New Chapter 8 Harmonic Limits and Filtering

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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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



Isc/IL


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CURRENT DISTORTION LIMITS

FOR HIGH VOLTAGE SYSTEMS (>161 KV)

AND DISPERSED GENERATION AND COGENERATION



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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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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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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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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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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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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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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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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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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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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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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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



(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



(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



(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.


45/46


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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Date post:	03-Jun-2018
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New Chapter 8 Harmonic Limits and Filtering

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