Merge pull request #141 from ejmiller2/master

Resolve #140 snlinverter night tare power issue

Author: wholmgren

docs/sphinx/source/whatsnew/v0.3.1.txt

@@ -15,8 +15,15 @@ Enhancements
 Bug fixes
 ~~~~~~~~~
 
+* Fixes night tare issue in snlinverter. When the DC input power (p_dc)
+  to an inverter is below the inversion startup power (Ps0), the model
+  should set the AC output (ac_power) to the night tare value (Pnt). 
+  The night tare power indicates the power consumed by the inverter to
+  sense PV array voltage. The model was erroneously comparing Ps0 with
+  the AC output power (ac_power), rather than the DC input power (p_dc).
 
 
 Contributors
 ~~~~~~~~~~~~
 
+* ejmiller2

pvlib/pvsystem.py

@@ -1662,7 +1662,7 @@ def snlinverter(inverter, v_dc, p_dc):
 
     ac_power = (Paco/(A-B) - C*(A-B)) * (p_dc-B) + C*((p_dc-B)**2)
     ac_power[ac_power > Paco] = Paco
-    ac_power[ac_power < Pso] = - 1.0 * abs(Pnt)
+    ac_power[p_dc < Pso] = - 1.0 * abs(Pnt)
 
     if len(ac_power) == 1:
         ac_power = ac_power.ix[0]

pvlib/test/test_pvsystem.py

@@ -384,6 +384,17 @@ def test_snlinverter_float():
     assert_almost_equals(pacs, 132.004278, 5)
 
 
+def test_snlinverter_Pnt_micro():
+    inverters = sam_data['cecinverter']
+    testinv = 'Enphase_Energy__M250_60_2LL_S2x___ZC____NA__208V_208V__CEC_2013_'
+    vdcs = pd.Series(np.linspace(0,50,3))
+    idcs = pd.Series(np.linspace(0,11,3))
+    pdcs = idcs * vdcs
+
+    pacs = pvsystem.snlinverter(inverters[testinv], vdcs, pdcs)
+    assert_series_equal(pacs, pd.Series([-0.043000, 132.545914746, 240.000000]))
+
+
 def test_PVSystem_creation():
     pv_system = pvsystem.PVSystem(module='blah', inverter='blarg')