So, today
we will continue explaining other steps for Power Factor Correction Capacitors Sizing
Calculations For Existing Installations:
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4- Power Factor Correction Capacitors
Sizing Calculations Steps
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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.
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4.1 Power Factor Correction Capacitors
Sizing Calculations Steps
For Existing Installations
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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
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First: Preliminary Evaluation Phase
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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
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Step#4: Make Preliminary Measurements For
Harmonics
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- This
step is used to help identify and report potential harmonic problems.
- 20% of industrial plants cannot operate
capacitors without some careful attention to the harmonics. This means that
80% of industrial plants will not have problems, but it is wise to study any
possibility for harmonic effects in the initial evaluation.
Measuring Harmonics
- The measuring instrument used to make
the preliminary measurements in step#2 should have the capability to report
harmonics as well.
- The key things to determine by measurements with respect to harmonic
distortion are:
- Main bus voltage distortion; Measure
the THD including all harmonics and, also, individual harmonic magnitudes in
percent, of fundamental.
- Harmonic content of load currents, particularly those in suspected
harmonic-producing loads such as adjustable-speed motor drives. The THD and rms values are needed as
well as magnitudes of individual harmonics.
- Representative harmonic current in
existing capacitor banks, at least the THD and rms values.
Analysis of Harmonics Measurements
Based on the above harmonics
measurements, a harmonics study will be required if any of the following
conditions are found:
- Harmonic-producing devices (such as
adjustable-speed drives and arc furnaces) are a large portion of the load,
- The main bus voltage distortion is greater than 2%,
- The distortion of the total plant current
is greater than 10%.
Important Notes:
- As you take the power intervals, take
periodic harmonic samples. If the load varies considerably, such as rolling
mills, veneer lathes, etc., take several measurements to try to capture the
worst case.
- If practical, measure the distortion of currents in existing capacitor banks.
Existing capacitors are the best locations to find harmonic currents that
might indicate that there is a harmonics problem. Unfortunately, capacitor
leads are not always easily accessible. However, it is strongly recommended
that you open one or two cabinets and measure the current. If you do not have
a meter capable of showing the individual harmonic, measure the rms current
with a suitable true rms ammeter. If the rms current measured is much higher
than the expected rated current, there is likely to be a harmonics problem.
- Lack of high current in capacitors does not indicate that there will be no
problems after the addition of capacitors. it only indicates that there are
no harmonics now.
- The instruments for monitoring harmonics range from permanently installed
3-phase monitors to hand-held single-phase devices. Most of the modern ones
are capable of transmitting data to a computer for further analysis.
- While there are
numerous instruments available for measuring harmonics, proper harmonic
measurements and the proper interpretation of the results will generally
require someone with special training in this field.
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Step#5: Repeat the Economic Screening
Worksheet
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- This
step is used to include the cost of studies and harmonic mitigation- if it is
required - in the cost of the capacitor costs worksheet.
- In
step#3, we ignore including costs of the harmonic study. Then, if there are significant
harmonic sources or existing distortion, come back to the capacitor costs
worksheet, add in the estimated study cost in step j & cell L23, and re-compute
the economics in the Economic Evaluation Worksheet.
- Harmonic
studies range from $3000 to $7000 for smaller industrial plants. Studies for
large plants might typically be $15000 and up.
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Step#6: Compare the Savings with the
Probable Cost of Capacitors' Installation
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This step is to determine if the
capacitors' installation process - after adding the harmonic study costs -
still economical or not? And we will have two cases as follows:
- Case#1: If the capacitors installation is still economical with
these considerations, you may proceed with reasonable confidence that an
economical and effective capacitors installation can be achieved and you can
proceed with the design phase.
- Case#2: If the capacitors installation is no longer economical
with the harmonic considerations, you have two options:
Option#1: Stop the process if you feel certain that harmonic
mitigation will be needed and not economically justifiable.
Option#2: Proceed uneconomically into the design phase with the hope
that the harmonic problems will not severely impact the installation, which
happens frequently, and with the understanding that it may not be possible to
find a capacitor scheme that is economical according to your criteria.
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Example#1
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 is uneconomical based on a three year evaluation
period and interest rate 7%. So, we can stop the process or proceed
uneconomically to the design phase.
- The simple payback period is predicted to
be about 3.12 years.
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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
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Step#1: Performing a Detailed Plant Survey
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This
step is used to collect sufficient data about the plant to determine the
optimal size of capacitor to place in each location. It includes the
following sub-steps:
- Step#1.A:
Review the one line diagram.
- Step#1.B:
Take into consideration the loads that produce harmonics.
- Step#1.C:
collect sufficient data Inventory by using measuring instruments.
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Step#1.A: Review the
one line diagram
- Review
the one line diagram of the facility to determine the most appropriate form
of power factor correction. The one-line diagram must show the main structure
of the electrical system from the utility interconnection up to at least the
main motor control centers and larger sub panels. You must note the Major types
of loads, step-down transformers, and approximate locations for existing
capacitor banks.
- Figure-4 shows an example of a simple one-line diagram. The main buses
are indicated by the heavy bars and are labeled with the rated bus voltage.
All significant transformers and loads (particularly nonlinear loads) should
be indicated.
- Complete the one-line diagram as you proceed with the detailed load inventory.
Supply the diagram to contractors and engineers you may choose to assist with
the application. This will enhance communications considerably.
Fig.4: Simple One-Line
Diagram
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Step#1.B: Take into
consideration the loads that produce harmonics
- Take
into consideration the loads that produce harmonics or have sophisticated
electronic controls that can interfere with power factor correction
efforts. These loads like:
- Adjustable
speed drives,
- Variable
frequency drives,
- DC
Drives,
- Arc
furnaces,
- large
amounts of fluorescent or sodium-vapor lighting
- Large
UPS systems.
- If
these loads exist in significant quantities in comparison to the size of the electrical
system, a detailed harmonics study should be performed. This is why it is a good
idea to obtain a meter capable of showing harmonic distortion for the initial
plant survey. A quick scan of the bus voltages and a few load or capacitor currents
will provide a good idea of the existence of harmonic-producing loads.
- Also,
if you found one of the following loads:
- frequently-switched major loads,
- greatly varying loads,
In
this case, more engineering studies must be made to ensure successful application
of power factor correction capacitors. Studies typically cost several thousand
dollars and additional costs may be incurred for filters or other specialized
equipment necessary for proper operation of power factor correction capacitors.
Also, We can notice the existing of harmonic problems from
visual inspections as follows:
- An
existing capacitor bank that cannot be used because it either blows fuses or interferes
with some process is almost a certain indication of harmonic problems. It
could also be an indication that the capacitor and/or the associated load is
switched often and might require some special considerations.
- Motors or transformers making unusual high-pitched sounds and lightly-loaded
transformers running much hotter than other similarly-loaded transformers are
signs for harmonic problems.
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Step#1.C: collect
sufficient data Inventory by using measuring instruments
Collect
sufficient data Inventory by using measuring instruments to understand how
the system behaves so that optimal capacitor configurations may be
determined. Sufficient data should be collected to enable a detailed computer
analysis. The below checklists in figure-5 contains all the specific data that
you need to collect.
Fig.5: checklists for initial and detailed
plant survey
The
following Important notes must be taken into consideration:
- Standard utility meters typically accumulate only kWh and kvarh over a month with
a "drag hand" function that records the maximum demand interval.
- For
the purposes of the detailed load inventory, you should obtain an instrument capable
of recording kW and kvar by a demand interval of, for example, 15 or 30 minutes. One type
of portable power profiling instrument is shown in Figure 6. The power profiling instrument for performing a detailed
plant survey should have the following recording capabilities:
- kW, kvar, PF by demand interval.
- Voltage and Current snapshots
- Voltage and Current harmonics; snapshots and trends.
Fig.6: portable power profiling
instrument
- Many
utilities have adopted electronic meters which can provide
you with this data for the whole plant. Some meters not only record the kW
and kvar but can also differentiate between leading and lagging vars, reporting
each for the interval. This is very useful for determining the amount of
capacitance needed and for verifying the effectiveness of capacitors after installation.
- The
utility metering typically collects only average data for the whole plant. While
these measurements some general idea of how much total capacitance is needed,
it provides little help in determining where to put it. For that, you will
need to profile the loads on each major feeder and branch circuit for at least
several demand intervals during typical loads.
- Figure-7 shows a modified version of the one-line diagram of Fig.4 with
the minimum number of metering points required for a detailed plant survey.
Meter at least each of the main feeds to the motor control centers (MCC) as well
as other major sections of the plant, such as the office load. You may extend
the metering farther out into the plant, to obtain the level of detail
desired, as your time and budget allow. It is usually sufficient for the
placement of power factor correction capacitors to meter just the major feeds
within the plant.
Fig.7: Metering
locations for minimum power profiling for power factor correction.
- It would be ideal to collect all this data simultaneously. While there are
some permanently-installed instrumentation systems with this capability, it is
available only in a few industrial facilities. Therefore, you are usually constrained
to using portable instruments that are moved from circuit-to-circuit. You may
connect the metering at any convenient place along a feed. This is usually at
the panel termination on either end. A power profiler must be connected to the three-phase bus voltages and three-phase line currents. This
requires access to terminal lugs for voltage and an unobstructed access to
the cables to install the current transformers.
- Permanently-mounted
monitoring devices may be used, but it is generally sufficient to use portable
meters with clip-on probes. On smaller
cables, the clip-on probes are usually sufficient. For large cables, you must
rely on existing CT's or be satisfied with clipping a probe on one cable in a
parallel bundle and assuming the others are the same.
- The sequence of taking the measurements is generally of secondary importance
unless there are particular load behaviors you are trying to capture.
- The
most important thing is that you set up a system for keeping track of what measurements
were made, when they were made and what was going on in the plant at the
time. Keep a detailed log book. Use features of the profiling instrument for labeling
any output.
- Leave
the instruments in place for several demand intervals whatever it takes to properly
characterize a load. For some loads, this may be as short as one hour or one duty
cycle. For others, it may be necessary to monitor kW and kvar demand for several
days to obtain definitions of the reactive power demand so that power factor
correction can be properly defined.
- Generally, most individual loads are sufficiently consistent that the measurements need only be made through a few typical duty cycles. The main
feeds should be monitored for at least a full typical working day. If practical,
a full cycle of normal days and idle days (a week, 10 days, or whatever)
should be monitored. This could be important later for determining whether or
not to switch capacitors.
- The
data collected are time interval data expressed as either kW and power factor
or kW and kvar demand for that interval, depending on the capabilities of the
instrument.
- The
monitoring should be sufficient to determine at least the load going into
each significant motor control center or subpanel. It is generally not
necessary to monitor each individual motor load. The exception might be for a
case where a few large motors comprised the bulk of a load at a particular motor control center.
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A Simplified method to collect sufficient data Inventory by
using measuring instruments
After you do the initial economic screening, you should have a good idea of what
you can afford to spend on studies, inventories, and the like. When the
budget for power factor correction does not justify a detailed inventory and analysis,
the following are suggestions for less extensive load measurements:
- Monitor
only the main feeds.
- Measure, at least, snapshots of the rms currents in the main cables to get an
idea of how the load inside the plant divides.
- Assume
the overall plant load shape for all loads.
- In lieu of demand interval data, several snapshots at different times will
suffice. Obviously, the more data collected, the more likely it is that the capacitors
will be added to portions of the plant where they will do the most good. The greatest
benefits will be achieved on lines that have high current with respect to ampacity,
poor power factor, and long cable runs. Therefore, make special note of loads
and cables with these characteristics.
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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:
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Subject Of
Previous Article
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Article
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- Glossary of Power Factor Correction Capacitors
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- Types of Loads,
- The Power
Triangle,
- What is a power
factor?
- Types of power
factor
- Why utilities
charge a power factor penalty?
- Billing Structure.
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- What causes
low power factor?
- Bad impacts
of low power factor,
- Benefits of
Power Factor correction.
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- How to make
Power Factor Correction?
- Types of
Power Factor Correction Capacitors
- Individual compensation
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- Group compensation,
- Central compensation,
- Hybrid compensation.
- Summary for Power Factor Correction
Capacitors Sizing Calculations Steps
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- Step#1: Collect Monthly Billing Data
- Step#2: Make Some Preliminary
Measurements For Current And Voltage
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- Step#3: Fill the Economic Screening
Worksheet
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