Harmonics Signals

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Power Quality Drives

ROBICON The Sine of Quality Robicon 1997

IntroductionIntroductiontotoHarmonicsHarmonics

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Harmonic IssuesHarmonic Issues What are Harmonics?

Where do they come from?

What are the effects of Harmonics? What are the current standards?

How do you measure Harmonics?

How do you know if theres a harmonic problem?

How can they be controlled or eliminated? Common myths and misconceptions

References

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Definition of HarmonicsDefinition of Harmonics They are AC currents or voltages at integer multiples

of the fundamental frequency

The fundamental is the lowest frequency in thewaveform, generally the repetition frequency

They cannot transfer power on the average

Harmonics are present in any non-sinusoidalwaveform

More rapid changes in the waveform require thepresence of higher order harmonics

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Square Wave Harmonic ContentSquare Wave Harmonic Content

Fund

3

3,5

3,5,7

3,5,7,9

3,5,7,9,11

3,5,7,9,11,13

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Sources of HarmonicsSources of Harmonics They arise from non-linear loads in which current is

not strictly proportional to voltage

Linear loads like resistors, capacitors and inductorsdo not produce harmonics

Since diodes and SCRs are non-linear, those circuitsgenerate harmonic currents

Other equipment which causes harmonics:

UPS, rectifiers, transformers, ballasts, welders, arc furnaces, andpersonal computers

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CSI Current WaveformCSI Current Waveform

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PWM Current WaveformPWM Current Waveform

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Effects of HarmonicsEffects of Harmonics Reduction of power system efficiency

Increased heating of transformers (K-factor)

Excitation of power system resonances Increased acoustical noise in motors

RFI generation

Interference with sensitive equipment

There are no good effects!

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Examples of Sensitive EquipmentExamples of Sensitive Equipment Carrier synchronized clocks

Audio/video recording equipment

Generator regulators and synchronizers Telephone equipment

Fluorescent lights

AM radio receivers

Medical equipment PLCs

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Harmonic StandardsHarmonic Standards

IEEE-519 1992

Definitions: Voltage total harmonic distortion (VTHD)

Current total harmonic distortion (CTHD) K-Factor

Point of Common Coupling

VTHD Limits, Table 10.2

CTHD Limits, Table 10.3 Dilution by linear loads

There are no Susceptibility Limits!

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THD Definitions from IEEE-519THD Definitions from IEEE-519Voltage Total Harmonic Distortion VTHD

Current Total Harmonic Distortion CTHD

Sum of squares of amplitudes of all voltage harmonics

Sum of squares of amplitudes of all current harmonics

Amplitude of fundamental voltage

Amplitude of fundamental Current

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Point of Common CouplingPoint of Common Coupling The point of common coupling is the location in the

power distribution system where harmonic distortionis to be measured, usually where harmonic currents

flow into a bus which feeds other equipment. Itslocation must be specified!

In the absence of a specified location, the POCC forcurrent harmonics is the plant-utility interface

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Definition of K-FactorDefinition of K-Factor

K Factor =

S (Square of P.U. Harmonic Current)*(Square of Harmonic Number)

K-Factor theoretically represents the increase in stray losses(conductor eddy currents) in a magnetic component

DITs need to have a K-Factor specification

CSI VFDs typically have a K-Factor of 13 PWM VFDs typically have a K-Factor of 6

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IEEE 519 Table 10.2IEEE 519 Table 10.2Low Voltage System Classification and Distortion Limits

Special

Applications

General

Systems

Dedicated

Systems

Notch Depth 10% 20% 50%

VTHD 3% 5% 10%

Notch Area 16400 22800 36500

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IEEE 519 Table 10.3IEEE 519 Table 10.3

Current Distortion Limits for General Distribution Systems

Isc/Il < 11(5, 7)11


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Harmonic AnalysisHarmonic Analysis

Tools and TechniquesTools and Techniques For simple cases, use Short-Circuit Ratio and look up

VTHD on a curve for that product Short Circuit Ratio is the short circuit current at the POCC divided

by the drive rated current

For simple cases, use TURBOSIM to make a moreprecise calculation of VTHD

There is no simple way of getting CTHD short of aspecific calculation

For complex multi-VFD cases, use VFDNET

Watch out for underlying assumptions in thecalculations!

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CSI Voltage Distortion ChartCSI Voltage Distortion Chart

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PWM Voltage Distortion ChartPWM Voltage Distortion Chart

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Current Distortion System ExampleCurrent Distortion System Example

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Example CalculationExample CalculationVTHD CTHD 5 th 7 th 1 1 th 1 3 th 1 7 th 1 9 th 2 3r d 2 5 th 2 9 th 3 1 s t 3 5 th 3 7 th

DRIVE T YPE

PWM 2.5% LR 2.7 1 5 .4 1 4 .2 4. 88 2 .7 4 1 .2 4 1 .1 6 0 .7 1 0. 6 0. 49 0. 34 0 .3 4 0 .2 3 0 .23

PWM 5% LR 2 1 2 .2 1 1 .5 3 .1 1 2 .2 9 1 .2 4 0 .8 3 0 .6 8 0 .3 8 0 .3 8 0 .2 6 0 .2 3 0 .1 9 0 .1 5

CS I 2.5% LR 4 1 0 .8 7 .88 4 .8 3. 3 2 .5 5 1 .9 9 1 .6 9 1.39 1.2 0 .98 0 .86 0 .71 0 .64

CS I 5% LR 3.1 10.3 7.8 4 .6 9 3 .1 1 2 .3 6 1 .7 6 1 .4 3 1.05 0. 86 0. 64 0 .5 3 0 .3 4 0. 3

CS I 12-Puls e 2.2 4.39 0 0 3.15 2.59 0 0 1.13 0.98 0 0 0.41 0.34

PWM 12-Puls e 1 2.78 0 0 2.4 1.24 0 0 0.41 0.41 0 0 0.19 0.15

Cle a np o we r 0.35 1.35 0.98 0.38 0.38 0.45 0.38 0.26 0.15 0.08 0 0 0.04 0

P e rfe c t Ha rmony 9 0.56 1.3 0 0 0 0 1.03 0.72 0 0 0 0 0.2 0.17

P e rfe c t Ha rmony 15 0.31 0.45 0 0 0 0 0 0 0 0 0.32 0.3 0 0

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Current Harmonics of Various DrivesCurrent Harmonics of Various DrivesCTHD5th 7th 11th 13th 17th 19th 23rd 25th 29th 31st 35th 37th

DRIVE TYPE

6-P PWM 2.5% LR 41 37.8 13 7.3 3.3 3.1 1.9 1.6 1.3 0.9 0.9 0.6 0.6

6-P PWM 5% LR 32.6 30.6 8.3 6.1 3.3 2.2 1.8 1 1 0.7 0.6 0.5 0.4

6-P CSI 2.5% LR 28.8 21 12.8 8.8 6.8 5.3 4.5 3.7 3.2 2.6 2.3 1.9 1.7

6-P CSI 5% LR 27.5 20.8 12.5 8.3 6.3 4.7 3.8 2.8 2.3 1.7 1.4 0.9 0.8

CSI 12-Pulse 11.7 0 0 8.4 6.9 0 0 3 2.6 0 0 1.1 0.9

PWM 12-Pulse 7.4 0 0 6.4 3.3 0 0 1.1 1.1 0 0 0.5 0.4

Cleanpower 3.6 2.6 1 1 1.2 1 0.7 0.4 0.2 0 0 0.1 0

Perfect Harmony 9 3.47 0.01 0 0 0 2.75 1.93 0 0 0 0 0.53 0.46

Perfect Harmony 15 1.21 0.01 0 0 0 0 0 0 0 0.85 0.81 0 0

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Harmonic Analysis GotchasHarmonic Analysis Gotchas Analysis assumes perfect phase and amplitude

balance in the power source!

Cable reactance is neglected unless specified -- it canhave a significant effect on the results

Transformers are assumed to have 5.75% impedanceunless otherwise specified

Pre-existing distortion is neglected -- this can be very

severe on generator sources Power factor correction capacitors are presumed not

to be present unless otherwise specified

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Harmonic Mitigation TechniquesHarmonic Mitigation Techniques Load segregation

Input Line Reactance

Harmonic Filters Higher Pulse Numbers

Perfect Harmony

Lowest Harmonics are Pulse Number +/- 1

Poor Mans Twelve Pulse

Input Switching Converter Cleanpower VFD

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Load SegregationLoad Segregation Load Segregation seeks to connect VFDs and other

harmonic producing loads to the power distributionsystem at the lowest impedance point rather thanconnecting to a higher impedance local bus.

It is frequently accomplished by using a DIT toconnect the drive directly to a medium voltage bus.

It can also include using a UPS to isolate sensitiveequipment.

O

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Load SegregationLoad Segregation

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Input Line ReactanceInput Line Reactance Input reactance reduces the level of VTHD

It can be added with a DIT or line reactor

It has only a small effect on CTHD It is mandatory on our PWM drives as it is a vital part

of the device protection scheme

If a DIT is used for this purpose it needs to have anappropriate K-Factor

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Harmonic FiltersHarmonic Filters A filter consisting of L-C-R components can be

designed to meet an harmonic requirement

Filter are specific to the power system characteristicsand must be re-designed for every application

Filters are large, expensive, wasteful of power andtime-consuming to design

They are especially hard to design when an

emergency generator is the source, or when multipleutility feeds are involved

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Harmonic FilterHarmonic Filter

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Higher Pulse NumbersHigher Pulse Numbers Using Higher Pulse Numbers is an effective way to

reduce harmonics.

It reduces the CTHD substantially

Magnetic components are required to provide phase-shifted sources

Additional input conversion devices (thyristors ordiodes are required)

This technique is not affected by power systemimpedance changes.

The Perfect Harmony uses this technique

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2300 VAC

INDUCTION

MOTOR

C3

A1

B1

C1

A2

B2

C2

A3

B3

POWER

CELL

POWER

CELLPOWER

CELL

POWER

CELL

POWER

CELL

POWER

CELL

POWER

CELL

POWER

CELLPOWER

CELL

INPUT POWER

Perfect Harmony CircuitPerfect Harmony Circuit

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Perfect Harmony InputPerfect Harmony Input

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Poor Mans Twelve-PulsePoor Mans Twelve-Pulse This technique is applicable only where there are a

number of similarly sized VFDs on a bus

About Half of the VFDs are connected through delta-wye DITs, while the other half are connected throughline reactors or delta-delta DITs

The phase shifting effects of the transformers resultsin significant harmonic cancellation of the fifth andseventh harmonic on the primary side

It is more effective for PWM drives than CSIs

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Poor Mans 12-PulsePoor Mans 12-Pulse

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ROBICONThe Sine of Quality

Robicon 1997

Input Switching ConvertersInput Switching Converters By using transistor switches on the input, the current

harmonics can be shifted to much higher frequency--typically above 35th harmonic.

They may then be removed by a very small filterintegral to the drive.

This technique results in unity power factor and theability to regenerate power back to the line.

The Cleanpower drive utilizes this technique.

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Robicon 1997

Clean Power CircuitClean Power Circuit

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Robicon 1997

Clean Power Input WaveformClean Power Input Waveform

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Clean Power Input WaveformClean Power Input Waveform

2V 10mV 2ms SAVE

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Comparison of CTHDsComparison of CTHDs 6-Pulse PWM with 2.5% LR 40%

6-Pulse CSI with 2.5% LR 30%

12-Pulse CSI with %5 DIT 15% 12 Pulse PWM with 5% LR 9%

Cleanpower VFD


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Harmonic MythsHarmonic Myths

and Misconceptionsand Misconceptions Diode Input circuits cause no harmonics

DITs Prevent Harmonics from Flowing into the powersystem

Higher Order Harmonics (>23) need not beconsidered

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ConclusionsConclusions Harmonic Control is a major issuewith consultants

and customers.

HRG has the Knowledge, Experience, and Productsto deal with any VFD harmonic issue.

We can exploit this advantage because many of ourcompetitors are not so well equipped.

The competition must not be permitted to get away

with avoiding or ignoring the harmonic issues onprojects.

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Square Wave HarmonicSquare Wave Harmonic

ContentContentFund

3

3,5

3,5,7

3,5,7,9

3,5,7,9,11

3,5,7,9,11,13

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Practical Motor Drive CircuitsPractical Motor Drive Circuits

Almost all motor drive circuits consist of three parts:

A input converter to change the AC to DC;

A DC link to store and filter the DC;

An output inverter to change the DC into AC. Both output voltage and frequency must be controlled together

for motor load.

AC-DCConversion

DC-ACConversion

DC Link

AC Input;

fixed Frequency,

fixed Voltage

AC Output;

variable Frequency,

variable Voltage

MotorCapacitor

or

Inductor

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Basic VFD TopologiesBasic VFD Topologies

The line-side converter determines the input

harmonic current and power factor; there is

almost no influence from the inverter, as it is

isolated by the DC line.

There are only two choices of topologies:

Current-Fed

Voltage-Fed circuits. Then, the line-side converter circuit is

determined.

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The Basic AC-DC RectifierThe Basic AC-DC Rectifier

The bridge rectifier is the

workhorse of power

electronics. It is used in 1

phase and 3 phase versions

most commonly.

The output voltage is a DC

voltage equal to 3/ * Vllpk

This is also used as the input

power conversion for PWM

AC drives.

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Properties of the Bridge RectifierProperties of the Bridge Rectifier

The positive bus is at the potential of the most positive line voltage,

while the negative bus is at the potential of the most negative line

voltage. (Its like an auction--the highest potential line wins)

The input current is quite distorted, with large fifth and seventh current

harmonics. But since the rate of change of current is low, there are fewhigher order harmonics.

This circuit is used as the building block for multi-phase arrangements

to reduce the current distortion.

The input displacement power factor is uniformly high.

This circuit cannot return energy to the line.

AC and DC side inductors are frequently used to reduce the input

harmonic current.

This is arguably the most basic and inexpensive power conversion unit.

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CommutationCommutation

Ls: Source

inductance

Line Commutation

IaIb

Va

Vb

0 0

Vba

Load

Commutation is the process of transferring current from one switching

element to another.

There are various ways that this is accomplished.

Line commutation: the AC line voltage causes the current to transfer

Forced commutation: Another circuit element acts to transfer the current.

Self commutation: the switching device turns off by itself.

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3-Phase Rectifier Input Current3-Phase Rectifier Input Current

Line-side Reactance OnlyLine-side Reactance Only

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6-Pulse Thyristor Converter6-Pulse Thyristor Converter

Input Current WaveformInput Current Waveform

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Current HarmonicsCurrent Harmonics

of Various Drivesof Various DrivesCTHD5th 7th 11th 13th 17th 19th 23rd 25th 29th 31st 35th 37th

DRIVE TYPE

6-P PWM 2.5% LR 41 37.8 13 7.3 3.3 3.1 1.9 1.6 1.3 0.9 0.9 0.6 0.6

6-P PWM 5% LR 32.6 30.6 8.3 6.1 3.3 2.2 1.8 1 1 0.7 0.6 0.5 0.4

6-P CSI 2.5% LR 28.8 21 12.8 8.8 6.8 5.3 4.5 3.7 3.2 2.6 2.3 1.9 1.7

6-P CSI 5% LR 27.5 20.8 12.5 8.3 6.3 4.7 3.8 2.8 2.3 1.7 1.4 0.9 0.8

CSI 12-Pulse 11.7 0 0 8.4 6.9 0 0 3 2.6 0 0 1.1 0.9

PWM 12-Pulse 7.4 0 0 6.4 3.3 0 0 1.1 1.1 0 0 0.5 0.4

Cleanpower 3.6 2.6 1 1 1.2 1 0.7 0.4 0.2 0 0 0.1 0

Perfect Harmony 9 3.47 0.01 0 0 0 2.75 1.93 0 0 0 0 0.53 0.46

Perfect Harmony 15 1.21 0.01 0 0 0 0 0 0 0 0.85 0.81 0 0

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Input Switching ConverterInput Switching Converter

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Input Switching ConvertersInput Switching Converters

By using transistor switches on the input, the current

harmonics can be shifted to much higher frequency--

typically above 35th harmonic. This is the same

technique used on the output. They may then be removed by a very small filter

integral to the drive.

This technique results in unity power factor (or any

power factor you want) and the ability to regenerate

power back to the line.

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Comparison of CurrentComparison of Current

Total Harmonic DistortionTotal Harmonic Distortion 6-Pulse PWM with 2.5% line reactor 40%

6-Pulse CSI with 2.5% line reactor 30%

12-Pulse CSI with %5 DIT 15% 12 Pulse PWM with 5% line reactor 9%

Switching Converter


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Twelve-Pulse Bridge RectifierTwelve-Pulse Bridge Rectifier

3-PHASE MV INPUT

INPUT FILTER FORHARMONIC CORRECTION

12 Pulse Rectifier

To Inverter

12recnv

To Inverter

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Harmonics of RectifiersHarmonics of Rectifiers

0

5

10

15

20

25

30

35

5 7 11 1 3 17 1 9 2 3 2 5 2 9 3 1 3 5 3 7

P ercent Harm onic Current

6-pu lse: All b a rs

12-pu lse: Black ba rs on ly

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The Controlled RectifierThe Controlled Rectifier

By substituting thyristors for

diodes, we can control the DC

voltage of the rectifier.

In this form, it is usable as a

regulated DC supply like a DCmotor drive.

It also is widely used as the

input stage for variable

frequency AC drives of the

current-fed type.

The output voltage is a functionof the input and the phase delay

of the turn on pulse to the

SCRs.

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3-PHASE MV INPUT

INPUT FILTER FOR

POWER FACTOR ANDHARMONIC CORRECTION

12 Pulse Thyristor Converter

To Inverter

12scrcnv

To Inverter

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WaveformsWaveforms

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Harmonics of ThyristorHarmonics of Thyristor

ConvertersConverters

0

5

10

15

20

25

5 7 11 13 17 19 23 25 29 31 35 37

Percent Harmonic Current

6-pulse: All b ars

12-pulse : Blac k bars only

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Voltage-Fed and Current-FedVoltage-Fed and Current-Fed

TopologiesTopologies Voltage-fed and current-fed refer to the two basic VFD

strategies of applying power to the motor. In Europe, these are

called voltage-impressed and current-impressed, which is a

clearer description.

In voltage-fed circuits, the output of the inverter is a voltage,

usually the DC link voltage. The motor and its load determines

the current that flows. The inverter doesnt care what the current

is. (within limits)

In current-fed circuits, the output of the inverter is a current,

usually the DC link current. The motor and its load determinesthe voltage. The inverter doesnt care what the voltage is.

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Comparison of Voltage-fed andComparison of Voltage-fed and

Current-fed circuitsCurrent-fed circuits Today, voltage-fed VFDs use a rectifier bridge. This gives them consistently

high P.F. and minimum high-order harmonics. The reactive power needs of the

motor come from the capacitor, and are not reflected to the line. The DC link

electrolytic capacitors can be a reliability and lifetime issue. Energy stored in the

link is very high compared to the CSIs, and a fault in the inverter can lead to

very high currents. The motors inherent inductance can be conveniently used tofilter a PWM voltage wave. On the other hand, very fast wavefronts have

become a concern to motor designers and users.

The preferred approach in current-fed inverters is to use a thyristor converter to

control the current. Thus the power factor is the load power factor times the PU

speed. The reactive power demand of the motor is passed back to the line. High

order harmonics are present due to the high di/dt. Link energy storage is

relatively low, and the DC link reactor provides immunity to faults and grounds.Since the current is regulated, inverter faults do not cause high currents. One

cannot change the motor current instantaneously, so all the CSI circuits require

a capacitive filter on the motor to absorb the high di/dt of the inverter.

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CF versus VF VFDsCF versus VF VFDs

Current-fed Type

Lower Cost at High HP

Four- Quadrant

P.F. = P.U. Speed*Load P.F.96.5% Efficiency

Immune to short circuits

Low-Cost Components

Large Magnetics

Lower motor noise

Non-Critical layout

30% Harmonic Current

Low dV/dt at output

Voltage-fed Type

Lower Cost at Low HP

Two-Quadrant

95% displacement P.F.96-97.5% Efficiency

Requires protection

Higher-cost Components

Small or no Magnetics

Higher Motor Noise

Critical Layout

40% Harmonic current

High dV/dt at output

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Harmonic StandardsHarmonic Standards

IEEE-519 1992

Definitions:

Voltage total harmonic distortion (VTHD)

Current total harmonic distortion (CTHD) K-Factor

Point of Common Coupling

VTHD Limits, Table 10.2

CTHD Limits, Table 10.3

Dilution by linear loads There are no Susceptibility Limits!

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THD Definitions from IEEE-519THD Definitions from IEEE-519

Voltage Total Harmonic Distortion VTHD

Current Total Harmonic Distortion CTHD

Sum of squares of amplitudes of all voltage harmonics

Sum of squares of amplitudes of all current harmonics

Amplitude of fundamental voltage

Amplitude of fundamental Current

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Point of Common CouplingPoint of Common Coupling

The point of common coupling is the location in the

power distribution system where harmonic distortion

is to be measured, usually where harmonic currents

flow into a bus which feeds other equipment. Itslocation must be specified!

In the absence of a specified location, the POCC for

current harmonics is the plant-utility interface

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Current DistortionCurrent Distortion

System ExampleSystem Example

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Example CalculationExample Calculation

VTHD CTHD 5 th 7 th 1 1 th 1 3 th 1 7 th 1 9 th 2 3r d 2 5 th 2 9 th 3 1 s t 3 5 th 3 7 th

DRIVE T YPE

PWM 2.5% LR 2.7 1 5 .4 1 4 .2 4. 88 2 .7 4 1 .2 4 1 .1 6 0 .7 1 0. 6 0. 49 0. 34 0 .3 4 0 .2 3 0 .23

PWM 5% LR 2 1 2 .2 1 1 .5 3 .1 1 2 .2 9 1 .2 4 0 .8 3 0 .6 8 0 .3 8 0 .3 8 0 .2 6 0 .2 3 0 .1 9 0 .1 5

CS I 2.5% LR 4 1 0 .8 7 .88 4 .8 3. 3 2 .5 5 1 .9 9 1 .6 9 1.39 1.2 0 .98 0 .86 0 .71 0 .64

CS I 5% LR 3.1 10.3 7.8 4 .6 9 3 .1 1 2 .3 6 1 .7 6 1 .4 3 1.05 0. 86 0. 64 0 .5 3 0 .3 4 0. 3

CS I 12-Puls e 2.2 4.39 0 0 3.15 2.59 0 0 1.13 0.98 0 0 0.41 0.34

PWM 12-Puls e 1 2.78 0 0 2.4 1.24 0 0 0.41 0.41 0 0 0.19 0.15

Cle a np o we r 0.35 1.35 0.98 0.38 0.38 0.45 0.38 0.26 0.15 0.08 0 0 0.04 0

P e rfe c t Ha rmony 9 0.56 1.3 0 0 0 0 1.03 0.72 0 0 0 0 0.2 0.17

P e rfe c t Ha rmony 15 0.31 0.45 0 0 0 0 0 0 0 0 0.32 0.3 0 0

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Current HarmonicsCurrent Harmonics

of Various Drivesof Various DrivesCTHD5th 7th 11th 13th 17th 19th 23rd 25th 29th 31st 35th 37th

DRIVE TYPE

6-P PWM 2.5% LR 41 37.8 13 7.3 3.3 3.1 1.9 1.6 1.3 0.9 0.9 0.6 0.6

6-P PWM 5% LR 32.6 30.6 8.3 6.1 3.3 2.2 1.8 1 1 0.7 0.6 0.5 0.4

6-P CSI 2.5% LR 28.8 21 12.8 8.8 6.8 5.3 4.5 3.7 3.2 2.6 2.3 1.9 1.7

6-P CSI 5% LR 27.5 20.8 12.5 8.3 6.3 4.7 3.8 2.8 2.3 1.7 1.4 0.9 0.8

CSI 12-Pulse 11.7 0 0 8.4 6.9 0 0 3 2.6 0 0 1.1 0.9

PWM 12-Pulse 7.4 0 0 6.4 3.3 0 0 1.1 1.1 0 0 0.5 0.4

Cleanpower 3.6 2.6 1 1 1.2 1 0.7 0.4 0.2 0 0 0.1 0

Perfect Harmony 9 3.47 0.01 0 0 0 2.75 1.93 0 0 0 0 0.53 0.46

Perfect Harmony 15 1.21 0.01 0 0 0 0 0 0 0 0.85 0.81 0 0

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Harmonic Analysis GotchasHarmonic Analysis Gotchas

Analysis assumes perfect phase and amplitude balance in the

power source!

Cable reactance is neglected unless specified -- it can have a

significant effect on the results

Transformers are assumed to have 5.75% impedance unless

otherwise specified

Pre-existing distortion is neglected -- this can be very severe on

generator sources

Power factor correction capacitors are presumed not to be

present unless otherwise specified

FORWARD BACK


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Harmonic FiltersHarmonic Filters

A filter consisting of L-C-R components can be

designed to meet an harmonic requirement

Filter are specific to the power system characteristics

and must be re-designed for every application Filters are large, expensive, wasteful of power and

time-consuming to design

They are especially hard to design when an

emergency generator is the source, or when multiple

utility feeds are involved

FORWARD BACK


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Higher Pulse NumbersHigher Pulse Numbers

Using Higher Pulse Numbers is an effective way to

reduce harmonics.

It reduces the CTHD substantially

Magnetic components are required to provide phase-shifted sources

Additional input conversion devices (thyristors or

diodes are required)

This technique is not affected by power systemimpedance changes.

The Perfect Harmony uses this technique

FORWARD BACK


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Harmonic Myths andHarmonic Myths and

MisconceptionsMisconceptions Diode Input circuits cause no harmonics

DITs Prevent Harmonics from Flowing into the power

system

Higher Order Harmonics (>23) need not beconsidered

BACK


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ConclusionsConclusions

Occasionally, VFDs can have an adverse effect on power

quality, severe enough to cause problems with other equipment.

But, there are a number of simple ways to minimize the effect of

a non-linear load.

Beginning in 1992, drive manufacturers have introduced new

technology to overcome these problems.

Today, one can obtain even the largest VFD of a design (Perfect

Harmony and 18-pulse Clean Power) which presents virtually a

linear, unity power factor load to the line.

Harmonics Signals

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