Copper Development Association
Power Quality Partnership
Harmonics in Power Systems
Copper Development Association
Power Quality Partnership
Copper Development Association
Fluke (UK) Ltd
MGE UPS Systems Ltd
Rhopoint Systems Ltd
Copper Development Association
Copper Development Association
• Established 1933
• website - www.cda.org.uk
• Technical helpline 01727 731200
• IEE Endorsed Provider
Copper Development Association
Harmonics in Power Systems
Background to Harmonics, Problems, Solutions and Standards
David Chapman, Copper Development Association
Harmonic Measurement and Power Quality Surveys
Ken West, Fluke (UK) Ltd
Total Harmonic Management
Shri Karve, MGE UPS Systems Ltd
Applying Predictive Techniques to Power Quality
David Bradley, Rhopoint Systems Ltd
Copper Development Association
-1.100
0.000
1.100
0 90 180 270 360
Fundamental
Third harmonic
Fifth harmonic
Fundamental with third and fifth harmonics
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-1.600
0.000
1.600
0 90 180 270 360
Composite waveform
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Switched mode power supplies (SMPS)
Electronic fluorescent lighting ballasts
Variable speed drives
Un-interruptible power supplies (UPS)
These are all non-linear loads
Loads that generate harmonics
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How harmonics are generated – linear load
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How harmonics are generated – non-linear load
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A Common non-linear load
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Current waveform for a typical Personal Computer
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Harmonic profile of a typical Personal Computer
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Harmonic profile for electronic fluorescent ballast
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Harmonic profile for magnetic fluorescent ballast
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Six-pulse bridge
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Typical harmonic profile - six-pulse bridge
Six pulse bridge - harmonic current
0
5
10
15
20
25
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25
Harmonic number
%
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Twelve-pulse bridge
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Typical harmonic profile - twelve-pulse bridge
Twelve pulse bridge - harmonic current
0
5
10
15
20
25
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25
Harmonic number
%
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Why have harmonics become so important?
Harmonic generating equipment has been in use for decades
• Increase in the number of loads
• Change in the nature of loads
• Increase in those producing triple-Ns
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Equivalent circuit of a harmonic generating load
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Harmonic Diversity
Fund3rd
5th7th
9th11th 1
25
1020
410
10
20
30
40
50
60
70
80
% wrt RMS
HarmonicNo of Units (pairs)
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THD
0
20
40
60
80
100
120
1 2 5 10 20 41
No of Units (pairs)
% w
rt F
un
dam
en
tal
Harmonic Diversity - THDI
Copper Development Association
Problems caused by harmonics
currents within the installation overloading of neutrals
overheating of transformers
nuisance tripping of circuit breakers
over-stressing of power factor correction capacitors
skin effect
voltages within the installation voltage distortion & zero-crossing noise
overheating of induction motors
currents in the supply
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In balanced three phase systems the fundamental current cancels out
But triple-N harmonics add arithmetically!
Non triple-N harmonics cancel in the neutral
Overheating of neutrals
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Harmonic neutral currents
-8 .0
-6 .0
-4 .0
-2 .0
0 .0
2 .0
4 .0
6 .0
8 .0
0 120 240 360 480 600 720
Phase 1 Phase 2 Phase 3
Phase 1 3rd harmonic
Phase 2 3rd harmonic
Phase 3 3rd harmonic
3rd harmonic neutral current
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Neutral conductor sizing
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Neutral conductor sizing
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Neutral conductor sizing
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Neutral currents can easily approach twice the phase currents - sometimes in a half-sized conductor.
IEEE 1100-1992 recommends that neutral busbars feeding non-linear loads should have a cross-sectional area not less than 173% that of the phase bars.
Neutral cables should have a cross-sectional area that is 200% that of the phases, e.g. by using twin single core cables.
Neutral conductor sizing
Copper Development Association
Sizing the neutral conductor
BS 7671:2001 - From January 2002
473 - 03 - 04
where neutral current is expected to exceed phase current
473 - 03 - 05where neutral cross-section is less than phase cross section
- neutral overcurrent protection is required
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Sizing the neutral conductor
For three phase circuits using single core cables:
• Use a neutral conductor sized for the actual neutral current
• If the neutral current is not known, use a double sized neutral cable
• Provide overcurrent protection
• But take account of the grouping factors!
• Take into account voltage drop
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Sizing the neutral conductor
For multi-core cables :
• Multi-core cables are rated for only three loaded cores - applies to both 4 and 5 core cables
• When harmonics are present the neutral is also a current carrying conductor
• Cable rated for three units of current is carrying more - three phases plus the neutral current
• It must be de-rated to avoid overheating
• Neutral must have overcurrent protection
• Grouping factor must be taken into account
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Sizing the neutral conductor - thermal
0.5
1.0
1.5
2.0
2.5
0 10 20 30 40 50 60 70
% third harmonic current in phase
Cab
le s
ize
mu
ltip
lier
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Sizing the neutral conductor - IEC
0.5
1.0
1.5
2.0
2.5
0 10 20 30 40 50 60 70
% third harmonic current in phase
Cab
le s
ize
mu
ltip
lier
Copper Development Association
Sizing the neutral conductor
0.5
1.0
1.5
2.0
2.5
0 10 20 30 40 50 60 70
% third harmonic current in phase
Cab
le s
ize
mu
ltip
lier
Thermal
IEC
Copper Development Association
Neutral conductor protection
Neutral conductors should be appropriately
sized and provided with over-current protection.
The protective device must break all the phases,
but does not necessarily need to break the
neutral itself.
This implies a future need for 4 pole breakers
with double rated neutral poles.
Copper Development Association
Transformers supplying harmonic loads must be appropriately de-rated
Harmonic currents, being of higher frequency, cause increased magnetic losses in the core and increased eddy current and skin effect losses in the windings
Triple-n harmonic currents circulate in delta windings, increasing resistive losses, operating temperature and reducing effective load capacity
Effect of harmonics on transformers
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Increased Eddy current losses in transformers
2hh
1h
2hefeh hIPP
max
where: Peh is the total eddy current loss
Pef is the eddy current loss at fundamental frequencyh is the harmonic orderIh is the RMS current at harmonic h as a percentage of rated fundamental current
Increase in eddy current loss can be calculated by:
Copper Development Association
K-Rating of Transformers
Two rating or de-rating systems for transformers:-
• American system, established by UL and manufacturers, specifies harmonic capability of transformer - known as K factor.
• European system, developed by IEC, defines de-rating factor for standard transformers - known as factor K.
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K-Rating of Transformers - US System
2hh
1h
2hhIK
max
where: h is the harmonic order Ih is the RMS current at h in per unit of rated load current
First, calculate the K factor of the load according to:
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For this typical PC load, the K factor is 11.6
(See IEE 1100-1992 for a worked example)
K-Rating of Transformers - US System
Copper Development Association
K-Rating of Transformers - US System
Next, select a transformer with a higher K rating:
standard ratings are 4, 9, 13, 20, 30, 40 and 50.
NB - for non K-rated transformers:
The K factor describes the increase in eddy
current losses, not total losses.
Copper Development Association
K-Rating of Transformers - European System
In Europe, the transformer de-rating factor is calculated according to the formulae in BS 7821 Part 4. The factor K is given by:
5.0
2
2
1
2
1
11
Nn
n
nq
I
In
I
I
e
eK
e is ratio of eddy current loss (50 Hz) to resistive loss
n is the harmonic order
q is dependent on winding type and frequency, typically 1.5 to 1.7
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K-Rating of Transformers - European System
For the same PC load, the de-rating factor is 78%
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K Factor
The methods for rating transformers
are discussed in CDA Publication 144
In addition, calculation software is
available on our web site:
www.cda.org.uk
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K-Rating - Calculation software
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K-Rating - Calculation software
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K Factor
0
2
4
6
8
10
12
14
16
1 2 5 10 20 41
No of Units (pairs)
K F
acto
r
Harmonic Diversity - K Factor
Copper Development Association
K-Rating or De-rating?
‘K-rated’ transformers are designed to supply harmonic
loads by :
• using stranded conductors to reduce eddy current
losses
• bringing secondary winding star point connections
out separately to provide a 300% neutral rating
Copper Development Association
‘De-rating’ a standard transformer has some disadvantages:-
primary over-current protection may be too high to protect the secondary and too low to survive the in-rush current
the neutral star point is likely to be rated at only 100% of the phase current
it is less efficient future increases in loading must take the de-rating
fully into account
K-Rating or De-rating?
Copper Development Association
Transformers supplying harmonic loads must be appropriately de-rated
Harmonic currents, being of higher frequency, cause increased magnetic losses in the core and increased eddy current and skin effect losses in the windings
Triple-n harmonic currents circulate in delta windings, increasing resistive losses, operating temperature and reducing effective load capacity
Effect of harmonics on transformers
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Effect of triple-n harmonics in transformers
Triple-n harmonic currents circulate in delta windings - they do not propagate back onto the supply network.
- but the transformer must be specified and rated to cope with the additional losses.
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Alternating current tends to flow on the outer surface of a conductor - skin effect - and is more pronounced at high frequencies.
At the seventh harmonic and above, skin effect will become significant, causing additional loss and heating.
Where harmonic currents are present, cables should be de-rated accordingly. Multiple cable cores or laminated busbars can be used.
Skin effect
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Skin effect - penetration depth
fd
51021
where:
d is the depth of penetration, mm
f is the frequency, Hz, and
is the resistivity of the conductor
At the fundamental, 50 Hz d = 9.32 mm
At the 11th harmonic, 550Hz d = 2.81 mm
Copper Development Association
Nuisance tripping can occur in the presence of harmonics for two reasons:
Residual current circuit breakers are electromechanical devices. They may not sum higher frequency components correctly and therefore trip erroneously.
The current flowing in the circuit will be higher than expected due to the presence of harmonic currents. Most portable measuring instruments do not read true RMS values.
Circuit breakers
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Problems caused by harmonics
currents within the installation overloading of neutrals
over-heating of transformers
over-stressing of power factor correction capacitors
skin effect
nuisance tripping of circuit breakers
voltages within the installation voltage distortion & zero-crossing noise
over-heating of induction motors
currents in the supply
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Voltage distortion
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Reducing Voltage Distortion by Circuit Separation
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Increased magnetic and copper losses
Each harmonic generates a field which may rotate forward (+), backward (-), or remain stationary (0)
1 2 3 4 5 6 7 8 9 10 11 12
+ - 0 + - 0 + - 0 + - 0
• Zero sequence harmonics produce a stationary field, causing over-heating and reduced efficiency
Effect of harmonics on induction motors
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• The negative and positive sequence harmonics together cause torque pulsing, vibration and reduced service life
• Harmonics are induced in the rotor leading to overheating and torque pulsing
Stator harmonic 1 5 7 11 13 17
19
Rotor harmonic - 6 6 12 12 18
18
Harmonic rotation F B F B F B
F
Effect of harmonics on induction motors
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Motor de-rating curve for harmonic voltages
0.7
0.75
0.8
0.85
0.9
0.95
1
0 2 4 6 8 10 12
Harmonic Voltage Factor (HVF)
De-
rati
ng
Fac
tor
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Harmonic voltage factorThe Harmonic Voltage Factor (HVF) is defined as:
n
5n
2n
n
VHVF
where:
Vn is the RMS voltage at the nth harmonic as a percentage of the fundamental, and
n is the order of each odd harmonic, excluding triple-Ns
Copper Development Association
Harmonic currents cause harmonic voltage distortion on the supply that can affect other customers. This distortion can propagate onto the 11 kV grid and spread widely.
There are limits for harmonic voltage distortion - a supplier may refuse to supply power to a site that exceeds them.
Harmonic problems affecting the supply
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Harmonic Standards
Electricity Association Engineering Recommendation G 5/4 (2001)
BS EN 61000
IEEE Std 519-1992 Recommended Practices and Requirements for Harmonic Control in Electrical Power Systems
ISBN 1 - 55937 - 239 - 7
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Why revise G5/3?
Levels at 132kV higher than Grid Code allows
Introduction of concept of Electromagnetic Compatibility
G5/3 didn’t include notching and burst harmonics
Introduction of the EU Compatibility Directive and developments in IEC and European Standards
Better information on network harmonic impedance (see ETR 112)
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The Electromagnetic Compatibility concept
Satisfactory operation of supply systems and users’ equipment only when electromagnetic compatibility exists between them
Emission limits help fulfil this objective
G5/4 seeks to limit harmonic distortion levels on the network at the time of connection to below the immunity levels of equipment
Enforced via the Electricity Supply Regs, Grid & Distribution Codes, and connection agreements
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Harmonic Compatibility
Disturbance Level
Total supply network disturbance
Pro
bab
ility
Den
sity
Compatibility Level
Susceptibility of local equipment
Immunity (test) levels
Planning levels
Emission limits for individual sources
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Compatibility levels v Planning levels
Compatibility levels in IEC 61000-2-2 & 61000-2-12, for 400V and 6.6kV to 33kV systems are based on the immunity of capacitors
The margins between planning levels and the compatibility levels depend on voltage level and range from 3% at lv and 5% at mv to 0.5% at ehv
The margins are necessary to make allowance for system resonance and for loads connected where there is no consent required from the DNO
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Stage 1
Applies only to lv connected loads
Requires reference to other IEC standards e.g. IEC 61000-3-2 emissions from lv connected
equipment <16A IEC 61000-3-4 ditto >16A (To be 61000-3-12)
Clarifies that levels may be modified by reference to relevant fault levels rather than the notional ones used to derive the table of emissions Table 7
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Aggregate loads
G5/4 requires that aggregate non-linear loads be considered• An individual non-linear equipment complying
with 61000-3-2 can be connected without consideration
• Groups of non-linear equipment with aggregate rated current <16A and complying can be connected
• For >16A either 61000-3-4 or 61000-3-6 should be used to assess emissions using diversity rules from 61000-3-6 if necessary
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Example of application - the problem
Connection of communication centre equipment– 15 off rectifier equipment type R2948-15– each equipment is rated at 12.37A– each equipment meets BS EN 61000-3-2– the connection will be at lv and single phase– future expansion expected to 30 units
Can they be connected?
The customer says that no data on emissions is available
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The solution
Data must be available - cannot claim BS EN 61000-3-2 compliance otherwise!
Data was obtained simply by e-mailing the manufacturer in New Zealand
Simplified calculations were carried out on a spreadsheet to check compliance
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Product data sheet
0.0
0.1
0.2
0.3
0.4
0.5
0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40
Harmonic (f1 = 50Hz)
IP C
urre
nty(
A)
EUT
EN61000-3-2
Product TestHarmonic Emissions
Product: R2948-15Serial #: 1040171Date tested: 07 October 1999
Test ParametersInput Voltage: 230v 50HzOutput Voltage: 54v at no loadOutput Current: 52AAmbient temperature: +20°C
Product Compliance Group
12.37A
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The calculations
As a first estimate the current emissions are
multiplied by the number of units, and the result
compared with the values in Table 7 of G5/4.
This shows that there is no problem
The spreadsheet calculations would show that
the future increase to 30 units would give values
of emissions greater than the limits for triple-Ns
above 21st
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Table 7: Stage 1 Max Harmonic RMS Current Emissions for aggregate loads and equipment rated >16A per phase
Harmonicorder ‘h’
Emissioncurrent Ih
Harmonicorder ‘h’
Emissioncurrent Ih
Harmonicorder ‘h’
Emissioncurrent Ih
Harmonicorder ‘h’
Emissioncurrent Ih
2 28.9 15 1.4 28 1.0 41 1.8
3 48.1 16 1.8 29 3.1 42 0.3
4 9.0 17 13.6 30 0.5 43 1.6
5 28.9 18 0.8 31 2.8 44 0.7
6 3.0 19 9.1 32 0.9 45 0.3
7 41.2 20 1.4 33 0.4 46 0.6
8 7.2 21 0.7 34 0.8 47 1.4
9 9.6 22 1.3 35 2.3 48 0.3
10 5.8 23 7.5 36 0.4 49 1.3
11 39.4 24 0.6 37 2.1 50 0.6
12 1.2 25 4.0 38 0.8
13 27.8 26 1.1 39 0.4
14 2.1 27 0.5 40 0.7
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Sample spreadsheet
Harmonicnumber
Emissionfrom EUT
Emissions15 units
Table 7emissions
Emissions30 units
3 0.42 6.3 48.1 12.6
5 0.21 3.1 28.9 6.2
7 0.16 2.3 41.2 4.7
9 0.11 1.65 9.6 3.3
15 0.03 0.43 1.4 0.8
21 0.035 0.525 0.7 1.05
Emissions in Amps (RMS)
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0
Example flow chart for lv connection
Less than 16A
Complies with 61000-3-2
Complies with 6.2
Complies with 61000-3-4
N
Complies with 61000-3-2
Y
Y
Y
N
N
N
N
Y
Y
YY
Mitigation required Connect to network
Complies with 6.3.1
3 phase <5 kVA
Complies with Table 6
NComplies
with Table 7
N N
N
Y
Y
Go to Stage 2
START
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Stage 2
This applies only to:
a load or aggregate load that doesn’t meet IEC 61000-3-2 and 61000-3-6, or Table 7 current emissions, i.e. Stage 1
PCC less than 33kV i.e. at 6.6, 11 or 22kV
Current emissions can be less than Table 12, or a simplified voltage assessment can be used based on the harmonic impedance just described
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Harmonic Measurements
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Assessment of the connection of new non-linear equipment under Stage 2
a) measure voltage distortion present at PCC
b) assess the voltage distortion which will be caused by the new equipment, and
c) predict the possible effect on harmonic voltage levels by an addition of the results of (a) and (b)
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Assessment of the connection of new non-linear equipment under Stage 2
If the results of (c) are less than
• the harmonic voltage planning levels for the
5th harmonic and
• the THD planning level
connection of the equipment is acceptable
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Combination rules
for harmonics up to and including the 5th and for all triple-Ns, the measured and calculated values of voltage distortion are assumed to peak at the same time and to be in phase - linear addition
for the other harmonics, an average phase difference of 90 is assumed at the time of maximum THD - rms addition
the THD is then given by the rms addition of all combined harmonics up to the 50th
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The Challenge
to keep the harmonic voltage distortion at the point of
common coupling below levels permitted by G5/4
to keep harmonic currents below levels
that cause equipment overload and damage within the
installation
that are permitted by G5/4
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Steps to be taken to reduce voltage distortion on the supply include, for example:
Passive harmonic filters
Isolation transformers
Active harmonic conditioners
Harmonic solutions
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Filters are useful when
the harmonic profile is well defined – such as motor controllers
the lowest harmonic is well above the fundamental frequency
- but filter design can be difficult and, especially for lower harmonics, the filters can be bulky and expensive
Passive harmonic filters
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Passive harmonic filter
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V I
0 2
IpIq
Power FactorPower Factor
Copper Development Association0 2
Ip
POWER
Power FactorPower Factor
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0 2
Iq
POWER
Power FactorPower Factor
Copper Development Association0 2
V
I1
I5
I 7
LI
Power FactorPower Factor
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Mactivepower
reactive power
G
Power FactorPower Factor
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Mactivepower
reactive power
CAPACITOR
Power Factor CorrectionPower Factor Correction
G
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Mactivepower
reactive power
CAPACITOR
G
Power Factor CorrectionPower Factor Correction
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M M M M
• Diversity• Self Excitation• Harmonics
Power Factor CorrectionPower Factor Correction
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M M M M
Control
Power Factor CorrectionPower Factor Correction
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M M M M
Control
• Transformer overloading
• Step voltage
• Bank Size
• Switch-fuse & Cable load ratings
• Load make/break rating of main isolator/switch-fuse
Power Factor CorrectionPower Factor Correction
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Power Factor Correction Bank Sizing
Required improvement in % wattess X kW Maximum Demand
equivalent to {tan(cos-1PFA) - tan(cos-1PFR)} X MD (kW)
or
kVArh (actual) - kVArh (required) running hours X load factor
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• Capacitor Discharge time required for standard Power Factor banks (1 minute)
• Rapidly switching loads require Zero crossing Thyristor or IGBT switched steps
e.g. Spot WeldersLift motorsCranes
Power Factor CorrectionPower Factor Correction
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M
CONVERTOR
TO POWER SYSTEM
HARMONICS
LV
Harmonic ResonanceHarmonic Resonance
AMPLIFIED HARMONICS
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Detuned or Blocking Banks
SOURCE IMPEDANCE WITH FILTER IN CIRCUIT
0.0000
0.0500
0.1000
0.1500
0.2000
0.2500
0.3000
100
112
124
136
148
160
172
184
196
208
220
232
244
256
268
280
292
304
316
328
340
352
364
376
388
400
412
424
436
448
460
472
484
496
508
520
532
544
556
568
580
592
Frequency
Y =
Ln (Z
+1)
Inductive
Capacitive
Fo = 189 to 204 Hz
5th 7th 11th
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SOURCE IMPEDANCE WITH FILTER IN CIRCUIT
0.0000
0.0500
0.1000
0.1500
0.2000
0.2500
0.3000
Frequency
Y =
Ln (Z
+1)
Inductive
Capacitive
Fo = 235 to 245Hz7th
Filter Banks - 5th Harmonic
Copper Development Association
Filter Banks 5th & 7th Harmonic
SOURCE IMPEDANCE WITH FILTER IN CIRCUIT
0.0000
0.0500
0.1000
0.1500
0.2000
0.2500
0.3000
0.3500
0.4000
0.4500
0.5000
100
112
124
136
148
160
172
184
196
208
220
232
244
256
268
280
292
304
316
328
340
352
364
376
388
400
412
424
436
448
460
472
484
496
508
520
532
544
556
568
580
592
Frequency
Y =
Ln (Z
+1)
5th 7th
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Third harmonic filtersThird harmonic filters
10 Amps
10 Amps
10 Amps
30 Amps
Load
R
N
S
T
E
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Third harmonic filtersThird harmonic filters
10 Amps
10 Amps
10 Amps
30 Amps
Load
R
N
S
T
E
v
I3 = 0 Amps
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R S T
N
R S T
N
Delta Interconnected-Star Transformer
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Load
Interconnected Star Transformer sized for
harmonic currents only
I3
Harmonic reduction transformers
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Delta-star isolating transformers reduce propagation of harmonic current into the supply.
Transformers should be adequately rated to cope with the harmonics
Although the transformer effectively establishes a new neutral, don’t use half-sized neutrals
Provide a well rated four wire feed so that the transformer can be isolated for service
Isolating transformers
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Isolating transformers
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Isolating transformers
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Isolating transformers
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Isolating transformers
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Where the harmonic profile is unpredictable or contains a high level of lower harmonics, active filters are useful
Active harmonic conditioners operate by injecting a compensating current to cancel the harmonic current
Active filters
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Keep circuit impedances low
Rate neutrals and multi-core cables generously - 1.73 to 2 times normal size
Always use true RMS meters
Provide a large number of separate circuits to isolate problem and sensitive loads
Take harmonics into account when rating transformers
Provide appropriate filtration where required
Harmonic solutions
Copper Development Association
Copper Development Association
www.cda.org.uk
Harmonics in Power Systems