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Power Quality Considerations Power Quality Considerations When Applying When Applying

Adjustable Frequency DrivesAdjustable Frequency Drives

Explanations and Various CountermeasuresExplanations and Various Countermeasures

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Power QualityPower Quality

Why the Renewed Interest in Power Quality? Why the Renewed Interest in Power Quality?

• Copy Machines

• Fax machines

• Computers

• Elevator Controls

• Solid State Lighting Ballasts

• Devices that incorporate Static Power Converters -SCRs, Diodes .. etc.

• Adjustable Frequency Drives

Common Issue among Common Devices

7/15/2002 PP.AFD.08 3 of 28

Power Power Quality TopicsQuality Topics

• What are Harmonics?

• What is Harmonic Distortion?

• Differences between current and voltage distortion

• Possible effects of Harmonics

• What Guidance is there in the Industry

• What Solutions does Yaskawa Offer?

Harmonics are important to understand the

relationship between Power Quality and switch

mode power supplies!

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Definition Definition of Harmonicsof Harmonics

• Harmonics are defined as currents or voltages with frequencies that are integer multiples of the fundamental power frequency

• SIMPLY PUT - Harmonics are used to mathematically describe the shape of a curve that is not sinusoidal.

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What is Harmonic What is Harmonic Distortion?Distortion?

• Harmonic Distortion is a mathematical way of describing how non-sinusoidal a wave shape appears

• Fourier Analysis - Sum of the Squares

Every Wave shape has Harmonic Distortion!

THD = 1.2%

THD = 78.3%

TVD Vhh z

==

∞

∑ 2

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Fourier Fourier AnalysisAnalysis

• Fourier Analysis of the waveforms found in a three phase diode rectifier shows low order harmonics including the 5th, 7th, 11th , 13th, etc.

• Calculation of true power factor considers the energies contained on these additional frequencies. Figure 6-2 shows the resulting harmonic spectrum based on Fourier analysis of the current waveform shown in figure 6-1.

Three phase diode rectifier, line voltage/current

-700

-600

-500-400

-300

-200

-100

0

100

200

300

400

500

600

700

Voltage Current

F igure 6.1

100.00%

30.38%

5.55% 7.16%4.83% 4.32% 3.59%

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19

Harmonic Order

Magnitude (as % of Fundamental

Normalized Harmonic Spectrum

F igure 6.2

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Types of Types of HarmonicsHarmonics

DC Drive - SCR Based AC Drive - Diode Rectifier

New Technology May Solve Old Power Quality Problems

SCR Rectification - Line Notching, Increases Voltage Distortion

Diode Rectification - Pulsed Current, Increases Current Distortion

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Possible Effects of Possible Effects of HarmonicsHarmonics

• Increased Transformer Heating

recommended K-Factor of 4 to 13 on new installations

• Increased Conductor Heatinglarger gauge wire

run two wires in parallel

• Electromagnetic Equipment

PLCs - more sensitive to Voltage Notching

• System resonance - Power Factor Correction utilize input reactors to reduce likelihood of resonance

• Lower Power Factor for System

( )PF PF Power Power Power PowerTrue Total al al act Harmonics

= = + +Re Re Re ./

Harmonic Distortion most likely will have no effect on

Power Distribution Performance

7/15/2002 PP.AFD.08 9 of 28

Consider estimating power factor at the

terminals of an AC Drive in a system with

low source impedance (high available short circuit current) with no input line

reactor or DC bus choke.

How Harmonics How Harmonics Lower EfficiencyLower Efficiency

-500

-400

-300

-200

-100

0

100

200

300

400

500

F igure 9.1

pf = kW/kVA

I THD = 92.8%

pf = 1/Sqrt(12+.9282)

pf = 73.3%

Power Factor Considering 92.8% I THD

F igure 9.2

True factor is improved, when current

distortion is limited by system

impedance. (Including reactors, or bus chokes.)

-500

-400

-300

-200

-100

0

100

200

300

400

500

F igure 9.1

pf = kW/kVA

I THD = 32.6%

pf = 1/Sqrt(12+.3262)

pf = 95.08!

Power Factor Considering 32.6% I THD

F igure 9.2

Impedance Improves Efficiency!

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Power Factor When Power Factor When Harmonics ExistHarmonics Exist

• From IEEE Std. 141-1993: Power is the product of in-phase current times the voltage or:

P60 = V60 * I60cos θθθθ

• In the case of harmonics:

Ph = Vh * Ihcos θ θ θ θ or S = (Sqrt(P2 + Q2 +D2)){R1]

• Where P = Real Power, Q = Reactive

Power and D = Distortion Power.

• System losses will be higher due to the harmonic components, than with equivalent 60 kVA.

Ph = I2h * Rh

(Ohm’s law in harmonic-land)

True Power Factor Representation - Expanded

P Real Power (kW)

QReactive

Power

X (kVAr)

F igure 10.1

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The Risk of Parallel The Risk of Parallel ResonanceResonance

• Hp - the harmonic order (per-unit frequency) at parallel resonant frequency

• MVAsc - the system short-circuit capacity

• MVArc - the power factor improvement capacitor

Hp - Sqrt(MVAsc / MVArc)

• Per IEEE Red Book (Std. 141-1993): “If the SCR (short circuit ratio is less than 20), and there is a parallel resonance condition near a characteristic harmonic of the non-linear load, there will be a problem.”

• Since all power systems have inductance and capacitance, they will resonate at a given frequency.

When an exciting energy at that frequency, in a quantity that is large enough to offset the natural dampening in the system, is present, resonance will occur. In a typical industrial or commercial building, the natural resonance frequency is likely to be in the range of 1000 Hz. It is a real world possibility that a system can resonate.

X

cXL

ih

ih

Resonance occurs when: Xc = XL

Power Factor Capacitors Relieve Load [R2]

F igure 11.1

Parallel Resonance

Current measured at the capacitor,

showing 660Hz, (11th harmonic resonance)F igure 11.2

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What Guidance What Guidance is there in Industry?is there in Industry?

IEEE 519 - 81

Table Two: Voltage Distortion Limits

Bus Voltage at PCC Individual

Voltage

Distortion (%)

Total Voltage

Distortion THD

(%)

69 kV and below 3.0 5.0

69.001 kV through 161 kV 1.5 2.5

161.01 kV and above 1.0 1.5

UTILITY

USER A USER B

VTHD A VTHD B

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IEEE519IEEE519--9292

Table Three: Current Distortion Limits for General Distribution Systems

(120 V through 69 kV)

Maximum Harmonic Current Distortion

in Percent of Load Current

ISC/IL <11 11≤h<17 17

≤h<23 23

≤h<35 35

≤h 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

Even harmonics are limited to 25% of the odd harmonic limits above.

where

ISC = Maximum short-circuit current at PCC.

IL = Maximum demand load current (fundamental frequency

component) at PCC.

Specification has created a problem

• Compare Short Circuit Capacity to Maximum Load Current

• Determine Point of Common Coupling

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IEEE519IEEE519--9292

• Addressed Current Distortion

• Does not clarify PCC

• Limits do not increase sufficiently closer to non-linear device

• Does not clarify injected harmonics

• Is not clear that goal is reasonable Voltage Distortion

Don’t Make Your Customer Pay for Poor Guidance

Results:Pass / Fail Dependant on Point of

Measurement

Specification is not intended to comply

internally

All electrical products are lumped together

Double the price of a VFD package to make a

waveform look Pretty!

Issues:

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Countermeasures with Countermeasures with Yaskawa DrivesYaskawa Drives

• Correctly Sized Input Transformer (Kfactor Rated)

• Standard AFD

• DC Link Choke (Standard from 30 Hp to 250 Hp)

• AC Input Reactor (Optional on All Drives)

• Custom Designed Low Pass or Broad Band Filters

(Optional on All Drives)

• 12-Pulse Transformer (Optional from 30 Hp thru 1000 Hp

The Solution to fit the Specification

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C u r re n t

A m p s

0

1 0 0

2 0 0

-1 0 0

-2 0 0

.

Standard Standard 66--Pulse Front EndPulse Front End

• Distortion Levels can vary from 60% to 130%

• Dependent on stiffness of power transformer

• Not a problem

• Input Current Waveform

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C u r r e n t

A m p s

0

5 0

1 0 0

- 5 0

- 1 0 0

.

DC Link ChokeDC Link Choke

• Reduce Distortion by 50% from standard 6 Pulse Drive

• Represent 3% input impedance to Line Power

• Current Waveform

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C u r r e n t

A m p s

0

5 0

1 0 0

- 5 0

- 1 0 0

.

AC Input ReactorAC Input Reactor

• Reduce Distortion by 50%, dependent on System Impedance

• Reduce Nuisance Trips from surge voltages

• Current waveform

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Low Pass / Low Pass / Broad Band FiltersBroad Band Filters

• Designed to take 5th and 7th Harmonic out of system

• Series Resonant Tuned

• Leading Power Factor

fLC

= 12( )π

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Shunt FiltersShunt Filters

The development of shunt filters to correct harmonic distortion has lead to the acceptance of two common technologies, under one heading, but with dramatic differences.

1.) Power factor capacitor providers often use

tuning reactors to de-tune power factor back, and apply it on a bus. These filters require careful field evaluation and harmonic analysis to assure

effectiveness and prevent against resonance. Some are tuned to 240Hz, and are simply called de-tuned p.f. banks, others are tuned to 300Hz, and act as a shunt filters for harmonic energies.

2.) Drives applied filters which include a 5% series

inductor and are applied in front of a single drive load, and correct harmonic distortion for that particular drive. Drive applied filters generally do

not require rigorous system analysis, and there is not a risk of resonance.

Typical de-tuned p.f. bank

F igure 20.1

Drive Applied Harmonic

Filter

F igure 20.2

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1212--Pulse TransformerPulse Transformer

• Reduce Distortion by 92%

• Lowest Levels in the Industry

• Input Current Waveform

C u r r e n t

A m p s

0

5 0

1 0 0

- 5 0

- 1 0 0

.

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MultiMulti--pulse Converters pulse Converters Using Using

Phase Shifting TransformersPhase Shifting Transformers

• Theoretical phase cancellation in transformer primary eliminates low harmonics. In practice, phase cancellation is dependant on current and voltage phase balance and current sharing between bridges.

• Can limit the level of current harmonic distortion to 5-15% depending on transformer configuration, bridge symmetry, and source impedance..

Twelve-pulse Drive using Zig-Zag Transformer

ToDCBus

FromDCBus

FromSource

Transformer

RectifiersF igure 22.2

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Example!Example!

Information Needed:

- Simplified One Line Diagram

- Electrical Schedule (preferred)

- Ratio of linear load / Non Linear Load

Quick Estimate based upon Electrical Schedule

System Properties:

Connected kVA = 769

Impedance: 5.75%

Non Linear kVA = 114

Xfmr Size = 1000 kVA

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PCC Service atPCC Service atEntrance of Bldg.Entrance of Bldg.

Power System Distribution Specification1000 Kva @ 5.75 % impedance PCC = Service Entrance

655 Hp Linear Load ISC/IL = 22.7

115 Hp MagneTek Drive Load

Harmonic Reduction

4%2% 1% 1% 1%

13%

5% 5%

2% 1%

0%

5%

10%

15%

6 Pulse DC Link AC Input

Reactor

Low Pass

Filter

12 Pulse

Xfmr

Power Distribution Rule of Thumb:

Keep Voltage Distortion Below 5% - Normal Conditions

7.5% Start Up, Unusual Conditions

IEEE519-92, ITHD < 8.0%

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PCC at PCC at VFD TerminalsVFD Terminals

Power System Distribution Specification1000 Kva @ 5.75 % impedance PCC = At VFD Terminals

115 Hp MagneTek Drive Load ISC/IL = 153

Harmonic Reduction

4% 2% 1% 1% 1%

87%

34% 33%

13%8%

0%

10%

20%

30%

40%

50%

60%70%

80%

90%

6 Pulse DC Link AC Input

Reactor

Low Pass

Filter

12 Pulse

Xfmr

Power Distribution Rule of Thumb:

Keep Voltage Distortion Below 5% - Normal Conditions

7.5% Start Up, Unusual Conditions

IEEE519-92, ITHD < 15.0%

7/15/2002 PP.AFD.08 26 of 28

Summary:Summary:Effects of HarmonicsEffects of Harmonics

#1 Effect of Harmonic is increased heating of supply transformer

System Properties:

Xfrm Rating: 2500 / 3125 FC kVA Connected HP: 2785

Impedance: 5.67% Demand Hp: 1950

Estimated ITHD = 23.9%

I Load = 3639 Amps

Estimated Current Harmonics : I Load * THD = 870 Amps

IRMS = (I Load^2 + I Harm ^2)^.5 = 3742 Amps

Supply Xfmr KVA = Voltage * Current * 1.73 = 480 * 3742 * 1.73 = 3111 kVA

Capacity Reduction due to Current HarmonicsKVA = Voltage * Current * 1.73 = 480 * 3639 * 1.73 = 3021 kVA

Difference = 3111 - 3021 = 89 kVA = 2.8 % Losses at Full Capacity

Simple Calculations to put End User at Ease!

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Comparative Cost of Comparative Cost of Harmonic Mitigation DevicesHarmonic Mitigation Devices

F igure 27.2

• Assumptions:• 10,000 installed base cost

of 6-pulse drive.• Values will vary for lower

HP drives.

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Benefits Outweigh Benefits Outweigh ChallengesChallenges

•• ImproveImproveBuilding Owner’s Bottom Line Thru Energy EfficiencyBuilding Owner’s Bottom Line Thru Energy Efficiency

•• Reduce Wear /TearReduce Wear /Tear on Pumps, Belts, Seals, Bearings …etcon Pumps, Belts, Seals, Bearings …etc

•• EliminateEliminateDemand charge due to Inrush from Line starting MotorDemand charge due to Inrush from Line starting Motor

•• Gain Gain Precise Control on Varying Climate in Building thru AutomationPrecise Control on Varying Climate in Building thru Automation

•• Educating on Power QualityEducating on Power Quality

•• Electrical Noise IssuesElectrical Noise Issues

•• Motor Design CompatibilityMotor Design Compatibility

SpeedSpeed

WindingsWindings

Long Lead LengthLong Lead Length

RetrofitsRetrofits