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Harmonics in Low Voltage Three-PhaseFour-Wire Electric Distribution Systems

and Filtering Solutions

Harmonics in Low Voltage Three-PhaseFour-Wire Electric Distribution Systems

and Filtering Solutions

Dr. Prasad EnjetiTexas A&M University

Power Electronics and Power Quality Laboratory

Dr. Prasad EnjetiTexas A&M University

Power Electronics and Power Quality Laboratory

PSERC Online Seminar

February 13, 2001 © 2001 Texas A&M University. All rights reserved.

PSERC Online Seminar

February 13, 2001 © 2001 Texas A&M University. All rights reserved.



Introduction

Introduction

Outline

Outline

1

11

Harmonic Current Sources

Harmonic Current Sources2

22

Case Studies

Case Studies3

33

Conclusions

Conclusions5

55

Solutions:- Derating- Passive Approach

- Active Approach

Solutions:- Derating- Passive Approach

- Active Approach

4

44



Introduction

Introduction



Typical Building Power Distribution SchemeTypical Building Power Distribution Scheme



Harmonic Current SourcesHarmonic Current Sources



Copy machines

Electric typewriters

Light fixture ballasts

Personal computers

Computer systems

Audio visual equipment

Recorders

Televisions

Video tape players

SCR drives for motors

SCR drives for elevators

UPS systemsComputer terminals

Harmonic Producing Equipment in Office BuildingsHarmonic Producing Equipment in Office Buildings

Laboratory testing equipment



Harmonic Related Problems in BuildingsHarmonic Related Problems in Buildings

•Overheating and damage to neutral conductors

•Overheating and damage to panel board feeders

•Line voltage distortion

•Higher Common mode voltage

•Nuisance tripping of circuit breakers

•Overheating and premature failure of distributiontransformers



Case StudiesCase Studies



Typical Current Waveforms of AppliancesTypical Current Waveforms of Appliances



Typical Current Waveforms of Appliances. Contd.Typical Current Waveforms of Appliances. Contd.







Eff f l l d



Effect of Triplen Harmonics in Neutral ConductorsEffect of Triplen Harmonics in Neutral Conductors

Ia

Ib

Ic

IN

2

,

2

,

2

,, rmscrmsbrmsarms N I I I I ++=

rms phaserms N I I ,, *3=



Survey results, Building ISurvey results, Building I



Survey results, Building IISurvey results, Building II



Building I

Building II

Building III

Building IV

Building V

66.8A

50.0A

12.8A

80.6A

23.0A

77.3A

65.6A

36.1A

77.2A

17.4A

72.5A

41.1A

24.5A

90.0A

26.0A

PhaseBuilding

A B C

Summary of Building MeasurementSummary of Building Measurement

120.0A

53.7A

27.5A

142.0A

12.7A

170%

103%

80.9%

171.3%

57.4%

NNeutral toPhase, %

Building VI 44.6A 41.2A 41.9A 60.7A 143%



OFFICE BUILDINGSOFFICE BUILDINGSOFFICE BUILDINGS

• 9 Buildings surveyed.• Typical loads include computers andoffice equipment.

• Average neutral to phase ratio of 1.13• Average crest factor of 2.12

Harmonic Profiles for Typical BuildingsHarmonic Profiles for Typical Buildings

MEDICAL FACILITIESMEDICAL FACILITIESMEDICAL FACILITIES

• 22 facilities surveyed.• Typical loads include computers, officeequipment, diagnostic and patient careequipment.

• Average crest factor of 2.02

INDUSTRIAL FACILITIESINDUSTRIAL FACILITIESINDUSTRIAL FACILITIES

• 14 sites surveyed.• Typical loads include computers, laboratoryequipment, telecommunication equipment.

• Average neutral to phase of 1.04• Average crest factor of 2.12



GOVERNMENT BUILDINGSGOVERNMENT BUILDINGSGOVERNMENT BUILDINGS

Harmonic Profiles for Typical Buildings. Contd.Harmonic Profiles for Typical Buildings. Contd.

AUDIO VISUAL STUDIOSAUDIO VISUAL STUDIOSAUDIO VISUAL STUDIOS

• 6 sites surveyed.

• Typical loads include computers, dataprocessing equipment, officeequipment.

• Average neutral to phase of 0.77

• Average crest of 1.78

•8 studios surveyed.•Average neutral to phase of 0.65•Average crest of 1.74

C M d N i B ildi I



Common Mode Noise, Building ICommon Mode Noise, Building I

Li V lt Di t ti B ildi I



Line Voltage Distortion, Building ILine Voltage Distortion, Building I



Harmonic Effects on TransformersHarmonic Effects on Transformers

• Excessive stress due to heating

• Insulation breakdown

• Lower operating efficiency

• Short life span

• Acoustic noise

• Losses in transformers are subdividedinto core and winding losses.

• Core loss is of minor concern since it

is due to flux generated in the core bythe bus voltage.

• Winding losses are increased due toI2R and stray losses.

TRANSFORMER LOSSES DUETO TRIPLEN HARMONICS

TRANSFORMER LOSSES DUETRANSFORMER LOSSES DUE

TO TRIPLEN HARMONICSTO TRIPLEN HARMONICSEFFECTS OF

TRIPLEN HARMONICS

EFFECTS OFEFFECTS OF

TRIPLEN HARMONICSTRIPLEN HARMONICS



Solutions:-Derating

-Passive Approach-Active Approach

Solutions:-Derating


d

R mm d ti s



Recommendationsby CBEMA

Recommendationsby CBEMA

1. Derate transformers or use K-Factor Transformers.

2. Oversize all neutral components for 1.73 times ratedfull load amps.

3. Use separate neutral conductors for nonlinear loadsand avoid shared neutral conductors where practical.

4. Use neutral over current sensors to trip phase

conductors.

5. Use true rms. ammeters and instruments withsufficient bandwidth for measurement.



Transformer Derating SchemesTransformer Derating Schemes

Derating is a means of determining the maximum load that may be

safely placed on a transformer that supplies harmonic loads.

The most common derating method is the CBEMA approved "crestfactor" method which provides a transformer harmonic derating

factor,THDF.

E l f T f D ti B ildi I



A

B

C

70A

76A

73A

178A

181A

180A

Phase True RMSPhase Current Peak Value ofPhase Current

HENCE, THE TRANSFORMER SHOULD BE DERATED

TO 57% OF ITS NAME PLATE RATING!!!

Example of Transformer Derating. Building IExample of Transformer Derating. Building I

K F t R t d T f



K Factor Rated TransformersK Factor Rated Transformers

•K factor was developed by Underwriters Laboratory in UL 1561.

•K factor transformers are designed to supply nonsinusoial loads.

•They contain enlarged primary windings to carry circulating triplenharmonic currents.

•The magnetic core has a lower flux density as it is designed withhigher grades of iron.

•K factor transformers use smaller, insulated, secondary

conductors in parallel to reduce skin effect.

•K factor transformers are more expensive than conventionaltransformers.

K F R d T f C d

K F t R t d T nsf m s C ntd



K Factor Rated Transformers. Contd.K Factor Rated Transformers. Contd.

K factor is a means of rating a transformer with respect to theharmonic magnitude and frequency of the load.

The K factor numbers do not linearly indicate transformerharmonic tolerance. For example, a K4 rated transformer hasfour times the eddy current tolerance as a K1 transformer. A K13rated transformer has approximately twice the tolerance of a K4and K30 has twice of a K13.

E l K F C l l i B ildi I

Example K Factor Calculation Building I



Example K Factor Calculation. Building IExample K Factor Calculation. Building I

K Factor = 8.84K Factor = 8.84

Solutions Neutral Cable

Solutions Neutral Cable



Solutions. Neutral CableSolutions. Neutral Cable

Super Neutral Cable®

Super Neutral Cable is a metal-clad, Type MC Cable,

manufactured with an oversized neutral conductor or one

neutral per phase for three-phase/four-wire power supply

systems to computers (with dc drive fan motors, tape anddisk drives) office machines, programmable controllers, and

similar electronic equipment where non-linear switching

loads produce additive, odd order harmonic currents which

may create overloaded neutral conductors.

The oversized neutral conductor(s) are sized 150% to 200%

of the phase conductor ampacity to minimize the effects of

harmonics generated by the non-linear loads. The neutral

per phase (striped with color to match the phase conductor)

accomplishes the same objective.

Super Neutral Cable can be used under computer room

floors, raised floors, or overhead in the space above hung

ceilings used for environmental air handling. It handles

branch and feeder, plus power, lighting, signal, and control

circuits in dry locations.

Super Neutral Cable®

Super Neutral Cable is a metal-clad, Type MC Cable,

manufactured with an oversized neutral conductor or one

neutral per phase for three-phase/four-wire power supply

systems to computers (with dc drive fan motors, tape anddisk drives) office machines, programmable controllers, and

similar electronic equipment where non-linear switching

loads produce additive, odd order harmonic currents which

may create overloaded neutral conductors.

The oversized neutral conductor(s) are sized 150% to 200%

of the phase conductor ampacity to minimize the effects of

harmonics generated by the non-linear loads. The neutral

per phase (striped with color to match the phase conductor)

accomplishes the same objective.

Super Neutral Cable can be used under computer room

floors, raised floors, or overhead in the space above hung

ceilings used for environmental air handling. It handles

branch and feeder, plus power, lighting, signal, and controlcircuits in dry locations.

S l ti T f

Solutions Transformers



Solutions. TransformersSolutions. Transformers

K-factor transformers:

Sola

Dongan

Disadvantages and Risks

Disadvantages and Risks



Disadvantages and RisksDisadvantages and Risks

Derating a transformer is a temporary fix and often translates intolower efficiency operation and increased heat for losses.

Derated transformers also run the risk of being perceived to be

partially loaded and future load additions are possible.

While derating removes some stresses from the transformer, a typicaldry type transformer is not designed to supply harmonic loads. Hence,

it may be subject to a shorter life span and lower efficiency.

Separate neutral conductors for computer loads is almost impossibleto implement, due to a wide scattering of data processing equipment

all over the building.

Separate neutral conductors for computer loads is almost impossible

to implement, due to a wide scattering of data processing equipment

all over the building.

Oversizing of transformers, or selection of unnecessarily high K factor

ratings of transformers, can increase the harmonic currents due to

lower impedance.



Solutions:-Derating


Solutions:-Derating


Passive Approach for Neutral Current Reduction

Passive Approach for Neutral Current Reduction



Circulates excessive triplen harmonics through thefilter and back to the load.

Protects the distribution transformer and neutral

conductor from excessive neutral currents.

Improves transformer power factor.


Protects the distribution transformer and neutralconductor from excessive neutral currents.


Passive Approach for Neutral Current ReductionPassive Approach for Neutral Current Reduction



T-Connected Transformer for Neutral

T-Connected Transformer for Neutral



T Connected Transformer for NeutralCurrent Reduction

f m fCurrent Reduction

VA Rating CalculationVA Rating CalculationVA Rating Calculation

kVA rating is higherkVA rating is higher

Star Delta Transformer for

Star-Delta Transformer for



Star-Delta Transformer forNeutral Current Reduction

Star-Delta Transformer forNeutral Current Reduction

VA Rating CalculationVA Rating CalculationVA Rating Calculation

Transformer kVA rating is higher.

Harmonic filtering is impedance sensitive.

Transformer kVA rating is higher.

Harmonic filtering is impedance sensitive.

Zi Z T f f N t l C t R d ti

Zig Zag Transformer for Neutral Current Reduction



Zero sequence impedance of the Zig-Zag transformer must be low. Thisrequires special design.

The effectiveness of the Zig-Zag transformer to divert 3rd. harmonic currentis highly dependent on the distribution system impedance. In most cases

only 50% reduction can be guaranteed.

Lower zero-sequence impedance increases the single-phase fault currentlevel. Therefore, fusing and circuit breaker re-sizing may be necessary.

The specially designed low impedance Zig-Zag transformer becomes a lowimpedance path for zero sequence currents from other parts of the system

Due to the reasons mentioned above the passive Zig-Zag transformerapproach is larger in size and weight.

Zero sequence impedance of the Zig-Zag transformer must be low. This

requires special design.

The effectiveness of the Zig-Zag transformer to divert 3rd. harmonic currentis highly dependent on the distribution system impedance. In most cases

only 50% reduction can be guaranteed.

Lower zero-sequence impedance increases the single-phase fault currentlevel. Therefore, fusing and circuit breaker re-sizing may be necessary.

The specially designed low impedance Zig-Zag transformer becomes a lowimpedance path for zero sequence currents from other parts of the system

Due to the reasons mentioned above the passive Zig-Zag transformer

approach is larger in size and weight.

Zig-Zag Transformer for Neutral Current ReductionZig-Zag Transformer for Neutral Current Reduction

DisadvantagesDisadvantagesDisadvantages





Solutions:-Derating


Solutions:-Derating


Active Neutral Current Filtering Scheme

Active Neutral Current Filtering Scheme



Active Neutral Current Filtering Schemeg




Circulates excessive triplen harmonics through the

filter and back to the load.



Single & Three-

Phase Linear and

Non-linear

Distributed Loads

HarmonixTM

:

Active HarmonicCancellation

System

- +

- +

- +

I ref = 0I n

A i N l C Fil i S h



Active Neutral Current Filtering SchemeActive Neutral Current Filtering Scheme

•Developed by Texas A&M University,

College Station & Current Technology,Inc. Dallas, Texas.

•Design to Cancel Zero SequenceHarmonic Current from a 3-Phase4-Wire Distribution System.

•Patented Technology.

A ti N t l C t Filt i S h



Active Neutral Current Filtering SchemeActive Neutral Current Filtering Scheme

Active Neutral Current Filtering Scheme.

Active Neutral Current Filtering Scheme.



Act ve Neutral Current F lter ng Scheme.Functional Block Diagram.

gFunctional Block Diagram.

S i n g le & T h r e e -

P h a s e L in e a r a n d

N o n - l i n e a rD i s t r i b u t e d L o a d s

- +

- +

- +

I s e n s e d

L f P W M F u ll -

B r i d g e

I n v e r t e r

3 - P h a s e

B r i d g e

R e c t i f ie r

+N

S e n s i n g

C i r c u i t r y

P W M

C o n t r o l l e r

E r r o r

A m p l if ie r6 0 H Z

N o t c h

F i l t e r

I r e f = 0

T 1I N

I N /3

I N /3

I N /3I nI n - I N

a

b

c

n

l F l

A ti N t l C t Filt



Active Neutral Current FilterActive Neutral Current Filter

Active Neutral Current Filter.




Features.Features.

•High Cancellation Effectiveness : 90 - 95%.

•Performance independent of System Impedance.

•Cancels neutral current harmonics by measurement

and close-loop control.

•No low-impedance path for Zero-Sequence 60 Hz.

•Built-in pulse-pulse current limit (No Overloading).

•Significant improvement in Voltage and Current THD’s.

•Fast-response characteristics.





Connection.Connection.

L o w

B a t t e r y

A l a rm

O n / O f f

T h e r m a l

O v e r lo a d

A l a r m

D i s a b l e d

I -m a x

F a u l t

I -H a r m o n i c

I -R M S

N o r m a lT e s t

F i l t e r

O n / O f f F i l t e r

O n4 . 5

1 0 2 . 8

H A R M O N I X

C u r r e n t T e c h n o l o g y H a r m o n i c C o n t r o l P a n e l

A c t i v e H a r m o n i c C a n c e l l a t i o n

S y s t e m / 1 0 0 a m p A m p s

A m p s

H a r m o n i c F i l te r

C a n c e l l a ti o n C u r r e n t

F a c i l i t y

N e u t ra l C u r r e n t

B r e a k e r

L o w

B a t t e r y

A l a rm

O n / O f f

T h e r m a l

O v e r lo a d

A l a r m

D i s a b l e d

I -m a x

F a u l t

I -H a r m o n i c

I -R M S

N o r m a l

T e s t

F i l t e r

O n / O f f

F i l t e r

O n4 . 5

1 0 2 . 8

H A R M O N I X

C u r r e n t T e c h n o l o g y H a r m o n i c C o n t r o l P a n e l

A c t i v e H a r m o n i c C a n c e l l a t i o n

S y s t e m / 1 0 0 a m p A m p s

A m p s

H a r m o n i c F i l te r

C a n c e l l a ti o n C u r r e n t

F a c i l i t y

N e u t ra l C u r r e n t

B r e a k e r3 Phase

Breaker

G

A B C N

Current

Resistor

Term.

Box

120/208 - 225 Amps

Distribution Panel

CurrentSensor

N

Active Neutral Current Filter.Connection

Active Neutral Current Filter.Connection



Connection.Connection.

Filter Specifications

Filter Specifications



Filter Specificationsp

Voltage

Current

Frequency

Power Efficiency

Cancellation Effectiveness

Topology

120/208, 220/380, 277/480

100 A rms per module

50/60 Hz

90%

90-95%

Up to 4 filters can be

connected in parallelParallelability

Parallel connection3-Phase, 4-Wire

Filter Specifications. Contd.

Filter Specifications. Contd.



Pulse-Pulse current protectionThermal shutdown

Overcurrent protectionProtection

Facility currentFilter current

Harmonics or total rms currentMonitoring

Indicator lights, Audible alarm,Max current indicator,

Dry contactsStatus Indication

21H x 24W x13DFloor mount stackable

Enclosure

200 lbs (approx.)Weight

Active Neutral Current Filter. Test Results





P C

P G 2

1 2 0 /2 0 8 D i st ri b u t io n

P a n e l B o a r d s

S i n g l e - p h a s e

L in e a r &N o n - L i n e a r

L o a d s

O n e - L i n e D ia g r a m - B e t a S ite

E l e c t r ic R o o m 1 0 3

P E

H a r m o n i xT M

: A c t iv e

H a r m o n i c

C a n c e l l a t i o n

S s te m

L o a d C e n te r





Neutral Current without Active Harmonic Cancellation System

Neutral Current with Active Harmonic Cancellation System





Neutral Current(A rms)

Zero-Sequence HarmonicCurrent in Neutral (A rms)

60 Hz Zero-SequenceCurrent in Neitral (A rms)

Voltage THD

(% Fundamental)

82.1

71.5

34.3

3.3

Cancellation Effectiveness -

Current THD(% Fundamental)

67.2

33.0

4.14

32.7

2.9

94.2

33.3

Variable Without Filter With Filter

Neutral Current Filtering Scheme

Neutral Current Filtering Scheme



Conclusions

Conclusions



The active filter employs a conventional zig-zag transformer (notoptimized for low zero sequence impedance) and cancels the neutralcurrent harmonics by measurement and closed loop current control.

The performance of the active filter is independent of the systemimpedance, hence is not location sensitive.

The active filter compensates only for harmonic currents in the neutraland does not become a low impedance path to unbalanced 60Hz currents.

The active filter has an electronic built in current limit and does not overload.

The proposed active filter is rugged and stable in operation. Notice the

power electronic components are not subjected to line disturbancessuch as over voltages, voltage spikes etc.

The kVA of the power electronic active current source in low as the voltage

between the neutral of the zig-zag transformer and the system neutral isnear zero.

The active filter employs a conventional zig-zag transformer (notoptimized for low zero sequence impedance) and cancels the neutralcurrent harmonics by measurement and closed loop current control.

The performance of the active filter is independent of the systemimpedance, hence is not location sensitive.

The active filter compensates only for harmonic currents in the neutraland does not become a low impedance path to unbalanced 60Hz currents.

The active filter has an electronic built in current limit and does not overload.

The proposed active filter is rugged and stable in operation. Notice the

power electronic components are not subjected to line disturbancessuch as over voltages, voltage spikes etc.

The kVA of the power electronic active current source in low as the voltagebetween the neutral of the zig-zag transformer and the system neutral isnear zero.

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