PQ Fundamentals

7/29/2019 PQ Fundamentals

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Copyright 2001 by Prof. S. S. Venkata.

Electric Power Quality

by

S. S. (Mani) Venkata

Iowa State University

Ames, Iowa


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OUTLINE

PART I: Power Quality Definitions & Concepts.

PART II: Sources & Mitigation Schemes.

PART III: Case Study of Practical Example.


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Electric Power Quality

TutorialPart I: Power Quali ty

Definitions & Conceptsby

S. S. (Mani) Venkata

Iowa State University

Ames, Iowa

December 29, 2001


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Copyright 2001 by Prof. S. S. Venkata. PQ TUTORIAL: PART I 4/57

Power Quality Definitions

What is Power Quality (PQ) ?

Why is it important to Energy Suppliers?

Why is it important to Customers?

Typical PQ Problems.

Impact of PQ on Utilities and Customers.

Focus on Three Aspects of Power Quality Harmonics

Voltage Sags

Voltage Flicker

Review of Power Concepts under Non-sinusoidalConditions.


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Classical Distribution Systems


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Future Distribution Systems


7/57Copyright 2001 by Prof. S. S. Venkata. PQ TUTORIAL: PART I 7/57

What is Power Quality?

Power quality broadly refers to the

delivery of a sufficiently high grade ofelectric service.

In general, it involves maintaining a

sinusoidal load bus voltage at stipulatedmagnitude and frequency.



Voltage/Current Waveforms at

a Veneer Plant



Typical Power Quality Problems

1. Disturbances

- Voltage Dip (SAG)

- Brief Voltage Increases (SWELLS)

- Outages

- Transients

- Voltage Notches

2. Unbalance

3. Distortion

- Voltage Harmonics

- Current Harmonics

4. Voltage Fluctuation- Step Voltage Changes (regular or irregular)

- Cyclic or Random Voltage Changes

5. Flicker



Typical Power Quality Problems (cont.)



Why is Power Quality

Important?

It affects both utilities assuppliers and customers as

users



Impact on Customer Side

Computers and communication equipment are

susceptible to power system disturbances which

can lead to loss of data and erratic operation.

Automated manufacturing processes such as

paper-making machinery, chip-making

assembly lines, etc. can shutdown in case of

even short voltage sags.


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Impact on Customer Side (cont.)

Induction and synchronous motors can have excessive

losses and heating.

Home electronic equipment are vulnerable to powerquality problems - e.g., blinking VCR machines and

digital clocks.

Equipment and process control malfunction translatesto dollars of expense for replacement parts and for

down time, impacting adversely on profitability and

product quality.


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Impact on Utility Side

Failure of power-factor correction capacitors

due to resonance conditions.

Increased losses in cables, transformers andconductors, especially neutral wires.

Errors in energy meters, which are calibrated

to operate under sinusoidal conditions.


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Impact on Utility Side (cont.)

Incorrect operation of protective relays,particularly in solid-state and microprocessor-

controlled systems.

Interference with ripple control and power line

carrier systems used for remote switching, load

control, etc.

Unhappy customers as well as malfunction and

failure of system components and control

systems, impacting adversely on profitability.


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Total Cost of PQ Problems &

Solutions Annual Cost of Problems

Estimated as $ 180 M to Users + $ 300 M to Utilities,

Total of $ 480 M per year

Total cost of solutions

Estimated as $3+ Billion by EPRI based on the amount

expended by the industry to mitigate PQ problems


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Focus on Three Aspects of Power

Quality

Harmonics

Voltage Sags

Voltage Flicker


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Harmonics and Their Impact

on Equipment and Systems Linear Loads: draw Currents proportional to

applied voltages.

Examples: incandescent lighting, heatingand motor loads

Non-linear loads: draw current only a part of

the voltage cycle.

Examples: computers, adjustable speed

drives and programmable logic converters.


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Harmonics and Their Impact

on Equipment and Systems The resulting current from nonlinear

loads contains 3rd, 5th, 7th, ...harmonics.

Harmonic currents permeate into source

currents.

Source currents having harmonic content

impact source voltage.


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Metering Errors

Induction Watt-Hour Meter

Induction watt-hour meters work on the same principles

as an induction motor.

Positive sequence harmonics aid disk rotation.

Negative sequence harmonics retard the disk.

1 2 3 4 5 6 7 8 9 10

+ - 0 + - 0 + - 0 +

0

0.2

0.4

0.6

0.8

1

0 120 240 360 480 600Hz

Reading/Actual

Meter Frequency Response


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Linear Load: the consumer pays for unused energy

due to voltage distortion.

Non-Linear Load: the consumer pays even more forunused energy due to both voltage and current

distortion

Both Cases: the metering error is more significantwhen the load is light and the harmonic energy is alarge % of the energy transferred through the meter

Metering Errors (cont.)

Linear

Load

>

>

W

>


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Harmonic Resonance

Parallel Resonance Series Resonance

HarmonicSource

EQUIVALENT CIRCUIT

Capacitor

Capacitor HarmonicSource

ONE-LINE DIAGRAM

Source

Impedance

Cap.

Capacitor

ONE-LINE DIAGRAM

Line

EQUIVALENT CIRCUIT

Line

impedance

Harmonic

Source

HarmonicSource

Source

Impedance


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Harmonic Resonance (cont.)

Series resonance caused bytransformer and secondarycapacitor.

Harmonic source on theprimary can cause a highvoltage distortion on thesecondary.

This can result in capacitor

failure if the capacity ofharmonic source(s) on theprimary is larger than thecapacitor rating.

Primary

Loads

EQUIVALENT CIRCU IT

Harmonic

Source

Capacitor

Transformer

Reactance

ONE-LINE DIAGRAM

Capacitor

Secondary

Harmonic

Source

Loads


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Example of Resonance


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Harmonics Analysis

The Fourier Series:

Any periodicwaveform can be

represented by aninfinite series of sinewaves havingfrequencies which aremultiples of the

fundamentalfrequency, i.e.,harmonics.


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Fourier Series-Basic Equations

Let f(t) be a periodic waveform with fundamental frequency 0

The Fourier Series Representation of f(t) is:

f(t) = a0 + a1cos0t + a2cos20t + + b1sin0t + b2sin20t + (1.1)

= a0 + ancos(n

0t) + bnsin(n

0t) (1.2)

1n

1n


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Fourier Series - Basic Equations (cont.)

where

a0 = (d.c. component) (1.3)

an = (1.4)

bn = (1.5)

T

0

dtf(t)

T

1

T

0

0dttnsinf(t)

T

2

dttncosf(t)T

2T

0

0


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We have from Eq. 1.3 to Eq. 1.5:

0.1.1T

1T

0

T2

T

0

dtdta i.e., no dc component

0.cos.1.cos.1T

2T

0

2

00

T

T

n dttndttna

T

0

T2

T

0n t.dtnsin.1t.dtnsin.1T

2b

0

n

4

if n is odd

0 if n is evenThus f(t) can be represented as :

...7sin7

15sin

5

13sin

3

1sin

4)(

0000tttttf


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Example of Fourier Series


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Harmonics Indices

THD: Total Harmonic Distortion: Ratio of rmsvalue of total harmonic content to rms value offundamental

TIF:Telephone Interference Factor

C-Message Weights

V.T and I.T Products


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Total Harmonic Distortion (THD)

The most commonly used power quality measure

It is defined as the ratio of the root-mean square of the

harmonic content to the root-mean square value of the

fundamental quantity. Frequently the THD is

expressed in percent

1

2

5

2

4

2

3

2

2 ...

V

VVVVTHD

(for voltage)

1

2

5

2

4

2

3

2

2 ...

I

IIII (for current)


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THD (cont.)

The THD is zero for a perfectly sinusoidal wave. It

increases indefinitely as the waveform distortion

increases.

A THD of5% is commonly cited as the border line

between high and low distortion for distributioncircuits.

Balanced THD includes only positive and negative

sequence signals

Residual THD includes only triplen or zero-sequence signals only.


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Telephone Influence Factor (TIF)

The TIF is a variation of the THD in which the

harmonic components are weighted by factors whichreflect:

The frequency response of the human ear and

The variation of the inductive coupling between adjacent

circuits with frequency.

It is defined as: TIF =

(Frequently, the TIF is expressed in percent.)

The ANSI 368 Standard recommends truncation of the

infinite series at 5.0 kHz.

.. .

.. .IIIII

2

5

2

4

2

3

2

2

2

1

2

5

2

4

2

3

2

2

2

12

5

2

4

2

3

2

2

2

1

IIIII

wwwww


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Telephone Influence Factor (TIF)

(cont.) The TIF is often used to asses interference of

power distribution circuits with audio

communication circuits.

It is useful for assessing interference withanalog telephone circuits, but is not indicative

of interference with circuits which usetechniques such as pulse code modulation(PCM).

Table. C-message and TIF weighting coefficients


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g g g

TIF weighting factors vs. frequency


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C-message weights

The C-message weighted index is similar to the TIF

except that weights ci are used instead of w

C=

The C-message weights are related to the TIF weights

as follows:

...

...IcIcIcIcIc2

5

2

4

2

3

2

2

2

1

252423222125

24

23

22

21

IIIII

ii wcif 05


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C-message weights

Unlike the TIF weights, the C-message

weights do not take into consideration

linear variation of mutual coupling ofcircuits with frequency.


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V.T and I.T products

The THD does not take into account the strength (level)

of the signal. The V.T product is an alternative index

which incorporates the voltage amplitude.

V.T =

ws are weights that are listed in Table 2.2 of Heydts

book

The I.T product is a similar index for line currents.

I.T =

...VVVVV2

5

2

4

2

3

2

2

2

125

24

23

22

21

wwwww

Vrms TIFV .

...IIIII2

5

2

4

2

3

2

2

2

12

5

2

4

2

3

2

2

2

1 wwwww

Irms TIFI .


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Voltage Sags

A voltage sagis when a customer experiences

temporary voltage levels lower than a specified

level (between 0.9 and 0.1 pu)

Causes:

Short-circuit conditions (Faults)

Starting large motors, etc.


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Voltage Sags

Effects:

Duration-dependent

Failure of computer equipment

Outages of sensitive process plants

Measures

CBEMA Curve (1978): less stringent restrictions ITIC Curve (1996): demands more severe

performance standards


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The CBEMA (Computer Business Equipment manufacturers

Association) Curve

0.0001 0.001 0.01 0.1 1 10 100 1000

-100

-50

0

50

100

150

200

250

TIME IN SECONDS

PERC

ENTCHANGEINBUSVOLTA

GE

8.33ms

OVERVOLTAGE C ON DITION S

UNDERVOLTAGE CONDITIONS

0.5CYCLE

RATED

VOLTAGE

ACCEPTABLE

POWER


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The ITIC (Information Technology Industry Council) Curve

0.0001 0.001 0.01 0.1 1 10 100 1000

-100

-50

0

50

100

150

200

250

TIME IN SECON DS

PERC

ENTCHANGEINBUSVOLTAGE

8.33ms

OVERVOLTAGE CONDITIONS

UNDERVOLTAGE CONDITIONS

0.5CYCLE

RATED

VOLTAGE

ACCEPT ABLE

POWER

10%+--


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Alternative Power Acceptability Curves

Curve Year Application Source

FIPS poweracceptability

curve

1978 Automatic dataprocessing

(ADP)

equipment

U.S. federalgovernment

CBEMAcurve

1978 Computer businessequipment

ComputerBusiness

Equipment

Manufacturers

AssociationITIC curve 1996 Information

technologyequipment

InformationTechnology

IndustryCouncil

Failure ratecurves for

industrial

loads

1972 Industrial loads IEEE Standard493

AC linevoltage

tolerances

1974 Mainframecomputers

IEEE Standard446

IEEEEmerald

Book

1992 Sensitiveelectronic

equipment

IEEE Standard1100


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If loads such as arc furnaces cause

variation in the distribution bus voltage

which has a spectral characteristicwhich lies between a fraction of a Hertz

and about one third of the system

frequency, this condition is calledflicker.

Voltage Flicker Definition


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Flicker is a characteristic where a high frequency

(0) sinusoid is modulated by a low frequency

sinusoid (f).

Mathematically,

v(t) = (1 + Vfcos(ft)) Vmcos (0t)

Side-band frequencies of (0f) will be present.



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Intensity of Flicker, F

where Sscf = short-circuit kVA at electrode tip

Ssc = short-circuit kVA at PCC(point of common coupling)

Perceptibility of Flicker depends on both

Vfand f.


SS

VV

sc

scf

m

f

Power Component


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Power Component

Definitions:P,Q,S,...

Sinusoidal System Power Concepts

tsin2 Vv ; )-t(sin2 Ii

t2sinsint2cos1cos* VIVIivp

t2sint2cos1

QP powerorcos activerealVIP

powersin reactiveVIQ

power22 apparentQPVIS

cos/.. SPfp

Graphical Interpretation


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Graphical Interpretation


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Physical Interpretation

Real power, P is the average value of instantaneous

power. It represents the useful power being

transmitted.

Reactive power, Q is the peak value of that power

component which travels back and forth on the line,

resulting in the zero average.

Apparent power, S determines the loading of the

system and is used for rating power apparatus.

Power factor of a system is an indicator of the

efficiency with which power is transmitted. It is

desirable to have a power factor close to 1.


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Non-Sinusoidal System Power Concepts

Total Harmonic Distortion (THD)

Rms voltage =V = VRMS =

Rms current = I = IRMS =

Apparent power, S = VI = VRMS IRMS

Real power, P =

1

2

4

2

3

2

2 ...

VVVV

.. .2

4

2

3

2

2

2

1 VVVV

...2

4

2

3

2

2

2

1 IIII

...coscoscos 333222111 IVIVIV


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Reactive Power = ?

Budeanu

s Definition:

Distortion power, D =

Fryzes Definition:

Reactive power,

...sinsinsin 333222111 IVIVIVQB

222QPS

22PSQF

222DQQ

BF


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Power Components for Non-sinusoidal

Conditions


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References

S. S. Venkata, G. T. Heydt, Proceedings of the NSF Workshop

on Electric Power Quality,Jan. 1991, Grand Canyon, AZ, USA.

2. J. Arrillaga, N. R. Watson, S. Chen, Power System Quality

Assessment,John Wi ley & Sons, England, 2000.

3. R. C. Dugan, M. F. McGranaghan, H. W. Beaty, Electrical

Power Systems Quality,McGraw-Hil l, USA,1996.

4. G. T. Heydt, Electric Power Quality,Stars in a Circle, USA,

1991.

5. E. Acha, M. Madrigal, Power Systems Harmonics: Computer

Modeling and Analysis,John Wi ley & Sons, England, 2001.

6. A. E. Emanuel,

IEEE Tutorial Course: NonsinusoidalSituations Effects on The Performance of Meters and

Definitions of Power,I EEE, USA,1990.

Documents

PQ Fundamentals