Protection Fundamentals
Kevin Hinkley
Technology and Innovation Manager
TransGrid
What are we doing in this topic?
Basics of protection
n  Policy for Protection
n  Backup (CBF) Protection
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What is a fault?
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Abnormal current flow greater than nominal
ratings of the equipment
–  Load current 1 kA, fault current 40kA
n
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Faults can be due to HV equipment
insulation failure, lightning or human
mistakes.
Types of faults – 3ph, 3ph-e, 1ph-e, 2ph-e,
ph-ph
What is a Fault?
Per unit voltage Circle
–  Blue QLD
–  Red NSW
–  Cyan VIC
–  Purple SA
Simulation By Michael Bradbery
Why do we have faults?
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To economically deliver power over large distances
–  X and R must be kept low (Losses)
–  The voltage is increased to lower losses
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Transformer copper losses are reduced by using a
larger conductor
Hence low X and R means large current if
connected to earth or phase to phase.
What does a fault do to
the power system?
Generators slip poles
n  Stress generator shafts
n  Conductors overheating
n  Over rating of equipment
n  Split power system
n
How do we detect and
control a fault?
CTs
n  VTs
n
CB’s and Relays
n  Fuses
n
How Quick do we need to
remove a fault?
500 kV
Near 80 ms, Far 100 ms
330 kV
Near 100 ms, Far 120 ms
132 kV
Near 120 ms, Far 220 ms
Protection Policy Objectives
To disconnect the faulty circuit with
minimum damage due to the fault
n  To leave the rest of the power system
in a healthy state
n  To prevent or minimise
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–  Injury to people
–  Damage to property
–  Loss of Supply
Design limitation of Protection
The protection system is designed to
clear a single fault despite the failure
of one protection scheme or CB.
n  This is: ‘N-1’
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Requirements of a Protection System
n
Reliability
–  Protection should always operate when
required
n
Security
–  Protection should refrain from operating
when not required
Basic Protection Policy
n
Any Fault must be:
–  Seen by two protections
–  Able to be cleared by two CB’s
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Allow for a single contingency (N-1)
–  Failure of one protection system or
–  Circuit breaker
n
Only remove the faulted element of
the power system
Requirements of a Protection System
Backup for relay and scheme failure
n  Backup for circuit breaker failure
n  No dead zones
n  No permanent damage due to
clearance times
n  Clearance time should not exceed
system stability time (CCT)
n
Protection Relay Policy
All main Protections are duplicated
n  Relays are of different manufacture
n
–  If same manufacturer, then they should
operate on different principles
–  Exceptions are allowed for some relays
with proven field service
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Conform to appropriate IEC Standards
Industry accepted risks
n
It is accepted that the following low
probability events may result in a loss of
system stability under certain operating
conditions:
–  2-Phase Phase-earth bus fault
–  Circuit breaker failure
–  Battery failure at single battery locations
–  Failure of a CB or disconnector pallet switch
–  Blindspot fault at critical locations
Blind spot fault
Fault between the CT and CB
n  Protection operates but does not clear
the fault (Total Operation time 100ms)
n  Circuit breaker fail logic used to clear
the fault (Total Clearance time 250ms)
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No.1 A Protection
No.1 B Protection
A Feeder
B Feeder
P1 P2
P1 P2 P2 P1
P2 P1
Backup Protection
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Backup is provide for:
–  Protection (relay & scheme) failure
–  Circuit breaker failure.
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Backup can be:
–  Remote Backup - 66 kV and below
–  Local Backup - 132 kV and above
Remote Backup
n
Backup for relay & circuit breaker is at
a remote location
–  suited to radial systems
–  Slower clearance, maximum disruption
–  Minimum complexity & inexpensive
Source
Relay A
Relay B
Ideal Local Backup
Relay & CB duplicated - ie, locally
backed up.
n  Faster clearance, minimal disruption
n  Complex circuitry & expensive
n
Source
P1 P2
Current Checked
Circuit Breaker Fail
C
D
E
F
A
Protection
Current
Check
B
Timer
Trip CB’s B,C,D,E & F
Protection of individual
systems
Feeder Protection
n  Transformer Protection
n  Busbar Protection
n  Reactor Protection
n  Capacitor Protection
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What we have done ?
Basics of protection
n  Policy for Protection
n  Backup (CBF) Protection
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