Electric Power Distribution

Electric Power Distribution 1

Electric Power Electric Power DistributionDistribution


Introductory QuestionIntroductory Question

Electric power reaches this city via Electric power reaches this city via high voltage transmission lines. What high voltage transmission lines. What fraction of the electric charges fraction of the electric charges traveling on those transmission lines traveling on those transmission lines pass through this room?pass through this room?

A.A. About 1%About 1%

B.B. About 0.01%About 0.01%

C.C. Exactly 0%Exactly 0%


Observations aboutObservations aboutElectric Power DistributionElectric Power Distribution

Household electricity is alternating Household electricity is alternating current (AC)current (AC)

Household voltages are typically Household voltages are typically 120V or 240V120V or 240V

Power is distributed at much higher Power is distributed at much higher voltagesvoltages

Power transformers are commonPower transformers are common Power substations are less commonPower substations are less common


4 Questions about4 Questions aboutElectric Power DistributionElectric Power Distribution

Why isn’t power transmitted at low Why isn’t power transmitted at low voltages?voltages?

Why isn’t power delivered at high Why isn’t power delivered at high voltages?voltages?

What is “alternating current” and What is “alternating current” and why use it?why use it?

How does a transformer transfer How does a transformer transfer power?power?


Question 1Question 1

Why isn’t power transmitted at low Why isn’t power transmitted at low voltages?voltages?


Electric Power and a Electric Power and a LightbulbLightbulb

An electric current passing through a lightbulbAn electric current passing through a lightbulb experiences a voltage drop in the filamentexperiences a voltage drop in the filament and converts electric power into thermal power.and converts electric power into thermal power.

The power provided to the lightbulb isThe power provided to the lightbulb is the product of the current in its filamentthe product of the current in its filament times the voltage drop that current experiences, ortimes the voltage drop that current experiences, or

power provided = current· voltage drop in filamentpower provided = current· voltage drop in filament.. Doubling voltage Doubling voltage oror current doubles bulb current doubles bulb

powerpower


Electric Power and a Electric Power and a WireWire

An electric current passing through a wireAn electric current passing through a wire wastes electric power as thermal power,wastes electric power as thermal power,

power wasted = current· voltage drop in wirepower wasted = current· voltage drop in wire.. Since the wire obeys Ohms law,Since the wire obeys Ohms law,

voltage drop in wire = resistance· currentvoltage drop in wire = resistance· current,, the power it wastes isthe power it wastes is

power wasted = resistance· currentpower wasted = resistance· current22.. Doubling current quadruples wasted Doubling current quadruples wasted

power!power!


Large Currents are Large Currents are WastefulWasteful

The goal of a power distribution system is toThe 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 citypower transmitted = current· voltage drop at city,, while wasting only a little electric power in the while wasting only a little electric power in the

wires,wires,

power wasted = resistance· currentpower wasted = resistance· current22.. That goal can be met byThat goal can be met by

using a small current and a huge voltage drop,using a small current and a huge voltage drop, together with low-resistance wires.together with low-resistance wires.



Why isn’t power delivered at high Why isn’t power delivered at high voltages?voltages?


High Voltages are High Voltages are DangerousDangerous

When large voltage drops are available,When large voltage drops are available, charges tend to jump toward the lower charges tend to jump toward the lower

voltagevoltage and currents tend to flow through and currents tend to flow through

unexpected paths.unexpected paths. High-voltage electrical power in a home High-voltage electrical power in a home

isis a spark hazard,a spark hazard, a fire hazard,a fire hazard, and a shock hazard.and a shock hazard.


The Voltage HierarchyThe Voltage Hierarchy

Large currents are too wasteful for Large currents are too wasteful for transmissiontransmission

High voltages are too dangerous for deliveryHigh voltages are too dangerous for delivery So electric power distribution uses a So electric power distribution uses a

hierarchy:hierarchy: high voltage circuits in the countrysidehigh voltage circuits in the countryside medium voltage circuits in citiesmedium voltage circuits in cities low voltage circuits in neighborhoods and homeslow voltage circuits in neighborhoods and homes

Transformers transfer power between Transformers transfer power between circuits!circuits!



What is “alternating current” and What is “alternating current” and why use it?why use it?


Alternating Current (AC)Alternating Current (AC)

In alternating current,In alternating current, the voltages of the power delivery wires the voltages of the power delivery wires

alternatealternate and the resulting currents normally and the resulting currents normally

alternate, too.alternate, too. Alternating voltage in the USAlternating voltage in the US

completes 60 cycles per second,completes 60 cycles per second, reversing every 1/120 second.reversing every 1/120 second.


AC and TransformersAC and Transformers

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

but it’s a nuisance for electronic devicesbut it’s a nuisance for electronic devices(e.g., computers, televisions, sound systems).(e.g., computers, televisions, sound systems).

AC produces alternating magnetic fieldsAC produces alternating magnetic fields and transformers use alternating magnetic fieldsand transformers use alternating magnetic fields to move power between AC circuits!to move power between AC circuits!



How does a transformer transfer How does a transformer transfer power?power?


Electromagnetism Electromagnetism (Version (Version 2)2)

Magnetic fields are produced byMagnetic fields are produced by magnetic poles (but free poles don’t seem magnetic poles (but free poles don’t seem

to exist),to exist), moving electric charges,moving electric charges, and changing electric fields [more later…]and changing electric fields [more later…]..

Electric fields are produced byElectric fields are produced by electric charges,electric charges, moving magnetic polesmoving magnetic poles,, and changing magnetic fieldsand changing magnetic fields..


Electromagnetic Electromagnetic InductionInduction

Moving poles or changing magnetic fieldsMoving poles or changing magnetic fields produce electric fields,produce electric fields, which propel currents through conductors,which propel currents through conductors, and those currents, in turn, produce magnetic and those currents, in turn, produce magnetic

fields.fields. Overall, changing magnetic effects Overall, changing magnetic effects induce induce

currents and thereby produce magnetic currents and thereby produce magnetic fields.fields.

Lenz’s law predicts the nature of those Lenz’s law predicts the nature of those fields:fields: ““The effects of magnetic induction oppose the The effects of magnetic induction oppose the

changes that produce them.”changes that produce them.”


TransformerTransformer

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

The transformerThe transformer transfers powertransfers power

between the circuitsbetween the circuits but it doesn’tbut it doesn’t

transfer any chargestransfer any chargesbetween the circuitsbetween the circuits


Current and VoltageCurrent and Voltage

Power arriving in the primary circuit Power arriving in the primary circuit must equal power leaving in the must equal power leaving in the secondary circuitsecondary circuit

Since power is the product of Since power is the product of voltage· current,voltage· current, a transformer can obey energy a transformer can obey energy

conservationconservation while exchanging voltage for currentwhile exchanging voltage for current or current for voltage!or current for voltage!


Step-Down TransformerStep-Down Transformer

A step-down transformerA step-down transformer has relatively few turns in its secondary has relatively few turns in its secondary

coilcoil so charge is pushed a shorter distanceso charge is pushed a shorter distance and experiences a smaller voltage riseand experiences a smaller voltage rise

A larger currentA larger currentat smaller voltageat smaller voltageflows in theflows in thesecondary circuitsecondary circuit


Step-Up TransformerStep-Up Transformer

A step-up transformerA step-up transformer has relatively many turns in its has relatively many turns in its

secondary coilsecondary coil so charge is pushed a longer distanceso charge is pushed a longer distance and experiences a larger voltage riseand experiences a larger voltage rise

A smaller currentA smaller currentat larger voltageat larger voltageflows in theflows in thesecondary circuitsecondary circuit


Power Distribution Power Distribution SystemSystem

A step-up transformer increases A step-up transformer increases voltagevoltagefor efficient long-distance for efficient long-distance transmissiontransmission

A step-down transformer decreases A step-down transformer decreases voltagevoltagefor safe delivery to communities and for safe delivery to communities and homeshomes


Introductory Question Introductory Question (revisited)(revisited)

Electric power reaches this city via Electric power reaches this city via high voltage transmission lines. What high voltage transmission lines. What fraction of the electric charges fraction of the electric charges traveling on those transmission lines traveling on those transmission lines pass through this room?pass through this room?

A.A. About 1%About 1%

B.B. About 0.01%About 0.01%

C.C. Exactly 0%Exactly 0%


Summary aboutSummary aboutElectric Power DistributionElectric Power Distribution

Electric power is transmitted at high Electric power is transmitted at high voltagesvoltages

Electric power is delivered at low Electric power is delivered at low voltagesvoltages

Transformers transfer power between Transformers transfer power between circuitscircuits

Transformers require AC power to Transformers require AC power to operateoperate

The power distribution system is ACThe power distribution system is AC

Electric Power Distribution

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