unit 3_on 24022012

IFETCE/EEE/VIIIsem/PQ/EE2028/PPT/VER 1.0

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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 AFD’s– Forced commutation causes line notches– But … modern AFDs use diode front ends– Self commutating … no notching


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• 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 is K-Factor?

• K-factor is a weighting of the harmonic load currents according to their effects on transformer heating, as

• The higher the K-factor, the greater the harmonic heating effects.


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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?”


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What are Harmonics?

Definition:

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


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What are Harmonics?

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

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+

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, any function 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:


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Common sources of Harmonics

Lighting ballasts

UPS systems

MAC and DC drives


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AC drives and Harmonics

ConverterDC bus

&smoothing

Inverter

Determine the line-sideharmonics

Determines load-sideharmonics


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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.


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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


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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%


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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


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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.


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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 excitesa system resonance, amplification of thatharmonic may occur.

Calculation of the harmonic resonance frequency for thesystem 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 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


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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 %


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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.


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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.


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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.


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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.


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


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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


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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.


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Harmonics

• Harmonics is a sinusoidal component of a periodic wave or quantity having a frequency that is an integral multiple of the fundamental power frequency.

• The equation representing a harmonic frequency (fh )is given by,

fh = f1 × h


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• Where f1 is the fundamental frequency and h is the harmonic order.

• For example, if the fundamental power frequency is 50 Hz, then the 2nd harmonic is 100 Hz; the 3rd harmonics is 150 Hz, etc.

• As their names imply, odd harmonics have odd numbers (e.g., 3, 5, 7, 9, 11), and even harmonics have even numbers (e.g., 2, 4, 6, 8, 10).

• Harmonic number 1 is assigned to the fundamental frequency component of the periodic wave.


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• Harmonic number 0 represents the constant or DC component of the waveform.

• The DC component is the net difference between the positive and negative halves of one complete waveform cycle.


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Waveform Distortion composed of fundamental and 5th harmonics


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Waveform Distortion composed of fundamental and 3rd harmonics


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• The majority of nonlinear loads produce harmonics that are odd multiples of the fundamental frequency.

• Certain conditions need to exist for production of even harmonics.

• Uneven current draw between the positive and negative half cycle of operation can generate even harmonics.

• The uneven operation may be due to the nature of the application or could indicate problems with the load circuitry.


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• Transformer magnetizing currents contain appreciable levels of even harmonic components and so do arc furnaces during startup.


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Sub-harmonics

• Sub-harmonics have frequencies below the fundamental frequency and are rare in power systems.

• When sub harmonics are present, the underlying cause is resonance between the harmonic currents or voltages with the power system capacitance and inductance.


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Sub-harmonics

• Sub-harmonics may be generated when a system is highly inductive (such as an arc furnace during startup) or if the power system also contains large capacitor banks for power factor correction or filtering.

• Such conditions produce slow oscillations that are relatively undamped, resulting in voltage sags and light flicker


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Fourier series representation


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Basics of the harmonics phenomena


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Harmonic content of a 6-pulse converter


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Harmonic components are added to the fundamental


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POWER DEFINITIONS UNDER NON-SINUSOIDAL CONDITIONS

• Apparent power S (volt-ampere-VA): The product of the RMS voltage and current.

• Active power P (watts-W): The average rate of delivery of energy.

• Reactive power Q (reactive volt ampere-VAR): The portion of the apparent power that is out of phase, or in quadrature, with the active power


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Displacement and True Power Factor


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• True power factor is calculated as the ratio between the total active power used in a circuit (including harmonics) and the total apparent power (including harmonics) supplied from the source

• True power factor = Total active power (P)/Total apparent power(S)


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Example of Linear loads


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Example of Non-linear load


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Harmonic distortion Wave form


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Voltage and Current Waveform Distortion

Phenomenon


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• IEEE 519-1992, “Recommended Practices and Requirements for Harmonic Control in Electrical Power Systems,” recognizes this by basically saying:


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IEEE 519-1992• The control over the amount of harmonic

current injected into the system takes place at the end-use application.

• Assuming the harmonic current injection is within reasonable limits, the control over the voltage distortion is exercised by the entity having control over the system impedance, which is often the electric utility.


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Effect of load voltage due to harmonic current flowing

through the system impedance


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Causes of Harmonics

• Harmonics can arise –in the generating system, –in the distribution system, and –from the loads connected to the network.

• If a generator produces a non-ideal sinusoidal waveform, the voltage waveform will contain a certain amount of harmonics.


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• Harmonic current generation from non-linear loads (Non-Linear loads are those in which the load does not draw a sinusoidal current) .


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Harmonics Effects

• The most notable effects that harmonics have on a power system are impact on the quality of the AC voltage waveform (i.e., it will become distorted), thus causing problems with other sensitive loads connected to the same supply.

• In transformers, harmonic currents cause the RMS current to be greater than its capacity, leading to increased conductor loss and heating.


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• In motors, decreased efficiency, excessive heating, and vibration are symptoms of harmonic voltage distortion.

• The tripping of protective relaying, telephone interference, and false meter readings are other consequences of harmonics in power systems.


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Common Sources of Harmonics

In general, sources of harmonics are divided into:

(a) Commercial loads

(b) Industrial loads

(c) Residential loads


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Common Sources of Harmonics

unit 3_on 24022012

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