ETAP - Coordination and protecion

© 2011 Operation Technology, Inc. PROPRIETARY & CONFIDENTIAL© 2011 Operation Technology, Inc. PROPRIETARY & CONFIDENTIAL
Protection and Coordination
Pirooz Barkhordar, PE
Operation Technology, Inc.

© 2011 Operation Technology, Inc. PROPRIETARY & CONFIDENTIAL
Protection & Coordination
• Agenda
• Objectives
• Equipment Protection
• Protection Types
• Overcurrent Protection
• STAR Overview
• Features and Capabilities
• Protective Device Types
• TCC Curves
• STAR Short-circuit
• PD Sequence of Operation
• Normalized TCC curves
• TCC Print and Settings Report
• Examples and Assignments

• Objectives
• Human Safety
• Prevent injury and fatality

Real Side of Failure in Safety

• Objectives
• Protection of Equipment
• Permit normal operation
• Isolate the equipment in case of abnormal conditions

• Objectives
• Protection of System (Stability Protection)
• Over / Under Voltage
• Over / Under Frequency
• Rate of Frequency Change
• Islanding of System

• Objectives
• Selectivity
• Minimal isolation of network with abnormal conditions
• Permit normal operation for rest of electrical network

• Objectives
• Reasonable Cost
• Maximum achievable reliability for protection and
coordination at minimal cost
• Science, Experience, and Art
• Sensitivity to faults and insensitivity to normal operation
• Fast fault clearance with proper selectivity
• Minimal isolation of faulty area
• Capability to operate correctly under all predictable
power system conditions

References
• IEEE Std. 242-2001, IEEE Recommended Practice
for Protection and Coordination of Industrial and
Commercial Power Systems (IEEE Buff Book)
for Electric Power Distribution for Industrial Plants
(IEEE Red Book)
for Industrial and Commercial Power Systems
Analysis (IEEE Brown Book)
• Other technical references

Study Procedure
• Prepare an accurate one-line diagram (relay
diagrams)
• Obtain the available system current spectrum
(operating load, overloads, fault kA)
• Determine the equipment protection criteria
• Select the appropriate protective devices / settings
• Plot the fixed points (operating/damage curves, FLA,
ampacity, etc.)
• Obtain / plot the device characteristics curves
• Analyze the results

Required Data
• One-line diagrams (Relay diagrams)
• Power Grid Fault Current Data and Protective Device Settings
• Generator Data
• Transformer Data
• Motor Data
• Load Data
• Fault Currents
• Cable / Conductor Data
• Bus / Switchgear Data
• Instrument Transformer Data (CT, VT)
• Protective Device (PD) Data

Protection of Equipments
• Major Equipments (apparatus)
• Induction Motor
• Synchronous Motor
• Cable
• Transformer
• Generator
• Bus
• Transmission/Distribution Line

Equipment Protection Criteria
• Permit: Normal Running Condition
• Max permitted current at working conditions
• Environment temperature, cooling media, elevation, etc.
• Protect: Abnormal Fault Condition
• Excessive through fault current caused by:
• Improper design, installation, or operation of equipment
• Incidents

Excessive Currents
• Excessive currents in abnormal conditions
• Overload current
• (100-160% Full Load Amps)
• Short-time overload current
• Short-circuit current

Capability / Damage Curves
t
I
I2
2
t
Gen
I2
t
Motor
Xfmr
I2
t
Cable
I2
t

Protection Types
• Overcurrent
• Inverse Time Over Current (TOC)
• Instantaneous Over Current (IOC)
• Directional
• Differential

Protection Types
• Impedance
• Distance
• Voltage
• Under/Over Voltage
• Frequency
• Under/Over Frequency
• Mechanical
• Pressure (Buchholz Relay)

Overcurrent Protection
Overcurrent Characteristics
• Inverse Time Over Current (TOC)
• Simple, cheap, and large
application in LV, and MV
• LV Breakers
• Represent tolerance band
• MCB, MCCB, ICCB, PCB
• Fuses
• Overload Heater
• Overload Relay
Time-Current-Characteristics (TCC)

Overcurrent Protection
Relay TOC Characteristics Relay TOC Curves
• Curve Shape Adaptation
• Equipment Protection
• Selectivity
• Time Margin at higher fault
currents

ETAP Star Overview
• Star Mode
• Creation of TCC and Star View
• Addition of devices to existing TCC
• Graphical and Editor adjustments
• Star View Options (top)
• Combine Curve (ETAP 11 enahncement)
• Star Mode and Star View difference

ETAP Star Overview
• Supported Protective Devices and Functions
• Overload - CT based & Inline (49)
• Phase, neutral, ground, and negative sequence overcurrent
(51/50)
• Voltage control and restraint overcurrent (51VC/51VR)
• Directional overcurrent (67)
• High impedance & percentage differential (87)
• Electronic & hydraulic reclosers (79)
• Relay interlock with HVCB, switch and contactor
• CT Ratio and multiple connections
*Reverse power (32) and under/over voltage (27/59) are supported in
Transient Stability

Low Voltage Protective Devices
• Low Voltage Circuit Breaker (LVCB)
• Power Circuit Breaker (PCB)
• UL 1066, ANSI C37.13, ANSI C37.16, ANSI C37.17
• IEC60947-2
• Insulated Case Circuit Breaker (ICCB)
• UL489 (Non-fused MCCB, 2 step stored energy closing mechanism,
electronic trip, and drawout construction)
• IEC60947-2
• Molded Case Circuit Breaker (MCCB)
• UL489 (integral unit and enclosed housing of insulating material)
• IEC60947-2
• Miniature Circuit Breaker (MCB)
• UL489, UL508, UL1077
• IEC60898

LVCB Differences

• LVCB Trip Units
• Thermal Magnetic
• Motor Circuit Protector (MCP)
• Solid State Trip (SST) or microprocessor based
• Electro-mechanical

LV Protective Devices
• MCCB Trip Units
– Thermal-Magnetic

LV Protective Devices
• MCCB Trip Units
– Magnetic Only

• LVCB Trip Units
• Solid State Trip (SST) or
microprocessor based
• Electro-mechanical
• Trip Unit Segments
• Long Time (LT ANSI; I> IEC)
• Short Time (ST ANSI; I>> IEC)
• Instantaneous (IT ANSI; I>>> IEC)

Fuse (Power Fuse)
• Non Adjustable Device (unless electronic)
• Continuous and Interrupting Rating
• Voltage Levels (Max kV)
• Interrupting Rating (sym, asym)
• Characteristic Curves
• Min. Melting
• Total Clearing
• Application (rating type: R, E, X, …)

Fuse Types
• Expulsion Fuse (Non-CLF)
• Current Limiting Fuse (CLF)
• Electronic Fuse (S&C Fault Fiter)

Minimum Melting
Time Curve
Total Clearing
Time Curve

Current Limiting Fuse (CLF)
• Limits the peak current of short-circuit
• Reduces magnetic stresses (mechanical damage)
• Reduces thermal energy

Current Limiting ActionCurrent(peakamps)
tm ta
Ip’
Ip
tc
ta = tc – tm
ta = Arcing Time
tm = Melting Time
tc = Clearing Time
Ip = Peak Current
Ip’ = Peak Let-thru Current
Time (cycles)

CLF Let-Through Chart

• Assumptions:
1. Short-circuit X/R ≤ Tested Short-circuit X/R, or
Short-circuit power factor ≥ tested power factor

• Assumptions
2. The fault is on the load terminal

• Impact of Downstream Breaker
• The fault current passing through both PDs
• The breaker may start to open representing a
dynamic impedance causing reduced let-through
current with different trip time
• A combination test is needed to make sure this is
not happening. This is a series rating test.

• Assumptions
3. The sum of motor full load currents contribution between the
series rated devices should not exceeds 1 percent of
interrupting rating of lowest rated device.

Symmetrical RMS Amperes
PeakLet-ThroughAmperes
100 A
60 A
7% PF (X/R = 14.3)
12,500
5,200
230,000
300 A
100,000
Let-Through Chart

Fuse
Generally:
• CLF is a better short-circuit protection
• Non-CLF (expulsion fuse) is a better Overload
protection
• Electronic fuses are typically easier to coordinate
due to the electronic control adjustments

Zones of Protection
• Protective devices and protected equipment
represent the “Protection Zone”

Motor Protection
• Motor Starting Curve
• Thermal Protection
• Locked Rotor Protection
• Fault Protection

Inrush Current
First half cycle current showing
current offset.
Beginning of run up current
showing load torque pulsations.
Starting Current of a 4000Hp, 12 kV, 1800 rpm Motor
Motor pull in current showing motor
reaching synchronous speed

Motor Protection
LV Motor Protection MV Motor Protection

Motor Protection
• Standards & References
• IEEE Std 620-1996 IEEE Guide for the Presentation of
Thermal Limit Curves for Squirrel Cage Induction
Machines.
• IEEE Std 1255-2000 IEEE Guide for Evaluation of Torque
Pulsations During Starting of Synchronous Motors
• ANSI/ IEEE C37.96-2000 Guide for AC Motor Protection
• NEMA MG-1 Motors and Generators
• The Art of Protective Relaying – General Electric

Overload Relay / Heater
• Motor overload protection is provided by a
device that models the temperature rise of the
winding
• When the temperature rise reaches a point
that will damage the motor, the motor is de-
energized
• Overload relays are either bimetallic, melting
alloy or electronic

Question
What is Class 10 and
Class 20 Thermal
OLR curves?

Answer
• At 600% Current Rating:
– Class 10 for fast trip, 10
seconds or less
– Class 20 for, 20 seconds or less
(commonly used)
– There is also Class 15, 30 for
long trip time (typically
provided with electronic
overload relays)
6
20

Answer

Overload Relay / Heater
• When the temperature at the combination motor starter is more than ±10
°C (±18 °F) different than the temperature at the motor, ambient
temperature correction of the motor current is required.
• An adjustment is required because the output that a motor can safely
deliver varies with temperature.
• The motor can deliver its full rated horsepower at an ambient temperature
specified by the motor manufacturers, normally + 40 °C. At high
temperatures (higher than + 40 °C) less than 100% of the normal rated
current can be drawn from the motor without shortening the insulation
life.
• At lower temperatures (less than + 40 °C) more than 100% of the normal
rated current could be drawn from the motor without shortening the
insulation life.

Motor Starting and Thermal Limit
Sample data provided by the manufacturer

Motor Protection - Overload Pickup
(NEC Art 430.32 – Continuous-Duty Motors)
• Thermal O/L (Device 49) Pickup
• Motors with marked Service Factor ≥ 1.15
• Pickup = 125% of FLA
• Motors with temp. rise not over 40°C
• Pickup = 125% of FLA
• All other motors
• 115% of FLA

Motor Protection – Inst. Pickup
LOCKED
ROTOR S d
1
I
X X "
PICK UP
LOCKED ROTOR
I
RELAY PICK UP 1.2 TO 1.2
I
PICK UP
LOCKED ROTOR
I
RELAY PICK UP 1.6 TO 2
I
with a time delay of 0.10 s (six cycles at 60 Hz)
Recommended Instantaneous Setting:
If the recommended setting criteria cannot be met, or where more sensitive
protection is desired, the instantaneous relay (or a second relay) can be set more
sensitively if delayed by a timer. This permits the asymmetrical starting component
to decay out. A typical setting for this is:

Locked Rotor Protection
• Thermal Locked Rotor (Device 51)
• Starting Time (TS < TLR)
• LRA
• LRA sym
• LRA asym (1.5-1.6 x LRA sym) + 10% margin

Fault Protection
(NEC Art / Table 430-52)
• Non-Time Delay Fuses
• 300% of FLA
• Dual Element (Time-Delay Fuses)
• 175% of FLA
• Instantaneous Trip Breaker
• 800% - 1300% of FLA*
• Inverse Time Breakers
• 250% of FLA
*can be set up to 1700% for Design B (energy efficient) Motor

Low Voltage Motor Protection
• Usually pre-engineered (selected from
Catalogs)
• Typically, motors larger than 2 Hp are
protected by combination starters
• Overload / Short-circuit protection

200 HP
MCP
O/L
Starting Curve
I
2
T
(49)
MCP (50)
(51)
ts
tLR
LRAs LRAasym

Low-voltage Motor
Ratings Range of ratings
Continuous amperes 9-250 —
Nominal voltage (V) 240-600 —
Horsepower 1.5-1000 —
Starter size (NEMA) — 00-9
Types of protection Quantity NEMA
designation
Overload: overload
relay elements
3 OL
Short circuit:
circuit breaker current
trip elements
3 CB
Fuses 3 FU
Undervoltage: inherent
with integral control
supply and three-wire
control circuit — —
Ground fault (when
specified): ground relay
with toroidal CT — —

Minimum Required Sizes of a NEMA Combination Motor
Starter System
MAXIMUM CONDUCTOR LENGTH FOR ABOVE AND
BELOW GROUND CONDUIT SYSTEMS. ABOVE GROUND
SYSTEMS HAVE DIRECT SOLAR EXPOSURE. 750
C
CONDUCTOR TEMPERATURE, 450
C AMBIENT
CIRCUIT BREAKER
SIZE
FUSESIZE
CLASSJ
FUSE
MOTORHP
460VNECFLC
STARTER
SIZE
MINIMUM
SIZE
GROUNDING
CONDUCTOR
FORA50%CURRENTCAPACITY
MINIMUM
WIRE
SIZE
MAXIMUM
LENGTHFOR1%
VOLTAGE
DROP
NEXT
LARGEST
WIRE
SIZE
USENEXT
LARGERGROUND
CONDUCTOR
MAXIMUM
LENGTHFOR1%
VOLTAGE
DROPWITH
LARGERWIRE
250% 200% 150%
1 2.1 0 12 12 759 10 1251 15 15 15 5
1½ 3 0 12 12 531 10 875 15 15 15 6
2 3.4 0 12 12 468 10 772 15 15 15 7
3 4.8 0 12 12 332 10 547 20 20 15 10
5 7.6 0 12 12 209 10 345 20 20 15 15
7½ 11 1 12 10 144 8 360 30 25 20 20
10 14 1 10 8 283 6 439 35 30 25 30
15 21 2 10 8 189 6 292 50 40 30 45
20 27 2 10 6 227 4 347 70 50 40 60
25 34 2 8 4 276 2 407 80 70 50 70
30 40 3 6 2 346 2/0 610 100 70 60 90
40 52 3 6 2 266 2/0 469 150 110 90 110
50 65 3 2 2/0 375 4/0 530 175 150 100 125
60 77 4 2 2/0 317 4/0 447 200 175 125 150
75 96 4 2 4/0 358 250 393 250 200 150 200
100 124 4 1 250 304 350 375 350 250 200 250
125 156 5 2/0 350 298 500 355 400 300 250 350
150 180 5 4/0 500 307 750 356 450 350 300 400

Required Data - Protection of a Medium Voltage
Motor
• Rated full load current
• Service factor
• Locked rotor current
• Maximum locked rotor time (thermal limit curve) with the motor at ambient and/or operating
temperature
• Minimum no load current
• Starting power factor
• Running power factor
• Motor and connected load accelerating time
• System phase rotation and nominal frequency
• Type and location of resistance temperature devices (RTDs), if used
• Expected fault current magnitudes
• First ½ cycle current
• Maximum motor starts per hour

Medium-Voltage Class E Motor Controller
Ratings
Class El (without
fuses)
Class E2 (with fuses)
Nominal system voltage 2300-6900 2300-6900
Horsepower 0-8000 0-8000
Symmetrical MVA interrupting capacity
at nominal system voltage
25-75 160-570
Types of Protective Devices Quantity NEMA Designation
Overload, or locked Rotor, or both:
Thermal overload relay
TOC relay
IOC relay plus time delay
3
3
3
OL OC TR/O
Thermal overload relay 3 OL
TOC relay 3 OC
IOC relay plus time delay 3 TR/OC
Short Circuit:
Fuses, Class E2 3 FU
IOC relay, Class E1 3 OC
Ground Fault
TOC residual relay 1 GP
Overcurrent relay with toroidal CT 1 GP
NEMA Class E2 medium
voltage starter
NEMA Class E1
medium voltage starter

Thermal Limit Curve

Thermal Limit Curve
Typical Curve

Cable Protection

Cable Protection
• IEEE Std. 242-2001, IEEE Recommended Practice for
Protection and Coordination of Industrial and Commercial
Power Systems (IEEE Buff Book)
• IEEE Std 835-1994 IEEE Standard Power Cable Ampacity
Tables
• IEEE Std 848-1996 IEEE Standard Procedure for the
Determination of the Ampacity Derating of Fire-Protected
Cables
• IEEE Std 738-1993 IEEE Standard for Calculating the
Current- Temperature Relationship of Bare Overhead
Conductors
• The Okonite Company Engineering Data for Copper and
Aluminum Conductor Electrical Cables, Bulletin EHB-98

Cable Protection
2
2
1
t
A
T 234
0.0297log
T 234
The actual temperature rise of a cable when exposed to
a short circuit current for a known time is calculated by:
Where:
A= Conductor area in circular-mils
I = Short circuit current in amps
t = Time of short circuit in seconds
T1= Initial operation temperature (750C)
T2=Maximum short circuit temperature
(1500C)

Cable Short-Circuit Heating Limits
Recommended
temperature rise:
B) CU 75-200C

Shielded
Cable
The normal tape
width is 1½
inches

NEC Section 110-14 C
• (c) Temperature limitations. The temperature rating associated with the ampacity
of a conductor shall be so selected and coordinated as to not exceed the lowest
temperature rating of any connected termination, conductor, or device. Conductors
with temperature ratings higher than specified for terminations shall be permitted
to be used for ampacity adjustment, correction, or both.
• (1) Termination provisions of equipment for circuits rated 100 amperes or less, or
marked for Nos. 14 through 1 conductors, shall be used only for conductors rated
60C (140F).
• Exception No. 1: Conductors with higher temperature ratings shall be permitted to
be used, provided the ampacity of such conductors is determined based on the
6OC (140F) ampacity of the conductor size used.
• Exception No. 2: Equipment termination provisions shall be permitted to be used
with higher rated conductors at the ampacity of the higher rated conductors,
provided the equipment is listed and identified for use with the higher rated
conductors.
• (2) Termination provisions of equipment for circuits rated over 100 amperes, or
marked for conductors larger than No. 1, shall be used only with conductors rated
75C (167F).

Transformer Protection

• National Electric Code 2011 Edition
• IEEE Std 242-1986; IEEE Recommended Practice for Protection and
Coordination of Industrial and Commercial Power Systems
• C37.91-2000; IEEE Guide for Protective Relay Applications to Power
Transformers
• C57.12.59; IEEE Guide for Dry-Type Transformer Through-Fault
Current Duration.
• C57.109-1985; IEEE Guide for Liquid-Immersed Transformer Through-
Fault-Current Duration
• APPLIED PROCTIVE RELAYING; J.L. Blackburn; Westinghouse Electric
Corp; 1976
• PROTECTIVE RELAYING, PRINCIPLES AND APPLICATIONS; J.L.
Blackburn; Marcel Dekker, Inc; 1987

Transformer Categories
ANSI/IEEE C-57.109
Minimum nameplate (kVA)
Category Single-phase Three-phase
I 5-500 15-500
II 501-1667 501-5000
III 1668-10,000 5001-30,000
IV above 10,000 above 30,000

Transformer Categories I, II

Transformer Category III

Transformer Category IV

Transformer
t
(sec)
I (pu)
Thermal200
2.5
I
2
t = 1250
2
25Isc
Mechanical
K=(1/Z)
2
t
(D-D LL) 0.87
(D-R LG) 0.58
Frequent Fault
Infrequent Fault
Inrush
FLA

MAXIMUM RATING OR SETTING FOR OVERCURRENT DEVICE
PRIMARY SECONDARY
Over 600 Volts Over 600 Volts 600 Volts or Below
Transformer
Rated
Impedance
Circuit
Breaker
Setting
Fuse
Rating
Circuit
Breaker
Setting
Fuse
Rating
Circuit Breaker
Setting or Fuse
Rating
Not more than
6%
600 % 300 % 300 % 250% 125%
(250% supervised)
More than 6%
and not more
than 10%
400 % 300 % 250% 225% 125%
(250% supervised)
Table 450-3(A) source: NEC

Recommended Minimum
Protective system
Winding and/or power system
grounded neutral grounded
Winding and/or power system
neutral ungrounded
Up to 10 MVA Above 10 MVA Up to 10 MVA
Above
10 MVA
Differential - √ - √
Time over current √ √ √ √
Instantaneous restricted
ground fault √ √ - -
Time delayed ground
fault √ √ - -
Gas detection
√ -
√
Over excitation -
√ √ √
Overheating -
√ -
√

Question
What is ANSI Transformer Shift Curve?

Transformer Shift Factor

Dyg Transformer Through Fault

Question
What is meant by Frequent and
Infrequent Faults for transformers?

Frequent and Infrequent Faults
* Should be selected by reference to the frequent-fault-incidence protection curve or for transformers
serving industrial, commercial and institutional power systems with secondary-side conductors
enclosed in conduit, bus duct, etc., the feeder protective device may be selected by reference to the
infrequent-fault-incidence protection curve.
(Frequent Fault = More than 10 through faults (lifetime) for category II and 5 faults for category III)
Source
Transformer primary-side protective device
(fuses, relayed circuit breakers, etc.) may be
selected by reference to the infrequent-fault-
incidence protection curve
Category II or III Transformer
Fault will be cleared by transformer
primary-side protective device
Optional main secondary –side protective device.
May be selected by reference to the infrequent-fault-
incidence protection curve
Feeder protective device
Fault will be cleared by transformer primary-side
protective device or by optional main secondary-
side protection device
Fault will be cleared by
feeder protective device
Infrequent-Fault
Incidence Zone*
Feeders
Frequent-Fault
Incidence Zone*

Selective Coordination
• Inherent Selective Devices
• Examples
• Differential Relays
• Pilot Wire Relays
• Transformer Sudden Pressure Relays
• More expensive
• Justified based on value or role of protected
equipment in supply of power

• Overcurrent Selectivity Rules
• Downstream device curve is located to the left and below
of upstream device curve for range of applicable currents
• Sufficient time margin for operation of downstream before
upstream

Margins for Selectivity
• Relay - Relay coordination requires
• Minimum of 0.25 to 0.40 seconds time margin between the relay curves at the
maximum fault current to account for the interrupting time of the circuit
breaker, relay over-travel time, relay tolerances, and a safety factor
• For induction disk relays, the minimum desired time margin for a 5 cycle
breaker is generally 0.30 seconds
• 5 cycle breaker 0.08 seconds
• relay over-travel 0.10 seconds
• CT ratio & safety factor 0.12 seconds
• Total = 0.30 seconds
• For digital relays, the minimum desired time margin for a 5 cycle breaker is
generally 0.25 seconds
• 5 cycle breaker 0.08 seconds
• relay accuracy +.02 sec. 0.04 seconds
• CT ratio & safety factor 0.13 seconds
• Total = 0.25 seconds
• Margin between pickup levels of > 10% for two devices in series.

• Electromechanical Relay - Fuse coordination requires a minimum 0.22 second time
margin between the curves.
• Electromechanical Relay - Low Voltage Breaker coordination requires a minimum
0.22 second time margin between the curves.
• Static Relay - Fuse coordination requires a minimum 0.12 second time margin
between the curves.
• Static Relay - Low Voltage Breaker coordination requires a minimum 0.12
second time margin between the curves.
• Fuse - Fuse coordination requires that the total clearing time of the downline
fuse curve be less than 75% of the minimum melt time of the upline fuse curve
to account for pre-loading.
• Fuse - Low Voltage Breaker coordination requires that the down-line breaker
maximum time curve be less than 75% of the minimum melt time of the up-line
fuse curve to account for pre-loading.

• Fuse - Relay coordination requires a minimum 0.3 second time margin
between the curves.
• Low Voltage Breaker - Fuse coordination requires a minimum 0.1 second
time margin between the curves to allow for temperature variations in
the fuse.
• Low Voltage Breaker - Low Voltage Breaker coordination requires only
that the plotted curves do not intersect since all tolerances and operating
times are included in the published characteristics.
• Low Voltage Breaker - Relay coordination requires a minimum 0.2 second
time margin between the curves.

Ground Fault Protection
• NEC Requirements for Solidly Grounded System
• Articles 215.10 (feeders), 230.95 (services), 240.13
(overcurrent protection), etc.
• 260 V (150 V, L-G) ≤ Line-Line Voltage ≤ 600 V
• Main disconnect is rated 1000 A or more
• GF Settings is limited to 1200 A pickup and 1 sec for
ground faults > 3000 A
• Industry Practice
• Grounded wye systems 2400 V or more

Ground Fault Detection
• General Concept
• Measurement of Residual (IR)
or Zero Sequence current (3I0)
• IR = 3I0 = Ia + Ib + Ic
(Vector Summation)
• Balanced Fault: Ia = Ib = Ic and
IR = 3I0 = 0
• Unbalanced system Ia ≠ Ib ≠ Ic
and IR = 3I0 > 0

• Direct (Ground, 50G/51G)
• Grounded-phase (3I0) current is detected directly
with a current transformer installed in the
grounded neutral conductor.

• Balance Flux (Ground, 50G/51G)
(Core Balance or Zero Sequence CT)
• Grounded-phase current (IR) is directly detected by a
doughnut-type current transformer installed around
the three phase conductors
Note: The equipment grounding conductors (including conductor shields)
must not be installed through the current transformer.

• Residual
• Grounded-phase current is detected as the
unbalance in the current produced by the phase
current transformers

ETAP Terminology
• Relay Ground Function
• Externally measured residual current (2 inputs)
• Relay Neutral Function
• Relay internally measured residual current (6 inputs)

Relay Ground Inputs

Relay Sensitive Ground Inputs

Relay Neutral Inputs

Relay Function Diagram

Ground Fault Coordination
• GF Selective Coordination
• Device ground fault overcurrent coordination with:
• Other devices with ground detection
• Other devices with phase overcurrent detection
• Combination of phase and ground fault detection
• Minimum and Maximum Fault
• Phase and single-line to ground fault coordination

Ground Fault Coordination
Individual Phase and GF Curves Phase and GF Curve Combination

Protective Devices
• Relays
• Microprocessor/electronic
• More expensive, faster, multiple functionality
• Electromechanical
• Simple, cheap, slower, limited functionality

Relay ANSI Device Numbers
• 21 – Distance
• 27 – Under Voltage
• 32 – Directional Power
• 49 – Thermal Overload
• 50 – Instantaneous Over Current
• 51 – AC Inverse Over Current
• 52 – AC Circuit Breaker
• 59 – Overvoltage
• 67 – AC Directional Over Current
• 79 – AC Recloser
• 81 – Frequency
• 87 – Differential
• P – Phase
• N – Neutral
• G – Ground
• SG – Sensitive Ground
• V – Voltage
• VC – Voltage Control
• VR – Voltage Restrained

ETAP - Coordination and protecion

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