protection types of schemes-fischer
DESCRIPTION
POWER TYPES OF SCHEMES, PROTECTIONTRANSCRIPT
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Harold FischerRobinson Sales, Inc.SEL Representative
March 25, 2014
Introduction to Power System Protection
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Introduction to Protection
• Power system review• Protection system purpose• Protection system components• Protection system measures• Types of protective relays
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Objectives
• Understand what protective relays are and how the basic types work
• Know the “terms” used in power system protection
• Relate SEL products to their typical applications
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Power System Elements
• Generators
• Transformers
• Power Lines
• Buses
• Capacitor Banks
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Generator
DistributionSubstation
Single Bus
Transmission Line
DistributionLine
CircuitBreaker
Capacitor
RingBusGS
S YS X
GeneratorStep Up
Transformer
Bulk PowerSubstation
S Z
WholesaleCustomer
Sub-Transmission
Transformer
Power System Elements
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Zones of Protection
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Protective Relay Functions
• Detect Faults• Short circuits
• Detect abnormal operating conditions• Overloads, unbalances, low voltage or
frequency• Control and supervision
• Automatic reclosing, bus transfer, synch-check
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Components of a Protection System
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Protective Relaying System
Relay
52
CommunicationsChannel
DC Supply
Current Transformers
Circuit Breaker
VoltageTransformers
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Protection System Elements
• Protective Relays• Circuit Breakers• Current and Voltage Transformers• Communication Channels• DC Supply System• Control Cables
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Instrument Transformers
• CT – Current Transformer• Current scaling• Isolation from primary voltage
• VT – Voltage Transformer• Voltage scaling• Isolation from primary voltage
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Measures Applied to Relays
How do relay engineers evaluate and classify relays;
and decide which ones to use?
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Protection Operation Classification
• Correct Operation
• Incorrect Operation• Failure to trip• False tripping
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Power System Protection Requirements
• Measures of Reliability• Dependability – ability of relay to
always trip for a fault in its protected zone
• Security – ability of relay to never trip for a fault outside its protected zone
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Power System Protection Requirements
• Measures of Performance• Sensitivity – ability of relay to
determine a faulted power system from a normal power system
• Selectivity – ability of a relay to determine what part of the power system is faulted
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Power System Protection Requirements
• Measures of Performance (cont.)• Dependability – ability of relay to
always trip for a fault in its protected zone
• Security – ability of relay to never trip for a fault outside its protected zone
• Speed – how fast can the relay determine that there is a fault in its zone
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• Simplicity
• Economics• Protection costs• Equipment costs• Outage costs
Power System Protection Requirements
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General Relationships
• Higher selectivity allows higher speed• Dependability is generally
proportional to sensitivity and speed
• Security is generally proportional to selectivity
• Higher speed and selectivity generally means higher cost and complexity
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Types of Protective Relays
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IEEE C37.2 Device Numbers
• 50 – Instantaneous Overcurrent Relay
• 51 – Time Overcurrent Relay
• 67 – Directional Overcurrent Relay
• 21 – Distance Relay
• 87 – Differential Relay
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Overcurrent Relays
• Detect current above normal and trip
• Two types• Instantaneous (ANSI Code 50)• Inverse Time (ANSI Code 51)
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Overcurrent Relay Types• Instantaneous (50, 50N)• Time-Delayed (51, 51N)
Inverse Time
Definite Time
IPU
T
I
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Inverse Time OC (ANSI Code 51)
• Works like a fuse• Trip fast for high overcurrent• Trip slow for small overcurrent
• Desirable attribute for many protection problems
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Application of OC Relays
• Commonly used for:• Primary fault protection for radial
feeders• Backup fault protection for more
sophisticated relay schemes• Supervisory elements for more
sophisticated relay schemes• Overload protection
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Overcurrent Application
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Overcurrent Application
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Overcurrent Application
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Overcurrent Application
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Summary for Overcurrent Relays
Measure RatingSensitivity PoorSelectivity PoorSpeed DependsSimplicity BestCost Best
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Directional Overcurrent Relays
• ANSI Code 67
• Two criteria• Determine that current is flowing
in a particular direction• Detect current above normal and
trip
• More selective
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How do we determine direction?
• Which way did he go?• AC current reverses direction every
half cycle!
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How do we determine direction?• We need a reference
• Called a polarizing quantity• Phase angle of current with respect to
voltage for example
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Application of Directional Relays
• Twice as selective as 50/51 relays
• Used in applications that need more selectivity such as:• Networked lines with fused loads• Bus mains and ties• Supervisory elements for more
sophisticated relay schemes
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Directional Relay Application
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Summary for Directional Relays
Measure RatingSensitivity DependsSelectivity GoodSpeed GoodSimplicity ModerateCost Higher
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Distance Relays
• ANSI Code 21
• Determines:• Not only the direction of the fault• But, also how far away it is
• Set the reach of the relay in ohms (Resistance and Reactance)
• Line has characteristics ohms/mile• Measured ohms = distance
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Types of Distance Relays
• Two types in general use today• Mho (used in the US)• Quadrilateral (Common in Europe)
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Mho Relay Characteristic
X
M3
R
B
A
M1
M2
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Application of Distance Relays
• Even more selective that 67 relays since they only respond to faults within their reach
• Used where more selectivity is required• Networked lines with critical clearing
times
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Distance Relay Application
Zone 1
Bus A Bus B Bus C
Zone 2
Zone 3
(Instantaneous)
(15-30 cycle delay)
(> 30 cycle delay)
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Improving Distance Relay Selectivity
• Distance relays are not accurate enough to reach exactly 100% of the line
• Use pilot (communication aided) protection
• POTT, PUTT, DCB, DCUB
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Pilot Protection
XMTR = Transmitter RCVR = Receiver
XMTR RCVR
XMTRRCVR
Relay Relay
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Pilot Protection
If each end of the line tells the other if the fault is forward or
reverse, they can compare notes and determine if the fault is inside
the protected zone
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Distance Relay Application (POTT)
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Distance Relay Application (DCB)
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Summary for Distance Relays
Measure RatingSensitivity GoodSelectivity BetterSpeed Better Simplicity PoorCost Higher
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Differential Relays
• ANSI Code 87
• Simple concept:• Measure current in vs. current out• If they are not equal, then trip
• Operates on the difference in current – not the total current, thus sensitivity is good
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Differential Relay Zone
Internal Fault
87
Protected zone is precisely determined by location of CT’s connected to relay
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Types of Differential Relays
• Differential Overcurrent• Operates on magnitude of difference
• Percent Restrained Differential• Difference must be a percentage of
the total current• Automatically adjusts sensitivity
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Types of Differential Relays
• Line Current Differential• Communicate current at remote and
of each zone through communications channel
• High Impedance Bus Differential
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Differential Relay Application
HV LV
800/1 2000/5
100/5(REF)
3 3
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Differential Relay Application
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Line Current Differential
IL IR
Exchange of current dataacross high speed channel
L R
CommunicationsChannelRelays Relays
T
R
T
R
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Differential Protection is Even Used in the Home!
• GFCI (Ground Fault Circuit Interrupter)• Compare the current going to an
appliance to the current returning• Trips for a very small difference• High sensitivity to keep you safe
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Application of Differential Relays
• Applied anywhere that high speed, high sensitivity, and high selectivity is required:• Transformers• Generators• Bus Sections• Lines
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Summary for Differential Relays
Measure RatingSensitivity BestSelectivity BestSpeed Best Simplicity ModerateCost Depends
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Summary
• There are four main relay types for detecting faults• 50/51, 67, 21, 87
• We decide on which types of relays to use based upon several factors• Sensitivity, selectivity, speed, cost,
simplicity• There are two measures for reliability
• Dependability, security
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Any Questions?