2. WHAT IS DISTORTION IN
ELECTRICAL SYSTEMS?
Alteration of the original
shape (or other
characteristic) of sound
or waveform.
3. WHAT IS HARMONICS?
“A COMPONENT FREQUENCY OF A HARMONIC MOTION OF
AN ELECTRO MAGNETIC WAVE i.e. AN INTEGRAL MULTIPLE
OF FUNDAMENTAL FREQUENCY”.
FUNDAMENTAL FREQUENCY in India is 50Hz.
4. WHAT CAUSES HARMONICS ?
NON-LINEAR LOAD ::: CURRENT IS NOT
PROPOTIONAL TO VOLTAGE
e.g . DIODE , TRANSISTOR , NON-LINEAR
AMPLIFIERS
ETC.
Some devices and loads:
Converters,
Devices which includes semi-conductor elements,
Generators,
Motors,
Computers,
Electronic ballasts,
Switching power supplies,
Welding machines,
Control circuits,
HVDC transmission systems,
Electrical Communication systems.
Transformers
6. TOTAL HARMONIC DISTORTION
(THD)
Total Harmonic Distortion(THD): ------ Measure of amount of
harmonics
THD of the load should be less than 7% for acceptable
performance
THD for intermodulation distortion can also be calculated in a similar
way.
7. INTERMODULATION
DISTORTION distortion (IMD) is the amplitude
Intermodulation or intermodulation
modulation of signals containing two or more different frequencies in a system
with nonlinearities
Intermodulation occurs when the input to a non-linear system is composed of
two or more frequencies. Consider an input signal that contains three frequency
components at fa ,fb and fc which may be expressed as :
8. Output signal
Y(t) = G(x(t))
Y(t) will contain three frequencies fa ,fb and fc of the input signal as well as
number of linear combinations of these fundamental frequencies:
Kafa+Kbfb +Kcfc
Where Ka Kb and Kc are arbitrary integers which can assume positive or
negative values. These are the intermodulation products (or IMPs).
Intermodulation order:
The order O of a given intermodulation product is the sum of the absolute
values of the coefficients
O= |Ka|+|Kb|+|Kc|
In our example 3rd order intermodulation products occur where
9.
10. The performance of an ideal amplifier can be represented by the transfer function:
V out
A0
A1V in
An amplifier with some distortion due to nonlinearities can be expressed by a transfer function in the form of a
power series expansion:
V out
A0
A1V in
A 2V in
An input signal with two frequencies
The first order term
A0
A 2V
2
A 2V 12
A 2V 2
A 2V1
2
2
2
DC terms
and
A3V in
2
3
A 4V in
2
4
....
V in
may be shown as:
A1V in gives the fundamental products
The second order term A 2V in
2
in
1
2
V out
V1 cos(
A0
1
t)
V 2 cos(
A1V1 cos(
1
t)
t
2
cos(
t)
A1V 2 cos(
determines the second order products:
2
2
cos( 2
t)
1
A 2V 2
cos( 2
2
2nd harmonic terms
t)
2
A 2V1V 2
2
[cos(
1
2
t)
2nd order IMD terms
1
t
2
t )]
2
t)
11. The third order term A 3V in
A3V
3
in
3 A3
V1V
2
A3V13
2
2
cos( 3
4
V1
3
determines the third order products:
3
cos(
2
1
t)
A3V 23
1
t)
3 A3
2
cos( 3
4
2
3
V2
2
cos V1 V 2
4
2
2
Fundamental frequency terms
t)
3rd harmonic terms
t)
2
3 A3V1 V 2
cos(
2
[cos( 2
1
t
2
t)
cos( 2
1
t
2
t )]
3 A3V1 V 2
4
[cos( 2
2
t
1
t)
cos( 2
2
t
1
t )]
3rd order IMD terms – The troublemakers
12. THIRD ORDER INTERCEPT POINT
The 3rd order products will be
the largest (loudest) of the
intermodulation products.
As a general rule, the 3rd order
products will increase (grow)
3-times faster than the
fundamental signal (the signal
of interest)
13. DISADVANTAGE
S
Increased losses on the distribution system due to increase in the
effective rms current
Overloads, vibration and premature ageing of the
generators, transformers and motors as well as increase in the noise
level.
Distortion of the supply voltage that can disturb the operation of the
sensitive loads
Disturbances in the communication networks and telephone lines
SOLUTIONS
Selective filters for low-order harmonics.
Active power filters before non-linear loads.
THD based power tarrifs methods.
