Electrical Power Distribution

Information obtained from howstuffworks.com

The Power Plant

• Power plants consist of a spinning electrical generator.

• Generator can be made to spin by steam from burning coal, steam from a nuclear reactor, water from a hydroelectric dam, etc.

• Generates 3-phase AC power

Single-PhaseAC (Alternating Current)

• What is in your house?• Oscillating power = alternating current• DC (direct current) is a steady stream of

electrons flowing one direction. Ex: a battery

Three-Phase Power• What is produced at a power plant?• Three Phases of AC Power offset 120 degrees

from each other.

• Why three phases? At any given moment one of the three phases is nearing a peak so maximum power output.

Power Plant• Four wires coming out of a power plant: three

phases plus a ground

Which one do you think is the ground?

Hint: It doesn’t need insulators!

Transmission Substation• Uses transformers to step up the voltage

Ex: 12,000 volts out of the power plant is stepped up to 500,000 volts for transmission.

• Why? Increasing the voltage reduces power loss when traveling long distances.

• Average transmission distance is 300 miles.

Typical High Voltage Transmission Lines

Do you see the sets of three??

Power Substation• Power coming off the transmission lines use

transformers to stepped-down the voltage to less than 10,000 volts.

• Distributes power in multiple directions.• Can be disconnected from the transmission grid

if necessary.

Distribution• Power goes from the transformer to the

“distribution bus”.• The distribution bus distributes power to two

separate distribution lines at two different voltages (standard line voltage ~7,200V and a higher voltage).

At your house…

• There is usually another transformer near your house stepping down the voltage even further.

• Only one of the three original phases will enter your house.

• A typical house uses 240V for large appliances like electric stoves and air conditioning systems and 120V for standard plug in the wall items like TVs and toasters.

At your house• The 240V enter your house through a watt-

hour meter.• This is how the power company charges you

for usage.

Fuses and Circuit Breakers

• Power enters your house through a fuse and/or circuit breaker.

• THESE ARE SAFETY DEVICES!• For example if wire comes loose,

a mouse chews through a wire, a motor overheats and melts, etc. a fuse will blow or a circuit will trip so power is cut off to avoid fires.

Review

Step 1: Electricity leaves Power Plant in ____ phases.Step 2: Voltage is stepped up for transmission to minimize _______.

Review

Step 3: Voltage is stepped-down at the __________.Step 4: Voltage is further stepped down and enters your house

at ____ Volts.

Check your answers!

• Step 1: Electricity leaves Power Plant in three phases.

• Step 2: Voltage is stepped up for transmission to minimize power loss.

• Step 3: Voltage is stepped-down at the substation.

• Step 4: Voltage is further stepped down and enters your house at 240 Volts.