Wrong function reference for BranchFlowPerPSTAngle factor

src/main/java/com/powsybl/openloadflow/sensi/DcSensitivityAnalysis.java

@@ -211,19 +211,25 @@ private void createBranchSensitivityValue(LfSensitivityFactor factor, DenseMatri
         } else {
             sensiValue = factor.getBaseSensitivityValue();
             flowValue = factor.getFunctionReference();
+            boolean zeroSensiValue = false;
             for (ComputedContingencyElement contingencyElement : contingencyElements) {
-                if (contingencyElement.getElement().getId().equals(functionBranchId)
-                        || contingencyElement.getElement().getId().equals(factor.getVariableId())) {
-                    // the sensitivity on a removed branch is 0, the sensitivity if the variable was a removed branch is 0
+                double contingencySensitivity = p1.calculateSensi(contingenciesStates, contingencyElement.getContingencyIndex());
+                flowValue += contingencyElement.getAlphaForFunctionReference() * contingencySensitivity;
+                sensiValue +=  contingencyElement.getAlphaForSensitivityValue() * contingencySensitivity;
+                if (contingencyElement.getElement().getId().equals(functionBranchId)) {
+                    // the monitored branch is in contingency, the sensitivity value equals to zero and its post-contingency flow
+                    // equals to zero too.
                     sensiValue = 0d;
                     flowValue = 0d;
                     break;
                 }
-                double contingencySensitivity = p1.calculateSensi(contingenciesStates, contingencyElement.getContingencyIndex());
-                flowValue += contingencyElement.getAlphaForFunctionReference() * contingencySensitivity;
-                sensiValue +=  contingencyElement.getAlphaForSensitivityValue() * contingencySensitivity;
+                if (contingencyElement.getElement().getId().equals(factor.getVariableId())) {
+                    // the equipment responsible for the variable is indeed in contingency, the sensitivity value equals to zero.
+                    // No assumption about the reference flow on the monitored branch.
+                    zeroSensiValue = true;
+                }
             }
-            if (contingency != null && contingency.getHvdcIdsToOpen().contains(factor.getVariableId())) {
+            if ((contingency != null && contingency.getHvdcIdsToOpen().contains(factor.getVariableId())) || zeroSensiValue) {
                 sensiValue = 0d;
             }
         }

src/test/java/com/powsybl/openloadflow/sensi/dc/DcSensitivityAnalysisContingenciesTest.java

@@ -12,6 +12,7 @@
 import com.powsybl.computation.local.LocalComputationManager;
 import com.powsybl.contingency.*;
 import com.powsybl.iidm.network.*;
+import com.powsybl.iidm.network.test.PhaseShifterTestCaseFactory;
 import com.powsybl.loadflow.LoadFlowParameters;
 import com.powsybl.openloadflow.network.*;
 import com.powsybl.openloadflow.sensi.*;
@@ -1801,4 +1802,19 @@ void testDanglingLineContingencyDistributedSlackOnGenerators() {
         assertEquals(2.0624, finalP, LoadFlowAssert.DELTA_POWER);
         assertEquals(0.1875, finalP - initialP, LoadFlowAssert.DELTA_POWER);
     }
+
+    @Test
+    void contingencyOnPhaseTapChangerTest() {
+        Network network = PhaseShifterTestCaseFactory.create();
+        SensitivityAnalysisParameters parameters = createParameters(true, "VL1_0", true);
+        parameters.getLoadFlowParameters().setBalanceType(LoadFlowParameters.BalanceType.PROPORTIONAL_TO_GENERATION_P_MAX);
+        SensitivityFactorsProvider factorsProvider2 = n -> List.of(new BranchFlowPerPSTAngle(new BranchFlow("L1", "L1", "L1"),
+                new PhaseTapChangerAngle("PS1", "PS1", "PS1")));
+        List<Contingency> contingencies = List.of(new Contingency("PS1", new BranchContingency("PS1")));
+        SensitivityAnalysisResult result = sensiProvider.run(network, VariantManagerConstants.INITIAL_VARIANT_ID, factorsProvider2, contingencies,
+                parameters, LocalComputationManager.getDefault())
+                .join();
+        assertEquals(100.0, getContingencyFunctionReference(result, "L1", "PS1"), LoadFlowAssert.DELTA_POWER);
+        assertEquals(0.0, getContingencyValue(result, "PS1", "PS1", "L1"), LoadFlowAssert.DELTA_POWER);
+    }
 }