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MT5 Multi-Tracer Operating Manual
page 80
Another problem arises when false power peaks are created due to rapid changes in
irradiance. This can confuse the power tracking. However, this is typically only an issue
when initially acquiring the peak-power point. Because the power varies directly with
irradiance levels, the operating voltage point is relatively immune to irradiance changes.
Therefore once the Multi-Tracer has locked on to the peak-power point, irradiance
changes have little effect.
Another potential issue is testing PV modules with unusual IV curves due to internal
failures. Damaged or extremely poorly functioning PV modules may have IV curve
characteristics with multiple, local peak-power points. It is possible for the Multi-Tracer
to lock on and track one of these false peaks.
There are two advanced parameters affecting peak-power tracking defined on the
module channel settings page, see section 4.3.2
“Module Channel Settings”. These are
the “Advanced Power Tracking” settings of “Min. Fraction of Voc” and “Max Step
Fraction of Voc”.
“Min. Fraction of Voc” defines the minimum possible peak-power, voltage as a fraction
of Voc. The default is 0.40 (40%) which is actually very low for typical PV modules.
During peak-power tracking, the Multi-Tracer will not attempt to power track at any
voltage below 40% of the Voc value. If the tracking reaches this po
int, it will “turn
around” and begin increasing the operating voltage. This lower limit avoids wasting time
at unrealistically low values of operating voltage. Although 40% is very low, unless there
is a reason to change this value, Raydec recommends leaving it at this value.
“Max Step Fraction of Voc” is the largest possible power-tracking step size as a fraction
of Voc. The default is 0.1 (10%). The larger this value the more quickly the Multi-Tracer
will find and begin power tracking. However, if this value is too large, there may be
difficulty in locating the peak power point for modules with very high fill-factors. High fill-
factor modules operate at levels approaching 90% of Voc. Therefore a 0.1 (10%) step
size may be too large. If working with high fill-factor modules and experiencing difficulty
with reliable peak-power tracking, reduce this value by half of its current setting.