Transcript Power factor in transmission system

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contents
Introduction
What is power factor
Importance of power factor
Disadvantages of low power factor
Calculation of power factor
Power factor correction method
Types of power factor correction
Power factor correction unit
Conclusion
Introduction
 Any industrial process using electric motors
(to drive pumps, fans, conveyors,
refrigeration plant etc.) introduces
inefficiencies into the electricity supply
network by drawing additional currents,
called "inductive reactive currents".
 This can be regulated by regulating an
important component of power supply
called “power factor”
AC power flow main three components:
1) Active power (P), measured in watts (W);
2) Apparent power (S), measured in volt-amperes (VA);
3) Reactive power (Q), measured in reactive
volt-amperes (VAr).
ACTIVE POWER

The actual amount of power being used, or
dissipated, in a circuit is called active power
 It is also called as true power
True power is a function of a circuit's dissipative
elements, usually resistances (R).
 It is measured in watts (symbolized by the capital
letter P, as always)
REACTIVE POWER
It is equal to the peak value of that power component
that travel back and forth on the line, resulting in zero
average ,and thus does no useful work.
Reactive loads such as inductors and capacitors dissipate
zero power
Yet the fact that they drop voltage and draw current
gives the deceptive impression that they actually do
dissipate power.
This “phantom power” is called reactive power
It is measured in a unit called Volt-Amps-Reactive (VAR)
APPARENT POWER
 The combination of reactive power and true power is
called apparent power
 It is the product of a circuit's voltage and current,
without reference to phase angle.
 Apparent power is measured in the unit of Volt-Amps
(VA) and is symbolized by the capital letter S.
P = True power = VIcosφ
Q = Reactive power = VIsinφ
S = Apparent power = P+jQ = |VI|
What is power factor
 It is defined as the ratio of the real power to the apparent
power, and is a number between 0 and 1.
Power factor = Active power/Apparent power
 Power factor is the percentage of electricity that is being
used to do useful work
EXPLANATION
In a purely resistive AC circuit, voltage and current
waveforms are in step (or in phase), changing polarity at
the same instant in each cycle.
Where reactive loads are present, such as with capacitors
or inductors, voltage and current waveforms are not in step
(or not in phase), thus result in a time difference between
the current and voltage waveforms.
Thus have energy storage in the loads which returns to the
source and is not available to do work at the load.
A circuit with a low power factor or with reactive loads will
have thus higher currents to transfer at a given quantity of
power than a circuit with a high power factor.
PURELY RESISTIVE SYSTEM
Instantaneous and average power calculated from AC voltage and
current with a unity power factor (φ=0, cosφ=1)
PURELY INDUCTIVE SYSTEM
Instantaneous and average power calculated from AC voltage and
current with a zero power factor (φ=90, cosφ=0)
SYSTEM WITH BOTH RESISTIVE AND INDUCTIVE LOAD
Instantaneous and average power calculated from AC
voltage and current with a lagging power factor (φ=45,
cosφ=0.71)
Importance of power factor IN
INDUSTRIES
 A power factor of one or "unity power factor" is the
goal of any electric utility company since if the power
factor is less than one, they have to supply more
current to the user for a given amount of power use.
 In so doing, they incur more line losses. They also must
have larger capacity equipment in place than would be
otherwise necessary.
DISADVANTAGES OF LOW POWER
FACTOR
 Direct costs of low power factor
 This increases generation and transmission costs
 Loss in distribution capacity
 Larger Investment
 Transformers
 Large size conductors
Power factor vs. conductor size
Power factor vs. losses
Power factor correction
method
Power factor correction (PFC) is a technique of
counteracting the undesirable effects of electric
loads that create a power factor that is less than 1
Power factor correction may be applied either by
an electrical power transmission utility to improve
the stability and efficiency of the transmission
network
Or correction may be installed by individual
electrical customers to reduce the costs charged to
them by their electricity supplier
Types of power factor
correction method
i.
Passive PFC
ii.
Active PFC
PASSIVE PFC
 This is a simple way of correcting the nonlinearity of a load
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by using capacitor banks
Power factor correction is achieved by the addition of
capacitors in parallel with the connected motor circuits
Depending on the load and power factor of the network, the
power factor controller will switch the necessary blocks of
capacitors in steps
The resulting capacitive current is leading current and is
used to cancel the lagging inductive current flowing from
the supply.
It is not as effective as active PFC Switching the capacitors
into or out of the circuit causes harmonics
LEAKAGE
CURRENT
CURRENT VECTORS WITHOUT CORRECTION
CURRENT VECTORS WITH CORRECTION
TYPES OF PASSIVE PFC
 STATIC METHOD
Capacitors connected at each starter and controlled by
each starter
 BULK METHOD
In this method capacitors are connected at a
distribution board and controlled independently from
the individual starters
Bulk method
Static method
Active PFC
 An Active Power Factor Corrector (active PFC) is a power
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electronic system that controls the amount of power drawn
by a load
In most applications, the active PFC controls the input
current of the load by addition of a boost converter that
forces a sinusoidal input so that the current waveform is
proportional to the mains voltage waveform (a sinewave).
This requires additional semiconductor switches and
control electronics, but permits cheaper and smaller passive
components
Active PFC is the most effective and can produce a PFC of
0.99 (99%).
That feature is particularly welcome in power supplies for
laptops and cell phones.
Power factor correction
unit
 An automatic power factor correction unit is equipment
used to improve power factor, in industrial networks.
 By increasing the power factor to near 1, current is reduced
in the power system and usually utility charges for
electrical power are reduced.
 It usually consists of a number of capacitors that are
switched by means of contactors.
BENEFITS OF POWER FACTOR CORRECTION
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Reduce Utility Power Bills.
Reduction in load current.
Increase in voltage level across the load.
Improve System Operating Characteristics. (Reduce
Line Losses)
 Reduces the size of conductor required.
CONCLUSION
Improvement of power factor is necessary in both
consumer and supplier point of view. In some
industries penalty is charged due to maintenance of
low power factor. In some industries where power
consumed by the load is low, high ratings of
equipments are required. SVC Technologies and
Synchronous Generator are used for the improvement
of power factor.