©
2017 Sensata Technologies
Page 20
Installation
2.5.3
Determining the Upper Output Voltage (V
OC-UPP
)
This section details how to determine the maximum number of modules allowed to be connected
in series that will stay below the High V
OC
range of the PT-100. The Open Circuit Voltage (V
OC
) of
the PV array should normally operate below the High V
OC
range of the PT-100, (which is also the
upper limit of the MMPT voltage range). The PT-100 will stop operating if the voltage is allowed
to rise into the High V
OC
range. While this is not as critical as exceeding the maximum V
OC
limit—
which can damage the PT-100—the energy from the array cannot be used because the PT-100 is
not operating.
Because the PV module voltage increases at low temperatures, use the following calculation to
determine the V
OC
of the individual module at the lowest average temperature at the installation
location. Once the module’s upper V
OC
level is calculated, you can determine the maximum
number of modules allowed to be connected in series that will stay below the Upper V
OC
limit of
the PT-100.
• 2A) Calculating Module’s Maximum Voltage (V
OC-UPP
):
To calculate the module’s upper output voltage (V
OC-UPP
) level, use the formula below:
V
OC-UPP
= V
OC
+ [(T
AVG-L
−
T
STC
) ×
α
V
OC
(V)]
• V
OC-UPP
= module’s upper output voltage
• V
OC
= module’s rated open circuit voltage at STC (25°C)
• T
AVG-L
= module’s temperature at the lowest average temperature
• T
STC
= module’s temperature at STC (25°C)
•
α
V
OC
(V) = temperature coef
fi
cient of V
OC
provided as V
Info:
When lower than average temperatures occur, the module’s V
OC
may rise above
the PT-100’s upper MPPT voltage limit. This is not of great concern because this is likely
to occur when irradiance is very low (i.e., a few hours before sunrise). If sized correctly,
once the sun rises and the temperature starts increasing, the array’s V
OC
should fall
within the MPPT voltage range and begin operating.
• 2B) Calculating Maximum Number of Modules in Series for Upper V
OC
:
After calculating the module’s upper V
OC-UPP
level, determine the maximum number of modules
that can be placed in series and still remain below the High V
OC
range (or below the upper V
MP
level) when operating in normal/average temperature conditions. Refer to Table 2-1 to
fi
nd the
upper MPPT voltage limit based on the particular battery bank connected to the PT-100.
Once the controller’s upper MPPT voltage limit has been determined, divide this value by the
V
OC-UPP
calculation and then round down to the nearest whole number. This whole number value is
the maximum number of modules that should be placed in a series string to ensure the PV array’s
V
OC
remains below the High V
OC
range of the PT-100.
