Power Systems
Elliot Olsen
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Overview of Electric Utility System
Definition of Primary Distribution
Substations
Feeder Configurations
Primary Distribution Systems
Protective Components
Power Quality
Underground vs. Pole Mounted
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Three stages: Generation, Transmission,
Distribution
Image Source: www.iaei.org
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Generation
 Coal, nuclear, renewable, etc.
 Output typically 50-1300 MVA @ 2-20 kV
 Generator step-up (GSU) transformer will increase
voltage/decrease current to reduce losses during
transmission
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Transmission
 Delivers energy from generators to distribution
system
 Provides energy interchange among utilities
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Begins at a substation and includes
everything up to the meter at customers’
premises.
Divided into two areas:
 Primary: Distribution substation
transformer
 Secondary: Distribution transformer
customers’ meters
distribution
Image Source: www.osha.gov
Transmission voltage (high) is stepped down to
distribution voltage (low)
 Distribution power is output via feeders connected to
distribution bus
 Substations are commonly constructed underground
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7 Classes of primary voltages
Class, kV
Voltage, kV
2.5
2.4
5
4.16
8.66
7.2
15
12.47
13.2
13.8
25
22.9
24.94
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34.5
34.5
50
46
15 kV is most common in the US
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Substation should be located near the center
of the loads it serves
N
location min(
Zi * di )
i 1
 Typically a substation is between 1-30 miles from
the furthest customer
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Loads should be supplied without undue
voltage regulation and with standard
equipment
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Substation design should allow for future
expansion
A substation failure or shutdown should not
affect a large number of customers
In an emergency, substations should be able
to allow other substations to tie to them and
vice versa
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Immediate output of a substation
3-Phase configured in 1 of 5 ways
 4-Wire Y-connected, multi-grounded
 4-Wire Y-connected, uni-grounded
 3-Wire Y-connected, uni-grounded
 3-Wire Δ-connected
 3-Wire Y-connected, ungrounded
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Used in most distribution systems
Substation is grounded via a 1 Ω impedance
to limit fault currents
Neutral is grounded at least 4 times every
mile
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Second most popular configuration
Popular in industrial settings
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Advantages of Y
 Greater feeder capacity
▪ Fewer substations are necessary
▪ Rural areas with long distances between consumers are
easier to service
 Since the neutral wire is grounded at many points,
voltage stresses on insulation is less
 Most devices used in existing Δ circuits can be
reused
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Disadvantages of Y
 Higher phase-to-phase voltages only apply to the
3-phase main, not branches
 Harder to balance phases
 Fault currents are much higher in Y configurations
 Higher voltage and many grounds in a Y circuit
may cause interference with communication
circuits in parallel with power circuits
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Feeders are primarily designed by maximum
voltage variation permissible at the farthest
consumer
The feeder has per unit impedance of Z and
current I, meaning a voltage drop of IZ per
unit length
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Voltage Regulation: Voltage at the receiving
end of a line, with NoLoad and FullLoad
conditions
%VR
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10% VR
load
| VL
NoLoad
| VL
| | VL
FullLoad
FullLoad
|
|
*100
+/- 5% of voltage source at the
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How to regulate the voltage?
 1. Voltage regulators
 2. Shunt capacitors
 3. Choose a proper conductor
▪ If there is not significant loss in the line, other regulating
methods may not be necessary
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Regulators maintain a constant voltage
Image Source: Electrical Distribution Engineering, 3rd Ed.
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Used at substation and/or on feeders
Voltage rise due to capacitors:
VR
IC X L
 Note: VR is not dependent on load
 However, low load will result in less voltage drop
along the line, therefore, during low load times,
capacitors are usually shut off
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Single-phase branch from a feeder main
Each branch is again stepped down in
secondary distribution for use by consumers
Balancing Branch Loads
 Power systems work best when the loads on all
three phases are equal
 Impedance and distance from substation are used
to determine how to balance each phase
N
line _ impedance
Z iL * d iL
i 1
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Economic Factors: Initial cost and upkeep
Local government regulations
Permissible voltage variations
Mechanical feasibility
Loads to be supplied
Image Source: Electrical Distribution Engineering, 3rd Ed.
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3 Possible configurations of distribution
 Radial
 Loop
 Primary Network Systems
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Feeders emanate radially from the substation
Most commonly used configuration
Typical in low-load-density areas
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Used where high reliability is important
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More protective devices are required to
isolate faults along the feeder
N.O.
fuse
N.O.
N.O.
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Feeder conductors must be large enough to
sustain entire load of loop
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Least common configuration
Found only in downtown areas of large cities
with high load densities
Multiple interconnected
substations
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Lightning Arrester: Limits surge voltages by
providing a path to ground
 Also helps prevent equipment faults
Image Source: www.osha.gov
Image Source:
www.lightningsource.com
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Similar to a circuit breaker… but they reclose
Most SC faults are temporary and resolve
themselves
 Tree branch shorts two phases, lines sway
together in the wind, etc
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A recloser will open during a
fault, then close to see if the
fault went away
Image Source: Brian Hayes, Flickr
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At initial fault detection, reclosers will open
as quickly as possible
 Assumes a temporary fault will be cleared
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Subsequent openings will be delayed
 This allows persistent faults to be cleared by fuses
downstream, which will isolate the fault to as few
customers as possible
IEEE 1100: “The concept of powering and
grounding electronic equipment in a manner
that is suitable for the operation of that
equipment.”
 Factors degrading power quality:
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Grounding
Surges
Load Switching
Harmonics
And many more…
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The Power Quality pyramid reveals the most
likely culprits of degraded power quality
Image Source: Eaton, Power Distribution Systems
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Merits of overhead vs. underground lines
 Reliability
▪ Frequency of outages (# of power outages/year)
▪ Duration of outages (minutes/year a customer has no
power)
 Cost
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Other factors
 Local perception
 Age of infrastructure
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Reliability
 80% of faults in pole mounted systems are
temporary faults
 Recall that temporary faults are caused by wind,
tree branches, animals, etc. They do not occur in
underground systems
Source: Power Distribution Engineering Fundamentals
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Cost
 Installing underground systems is much more
expensive than pole mounted
 Above ground, phase wires are insulated by air
 Insulation adds to costs of conductors, and more
sophisticated equipment is needed
 Extra costs are usually offset by developers, who
prefer aesthetics of underground systems
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Reliability
 Frequency of outages was 50% less for
underground
 Duration of outages was 58% lower for overhead
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Cost
 4 to 6 times more expensive to build new
underground lines
 Cost to place existing overhead lines
underground: $41 billion 125% rate increase
▪ 77% of SCE&G’s distribution system is overhead
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J. Glover, M. Sarma, T. Overbye Power System Analysis & Design, 5th ed. Stamford, CT:
CENGAGE Learning, 2012.
A. Pansini Electrical Distribution Engineering, 3rd ed. Lilburn, GA: The Fairmont Press, 2007.
J. Glover, “Electric Power Distribution,” Encyclopedia of Energy Technology and the
Environment. New York: John Wiley & Sons, 1995.
T. Gonen, Power Distribution Engineering. New York: Wiley, 1986.
SCE&G, “Underground vs. Overhead Power Lines.”
Eaton, “Power Distribution Systems.” 2011.
M. Fard. (6 May 2011) “Lightning Arresters.” [Online]. Available:
http://lightningsource.wordpress.com/