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8/12/2019 New Chapter 8 Harmonic Limits and Filtering
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Chapter 8 Page 1 Harmonic Limits and Filtering
CHAPTER 8
HARMONIC LIMITS AND
FILTERING
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Chapter 8 Page 2 Harmonic Limits and Filtering
INTRODUCTION
As the number and power ratings of nonlinear loads connected to the utilitysystem increases, so does the concern over harmonics. This concern is, first,with the effect of harmonics on the quality of power being furnished to the
customer and, second, with the effects of harmonics on the operation of othersystems such as the telephone.
These concerns have lead to the formulation of utility guidelines for the followingitems:
Voltage Distortion Factor
Current Distortion Factor
Telephone Interference
Moreover, each utility may specify additional harmonic limitations.
SUMMARY OF THE IEEE 519 STANDARD
The following is a summary of the content of the IEEE 519-1992 Standard.
Scope
"This Recommended Practice intends to establish goals for the design of
electrical systems which include both linear and nonlinear loads. The voltage and
current waveforms which may exist throughout the system are described andwaveform distortion goals for the system designer are established. The interface
between sources and loads is described as the point of common coupling, and
observance of the design goals will minimize interference between electrical
equipment."
"The recommended practice addresses steady-state limitation. Transient
conditions exceeding these limitations may be encountered. It sets the
quality of power that is to be provided at the point of common coupling. This
document does not cover the effects of radio-frequency interference, but does
include electromagnetic interference with communications systems."
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Applicat ion of the Standard
"This standard is to be used for guidance in the design of power systems with
nonlinear loads. The limits set are for steady-state operation and are
recommended for the 'worst case' conditions. Transient conditions exceeding
these limits may be encountered."
IEEE 519-1992 STANDARD - TERMS
TDD - Total Demand Distortion (RMS), harmonic current distortion in % of
maximum demand load current (15 or 30 minute demand).
Point of Common Coupling (PCC) -A point of metering, or any point as long
as both the utility and the consumer can either access the point for direct
measurement of the harmonic indices meaningful to both or estimate the
harmonic indices at the point of interference through mutually agreeable
methods. Within an industrial load, the point of common coupling is the point
between the nonlinear load and other loads.
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VOLTAGE DISTORTION FACTOR
DEFINITION
The voltage distortion factor is defined as follows:
VDFV
Vnn
=
=
1
1
2
2
1 2/
where: VDF = Voltage Distortion Factor
V1= rms Value of the fundamental voltage
Vn= rms Value of the nthharmonic voltage
LIMITSLimits on voltage distortion for medium and high voltage systems are listed inTable 8-1.
Bus Voltage at PCCIndividual Voltage
Distortion (%)
Total Voltage
Distortion THD (%)
69 kV and below
69 001 kV through 161 kV
161 001 kV and above
3.0
1.5
1.0
5.0
2.5
1.5Note: High-voltage systems can have up to 2.0% THD where the cause is an
HVDC terminal that will attenuate by the time it is tapped for a user.
Table 8-1. Limi ts on Voltage Distortion
These limits should be used for worst case continuous operation lasting longerthan one hour. For shorter periods, during startup or unusual conditions, theselimits may be exceeded by 50%.
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CURRENT DISTORTION FACTOR
DEFINITION
The current distortion factor is defined as follows:
CDFI
Inn
=
=
1
1
2
2
1 2/
where: CDF = Current Distortion Factor
I1= rms value of the fundamental current
In= rms value of the nth
harmonic current
LIMITS
The following limits should be used for worst case continuous operation lastinglonger than one hour. For shorter periods, during startup or unusual conditions,these limits may be exceeded by 50%.
These limits are permissible provided the transformer connecting the user to theutility will not be subjected to harmonics in excess of 5% CDF.
The harmonic current limits are based on the power requirements of the user withrespect to the size of the utility power system.
As the size of user load decreases with respect to the utility, the larger is thepercentage of harmonic current that can be injected into the utility system.
When the CDF limit exceeds 5%, the heating effect in the transformer should be
evaluated by applying the methodology contained in IEEE Standard 57.110-1986.
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CURRENT DISTORTION LIMITS
FOR GENERAL DISTRIBUTION SYSTEMS
(120 V - 69 KV)
MAXIMUM HARMONIC CURRENT DISTORTION IN % OF IL
INDIVIDUAL HARMONIC ORDER (ODD HARMONICS)
Isc/IL
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CURRENT DISTORTION LIMITS FOR
GENERAL SUBTRANSMISSION SYSTEMS
MAXIMUM HARMONIC CURRENT DISTORTION IN % OF IL
INDIVIDUAL HARMONIC ORDER (ODD HARMONICS)
Isc/IL
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CURRENT DISTORTION LIMITS
FOR HIGH VOLTAGE SYSTEMS (>161 KV)
AND DISPERSED GENERATION AND COGENERATION
MAXIMUM HARMONIC CURRENT DISTORTION IN % OF IL
INDIVIDUAL HARMONIC ORDER (ODD HARMONICS)
Isc/IL
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TELEPHONE INTERFERENCE
INTRODUCTION
Harmonic currents flowing in a utility transmission system produce magnetic fieldsthat induce extraneous currents into adjacent telephone lines. These extraneouscurrents interfere with telephone transmission if they are in the audio frequencyrange and of appreciable magnitude.
Telephone interference is expressed by an IT value
( )=
==1
2**
n
nnWITIFITI
where Inis the rms value of the nth
harmonic current flowing in the utility system andWnis a TIF weighting factor that is dependent on frequency.
TIF WEIGHTING FACTORS
TIF weighting factors, Wn, are listed in Table 8-2.
FREQ TIF FREQ TIF FREQ TIF FREQ TIF
60
180
300
360
420
540
660
720
780
900
1000
0.5
30
225
400
650
1320
2260
2760
3360
43505000
1020
1080
1140
1260
1380
1440
1500
1620
1740
1800
-
5100
5400
5630
6050
6370
6650
6680
6970
7320
7570-
1860
1980
2100
2160
2220
2340
2460
2580
2820
2940
-
7820
8330
8830
9080
9330
9840
10340
10600
10210
9820-
3000
3180
3300
3540
3660
3900
4020
4260
4380
5000
-
9670
8740
8090
6730
6130
4400
3700
2750
2190
840-
Table 8-2. TIF Weight ing Factors
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Figure 8-1. TIF Weight ing Factors
Example A: A hypothetical load on the primary side of a 115 kV-rated transformer islisted in Table 8-3. Calculate the I * T factor.
HARMONI
C
NO.
FREQ.
(HZ)
IN
(AMPS) WN INWN (INWN)2
1 60 50.0 0.5 25 625
5 300 5.0 225 1125 1265625
7 420 2.5 650 1625 2640625
11 660 1.0 2260 2260 5107600
13 780 1.0 3220 3220 10368400
17 1020 0.5 5100 2550 6502500
19 1140 0.5 5630 2815 10660225
23 1380 0.5 6370 3185 10144225
25 1500 0.5 6680 3340 11155600
Table 8-3. Example A Answer
I T ( I W )n
n n* , ,= =
=
1
252 57 845 425 7606=
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LIMITS
Limits on telephone interference are presented below
Category Descript ion I * T
I Levels most unlikely tocause interference
Up to 10,000
II Levels that might causeinterference
10,000 to 25,000
III Levels that probably willcause interference
greater than 25,000
Note: These values of I*T product are for circuits with an exposurebetween overhead systems, both power and telephone.Within an industrial plant or commercial building, the
exposure between power distribution in cables andtelephone lines in cable with twisted pairs is extremely lowand no interference is normally encountered. I*T productssimilar to those of Table 8-4 should be used within plantsand buildings.
For some areas that use a ground retrun for eithertelephone or power circuits, this value may be as low as1500.
Table 8-4. Balanced I*T Guidelines for Converter Installations,Tie (Supply) Lines
Balanced I * T is the I * T value of the phase currents.
Residual I * T is the I * T value of the neutral (ground return) currents.
The above guidelines are applicable to balanced rather than residual I * Tvalues.IEC 555-2
Disturbances in Supply Systems Causes by Household and Similar Equipment
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This standard defines limits for harmonic production of individual loads having an
input current up to 16 A. It classifies equipment into four categories:
Class A Balanced three phase equipment and that equipment that
does not fit any other class as defined below.
Class B Portable and similar tools.
Class C Lighting equipment, including dimmers.
Class D Equipment having an input current with a special waveshape
(electronic power supplies).
Both absolute and relative harmonic limits are specified, see Tables below. Note for
Class A the absolute limits allow impose severe restrictions on larger equipment.
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HARMONIC CURRENT LIMITS FOR CLASS A EQUIPMENT AND CERTAIN
CLASS C EQUIPMENT WITH PHASE CONTROLLED LAMP DIMMERS
HARMONIC ORDER MAXIMUM PERMISSIBLE
HARMONIC CURRENT (A)
Odd Harmonics
3 2.3
5 1.14.
7 0.77
9 0.4
11 0.33
13 0.21
15-39 0.15 x (15/n)
Even Harmonics
2 1.08
4 0.43
6 0.30
8-40 0.23 x ( 8/n)
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HARMONIC LIMITS FOR CLASS C EQUIPMENT
LARGER THAN 25 WATTS
HARMONIC ORDER MAXIMUM PERCENT
2 2
3 30
5 10
7 7
9 5
11-39 3
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HARMONIC LIMITS
FOR CLASS D AND CLASS C EQUIPMENT
LARGER THAN 25 WATTS (RELATIVE LIMITS 300 WATTS)
HARMONIC ORDER RELATIVE mA/W ABSOLUTE (A)
Odd Harmonic
3 3.6 1.08
5 2.0 0.60
7 1.5 0.45
9 1.0 0.30
11-39 0.6 x (11/n) 0.18 x (11/n)
Even Harmonic
2 1.0 0.30
4 0.5 0.15
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NON-FILTER METHODS
ALTER SYSTEM OPERATING CONDITIONS
Simplest and least expensive.
Often impractical or undesirable.
CHANGE LOCATION OR SIZE OF POWER FACTOR CAPACITOR
Changes resonant frequency.
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Chapter 8 Page 23 Harmonic Limits and Filtering
ACTIVE FILTERS
use IGBTs (Insulated Gate Bipolar Transistor) switching up to 20 kHz
supply a current waveform (opposite polarity) that is added to the harmonic
current to produce a nearly sinusoidal wave shape
advantages
instant adaptation to changing load and source conditions
can be located in close proximity to the non-linear load
can be used on single or three phase systems
eliminates a broad spectrum of harmonics (up to 50th
)
harmonics in the system are nearly zero
increases system capacity
Figure 8-4. Active Filter
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Chapter 8 Page 24 Harmonic Limits and Filtering
ACTIVE FRONT ENDS
active filter attached to the input of a VFD
uses IGBTs instead of diode rectifier
ensures good power quality over the drives complete operating range
advantages
sinusoidal line current drawn from system, thus low harmonics
power factor adjustable from 0.8 capacitive to 0.8 inductive
can compensate for line supply undervoltage
no interference with p.f. correction equipment (no resonance)
can be installed without a detailed analysis of power system
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Chapter 8 Page 25 Harmonic Limits and Filtering
PHASE MULTIPLICATION
Harmonics can be canceled by phase multiplication (Figure 8-5). For cancellation tooccur using m six-pulse rectifiers the following five conditions must exist:
(1) Transformers must have the same turn ratios.
(2) Transformers must have identical impedances.
(3) Secondary voltages must be phase shifted 360/6m degrees from each other.
(4) Rectifiers must be controlled at the same delay angle.
(5) Rectifiers must share the dc load equally.
Because no two rectifiers are exactly the same, cancellation is not complete.Assume that 10 percent of the uncancelled harmonic will remain.
Figure 8-5. Phase Multip lication
12 Phase
6- Pulse 6- Pulse
(a) 12 Pulse
24 Pulse
(a) 24 Phase
Bus C
6- Pulse
12 Pulse
6- Pulse
Bus D
6- Pulse
12 Pulse
6- Pulse
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Chapter 8 Page 26 Harmonic Limits and Filtering
FILTER METHODS
PROVIDE SHUNT LC FILTERS
Located near the harmonic source.
Tuned to most troublesome harmonics (lowest order).
Provides low impedance path for harmonic current flow.
For large systems separate filter sections may be required for major harmonics(e.g., 2
nd, 3
rd, 4
th, 5
th, etc.).
Various filter arrangements can be employed.
Arrangement 8-6a - Simplest, least expensive, generally tuned to N =
4.7 - 4.8 harmonic.
Arrangement 8-6b - Bypass resistor added to provide high frequencyfiltering.
Arrangement 8-6c - Required when stringent harmonic specifications areimposed. Includes separate filter sections tuned to each of the lowestorder harmonics plus a high frequency section.
Figure 8-6. Band Pass Filters
SINGLE-TUNED FILTERS
IMPEDANCE
The impedance of single-tuned filters consists of a capacitor and an inductor that areconnected in series, as represented by the following formula.
Z Rf = + j ( L -1
C )
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Chapter 8 Page 27 Harmonic Limits and Filtering
where: XL= L is the inductance of the reactor in ohms at 60 Hz
XC= 1/(C) is the reactance of the capacitor in ohms at 60 Hz
R = filter resistance in ohms
n= 2fn=1
LC = tuned angular frequency in radians/second
Figure 8-7. Single-Tuned Filter
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Chapter 8 Page 28 Harmonic Limits and Filtering
QUALITY FACTOR
The quality factor or sharpness of the filter tuning (larger Q, sharper tuning) isrepresented by the following formula.
Q = = =X
R
L
R R Cn n
n
1
FREQUENCY DEVIATION (DETUNING)
In practice a filter is not always tuned exactly to the frequency of the harmonic that itis intended to suppress. This is the result of several factors.
(1) The power system frequency may change, which causes the harmonic frequencyto change proportionally.
(2) The inductance and capacitance values may change. Of the two values, the
capacitance value can change the most because of aging and changes in ambienttemperature or self-heating.
(3) The initial tuning may be off because of finite size capacitor units.
The total detuning or equivalent frequency deviation is represented by the followingformula:
=
= + +
n
n
f
f
L
L
C
C
12
A change of L or C of 2% causes the same detuning as a change of 1% in thesystem frequency.
A plot of filter impedance versus detuning is shown in Figure 8-8.
Figure 8-8. Filter Impedance vs. Detuning
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Chapter 8 Page 29 Harmonic Limits and Filtering
The filter passband, PB, is bounded by frequencies at which Zf = 2 R, as
represented by the following formula.
= = =
1
2 2 2Q
R
L
R C
n
n
A tuned filter can be made less susceptible to changes in f, L, or C by (1) increasingC and (2) reducing Q.
DOUBLE-TUNED FILTERS
CONFIGURATION
A double-tuned filter can be achieved in two configurations, as shown in Figures 8-9(a) or (b). Because of the reduction in the number of inductors subjected to full lineimpulse voltages, the main advantage of the configuration shown in Figure 8-9(b) is
for high voltage applications.
Figure 8-9. Double-Tuned Filter
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COMPOSITE FILTERS
CONFIGURATION
Composite filters (Figure 8-10) are made up of various tuned filters and a high pass
filter. For example, F1 and F2 would each be tuned to an individual low orderharmonic, while F3would be designed to provide high frequency filtering. Compositefiltering is the most popular method of filtering.
Figure 8-10. Composi te Filter
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Chapter 8 Page 31 Harmonic Limits and Filtering
Figure 8-11 shows a typical composite filter that is used with 6-pulse rectifiers.
Figure 8-11. Filter for 6-Pulse Recti fiers
Figure 8-12 shows a typical composite filter that is used with an arc furnaceinstallation.
Figure 8-12. Filter for an Arc Furnace Installation
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Chapter 8 Page 32 Harmonic Limits and Filtering
At
Resonant
Frequency
At
Fundament
al
FrequencyReactor
Voltage
Capacitor
Voltage
Reactor
Voltage
Capacitor
Voltage
Capacitor Voltage
Reactor Voltage
System
Voltage
hXL
(-hXL)
XL
(-h2XL)
h2
h2-1* VF
1
h2-1* VF
VF
VF
I
CAPACITOR VOLTAGE
VERSUS SYSTEM
VOLTAGE
At
Resonant
Frequency
At
Fundament
al
FrequencyReactor
Voltage
Capacitor
Voltage
Reactor
Voltage
Capacitor
Voltage
Capacitor Voltage
Reactor Voltage
System
Voltage
hXL
(-hXL)
XL
(-h2XL)
h2
h2-1* VF
1
h2-1* VF
VF
VF
I
CAPACITOR VOLTAGE
VERSUS SYSTEM
VOLTAGE
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Chapter 8 Page 33 Harmonic Limits and Filtering
13.8
14.091.027
14.191.036
14.381.045
14.721.074
15.531.133
18.401.332
be rated for (kV)of System VoltageFilter is Tuned
shouldin P.U.To Which
CapacitorCapacitor VoltageHarmonic Frequency
13.8
14.091.027
14.191.036
14.381.045
14.721.074
15.531.133
18.401.332
be rated for (kV)of System VoltageFilter is Tuned
shouldin P.U.To Which
CapacitorCapacitor VoltageHarmonic Frequency
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Chapter 8 Page 34 Harmonic Limits and Filtering
SYSTEM WITH NO FILTERING
1 2 3 4 5 6 7
ParallelReactance
Parallel
Reactance
System Reactance
Capacitor
Reactance
Harmonic
Reactance
System Reactance
(a) Typical System One Line Diagram
(b) System Representation
(c) Frequency Plot
of
System
Impedance
SYSTEM WITH NO FILTERING
1 2 3 4 5 6 7
ParallelReactance
Parallel
Reactance
System Reactance
Capacitor
Reactance
Harmonic
Reactance
System Reactance
(a) Typical System One Line Diagram
(b) System Representation
(c) Frequency Plot
of
System
Impedance
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SYSTEM WITH FILTERING
1 2 3 4 5 6 7
Parallel
Reactance
(no filter)
Parallel
Reactance
(no filter)
System Reactance
Capacitor
Reactance
Harmonic
Reactance
System Reactance
(a) Typical System One Line Diagram
(b) System Representation
(c) Frequency Plot
of
System
Impedance
Filter
Reactance
Parallel
Reactance
(with filter)
Critical Area
SYSTEM WITH FILTERING
1 2 3 4 5 6 7
Parallel
Reactance
(no filter)
Parallel
Reactance
(no filter)
System Reactance
Capacitor
Reactance
Harmonic
Reactance
System Reactance
(a) Typical System One Line Diagram
(b) System Representation
(c) Frequency Plot
of
System
Impedance
Filter
Reactance
Parallel
Reactance
(with filter)
Critical Area
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TUNED FILTER DESIGN
GENERAL
Filter design is a step-by-step procedural process. The remainder of this tab will
describe that process by following an example problem.
SAMPLE CALCULATION
A 6600 kVAR capacitor bank is required to provide power factor correction on a 34.5kV power system. To avoid possible resonance problems, an inductor is connectedin series with the capacitor to form a filter that is tuned to N = 4.7. The filter is wye-connected. See Figure 8-13. The step-by-step design process is as follows.
Figure 8-13. Sample Calculation One-Line Diagrams
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Step 1: At the nominal system voltage of 34.5 kvLL:
kV
kV
LL
LN
VN
Nx V x kVCF F LL=
=
=
= =
2
2
2
21
4 7
4 7 134 5
36136
36136
32086
( . )
( . ).
.
..
Use a standard 21.6 kV capacitor can.
The rated capacitor voltage is then
VC= 21.6 x 3 = 37.4 kVLL
Step 2: Tuning reactor required for power factor correction:
kVARN
x kVAC
x
=
=
1
1
1
4 7 1
2
2
6600 = 313 kVAR( . )
Step 3: Required capacitor operating kVAR :
kVAR = 6600 + 313 = 6913 kVAR at 36.136 kVLL
At rated capacitor voltage:
kVAR =37 4
361366913 7405
2.
.
=x kVAC
Conclusion: One needs 7405 kVAR @ 37.4 kV to get the required6913 kVAC @ 36.136 kV
Step 4: Capacitor bank electrical parameters:
Choose 7.5 MVAR at 37.4 kVLL.
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Step 5: Capacitor reactance @ 50 Hz :
Step 6: Tuning Reactor reactance @ 50 Hz :
Step 7: Actual capacitor operating MVAC @ 36.136 kV:
MVAC x MVAC=
=
36136
37 4
7 5 7 0
2
.
.
. .Note: 6913 kVAR of capacitors was initially required.
Capacitor operating current = reactor operating current
I VA
= = =3
7000
3112
V * 36.136amperes
LL
Step 8: The tuning reactor electrical parameters are as follows:
1. Voltage rating = 37.4 kV (L-L)
2. XL= 8.44 at 50 Hz
3. L = 26.9 mH
4. Q = minimum of 50 at 50 Hz
5. Current carrying duty
Fundamental = 112 amperes
For harmonic currents, a harmonic analysis study should beperformed to determine total current rating.
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ACTIVE FILTER DESIGN AND SPECIFICATION
FOR CONTROL OF HARMONICS IN INDUSTRIAL AND COMMERCIAL
FACILITIES
Mark McGranaghan
Electrotek Concepts, Inc.
Knoxville TN, USA
The increasing use of power electronics-based loads (adjustable speed drives,switch mode power supplies, etc.) to improve system efficiency and controllability isincreasing the concern for harmonic distortion levels in end use facilities and on the
overall power system.
The application of passive tuned filters creates new system resonances which aredependent on specific system conditions. Also, passive filters often need to besignificantly overrated to account for possible harmonic absorption from the powersystem.
Passive filter ratings must be coordinated with reactive power requirements of theloads and it is often difficult to design the filters to avoid leading power factoroperation for some load conditions. Active filters have the advantage of being able tocompensate for harmonics without fundamental frequency reactive power concerns.
This means that the rating of the active power can be less than a conquerablepassive filter for the same nonlinear load and the active filter will not introducesystem resonances that can move a harmonic problem from one frequency toanother.
The active filter concept uses power electronics to produce harmonic componentswhich cancel the harmonic components from the nonlinear loads. These active filtersare relatively new and a number of different topologies are being proposed.
Active Filter Configuration
The active filter uses power electronic switching to generate harmonic currents thatcancel the harmonic currents from a nonlinear load. The active filter configurationinvestigated in this paper is based on a pulse-width modulated (PWM) voltagesource inverter that interfaces to the system through a system interface filter asshown in Figure 1. In this configuration, the filter is connected in parallel with theload being compensated. Therefore, the configuration is often referred to as anactive parallel filter.
Figure 1 illustrates the concept of the harmonic current cancellation so that thecurrent being supplied from the source is sinusoidal.
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The active filter does not need to provide any real power to cancel harmonic currentsfrom the load. The harmonic currents to be canceled show up as reactive power.Reduction in the harmonic voltage distortion occurs because the harmonic currentsflowing through the source impedance are reduced.
Therefore, the dc capacitors and the filter components must be rated based on thereactive power associated with the harmonics to be canceled and on the actualcurrent waveform (rms and peak current magnitude) that must be generated toachieve the cancellation.
The current waveform for canceling harmonics is achieved with the voltage sourceinverter and an interfacing filter. The filter consists of a relatively large isolationinductance to convert the voltage signal created by the inverter to a current signal forcanceling harmonics. The rest of the filter provides smoothing and isolation for highfrequency components. The desired current waveform is obtained by accuratelycontrolling the switching of the insulated gate bipolar transistors (IGBTs) in the
inverter. Control of the current wave shape is limited by the switching frequency ofthe inverter and by the available driving voltage across the interfacing inductance.
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Example System for Act ive Filter Performance Evaluation
A simple example system was modeled to evaluate the active filter performance fordifferent types of loads and to evaluate the impact of system switching events on thedesign requirements for the active filter. A typical distribution circuit as shown in
Figure 4 was selected for this evaluation. Important parameters are as follows:
Source strength at transmission supply point = 200 MVA
138/13.8 kV Transformer: 10 MVA, 7% impedance
Substation capacitor bank size = 3.0 Mvar (switched)
Equivalent load for parallel feeders = 3.0 MW
Modeled feeder circuit: 3.0 miles to example customer
Feeder capacitor bank on 13.8 kV side at example customer: different sizesevaluated
Customer low voltage capacitor bank: varied
Customer service transformer: 1500 kVA, 6% impedance
Customer load = 1.0 MW
Active Filter size = 400 Vrms, 30 Arms
Nonlinear load: different loads evaluated
Determining Active Filter Ratings for Nonlinear Load Types
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One of the confusing aspects of applying active filters is trying to figure out the activefilter rating that is required to compensate for the harmonics from a particular load. Aparallel-connected active filter should be rated in terms of the rms current it canprovide. Then the task is to figure out the rms current required to compensate for theharmonics from different types of loads. Simulations were performed for a number of
typical nonlinear loads to develop some guidelines for active filter ratings.
One advantage of the parallel-connected active filter, as compared to passive filters,is that it is self-limiting in terms of the harmonic cancellation provided. there is noconcern for overloading the filter due to harmonics from the utility supply system orunder-rating the filter for the loads involved. The worst case scenario if the filter isunder-rated is that it just wont completely compensate for all the nonlinear loadcurrent harmonics. In fact, it may not be necessary to compensate for all theharmonics from a nonlinear load. With the active filter, the size can be selected toachieve any desired level of cancellation. One good way to use this concept wouldbe to provide only enough compensation so that the load/filter compensation waswithin some specified guidelines for harmonic generation (e.g. IEEE 519-1992).
Effect of Load Waveform on Filtering Effectiveness
The effectiveness of the active filter in compensating for harmonic components ofthe load current depends on the specific load current waveform involved. Twodifferent waveforms may have the same rms harmonic content but the active filtermay do a better job of compensating for one of the waveforms because of the waveshapes involved.
An ac voltage regulator is used for illustration. Two cases are compared in Figure 5.The only difference between the two cases is the load of the ac regulator.
In the waveforms on the left side of the figure, the load is a pure resistance. Thewaveforms on the right side are for the case where the load is a series combinationof resistance and reactance. The performance is much better for the smoother loadcurrent waveform (RL load). It is worthwhile to note that the majority of applicationsfor the active filter will involve waveforms like those on the right hand side of Figure 6(e.g. adjustable speed drives with diode bridge rectifiers or single phase electronicloads), rather than the left side.
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In general, the current waveform of an ac regulator with resistive load is an exampleof the wave shape that poses the severest challenge for an active filter.
The problem is the high di/dt that is required of the filter to compensate for the highdi/dt at turn on of the regulator. The problem is most severe when the regulator isturned on with a firing angle close to 90 degrees because this is when the availabledriving voltage stored on the dc capacitor is at a minimum.
The output di/dt capability can be raised either by increasing the dc voltage settingor by reducing the size of the interfacing inductance. The limiting factor forincreasing the dc voltage is the voltage withstand capability of the IGBT devices.The limiting factors for reducing the interfacing inductance include the IGBT di/dtwithstand capability, control requirements, the interface passive filter requirement,and overall system stability. If the interfacing inductance becomes too small, the dcvoltage cannot be kept constant for normal operation.
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Steady-State Rating Requirements and Act ive Filter Effectiveness
The best way to provide a rating for an active filter is in terms of the rms current thatit must provide to compensate for harmonics from nonlinear loads.
Table 1 provides a convenient summary of different nonlinear load types withexample waveforms and typical levels of harmonic current distortion associated witheach load. Using these typical waveforms, it is possible to calculate a theoreticalvalue for the required harmonic compensation from the active filter. The summaryincludes the THD for each nonlinear load waveform and the required active filterrating in rms amps per kVA of load rating. These ratings assume that the active filterrating should be based on the total rms harmonic current content of the load. Asimulation waveform illustrating the active filter effectiveness for each of thesewaveforms is also provided. The ratings in Table 1 assume ideal active filtercharacteristics. That is, they assume that the active filter can compensate for everyamp of harmonic current created by the nonlinear load. It is clear from the simulation
result waveforms also included in the table that the harmonic cancellation is notperfect. The distortion in the supply current is also provided in the table to illustratethe effectiveness of the active filter.
It is important to note that these simulations were for steady state conditions (loadwas not changing).
Therefore, the effect of the response time associated with the FFT control was not afactor.
A number of important observations can be made based on the results summarizedin Table 1:
The overall filtering effectiveness depends significantly on the types ofloads being compensated. There is no simple relationship between theload current THD and the filter effectiveness.
The active filter is most effective when the load current waveform does nothave abrupt changes. As a result, it is very effective for most voltagesource inverter-type loads, even when the distortion is high.
The active filter effectiveness was not as good for 12 pulse loads. This iscaused by the fact that the higher frequency components are moredominant in these loads.
The rating requirement for the passive filter capacitor is also dependent onthe load current characteristics. Load current waveforms with more highfrequency content (e.g. ac regulator with resistive load or 12 pulseconverters) result in higher duties on the filter capacitor.
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