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:
 For Existing Installations,
 For New Designs.

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:
 Preliminary
Evaluation Phase,
 Design Phase.
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

Step#3: Fill the Economic Screening
Worksheet

 This
step is used to determine the need for power factor correction capacitors for
existing installation by determination of the approximate savings possible
through power factor penalty reduction and loss reduction. Then the savings
are compared with the probable cost of capacitors to determine if it is
economical.
How it works?
 The
worksheets assume that the capacitors will completely eliminate the monthly
power factor penalty and that they will be distributed throughout the plant
in such a manner that the maximum possible loss savings is achieved.
 Then,
using the desired time period and interest rate (selected by the
purchaser), the annual power factor penalty savings and loss savings are
converted to their equivalent present value for comparison with the capacitor
installation cost.
To
be economical, we must have:
The equivalent present
savings ˃ the cost of the
capacitors
 If
it is economical, we will proceed to the design phase. If not, we can
stop/abort the process or continue with the design phase knowing that the
installed PFCC isn’t economical.
Worksheet Construction
The
economic screening worksheet is actually made up of (4) smaller worksheets as
follows:
 The
Cover showing the name of the worksheet and its usage, see Fig.3.
 The
Capacitor Costs Worksheet, which is used to Estimate Total Capacitor Cost, see
Fig.4.
 The
Loss Savings Worksheet, which is used Estimate loss savings, see Fig.5.
 The
Power Factor Penalty Savings Worksheet, which is used to Estimate power
factor penalty savings, see Fig.6.
 The
Economic Evaluation Worksheet, which is used to Evaluate economics and
calculate the payback, present worth and benefit/cost. see Fig.7.
Fig.3: The Cover Worksheet
Fig.4: The Capacitor Costs
Worksheet
Fig.5: The Loss Savings
Worksheet
Fig.6: The Power Factor
Penalty Savings Worksheet
Fig.7: The Economic
Evaluation Worksheet
Important
notes for Loss Savings Worksheet
Losses
are estimated by summing estimates of the transformer and cable losses.
Losses = Transformer
losses + cables losses
1 Transformer losses:
 Transformer
losses are straightforward. The impedance can be determined from the
nameplate and the current magnitude can be determined by measurements or
existing panel meters.
 Transformers
within the plant should also be accounted for in the loss calculation.
However, the facility service entrance transformer should not be included if
it is on the utility side of the meter because reductions in transformer
losses would not benefit the end user. Include only those stepdown
transformers on which loadside capacitors will be installed. A primaryside
capacitor will not reduce the transformer's currents and therefore, does not affect the losses. The average kW losses are estimated
for each transformer that fits the criteria and then multiplied by 730 (avg.
no. of hours in a month) to determine the monthly kWh losses
2 Cables losses
 The
cable losses are difficult to accurately quantify because cable sizes and lengths vary widely. The cable loss formula used in the
worksheet is based on the empirical observation that with cable sizes and
lengths typical of industrial systems, cable losses would be about 2% if all
cables were operated near rated cable ampacity.
 Assuming
that the cables were loaded to ampacity. Then the losses are referred to the
total load that this current would yield. In both cases, the losses would
have been approximately 2%. Thus, this was chosen as a round number for
estimating purposes. Multiplying this by the square of the actual average per
unit loading yields a number close to the actual losses. These assumptions
make the estimated cable loss savings very approximate, but a more precise
calculation would be too cumbersome for a hand calculation and require a
detailed plant survey.
 The
worksheet is designed to be used with data that can be obtained through
relatively simple measurements. The percent loading of the cable is designed
to be estimated from rms current measurements of the main feeder cables. The
recommended procedure is to measure the average current flowing in several
important cables and compare that to the ampacity of the cable. The ratio of
average current to ampacity is entered into the Loss Savings Worksheet on
line (b). Obviously, some cables will be more heavily loaded than others. The worksheet is intended to use an
average loading level for the plant. It should be possible to make this
judgment by determining the currents in a few of the main cables.

Again we will explain How
to calculate the average power factor for an existing installation which will
be needed in step a in the Capacitor Costs Worksheet.
How to calculate the average power
factor for an existing installation?
 The power factor used in billing is generally an average power factor determined over the entire month, although a few utilities bill intervalbyinterval.
 There are two usual procedure for determining the
power factor for existing buildings by using one of the following measuring
devices:
 the kilovar hours (kvarh) meter as well as the
kilowatthours (kWh) meter,
 A clamp on power factor meter.
Method#1: The kilovarhours (kvarh) meter as well
as the kilowatthours (kWh) meter.
 This may be done by two separate meters or may be
contained within one electronic meter. The kvarh are then combined with the
kWh to obtain an equivalent kilovoltamperehours (kVAh):
kVAh = Ö( kWh^{2} + kvarh^{2} )
The average power factor is then:
PF = kWh / kVAh
 The kvarh meter is usually “detented” so that it only records lagging vars;
that is, the vars drawn by motors. No credit is given for leading vars.
Note:
 Many utilities are now considering billing for kvarh
similarly to kWh. Existing meter technology can separately track leading and
lagging kvarh. This provides the opportunity to have flexible rate structures
to create more incentives for industrial end users to control var consumption
and production.
Method#2: A clamp on
power factor meter
 Since each load has its own
power factor, the measurements should start with each individual machine and
move upward to each distribution panel and finish at the feeder and then to
transformer as shown in the Fig.8 below.
Fig.8
 Measuring power factor is a
costly procedure when it is required to shut the load down and connect in a
metering system to measure the current, voltage and power. So as to avoid the
costly shutdown and time consuming measurement, it is preferable to use a clamp
on power factor meter.
 To connect the meter, the
voltage leads are first connected to the meter and then clipped to the phases
supplying the load. The clampon current transformer is then clamped on to
the phase supplying the load. To select the appropriate clamp on CT, a
conventional clamp tester is used to measure the load current. The voltage is
also measured. Now using the clampon, power factor meter with appropriate
CT, the power factor reading is noted.

Example#1 for Economic
Screening Worksheet
 A food processing
facility fed at 480 V through a primarymetered 5 MVA transformer. The
facility consists of three main processes which run independently. The kW
level at each plant does not vary significantly. Thus, the overall kW level
tends to be relatively constant for extended periods of time, with
significant changes occurring only when a plant is started up or shut down.
 The typical peak demand at present is 1000 kW, which is much less than the 5
MVA transformer capacity.
 The system originally
had considerably more compressors than it currently does. Fig.9 shows
a simplified oneline diagram of the facility. The capacitor locations
selected are shown on the diagram.
Fig.9: Single Line Diagram for a Food Processing
Facility
Preliminary Evaluation
The Economic Screening Worksheets for the preliminary evaluation of this site
are filled out for this case as an example and shown in below. The
information known about the plant before the detailed plant survey was that:
 The average power factor
was about 78% and the desired power factor is 0.95.
 The typical demand was
approximately 1000 kW.
 Assumed Cost/kvar for
fixed capacitors $30 per kvar.
 No harmonic study
required.
 Billed demand 1240 KW
and the actual demand 1020 KW from the bills.
 The billing schedule
did not have a demand charge specifically identified, but an equivalent
charge of approximately
$2.90 per kW was derived from one of the bills by computing the bill assuming
the power factor was corrected.
 Monthly
energy usage from the bill is 482900 KWH
 Cable
Capacity Factor to be used as 0.2
 The per
unit resistance from the transformer nameplate is 0.00966.
 Note that the losses
were evaluated at the marginal energy rate of $0,007 per kWh, which is the
lowest rate at the highest level of energy usage. This is the proper value
for evaluating the effect of loss reduction because the losses are subtracted
out at the margin.
 The used fixed
capacitors will be studied for 3 years and 7% interest rate.
Solution
The solution by using
our Economic Screening Worksheet is shown in below images:
Results
The worksheets indicate
that a relatively simple fixed capacitor installation that costs
approximately $30 per kvar should be quite economical based on a three year
evaluation period and interest rate 7%. So, we can proceed to the design
phase and The simple payback period is predicted to be about 1.8 years.

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In the
next article, we will continue explaining other steps
for Power
Factor Correction Capacitors Sizing Calculations for Existing Installations. Please,
keep following.
The
previous and related articles are listed in below table:
Subject Of
Previous Article

Article

 Glossary of Power Factor Correction Capacitors


 Types of Loads,
 The Power
Triangle,
 What is a power
factor?
 Types of power factor
 Why utilities
charge a power factor penalty?
 Billing Structure.


 What causes
low power factor?
 Bad impacts
of low power factor,
 Benefits of
Power Factor correction.


 How to make
Power Factor Correction?
 Types of
Power Factor Correction Capacitors
 Individual compensation


 Group compensation,
 Central compensation,
 Hybrid compensation.
 Summary for Power Factor Correction
Capacitors Sizing Calculations Steps


 Step#1: Collect Monthly Billing Data
 Step#2: Make Some Preliminary
Measurements For Current And Voltage


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