Solmetric PVA-1000S, User Manual

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8 Translation of I-V Data to Standard Test Conditions
8-1
8 Translation of I-V Data to Standard Test
Conditions
The PVA PC software provides a feature for translating the displayed I-V curve to
Standard Test Conditions (STC) of 1000W/m
2
and 25

C. The software also translates the
key performance parameters in the Table view (tab).
The primary application for these features is the analysis of I-V data collected during the
commissioning of commercial scale PV arrays. As these measurements are usually
performed in a 4-hour time span centered about solar noon, the measured I-V curves
reflect the changes in irradiance and cell temperature that take place over this time
period. The translation features remove these effects to a first order by translating key
performance parameters derived from the I-V curves to STC conditions. Fast changes in
irradiance and temperature caused by rapidly moving clouds will be difficult to correct
accurately, so clear days are still required for quality end results.
Translation will introduce error in proportion to the span of the translation. This should
be taken into account when assessing the consistency of performance across a population
of PV strings.
Parameter definitions
The following definitions are extracted from the Sandia PV Array Model (D. L. King)
paper:
I
sc
= Short-circuit current (A)
I
mp
= Current at the maximum-power point (A)
V
mp
= Voltage at maximum-power point (V)
V
oc
= Open-circuit voltage (V)
P
mp
= Power at the maximum power point (W)
α
Isc
= Normalized temperature coefficient for I
sc
, (%/°C). This parameter is ‘normalized’
by dividing the temperature dependence (A/°C) measured for a particular standard solar
spectrum and irradiance level by the module short-circuit current at the standard