Harmonic Distortion - Sept 21 2011

Power Quality Management - Harmonic Distortion

and Variable Frequency Drives

Voltage and Current Harmonic Distortion Cause and Effect

23/09/2011 Marek Farbis, Mirus International Inc. 1

We will focus on

• Introduction: the ideal vs. distorted waveform • Definition of harmonics • Effects of harmonic distortion • What is a cause for harmonic voltage distortion? • Definition and calculation of THD • VFDs and harmonics • Standards and recommendations • Harmonic mitigation techniques • Applications


Introduction • Electricity generation is

normally produced at constant frequencies of 50 Hz or 60 Hz and can be considered practically sinusoidal.

• Ideally, an electricity supply should invariably show a perfectly sinusoidal voltage signal at every customer location.

• In reality however these signals contain many types of disturbances.

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0

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0.000 0.002 0.004 0.006 0.008 0.010 0.012 0.014 0.016 0.018

Vo

lts

Time [sec]

3-Phase, 480V, 60Hz Power Supply

V(A,B) V(B,C) V(C,A)

Marek Farbis, Mirus International Inc.

Introduction

• The deviation of the voltage and current waveforms from sinusoidal is described in terms of the waveform distortion, often expressed as harmonic distortion.

• In nearly all cases harmonic distortion is produced by a customer’s equipment (non-linear loads) injecting electrical noise into the power system i.e. Variable Frequency Drives.

Marek Farbis, Mirus International Inc.

Definition of Harmonics • In a periodic signal the primary, desired frequency is

the "Fundamental Frequency“. • The term “harmonics” was originated in the field of

acoustics, where it was related to the vibration of a string or an air column at a frequency that is a multiple of the base frequency.

• A harmonic component in an AC power system is defined as a sinusoidal component of a periodic waveform that has a frequency equal to an integer multiple of the fundamental frequency of the system.

• French mathematician Jean Baptiste Joseph Fourier (1768-1830) found that any function of a variable can be expanded in a series of sines of multiples of the variable.


Distorted Waveform

-2

-1.5

-1

-0.5

0

0.5

1

1.5

2

Harmonics - Components of a Distorted Waveform

Fourier Series f(t) = Ao+A1sin(wt+q1)+A2sin(2wt+q2)+A3sin(3wt+q3) ...

Fundamental - 60 Hz

-1.5

-1

-0.5

0

0.5

1

1.5

5th Harmonic - 300 Hz

-1.5

-1

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0

0.5

1

1.5

7th Harmonic - 420 Hz

-1.5

-1

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0

0.5

1

1.5

Resultant Waveform

-2

-1.5

-1

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0

0.5

1

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2

Resultant Waveform

-2

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-1

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0

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1

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2

Resultant Waveform

-2

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-1

-0.5

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1

1.5

2

Harmonic Spectrum

0

20

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60

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100

1 3 5 7 9 11 13

Harmonic #

% o

f F

un

dam

en

tal

Harmonic Spectrum

0

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40

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100

1 3 5 7 9 11 13

Harmonic #

% o

f F

un

dam

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tal

Harmonic Spectrum

0

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40

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100

1 3 5 7 9 11 13

Harmonic #

% o

f F

un

dam

en

tal

Time domain

Frequency domain

300 Hz

420 Hz

60 Hz


Why is the harmonic distortion bad?

• Effect of penetration in the electrical system affecting adjacent installations.

• Thermal effect on electric rotating machines, transformers, capacitors, and cables (extra losses).

• Pulsating torques in rotating machines. • Neutral conductor overloading. • Increased risk of faults from overvoltage conditions

developed on power factor correction capacitors and resonant conditions.

• Unexpected Fuse Operation. • Abnormal operation of electronic relays. • Abnormal operation of solid-state devices. • Lower system power factor preventing effective utilization.


What causes a voltage distortion?

• Relationship between System Impedance and Voltage Distortion.


Relationship between System Impedance and Voltage Distortion.

~

ZSh

ZTh ZCh

Ih

Harmonic Current Source

Sinusoidal Voltage Source

VS VT VL

Where:

ZSh – impedance of the source at harmonic h,

ZTh – impedance of the transformer at harmonic h,

ZCh – impedance of cables at harmonic h,

VS = Ih x ZSh <- The voltage will be the least distorted nearest to the

source.

VT = Ih x (ZSh+ZTh) <- Voltage Distortion at the Transformer at h

VL = Ih x (ZSh+ZTh+ZCh) <- more distorted nearer the load, as the

harmonic current flows through larger amounts of impedance.

cable xfmr

Non-linear Load

Ohm’s Law:

Vh = Ih x Zh Where: Zh – impedance at hth harmonic, Ih – current of hth harmonic, Vh – voltage of hth harmonic,

ZSh

ZCh

ZTh

CUSTOMER/UTILITY

UTILITY

VFD1


While current travels only along the power path of the non-linear load, voltage distortion affects all loads connected to that particular bus or phase.

Harmonic voltage distortion is caused by the flow of harmonic currents through system impedance.

Total Harmonic Distortion

• “Fundamental Current” refers to the current carried in the fundamental frequency, Ih1 (60 Hz).

• “current Total Harmonic Distortion” refers to the ratio of all harmonic currents to the fundamental current.

%100

1

2

2max

h

h

h

h

I

I

iTHD

Ratio of the root-sum-square (RSS) value of the harmonic content of the current to the RMS value of the fundamental current.


Standard Variable Frequency Drive (PWM)

IGBT ‘S = FAST KNIFE SWITCHES IGBT = Insulated-Gate Bipolar Transistor

CONTROL VOLTAGE & FREQUENCY

DIODE BRIDGE


VFD, 6-Pulse Rectifier Current Waveform

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300.0

0.017 0.022 0.027 0.032

Cu

rre

nt

[Am

ps]

time [msec]

VFD input current


VFD, 6-Pulse Rectifier and Harmonics

For simple diode bridge rectifiers:

When, p = 6

h = n · 6 ± 1 h = -- 5,7,--,11,13,--,17,19...

0

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1 3 5 7 9 11 13 15 17 19 21 23 25

harmonic

% F

un

d.

. ia

Current Waveform and Spectrum

h = n · p ± 1 h = harmonic number p = # of pulses in rectification scheme n = any integer (1, 2, 3, etc.)


Harmonic distortion limits

• IEEE Standard 519 – 1992

– IEEE Recommended Practices and Requirements for Harmonic Control in Power Systems.

• IEEE Standard C57.110 – 1986

– IEEE Recommended Practice for Establishing Transformer Capability When Supplying Non-sinusoidal Load Currents.


IEEE Standard 519 General Overview

Introduced in 1981 (Latest revision 1992)

‘Recommended Practices and Requirements for Harmonic Control in Electrical Power Systems’ o Sets limits for voltage and current distortion at Point

of Common Coupling.

o Recognizes responsibility of both User and Utility.

Widely adopted in N. America Becoming more common globally


Definition of Terms

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 can estimate the harmonic indices at point of interference.

Within an industrial plant the PCC is the point between the nonlinear load and the other loads.

XFMR

XFMR

ZTh

UTILITY

MOTOR1

ZCh3 ZCh4

CUSTOMER/UTILITY

VFD2

ZCh2

MOTOR2

MOTOR3

ZCh1

ZSh

VFD1


IEEE Standard 519-1992, “Recommended Practices and Requirements for Harmonic Control in Electrical Power Systems”

Definition of Terms

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 can estimate the harmonic indices at point of interference. Within an industrial plant the

PCC is the point between the nonlinear load and the other loads.

XFMR

XFMR

ZTh

UTILITY

MOTOR1

ZCh3 ZCh4

CUSTOMER/UTILITY

VFD2

ZCh2

MOTOR2

MOTOR3

ZCh1

ZSh

VFD1

Rarely

convenient to

measure on

Utility Side.



Definition of Terms

Short-Circuit Ratio (ISC/IL): Ratio of the short-circuit current (ISC) available at the PCC to the maximum fundamental load current (IL).

Maximum Load Current (IL): Recommended to be the average current of the maximum demand for the preceding 12 months.

XFMR

XFMR

ZTh

UTILITY

MOTOR1

ZCh3 ZCh4

CUSTOMER/UTILITY

VFD2

ZCh2

MOTOR2

MOTOR3

ZCh1

ZSh

VFD1


The harmonic current limits are based on the size of the load with respect to the size of the power system to which the load is connected.


ISC

IL

Recommended Current Distortion Limits

Table 10.3, p72

Current Distortion Limits for General Distribution Systems (120 V Through 69,000 V)

Where:

ISC = maximum short-circuit current at PCC.

IL = maximum demand load current (fundamental frequency component) at PCC.

TDD = Total Demand Distortion (harmonic current distortion calculated in % of maximum demand load current)

THD = Total Harmonic Distortion (calculated based on actual load)

Maximum Harmonic Current Distortion in Percent of IL

Individual Harmonic Order (Odd Harmonics)

ISC/IL <11 11h<17 17h<23 23h<35 35h TDD

<20* 4.0 2.0 1.5 0.6 0.3 5.0

20<50 7.0 3.5 2.5 1.0 0.5 8.0

50<100 10.0 4.5 4.0 1.5 0.7 12.0

100<1000 12.0 5.5 5.0 2.0 1.0 15.0

>1000 15.0 7.0 6.0 2.5 1.4 20.0



Recommended Voltage Distortion Limits

Table 10.2, p70

Low-Voltage System Classification and Distortion Limits

Special

Applications1

General

System

Dedicated

System2

Notch Depth 10% 20% 50%

THD (voltage) 3% 5% 10%

Notch Area (AN)3 16 400 22 800 36 500

NOTE: The Value AN for other than 480 V systems should be multiplied by V/480 1 Special applications include hospitals and airports. 2 A dedicated system is exclusively dedicated to the converter load. 3 In volt-microseconds at rated voltage and current.



Current Distortion Criteria

• Intended to limit for the harmonic current injection from individual customers, so they will not cause unacceptable voltage distortion levels. – Current harmonics will distort voltage in proportion to

impedance of power system.

• Short circuit current, ISC is a measure of system impedance. – Higher ISC means lower impedance, therefore lower voltage

distortion.

• Short circuit ratio, ISC/IL allows for higher distortion levels at lighter loads. – As the size of the user load decreases with respect to the size of

the system, the % of harmonic current that the user is allowed to inject into the utility increases.



Effect of a Stiff Source, ISC/IL > 100

vTHD = 2 % iTHD = 127 %

400 Hp VFD


Effect of a Weak Source, ISC/IL = 8

vTHD = 16 % iTHD = 25 %

400 Hp VFD


Load Level contribution to Harmonics

• A load’s maximum contribution to harmonic distortion is at rated load. – Harmonic current in Amps at full load is highest even

if iTHD might be higher at lighter loads. IEEE Std 519 uses TDD (Total Demand Distortion) for this purpose.

– If IEEE Std 519 limits can be met at full load, then both voltage distortion and harmonic overheating would be satisfied at all load levels.

• More practical to use a load’s rated current as the Demand Current.


WHAT METHODS ARE USED TODAY TO MITIGATE HARMONIC CURRENTS GENERATED BY PWM VFD’S ?

1.) DO NOTHING.

2.) ADD AC LINE REACTORS OR DC LINK CHOKES

3.) TUNED TRAP FILTER

4.) LOW PASS FILTERS

5.) 18-PULSE VFD’S

6.) VFD/CW ACTIVE FRONT ENDS (AFE).

7.) VFD WITH PARALLEL ACTIVE HARMONIC FILTER

8.) MIRUS LINEATOR™ AUHF.


LINEATOR™ Advanced Universal Harmonic Filter with 600V-480V AUTOXFMR



APPLICATIONS

Case Study, Current Distortion

McQuay 400Hp VFD Chiller with Mirus Lineator.

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A

1:33:27.264 PM

7/20/2011

1:33:27.281 PM

7/20/2011

3 ms/Div

16.669 (ms)

Competitive 500Hp VFD chiller with 5% AC Line reactor.

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1:39:39.267 PM

7/20/2011

1:39:39.284 PM

7/20/2011

3 ms/Div

16.672 (ms)

A1 Waveform

163.27 Arms, 7.69 %THD

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7/20/2011 - 1:33:27.264 PM

7.7 % THD at 65% Load = 5% TDD

A1 Waveform

216.05 Arms, 42.81 %THD

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7/20/2011 - 1:39:39.267 PM

42.8 % THD at 65% Load = 27.8% TDD


U1 Waveform

466.20 Vrms, 3.55 %THD

0

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1 5 10 15 20 25 30 35 40 45 50

8/23/2011 - 11:54:34.219 AM

U1 Waveform

587.71 Vrms, 0.86 %THD

0

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1 5 10 15 20 25 30 35 40 45 50

7/20/2011 - 1:33:27.264 PM

Case Study, Voltage Distortion result

McQuay 400Hp VFD chiller with Mirus Lineator.

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V

1:33:27.264 PM

7/20/2011

1:33:27.281 PM

7/20/2011

3 ms/Div

16.669 (ms)

Competitive 500Hp VFD chiller with 5% AC Line reactor.

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0.000

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600.0

V

11:54:34.219 AM

8/23/2011

11:54:34.236 AM

8/23/2011

3 ms/Div

16.669 (ms)


Summary • By drawing non-sinusoidal current, VFD’s

generate harmonics. • The flow of harmonic currents through the power

system impedance creates voltage distortion. • Excessive voltage distortion will cause equipment

malfunction. • IEEE Std 519 harmonic limits can be met by

application of appropriately designed harmonic treatment.

• Harmonic treatment method must perform in ‘Real World’ conditions and when supplied by generator.


How to Ensure your VFD Installation Meets IEEE 519 Limits

• Perform harmonic survey to determine existing conditions

• Obtain harmonic spectrum of load from manufacturer

• Use modeling software to analyze various treatment methods and system conditions

• Analyze impact on Generator or UPS

• Anticipate effect of future system or load changes

• Specify LINEATOR™ FOR

ALL LARGER VFD APPLICATION

Or


Thank you


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Harmonic Distortion - Sept 21 2011

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