {
+                // depending on the switch status, we activate either v1 = v2, ph1 = ph2 equations
+                // or equations that set dummy p and q variable to zero
+                equationSystem.getEquation(element.getNum(), AcEquationType.ZERO_PHI)
+                        .orElseThrow()
+                        .setActive(enable);
+                equationSystem.getEquation(element.getNum(), AcEquationType.DUMMY_TARGET_P)
+                        .orElseThrow()
+                        .setActive(!enable);
+
+                equationSystem.getEquation(element.getNum(), AcEquationType.ZERO_V)
+                        .orElseThrow()
+                        .setActive(enable);
+                equationSystem.getEquation(element.getNum(), AcEquationType.DUMMY_TARGET_Q)
+                        .orElseThrow()
+                        .setActive(!enable);
+            }
+        } else {
+            // update all equations related to the element
+            for (var equation : equationSystem.getEquations(element.getType(), element.getNum())) {
+                if (equation.isActive() != enable) {
+                    equation.setActive(enable);
+                }
             }
-        }
 
-        // update all equation terms related to the element
-        for (var equationTerm : equationSystem.getEquationTerms(element.getType(), element.getNum())) {
-            if (equationTerm.isActive() != enable) {
-                equationTerm.setActive(enable);
+            // update all equation terms related to the element
+            for (var equationTerm : equationSystem.getEquationTerms(element.getType(), element.getNum())) {
+                if (equationTerm.isActive() != enable) {
+                    equationTerm.setActive(enable);
+                }
             }
         }
     }

src/main/java/com/powsybl/openloadflow/ac/equations/AcEquationType.java

@@ -27,7 +27,9 @@ public enum AcEquationType implements Quantity {
     ZERO_V("zero_v", ElementType.BRANCH), // zero impedance branch, voltage magnitude equality
     ZERO_PHI("zero_\u03C6", ElementType.BRANCH), // zero impedance branch, voltage angle equality
     DISTR_RHO("distr_\u03C1", ElementType.BRANCH), // remote transformer voltage control ratio distribution
-    DISTR_SHUNT_B("distr_b", ElementType.SHUNT_COMPENSATOR); // shunt remote voltage control susceptance distribution
+    DISTR_SHUNT_B("distr_b", ElementType.SHUNT_COMPENSATOR), // shunt remote voltage control susceptance distribution
+    DUMMY_TARGET_P("dummy_target_p", ElementType.BRANCH),
+    DUMMY_TARGET_Q("dummy_target_q", ElementType.BRANCH);
 
     private final String symbol;
 

src/main/java/com/powsybl/openloadflow/ac/outerloop/AcTargetVector.java

@@ -120,6 +120,8 @@ public static void init(Equation<AcVariableType, AcEquationType> equation, LfNet
 
             case DISTR_RHO:
             case DISTR_SHUNT_B:
+            case DUMMY_TARGET_P:
+            case DUMMY_TARGET_Q:
                 targets[equation.getColumn()] = 0;
                 break;
 

src/test/java/com/powsybl/openloadflow/ac/NonImpedantBranchDisablingTest.java

@@ -0,0 +1,63 @@
+/**
+ * Copyright (c) 2022, RTE (http://www.rte-france.com)
+ * This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/.
+ */
+package com.powsybl.openloadflow.ac;
+
+import com.powsybl.commons.reporter.Reporter;
+import com.powsybl.iidm.network.Network;
+import com.powsybl.loadflow.LoadFlowParameters;
+import com.powsybl.math.matrix.DenseMatrixFactory;
+import com.powsybl.openloadflow.OpenLoadFlowParameters;
+import com.powsybl.openloadflow.ac.outerloop.AcLoadFlowContext;
+import com.powsybl.openloadflow.ac.outerloop.AcLoadFlowParameters;
+import com.powsybl.openloadflow.ac.outerloop.AcloadFlowEngine;
+import com.powsybl.openloadflow.graph.EvenShiloachGraphDecrementalConnectivityFactory;
+import com.powsybl.openloadflow.network.LfNetwork;
+import com.powsybl.openloadflow.network.NameSlackBusSelector;
+import com.powsybl.openloadflow.network.NodeBreakerNetworkFactory;
+import com.powsybl.openloadflow.network.impl.LfNetworkLoaderImpl;
+import org.junit.jupiter.api.Test;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+
+/**
+ * @author Geoffroy Jamgotchian <geoffroy.jamgotchian at rte-france.com>
+ */
+class NonImpedantBranchDisablingTest {
+
+    @Test
+    void test() {
+        Network network = NodeBreakerNetworkFactory.create();
+        network.getSwitch("B3").setRetained(false);
+        network.getSwitch("C").setRetained(true);
+        AcLoadFlowParameters parameters = OpenLoadFlowParameters.createAcParameters(new LoadFlowParameters(),
+                                                                                    new OpenLoadFlowParameters(),
+                                                                                    new DenseMatrixFactory(),
+                                                                                    new EvenShiloachGraphDecrementalConnectivityFactory<>(),
+                                                                                    Reporter.NO_OP,
+                                                                                    true,
+                                                                                    false);
+        parameters.getNetworkParameters().setSlackBusSelector(new NameSlackBusSelector("VL1_1"));
+        LfNetwork lfNetwork = LfNetwork.load(network, new LfNetworkLoaderImpl(), parameters.getNetworkParameters()).get(0);
+
+        try (AcLoadFlowContext context = new AcLoadFlowContext(lfNetwork, parameters)) {
+            var engine = new AcloadFlowEngine(context);
+            engine.run();
+            assertEquals(8, context.getEquationSystem().getSortedVariablesToFind().size());
+            var l1 = lfNetwork.getBranchById("L1");
+            var l2 = lfNetwork.getBranchById("L2");
+            assertEquals(3.01884, l1.getP1().eval(), 10e-5);
+            assertEquals(3.01884, l2.getP1().eval(), 10e-5);
+
+            lfNetwork.getBranchById("C").setDisabled(true);
+
+            engine.run();
+            assertEquals(8, context.getEquationSystem().getSortedVariablesToFind().size()); // we have kept same variables!!!
+            assertEquals(0, l1.getP1().eval(), 10e-5);
+            assertEquals(6.07782, l2.getP1().eval(), 10e-5);
+        }
+    }
+}

src/test/java/com/powsybl/openloadflow/sa/OpenSecurityAnalysisTest.java

@@ -8,6 +8,7 @@
 
 import com.powsybl.computation.ComputationManager;
 import com.powsybl.contingency.*;
+import com.powsybl.ieeecdf.converter.IeeeCdfNetworkFactory;
 import com.powsybl.iidm.network.*;
 import com.powsybl.iidm.network.extensions.LoadDetailAdder;
 import com.powsybl.iidm.network.test.EurostagTutorialExample1Factory;
@@ -1261,4 +1262,18 @@ void testPhaseShifterNecessaryForConnectivity() {
         assertEquals(100.3689, l1ContingencyResult.getBranchResult("PS1").getP1(), LoadFlowAssert.DELTA_POWER);
         assertEquals(-100.1844, l1ContingencyResult.getBranchResult("PS1").getP2(), LoadFlowAssert.DELTA_POWER);
     }
+
+    @Test
+    void testWithNonImpedantLineConnectedToSlackBus() {
+        Network network = IeeeCdfNetworkFactory.create14();
+        network.getLine("L1-2-1").setR(0).setX(0);
+        network.getLine("L4-5-1").setR(0).setX(0);
+
+        List<Contingency> contingencies = allBranches(network);
+
+        List<StateMonitor> monitors = Collections.emptyList();
+
+        SecurityAnalysisResult result = runSecurityAnalysis(network, contingencies, monitors);
+        assertEquals(20, result.getPostContingencyResults().size()); // assert there is no contingency simulation failure
+    }
 }