Fix DC Load Flow bus imbalance

src/main/java/com/powsybl/openloadflow/dc/equations/AbstractClosedBranchDcFlowEquationTerm.java

@@ -97,7 +97,8 @@ public double calculateSensi(DenseMatrix dx, int column) {
         double dph1 = dx.get(ph1Var.getRow(), column);
         double dph2 = dx.get(ph2Var.getRow(), column);
         double da1 = a1Var != null ? dx.get(a1Var.getRow(), column) : 0;
-        return calculateSensi(dph1, dph2, da1);
+        // - eval(0,0,0) to have an exact epression and remove the constant term of the affine function (wich is 0 in practe because A2 = 0)
+        return eval(dph1, dph2, da1) - eval(0, 0, 0);
     }
 
     protected double ph1(StateVector sv) {
@@ -116,16 +117,11 @@ protected double ph2() {
         return ph2(sv);
     }
 
-    protected abstract double calculateSensi(double ph1, double ph2, double a1);
+    protected abstract double eval(double ph1, double ph2, double a1);
 
     @Override
     public double eval() {
-        return calculateSensi(ph1(), ph2(), a1());
-    }
-
-    @Override
-    public double eval(StateVector sv) {
-        return calculateSensi(ph1(sv), ph2(sv), a1(sv));
+        return eval(ph1(), ph2(), a1());
     }
 
     protected double a1(StateVector sv) {

src/main/java/com/powsybl/openloadflow/dc/equations/ClosedBranchSide1DcFlowEquationTerm.java

@@ -36,7 +36,7 @@ public static ClosedBranchSide1DcFlowEquationTerm create(LfBranch branch, LfBus
     }
 
     @Override
-    protected double calculateSensi(double ph1, double ph2, double a1) {
+    protected double eval(double ph1, double ph2, double a1) {
         double deltaPhase = ph2 - ph1 + A2 - a1;
         return -getPower() * deltaPhase;
     }

src/main/java/com/powsybl/openloadflow/dc/equations/ClosedBranchSide2DcFlowEquationTerm.java

@@ -36,7 +36,7 @@ public static ClosedBranchSide2DcFlowEquationTerm create(LfBranch branch, LfBus
     }
 
     @Override
-    protected double calculateSensi(double ph1, double ph2, double a1) {
+    protected double eval(double ph1, double ph2, double a1) {
         double deltaPhase = ph2 - ph1 + A2 - a1;
         return getPower() * deltaPhase;
     }

src/main/java/com/powsybl/openloadflow/equations/Equation.java

@@ -156,9 +156,16 @@ public double eval() {
         for (EquationTerm<V, E> term : terms) {
             if (term.isActive()) {
                 value += term.eval();
-                if (term.hasRhs()) {
-                    value -= term.rhs();
-                }
+            }
+        }
+        return value;
+    }
+
+    public double evalLhs() {
+        double value = 0;
+        for (EquationTerm<V, E> term : terms) {
+            if (term.isActive()) {
+                value += term.evalLhs();
             }
         }
         return value;

src/main/java/com/powsybl/openloadflow/equations/EquationTerm.java

@@ -178,11 +178,11 @@ static <V extends Enum<V> & Quantity, E extends Enum<E> & Quantity> EquationTerm
     double eval();
 
     /**
-     * Evaluate the equation term with an alternative state vector.
+     * Evaluate equation lhs of the equation term
      * @return value of the equation term
      */
-    default double eval(StateVector sv) {
-        throw new UnsupportedOperationException("Not implemented");
+    default double evalLhs() {
+        return eval() - (hasRhs() ? rhs() : 0);
     }
 
     /**

src/main/java/com/powsybl/openloadflow/equations/EquationVector.java

@@ -42,10 +42,10 @@ protected double[] createArray() {
         return array;
     }
 
-    private void eval(double[] array, List<Equation<V, E>> equations) {
+    private void evalLhs(double[] array, List<Equation<V, E>> equations) {
         Arrays.fill(array, 0); // necessary?
         for (Equation<V, E> equation : equations) {
-            array[equation.getColumn()] = equation.eval();
+            array[equation.getColumn()] = equation.evalLhs();
         }
     }
 
@@ -59,7 +59,7 @@ protected void updateArray(double[] array) {
             throw new IllegalArgumentException("Bad equation vector length: " + array.length);
         }
 
-        eval(array, equations);
+        evalLhs(array, equations);
 
         LOGGER.debug(PERFORMANCE_MARKER, "Equation vector updated in {} us", stopwatch.elapsed(TimeUnit.MICROSECONDS));
     }

src/main/java/com/powsybl/openloadflow/equations/InjectionDerivable.java

@@ -62,12 +62,4 @@ public boolean isActive() {
         return getBranchTermStream().anyMatch(EquationTerm::isActive);
     }
 
-    @Override
-    public double eval(StateVector sv) {
-        // The equation is
-        // rhs = VariableInjectionPart+ branchPart
-        // with rhs = - (cte injectionPart)  (for ex sum of targetQ)
-        // -branchPart = variableInjectionPart + cteInjectionPart
-        return -getBranchTermStream().mapToDouble(t -> t.eval(sv)).sum();
-    }
 }

src/main/java/com/powsybl/openloadflow/util/Derivable.java

@@ -9,7 +9,6 @@
 
 import com.powsybl.math.matrix.DenseMatrix;
 import com.powsybl.openloadflow.equations.Quantity;
-import com.powsybl.openloadflow.equations.StateVector;
 import com.powsybl.openloadflow.equations.Variable;
 
 /**
@@ -23,5 +22,4 @@ public interface Derivable <V extends Enum<V> & Quantity> extends Evaluable {
 
     boolean isActive();
 
-    double eval(StateVector sv);
 }

src/test/java/com/powsybl/openloadflow/dc/DcLoadFlowTest.java

@@ -197,6 +197,53 @@ void nonImpedantBranchTest() {
         assertEquals(33.3333, network.getLine("L1").getTerminal1().getP(), 0.01);
     }
 
+    @Test
+        void nonImpedantBranchAndPhaseShiftingTest() {
+        Network network = PhaseShifterTestCaseFactory.create();
+        network.getLine("L2").setX(0).setR(0);
+        network.getTwoWindingsTransformer("PS1").getPhaseTapChanger().getStep(1).setAlpha(2);
+        loadFlowRunner.run(network, parameters);
+
+        assertEquals(16.5316, network.getLine("L1").getTerminal1().getP(), 0.01);
+        assertEquals(83.4683, network.getLine("L2").getTerminal1().getP(), 0.01); // Temporary comment : P without fix = 133.87
+        assertEquals(-83.4683, network.getTwoWindingsTransformer("PS1").getTerminal2().getP(), 0.01);
+
+        // With e second zero impedance line and a second load
+        VoltageLevel vl2 = network.getVoltageLevel("VL2");
+        vl2.getBusBreakerView().newBus()
+                .setId("B2Bis")
+                .add();
+        vl2.newLoad()
+                .setId("LD2Bis")
+                .setConnectableBus("B2Bis")
+                .setBus("B2Bis")
+                .setP0(100.0)
+                .setQ0(50.0)
+                .add();
+        network.newLine()
+                .setId("L2Bis")
+                .setVoltageLevel1("VL3")
+                .setConnectableBus1("B3")
+                .setBus1("B3")
+                .setVoltageLevel2("VL2")
+                .setConnectableBus2("B2Bis")
+                .setBus2("B2Bis")
+                .setR(0.0)
+                .setX(0.0)
+                .setG1(0.0)
+                .setB1(0.0)
+                .setG2(0.0)
+                .setB2(0.0)
+                .add();
+        network.getGenerator("G1").setMaxP(500);
+        LoadFlowResult result = loadFlowRunner.run(network, parameters);
+        assertTrue(result.isFullyConverged());
+        assertEquals(49.86, network.getLine("L1").getTerminal1().getP(), 0.01);
+        assertEquals(150.13, network.getTwoWindingsTransformer("PS1").getTerminal1().getP(), 0.01);
+        assertEquals(0, network.getTwoWindingsTransformer("PS1").getTerminal2().getP() + network.getLine("L2").getTerminal1().getP() + network.getLine("L2Bis").getTerminal1().getP(), 0.01); // Temporary comment : P without fix = 133.87
+        assertEquals(-200, network.getGenerator("G1").getTerminal().getP());
+    }
+
     @Test
     void multiCcTest() {
         Network network = IeeeCdfNetworkFactory.create14();