isa northern california section, south bay october 14, 2003 craig chidester 909 288 7990

ISA Northern California Section, South Bay

October 14, 2003

Craig Chidester909 288 7990

AFDs and Their Effect on Power Quality

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


• Incoming Sine Wave Notching?– Arises from SCR front ends on AFD’s

– Forced commutation causes line notches

– But … modern AFDs use diode front ends

– Self commutating … no notching


• 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

What kind of Power Quality Effects?

• Harmonic Distortion

– AFDs, DC Drives, UPSs, DC power supplies (computers, duplicators, fax’s) will cause current (and voltage) harmonics

• Single phase – 3rd, 6th, etc (triplens) can cause transformer neutral conductor overheating

• Three phase – 5th, 7th, 11th, 13th, etc can cause equipment malfunctions

• Big questions – “How much?” and “How much is too much?”

What are Harmonics?

Definition:

Harmonics are integral multiples of some fundamentalfrequency that, when added together, result in adistorted waveform.

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 =

What are Harmonics?

In fact, any function may be constructed from a sine wave and some number of its harmonics:

Where do they come from?

The power company typically supplies a reasonablysmooth sinusoidal waveform:

Where do they come from?

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

Common sources of Harmonics

Lighting ballasts

UPS systems

MAC and DC drives

AC drives and Harmonics

ConverterDC bus

&smoothing

Inverter

Determine the line-sideharmonics

Determines load-sideharmonics


Inverter

Determines load-sideharmonics

EFFECTS OF LOAD-SIDE HARMONICS:

Have implications for the motor insulation and windings.

Essentially have zero effect on other equipment on the powersystem.


ConverterDC bus

&smoothing

Determine the line-sideharmonics

LINE-SIDE HARMONICS CAN HAVE FAR-REACHING EFFECTS ON THE POWER SYSTEM:

Distribution transformers

Standby generators

Communications equipment

Switchgear and relays

Computers, computer systems

Diagnostic equipment


Typical 6-step converter waveform:

Harmonic ContentI5 = 22.5%I7 = 9.38%I11 = 6.10%I13 = 4.06%I17 = 2.26%I19 = 1.77%I23 = 1.12%I25 = 0.86%

Harmonics and transformers

Transformer overheating and potentialinsulation failure result from severalconditions caused by harmonics:

Increased skin and proximity effects

Harmonics circulating in the primarywinding

Increased hysteresis losses

Increased eddy current losses

DC in the primary windings

AFCAFC

Harmonics and transformers

AFCAFC

Many transformers are rated by “K factor” which simply describes their ability to withstand harmonics.

Transformers may also be deratedto compensate for the additionalheating caused by harmonics.

Improved transformer designs have also been developed, with oversized neutral busses, special cores, and specially designed coils.

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 excitesa system resonance, amplification of thatharmonic may occur.

Calculation of the harmonic resonance frequency for thesystem can give an indication of potential resonance problems.

Harmonics and power-correction capacitors

EXAMPLE:

Assume a 1500 kVA supply xfmr, with a 5.75% impedance.

Also assume 600 kVA of powercorrection capacitors on the system

1500 kVA5.75%

600 kVAC

The harmonic resonance frequency is defined by:

= hr =

kVAsc

kVAC

1500 / 0.0575 = 6.6

600

Recommended limits - IEEE 519

The Institute of Electrical and Electronics Engineers (IEEE)has set recommended limits on both current and voltagedistortion in IEEE 519-1992.

Voltage distortion limits (@ low-voltage bus):

Application class THD (voltage)

Special system 3 %

General system 5 %

Dedicated system 10 %

Recommended limits - IEEE 519

MAXIMUM HARMONIC CURRENT DISTORTION in percent of IL

Individual harmonic number (odd harmonics) Isc/IL <11 11<h<17 17<h<23 23<h<35 TDD <20 4.0 2.0 1.5 0.6 5.0 20-50 7.0 3.5 2.5 1.0 8.0 50-100 10.0 4.5 4.0 1.5 12.0 100-1000 12.0 5.5 5.0 2.0 15.0 >1000 15.0 7.0 6.0 2.5 20.0

Isc: Maximum short-circuit current at the Point of CommonCoupling (PCC).

IL: Maximum demand load current (fundamental) at the PCC.

Attenuation of Harmonics

Inductive Reactance

Method: Add a line reactor or isolation transformerto attenuate harmonics.

Benefits: Low cost.

Technically simple.

Concerns: Tends to offer reductions in only higherorder 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.


Passive Filters

Method: Provide a low-impedance path to groundfor the harmonic frequencies.

Benefits: May be tuned to afrequency between two prevalent harmonicsso as to help attenuate both.

Concerns: Tuning the filters may be a labor-intensive process.

Filters are difficult to size, because they offera path for harmonics from any source.

Quite sensitive to any future system changes.


Active Filters

Method: Inject equal and opposite harmonics onto thepower system to cancel those generated by other equipment.

Benefits: Have proven very effective in reducingharmonics 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.


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 everycase.Does little to attenuate the 11th and 13th harmonics.


18-pulse Rectifier

Method: An integral phase-shift transformer and rectifier Input which draws an almost purely sinusoidalwaveform 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 HP’s

Comparison of waveforms

6-pulse converter

12-pulse converter

18-pulse converter

note the level of distortionand steep current rise.

the waveform appears moresinusoidal, but still not very smooth.

virtually indistinguishablefrom the source currentwaveform.

isa northern california section, south bay october 14, 2003 craig chidester 909 288 7990

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