Different techniques to improve power factor

MD.ASIM IQBAL

RC4903B40

10902821

C4903

Introduction useful work output. The ideal power factor is unity, or one.
Anything less than one
Power factor is a measure of how efficiently, or inefficiently, means that extra power is
that electrical power is used by a customer. For industrial required to achieve the actual task at hand. All current flow
customers, a low power factor is generally caused by causes losses both in the
supply and distribution system. A load with a power factor of
inductive loads such as transformers, electric motors and high-
1.0 results in the most
intensity discharge lighting. Customers that do not use efficient loading of the supply. A load with a power factor of,
electrical power efficiently are being charged additional fees say, 0.8, results in much
for the inefficient use of power by their electric utility higher losses in the supply system and a higher bill for the
company. consumer. A comparatively
small improvement in power factor can bring about a
All inductive loads require two different types of power for significant reduction in losses
the motor to operate: since losses are proportional to the
square of the current.
When the power factor is less than one the ‘missing’ power is
Active power (measured in kW or kilowatts) - this power
known as reactive power
produces the motive force which unfortunately is necessary to provide a magnetizing
Reactive power (kvar) - this energizes the magnetic field of field required by motors and
the motor. other inductive loads to perform their desired functions.
Reactive power can also be
The operating power from the distribution system is composed interpreted as wattles, magnetizing or wasted power and it
of both active (working) and reactive (non-working) elements. represents an extra burden
on the electricity supply system and on the consumer’s bill. A
The active power does useful work in driving the motor
poor power factor is
whereas the reactive power only provides the magnetic field. usually the result of a significant phase difference between the
Unfortunately, electric utility's customers are charged for both voltage and current at
active and reactive power. the load terminals, or it can be due to a high harmonic content
or a distorted current
Example: A customer's power factor drops, the system waveform. A poor power factor is generally the result of an
becomes less efficient. A drop from 1.0 to 0.9 results in inductive load such as an
15% more current being required for the same load. A power induction motor, a power transformer, a ballast in a luminaire,
factor of 0.7 requires approximately 40% more current; and a a welding set or an
induction furnace. A distorted current waveform can be the
power factor of 0.5 requires approximately 100% (twice as result of a rectifier, an
much) to handle the same load. The answer to these problems inverter, a variable speed drive, a switched mode power
is to reduce the reactive power drawn from the supply by supply, discharge lighting or
improving the power factor. other electronic loads. A poor power factor due to inductive
loads can be improved by
Power factor correction the addition of power factor correction equipment, but a poor
power factor due to a
It is the ratio between the useful (true) power (kW) to the total distorted current waveform requires a change in equipment
(apparent) power (kVA) consumed by an item of a.c. design or the addition of harmonic filters. Some inverters are
electrical equipment or a complete quoted as having a power factor of better than 0.95
electrical installation. It is a measure of how efficiently when, in reality, the true power factor is between 0.5 and 0.75.
electrical power is converted into The figure of 0.95 is
based on the cosine of the angle between the voltage and to the supply, the current flowing would be in-phase with the
current but does not take into voltage. This is not the
account that the current waveform is discontinuous and case. The motor has a magnet and the magnetizing current is
therefore contributes to not in phase with the
increased losses. An inductive load requires a magnetic field voltage. The magnetizing current is the current that establishes
to operate and in creating the flux in the iron and,
such a magnetic field causes the current to be out of phase being out of phase, causes the shaft of the motor to rotate. The
with the voltage (the current magnetizing current is
lags the voltage). Power factor correction is the process of independent of the load on the motor and will typically be
compensating for the lagging between 20% and 60% of the
current by creating a leading current by connecting capacitors rated full load current of the motor. The magnetizing current
to the supply. does not contribute to the
work output of the motor. Consider a motor with a current
A sufficient capacitance is connected so that the power factor draw of 10 Amps and a
is adjusted to be as power factor of 0.75. The useful current is 7.5 A. The useful
close to unity as possible. power from the motor is
230X7.5 = 1.725kW but the total power that has to be
Power factor correction is the term given to a technology that supplied is 230X10 =2.3 kVA.
has been used since the Without power factor correction, to achieve the required
turn of the 20th century to restore the power factor to as close output of 1.725 kW (7.5 A) a
to unity as is power of 2.3 kVA (10 A) has to be supplied. A current of 10
economically viable. This is normally achieved by the A is flowing but only 7.5 A
addition of capacitors to the of that current is producing useful output.
electrical network which compensate for the reactive power The power factor can be expressed in two ways:
demand of the inductive 1.Power factor (pf) = Useful power (kW) divided by the
load and thus reduce the burden on the supply. There should total power (kVA),
be no effect on the
operation of the equipment. To reduce losses in the or
distribution system, and to reduce the electricity bill, power 2.Power factor (pf) = The cosine of the angle between
factor correction, usually in the form of capacitors, is added to useful power and total power= cos ø
neutralize as much of the magnetizing current as possible.
Capacitors contained in most
power factor correction equipment draw current that leads the
voltage, thus producing
a leading power factor. If capacitors are connected to a circuit
that operates at a
nominally lagging power factor, the extent that the circuit lags
is reduced
proportionately. Typically the corrected power factor will be
0.92 to 0.95. Some power
distributors offer incentives for operating with a power factor
of better than 0.9, for
example, and some penalize consumers with a poor power
factor. There are many ways
that this is metered but the net result is that in order to reduce
wasted energy in the
distribution system, the consumer is encouraged to apply
power factor correction. Most
Network Operating companies now penalize for power factors
below 0.95 or 0.9.

Power factor explained

Consider a single-phase induction motor. If the motor

presented a purely resistive load
 capacitors are not subjected torpid on-off-on conditions.
Typically the correction would
be placed on either the Main or Delta contactor circuits.
Power factor correction applied at the origin of the installation
consists of a controller
monitoring the VAr’s and this controllers witches capacitors
in or out to maintain the
power factor better than a preset limit (typically 0.95). Where
‘bulk’ power factor
correction is installed, other loads can in theory be connected
anywhere on the network.

How to improve power factor.

Power factor correction is achieved by the addition of Power Factor for a Three-Phase Motor
capacitors in parallel with the
connected motor or lighting circuits and can be applied at the The total power required by an inductive device as a motor or
equipment, distribution similar consists of
board or at the origin of the installation. Static power factor • Active (true or real) power (measured in kilowatts, kW)
correction can be applied at • Reactive power - the nonworking power caused by the
each individual motor by connecting the correction capacitors magnetizing current,
to the motor starter. A required to operate the device (measured in kilovars, kVAR)
disadvantage can occur when the load on the motor changes
and can result in under or The power factor for a three-phase electric motor can be
over correction. Static power factor correction must not be expressed as:
applied at the output of a PF = P / [(3)1/2 U I] (2)
variable speed drive, solid state soft starter or inverter as the where
capacitors can cause PF = power factor
serious damage to the electronic components. P = power applied (W, watts)
Over-correction should not occur if the power factor U = voltage (V)
correction is correctly sized. I = current (A, amps)
Typically the power factor correction for an individual motor
is based on the non load
(magnetizing) power since the reactive load of a motor is
comparatively constant compared to actual kW load over Methods of Power Factor Correction ( P.F.C)
compensation should be avoided. Care should be
taken when applying power factor correction star/delta type There are a couple of methods of (P.F.C) listed below are the
control so that the most common
1) Individual Load correction using capacitors
2) Group or Bulk using capacitors means of a timer. At peak time all capacitors would be
3) Automatic switching using capacitors switched in
and would be switched back out at a later time. It is only
1) In this method a capacitor is connected to the load allowed to be
commonly we see permanently connected in installations up to 25 kVAr
this in the case fluorescent light fittings thus improving the
power
factor at the source. This method is also done with large
motors
commonly

3) Automatic switching of capacitors may be used also. This

is
the most common method. It is achieved via the monitoring of
the
reactive power (kVAr) either electronically or by a relay
based form
when this reaches a certain value then capacitors are stepped
in
accordingly switched in via contactors.

2) For larger installations the above method may not be so

practical. This
is an alternative a large bank of capacitors is connected to the
supply
at the distribution board they are switched in and out of the
supply by
 a 20, 20, 10, 10, 5, 5 and 1 kVAr capacitors could be used.
The automatic
switching method will be used therefore if the power factor
drops below a
certain value certain values of capacitance shall be switched in
accordingly

Cable required for Capacitor

Current in capacitor (I) = 71000 = 102.47Amps

√3*400
Also the allowance of 33% for harmonics =136.28 Amps
The switch fuse size for the capacitors is 150 Amp
The capacitor bank is located 15 meters from the supply the
cable size
required to supply the capacitor is given as
16000 = 7.82mV/A/m
136.28*15
From the latest ECTI regulations a cable of 35mm2 XLPE
would be adequate
to supply the capacitor.

Power factor in linear circuit

For the factory I chose the power factor was 0.849 lagging.
Considering the In a purely resistive AC circuit, voltage and current
cost from the supply company in power factor surcharges I waveforms are in step (or in phase), changing polarity at the
decided it would same instant in each cycle. Where reactive loads are
be more economically viable to correct the power factor to present, such as with capacitors or inductors, energy storage
0.98 in the loads result in a time difference between the current
recommended by the supply company thus reliving us from and voltage waveforms. This stored energy returns to the
the power factor source and is not available to do work at the load. Thus, a
surcharge. circuit with a low power factor will have higher currents to
Shown here is a table of the installations loads transfer a given quantity of real power than a circuit with a
kW kVA kVAr P.F high power factor. A linear load does not change the shape
Single Phase Power 31390 32945.69 10004.4 0.952 of the waveform of the current, but may change the relative
Three Phase Power 138410 168165.1 95510.1 0.823 timing (phase) between voltage and current.
Totals 169800 199913.4 105514.5 0.849 Circuits containing purely resistive heating elements
The amount of correction required is given as: (filament lamps, strip heaters, cooking stoves, etc.) have a
KVAr = kW (Tanθ1 – Tanθ2) power factor of 1.0. Circuits containing inductive or
kW = total installed load in kilowatts capacitive elements (compact fluorescent lamps,[3] lamp
Tanθ1 = the original power factor of the installation ballasts, motors, etc.) often have a power factor below 1.0.
Tanθ2 = the new required power factor of the installation
kW = 169.8 kW Definition and calculation
Tanθ1 = cos-1 0.849 = 31.89° tan = 0.622
Tanθ2 = cos-1 0.98 = 11.47° tan = 0.203 AC power flow has the three components: real power (P),
kVAr required is equal to measured in watts (W); apparent power (S),
kW (Tanθ1 – Tanθ2 measured in volt-amperes (VA); and reactive power (Q),
169.8 (0.622 - 0.203) = 169.8 (0.419) = 71.14 kVAr capacitor measured in reactive volt-amperes (var).
Therefore the value of a capacitor required to correct the The power factor is defined as:
power factor from P/S
0.849 lagging to 0.98 lagging is 71.14 kVAr. As capacitors are .
build to only (λ=P/S)
to standard sizes i.e. 10, 25, 50 and 100 kVAr and so on we In the case of a perfectly sinusoidal waveform, P, Q and S can
cannot get a be expressed as vectors that form a vector
71.14 kVAr capacitor but a selection of the above could be triangle such that:
used for example
S2=P2+Q2
 Power factor correction may be applied by an electrical power
If φ is the phase angle between the current and voltage, then transmission utility to improve the stability and
the power factor is equal to |cos α|, and: efficiency of the transmission network. Correction equipment
may be installed by individual electrical
P=S|cos α | customers to reduce the costs charged to them by their
electricity supplier. A high power factor is generally
Since the units are consistent, the power factor is by definition desirable in a transmission system to reduce transmission
a dimensionless number between 0 and 1. When losses and improve voltage regulation at the load.
power factor is equal to 0, the energy flow is entirely reactive, Power factor correction brings the power factor of an AC
and stored energy in the load returns to the power circuit closer to 1 by supplying reactive power
source on each cycle. When the power factor is 1, all the of opposite sign, adding capacitors or inductors which act to
energy supplied by the source is consumed by the cancel the inductive or capacitive effects of the
load. Power factors are usually stated as "leading" or load, respectively. For example, the inductive effect of motor
"lagging" to show the sign of the phase angle. loads may be offset by locally connected
If a purely resistive load is connected to a power supply, capacitors. If a load had a capacitive value, inductors (also
current and voltage will change polarity in step, the known as reactors in this context) are connected to
power factor will be unity (1), and the electrical energy flows correct the power factor. In the electricity industry, inductors
in a single direction across the network in each are said to consume reactive power and capacitors
cycle. Inductive loads such as transformers and motors (any are said to supply it, even though the reactive power is
type of wound coil) consume reactive power with actually just moving back and forth on each AC cycle.
current waveform lagging the voltage. Capacitive loads such The reactive elements can create voltage fluctuations and
as capacitor banks or buried cable generate harmonic noise when switched on or off. They will
reactive power with current phase leading the voltage. Both supply or sink reactive power regardless of whether there is a
types of loads will absorb energy during part of the corresponding load operating nearby, increasing
AC cycle, which is stored in the device's magnetic or electric the system's no-load losses. In a worst case, reactive elements
field, only to return this energy back to the source can interact with the system and with each other
during the rest of the cycle. to create resonant conditions, resulting in system instability
For example, to get 1 kW of real power, if the power factor is and severe overvoltage fluctuations. As such,
unity, 1 kVA of apparent power needs to be reactive elements cannot simply be applied at will, and power
transferred (1 kW ÷ 1 = 1 kVA). At low values of power factor correction is normally subject to
factor, more apparent power needs to be transferred to engineering analysis.
get the same real power. To get 1 kW of real power at 0.2
power factor, 5 kVA of apparent power needs to be An automatic power factor correction unit is used to improve
transferred (1 kW ÷ 0.2 = 5 kVA). This apparent power must power
be produced and transmitted to the load in the factor. A power factor correction unit usually consists of a
conventional fashion, and is subject to the usual distributed number of
losses in the production and transmission processes. capacitors that are switched by means of contactors. These
contactors
Linear loads are controlled by a regulator that measures power factor in an
electrical network. To be able to measure 'power factor', the
Electrical loads consuming alternating current power consume regulator
both real power and reactive power. The vector uses a CT (Current transformer) to measure the current in one
sum of real and reactive power is the apparent power. The phase.
presence of reactive power causes the real power to Depending on the load and power factor of the network, the
be less than the apparent power, and so, the electric load has a power
power factor of less than 1. factor controller will switch the necessary blocks of capacitors
in steps
Power factor correction of linear loads to make sure the power factor stays above 0.9 or other
selected values
It is often desirable to adjust the power factor of a system to (usually demanded by the energy supplier).
near 1.0. This power factor correction is achieved
by switching in or out banks of inductors or capacitors. For Instead of using a set of switched capacitors, an unloaded
example the inductive effect of motor loads may be synchronous
offset by locally connected capacitors. When reactive motor can supply reactive power. The reactive power drawn
elements supply or absorb reactive power near the load, by the
the apparent power is reduced. synchronous motor is a function of its field excitation. This is
referred
to as a synchronous condenser. It is started and connected to References
the
electrical network. It operates at full leading power factor and
puts (1)http://en.wikipedia.org/wiki/Power_factor
vars onto the network as required to support a system’s
voltage or to (2)http://www.cogeneration.net/power_factor_correction.htm
maintain the system power factor at a specified level. The
condenser’s installation and operation are identical to (3)http://www.scribd.com
large electric motors. Its principal advantage is the ease with
which the amount of correction can be adjusted; it (4) http://www.energy-in-motion.com/PFC.html
behaves like an electrically variable capacitor. Unlike
capacitors, the amount of reactive power supplied is Conclusion
proportional to voltage, not the square of voltage; this
improves voltage stability on large networks. Power Factor is a measure of how efficiently electrical power
Synchronous condensors are often used in connection with
is consumed. In the ideal world Power Factor would be unity
high voltage direct current transmission projects or
in large industrial plants such as steel mills. (or 1). Unfortunately in the real world Power Factor is

Why improve power factor ? reduced by highly inductive loads to 0.7 or less. This
induction is caused by equipment such as lightly loaded
The benefits that can be achieved by applying the correct
power factor correction are electric motors, luminaire transformers and fluorescent
• Environmental benefit. Reduction of power consumption lighting ballasts and welding sets, etc.
due to improved
energy efficiency. Reduced power consumption means less In a 3 phase supply, kW consumed is (VOLTS x AMPS x
greenhouse gas
emissions and fossil fuel depletion by power stations. 1.73 x Power Factor) / 1000. The Electricity Company supply
• Reduction of electricity bills
you VOLTS x AMPS and they have to supply extra to make
• Extra kVA available from the existing supply
• Reduction of I2R losses in transformers and distribution up for the loss caused by poor Power Factor. When the power
equipment
• Reduction of voltage drop in long cables. factor falls below a set figure, the electricity supply companies
• Extended equipment life – Reduced electrical burden on charge a premium on the kW being consumed, or, charge for
cables
and electrical components. the whole supply as kVA.

The significance of power factor lies in the fact that utility Inductive loads cause the AMPS to lag behind the VOLTS.
companies supply customers with volt-amperes, but The wave forms of VOLTS and AMPS are then "out of phase"
bill them for watts. Power factors below 1.0 require a utility to
generate more than the minimum volt-amperes with each other. The more out of phase they become then the
necessary to supply the real power (watts). This increases
lower the Power Factor. Power Factor is usually expressed as
generation and transmission costs. For example, if the
load power factor were as low as 0.7, the apparent power Cos Phi. (Ø)
would be 1.4 times the real power used by the load.
Line current in the circuit would also be 1.4 times the current Capacitive Power Factor correction (PFC) is applied to
required at 1.0 power factor, so the losses in the
circuit would be doubled (since they are proportional to the electric circuits as a means of minimising the inductive
square of the current). Alternatively all components component of the current and thereby reducing the losses in
of the system such as generators, conductors, transformers,
and switchgear would be increased in size (and cost) the supply.
to carry the extra current. The introduction of Power Factor Correction capacitors is a
Utilities typically charge additional costs to customers who
have a power factor below some limit, which is widely recognised method of reducing an electrical load, thus
typically 0.9 to 0.95. Engineers are often interested in the
minimising wasted energy and hence improving the efficiency
power factor of a load as one of the factors that affect
the efficiency of power transmission. of a plant and reducing the electricity bill.
It is not usually necessary to reach unity, ie Power Factor 1,
since most supply companies are happy with a PF of 0.95 to
0.98

By installing suitably sized switched capacitors into the

circuit, the Power Factor is improved and the value becomes
nearer to 1 thus minimising wasted energy and improving the
efficiency of a plant