Download AC Power Distribution 6-9-10

Electrical Power
Distribution
An AC System
Louis E. Frenzel
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Summary
• Course use: AC circuits, combined DC/AC course. Inclass presentation.
• Objective: To give a relevant system example early in the
electronics curriculum.
• Content: Provides a general introduction to AC power
generation, transmission, and distribution. Introduce
conventional AC house wiring and related hardware as a
system.
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An Introduction to AC Power Wiring
• Most electronic equipment gets its power from the AC
power lines.
• The AC line provides the input for power supplies that
generate one or more DC voltage to operate the circuits in
electronic equipment.
• Most technicians will encounter AC power distribution in
their work and at home.
• It is useful to understand the fundamentals of AC wiring.
• AC wiring in a home is a good example of an AC system.
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The AC Power Distribution Module
• This module is designed to be used as part of an
AC circuits course.
• The prerequisite to this module is a basic understanding of
AC theory including transformers.
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Objectives
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Understand the basic process of AC power generation.
Understand the basic process of AC power distribution.
Explain the rationale for the use of high voltage distribution.
Name the contents of the service entrance in a typical home.
Indicate the voltage levels in home power distribution.
Trace or wire the connections in an AC outlet, switch, and light fixture.
Indicate common wire sizes, color codes, and current capacity.
State the basic conditions for electrical shock in a human.
Explain how the separate ground wire protects humans and
equipment.
• State how a ground fault interrupter works.
• Name the source of the rules and regulation of electrical wiring.
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Part 1
POWER GENERATION AND
TRANSMISSION
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AC Generation
• AC power is generated by huge electromechanical
generators at a utility’s power station.
• The power comes from heat generated by coal, natural
gas, oil, nuclear, or water energy.
• The heat produces steam that drives a turbine.
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AC Generation
• Refer to Fig. 1. The heat turns the turbine that, in turn,
rotates the generator.
• A DC generator is also rotated to create the power for the
AC generator field coils.
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Figure 1
AC Generator
• Fig. 2 shows how the
AC generator works.
• Fixed stator poles and
coils generate the 3phase voltages.
• A center rotating
magnetic field induces
voltages into the stator
coils as it turns.
• The field magnet gets
its power from the DC
generator.
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Figure 2
3-Phase Voltages
• Fig. 3 shows the three sine wave voltages
produced by the generator.
• There is a 120 degree time difference
between the three sine wave outputs.
Figure 3
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Generator Connections
• There are two ways to
connect the three
generator output coils:
delta (Δ) and wye (Y).
See Fig. 4.
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Figure 4
Generator Connections
• With the delta connection, each phase output (Vo) is the
same as the generator coil voltage (Vg).
• With the Y connection, each coil is in series with another
coil so each pair of coil voltages (Vg) add. However,
because the voltages are out of phase, the coil voltages do
not add directly.
• The voltage between any two Y output connections (Vo) is
equal to the voltage of one coil multiplied by 1.732.
• If the voltage from one coil is 16,000 volts, the voltage
between each Y connection is 16,000 x 1.732 = 27,712
volts.
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Long Distance Transmission
• The AC generator produces a very high output voltage in
the 13 kV to 40 kV range.
• For long distance transmission, the voltage is further
stepped up with a transformer to an even higher voltage.
See Fig.5 on the next slide.
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Long Distance Transmission
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Figure 5
Long Distance Transmission
• Common values are 138 kV, 550 kV and 765 kV. Newer
systems use mega volt levels.
• This step up in voltage is done to minimize power loss in
the long lines.
• To illustrate how power is lost in the transmission lines,
assume we wish to transmit 100 volts one mile to a 10 ohm
load. See Fig. 6.
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Figure 6
High Loss at Low Voltages
• With no line resistance, the load would receive 100 volts
and produce a current in the line of
V/R = 100/10 = 10 amperes.
• A power of V2/R = 1000 watts is transmitted.
• However, the transmission lines have resistance.
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High Loss at Low Voltages (continued)
• Assume a size #0 wire with a resistance of 0.528 ohms per
1 mile. With two wires, the total line resistance is 1.056
ohms.
• Total circuit resistance is 10 + 1.056 = 11.056 ohms.
• With 100 volts applied, the current in the lines and load is
100/11.056 = 9.04 amps.
• There is a voltage drop across each line of 9.04 x 0.528 =
4.77 volts.
• The total line voltage loss is 9.54 volts.
• The voltage at the load is only 100 – 9.54 = 90.46 volts.
• The power delivered to the load is only 90.46 x 9.04 = 818
watts.
• 182 watts is lost as heat in the transmission lines.
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Low Loss at High Voltages
• The secret to minimizing line voltage losses is to step the
voltage up to a very high voltage first.
• Assume the 100 volts is stepped up to 10,000 volts as
shown in Fig. 7.
Figure 7
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Low Loss at High Voltages
• The current in the lines is only 0.1 amp.
• The voltage drop across each line is only 0.1 x .528 =
0.0528 volts or 0.1056 total out of 10,000. The loss is
negligible.
• The total power lost is only 0.1056 x 0.1 = 0.01056 watts.
• The voltage is stepped down to 100 volts at the load so
virtually all of the original power is received.
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Sub-Stations
• As the transmitted power gets near the area to be served,
it is stepped down to a lower voltage at a sub-station by a
transformer.
• Two or more substations may be used resulting in even
lower voltage. See Fig. 5.
• The resulting lower voltage will then be distributed to the
local neighborhoods and other areas.
• The voltage will be in the 2000 to 7000 volt range, 4100
volts here.
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Local Distribution
• The lower voltage
is sent on wires to
local areas. There
it is stepped down
to the desired
voltage.
• One arrangement
is shown in Fig. 8.
A transformer on a
pole steps the
4100 volts on the
line down to 240
volts.
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Figure 8
Local Distribution
• Some local
distribution is by
underground wiring
and a transformer on
a concrete pad.
See Fig 9.
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Figure 9
Source Distribution
• Fig. 10 shows how the local high
voltage is distributed.
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Figure 10
Source Distribution
• The voltage to homes is from a 240 volts center-tapped
secondary winding on the transformer. This provides two
120 volt circuits.
• Some offices or factories get different voltages depending
on their need. Voltages or 208 and 480 volts are typical.
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Part 2
IN-HOME DISTRIBUTION
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Service Entrance
• The service entrance refers to the connections to a central
distribution and connection point where the power line
enters the home.
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Service Entrance
• The service entrance
includes the line from
the transformer, a
kilowatt hour meter
and the service box
that contains all of
the circuit breakers
and connections to
all wiring throughout
the house.
See Fig. 11.
• Note the bus bars to
which are screwed
all of the wiring
circuits.
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Figure 11
Service Box Wiring
• Another view of the
service box is given in
Fig. 12.
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Figure 12
Service Box Wiring
• Note that the center tap of the incoming transformer
connections is connected to earth ground. This becomes
the neutral connection.
• The voltage is then distributed to multiple circuits
throughout the house or building. Each circuit feeds
several AC outlets and lights that receive 120 volts.
• There are several special circuits that receive 240 volts for
a water heater, clothes dryer, or heating and air
conditioning system.
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Circuit Breakers
• Each circuit is protected by a circuit breaker, a
switch that connects in series with the hot wire
of each circuit. See Fig. 13.
• When used as a switch the circuit breaker
provides a way to turn off the voltage to a
circuit while work is being done.
• The circuit breaker is designed to pass a
specific maximum current. If the current
exceeds that value, the switch opens and
turns off the voltage.
• The circuit breaker protects against current
overloads that can produce sufficient heat to
start a fire.
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Figure 13
Circuit Breakers (continued)
• When the circuit breaker switch is on, current passes
through an electromagnet. If the current exceeds the rated
current value, the electromagnet has sufficient pull to
disengage the switch turning off the voltage to that circuit.
• Circuit breakers are available in current ratings of 15, 20,
30 and 40 amperes. Large breakers of 100 to 200 amps
are used as the main switch in each service box.
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Circuit Breakers (continued)
• Fig. 14 shows the typical circuit breaker wiring symbol and
connections.
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Figure 14
Wire
• The circuit wiring is done with cable designed for the
purpose. It contains three insulated solid copper wires
contained in an insulating sheath. See Fig. 15. The wire
is often referred to as Romex.
• The wires are color coded for specific purposes. The
neutral wire has white insulation. The hot wire insulation is
usually black. A separate ground wire has green insulation
or may be bare.
• The specifications are typically printed on the outer plastic
covering.
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Figure 15
Wire Current Rating
• The size of the wire determines its maximum current carrying capacity.
• Size #14 wire is the most common for most circuits.
• Larger wire like #12 or #10 is used on circuits that must handle higher
current.
• For copper wire the maximum current is:
–
–
–
–
#14
#12
#10
#8
15 amperes
20 amperes
30 amperes
40 amperes
• Such specifications ensure that high currents do not produce heat
sufficient to start a fire.
• Special armored cable like that in
Fig. 16 is used for outdoor wiring
and for wiring in harsh environments.
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Figure 16
Outlet Wiring
• Fig. 17 shows the wiring of a common AC outlet.
• The wire from the cable attaches with screws to the right
side. The neutral screw is usually
silver colored and the hot wire
screw is usually brass colored.
• Note that the neutral wire
connects to the slot that is
slightly larger to help ensure
correct alignment of the plug.
• The holes below the slots
connect to the green copper wire.
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Figure 17
Switch Wiring
• Common switch wiring is shown in Fig. 18. The switch itself
and its wiring are contained in a box like the AC outlet.
• The switch is connected in
series with the hot wire (black).
• Note that wire connections
are by wire nuts. These
are screwed down on the
solid wires that have
been twisted together first.
• The ground wire
connects to the switch
metallic frame.
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Figure 18
Light Fixture Wiring
• Fig. 19 shows the wiring for a typical ceiling light fixture.
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Figure 19
Light Fixture Wiring
• Then trace the switch wiring to the fixture. See Fig. 20.
Figure 20
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240 Volt Connections
• Fig. 21 shows typical plugs and outlets for use with high
current 120 volt circuits and 240 volts circuits.
• From left to right:
– A.
120 V, 20 A
– B.
240 V, 30 A
– C.
240 V, 40 A
– D.
240 V
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Figure 21
Part 3
SAFETY
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Safety First
• All electrical wiring is designed and installed to rigid
specifications and standards. The reason for this is safety.
• Safety precautions are taken to prevent electrical shock
which can kill a person and for protection against fire.
• Wiring specifications are dictated by the National Fire
Protection Agency (NFPA) via their extensive National
Electrical Code (NEC).
• Most states and local governments follow the NEC
guidelines but many have their own special rules and
regulations.
• A copy of the NEC book is a useful reference for anyone
doing or working with electrical components and wiring.
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Electrical Shock
• 120 and 240 volt power is sufficient to kill a human if he/
she comes into direct contact with the voltage.
• The contact resistance of dry skin is in the 100K to 500K
ohm range. Wet skin has much lower resistance.
• It is the amount of current that flows through a person that
determines the degree of lethality.
• At a current of 1 mA a human can feel a tingling. At a
current of 5 mA mild shock occurs.
• Usually any current above 50 mA is sufficient to cause
heart stoppage.
• As a rule of thumb, any voltage over about 40 volts can
cause such current levels.
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How Grounds Protect
• Since the neutral side of the AC voltage is connected to
ground, any contact between the hot side of the line and
ground will cause shock. See Fig. 22.
• The separate ground wire (green)
is included to prevent shock
and electrical overload.
• This separate ground wire
is connected to the housing
of most appliances or
tools with metal
enclosures.
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Figure 22
How Grounds Protect (continued)
• Most appliances contain some kind of AC motor. If a short
or other malfunction in the motor causes the hot side of the
line to touch the metallic housing of the motor or appliance,
a person will be shocked if he/she touches the appliance
(hot side) while standing on ground (neutral).
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How Grounds Protect (continued)
• The separate ground
wire will cause a short
to the circuit and trip the
breaker thereby
preventing a shock.
See Fig. 23.
Figure 23
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Ground Fault Interrupter
• Another potential shock hazard is where a low current path
is created between the hot side of the line and ground.
Such a high resistance path, which could be through a
human, may not be lethal but it could cause a hurtful
shock. Fig. 24 show such a path.
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Figure 24
Ground Fault Interrupter
• This path is usually created when there is a source of
moisture present.
• A device designed to protect against such conditions is the
ground fault interrupter (GFI).
• Fig. 25 shows the basic idea of a GFI.
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Figure 25
Ground Fault Interrupter (continued)
• The AC line to the load usually connected to an AC outlet is passed
through a magnetic core.
• If there is no fault, the current in the hot and neutral leads are the same
so no magnetic field is produced in the magnetic core of a transformer.
• If a high resistance path is created between the hot side and ground,
the currents in the two AC leads will be unequal. A magnetic field will
be produced in the transformer core inducing a voltage in the coil.
• The coil voltage is converted to DC and used to control a relay that
opens the connections to the AC line.
• Most GFI units are installed in AC outlets where moisture is present as
in the kitchen, bathrooms, garage and outdoors.
• Most GFI’s are very sensitive and trip with a current of 1 or 2 mA.
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