©
2017 Sensata Technologies
Page 18
Installation
2.5.2
Determining the Maximum PV Array Voltage
In order to size the PV array so that the output voltage never exceeds the maximum PV input
voltage limit on the PT-100, the maximum number of modules in series need to be determined. To
do this, you must
fi
rst determine—from the installation location—the V
OC
of the individual module
at the lowest expected temperature.
CAUTION
: PV array voltage that exceeds the maximum PV input voltage rating can
damage components in the PT-100 and is not covered under warranty. When sizing
the array, the NEC says to use the “lowest expected ambient temperature”. It is highly
recommended that you
fi
nd the historical record or extreme low temperature for your
location and use this value for the “lowest expected ambient temperature” calculations.
This is a conservative approach, but is justi
fi
ed to prevent high voltage damage to
the PT-100. Be aware that the maximum PV voltage level is recorded in the PT-100’s
internal memory, allowing this voltage to be checked if repair is required.
Info:
If the PV module manufacturer does not provide a temperature coef
fi
cient rating,
Table 690.7 in the NEC can be used if it is a crystalline module. This table gives a
correction factor that can be applied to determine the temperature compensated voltage.
• 1A) Calculating Module’s Maximum Output Voltage (V
OC-MAX
):
To calculate the module’s maximum output voltage based on the module manufacturer’s temperature
coef
fi
cient method—at the lowest recorded local temperature, use one of the formulas below:
a) If coef
fi
cient in percentage (%) use: V
OC-MAX
= V
OC
×
⟨
1 + [(T
LOW
−
T
STC
) ×
α
V
OC
(%)]
⟩
b) If coef
fi
cient in voltage (V) use: V
OC-MAX
= V
OC
+ [(T
LOW
−
T
STC
) ×
α
V
OC
(V)]
• V
OC-MAX
= module’s maximum output voltage
• V
OC
= module’s rated open circuit voltage at STC (25°C)
• T
LOW
= module’s temperature at the lowest recorded local temperature
• T
STC
= module’s temperature at STC (25°C)
•
α
V
OC
(%) = temperature coef
fi
cient of V
OC
provided as %
•
α
V
OC
(V) = temperature coef
fi
cient of V
OC
provided as V
• 1B) Calculating Absolute Maximum Number of Modules in Series:
After calculating the module’s maximum output voltage (V
OC-MAX
), determine the maximum number
of modules that can be placed in series. Refer to Table 2-1 to
fi
nd the maximum PV input voltage
based on the battery bank connected to the PT-100.
Once the maximum PV input voltage has been determined, divide this value by V
OC-MAX
, and then
round down to the nearest whole number. This whole number value is the absolute maximum
number of modules that can be placed in a series string to ensure the Open Circuit Voltage (V
OC
)
of the PV array does not exceed the maximum PV input voltage level of the PT-100.
