Remove transformer voltage controls when no PV buses at non controlled side

src/main/java/com/powsybl/openloadflow/ac/SimpleTransformerVoltageControlOuterLoop.java

@@ -26,6 +26,7 @@ public void initialize(OuterLoopContext context) {
         for (LfBranch controllerBranch : getControllerBranches(context.getNetwork())) {
             controllerBranch.setVoltageControlEnabled(true);
         }
+        context.getNetwork().fixTransformerVoltageControls();
     }
 
     @Override

src/main/java/com/powsybl/openloadflow/ac/TransformerVoltageControlOuterLoop.java

@@ -92,6 +92,7 @@ public OuterLoopStatus check(OuterLoopContext context, Reporter reporter) {
                 branch.setVoltageControlEnabled(true);
                 status = OuterLoopStatus.UNSTABLE;
             }
+            context.getNetwork().fixTransformerVoltageControls();
         }
 
         // At second outer loop iteration, the transformers are rounded. The generator voltage controls that were

src/main/java/com/powsybl/openloadflow/network/LfNetwork.java

@@ -591,6 +591,52 @@ public boolean isValid() {
         return valid;
     }
 
+    /**
+     * Disable transformer voltage control when there is no generator controlling voltage on the connected component
+     * that belong to the not controlled side of the transformer.
+     */
+    public void fixTransformerVoltageControls() {
+        List<LfBranch> controllerBranches = new ArrayList<>(1);
+        for (LfBranch branch : branches) {
+            if (!branch.isDisabled() && branch.isVoltageController() && branch.isVoltageControlEnabled()) {
+                controllerBranches.add(branch);
+            }
+            if (branch.isDisabled()) {
+                // apply contingency (in case we are inside a security analysis)
+                getConnectivity().cut(branch);
+            }
+        }
+        for (LfBranch branch : controllerBranches) {
+            getConnectivity().cut(branch);
+        }
+        int disabledTransformerCount = 0;
+        Map<Integer, Boolean> componentNoPVBusesMap = new HashMap<>();
+        for (LfBranch branch : controllerBranches) {
+            var voltageControl = branch.getVoltageControl().orElseThrow();
+            LfBus notControlledSide;
+            if (voltageControl.getControlled() == branch.getBus1()) {
+                notControlledSide = branch.getBus2();
+            } else if (voltageControl.getControlled() == branch.getBus2()) {
+                notControlledSide = branch.getBus1();
+            } else {
+                continue;
+            }
+            boolean noPvBusesInComponent = componentNoPVBusesMap.computeIfAbsent(getConnectivity().getComponentNumber(notControlledSide),
+                k -> getConnectivity().getConnectedComponent(notControlledSide).stream().noneMatch(LfBus::isVoltageControlled));
+            if (noPvBusesInComponent) {
+                branch.setVoltageControlEnabled(false);
+                LOGGER.trace("Transformer {} voltage control has been disabled because no PV buses on not controlled side connected component",
+                        branch.getId());
+                disabledTransformerCount++;
+            }
+        }
+        getConnectivity().reset();
+        if (disabledTransformerCount > 0) {
+            LOGGER.warn("{} transformer voltage controls have been disabled because no PV buses on not controlled side connected component",
+                    disabledTransformerCount);
+        }
+    }
+
     @Override
     public String toString() {
         return "{CC" + numCC +

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

@@ -133,6 +133,7 @@ private void calculatePostContingencySensitivityValues(List<LfSensitivityFactor<
             for (LfBranch branch : lfNetwork.getBranches()) {
                 branch.getVoltageControl().ifPresent(vc -> branch.setVoltageControlEnabled(true));
             }
+            lfNetwork.fixTransformerVoltageControls();
         }
 
         // we make the assumption that we ran a loadflow before, and thus this jacobian is the right one
@@ -245,6 +246,7 @@ public void analyse(Network network, List<PropagatedContingency> contingencies,
                 for (LfBranch branch : lfNetwork.getBranches()) {
                     branch.getVoltageControl().ifPresent(vc -> branch.setVoltageControlEnabled(true));
                 }
+                lfNetwork.fixTransformerVoltageControls();
             }
 
             // we make the assumption that we ran a loadflow before, and thus this jacobian is the right one

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

@@ -128,7 +128,7 @@ void voltageControlT2wtTest3() {
                 .setTargetV(135.0);
 
         LoadFlowResult result = loadFlowRunner.run(network, parameters);
-        assertFalse(result.isOk());
+        assertTrue(result.isOk());
 
         parametersExt.setTransformerVoltageControlMode(OpenLoadFlowParameters.TransformerVoltageControlMode.AFTER_GENERATOR_VOLTAGE_CONTROL);
         LoadFlowResult result2 = loadFlowRunner.run(network, parameters);

src/test/java/com/powsybl/openloadflow/sensi/ac/AcSensitivityAnalysisTest.java

@@ -478,6 +478,26 @@ void testBusVoltagePerTargetVTwt() {
         assertEquals(0.035205d,  result.getBusVoltageSensitivityValue("T2wT", "BUS_2"), LoadFlowAssert.DELTA_V);
         assertEquals(1d,  result.getBusVoltageSensitivityValue("T2wT", "BUS_3"), LoadFlowAssert.DELTA_V);
         assertEquals(1.055117d,  result.getBusVoltageSensitivityValue("T2wT", "BUS_4"), LoadFlowAssert.DELTA_V);
+
+        t2wt.getRatioTapChanger()
+                .setTargetDeadband(0)
+                .setRegulating(true)
+                .setTapPosition(3)
+                .setRegulationTerminal(t2wt.getTerminal1()) // control will be disabled.
+                .setTargetV(135.0);
+
+        SensitivityAnalysisResult result2 = sensiRunner.run(network, factors, Collections.emptyList(), Collections.emptyList(), sensiParameters);
+
+        assertEquals(4, result2.getValues().size());
+        assertEquals(0d, result2.getBusVoltageSensitivityValue("T2wT", "BUS_1"), LoadFlowAssert.DELTA_V);
+        assertEquals(0d,  result2.getBusVoltageSensitivityValue("T2wT", "BUS_2"), LoadFlowAssert.DELTA_V);
+        assertEquals(0d,  result2.getBusVoltageSensitivityValue("T2wT", "BUS_3"), LoadFlowAssert.DELTA_V);
+        assertEquals(0d,  result2.getBusVoltageSensitivityValue("T2wT", "BUS_4"), LoadFlowAssert.DELTA_V);
+
+        assertEquals(135.0, result2.getBusVoltageFunctionReferenceValue("BUS_1"), LoadFlowAssert.DELTA_V);
+        assertEquals(133.77,  result2.getBusVoltageFunctionReferenceValue("BUS_2"), LoadFlowAssert.DELTA_V);
+        assertEquals(25.88,  result2.getBusVoltageFunctionReferenceValue("BUS_3"), LoadFlowAssert.DELTA_V);
+        assertEquals(25.16,  result2.getBusVoltageFunctionReferenceValue("BUS_4"), LoadFlowAssert.DELTA_V);
     }
 
     @Test