1

Why?

Why AC and not DC ? Where AC comes from ? Why 60 Hz (or 50 Hz) ? Why three-phase systems ?

2

Why do we use AC and not DC ?

Construction of Generators: Key component is the 3 generator Simple in raising and lowering voltages:

Generators limited to about 25kV Transmission at 230, 345, 500 and 765kV (low losses) Subtransmission at 138, 69, 34.5kV Distribution at 13.8, 4.16kV Key component: power transformer

3

Why do we use AC and not DC ? (contd)

Break-even distance for HVDC Key component: power transmission

4

Where does AC come from?

AC voltages and currents are usually produced by rotating generators in a power system and are represented by sine waves. Three phase is generated by a generator with three sets of independent windings which are physically spaced 120 degrees around the stator. AC voltages and currents can also be produced by electronics.

5

The choice of Frequency

60 Hz and 50 Hz Nowadays:

60 Hz in Ecuador, North America, Brazil and Japan (has also 50 Hz!) 50 Hz in other countries (China) 25 Hz, 16 Hz Railways 400 Hz Oil rigs, ships and airplanes

6

The choice of Frequency (contd)

A too low frequency, like 10 or 20 Hz causes flicker A too high frequency:

Increases the hysteresis losses Increases the eddy current losses Increases the cable and line impedance

7

Why use 3 phases?

Smooth torque on generator shaft Delivery of constant power to a 3 phase load 3 Wires and not 6

8

Balanced three phase system

Voltages in the 3 phases have the same amplitude, but differ 120 electrical degrees in phase

9

Va

Ia

Vb

Ib

120

Vc Ic

120

120

0= + + =n a b cI I I I

Ia Ic

Ib

0

Balanced system -> Single phase calculation

Equal impedances in the 3 phases

10

Simple Power System

Every power system has three major components

generation: source of power, ideally with a specified voltage and frequency

transmission system: transmits power; ideally as a perfect conductor

load: consumes power; ideally with a constant resistive value

11

Complications

No ideal voltage sources exist Transmission system has resistance, inductance, capacitance and flow limitations Loads are seldom constant Simple system has no redundancy so power system will not work if any component fails

12

The BIG picture Generation (11 36 kV) Transmission and distribution (110 765 kV) Load (0.12 138 kV)

Industrial customer, 23 138 kV Commercial customer, 4.16 34.5 kV Residential customer , 120 240 V

13

Power Generation

14

Generator Prime Mover- Turbine

Fossil-fuel (coal/NG), Nuclear, Biomass

Wind, Tidal, Hydro

High Pressure &

Temp. steam or hot air

Electricity

Mechanical System Electrical System

Transmission & Distribution Network

15

Ring (mesh/loop) network topology

Radial or weakly mesh network topology

Source: FUNDAMENTALS OF ELECTRIC POWER SYSTEMS by Xiao - Ping Zhang

Electric Load

Ecuadorian Load at a 69kV Bus in 2013

Varies with time Moment-to-moment fluctuations Hour-to-hour changes Daily Weekly Seasonal

Base load counts for almost half of peak load.

16

US Electric Power Grid

17

European Electric Power Grid

18

Brief History of Electric Power

Early 1880s Edison introduced Pearl Street dc system in Manhattan supplying 59 customers 1884 Sprague produces practical dc motor 1885 William Stanleys development of a commercially practical transformer Mid 1880s Westinghouse/Tesla introduce rival ac system Late 1880s Tesla invents ac induction motor 1893 First 3 phase transmission line operating at 2.3 kV 1896 ac lines deliver electricity from hydro generation at Niagara Falls to Buffalo, 20 miles away

19

History (contd)

20

References

Burn: An Energy Project, How the grid works || BURN Radio. Available at: https://www.youtube.com/watch?v=1dZjohZPIqE Follow Electricity's Journey Through the Transmission System. Available at: https://www.youtube.com/watch?v=TQg2Y0kp2vI J.D. Glover, M.S. Sarma, T.J. Overbye, Power System Analysis and Design, 5th Edition, CENGAGE Learning, 2012. J. Casazza, F. Delea, Understanding Electric Power Systems, John Wiley, 2003.

21

Slide Number 1Why?Why do we use AC and not DC ?Why do we use AC and not DC ? (contd)Where does AC come from?The choice of FrequencyThe choice of Frequency (contd)Why use 3 phases?Balanced three phase systemBalanced system -> Single phase calculationSimple Power SystemComplicationsThe BIG picturePowerGenerationTransmission & Distribution NetworkElectric LoadUS Electric Power GridEuropean Electric Power GridBrief History of Electric PowerHistory (contd)References