Solar modules are connected in
series or parallel or series-parallel combination to get the required voltage,
current and power. Series connection will increase the string voltage while
keeping the current constant. On the other hand parallel connection will
increase the current whereas the voltage remains the same. Now what about the
voltage, current and power in the series-parallel connection? Yes you are correct, all the three will increase.

The capacity of the solar PV
plant depends on the roof space available and the individual requirement. Once
the PV module capacity is decided we can size the inverter. Normally the inverter
size is smaller than the rated output of the PV array at Standard Test
Conditions (STC). This is due to the losses or de-rating factors such as panel
dissimilarity, dust, losses in cables etc. An inverter of 80 % size of the PV
array is quite normal.

###
**What is a string in the solar PV system?**

Number of modules connected in
series is called the “string”. Size of the string determines the voltage input
to the inverter. The maximum and minimum number of modules in the string
depends on the maximum and minimum voltage of the inverter.

The output voltage generated from
the PV should never damage the inverter that’s why we have to calculate the
maximum system voltage. The maximum system voltage should not be more than the
highest acceptable inverter voltage.

Similarly we have to calculate
the minimum number of modules in the string so that in worst case scenario the
PV system’s output voltage is sufficient enough to turn the inverter ‘ON’.

###
**Relationship between Ambient Temperature
and String Output:**

As expected, a certain relation
exists between the ambient temperature and the string voltage, which is to be
considered while calculating or designing the string size. The PV output is inverse to the ambient
temperature i.e. with the decrease in ambient temperature; there is a certain
increase in string voltage and power. The vice-versa happens during summer
season. With a correctly sized PV array, the DC output will remain within the
optimum operating range of the inverter under different working and ambient
conditions.

###
**What should be the minimum string size in
the solar PV system?**

Softwares are also used in the design, construction, operation and maintenance of Solar PV plants. These softwares help to optimize the design configuration and system layouts.

In the following paragraphs, manual calculations of string size is given for the easy understanding of concepts. They are easy enough to be done manually.

In the following paragraphs, manual calculations of string size is given for the easy understanding of concepts. They are easy enough to be done manually.

For the calculation we need some
data from the data-sheet of the module. Suppose the;

Voltage at
Open Circuit, V

_{OC}= 43.4 V,
Temperature
at Standard Test Condition (STC) = 25

^{o}C,
Temperature Coefficient
at V

_{OC}= (-) 0.15 V/^{o}C,
Voltage at
Maximum Power, V

_{mp}= 35.4 V,
Temperature Coefficient at V

_{mp}= (-) 0.17 V/^{o}C,
Now from the inverter data sheet,
we have to get the maximum input DC voltage and the start ( or strike) voltage
of the inverter. From the environmental data we have to collect the hottest
day-time temperature and coldest day-time temperature of the location where the
PV system is to be installed.

Suppose that the effective cell
temperature during the hottest day is 70

^{o}C, which is 45^{o}C above the temperature at Standard Test Condition (STC) of 25^{o}C.
Therefore, the V

_{mp}voltage would be changed (reduced) by 45 x (-) 0.17 = (-) 7.65 V
Hence the V

_{mp}at 70^{o}C cell temperature would be 35.4 + (– 7.65) = 27.75 V.
Next we should consider the
de-rating factor which is due to the earlier mentioned factors such as module
mismatch, dust & dirt, cable loss etc. Let it be 0.88.

So the effective minimum V

_{mp}for each module at the inverter input would be 27.75 x 0.88 = 24.4 V.
From the above results we can
calculate the minimum permissible number of modules in the string.

Assume that the minimum start (or
strike) voltage for the inverter is 140 V. Taking a safety margin of 10%, i.e.
140 x 1.1 = 154 V. This means that the string size should be so selected that a minimum of
154 V is maintained at the inverter input terminals in the worst case scenario.

Hence the minimum number of
modules in the string to surpass the start voltage is

154/ 24.4 = 6.31 rounded up to 7 modules.

###
**What should be the maximum string size in the solar PV
system?**

As mentioned earlier, the output voltage of a solar PV module increases as the ambient temperature drops below the temperature at STC. At the coldest day-time
temperature the V

_{OC}of the array shall never be greater than the maximum allowed input voltage of the inverter. Now we have to calculate the maximum number of modules in the string. Oversizing the string can damage the inverter, cancel warranties and violate the safety codes.
Let’s suppose that during the
coldest day, the effective cell temperature is 15

^{o}C, which is 10^{o}C below the temperature at STC of 25^{o}C.
Therefore the V

_{OC}should be increased by (15 – 25) x (-) 0.15 = 1.5 V. You can say that the Voc at 15^{o}C is 43.4 + 1.5 = 44.9 V.
Assume that the maximum safe
working voltage allowed by the inverter is 400 V. Then the maximum number of
modules permissible in the string is 400/ 44.9 = 8.9 rounded down to 8 modules.

The goal of this article was to
convey the basic process for sizing the PV string for a grid connected system. Let’s
promote Solar PV system in our vicinity.