Add support for constant cost functions in correct_cost_functions (#816)

* add support for constant cost functions in correct_cost_functions
* remove error test for identical points in pwl functions which is now supported

CHANGELOG.md

@@ -2,7 +2,7 @@ PowerModels.jl Change Log
 =========================
 
 ### Staged
-- nothing
+- Update `correct_cost_functions!` to support PWL costs with a constant power value
 
 ### v0.19.4
 - Fix `sol_component_value_edge` implementation and usage (#811)

src/core/data.jl

@@ -1910,12 +1910,13 @@ function _correct_cost_function!(id, comp, type_name, pmin_key, pmax_key)
             _remove_pwl_cost_duplicates!(id, comp, type_name)
 
             for i in 3:2:length(comp["cost"])
-                if comp["cost"][i-2] >= comp["cost"][i]
+                if comp["cost"][i-2] > comp["cost"][i]
                     Memento.error(_LOGGER, "non-increasing x values in pwl cost model on $(type_name) $(id)")
                 end
             end
 
             _simplify_pwl_cost!(id, comp, type_name)
+
         elseif comp["model"] == 2
             if length(comp["cost"]) != comp["ncost"]
                 Memento.error(_LOGGER, "ncost of $(comp["ncost"]) not consistent with $(length(comp["cost"])) cost values on $(type_name) $(id)")

test/data.jl

@@ -689,15 +689,6 @@ end
         @test isapprox(result["objective"], gen_cost + dcline_cost; atol=1e-1)
     end
 
-     @testset "5-bus pwlc gen cost identical points" begin
-        data = PowerModels.parse_file("../test/data/matpower/case5_pwlc.m")
-
-        data["gen"]["1"]["ncost"] = 2
-        data["gen"]["1"]["cost"] = [0.0, 0.0, 0.0, 0.0]
-
-        @test_throws(TESTLOG, ErrorException, correct_network_data!(data))
-    end
-
      @testset "5-bus polynomial gen and dcline cost" begin
         data = PowerModels.parse_file("../test/data/matpower/case5_dc.m")
         result = run_opf(data, ACPPowerModel, nlp_solver)