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Protection BasicsPresented by
John S. Levine, P.E.
Levine Lectronics and Lectric, Inc.
770 565-1556
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Protection Fundamentals
By
John Levine
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Outline
Introductions Tools
Enervista Launchpad
OnLine Store Demo Relays at Levine
ANSI number
Training CDs Protection Fundamentals
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Objective
We are here to help make your job easier.This is very informal and designed around
Applications. Please ask question. We
are not here to preach to you. The knowledge base in the room varies
greatly. If you have a question, there is agood chance there are 3 or 4 other peoplethat have the same question. Please askit.
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Tools
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Demo Relays at L-3
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Relays at L-3
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GE Multilin Training CDs
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ANSI Symbols
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Conversion of Electro-Mechanical
to Electronic sheet
P P i t t ti t
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PowerPoint presentations at:
http://l-3.com/private/ieee/
http://l-3.com/private/ieee/http://l-3.com/private/ieee/http://l-3.com/private/ieee/http://l-3.com/private/ieee/5/22/2018 Protection Basics r3
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Protection Fundamentals
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Desirable Protection Attributes
Reliability: System operate properly Security: Dont trip when you shouldnt
Dependability: Trip when you should
Selectivity: Trip the minimal amount to clear thefault or abnormal operating condition
Speed: Usually the faster the better in terms ofminimizing equipment damage and maintaining
system integrity Simplicity: KISS
Economics: Dont break the bank
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Selection of protective relays requires compromises: Maximum and Reliable protection at minimum
equipment cost
High Sensitivity to faults and insensitivity to maximum
load currents
High-speed fault clearance with correct selectivity
Selectivity in isolating small faulty area
Ability to operate correctly under all predictable power
system conditions
Art & Science of Protect ion
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Cost of protective relays should be balancedagainst risks involved if protection is not
sufficient and not enough redundancy.
Primary objectives is to have faulted zonesprimary protection operate first, but if there are
protective relays failures, some form of
backup protection is provided.
Backup protection is local (if local primary
protection fails to clear fault) and remote (if
remote protection fails to operate to clear
fault)
Art & Science of Protect ion
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Primary Equipment & Components
Transformers - to step up or step down voltage level
Breakers - to energize equipment and interrupt fault current
to isolate faulted equipment
Insulators - to insulate equipment from ground and other
phases
Isolators (switches) - to create a visible and permanent
isolation of primary equipment for maintenance purposes
and route power flow over certain buses.
Bus - to allow multiple connections (feeders) to the same
source of power (transformer).
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Primary Equipment & Components Grounding - to operate and maintain equipment safely
Arrester - to protect primary equipment of sudden
overvoltage (lightning strike).
Switchgearintegrated components to switch, protect,
meter and control power flow
Reactors - to limit fault current (series) or compensate for
charge current (shunt)
VT and CT - to measure primary current and voltage and
supply scaled down values to P&C, metering, SCADA, etc.
Regulators - voltage, current, VAR, phase angle, etc.
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Types of Protection
Overcurrent Uses current to determine magnitude of fault Simple
May employ definite time or inverse time curves
May be slow Selectivity at the cost of speed (coordination stacks)
Inexpensive
May use various polarizing voltages or ground current
for directionality Communication aided schemes make more selective
I t t O t P t ti (IOC) &
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Instantaneous Overcurrent Protection (IOC) &
Definite Time Overcurrent
Relay closest to fault operates
first Relays closer to source
operate slower
Time between operating forsame current is called CTI(Clearing Time Interval)
Distribution
Substation
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(TOC) Coordination
Relay closest to fault operates
first Relays closer to source
operate slower
Time between operating forsame current is called CTI
Distribution
Substation
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Selection of the curves
uses what is termed as a
time multiplier or
time dial to effectively
shift the curve up or
down on the time axis
Operate region lies
above selected curve,
while no-operate region
lies below it
Inverse curves can
approximate fuse curve
shapes
Time Overcurrent Protection (TOC)
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Multiples of pick-up
Time Overcurrent Protection
(51, 51N, 51G)
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Types of Protection
Differential current in = current out
Simple
Very fast
Very defined clearing area
Expensive
Practical distance limitations Line differential systems overcome this using
digital communications
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Differential
Note CT polarity
dots
This is athrough-current
representation
Perfect
waveforms, nosaturation
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Differential
Note CT
polarity dots
This is aninternal fault
representation
Perfectwaveforms, no
saturation
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Types of Protection
Voltage Uses voltage to infer fault or abnormal
condition
May employ definite time or inverse time
curves May also be used for undervoltage load
shedding Simple
May be slow
Selectivity at the cost of speed (coordinationstacks)
Inexpensive
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Types of Protection
Frequency Uses frequency of voltage to detect power
balance condition
May employ definite time or inverse time
curves Used for load shedding & machinery
under/overspeed protection Simple
May be slow
Selectivity at the cost of speed can be expensive
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Types of Protection
Power Uses voltage and current to determine
power flow magnitude and direction
Typically definite time Complex
May be slow
Accuracy important for many applications
Can be expensive
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Types of Protection
Distance (Impedance) Uses voltage and current to determine impedance of
fault
Set on impedance [R-X] plane
Uses definite time Impedance related to distance from relay
Complicated
Fast
Somewhat defined clearing area with reasonableaccuracy
Expensive
Communication aided schemes make more selective
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Impedance
Relay in Zone 1 operates first
Time between Zones is calledCTI
Source
A B
21 21
T1
T2
ZA
ZB
R
X ZL
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Generation-typically at 4-20kV
Transmission-typically at 230-765kV
Subtransmission-typically at 69-161kV
Receives power from transmission system and
transforms into subtransmission level
Receives power from subtransmission system
and transforms into primary feeder voltage
Distribution network-typically 2.4-69kV
Low voltage (service)-typically 120-600V
Typical
Bulk
PowerSystem
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1. Generator or Generator-Transformer Units
2. Transformers
3. Buses
4. Lines (transmission and distribution)5. Utilization equipment (motors, static loads, etc.)
6. Capacitor or reactor (when separately protected)
Unit Generator-Tx zone
Bus zone
Line zone
Bus zone
Transformer zone Transformer zone
Bus zone
Generator
~
XFMR Bus Line Bus XFMR Bus Motor
Motor zone
ProtectionZones
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1. Overlap is accomplished by the locations of CTs, the key source forprotective relays.
2. In some cases a fault might involve a CT or a circuit breaker itself, which
means it can not be cleared until adjacent breakers (local or remote) are
opened.
Zone A Zone B
Relay Zone A
Relay Zone B
CTs are located at both sides of CB-fault between CTs is cleared from both remote
sides
Zone A Zone B
Relay Zone A
Relay Zone B
CTs are located at one side of CB-fault between CTs is sensed by both relays,
remote right side operate only.
Zone Overlap
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1. One-line diagram of the system or area involved
2. Impedances and connections of power equipment, system
frequency, voltage level and phase sequence
3. Existing schemes
4. Operating procedures and practices affecting protection
5. Importance of protection required and maximum allowed
clearance times
6. System fault studies
7. Maximum load and system swing limits
8. CTs and VTs locations, connections and ratios
9. Future expansion expectance
10. Any special considerations for application.
What Info is Required to Apply Protection
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C37.2:
Device
Numbers
Partial listing
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One Line Diagram
Non-dimensioned diagram showing how
pieces of electrical equipment are
connected
Simplification of actual system
Equipment is shown as boxes, circles and
other simple graphic symbols
Symbols should follow ANSI or IEC
conventions
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1-Line Symbols [1]
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1-Line Symbols [2]
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1-Line Symbols [3]
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1-Line Symbols [4]
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1-Line [1]
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1-Line [2]
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3-Line
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CB Trip Circuit (Simplified)
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Lock Out Relay
86b
PR
86b
Shown in RESET position
86
TC 86a
CB C il Ci it M it i
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CB Coil Circuit Monitoring:
T with CB Closed; C with CB Opened
CB C il Ci it M it i
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CB Coil Circuit Monitoring:
Both T&C Regardless of CB state
Current Transformers
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Current transformers are used to step primary system currents
to values usable by relays, meters, SCADA, transducers, etc.
CT ratios are expressed as primary to secondary; 2000:5, 1200:5,
600:5, 300:5
A 2000:5 CT has a CTR of 400
Current Transformers
St d d IEEE CT R l
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IEEE relay class is defined in terms of the voltage a CT
can deliver at 20 times the nominal current rating
without exceeding a 10% composite ratio error.
For example, a relay class of C100 on a 1200:5 CT means that
the CT can develop 100 volts at 24,000 primary amps
(1200*20) without exceeding a 10% ratio error. Maximum
burden = 1 ohm.
100 V = 20 * 5 * (1ohm)
200 V = 20 * 5 * (2 ohms)
400 V = 20 * 5 * (4 ohms)
800 V = 20 * 5 * (8 ohms)
Standard IEEE CT Relay
Accuracy
Standard IEEE CT B rdens (5 Amp)
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Standard IEEE CT Burdens (5 Amp)(Per IEEE Std. C57.13-1993)
Voltage Transformers
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VP
VS
Relay
Voltage (potential) transformers are used to isolate and step
down and accurately reproduce the scaled voltage for the
protective device or relay
VT ratios are typically expressed as primary to secondary;
14400:120, 7200:120
A 4160:120 VT has a VTR of 34.66
Voltage Transformers
T i l CT/VT Ci it
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Typical CT/VT Circuits
Courtesy of Blackburn, Protective Relay: Principles and Applications
CT/VT Ci it C i G d
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CT/VT Circuit vs. Casing Ground
Case ground made at IT location
Secondary circuit ground made at first point of
use
Case
Secondary Circuit
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Equipment Grounding
Prevents shock exposure of personnel
Provides current carrying capability for the
ground-fault current
Grounding includes design and construction ofsubstation ground mat and CT and VT safety
grounding
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System Grounding
Limits overvoltages
Limits difference in electric potential through local
area conducting objects
Several methods
Ungrounded
Reactance Coil Grounded
High Z Grounded Low Z Grounded
Solidly Grounded
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1. Ungrounded: There is no intentional
ground applied to the system-
however its grounded through
natural capacitance. Found in 2.4-
15kV systems.
2. Reactance Grounded: Total system
capacitance is cancelled by equal
inductance. This decreases thecurrent at the fault and limits voltage
across the arc at the fault to decrease
damage.
X0
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3. High Resistance Grounded: Limitsground fault current to 10A-20A.
Used to limit transient overvoltages
due to arcing ground faults.
R0= 2X0
System Grounding
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5. Solidly Grounded: There is a
connection of transformer orgenerator neutral directly to station
ground.
Effectively Grounded: R0
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Basic Current Connections:How System is Grounded
Determines How Ground Fault is Detected
Medium/High
Resistance
Ground
Low/No
Resistance
Ground
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Substation Types
Single Supply
Multiple Supply
Mobile Substations for emergencies
Types are defined by number of
transformers, buses, breakers to provide
adequate service for application
Industrial Substation Arrangements
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Industrial Substation Arrangements(Typical)
Industrial Substation Arrangements
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Industrial Substation Arrangements(Typical)
Utility Substation Arrangements
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Utility Substation Arrangements
Single Bus, 1 Tx, Dual supply Single Bus, 2 Tx, Dual
Supply
2-sections Bus with HS Tie-Breaker,
2 Tx, Dual Supply
(Typical)
Utility Substation Arrangements
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Breaker-and-a-halfallows reduction of
equipment cost by using 3 breakers for
each 2 circuits. For load transfer and
operation is simple, but relaying is
complex as middle breaker is responsible
to both circuits
Utility Substation Arrangements
Bus
1
Bus 2
Ring busadvantage that one
breaker per circuit. Also each
outgoing circuit (Tx) has 2 sources
of supply. Any breaker can be taken
from service without disrupting
others.
(Typical)
Utility Substation Arrangements
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Double Bus:Upper Main and
Transfer, bottom Double Main bus
Main bus
Aux. bus
Bus 1
Bus 2
Tie
breaker
Utility Substation Arrangements
Main
ReserveTransfer
Main-Reserved and Transfer
Bus: Allows maintenance of any
bus and any breaker
(Typical)
Switchgear Defined
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Switchgear Defined Assemblies containing electrical switching,
protection, metering and management devices Used in three-phase, high-power industrial,
commercial and utility applications
Covers a variety of actual uses, including motor
control, distribution panels and outdoorswitchyards
The term "switchgear" is plural, even whenreferring to a single switchgear assembly (never
say, "switchgears") May be a described in terms of use:
"the generator switchgear"
"the stamping line switchgear"
Switchgear Examples
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Switchgear Examples
A Good Day in System
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A Good Day in System
Protection
CTs and VTs bring electrical info to relays
Relays sense current and voltage and declare
fault
Relays send signals through control circuits tocircuit breakers
Circuit breaker(s) correctly trip
What Cou ld Go Wrong Here????
A Bad Day in System
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y y
Protection
CTs or VTs are shorted, opened, or their wiring is Relays do not declare fault due to setting errors,
faulty relay, CT saturation
Control wires cut or batteries dead so no signal issent from relay to circuit breaker
Circuit breakers do not have power, burnt trip coil
or otherwise fail to trip
Protect ion Systems Typ ical ly are
Designed for N-1
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Protection Performance Statistics
Correct and desired: 92.2%
Correct but undesired: 5.3% Incorrect: 2.1%
Fail to trip: 0.4%
Contribution to Faults
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Contribution to Faults
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Fault Types (Shunt)
AC & DC Current Components
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AC & DC Current Components
of Fault Current
Useful Conversions
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Useful Conversions
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Per Unit System
Establish two base quantities:
Standard practice is to define
Base power3 phase
Base voltageline to line
Other quantities derived with basic powerequations
Per Unit Basics
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Per Unit Basics
Short Circuit Calculations
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Short Circuit Calculations
Per Unit System
Per Unit Value = Actual QuantityBase Quantity
Vpu
= Vactual
Vbase
Ipu = Iactual
Ibase
Zpu = ZactualZbase
Sh t Ci it C l l ti
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Short Circuit Calculations
Per Unit System
Short Circuit Calculations
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Short Circuit Calculations
Per Unit SystemBase Conversion
Zpu = ZactualZbase
Zbase = kV2base
MVAbase
Zpu1= MVAbase1kV 2base1
XZactual
Zpu2= MVAbase2kV 2base2
XZactual
Zpu2 =Zpu1 x kV2base1 xMVAbase2
kV 2base2 MVAbase1
A St d f F lt
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A Study of a Fault.
A Fl h H d
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Arc Flash Hazard
Protective Relaying Methods
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of Reducing Arc Flash Hazard
Bus differential protection (thisreduces the arc flash energy by
reducing the clearing time
Zone interlock schemes where
bus relay selectively is allowed
to trip or block depending onlocation of faults as identified
from feeder relays
Temporary setting changes to
reduce clearing time during
maintenance
Sacrifices coordination
FlexCurve for improvedcoordination opportunities
Employ 51VC/VR on feeders
fed from small generation to
improve sensitivity and
coordination Employ UV light detectors with
current disturbance detectors
for selective gear tripping
Arc Flash Hazards
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Arc Flash Hazards
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Arc Pressure Wave
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Arc Pressure Wave
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Copy of this presentation are at:
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www.L-3.com\private\IEEE
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Protection Fundamentals
QUESTIONS?