Drive Harmonics Ieee 519 1992

8/6/2019 Drive Harmonics Ieee 519 1992

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ISA Northern California

Section, South Bay

October 14, 2003

Craig Chidester909 288 7990


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AFDs and Their Effect on

Power Quality


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What Kind of Power Quality Effects?

Power factor? PF = kW / kVA

High motor content means lagging PF

100HP motor, 460V, 93% eff, 119A

(100HP x 0.746kW/HP) / 0.93 = 80.2kW

119A x 460V x 1.73 / 1000 = 94.8kVA

PF = 80.2kW / 94.8kVA = 84.6% @ FL

But at actual load, more like 70% or less

PF is improved with AFDs to 90 95%

AFDs seen as resistive load


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Incoming Sine Wave Notching?

Arises from SCR front ends on AFDs

Forced commutation causes line notches

But modern AFDs use diode front ends Self commutating no notching


5/28


Voltage sag? Standard motor starters allow 650% inrush

Weak power systems are affected

500HP motor on 1000kVA, 5.75%Z Xfmr

650% X (500 / 1000) X 0.0575 = 19% sag

AFD limits inrush to 110% (or 150%)

110% X (500 / 1000) X 0.0575 = 3% sag


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What kind of Power Quality Effects?

Harmonic Distortion

AFDs, DC Drives, UPSs, DC power supplies (computers,duplicators, faxs) willcause current (and voltage)harmonics

Single phase 3rd, 6th, etc (triplens) can cause transformerneutral conductor overheating

Three phase 5th, 7th, 11th, 13th, etc can cause equipmentmalfunctions

Big questions How much? and How much is too much?


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What are Harmonics?

Definition:

Harmonics are integral multiples of some fundamentalfrequency that, when added together, result in a

distorted waveform.


8/28

What are Harmonics?

f(x) = sin(x) f(x) = sin(5x)5

+

The resulting wave shows a strong departure from the smooth

waves comprising it:

f(x) = sin(x) +sin(5x)

5=


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What are Harmonics?

In fact, anyfunction may be constructed from a

sine wave and some number of its harmonics:


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Where do they come from?

The power company typically supplies a reasonablysmooth sinusoidal waveform:


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Where do they come from?

...but nonlinear devices will draw distorted waveforms,which are comprised of harmonics of the source:


12/28

Common sources of Harmonics

Lighting ballasts

UPS systems

MAC and DC drives


13/28

AC drives and Harmonics

ConverterDC bus

&smoothing

Inverter

Determine the line-side

harmonicsDetermines load-side

harmonics


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Inverter

Determines load-side

harmonics

EFFECTS OF LOAD-SIDE

HARMONICS:

Have implications for the motor

insulation and windings.

Essentially have zero effect on

other equipment on the power

system.


15/28


ConverterDC bus

&smoothing

Determine the line-sideharmonics

LINE-SIDE HARMONICS CAN HAVE

FAR-REACHING EFFECTS ON THEPOWER SYSTEM:

Distribution transformers

Standby generators

Communications equipment

Switchgear and relays

Computers, computer systems

Diagnostic equipment


16/28


Typical 6-step converter waveform:

Harmonic Content

I5 = 22.5%

I7 = 9.38%

I11 = 6.10%

I13 = 4.06%

I17 = 2.26%

I19 = 1.77%

I23 = 1.12%

I25 = 0.86%


17/28

Harmonics and transformers

Transformer overheating and potential

insulation failure result from several

conditions caused by harmonics:

Increased skin and proximity effects

Harmonics circulating in the primary

winding

Increased hysteresis losses

Increased eddy current losses

DC in the primary windings

AFCAFC


18/28

Harmonics and transformers

AFCAFC

Many transformers are rated by

K factor which simply describes

their ability to withstand harmonics.

Transformers may also be derated

to compensate for the additionalheating caused by harmonics.

Improved transformer designs have

also been developed, with oversized

neutral busses, special cores, andspecially designed coils.


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Harmonics and power-correction capacitors

Power correction capacitors can cause

series and parallel resonance effects on

a power system.

If a harmonic is generated which excites

a system resonance, amplification of thatharmonic may occur.

Calculation of the harmonic resonance frequency for the

system can give an indication of potential resonance

problems.


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Harmonics and power-correction capacitors

EXAMPLE:

Assume a 1500 kVA supply xfmr,

with a 5.75% impedance.

Also assume 600 kVA of power

correction capacitors on the system

1500 kVA

5.75%

600 kVAC

The harmonic resonance frequency is defined by:

=hr=

kVAsckVAC

1500 / 0.0575 = 6.6

600


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Recommended limits - IEEE 519

The Institute of Electrical and Electronics Engineers (IEEE)

has set recommended limits on both current and voltage

distortion in IEEE 519-1992.

Voltage distortion limits (@ low-voltage bus):

Application class THD (voltage)

Special system 3 %

General system 5 %

Dedicated system 10 %


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Recommended limits - IEEE 519

MAXIMUM HARMONIC CURRENT DISTORTIONin percent of IL

Individual harmonic number (odd harmonics)

Isc/IL


23/28

Attenuation of Harmonics

Inductive Reactance

Method: Add a line reactor or isolation transformer

to attenuate harmonics.

Benefits: Low cost.

Technically simple.

Concerns: Tends to offer reductions in only higher

order harmonics. Has little effect on the 5th

and 7th harmonics.

Because of the associated voltage drop,

there are limits to the amount of reactance

that may be added.


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Passive Filters

Method: Provide a low-impedance path to ground

for the harmonic frequencies.

Benefits: May be tuned to a

frequency between two prevalent harmonics

so as to help attenuate both.

Concerns: Tuning the filters may be a labor-intensive

process.

Filters are difficult to size, because they offer

a path for harmonics from any source.

Quite sensitive to any future system changes.


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Active Filters

Method: Inject equal and opposite harmonics onto the

power system to cancel those generated by

other equipment.

Benefits: Have proven very effective in reducing

harmonics well below required levels.

Concerns: The high performance inverter required for the

harmonic injection is costly.

Power transistors are exposed to conditions

of the line, so reliability may be a problem.


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12-pulse Rectifiers

Method: Two separate rectifier bridges supply a singleDC bus. The two bridges are fed from phase-

shifted supplies.

Benefits: Very effective in the elimination of 5th and 7th

harmonics.

Stops harmonics at the source.Insensitive to future system changes.

Concerns: May not meet the IEEE standards in every

case.

Does little to attenuate the 11th and 13thharmonics.


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18-pulse Rectifier

Method: An integral phase-shift transformer and rectifier

Input which draws an almost purely sinusoidal

waveform from the source.

Benefits: Meets the IEEE standards in every case!

Attenuates all harmonics up to the 35th.

Stops harmonics at the source.

Insensitive to future system changes.

Concerns: Can be expensive at smaller HPs


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Comparison of waveforms

6-pulse converter

12-pulse converter

18-pulse converter

note the level of distortion

and steep current rise.

the waveform appears more

sinusoidal, but still not very

smooth.

virtually indistinguishable

from the source current

waveform.

Documents

Drive Harmonics Ieee 519 1992