nkoukpaizan · Oct 28, 2026 · Nov 2, 2026 · Nov 2, 2026 · Nov 4, 2026 · Nov 4, 2026
diff --git a/CHANGELOG.md b/CHANGELOG.md
@@ -1,6 +1,6 @@
 # Release Changelog
 
-## v.03
+## v0.3
 
 - Added `output_layout` solver option for long monitor output and a PostgreSQL example.
 - Restricted monitored bus names and device ids to letters, digits, spaces, `_`, `.` and `-`.
@@ -24,6 +24,8 @@
 - Fixed ESDC1A rate feedback to use the limited field-voltage rate.
 - Fixed REGCA to hold the terminal voltage at `Vhvmax`, as PowerWorld does; `Khv` is accepted but unused.
 - Added the FourBusLimits validation case, which drives the REPCA, REECB, REGCA, and ESDC1A limits.
+- Added a `TwoArea` integration test comparing flat and hierarchical PhasorDynamics systems.
+- Added rate feedback (`Kf`, `Tf`) to the IEEET1 exciters in the `TwoArea` case, whose generators lost synchronism without a fault.
 
 ## v0.2
 

diff --git a/GridKit/Model/PhasorDynamics/Component.hpp b/GridKit/Model/PhasorDynamics/Component.hpp
@@ -608,8 +608,8 @@ namespace GridKit
       /// Global indices of attached external signals
       std::vector<IdxT> ws_indices_;
 
-      RealT time_;
-      RealT alpha_;
+      RealT time_{0};
+      RealT alpha_{0};
 
       RealT freq_system_base_{60.0};
       RealT va_system_base_{100.0e6};

diff --git a/GridKit/Model/PhasorDynamics/SystemModel.hpp b/GridKit/Model/PhasorDynamics/SystemModel.hpp
@@ -125,6 +125,8 @@ namespace GridKit
       ComponentT*  getComponent(const std::string& label);
       BranchT*     getBranch(const std::string& id);
 
+      IdxT               numBuses() const;
+      IdxT               getBusId(IdxT gridkit_bus_id) const;
       IdxT               numComponents() const;
       const std::string& getComponentLabel(IdxT gridkit_component_id) const;
       IdxT               numSubsystems() const;

diff --git a/GridKit/Model/PhasorDynamics/SystemModelImpl.hpp b/GridKit/Model/PhasorDynamics/SystemModelImpl.hpp
@@ -1068,6 +1068,27 @@ namespace GridKit
       return bus;
     }
 
+    /**
+     * @brief Return the number of this system's own buses, without those of
+     * its subsystems
+     */
+    template <typename scalar_type, typename index_type>
+    typename SystemModel<scalar_type, index_type>::IdxT
+    SystemModel<scalar_type, index_type>::numBuses() const
+    {
+      return static_cast<IdxT>(buses_.size());
+    }
+
+    /**
+     * @brief Return the bus_id of one of this system's own buses
+     */
+    template <typename scalar_type, typename index_type>
+    typename SystemModel<scalar_type, index_type>::IdxT
+    SystemModel<scalar_type, index_type>::getBusId(IdxT gridkit_bus_id) const
+    {
+      return buses_.at(gridkit_bus_id)->busID();
+    }
+
     template <typename scalar_type, typename index_type>
     typename SystemModel<scalar_type, index_type>::IdxT
     SystemModel<scalar_type, index_type>::numComponents() const

diff --git a/cases/PhasorDynamics/TwoArea/README.md b/cases/PhasorDynamics/TwoArea/README.md
@@ -10,6 +10,8 @@ Figure 1: One-line diagram of the TwoArea case.[^1]
 
 This case is derived from the Kundur two-area system and includes exciters for use in GridKit.
 
+The exciters use the rate feedback `Kf = 0.1` and `Tf = 1.0` from the DC1C sample data in IEEE 421.5-2016, Annex H, because the generators lose synchronism without it. These values are not specific to this case. All four exciters use `Vrmax = 3.0` for consistency.
+
 Simulation and validation studies have not yet been implemented for this case.
 
 [^1]: PowerWorld, [Simulator Help](https://www.powerworld.com/WebHelp/).
diff --git a/cases/PhasorDynamics/TwoArea/TwoArea.case.json b/cases/PhasorDynamics/TwoArea/TwoArea.case.json
@@ -681,8 +681,8 @@
         "Ta": 1.0,
         "Ke": 0.0,
         "Te": 0.0,
-        "Kf": 0.0,
-        "Tf": 0.0,
+        "Kf": 0.1,
+        "Tf": 1.0,
         "Vrmin": -3.0,
         "Vrmax": 3.0,
         "E1": 0.0,
@@ -706,8 +706,8 @@
         "Ta": 1.0,
         "Ke": 0.0,
         "Te": 0.0,
-        "Kf": 0.0,
-        "Tf": 0.0,
+        "Kf": 0.1,
+        "Tf": 1.0,
         "Vrmin": -3.0,
         "Vrmax": 3.0,
         "E1": 0.0,
@@ -731,10 +731,10 @@
         "Ta": 1.0,
         "Ke": 0.0,
         "Te": 0.0,
-        "Kf": 0.0,
-        "Tf": 0.0,
+        "Kf": 0.1,
+        "Tf": 1.0,
         "Vrmin": -3.0,
-        "Vrmax": 0.0,
+        "Vrmax": 3.0,
         "E1": 0.0,
         "E2": 0.0,
         "Se1": 0.0,
@@ -756,8 +756,8 @@
         "Ta": 1.0,
         "Ke": 0.0,
         "Te": 0.0,
-        "Kf": 0.0,
-        "Tf": 0.0,
+        "Kf": 0.1,
+        "Tf": 1.0,
         "Vrmin": -3.0,
         "Vrmax": 3.0,
         "E1": 0.0,

diff --git a/tests/IntegrationTests/PhasorDynamics/CMakeLists.txt b/tests/IntegrationTests/PhasorDynamics/CMakeLists.txt
@@ -11,6 +11,7 @@ if(TARGET SUNDIALS::idas)
   install(TARGETS test_pd_integration)
 
   add_subdirectory(TenGen)
+  add_subdirectory(TwoArea)
 endif()
 
 if(GRIDKIT_ENABLE_ENZYME)

diff --git a/tests/IntegrationTests/PhasorDynamics/TwoArea/CMakeLists.txt b/tests/IntegrationTests/PhasorDynamics/TwoArea/CMakeLists.txt
@@ -0,0 +1,14 @@
+add_executable(test_pd_two_area_hierarchical runTwoAreaHierarchicalTests.cpp)
+target_link_libraries(
+  test_pd_two_area_hierarchical
+  PRIVATE GridKit::phasor_dynamics_systemmodel GridKit::solvers_dyn GridKit::testing)
+
+configure_file(${CMAKE_SOURCE_DIR}/cases/PhasorDynamics/TwoArea/TwoArea.case.json TwoArea.case.json
+               COPYONLY)
+configure_file(TwoAreaHierarchical.case.json TwoAreaHierarchical.case.json COPYONLY)
+
+add_test(NAME PhasorDynamicsIntegrationTwoAreaHierarchical COMMAND test_pd_two_area_hierarchical)
+
+install(TARGETS test_pd_two_area_hierarchical)
+install(FILES ${CMAKE_SOURCE_DIR}/cases/PhasorDynamics/TwoArea/TwoArea.case.json
+              TwoAreaHierarchical.case.json DESTINATION share/gridkit/tests/PhasorDynamics/TwoArea)
diff --git a/tests/IntegrationTests/PhasorDynamics/TwoArea/README.md b/tests/IntegrationTests/PhasorDynamics/TwoArea/README.md
@@ -0,0 +1,13 @@
+# TwoArea Hierarchical Test
+
+`TwoAreaHierarchical.case.json` has the same system parameters, buses and devices as the [TwoArea case](../../../../cases/PhasorDynamics/TwoArea/README.md) ([`TwoArea.case.json`](../../../../cases/PhasorDynamics/TwoArea/TwoArea.case.json)), split into the [subsystems](../../../../GridKit/Model/PhasorDynamics/INPUT_FORMAT.md#subsystems) below. The test `PhasorDynamicsIntegrationTwoAreaHierarchical` compares the two files.
+
+System | Buses | Devices
+-- | -- | --
+Top level | 8 | None
+`Area1` | 1, 2, 5, 6, 7 | Generators on buses 1 and 2 with their exciters and governors, the load on bus 7, and the branches between these buses and to bus 8
+`Area2` | 3, 4, 9, 10 | Generators on buses 3 and 4 with their exciters and governors, the load on bus 8, and the branches between these buses and to bus 8
+
+Bus 8 connects the two areas, so it stays at the top level, where both areas can use it. The load on bus 8 is in `Area2` because a system with subsystems cannot have devices of its own.
+
+The hierarchical file is a separate copy of the data, so a change to the case must be made in both files.