Refactor Grid interface

Previously, the Grid interface returned `None` if the microgrid was
configured as an island.
This update ensures that the interface consistently returns a Grid
instance.
This change is necessary to treat the grid connection uniformly within
the microgrid, regardless of whether the microgrid is configured as
an island or not. Moreover, it guarantees that the `microgrid.grid()`
function always yields a Grid instance for metric retrieval.

Now the rated current of the fuse is set to zero in case of an
islanded microgrid.
And the fuse of the grid connection point is now set to `None` when
the grid connection component metadata lacks information about the
fuse. Thus, a fuse set to `None` won't prevent the initialization
of the Grid instance.

Signed-off-by: Daniel Zullo <[email protected]>

Author: daniel-zullo-frequenz

src/frequenz/sdk/microgrid/_data_pipeline.py

@@ -191,9 +191,7 @@ def ev_charger_pool(
             )
         return self._ev_charger_pools[key]
 
-    def grid(
-        self,
-    ) -> Grid | None:
+    def grid(self) -> Grid:
         """Return the grid instance.
 
         If a Grid instance doesn't exist, a new one is created and returned.
@@ -479,7 +477,7 @@ def battery_pool(
     return _get().battery_pool(battery_ids, name, priority)
 
 
-def grid() -> Grid | None:
+def grid() -> Grid:
     """Return the grid instance.
 
     Returns:

src/frequenz/sdk/timeseries/grid.py

@@ -53,7 +53,6 @@ class Grid:
         )
 
         grid = microgrid.grid()
-        assert grid, "Grid is not initialized"
 
         # Get a receiver for a builtin formula
         grid_power_recv = grid.power.new_receiver()
@@ -62,8 +61,14 @@ class Grid:
         ```
     """
 
-    fuse: Fuse
-    """The fuse protecting the grid connection point."""
+    fuse: Fuse | None
+    """The fuse protecting the grid connection point.
+
+    The rated current of the fuse is set to zero in case of an islanded
+    microgrid.
+    And the fuse is set to `None` when the grid connection component metadata
+    lacks information about the fuse.
+    """
 
     _formula_pool: FormulaEnginePool
     """The formula engine pool to generate grid metrics."""
@@ -134,8 +139,7 @@ def initialize(
 
     Raises:
         RuntimeError: If there is more than 1 grid connection point in the
-            microgrid, or if the grid connection point is not initialized,
-            or if the grid connection point does not have a fuse.
+            microgrid, or if the grid connection point is not initialized.
     """
     global _GRID  # pylint: disable=global-statement
 
@@ -147,7 +151,10 @@ def initialize(
 
     grid_connections_count = len(grid_connections)
 
+    fuse: Fuse | None = None
+
     if grid_connections_count == 0:
+        fuse = Fuse(max_current=Current.zero())
         _logger.info(
             "No grid connection found for this microgrid. This is normal for an islanded microgrid."
         )
@@ -159,8 +166,6 @@ def initialize(
         if grid_connections[0].metadata is None:
             raise RuntimeError("Grid metadata is None")
 
-        fuse: Fuse | None = None
-
         # The current implementation of the Component Graph fails to
         # effectively convert components from a dictionary representation to
         # the expected Component object.
@@ -172,25 +177,28 @@ def initialize(
             fuse = Fuse(**fuse_dict) if fuse_dict else None
 
         if fuse is None:
-            raise RuntimeError("Grid fuse is None")
+            _logger.warning("The grid connection point does not have a fuse")
 
-        namespace = f"grid-{uuid.uuid4()}"
-        formula_pool = FormulaEnginePool(
-            namespace,
-            channel_registry,
-            resampler_subscription_sender,
-        )
+    namespace = f"grid-{uuid.uuid4()}"
+    formula_pool = FormulaEnginePool(
+        namespace,
+        channel_registry,
+        resampler_subscription_sender,
+    )
 
-        _GRID = Grid(fuse, formula_pool)
+    _GRID = Grid(fuse, formula_pool)
 
 
-def get() -> Grid | None:
+def get() -> Grid:
     """Get the grid connection.
 
-    Note that a microgrid configured as an island will not have a grid
-    connection point. For such microgrids, this function will return `None`.
+    Note that the rated current of the fuse is set to zero in case of an
+    islanded microgrid.
+    And the fuse is set to `None` when the grid connection component metadata
+    lacks information about the fuse.
 
     Returns:
         The grid connection.
     """
+    assert _GRID, "Grid is not initialized"
     return _GRID

tests/microgrid/test_grid.py

@@ -41,7 +41,9 @@ async def test_grid_1(mocker: MockerFixture) -> None:
     await mockgrid.start(mocker)
     grid = microgrid.grid()
 
-    assert grid is None
+    assert grid
+    assert grid.fuse
+    assert grid.fuse.max_current == Current.from_amperes(0.0)
     await mockgrid.cleanup()
 
 
@@ -81,6 +83,27 @@ async def test_grid_2(mocker: MockerFixture) -> None:
     assert grid.fuse == expected_fuse
 
 
+async def test_grid_3(mocker: MockerFixture) -> None:
+    """Validate that microgrids with a grid connection without a fuse are instantiated."""
+    components = {
+        Component(1, ComponentCategory.GRID, None, GridMetadata(None)),
+        Component(2, ComponentCategory.METER),
+    }
+    connections = {
+        Connection(1, 2),
+    }
+
+    # pylint: disable=protected-access
+    graph = gr._MicrogridComponentGraph(components=components, connections=connections)
+
+    mockgrid = MockMicrogrid(graph=graph)
+    await mockgrid.start(mocker)
+
+    grid = microgrid.grid()
+    assert grid is not None
+    assert grid.fuse is None
+
+
 async def test_grid_power_1(mocker: MockerFixture) -> None:
     """Test the grid power formula with a grid side meter."""
     mockgrid = MockMicrogrid(grid_meter=True)