Force area boundary branches to min impedance

docs/loadflow/loadflow.md

@@ -276,3 +276,10 @@ Indeed, in this case the slack injection can be seen as an interchange to 'the v
     - Connected to only buses that have an area:
         - All connected branches are boundaries of those areas: Not attributed to anyone, the mismatch will already be present in the interchange mismatch
         - Some connected branches are not declared as boundaries of the areas: Amount of mismatch to distribute is split equally among the areas (added to their "total mismatch")
+
+### Zero impedance boundary branches
+The following applies when the [`lowImpedanceBranchMode`](parameters.md) is set to `REPLACE_BY_ZERO_IMPEDANCE_LINE`.
+Currently, computations involving zero-impedance branches used as boundary branches are not supported.
+However, it is still possible to submit network models that include zero-impedance boundary branches.  
+If a terminal of a zero-impedance branch is designated as a boundary, Open LoadFlow will internally assign the branch
+an impedance value equal to the [`lowImpedanceThreshold`](parameters.md) parameter.

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

@@ -666,6 +666,11 @@ public void fix(boolean minImpedance, double lowImpedanceThreshold) {
                             || b.isTransformerReactivePowerController() || b.isTransformerReactivePowerControlled()
                             || b.getGeneratorReactivePowerControl().isPresent())
                     .forEach(branch -> branch.setMinZ(lowImpedanceThreshold));
+            // zero impedance boundary branch is not supported
+            areas.stream()
+                    .flatMap(a -> a.getBoundaries().stream())
+                    .map(LfArea.Boundary::getBranch)
+                    .forEach(branch -> branch.setMinZ(lowImpedanceThreshold));
         }
     }
 

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

@@ -16,6 +16,7 @@
 import com.powsybl.openloadflow.OpenLoadFlowParameters;
 import com.powsybl.openloadflow.OpenLoadFlowProvider;
 import com.powsybl.openloadflow.network.*;
+import com.powsybl.openloadflow.network.impl.Networks;
 import org.junit.jupiter.api.BeforeEach;
 import org.junit.jupiter.api.Test;
 import org.junit.jupiter.params.ParameterizedTest;
@@ -66,6 +67,40 @@ void twoAreasWithUnpairedDanglingLine() {
         runLfTwoAreas(network, interchangeTarget1, interchangeTarget2, -10, 0);
     }
 
+    @Test
+    void zeroImpedanceBoundaryBranchesNetworkConversion() {
+        Network network = MultiAreaNetworkFactory.createTwoAreasWithDanglingLine();
+        network.getLine("l23_A1").setX(0);          // boundary
+        network.getDanglingLine("dl1").setX(0);     // boundary
+        network.getLine("l12").setX(0);             // not boundary
+
+        LfNetwork lfNetwork = Networks.load(network, new LfNetworkParameters().setAreaInterchangeControl(false)).get(0);
+        assertTrue(lfNetwork.getBranchById("dl1").isZeroImpedance(LoadFlowModel.AC));
+        assertTrue(lfNetwork.getBranchById("l23_A1").isZeroImpedance(LoadFlowModel.AC));
+        assertTrue(lfNetwork.getBranchById("l12").isZeroImpedance(LoadFlowModel.AC));
+
+        lfNetwork = Networks.load(network, new LfNetworkParameters().setAreaInterchangeControl(true)).get(0);
+        assertFalse(lfNetwork.getBranchById("dl1").isZeroImpedance(LoadFlowModel.AC));
+        assertEquals(LfNetworkParameters.LOW_IMPEDANCE_THRESHOLD_DEFAULT_VALUE, lfNetwork.getBranchById("dl1").getPiModel().getX());
+        assertFalse(lfNetwork.getBranchById("l23_A1").isZeroImpedance(LoadFlowModel.AC));
+        assertEquals(LfNetworkParameters.LOW_IMPEDANCE_THRESHOLD_DEFAULT_VALUE, lfNetwork.getBranchById("l23_A1").getPiModel().getX());
+        assertTrue(lfNetwork.getBranchById("l12").isZeroImpedance(LoadFlowModel.AC));
+
+    }
+
+    @Test
+    void twoAreasWithZeroImpedanceBoundaryBranches() {
+        Network network = MultiAreaNetworkFactory.createTwoAreasWithDanglingLine();
+        double interchangeTarget1 = -40;
+        double interchangeTarget2 = 20;
+        network.getLine("l23_A1").setX(0);
+        network.getDanglingLine("dl1").setX(0);
+        parametersExt.setLowImpedanceBranchMode(OpenLoadFlowParameters.LowImpedanceBranchMode.REPLACE_BY_ZERO_IMPEDANCE_LINE);
+        runLfTwoAreas(network, interchangeTarget1, interchangeTarget2, -10, 2);
+        parameters.setDc(true);
+        runLfTwoAreas(network, interchangeTarget1, interchangeTarget2, -10, 0);
+    }
+
     @Test
     void twoAreasWithTieLineTest() {
         Network network = MultiAreaNetworkFactory.createTwoAreasWithTieLine();