chore: more robust surge control margin calculation (#229)

* chore: more robust surge control margin calculation

docs/docs/about/modelling/setup/models/compressor_modelling/compressor_charts/index.md

@@ -256,9 +256,9 @@ MODELS:
 
 
 For a variable speed compressor chart it is possible to add a surge control margin. This is currently done by giving a
-fraction or percentage as input. This percentage/fraction is applied to the average of the minimum flow rate for all
-speed curves in the compressor chart, to find how much the minimum flow should be increased. The increase in minimum
-flow will be the same for all speed curves. The corresponding head and efficiency values for the new minimum flow rate
+fraction or percentage as input. The control margin is used to calculate the increase in minimum flow, i.e. as a percentage 
+or fraction of the rate difference between minimum- and maximum flow, for the given speed. The increase in minimum
+flow is calculated individually for each speed curve. The corresponding head and efficiency values for the new minimum flow rate
 is found by interpolation along the speed curves. The same compressor chart can be used for multiple compressor stages,
 but with different surge control margins. Hence, the surge control margin is defined when setting up the stages in a
 [`Variable speed compressor train model`](/about/modelling/setup/models/compressor_modelling/compressor_models_types/variable_speed_compressor_train_model.md) or [`Variable speed compressor train model with multiple streams and pressures`](/about/modelling/setup/models/compressor_modelling/compressor_models_types/variable_speed_compressor_train_model_with_multiple_streams_and_pressures.md).

src/libecalc/core/models/compressor/train/chart/variable_speed_compressor_chart.py

@@ -36,8 +36,8 @@ def _get_new_point(x: List[float], y: List[float], new_x_value) -> float:
             "Any usage of this functionality is at your own risk."
         )
         new_curves = []
-        adjust_minimum_rate_by = np.average([np.min(curve.rate) for curve in self.curves]) * control_margin
         for curve in self.curves:
+            adjust_minimum_rate_by = (np.max(curve.rate) - np.min(curve.rate)) * control_margin
             new_minimum_rate = np.min(curve.rate) + adjust_minimum_rate_by
             rate_head_efficiency_array = np.vstack((curve.rate, curve.head, curve.efficiency))
             # remove points with rate less than the new minimum rate (i.e. chop off left part of chart curve)

src/tests/libecalc/core/models/compressor_modelling/test_compressor_chart.py

@@ -341,8 +341,18 @@ def test_variable_speed_compressor_chart_control_margin():
     control_margin = 0.1
     compressor_chart_adjusted = compressor_chart.get_chart_adjusted_for_control_margin(control_margin=control_margin)
 
-    assert compressor_chart_adjusted.curves[0].rate_actual_m3_hour[0] == 1.25
-    assert compressor_chart_adjusted.curves[1].rate_actual_m3_hour[0] == 4.25
+    adjust_minimum_rate_by_speed1 = (
+        compressor_chart.curves[0].rate_actual_m3_hour[-1] - compressor_chart.curves[0].rate_actual_m3_hour[0]
+    ) * control_margin
+    adjust_minimum_rate_by_speed2 = (
+        compressor_chart.curves[1].rate_actual_m3_hour[-1] - compressor_chart.curves[1].rate_actual_m3_hour[0]
+    ) * control_margin
+
+    new_minimum_rate_speed1 = compressor_chart.curves[0].rate_actual_m3_hour[0] + adjust_minimum_rate_by_speed1
+    new_minimum_rate_speed2 = compressor_chart.curves[1].rate_actual_m3_hour[0] + adjust_minimum_rate_by_speed2
+
+    assert compressor_chart_adjusted.curves[0].rate_actual_m3_hour[0] == new_minimum_rate_speed1
+    assert compressor_chart_adjusted.curves[1].rate_actual_m3_hour[0] == new_minimum_rate_speed2
 
 
 def test_compare_variable_speed_compressor_chart_head_and_efficiency_known_point_compared_to_interpolation(