Today, we will continue
explaining Power
Factor Correction Capacitors Sizing Calculations Steps in detail
as in the previous article “Power Factor Correction Capacitors Sizing Calculations – Part Five”, we only listed the Power Factor Correction Capacitors Sizing Calculations
Steps.
So, today
we will continue explaining other steps for Power Factor Correction Capacitors Sizing
Calculations For Existing Installations.
4 Power Factor Correction Capacitors Sizing Calculations Steps 
Now,
we are going to explain the Power Factor Correction Capacitors Sizing
Calculations Steps for Different Cases of Installations:

4.1 Power Factor Correction Capacitors
Sizing Calculations Steps
For Existing Installations

the
Power Factor Correction Capacitors Sizing Calculations Steps For Existing
Installations include two phases as follows:
Fig.1 shows the phases of
Power Factor Correction Capacitors Sizing Calculations For Existing
Installations.
Fig.1

First: Preliminary Evaluation Phase

The
preliminary evaluation is performed to determine if the application of power
factor correction capacitors is likely to be economical or not. Fig.2 shows
a typical flow chart
for the preliminary evaluation process.
Fig.2

Second: Design Phase

In
this phase, we will design and select the best economical, efficient and
applicable power factor capacitor scheme for an installation. Fig.3
shows a typical flow chart for the Design process.
Fig.3

Step#2: Select
Economical Capacitor Scheme

How to Develop a Fixed Capacitor Scheme?
There are several strategies you may
employ for developing a fixed capacitor scheme:
First scheme:
Sum the required amount of capacitors at the main bus (see fig.4)
This will eliminate the power factor
penalty, but will not reduce the losses within the plant. One lumped bank is
also the configuration most affected by harmonic resonance because there is
less damping than when the capacitors are distributed throughout the plant.
Fig.4
Please check the title "Central Compensation"
in article "Power Factor Correction Capacitors Sizing Calculations – Part Five".
Second scheme:
Distribute the capacitors to the motor control centers and sub panels proportionally
to average load (see fig.5)
Lacking better information, this
will generally achieve a good capacitor scheme with respect to losses,
although, it may be not optimal.
Fig.5
Please check the title "Group Compensation" in
article "Power Factor Correction Capacitors Sizing Calculations – Part Five".
Third scheme:
Distribute the capacitors using motor sizes and the ANSI/NEMA tables (see
fig.6)
This procedure is acceptable, but
does not reflect the need for more released capacity or loss reduction in
another part of the plant. Capacitors sized for small motors are often
proportionately much more expensive than larger fixed capacitors because of
installation costs. We found in the case studies that for moderately to
heavily loaded systems, most of the loss savings are achieved in the cables
and transformers feeding the motor control centers. Therefore, the procedure
here begins with a fixed Capacitor installation at the motor control centers.
Then, if there is a need to switch the capacitors, we will consider
installing them on motors. Of course, capacitors on larger motors are often
as economical as fixed banks of similar sizes.
Fig.6
Please check the title "Individual
Compensation" in article "Power Factor Correction Capacitors Sizing Calculations – Part Four".
Fig.7
Please check the title "Hybrid Compensation" in
article "Power Factor Correction Capacitors Sizing Calculations – Part Five".
How to select the best Economical Capacitor
Scheme?
Note:

Step#3:
Checking the "No Load" Voltage Rise

The limit on the steady state voltage is generally about 110%.
%ΔV = (Kvar x %Z) / KVA
Example#1:
Calculate the voltage rise that will
result from applying a 350 kvar capacitor at the secondary of a 1000 kVA
transformer with an impedance of7%.
Solution:
%ΔV = (Kvar x %Z) / KVA
%ΔV = 350 x 7 / 1000 = 2.45%
Also, from the following curve (fig.8)
you can find the % voltage drop for any power factor improvement.
Fig.8
Example#2:
Improve power
factor from 60 percent to 90 percent, calculate the reduction in voltage
drop.
Solution:
From the above
curve, at PF = 0.6 the, Voltage drop = 5.1%
And at PF =
0.9, the voltage drop = 3.6%
So, the
reduction in voltage drop = 5.13.6 = 1.5%

Step#4: Select
Capacitor Switching Options

Two types of capacitor switching
schemes are available as follows:
You can review article " Power Factor Correction Capacitors Sizing Calculations – Part Four" to know when and why we need to use one of
the above switching schemes.

Step#5: Check
the Harmonic Distortion and make Harmonic Mitigation Options

General Rule
If the estimated current in the
capacitors exceeds 135% rms or the bus voltage exceeds 5% THD, some form of
mitigation is required.
Harmonic Mitigation Options
Often, the harmonic currents are high simply
because the optimal capacitor value with respect to power factor and losses
exactly tunes the system to some undesirable harmonic. To Mitigate the harmonics, you need to use one of the following options:
First
Mitigation option:
Second
Mitigation option:
Third
Mitigation option:
Fourth Mitigation option:
How to select between different Mitigation
options?
Now the evaluation becomes an
economic tradeoff between the above options and you need to select the
economic solution taking into consideration the following notes:

Step#6: Use the
Economic Screening Worksheet again

This step is used to evaluate the
economics of the selected capacitor scheme by using the Economic Screening
Worksheet explain before in article " Power Factor Correction Capacitors Sizing Calculations – Part Seven", then:
How to Develop Lower Cost Capacitor Scheme
Solutions?
Solution#1:
Check with power factor capacitor and filter suppliers to
learn if there are more economical combinations that have not been
considered.
Solution#2:
It may be possible that by sacrificing
some power factor correction or loss improvement, a much less costly
installation may be achieved.
Solution#3:
it may be economical to install a few
large capacitor banks while it is not economical to install several smaller
ones distributed throughout the plant.
Solution#4:
it may be possible to combine several
capacitors under one power factor controller rather than having several controllers.
Solution#5:
Consider putting the capacitors at a
higher voltage level, if available. Many industrial plants have 4 to 15 kV
circuits as well as 480 volt circuits. Of course, there is considerably more
energy to be saved on the 480 volt circuits, but the capacitors and filters
on the higher voltage systems are often much lower per unit cost. At least,
the power factor penalty may be saved.
Solution#6:
Look at the root cause of the problem. If
it is harmonics, is it possible to mitigate the harmonics in some other way?
Solution#7:
Can the capacitors be tied in with other
plant improvements that would make it economical to install a practical
scheme?
Solution#8:
stepdown transformers that are too small may be the root cause of having to switch the capacitors because of the no
load voltage rise. Consider upgrading the transformer.

In the next article, we will start explaining steps for Power Factor Correction Capacitors Sizing Calculations for New Installations. Please, keep following.
The
previous and related articles are listed in below table:
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