©
2017 Sensata Technologies
Page 22
Installation
3B) Calculating Minimum Number of Modules in Series for Lower V
MP
:
After calculating the module’s lower V
MP-LOW
level, determine the minimum number of modules
that can be placed in series. Refer to Table 2-1 to
fi
nd the PT-100’s lower MPPT voltage limit based
on the particular battery bank connected to the PT-100.
Once the controller’s lower MPPT voltage limit has been determined, divide this value by the
V
MP-LOW
calculation and round up to the nearest whole number. This whole number value is the
minimum number of modules that can be placed in a series string to ensure the PV array’s V
MP
remains above the PT-100’s lower V
MP
level.
Examples
- Determining the array’s upper and lower voltage levels.
Using the values in the scenario below, determine the module’s upper V
OC
level (V
OC-UPP
) level,
and then the lower V
MP
level (V
MP-LOW
). Use these values to determine the minimum and maximum
number of modules that should be connected in series.
Scenario:
A module has an open circuit voltage of 37.8 V
OC
and a maximum power voltage
rating of 31.4V
MP
at STC (25°C), and the V
MP
temperature coef
fi
cient is -.141V/°C. The modules
are going to be installed in an area where the temperature rises to an annual average high
of 24°C and falls to an average low 1°C. The array will be mounted
fl
at on the roof with
minimum clearance (+35°C). The controller is connected to a 48-volt nominal battery bank
where the highest charging voltage required will be 66 volts.
A. Find the maximum number of PV modules that can be connected in series that
will stay below the upper V
OC
level.
1. What is the module’s upper V
OC
level (V
OC-UPP
)? 41.18V (as calculated below)
V
OC-UPP
= V
OC
+ [(T
AVG-L
−
T
STC
) ×
α
V
OC
(V)]
V
OC-UPP
= 37.8V + [(1°C - 25°C) × -.141V/°C]
V
OC-UPP
= 37.8V + [(-24°C) × -.141V/°C]
V
OC-UPP
= 37.8V + [3.38] = 41.18
V
OC-UPP
=
41.18V
2. What is the PT-100’s upper MPPT Voltage level (see Table 2.1)? 187V (48-volt system)
3. What is the maximum number of modules that may be installed in series? 187V ÷
41.18V = 4.54 = 4 modules (rounded down to the next whole number).
Answer: You should connect no more than
4
of these modules in each series string to remain
below the PT-100’s upper V
MP
level.
B. Find the minimum number of PV modules that can be connected in series that
will stay above the lower V
MP
level.
1. What is the module’s lower V
MP
level (V
MP-LOW
)? 21.3V (as calculated below)
V
MP-LOW
=
⟨
V
MP
+ [(T
AVG-H
+ T
Rise
−
T
STC
) ×
α
V
MP
(V)]
⟩
x 0.80
V
MP-LOW
=
⟨
31.4V + [(24°C + 35°C - 25°C) × -.141V/°C]
⟩
x 0.80
V
MP-LOW
=
⟨
31.4V + [(34°C) × -.141V/°C]
⟩
x 0.80
V
MP-LOW
=
⟨
31.4V + [-4.79]
⟩
x 0.80
V
MP-LOW
=
⟨
26.61
⟩
x 0.80
= 21.29
V
MP-LOW
=
21.3V
2. What is the PT-100’s lower MPPT Voltage level (see Table 2-1)? 73V (48-volt system)
3. What is the minimum number of modules that may be installed in series? 73V ÷ 21.3V
= 3.43 = 4 modules (rounded up to the next whole number).
Answer: You should connect at least
4
of these modules in each series string to remain
above the PT-100’s lower V
MP
level.
