Correction or improvement of
the power factor of electrical equipment like transformers is done to reduce
the required electric power and thereby save electricity. Here, we discuss how
the power factor correction is done and also the calculation steps to process
the method.
All electrical equipment works
with a degree of efficiency which may be poor or excellent. The efficiency of
electrical equipment like transformers and induction motors is indicated by the
term power factor. In the technical world, the power factor is defined as ratio
of the true power of the equipment and the apparent power.
If you want to understand the
calculation of power factor and its correction, you need to understand a few
terms, which we'll discuss here.
○ KW is the amount of power by which the
transformer does useful work. It is also called active power, real power, or
actual power.
○ KVAR is used to denote reactive power.
This is the power that the transformer needs to develop magnetized flux.
○ KVA denotes the apparent power. It is the
vector sum of reactive power and actual power.
As mentioned above, the power
factor is equal to the working power and apparent power.
Power Factor= KW / KVA
Therefore, we can say that the
value of power factor is always less than one and depends upon the value of
KVA. The more the value of KVA, the less will be the value of power factor.
To determine the value of KVA
we use the right angle rule. It is square root of the sum of KW and KVAR.
KVA= Sqrt (KW2 + KVAR2)
To increase the power factor,
we need to reduce the amount of required KVA. To do so, we have to shorten the
line of KVAR, and this work is done by introducing a capacitor. The capacitor
subtracts the extra KVAR and improves the power factor.
These capacitors are available
on the market based on how much KVAR value you need to reduce to get the
desired power factor.
Let we take a look how the
calculation of the correction is done.
First of all we need to
measure the amount of apparent power or KVA required for the transformer to
work. After getting this value, you need to determine what the power factor of
the transformer is and how much power factor you want to get.
Suppose, your transformer
requires 560 KVA to operate and it has the power factor 0.55. You want to
increase this power factor to 0.95.
You need to use the formula
given below to determine the value of KVAR.
KVAR = KVA*(1-pf2)
Now, follow the three steps
given below to determine how much KVAR value you need to reduce.
○ Calculate the value of KVAR using value
of the present power factor.
KVAR1 = 560*(1-0.552)
KVAR1 = 467.
○ Calculate the value of KVAR using desired
value of power factor.
KVAR2 = 560*(1-0.952)
KVAR2 = 175
○ Now, subtract KVAR1 by KVAR2. You will
get 292 KVAR.
You can use this value to
purchase the exact capacitor so as you would get the desired value of power
factor.
You can also use the power factor table to determine the required reduction
in KVAR. Follow the steps given below to use this table.
○ Determine the original power factor of
the transformer and also the desired power factor.
○ Get the value in the table where the row
of original power factor and column of desired power factor meet.
○ Determine the value of KW. If you have
KVA value, divide the KVA by the original power factor to get KW.
○ Now multiply KW with the value you have
gotten in the table. This is the KVAR that is required to reduce.
○ The power factor may reduce if the
transformer works under a lighter load. Moreover, the efficiency of the
transformer reduces if it is used below or above its load range, but lighter
load affects it more. You have to operate the transformer under the load range
for which it is made. It will increase the power factor and service life of the
transformer.
○ You have to operate the transformer at
rated voltage. If you don’t follow this instruction, the service life of the
transformer will definitely be reduced and the power factor, too. Using the
transformer at rated voltage would increase the power factor with time.
This is how the transformer
power factor calculation and correction is done to increase efficiency. Use of
a capacitor is a very effective way of correcting or improving the power factor
of transformers and other electrical equipment. This method is cheap; however,
it should be done only by a professional electrician. You should always operate
the transformer at the rated voltage and load range to avoid any reduction in
service life and power factor.