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Protection Fundamentals
Kevin Hinkley Technology and Innovation Manager TransGrid
What are we doing in this topic?
n Basics of protection n Policy for Protection n Backup (CBF) Protection
What is a fault?
n Abnormal current flow greater than nominal ratings of the equipment – Load current 1 kA, fault current 40kA
n Faults can be due to HV equipment insulation failure, lightning or human mistakes.
n 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?
n To economically deliver power over large distances – X and R must be kept low (Losses) – The voltage is increased to lower losses
n Transformer copper losses are reduced by using a larger conductor
n 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?
n Generators slip poles n Stress generator shafts n Conductors overheating n Over rating of equipment n Split power system
How do we detect and control a fault?
n CTs n VTs
n CB’s and Relays n Fuses
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
n 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 – Injury to people – Damage to property – Loss of Supply
Design limitation of Protection
n The protection system is designed to clear a single fault despite the failure of one protection scheme or CB.
n This is: ‘N-1’
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
n 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
n 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)
Protection Relay Policy
n All main Protections are duplicated n Relays are of different manufacture
– If same manufacturer, then they should operate on different principles
– Exceptions are allowed for some relays with proven field service
n 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
No.1 A Protection
Blind spot fault
n 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)
P1 P2 P2 P1 P1 P2 P2 P1
No.1 B Protection
B Feeder A Feeder
Backup Protection
n Backup is provide for: – Protection (relay & scheme) failure – Circuit breaker failure.
n 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
n Relay & CB duplicated - ie, locally backed up.
n Faster clearance, minimal disruption n Complex circuitry & expensive
Source P1 P2
Current Checked Circuit Breaker Fail
Trip CB’s B,C,D,E & F
Protection Current Check Timer
A B
C D E
F
Protection of individual systems
n Feeder Protection n Transformer Protection n Busbar Protection n Reactor Protection n Capacitor Protection
What we have done ?
n Basics of protection n Policy for Protection n Backup (CBF) Protection
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