PowerSystems Introduction for Non Engineer

8/12/2019 PowerSystems Introduction for Non Engineer

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Power Systems

for theNon Power Engineer

W.O. (Bill) Kennedy, P.Eng., FEIC

Copyright 2004 W.O. (Bill) Kennedy


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PurposeGive a basic understanding of how

power systems are put togetherand how they work

Concepts will be emphasizedMathematics will be kept to a

minimumMathematics only when necessary


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IntroductionTwo parts

First part covers power systemcomponents

Second part covers how thecomponents fit together and work

along with some measures ofpower system performance


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A little bit of PhysicsHans Christian Oerstead discovered the

relationship between magnetism andelectricity

Michael Faraday discovered that avoltage is induced on a wire when itsmoved in or through a magnetic field

James Clerk Maxwell developed themathematics of electromagnetics


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Real and Reactive PowerReal power does the work

Reactive power helps real power

do the work

Power systems need both or they

wont work

What is reactive power?


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Reactive power Quarterback can

throw a bullet, butnot very far

For long distances,

throws in an arc Real power is the

bullet

Reactive power isthe height of the arc


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Reactive Power Capacitors store energy equal

to CV2

Capacitor banks are used to

boost or raise voltage

Reactors use energy equal to

LI2

Motors and fluorescent lights

require reactive power


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Part 1 - Equipment

Generators

TransformersTransmission Lines

Loads


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Generators


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GeneratorsFundamental Law

E = N d/dt

Where is the flux

Magnetic example

High school physics

Faraday's discovery motionMaxwell mathematical theory


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GeneratorsRotor turns inside of the generator

satisfying Faradays LawVoltage induced on the stator follows

a sine waveTake advantage of space and put three

coils equally spaced, 120o apart


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GeneratorsThree Phase

-1.5

-1.0

-0.5

0.0

0.5

1.0

1.5

0 45 90 135 180 225 270 315 360

Degrees

Magnitude

Phase A

Phase B

Phase C

Motion of rotor induces a voltage on the stator

Stator doesnt move and waveform reflects effect of

rotor field as it moves inside the machine


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GeneratorsControl

Terminal voltageSpeed

Terminal voltage controlled by varyingthe voltage applied to the dc field of therotor

Speed controlled by governor, as loadincreases, fuel supply increases


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GeneratorsSpeed and frequency (60 Hz)

Frequency (f) = n/60 * p/2Poles are in pairs, hence divide by 2

Speed in revolutions per minute, whereasfrequency in cycles per second, hence

divide by 60

Steam sets high speed, small rotors

Hydro sets low speed, big rotors


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Generators

Generation by Fuel Type (Alberta)

44%

39%

9%

8%

coal

gas

renewables

import

Fuel sources in

Alberta Coal plants west

of Edmonton

Gas variouslocations

Renewables include

water and wind Import from BC and

SK


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Generators Capability curve

Limits Stator heating

Rotor heating

Stability

Whats required

Whats used

Generator Capability Curve

-1-0.8

-0.6

-0.4

-0.2

0

0.2

0.4

0.6

0.8

1

0.00 0.25 0.50 0.75 1.00

Real PowerReactiveP

ower


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Generator Capability Curve

-1

-0.8

-0.6

-0.4

-0.2

0

0.2

0.4

0.6

0.8

1

0.00 0.25 0.50 0.75 1.00

Real PowerReactive

Power


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TransformersFollow Faradays Law

E1=N1d/dt & E2=N2d/dt

Flux (d/dt) is constant

Therefore voltage change depends onnumber of turns, and basic equations

can be equated with the result:E1/N1 = E2/N2


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Transformers Since conservation

of energy must bepreserved and

voltage varies

inversely, currentmust vary directly

I1N1 = I2N2


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Transformers

Usual connection for the transmission systemis WYE grounded at the high voltage

Generators connected DELTA

Loads can be both


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Surge Impedance Loading


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(SIL) Transmission line

consists of:

Shunt capacitance

Series resistance and

inductance

Distributed along lengthof line

Treat as distributed

lumped elements Can ignore resistance



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(SIL) Close the breaker at

sending end

Shunt capacitance

charges to CV2

Close the breaker at

receiving end and feed

the load

Series inductance usesenergy at LI2

Load

Load



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(SIL)Equating shunt and series energies

CV2 = LI2

Performing the math yields

SIL (power) = V2/SI


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Properties of Surge Impedance (SI) Remains fairly constant over a wide range of

voltages Starts around 400 at lower voltages and

decreases with bundling to around 225 at

1500 kV Capacitance and inductance also remain

constant

Using this we can construct the followingtable


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Properties of Transmission LinesVoltage (kV) SI () R (/km) X (/km) Charging

(kVAr/km)SIL

(MW)X/R

69/72 370 0.4 0.5 15 13/14 1.2

138/144 370 0.2 0.5 70 50/55 2.5

230/240single

340 0.07 0.45 225 170 6

230/240bundled

300 0.07 0.4 290 180/195 6

345 bundled 285 0.026 0.365 525 415 14

500 bundled 250 0.018 0.345 1340 990 20


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0.000.25

0.50

0.75

1.00

1.25

1.50

1.75

2.00

2.25

2.50

2.75

3.00

100

200

300

400

500

600

700

800

900

1000

Length (km)

Lin

e

Loading

(SIL)

St. Clair Curve

3.25


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Loads Three types of load models

Constant MVA motors

Constant current resistive loads

Constant impedance reactor & capacitorbanks

For power flow use constant MVA

For transient studies need a combination and

may require frequency


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Summary Part 1Generators make the product

Transformers raise and lower voltageto allow efficient transport of product

Transmission lines are the highwaysLoads are the end user of the product


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Dinner BreakDinner Break


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Part 2 how the power system worksFundamental rules

Maintain reactive power balance andvoltages will be in required range

typically +/- 5% of nominalMaintain load/generation balance and

frequency or speed remains constant

typically 60 Hz +/- 0.02 Hz


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Characteristics of power systems Generation is usually remote from loads

Transmission needed to connect generationto load

Transformers needed to raise/lower voltage

Want as high a voltage as practical fortransmission minimizes losses

Use load size, generator size and line SIL to

get line voltage In Alberta, lines are typically 150 km long

At that distance loading 2 times SIL


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Putting it all togetherGenerators produce real power (P)

Generators produce/consumereactive power (Q)

Generator Q for underexcited

operation is around half overexcited

ability


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Putting it all together Transmission lines consume P in form of

losses, typically 5% to 7% of generation Lines produce/consume Q depending on

power flow on the line as a fraction of SIL

< SIL VArs flow out of line

> SIL VArs flow into line

Half from each end, if voltages are equal


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Putting it all together Loads consume P & Q

P required for resistive loads Q required for reactive loads induction motors

Synchronous motors can produce/consume Q

Switching and/or load stations Use shunt reactor/capacitor banks to

produce/absorb Q

Primarily for voltage control


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Breakers

Breakers used toconnect/disconnect

equipment

Breakers must becapable of picking

up and dropping

loads


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Breakers

Breakers must becapable of switchingunloadedtransmission lines

Breakers must becapable of

interrupting thesymmetrical faultplus any dc offset


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Power flowNeed a model of the system

Per unit system is bestMust have consistent voltage ratios

Base impedances on voltage levelMost models involve some lumping, i.e.

not practical to model every detail

However, this depends on the type ofstudy


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Power flow To solve a power flow need to solve for four

variables at each bus Bus voltage V

Bus angle

Real power P Reactive power Q

However, some variables already known

Load P & Q

Generator bus V


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Solution methodsFour solution methods

Gauss-Siedel solves phasor equationsNewton-Raphson solve for P & Q by

separation of variables

dc solves circuit as a dc circuit by

treating jX as a resistance

Decoupled load flow variant of Newton-Raphson. Separates V &


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Solution methods Solution results

Balance generation with load and lossesKeep all bus voltages within tolerance +/-

5%

Require a slack or swing bus. Can be afictitious generator to supply/absorb P & Q

Solution achieved when swing bus P & Q

equal zero Not practical, therefore minimize swing bus

P & Q


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Types of studiesDynamic studies

All of the above: Operations, Planning &Fault

Transients what happens as powersystem moves from one steady state toanother

Additional studies determine equipmentratings, e.g. breaker duty


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ContingenciesContingencies test the system for

robustnessContingency loss of one or more

components at a time

Costs escalate if system designed formore than two contingencies

Example loss of a generator and line ortransformer N-G-1


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Power system exampleGo to example


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Power System PerformanceLosses weve ignored losses up

to this pointMeasuring outages

Lines & Stations

Delivery Point measures


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Transmission LossesTransmission Losses

0

100

200

300

400

500

4750 5000 5250 5500 5750 6000 6250 6500 6750 7000 7250 7500 7750

Net Generation to Supply Alberta Load (MW)

Losses

M

W

Losses are

stochastic Simple system

losses vary as a

square of current Complex system

losses display a

linear variance


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Transmission LossesTransmission Losses Histogram

0

100

200

300

400

500

197

210

223

236

249

262

275

288

301

314

327

340

353

366

379

392

405

418

431

Losses (MW)

Count

Histogram demonstrates a normal

distribution pattern for losses


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Transmission lossesTransmission Generation, Load and Losses by Day

4000

4500

5000

5500

6000

6500

7000

7500

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24

Hour

Geenration&Load(MW)

0

100

200

300

400

500

600

700

800

900

1000

Losses(MW)

Net Gen

Net Load

Losses

+3-sigma

-3-sigma

Ave Losses

Losses on AIES are very linear


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Power system performanceNeed measure system performance

Measure frequency and duration ofoutages

Reason outages occur infrequently

Measures of performance look at allcomponents and causes

Usually stated as an average of wholesystem


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PerformanceFor Alberta, AESO publishes data to its

website on line and terminal outages asan overall average for the voltage class

For Delivery Points frequency and

duration data also published as asystem average

For comparison, all Canada data isincluded for Delivery Points


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PerformanceTwo types of duration are measured

Momentary < 1 minuteSustained > 1 minute

Following are examples of chartspublished on the AESO website

http://www.aeso.ca/transmission/5548.html
http://www.aeso.ca/transmission/5548.htmlhttp://www.aeso.ca/transmission/5548.htmlhttp://www.aeso.ca/transmission/5548.html


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Transmission - line

1.721,7010.05%6.074,5980.7675798,997Total

5.96950.03%2.64370.88141,595500

0.943200.04%4.931,1590.6923533,968240

1.266850.05%7.062,2720.5932254,417138/144

6.676010.14%6.081,1302.061869,01769/72

Frequency

per 100 km.a(faults/100

km.a)

Number ofMomentary

Faults

Unavailabilityper 100 km.a

(%)

Average

OutageDuration

(hrs/fault)

Total

OutageDuration

(hours)

Frequency

per 100 km.a(faults100

km.a)

Number ofSustained

Faults

KilometerYears

(km.a)

VoltageClass (kV)

For the Period From 1997 - 2001

Summary for Line Related Forced Outages

Transmission Outage Statistics

Alberta Interconnected Electric System


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System Average Interruption FrequencySAIFI-MI

0.0

0.4

0.8

1.2

1.6

1997 1998 1999 2000 2001

Year

Frequency

Alberta

Canada

Ice Storm

Removed


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System Average Interruption DurationSAIDI

0

100

200

300

400

1997 1998 1999 2000 2001

Year

Duration

(minutes) Alberta

Canada

Ice Storm

Removed


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Summary Part 2Power flow studies model and test

the system for robustnessyesterday, today and tomorrow

N-G-1 is used to test the system foroperation today and into the future


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Summary Part 2Losses are an important part of

power system design and operationHigher voltage lines reduce losses

However, losses are fixed when theconductor is chosen

For a system like Albertas, lossesare fairly flat


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Summary Part 2Outages are measured using

frequency and duration techniquesPresented as system average

numbersAlbertas performance not bad

when compared to rest of Canada


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Thats all folks!

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