add variable to Copperplate model

src/devices_models/device_constructors/branch_constructor.jl

@@ -541,6 +541,37 @@ function construct_device!(
     return
 end
 
+function construct_device!(
+    container::OptimizationContainer,
+    sys::PSY.System,
+    ::ArgumentConstructStage,
+    model::DeviceModel{T, HVDCTwoTerminalUnbounded},
+    network_model::NetworkModel{CopperPlatePowerModel},
+) where {T <: TwoTerminalHVDCTypes}
+    devices = get_available_components(model, sys)
+    add_variables!(container, FlowActivePowerVariable, devices, HVDCTwoTerminalUnbounded())
+    add_to_expression!(
+        container,
+        ActivePowerBalance,
+        FlowActivePowerVariable,
+        devices,
+        model,
+        network_model,
+    )
+    return
+end
+
+function construct_device!(
+    container::OptimizationContainer,
+    sys::PSY.System,
+    ::ModelConstructStage,
+    device_model::DeviceModel{<:TwoTerminalHVDCTypes, HVDCTwoTerminalUnbounded},
+    ::NetworkModel{CopperPlatePowerModel},
+)
+    add_constraint_dual!(container, sys, device_model)
+    return
+end
+
 # Repeated method to avoid ambiguity between HVDCTwoTerminalUnbounded and HVDCTwoTerminalLossless
 function construct_device!(
     container::OptimizationContainer,