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