Electric Power System
A H Chowdhury, PhD
Professor
Dept. of EEE, BUET
Electric Power System is the largest and most complex machine ever
devised by man
Basic elements of a
power system
Location of
switchgear in a
typical power
system
Substation Layout Diagram
Transmission Substation Switchyard
Transmission Substation Switchyard
Transmission Substation Switchyard
Transmission Bus
Transformers on Rail
Transformers on Rail
Transformer Protection
PT
CB
Circuit Breaker
Current Transformer
Battery Room
Components of Overhead Lines
Components of Overhead Lines
Insulators
Dampers
Typical HV Transmission Line
Bundled Conductors
Transmission Line Insulators
Mechanical Design
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•
230 kV transmission tower collapse at Bhairab, Kishoreganj on 1 May 2017
25 power plants shutdown, 37 districts without electricity for 5 hours
Birds on a Transmission Tower
Barapukuria, Dinajpur
Earth wire
Great islands of
Bangladesh
BPSN
https://www.pgcb.org.bd
Lightning
• 2,000 active thunderstorms at a given
moment
• Over 40,000 thunderstorms each day
• 50 - 100 flashes per second
• 1.5 – 3 billion flashes a year
• 24,000 deaths per year
• 240,000 injuries per year
• Millions of dollars in property damage
Lightning
Isokeraunic map of the world
Keraunic level (Nk) corresponds to the number of thunders heard in a defined area
Lightning
Global annual distribution of lightning activity (flashes/km2 year )
Lightning Effects on Power System
• Lightning is a significant disturbance of functioning of all
electrical installations
 All power and voltage levels are concerned
 May cause temporary disruption
 May cause destruction of equipment
 Danger to persons (not voltage, elevated potential of ground)
 Generates corona which causes loss of energy
• Lightning is the major cause of faults on overhead lines
Lightning Effects on Power System
• Lightning flash generally consists of several strokes
• First stroke is most often more severe than subsequent strokes
• Typical lightning voltage: Front t = 1.2 μS, Tail t = 50 μS
Lightning Effects on Transmission Line
• Voltages caused by direct lightning strike on power lines can
cause damages to power system
– particularly to transformers, electronic control and management
system
• Indirect lightning, hitting ground near a transmission lines,
are more frequent and can cause damage
• Underground cable also damaged by lightning
– induced overvoltage environment around cables can be nearly as
severe as overhead-line conditions
Lightning Effects on Transmission Line
• When lightning hits a power line directly
– It is like closing a “big switch” between a large
current source and the power line
– Causes a large transients
• When lightning hits proximity of power line
– Large magnetic field produced by lightning current
– Cause mutual linkage between power line and
lightning
– Generates electrical transient
Lightning Effects on Transmission Line
Traveling wave due to lightning strikes
a shield wire
The current passes along the tower
to the ground
Lightning Effects on Transmission Line
• Lightning strike on shield wire
 Most common on high voltage
transmission line
 High ground potential rise in tower body
due to higher earth resistance of tower
 Flashover from cross arm to phase
conductor across insulator string
 Can induce damaging voltage on phase
conductor (shield failure)
Lightning Effects on Transmission Line
• Lightning strike on phase conductor
 Lightning surge flows along line in both directions
 Damages insulation and weakens pole unit
 If strike adjacent to substation - damage to substation equipment
• Lightning strike on tower
 Lightning current flows to the earth, large potential difference
between tower top and bottom
 Insulator flashover occurs between tower and conductor
Lightning Effects on Transmission Line
• What happens when lightning strikes a tower?
– A traveling voltage is generated which travels back and forth along tower
– The wave is reflected at tower footing and at tower top
– This raises voltage at cross-arms and stresses insulators
• Insulator will flashover if transient voltage exceeds its withstand
level (back flash-over)
• Back flash-over voltage generated by multiple reflections along the
struck tower and also along shield wire at adjacent towers
Lightning and Distribution Line
Propagation of overvoltage wave at a lightning strike into a 11 kV
overhead line
Canadian Electricity
Association data:
 50 failures per 100 km of
distribution line per year
 Lightning contributing
roughly 15 of these
failures
Lightning and Distribution Line
Propagation of overvoltage wave at a lightning strike into a 11 kV
overhead line
Canadian Electricity
Association data:
 50 failures per 100 km of
distribution line per year
 Lightning contributing
roughly 15 of these
failures
Power System Lightning Protection
• Lightning can be protected against by:
 Having a high system BIL (Basic Insulation Level)
- e.g. improve critical flashover of (CFO) insulators
BIL is level designed to
withstand surge voltages
 Using shield wires
 Having a low impedance ground
- reduce tower footing resistance (TFR)
 Using surge arrestors to clamp voltages across equipment
• Direct strikes virtually impossible to protect against
Power System Lightning Protection
• Protection devices includes
 Arresters
 Spark gaps
 Guard wires
 Rods and mesh cages
• Protection with devices is very local
Shield Conductor
Shield wire or Earth wire for lightning strike protection
Power System Lightning Protection
Shield wire
String insulator
Bus
CT
CB
DS
PT
Power System Lightning Protection
Shield wire
Tower Earthing
• Needle electrode for better
dissipation of charges
• Tower footing resistance (TFR)
– resistance from tower top to
ground offered by metal parts of
tower + ground resistance
Lightning Protection Reliability
• Lightning protection design has
changed little for over two
centuries
• Photograph shows lightning
struck Moscow television tower
200 m below its top, i.e., tower
could not protect itself
• This is not an exception to the
rule
– Over the years most
descending discharges missed
the Tower top, contrary to
what had been expected
Grid failure leading to blackout
Karwan Bazar, Dhaka, 14 November 2014
Faults and Abnormal Conditions
Faults
Type of electrical failure that
causes greatest concern is the
short circuit or ‘fault’
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•
•
•
•
Three phase fault
Single line to ground fault
Line to line fault
Double line to ground fault
Open circuit fault
Faults and Abnormal Conditions
Abnormal conditions
• Under/over voltage
• Under/over frequency
• Voltage and current unbalance
• Power reversal
• Power swing
• Temperature rise
• Instability (rotor angle, frequency, voltage)
Abnormal Conditions in System
Grid voltage at Haripur 132 kV
May 1 and 2, 2014
142
140
138
Voltage, kV
136
134
132
130
128
126
0
100
200
300
Sample no.
400
500
600
Abnormal Conditions in System
Frequency variation at Haripur 132 kV
May 1 and 2, 2014
52
51.5
Frequency, Hz
51
50.5
50
49.5
49
48.5
0
100
200
300
Sample no.
400
500
600
Whose Fault?
System failures are result of multiple failures of following
components
• Design
• Equipment
• Procedures
• Operators
• Supplies and materials
• Environment
Design to Prevent Fault
• Adequate insulation
Design to Prevent Fault
Design to Prevent Fault
Design to Prevent Fault
A vacuum circuit breaker
A burnt vacuum circuit breaker
SCADA