[PDF] - Electric Power Electric Power Household voltages are typically 120V PDF Document

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Electric Power Distribution 1

Electric Power Electric Power Distribution Distribution

Electric Power Distribution 2

Observations about Observations about Electric Power Distribution Electric Power Distribution

Household electricity is alternating current (AC)

Household voltages are typically 120V or 240V

Power is distributed at much higher voltages

Power transformers are common around us

Power substations are there, but harder to find

Electric Power Distribution 3

4 Questions about 4 Questions about Electric Power Distribution Electric Power Distribution

Why isn’t power transmitted at low voltages?

Why isn’t power delivered at high voltages?

What is “alternating current” and why use it?

How does a transformer transfer power?

Electric Power Distribution 4

Question 1 Question 1

Why isn’t power transmitted at low voltages?

Electric Power Distribution 5

Electric Power and a Wire Electric Power and a Wire

An electric current passing through a wire

An electric current passing through a wire converts electrical power in thermal power converts electrical power in thermal power

power wasted = current · voltage drop in wire. power wasted = current · voltage drop in wire.

Si

th i b Oh l Si th i b Oh l

Since the wire obeys Ohms law,

voltage drop in wire = resistance · current voltage drop in wire = resistance · current, ,

the power that wire wastes is

power wasted = resistance · current power wasted = resistance · current2. .

Doubling current quadruples wasted power!

Electric Power Distribution 6

Large Currents are Wasteful Large Currents are Wasteful

The goal of a power distribution system is to

The goal of a power distribution system is to transmit lots of electric power to a city, transmit lots of electric power to a city,

power transmitted = current · voltage drop at city power transmitted = current · voltage drop at city, ,

hil

ti littl l t i i th i hil ti littl l t i i th i

while wasting little electric power in the wires,

power wasted = resistance · current power wasted = resistance · current2

2.

.

That energy efficiency can be achieved by using

a small current,

a huge voltage drop,

and low

and low-

resistance wires.

resistance wires.

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Electric Power Distribution 7

Question 2 Question 2

Why isn’t power delivered at high voltages?

Electric Power Distribution 8

High Voltages are Dangerous High Voltages are Dangerous

When large voltage drops are available,

strong electric fields are present,

charges experience enormous forces,

nd

rr nt t nd t fl thr h n p t d p th nd rr nt t nd t fl thr h n p t d p th

and currents tend to flow through unexpected paths.

High

High-

voltage electrical power in a home is

voltage electrical power in a home is

a spark hazard,

a fire hazard,

and a shock hazard.

Electric Power Distribution 9

The Voltage Hierarchy The Voltage Hierarchy

Large currents are too wasteful for transmission

High voltages are too dangerous for delivery

So electric power distribution uses a hierarchy:

high

high-

voltage circuits in the countryside

voltage circuits in the countryside

medium

medium-

voltage circuits in cities

voltage circuits in cities

low

low-

voltage circuits in neighborhoods and homes

voltage circuits in neighborhoods and homes

Transformers transfer power between circuits!

Transformers transfer power between circuits!

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Question 3 Question 3

What is “alternating current” and why use it?

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Alternating Current (AC) Alternating Current (AC)

In alternating current,

In alternating current,

the voltages of the power delivery wires alternate

and the resulting currents normally alternate, too.

Al i l i h US Al i l i h US

Alternating voltage in the US

completes 60 cycles per second,

reversing every 1/120 second.

Electric Power Distribution 12

AC and Transformers AC and Transformers

AC has little effect on simple electric devices

AC has little effect on simple electric devices (e.g., (e.g., lightbulbs lightbulbs, space heaters, toasters) , space heaters, toasters)

AC is a nuisance for electronic devices

AC is a nuisance for electronic devices (e g comp ters televisions so nd systems) (e g comp ters televisions so nd systems) (e.g., computers, televisions, sound systems) (e.g., computers, televisions, sound systems)

AC permits the easy use of transformers,

which can move power between circuits:

from a low

from a low-

voltage circuit to a high

voltage circuit to a high-

voltage circuit

voltage circuit

from a high

from a high-

voltage circuit to a low

voltage circuit to a low-

voltage circuit

voltage circuit

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Electric Power Distribution 13

Question 4 Question 4

How does a transformer transfer power?

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Electromagnetism Electromagnetism (Version 2)

(Version 2)

Magnetic fields are produced by

magnetic poles (but free poles don’t seem to exist),

moving electric charges,

nd h n in

l tri fi ld [m r l t r ] nd h n in l tri fi ld [m r l t r ]

and changing electric fields [more later…]

and changing electric fields [more later…]. .

Electric fields are produced by

electric charges,

moving magnetic poles

moving magnetic poles, ,

and changing magnetic fields

and changing magnetic fields. .

Electric Power Distribution 15

Electromagnetic Induction Electromagnetic Induction

Moving poles or changing magnetic fields

Moving poles or changing magnetic fields

produce electric fields,

which propel currents through conductors,

which produce magnetic fields.

which produce magnetic fields. p g p g

Changing magnetic effects

Changing magnetic effects induce induce currents currents

Induced currents produce magnetic fields

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Lenz’s Law Lenz’s Law

Lenz’s law predicts the nature of the induced

Lenz’s law predicts the nature of the induced magnetic fields: magnetic fields:

“When a changing magnetic field induces a current in a “When a changing magnetic field induces a current in a g g g g g g conductor, the magnetic field from that current conductor, the magnetic field from that current

pposes the change that induced it.”
pposes the change that induced it.”

Electric Power Distribution 17

Transformer Transformer

Alternating current in one circuit can induce an

Alternating current in one circuit can induce an alternating current in a second circuit alternating current in a second circuit

A transformer

i d i i d i

uses induction

to transfer power

to transfer power between its circuits between its circuits

but doesn’t

but doesn’t transfer any charges transfer any charges between its circuits between its circuits

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Current and Voltage Current and Voltage

A transformer must obey energy conservation

Power arriving in its primary circuit must equal

Power arriving in its primary circuit must equal power leaving in its secondary circuit power leaving in its secondary circuit Si i h d f Si i h d f l

Since power is the product of

Since power is the product of voltage · current voltage · current, ,

a transformer can exchanging voltage for current

r current for voltage!
r current for voltage!

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Electric Power Distribution 19

Step Step-

Down Transformer

Down Transformer

A step

A step-

down transformer

down transformer

has relatively few turns in its secondary coil

so charge is pushed a shorter distance

nd

p ri n m ll r lt ri nd p ri n m ll r lt ri

and experiences a smaller voltage rise

A larger current

A larger current at smaller voltage at smaller voltage flows in the flows in the secondary circuit secondary circuit

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Step Step-

Up Transformer

Up Transformer

A step

A step-

up transformer

up transformer

has relatively many turns in its secondary coil

so charge is pushed a longer distance

nd

p ri n l r r lt ri nd p ri n l r r lt ri

and experiences a larger voltage rise

A smaller current

A smaller current at larger voltage at larger voltage flows in the flows in the secondary circuit secondary circuit

Electric Power Distribution 21

Power Distribution System Power Distribution System

A step

A step-

up transformer increases the voltage

up transformer increases the voltage for efficient long for efficient long-

distance transmission

distance transmission

A step

A step-

down transformer decreases the voltage

down transformer decreases the voltage for safe delivery to comm nities and homes for safe delivery to comm nities and homes for safe delivery to communities and homes for safe delivery to communities and homes

Electric Power Distribution 22

Summary about Summary about Electric Power Distribution Electric Power Distribution

Electric power is transmitted at high voltages

Electric power is delivered at low voltages

Transformers transfer power between circuits

Transformers require AC power to operate

The power distribution system is AC