Loads
in electrical distributer systems are inductive Loads which need a magnetic
field to operate, Such as motors, transformers, ballasts and furnace.
In
first we should know what is the two kinds of power that be needed in the
inductive loads;
Active Power: to
perform the actual the actual work such as light, its unit is (KW). It Consumes
watts and can be read on a wattmeter. It is measured in kilowatts (kW)
Reactive Power: to
sustain the magnetic field, its unit (KVAR), doesn’t perform useful “work,” but
circulates between the generator and the load. It places a heavier drain on the
power source, as well as on the power source’s distribution system. Reactive
power is measured in kilovolt-amperes-reactive (kVAR)
Active
power and reactive power together make up apparent power.
Apparent power is measured in kilovolt-amperes (kVA).
What is the Power Factor?
Power
factor is the ratio of active power to apparent power. It measures how
effectively electrical power is being used. A high power factor signals
efficient utilization of electrical power, while low power factor indicates
poor utilization of electrical power.To determine power factor (PF), divide
working power (kW) by apparent power (kVA). In a linear or sinusoidal system,
the result is also referred to as the cosine θ
PF
= KW = cosine θ
KVA
Low Power Factor
The
main cause of low Power factor is Inductive Load. As in pure inductive
circuit, Current lags 90° from Voltage, this large difference of phase angle
between current and voltage causes zero power factor. Basically, all those
circuit having Capacitance and inductance (except resonance circuit (or Tune
Circuit) where inductive reactance = capacitive reactance (XL = Xc), so
the circuit becomes a resistive circuit), power factor would be exist over
there because Capacitance and inductance causes in difference of phase angle
(θ) between current and voltage.
There
are a lot of disadvantages of low Pf and we must improve
Pf .
Following are the causes of low Power factor:
·
Single phase and three phase induction Motors
(Usually, Induction motor
works at
poor power factor i.e. at:
Full
load, Pf = 0.8 -0.9
Small
load, Pf = 0.2 -0.3
No Load, Pf may come to Zero (0).
·
Varying Load in Power System
(As
we know that load on power system is varying. During low load period, supply
voltage is increased which increase the magnetizing current which cause the
decreased power factor)
·
Industrial heating furnaces
·
Electrical discharge lamps
(High intensity discharge
lighting) Arc lamps (operate a very low power factor)
·
Transformers
·
Harmonic Currents
What are the disadvantages of low power factor?
·
Large Line
Losses (Copper Losses):
We
know that Line Losses is directly proportional to the squire of Current “I2”
Power
Loss = I2xR i.e., the larger the current, the greater the line losses i.e.
I>>Line Losses
In
other words,
Power
Loss = I2xR = 1/CosФ2 ….. Refer to Equation “I ∝ 1/CosФ”….… (1)
Thus,
if Power factor = 0.8, then losses on this power factor =1/CosФ2 = 1/ 0.82 =
1.56 times will be greater than losses on Unity power factor.
·
Large kVA
rating and Size of Electrical Equipment:
As
we know that almost all Electrical Machinery (Transformer, Alternator,
Switchgears etc) rated in kVA. But, it is clear from the following formula that
Power factor is inversely proportional to the kVA i.e.
Therefore,
The Lower the Power factor, the larger the kVA rating of Machines also, the
larger the kVA rating of Machines, The larger the Size of Machines and The
Larger the size of Machines, The Larger the Cost of machines.
·
Greater
Conductor Size and Cost:
In
case of low power factor, current will be increased, thus, to transmit this
high current, we need the larger size of conductor. Also, the cost of large
size of conductor will be increased.
·
Poor Voltage
Regulation and Large Voltage Drop:
Voltage
Drop = V = IZ.
Now
in case of Low Power factor, Current will be increased. So the Larger the
current, the Larger the Voltage Drop.
Also
Voltage Regulation = V.R = (VNo Load – VFull Load)/ VFull Load
In
case of Low Power Factor (lagging Power factor) there would be large voltage
drop which cause low voltage regulation. Therefore, keeping Voltage drop in the
particular limit, we need to install Extra regulation equipments i.e. Voltage
regulators.
·
Low
Efficiency:
In
case of low Power Factor, there would be large voltage drop and large line
losses and this will cause the system or equipments efficiency too low. For
instant, due to low power factor, there would be large line losses; therefore,
alternator needs high excitation, thus, generation efficiency would be low.
·
Penalty from
Electric Power Supply Company on Low Power factor
Electrical
Power supply Company imposes a penalty of power factor below 0.95
lagging in Electric power bill. So you must improve Pf above
0.95.
We will discuss in the nest topics how to improve
power factor (Power factor correction )
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