Electric Power Grid
Power Transmission &
PPT prepared by Prof. Z. Jan Bochynski
EE3824 Course Instructor
The Lineman's and Cableman's Handbook, Shoemaker, T. M., Mack, J. E.,
Tenth Edition 2002, McGraw-Hill
Electrical Energy Resources
•Heat (thermal) energy generated from
burning fossil fuels: coal, petroleum,
•Thermal energy inside the Earth
•Nuclear reaction energy
•Potential energy of falling water
•Wind kinetic energy
•Solar electric from solar (photovoltaic)
•Thermal energy from the Sun radiation
Wind power towers
Power Generation Plants
Nuclear power plant Geothermal power plant
Hydroelectric power plant Fossil fuel power plant
Additional information: The Lineman's and Cableman's Handbook, Shoemaker, T. M., Mack, J. E., Tenth Edition 2002, McGraw-Hill.
A transformer substation as a part of the transmission system is a facility
where voltage increases (or decreases), but the transmitted power and
voltage frequency remain unchanged.
A transmission bus is used to distribute electric power to one or more transmission
lines. A substation can have circuit breakers that are used to switch generation
and transmission circuits in and out of service as needed.
Transformers Substation Functions
• Change voltage from one level to another
• Regulate voltage to compensate for system voltage changes
• Switch transmission and distribution circuits into and out of the grid system
• Measure electric power qualities flowing in the circuits
• Connect communication signals to the circuits
• Eliminate lightning and other electrical surges from the system
• Connect electric generation plants to the system
• Make interconnections between the electric systems of more than one utility
• Control reactive kilovolt-amperes supplied to and the flow of reactive kilovolt-amperes
in the circuits
Step-up / Step-down Substations
A step-up transmission substation receives electric power from a nearby
generating facility and uses a large power transformer to increase the
voltage for transmission to distant locations.
Step-up substationStep-down power transformer
The step-down substation can change the transmission voltage to a sub-
transmission voltage, usually 69 kV. The sub-transmission voltage lines
can serve as a source to distribution substations.
The specific voltages leaving a transmission substation are determined by
the customer needs of the utility supplying power and to the requirements
of any connections to regional grids.
Underground Distribution Substation
Distribution substation transformers change the subtransmission voltage to the level
use by the consumer. Typical distribution voltages vary from 34,500Y/19,920Y volts
to 4,160Y/2400Y volts.
Underground distribution substations are at the
Manhole is also called a
splicing chamber or cable
vault. They are of various
sizes usually from 2 to 6
inches in diameter.
Underground System Components
Conduits are hollow tubes running from manhole to manhole in an underground
transmission or distribution system. Conduits can be made of plastic (PVC), fiberglass,
fiber, tile, concrete, or steel.
PVC and fiberglass are most commonly used
Conduit on a gradeDuct run within conduit showing drainage in
Electrical cables are run through ducts. The diameter of a duct should be at least 1/2 to
3/4 inch greater than the diameter of the cable(s) installed in the duct. They can be
made of plastic (PVC), fiberglass, fiber, tile, concrete, or steel. PVC and fiberglass are
most commonly used.
Underground High-Voltage Underground
High-Voltage underground cables are usually shielded cables. They are made with a
conductor, conductor-strand shielding, insulation, semi-conducting insulation shielding,
metallic insulation shielding, and a sheath. The sheath can be metallic and may then serve
as the metallic insulation shielding and be covered with a nonmetallic jacket to protect the
sheath. This sheath helps to reduce or eliminate inductive reactance. Such cables are
commonly used in circuits operating at 2400 volts or higher
Underground Transformer Vault
A transformer vault is a structure or room in which power transformers,
network protectors, voltage regulators, circuit breakers, meters, etc. are
Riser for Underground System
A riser is a set of devices that connects an overhead line to an underground line.
A riser has a conduit from the ground up the pole where potheads are used to
connect to the overhead lines.
Transformers in Underground System
A vault, pad-mounted, submersible, and direct-buried transformers are used in
are installed on a concrete
pad on the surface near the
Vault type transformer in
Underground transformers are essentially the same as an aboveground transformers
and are constructed for the particular needs of underground installation.
Transmission Lines Characteristic
A transmission line can be either overhead or underground line. Transmission
lines carry electric energy from power plants to end-users.
They can carry:
• Alternating current AC or
• Direct current DC or
• Both AC and DC voltages.
The main characteristics that distinguish a transmission line from a distribution
• the transmission line operates at high voltages,
• the transmission line transmit large quantities of power
• the transmission line transmit the power over much larger distance.
Transmission Lines Type:
Overhead Transmission Lines
The 3-phase AC transmission line transmitting power of greater
voltage from 69 kV to 765 kV and has three wires.
Power Transmitting Poles
The DC voltage transmission line
(Two wires: positive and negative.)
The sub-transmission line carries voltage
reduced from the major transmission line
Overhead high voltage
Sub-transmission & Distribution Lines
Sub-transmission lines carry voltages reduced from the major transmission line system for use in
industrial or large commercial facilities.
The 34.5 kV - 69 kV voltage is sent to regional distribution substations.
Underground Transmission Lines
The underground transmission line run through populated
areas and cities. They may be buried with no protection, or
placed in conduit, trenches, or tunnels.
Transmission lines installed in a trench and different way in tunnels.
Power Distribution System
A distribution system originates at a distribution substation and includes the lines,
poles, transformers and other equipment needed to deliver electric power to
Industrial Customer Transportation Customer
Residential Customer Commercial Customer
2.4 – 4.16 kV
14.4 - 7.2 kV
Air Circuit Breaker Distribution Bus Potheads
Batteries Power-line Carrier
Bus Support Insulators Frequency Changers Power Transformers
Capacitor Bank Grounding Resistors Rectifiers
Circuit Switchers Grounding Transformers Relays
High-Voltage Underground Cables SF6 Circuit Breakers
High-Voltage Fuses Shunt Reactors
Lightning Arresters Steel Superstructures
Control Panels Supervisory Control
Control Wires Metal-clad Switchgear Suspension Insulators
Converter Stations Meters Synchronous Condensers
Coupling Capacitors Microwave Transmission Bus
Current Transformers Oil Circuit Breakers Vacuum Circuit Breakers
Disconnect Switches Potential Transformers
Power Transmission System
Lines Representation and Equations
Prof. Z. Jan Bochynski
Linear Diagram of AC 3-Phase
This power system has two synchronous machines, two loads, two busses, two
transformers, and transmission lines to connect the busses together.
One-line Diagram of a Typical Coal-fired
Dual 345 kV transmission
lines on a steel tower
Dual 110 kV transmission
lines on wooden poles
A 13.8 kV distribution
lines with the ground
wire above the three
A distribution line
with no ground
Transmission system voltages are typically from 69KV up to 765KV.
Distribution systems typically operate in a voltage range of 4KV to 46KV.
There are two types of transmission lines: overhead lines and buried
An overhead transmission line usually consist of three conductors or bundles
of conductors containing the three phases of the power system.
The conductors are aluminum cable steel reinforced (ACSR), with a steel core
and aluminum conductors wrapped around the core.
Cable lines are designed to be placed underground or under the water. In
cables conductors are insulated from one another and surronded by the
As a rule of thumb, the power handling capability of a transmission line is
proportional to the square of the voltage on the line.
Therefore, very high voltage transmission lines are used to transmit electric
power over long distance. The lower voltage lines are distribution lines
used to deliver power to the individual custmers.
Transmission Lines Characteristics
Transmission lines are characterized by a series of resistance and inductance and by a
The DC resistance of the line conductor:
The transmission line inductance:
ρ = resistivity of the conductor
A = cross sectional area of the conductor
l = length of the conductor
Φ = number of flux linkages produced
by the current through the line
The line capacitance between the pair of
conductors in F
q = the charges on the conductors in coulombs
V = the voltage between the conductors in volts
The AC resistance of a line conductor is always higher than its DC resistance because of
In AC line as frequency increases, more of the current is concentrated near the outer
surface of the conductor.
ρ is wire electrical conductivity
μ is magnetic permeability of the flux medium
f is frequency
Skin Effect Samples
Source: M. R. Patel “Introduction to Electrical Power and Power Electronics” CRC Press, 2013
Two round conductors get current
concentration near facing area
Two bus bars with facing flats
get current concentration neat
Internal Inductance of T L
For the conductor of a radius r carrying the current I the magnetic field
intensity at the distance x from the center of this conductor is:
Outside a conductor
Inside a conductor
The flux density Bx at the distance x from the
center of the conductor:
𝟐𝟐π𝒓𝒓𝟐𝟐 𝑰𝑰 in Wb
The internal inductance per meter of line length:
𝒍𝒍𝒊𝒊 𝒊𝒊𝒊𝒊 =
For cooper and aluminum (non-ferromagnetic) this inductance is: Lint =0.5 x 10-7 H/m
T L External Inductance
The magnetic intensity at the distance x from the center of the conductor:
The flux density Bx at a distance x from the
center of the conductor:
𝑩𝑩𝒙𝒙 = µ𝑯𝑯𝒙𝒙 = µ 𝑰𝑰
The external inductance per meter due to the
flux between P1 and P2 :
𝐥𝐥 𝐥𝐥 𝑫𝑫𝟐𝟐
Transmission Line Inductance of
a Single-Phase Two-Wires Line
The two-wire transmission line inductance
per line length:
l = 𝒍𝒍𝒊𝒊 𝒊𝒊𝒊𝒊 + 𝒍𝒍𝒆𝒆𝒆𝒆𝒆𝒆 = (
+ 𝒍𝒍 𝒍𝒍
The series inductive reactance of
X = 𝑗𝑗 𝜔𝜔 𝑙𝑙 = j 2π f l
Inductance of a Transmission Lines
Inductance per unit length l of a single-phase, two-wire transmission line is
proportional to the ratio D/r , where D is a distance between the centers of the two
conductors and r is the wires radius.
The greater the spacing between the phases of the transmission line, the greater
the inductance of the line. A single conductor of a high-voltage line will tend to
have a higher inductance than a single-conductor of a low-voltage line.
The greater the radius of the conductor in a transmission line, the lower the
inductance of the line.
The series inductance of buried cables will be much smaller than the inductance
of overhead transmission lines.
In practical transmission lines do not use conductors of extremely large radius,
because they would be very heavy, inflexible, and expensive. Instead, is practiced
bundling two, or more conductors together in each phase.
Transmission Lines with
Conductors in Bundle
with two conductors
Transmission line with
four conductors in bundle
Capacitance and Capacitive
Reactance of a Transmission Line
When a voltage is applied to a pair of conductors separated by a nonconducting dielectric
medium, opposite electric charges accumulate on the surface of the conductors of equal
magnitude that is proportional to the line voltage.
q = C V
The electric field density at the wire surface is D = εr ε0 E
εr is relative permeability of the material is 8,85 x 10-12 F
ε0 is one (1) for the air
𝟐𝟐 𝝅𝝅 𝜺𝜺 𝒙𝒙
E - the electric field intensity at the point always radially
outward from the conductor
𝟐𝟐 𝝅𝝅 𝒙𝒙
Capacitance of a Single-Phase
Two-Wire Transmission Line
The capacitance per unit length between the two conductors of the transmission line:
The capacitance to the ground of a single-phase
𝑐𝑐𝑛𝑛 = 𝑐𝑐𝑎𝑎𝑎𝑎 = 𝑐𝑐𝑏𝑏𝑏𝑏 =
2 π ε
The shunt capacitive admittance of a transmission line
depends on the capacitance of the line and the frequency of
the power system.
Yc = y d = (j2π f c) d
y - is the shunt admittance per
unit length of the transmission
d – is the length of the line
The capacitive reactance is: 𝑋𝑋𝑐𝑐 =
Transmission Line Capacitance
If the transmission line capacitance is proportional to the ratio D/r , therefore ::
The greater the spacing between the phases of the transmission line, the lower is
the line capacitance.
The shunt capacitance of buried cables will be much larger than the shunt
capacitance of overhead transmission line because of very small spacing between
conductors in a cable.
A single-conductor high-voltage line will tend to have a lower capacitance than
a single-conductor low-voltage line.
The greater the radius of the conductors in a transmission line, the higher the
capacitance of the line.
Transmission Line Models
A transmission line is characterized by a series resistance per unit length, a series
inductance per unit length, and shunt capacitance per unit length.
For short transmission line (less than 50 miles), the shunt capacitance can be neglected.
For medium-length transmission lines (50 to 150 miles), the shunt capacitance can be
devided into two lumped components, one before and one after the series impedances.
The Phasor Diagram of a Short
VS = VR + ZI = VR + I(R + j XL)
VR = VS – RI - jXLI
The phasor diagram of a transmission line
p.f. cosφ<1, +Q)
Unity p.f. =1
(cosφ=1, Q = 0)
p.f. cos φ>1, -Q)
Power Flows in Transmission Line
Pin = 3 VS IS cos θS = √3 V S
LL IS cos θS
VS is the magnitude of the source line-to-neutral voltage
VLL is the magnitude of the source line-to-line voltage
Pout =3 VR IR cos θR =√3 VR
LL IR cos θR
Voltage regulation VR:
𝑉𝑉𝑛𝑛𝑛𝑛 − 𝑉𝑉𝑓𝑓𝑓𝑓
Vnl no-load voltage
Vfl full load voltage
Qin = 3 VS IS cos θS = √3VS
LL IS cos θS
Qout = 3 VR IR cos θR = √3VR
LL IR cos θR
Reactive power in and out
Sout = 3 VR IR = √3 VR
Sin = 3 VS IS = √3 VS
Transmission Line efficiency
EMF Overhead T L
Electric field under an overhead
Magnetic flux density near
overhead transmission line
Magnetic Flux Density around Overhead
Line and Cable
The magnetic fields arising at/or close to ground level from underground HV cables and
overhead lines carrying similar currents.
Underground cable (1 meter underground) produces greater flux density above but is
decreasing very fast within the distance from its center.
Shielding Electric Fields from Power Lines
Electric field from power line can be reduced by building walls, trees and shield made of
Magnetic Fields from Home Appliances
T L Voltage and Electric Fields
Electric fields depend on the magnitude of the voltage and
distance from the source.
Magnetic Fields from Appliances
Electric Field and Human Body
The electric fields from power line induce electric charges at the body surface.
At the highest electric field strength
encountered, hair movement and small shocks
may be sensed by some people.
Electric fields induced
currents in the body
Magnetic Field and Human Body
Magnetic fields induce
circulating currents in the
Induced currents in large
quantity may interfere with
the nervous system and
flashes of light may be
noticed in the eye.
People having a pacemaker
should avoid the exposure to
the magnetic fields.