Today, we will continue
explaining the technical part for Power
Factor Correction Capacitors Sizing Calculations. We will explain the following
topics:
- How to make Power Factor Correction?
- Types of Power Factor Correction
Capacitors
1- How to make
Power Factor Correction?
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Power factor correction can be made in two ways:
- Reduce the amount of reactive energy,
- Compensate artificially for the consumption of reactive energy.
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1.1 Reduce the amount of
reactive energy
- Avoid supplying equipment with voltage in excess of the rated
voltage;
- Use the highest-speed motor that an application can accommodate.
Two-pole (nominal 3600 rpm) motors have the highest power factors. Note that
power factor decreases as the number of poles increases;
- Eliminate unloaded motors and transformers. Size motors as close
as possible to the horsepower demands of the load. A lightly loaded motor
requires little real power. A heavily loaded motor requires more real power.
Since the reactive power is almost constant, the ratio of real power to
reactive power varies with induction motor load, and ranges from about 10
percent at no load to as high as 85 percent or more at full load (see Fig.1).
An oversized motor, therefore, draws more reactive current at light load than
does a smaller motor at full load;
Fig.1
- Minimize the need for cycling loading of motors because the Low
power factor results when motors are operated at less than full load. This often
occurs in cyclic processes (such as circular saws, ball mills, conveyors,
compressors, grinders, extruders, or punch presses) where motors are sized
for the heaviest load. In these applications, power factor varies from moment
to moment. Examples of situations include a surface grinder performing a
light cut, an unloaded air compressor, and a circular saw spinning without
cutting.
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1.2 Compensate artificially
for the consumption of reactive energy
This can be done using one of the following equipment:
- Power Factor Capacitors,
- Rotary Machines.
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1.2.A Power Factor Capacitors
- By nature of its
electrostatic field, the capacitor stores energy whenever the voltage applied
across the capacitor is moving away from zero; it gives up energy after the
voltage has crested. This sequence is opposite to that of the magnetic field,
so the capacitor can be used to supply magnetizing current that would
otherwise be drawn from the utility source.
- Power factor correction is achieved by the addition of
capacitors in parallel with the connected motor circuits and can be applied
at the starter, or applied at the switchboard or distribution panel.
- Capacitors connected at each starter and controlled by each
starter is known as "Static Power Factor Correction" while
capacitors connected at a distribution board and controlled independently
from the individual starters are known as "Bulk Correction".
- Power capacitors serve as leading reactive current generators
and counter the lagging reactive current in the system. By providing reactive
current, they reduce the total amount of current your system must draw from
the utility.
- Capacitors are
generally the most economical source of reactive compensation. Other
advantages include:
- Low losses (less
than ¼ watt/KVAR),
- Essentially low
maintenance,
- Light, compact units which can be
combined as needed, make capacitors relatively easy to install and modify as
reactive compensation need change.
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1.2.B Rotary Machines
Such as phase advancers, synchronous machines and synchronous
condensers.
1.2.B.1
Synchronous Machines
Both synchronous
motors and generators can provide reactive power by increasing the excitation
field sufficiently. The KVAR available from fully loaded machine depends on
the rated KW and power factor:
KVAR = KW. Sin (acos (pf))
More KVAR is
available if the machine is not fully loaded. For example, A 1.0 PF, 100 KW
motor can provide 0 to 30 KVAR from full load down to no load by operating in
a leading mode.
Where auto-synchronous motors are employed, the power
factor correction may be a secondary function.
1.2.B.2 Synchronous Condensers
A synchronous condenser (see Fig.2)
is essentially an unloaded motor whose sole task is to provide reactive
power. Synchronous condensers are continuously variable within wide limits to
generate or consume KVAR. Due to high initial costs, losses, and maintenance
costs, synchronous condensers are not generally used for power factor
correction unless their voltage stabilizing effects and influence on the
short circuit capacity are needed. However, they do have the advantage that
they do not cause harmonic resonance as capacitors sometimes. Therefore, they
are used in certain difficult situations where the extra costs are
justifiable.
Fig.2
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Table-1 shows a Comparison between Rotary machines and Power
factor Capacitors.
Rotary machines
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Power factor Capacitors
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High initial cost makes its use
uneconomical, except where one is using rotating plant for a dual function:
drive and power factor correction
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the initial cost is very low
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the wear and tear inherent in all
rotating machines involves additional expense for upkeep and maintenance
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Upkeep and maintenance costs are minimal
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They
are used in certain difficult situations where the extra costs are
justifiable.
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they can be used with the same high
efficiency on all sizes of installation
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They
are not generally used for power factor correction unless their voltage
stabilizing effects and influence on the short circuit capacity are needed.
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They are compact, reliable, highly efficient & convenient
to install and lend themselves to individual, group or automatic method of
correction.
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They
do not cause harmonic resonance.
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Sometimes cause
harmonic resonance.
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Table-1
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Notes for This Course
- In this course, we will concentrate
on power factor correction using capacitors only. Other methods for power
factor correction will be explained later in other courses.
- The static VAR compensators used for providing fast acting
reactive power on high voltage transmission systems to regulate the
transmission voltage or to improve power factor for large industrial loads
are not included in this course.
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2- Types of Power
Factor Correction Capacitors
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There are
many types of Power Factor Correction Capacitors which can be categorized
according to the following:
- According to complexity
of control,
- According to
location.
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2.1
According To Complexity of Control
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Two types of compensation shall be adopted depending on the
complexity of control as follows:
- Single switching control,
- Selective switching
control.
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2.1.A Single
Switching Control
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In this case a fixed compensation is used by connection of a
fixed-value capacitor bank; this arrangement uses one or more capacitor(s) to
provide a constant level of compensation. Control may be:
- Manual: by circuit-breaker or load-break switch,
- Semi-automatic: by contactor,
- Direct connection to an appliance and switched with it.
These capacitors are applied in the following:
- At the terminals of inductive loads (mainly motors),
- At bus bars supplying numerous small motors and inductive appliances
for which individual compensation would be too costly,
- In cases where the load factor is reasonably constant.
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2.1.B
Selective
Switching Control
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Selective Switching
Control can be categorized to two types:
- Automatic compensation,
- Dynamic compensation.
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2.1.B.1
Automatic
Compensation
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- In most
installations there is not a constant absorption of reactive power due to
working cycles of machines with different electrical characteristics, these
fluctuating loads result in fluctuating power factor.
- In such
installations, automatic power factor correction systems which use monitoring
devices and power factor regulators to allow the automatic switching of
different capacitor banks, thus following the variations of the absorbed
reactive power and keeping the power factor of the installation constant.
Automatic Compensation
Main Parts
An
automatic compensation system is formed by (see Fig.3):
- Some
sensors detecting current and voltage signals;
- An
intelligent unit which compares the measured power factor with the desired
one and operates the connection and disconnection of the capacitor banks with
the necessary reactive power (power factor regulator);
- An electric
power board comprising switching and protection devices;
- Some
capacitor banks.
Fig.3
- Automatic compensation can be done by connection of different number
of capacitor steps, allowing the adjustment of the reactive energy to the
requested value.
- Automatic compensation is applied at points in an installation where
the active-power and/or reactive power variations are relatively large, for
example:
- At the busbars of a main distribution switch-board,
- At the terminals of a heavily-loaded feeder cable.
- Control of Automatic compensation is usually provided by contactors.
For compensation of highly fluctuating loads, fast and highly repetitive
connection of capacitors is necessary, and static switches must be used.
Advantages of Automatic Compensation:
- Regulation of power Factor to the requested value.
- Better utilization of the capacitor compared to individual
compensation.
- Eliminates the possibility of overcompensation.
- Possibility to extend existing banks without changes in the control
equipment.
Disadvantages of Automatic Compensation:
- Insensitivity to harmonics.
General Notes for Automatic Compensation:
The major important points in design of Automatic compensation are:
- Where the KVAR rating of the capacitors is less than, or equal to
15% of the supply transformer rating, a fixed value of compensation is
appropriate. Above the 15% level, it is advisable to install an automatically-controlled
bank of capacitors.
- Choice of regulator characteristics (c/k and tolerance) and step
size to avoid hunting.
- Correlation between minimum interval between switching and discharge
device of capacitor to avoid energizing a charged capacitor.
- Disconnecting of all steps in case of a mains outage.
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2.1.B.2 Dynamic
Compensation
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- This kind of compensation is requested when highly fluctuating loads
are present, and voltage fluctuations should be avoided.
- The principle of dynamic compensation is to associate a fixed
capacitor bank, an electronic VAR compensator and thyristors switching
modules to quickly connect or disconnect capacitors or inductors providing
either leading or lagging reactive currents.
- The result is a continuously varying and fast compensation,
perfectly suitable for loads such as lifts, crushers and spot welding.
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2.2
According
To Location
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- The location of low-voltage capacitors in an installation
constitutes the mode of compensation; Individual, Group, Central or Hybrid
compensation.
- There are
no general rules applicable to every type of installation and, in theory,
capacitors can be installed at any point, but it is necessary to evaluate the
relevant practical and economic feasibility.
- In principle, the ideal compensation is applied at a point of
consumption and at the level required at any instant.
- The successful operation
of a power factor correction depends largely on the correct positioning of
the capacitors in the network.
- The place for connection of capacitor banks in the electrical
network is determined by:
- Global objective (avoid penalties on reactive energy, relieve of
transformer or cables, avoid voltage drops and sags),
- Operating mode (stable or fluctuating loads),
- Foreseeable influence of capacitors on the network characteristics,
- Installation cost,
- Tariff in force,
- Metering point location;
the physical location of the utility meter should be determined since all
power capacitors must be installed “downstream” of the meter.
- Details of light, average
and full load KVA, KW and power factor,
- Position of motors,
welding equipment, transformers or other equipment causing bad power factor,
- Supply system problems
such as harmonics.
According to
the location of the capacitors, the main methods of power factor correction
are:
- Individual compensation,
- Group compensation,
- Central compensation,
- Hybrid compensation.
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2.2.A
Individual
Compensation
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- This
type of compensation has many other names like Fixed, static, single or
distributed compensation.
- Individual compensation
provides a constant amount of reactive power to compensate for the poor power
factor at the level of each machine (see Fig.4). This is the technical ideal configuration, as the reactive energy
is produced exactly where it is needed, and adjusted to the demand.
- This configuration is well adapted when the load power is
significant compared to the subscribed power (for example: motor load >
50kW).
Fig.4
- Fixed
capacitors are suitable for indoor or outdoor use. Fixed capacitors are
available in low voltages (832 volt and below), from 0.5 KVAR up to 400 KVAR
(If more than 400 KVAR is required, smaller units are paralleled together).
Applications of Individual compensation:
- Individual power
factor correction is advisable in the case of large electrical equipment with
constant load and power and long connection times and it is generally used for
motors and fluorescent lamps.
- To compensate the
no-load reactive power of transformers. Fixed compensation of transformers (+/-10 % of
transformer rating).
- For Individual
compensation of large motors (>50kW = 70hp).
- For drives in
continuous operation.
- For drives with
long power supply cables or cables whose cross section allows no margin for
error.
Notes:
- Individual
correction is commonly applied by using one contactor to control both the
motor and the capacitors. It is better practice to use two contactors, one
for the motor and one for the capacitors. Where one contactor is employed, it
should be upsized for the capacitive load. The use of a second contactor
eliminates the problems of resonance between the motor and the capacitors.
- Where
not to use fixed Power Factor Correction:
- Inverter: fixed Power
factor correction must not be used when the motor is controlled by a variable
speed drive or inverter.
- Solid
State Soft Starter: fixed Power Factor correction capacitors must not be connected to
the output of a solid state soft starter. When a solid state soft starter is
used, the capacitors must be controlled by a separate contactor, and switched
in when the soft starter output voltage has reached line voltage. Many soft
starters provide a "top of ramp" or "bypass contactor control"
which can be used to control the power factor correction capacitors.
Advantages of Individual
Compensation
- Complete
control. Capacitors don’t cause problems on the line during light load
conditions;
- No need for separate
switching. The motor always operates with its capacitor; capacitor
and load can use the same protective devices against over currents and are
connected and disconnected simultaneously.
- Improved
motor performance due to reduced voltage drops;
- Motors
and capacitors can be easily relocated together;
- Easier
to select the right capacitor for the load;
- Increased
system capacity.
- Capacitors
installed near the loads in a plant provide spot delivery of magnetizing
current (KVAR) just at the load, which eliminates unnecessary reactive
current in the feeder lines thereby reducing the line losses,
- This
is one of the most economical and efficient way of supplying KVARs, which
relieves both you and your utility of the cost of carrying this extra KVARs
load.
- Low
costs per KVAR
Disadvantages of
Individual Compensation
- Large
number of capacitors may be needed for individual motor correction,
increasing the installation costs ($ per KVAR compensation).
- Also
overload relay settings need to be changed to account for lower motor current
draw. If the capacitors are installed between the contactor and the overload
relay, the overload relay can be set for nameplate full load current of
motor.
- The capacitor is
only utilized during the time that its associated consumer is in operation. A
larger overall capacitor power rating is required as the coincidence factor
cannot be taken into account,
- It is not always
easy to install the capacitors directly adjacent to the machines that they
compensate (space constraints, installation costs).
- The power factor
correction system is distributed throughout the entire facility
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2.2.A.1
Methods of wiring the Individual power factor correction to Motor
Circuits
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Figure
shows the common connection diagrams for the power factor correction of motors,
which are (see Fig.5):
- Option#1:
On the secondary of the overload relay,
- Option#2:
Between the contactor and the overload relay,
- Option#3:
Between the circuit breaker and the contactor.
Fig.5
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Option#1:
On the secondary of the overload relay
This
method of installation commonly referred to as “at the load” or “motor
switched” When the capacitors are installed directly at the induction motor
terminals (on the secondary of the overload relay), the capacitors are turned
on and off with the motors, eliminating the need for separate switching
devices or over current protection. The capacitors are only energized when
then motor is running (see Fig.5).
Advantages
of Option#1:
- The
recommended location for new and existing motors; these capacitor ratings
normally correct the motor no-load power factor to unity which in turn
generally results in a full-load power factor of 94%-96%.
- This
is the most efficient location since the reactive power (KVAR) is produced at
the same spot where it is consumed.
- Line
losses and voltage drop are minimized.
- The
capacitor is switched automatically by the motor starter, so it is only
energized when the motor is running.
- No
separate switching device or overcurrent protection is required because of
the presence of the motor starter components.
Disadvantages
of Option#1:
- Care
must be taken in setting the overload relay since the capacitor will bring
about a reduction in amps through the overload. Therefore, to give the same
protection to the motor, the overload relay's trip setting should be
readjusted or the heater elements should be resized.
- After the
disconnection from the supply, the motor will continue to rotate (residual
kinetic energy) and self-excite with the reactive energy drawn from the
capacitor bank, and may turn into an asynchronous generator. In this case,
the voltage on the load side of the switching and control device is
maintained, with the risk of dangerous over voltages (up to twice the rated
voltage value).
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Option#2:
Between the contactor and the overload relay (see Fig.5)
This
installation method is normally preferred by motor control center and
switchgear builders since the overload setting is simplified.
Advantages of
Option#2
- The
advantages are the same as Option#1.
Disadvantages
of Option#2
- Disadvantages
are the same as Option#1 except the overload relay can now be set to the full
load amps as shown on the motor nameplate.
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Option#3:
Between the circuit breaker and the contactor
(see
Fig.5)
- The
compensation bank is connected only after the motor has been started and
disconnected in advance with respect to the switching off of the motor
supply.
- Where
there are multiple motors with low horsepower ratings, or motors which do not
run continuously, the capacitors should be connected directly to feeders in
the facility through an appropriate switching device to serve as a disconnect
for servicing, or light loads. Locations should be as far downstream in the
facility as possible for maximum benefit.
Advantages
of Option#3
- Since
the capacitor is not switched by the contactor, it can act as a central kvar
source for several motors fed by the same circuit breaker.
- This
location is recommended for jogging, plugging and reversing applications.
Disadvantages
of Option#3
- There
is a risk that the capacitor remains energized even when the motor or motors
are not running, there exists the possibility of overcorrection and leading
power factor during lightly loaded periods. Losses are higher than with
Options#1&2 as the reactive current must be carried further.
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General
Notes:
- Installations
may be made at load centers when it is difficult to connect the capacitors
directly across motor terminals or to feeders. Again, switching is a
recommended practice.
- If
only power bill penalties are to be offset, the total capacitor requirement
can be installed on the load side of metering equipment. Such a location does
not increase the capacity of the facility distribution system.
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Power Factor Correction
Capacitor connection locations with different motor starter
types (Auto-transformer, part-winding, wye-delta) and with multi-speed
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Fig.6 in Below
show the wiring diagrams of the Power Factor Correction Capacitor connection
locations with
different motor starter types (Autotransformer, part-winding, wye-delta) and
with multi-speed.
Fig.6
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In the next article, we will continue
explaining other Types of Power Factor Correction Capacitors. Please,
keep following.
The
previous and related articles are listed in below table:
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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