Harmonics in Power System

Document By
SANTOSH BHARADWAJ REDDY

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What Are Harmonics ?
• Distorted sine wave cause harmonics.
• Distorted current wave cause current harmonics.
• Distorted voltage wave cause voltage
harmonics.
• Fourier expansion result in integral multiples of
fundamental frequency components.
• Nth order harmonics is of n.Fs frequency.
• Generally odd harmonics are prevalent because
of half wave symmetry.
How Harmonics Are
Generated ?
• Non-linear loads generate current harmonics.
• Harmonic currents flow largely through capacitors.
• Harmonic currents also flows through network.
• The flow of harmonic currents cause voltage harmonics.
• Harmonics are thus injected to other linear loads
connected in the same bus.
• Harmonics injected into the network flow towards other
users connected to the network.
What Loads Generate
Harmonics
• Equipment using switched mode power supply
- Television
- Computers, other IT loads
• Equipment using power electronic devices
- AC & DC drives
- Frequency converters
- Rectifiers
- Arc & induction furnaces
- UPS
- Compact fluorescent & other discharge lamps
Sources of Harmonics
Following are some of the non-linear loads which generates
harmonics:
•Static Power Converters and Rectifiers, which are used in UPS,
Battery chargers, etc.
•Arc furnaces.
•Power Electronics for motor controls (AC/DC Drives)
•Computers.
•Television receivers
•Saturated Transformers
•Fluorescent Lighting with electronic ballast.
•Telecommunication equipment.
Type of Harmonics
Characteristic harmonics
- Related to circuit configuration.
- Fairly predictable frequency spectrum.
- Frequency spectrum given by k*p+1 ; k =
1,2,3….
- For ex. 5&7 for 6 pulse, 11 & 13 for 12 pulse.
- Magnitude inversely proportional to order.
Non-characteristic harmonics
- Caused by frequency converters.
- System imbalance (voltage & impedance)
Triplen harmonics
- 3.(2n+1) order n = 0,1,2… i.E 3,9,15,21.. Etc.
- Zero sequence in nature.
- Accumulates as neutral current.
Harmonic Order & Phase
Sequence
• Each harmonic order has a particular phase
sequence relationship with respect to
fundamental.
• By convention the fundamental is assumed to

have positive phase sequence.
• All higher order harmonics have either

positive,negative or zero phase sequence with
respect to fundamental.
How to Determine the Phase
Sequence of Each
Harmonics ? R
Positive Sequence
Y
Phase Sequence of RYB (+ Seq.)
For Fundamental Component
R Y B B
Fundamental +120o 0o -120o B

Negative Sequence
Second +240 o
0o
-240
o
Y
Harmonic -120o 0o +120o
Thus Second Harmonic Behaves As

a Negative Sequence Component. R
How to determine the phase
sequence of each harmonics ?
R Y B
Fundamental +1200 00 -1200
Third
Harmonic +3600 00 -3600
00 00 00
Fundamental Third Harmonics
R
Zero Sequence
Positive Sequence
Y R
Y
B
Thus third harmonic behaves as
B a zero sequence component.
Accumulation of 3rd harmonic current in the neutral
Three phase system
Wave forms of balanced three phase

fundamental currents.
Time.
R - phase.
R-Phase current with its third

harmonic component.
Time.
Y - phase.
Y-Phase current with its third

harmonic component.
Time.
B - phase.
B-Phase current with its third

harmonic component.
Time.
Addition of third harmonics in

Neutral conductor
Third harmonic currents of R,Y&B

phases are in phase with each other
and hence adds up, without
Time. cancellation in the neutral conductor.
How to Determine the Phase
Sequence of Harmonics ?
Harmonic
order 1 2 3 4 5 6 7 8 9
Phase
Sequence + - 0 + - 0 + - 0
Positive Negative
Zero Sequence
Sequence Sequence
Fundamental 2nd Harmonic 3rd Harmonic
4th Harmonic 5th Harmonic 6th Harmonic
3n+1 3n+2 3n+3
Div. by 3 Rem. 1 Div. by 3 Rem. 2 Divisible by 3
Characteristics of Harmonics
Positive Negative
Zero Sequence
Sequence Sequence
Causes over heating Causes over heating Causes over heating
due to ‘Skin effect’ due to ‘Skin effect’ due to ‘Skin effect’
Opposes the Accumulates in the

Aids the fundamental
fundamental neutral
Moderate heating Excessive heating Creates ‘hot neutral’

Responsible for
neutral to earth
Relatively less harmful Most harmful
voltage and open
neutral condition.
Skin effect
Cross-section of current carrying conductor
DC current flow Low frequency High frequency

AC current flow AC current flow
The effective area of the conductor, available for current flow,
reduces as the frequency of the AC current increases. Hence, the
resistance of the conductor increases, at higher frequencies, as it
is inversely proportional to its area of cross-section.
ρL
R =
A
Skin effect explanation
Magnified view of current carrying conductor
Enhanced impedance due
to high mutual inductance.
Hence least current flow.
Moderate impedance due to

medium mutual inductance.
Hence moderate current flow.
Least impedance due to

low mutual inductance.
Hence max. current flow.
Effect of Harmonics
Type of equipment Effect of harmonics
• Rotating machines
• Increased losses, over
heating due to skin effect.
• Pulsating torque
• Transformer, switch-gear, power
cables • Over heating, increased
• Protective relays
power consumption
• Mal-operation, nuisance
• Power electronics tripping
• Control & automation
• Power capacitors • Mal-operation, failure
• Erratic operation
• High currents & failure
due to overload
Effect of Harmonics on
Protective Relays
Mal-operation Nuisance tripping
Trip level set lower than the fundamental value. Trip level set higher than the fundamental value.
The relay should trip as the fundamental value The relay should not trip as the fundamental
is higher than the trip level. But the presence of value is lower than the trip level. But the presence
harmonics has reduced the peak value. Hence of harmonics has increased the peak value. Hence
the protective relay will not trip. the protective relay will trip.
Circuit configuration of
six pulse drive
Current spectrum of six pulse drive for
star-star & star-delta configuration
Twelve pulse drive configuration
Current spectrum of twelve pulse drive
How Capacitors &
Harmonics Are Related
-1
Network behaviour without capacitors
• Network do not reveal harmonics.
• Most of the harmonic currents internal to
network go to the grid.
• No resonance at harmonic frequencies.
• Network power factor is unacceptably
low.
Network Without
Capacitors
Harmonic currents flow towards Grid.
GRID
Min. Import of Harmonics from Grid.
No Resonance at harmonic frequencies.

BUS
Hence least Harmonic Problem.
Non Linear
Power Factor Very Low.

Load
M
How Capacitors &
Harmonics Are Related
-2
Network behaviour with capacitors
• Network start revealing harmonics
• Internally generated harmonic currents may
amplify due to parallel resonance
• Externally generated harmonics enter
capacitors due to series resonance
• May increase harmonic distortions.
• Capacitors draw excessive currents & fail
• Network power factor improves

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Document By

SANTOSH BHARADWAJ REDDY

• By convention the fundamental is assumed to

• All higher order harmonics have either

Fundamental +120o 0o -120o B

Thus Second Harmonic Behaves As

Wave forms of balanced three phase

R-Phase current with its third

Y-Phase current with its third

B-Phase current with its third

Addition of third harmonics in

Third harmonic currents of R,Y&B

Opposes the Accumulates in the

Moderate heating Excessive heating Creates ‘hot neutral’

DC current flow Low frequency High frequency

Moderate impedance due to

Least impedance due to

No Resonance at harmonic frequencies.

Power Factor Very Low.

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