Protection and Coordination

This document and the information within it are confidential and proprietary to ETAP/Operation Technology, Inc.
and may not be modified, copied, published, disclosed, distributed, displayed or exhibited in either electronic
or printed formats without the written authorization of ETAP/Operation Technology, Inc., 17 Goodyear, Irvine, CA 92618, USA.
© 2021 ETAP/Operation Technology, Inc. All Rights Reserved.

© 2017-2021 ETAP/Operation Technology, Inc. All Rights Reserved.

 Protection & Coordination
• Objectives
– Human Safety
• Prevent injury and fatality
– Protection of Equipment
• Permit normal operation
• Isolate the equipment in case of abnormal conditions
– Selectivity / Grading / Discrimination
• Minimal isolation of network with abnormal conditions
• Permit normal operation for rest of electrical network
– Reliability and Cost
• Maximum achievable reliability for protection and coordination at
minimal cost

© 2017-2021 ETAP/Operation Technology, Inc. All Rights Reserved. Protection and Coordination
 References
• IEEE Std. 242-2001, IEEE Recommended Practice for
Protection and Coordination of Industrial and Commercial
Power Systems (IEEE Buff Book)
• IEEE Std. 141-1993, IEEE Recommended Practice for Electric
Power Distribution for Industrial Plants (IEEE Red Book)
• IEEE Std. 399-1997, IEEE Recommended Practice for
Industrial and Commercial Power Systems Analysis (IEEE
Brown Book)
• Other technical references

© 2017-2021 ETAP/Operation Technology, Inc. All Rights Reserved. Protection and Coordination
 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

© 2017-2021 ETAP/Operation Technology, Inc. All Rights Reserved. Protection and Coordination
 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
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 Overcurrent Protection
• Major Equipments (apparatus)
– Induction Motor
– Synchronous Motor
– Cable
– Transformer
– Generator
– Bus
– Transmission/Distribution Line

© 2017-2021 ETAP/Operation Technology, Inc. All Rights Reserved. Protection and Coordination
 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

© 2017-2021 ETAP/Operation Technology, Inc. All Rights Reserved. Protection and Coordination
 Excessive Currents
• Excessive currents in abnormal conditions
– Overload current
• (100-160% Full Load Amps)
– Short-time overload current
• (300-1000% Full Load Amps)
– Short-circuit current
• (300-1200% Full Load Amps)

© 2017-2021 ETAP/Operation Technology, Inc. All Rights Reserved. Protection and Coordination
 Capability / Damage Curves
2
It I2t I2t
t
I22t

Motor
Xfmr Cable
Gen

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 Protective Devices
Overcurrent Characteristics Time-Current-Characteristics (TCC)
• Inverse Time Over Current
• Simple, cheap, and large application in
LV, and MV
• Relays (OC & OL)
• No tolerance band
• TOC, IOC
• Single / Multiple Function
• LV Breakers
• Represent tolerance band
• PCB/ACB, ICCB, MCCB, MCB
• Fuses
• Overload Heater

© 2017-2021 ETAP/Operation Technology, Inc. All Rights Reserved. Protection and Coordination
 Overcurrent Protection
Protection Concept Motor Protection Example
• Protective Device TCC
– To the left and below of equipment
damage curve

– To the right and above of

equipment operating curve

© 2017-2021 ETAP/Operation Technology, Inc. All Rights Reserved. Protection and Coordination
 Selective Coordination
• 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

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 Selective Coordination

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 Margins for Selectivity*
• Relay - Relay
– 0.12 to 0.22 seconds + downstream breaker opening time
• Relay – Fuse / Low Voltage Circuit Breaker
– 0.12 to 0.22 seconds
• Fuse / Low Voltage Circuit Breaker - Relay
– 0.12 seconds + downstream breaker opening time
• Fuse / Low Voltage Circuit Breaker - Fuse / Low Voltage Circuit Breaker
– Clear space between curves**

* Calibrated protective devices

* *Possible adjustment for upstream fuse preloading and safety factors

© 2017-2021 ETAP/Operation Technology, Inc. All Rights Reserved. Protection and Coordination
 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

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 ETAP Star Overview
• Star Mode
– Star Mode and Star View difference
– Creation of TCC and Star View
– Addition of devices to existing TCC
– Graphical and Editor adjustments
– Star View Toolbars and Plot Options

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 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
– Under / Over Voltage (27/59)
*Reverse power (32) and under/over Frequency (81) are supported in Transient Stability

© 2017-2021 ETAP/Operation Technology, Inc. All Rights Reserved. Protection and Coordination
 Protective Devices
• Relays
– Microprocessor/electronic
• More expensive, faster, multiple functionality
– Electromechanical
• Simple, cheap, slower, limited functionality

© 2017-2021 ETAP/Operation Technology, Inc. All Rights Reserved. Protection and Coordination
 Relay ANSI Device Numbers
• 21 – Distance • P – Phase
• 27 – Under Voltage • N – Neutral
• 32 – Directional Power • G – Ground
• 49 – Thermal Overload • SG – Sensitive Ground
• 50 – Instantaneous Over Current • V – Voltage
• VC – Voltage Control
• 51 – AC Inverse Over Current
• VR – Voltage Restrained
• 52 – AC Circuit Breaker
• 59 – Overvoltage
• 67 – AC Directional Over Current
• 79 – AC Recloser
• 81 – Frequency
• 87 – Differential

© 2017-2021 ETAP/Operation Technology, Inc. All Rights Reserved. Protection and Coordination
 Relay Characteristics
Relay TOC Characteristics Relay TOC Curves
• Curve Shape Adaptation
• Equipment Protection

• Selectivity
• Time Margin at higher fault currents

© 2017-2021 ETAP/Operation Technology, Inc. All Rights Reserved. Protection and Coordination
 Low Voltage Protective Devices
• Low Voltage Circuit Breaker (LVCB)
– Power Circuit Breaker (PCB) / Air Circuit Breaker (ACB)
• 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)
– Molded Case Circuit Breaker (MCCB)
• UL489 (integral unit and enclosed housing of insulating material)
• IEC60947-2
– Miniature Circuit Breaker (MCB)
• UL489, UL508, UL1077
• IEC60898

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 LVCB Differences

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

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 LV Protective Devices
• MCCB Trip Units
– Thermal-Magnetic

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 LV Protective Devices
• MCCB Trip Units
– Magnetic Only

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 Low Voltage Protective Devices
• 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)

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 Low Voltage Solid State Trip Device

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 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, …)

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

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 Fuse Characteristics

Total Clearing
Time Curve

Minimum Melting
Time Curve

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 Current Limiting Fuse (CLF)
• Limits the peak current of short-circuit

• Reduces magnetic stresses (mechanical damage)

• Reduces thermal energy

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 Current Limiting Action
Ip
Current (peak amps)

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

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 CLF Let-Through Chart

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

© 2017-2021 ETAP/Operation Technology, Inc. All Rights Reserved. Protection and Coordination
 CLF Let-Through Chart
• Assumptions
2. The fault is on the load terminal

© 2017-2021 ETAP/Operation Technology, Inc. All Rights Reserved. Protection and Coordination
 CLF Let-Through Chart
• 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.

© 2017-2021 ETAP/Operation Technology, Inc. All Rights Reserved. Protection and Coordination
 CLF Let-Through Chart
• 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.

© 2017-2021 ETAP/Operation Technology, Inc. All Rights Reserved. Protection and Coordination
 Let-Through Chart
7% PF (X/R = 14.3)

Peak Let-Through Amperes

230,000

300 A

12,500 100 A

60 A

5,200 100,000

Symmetrical RMS Amperes

© 2017-2021 ETAP/Operation Technology, Inc. All Rights Reserved. Protection and Coordination
 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

© 2017-2021 ETAP/Operation Technology, Inc. All Rights Reserved. Protection and Coordination
 Zones of Protection
• Protective devices and protected equipment
represent the “Protection Zone”

© 2017-2021 ETAP/Operation Technology, Inc. All Rights Reserved. Protection and Coordination
 Motor Protection
• Motor Starting Curve

• Thermal Protection

• Locked Rotor Protection

• Fault Protection

© 2017-2021 ETAP/Operation Technology, Inc. All Rights Reserved. Protection and Coordination
 Inrush Current
Starting Current of a 4000Hp, 12 kV, 1800 rpm Motor
First half cycle current showing
current offset.

Beginning of run up current

showing load torque pulsations.

Motor pull in current showing motor

reaching synchronous speed

© 2017-2021 ETAP/Operation Technology, Inc. All Rights Reserved. Protection and Coordination
 Motor Protection
LV Motor Protection MV Motor Protection

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 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

© 2017-2021 ETAP/Operation Technology, Inc. All Rights Reserved. Protection and Coordination
 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

© 2017-2021 ETAP/Operation Technology, Inc. All Rights Reserved. Protection and Coordination
 Question
What are Class 10 and
Class 20 Thermal
OLR curves?

© 2017-2021 ETAP/Operation Technology, Inc. All Rights Reserved. Protection and Coordination
 Answer
• At 600% Current Rating:
– Class 10 for fast trip,
10 seconds or less
– Class 20 for, 20 seconds 20

or less (commonly used)

– There is also Class 15, 30
for longer trip time
(typically provided with
electronic overload relays) 6

© 2017-2021 ETAP/Operation Technology, Inc. All Rights Reserved. Protection and Coordination
 Answer

© 2017-2021 ETAP/Operation Technology, Inc. All Rights Reserved. Protection and Coordination
 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.

© 2017-2021 ETAP/Operation Technology, Inc. All Rights Reserved. Protection and Coordination
 Motor Starting and Thermal Limit
Sample data provided by the manufacturer

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 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

© 2017-2021 ETAP/Operation Technology, Inc. All Rights Reserved. Protection and Coordination
 Motor Protection – Inst. Pickup
1
I =
XS + X d "
LOCKED
ROTOR

Recommended Instantaneous Setting:

I PICK UP
RELAY PICK UP =  1.6 TO 2
I LOCKED ROTOR
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:
I PICK UP
RELAY PICK UP =  1.2 TO 1.2
I LOCKED ROTOR
with a time delay of 0.10 s (six cycles at 60 Hz)

© 2017-2021 ETAP/Operation Technology, Inc. All Rights Reserved. Protection and Coordination
 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

© 2017-2021 ETAP/Operation Technology, Inc. All Rights Reserved. Protection and Coordination
 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

© 2017-2021 ETAP/Operation Technology, Inc. All Rights Reserved. Protection and Coordination
 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

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 (49)
I2T

O/L
tLR MCP

ts (51) 200 HP

Starting Curve
MCP (50)

LRAs LRAasym

© 2017-2021 ETAP/Operation Technology, Inc. All Rights Reserved. Protection and Coordination
 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 Quantity NEMA
protection designation

Overload: overload
relay elements
3 OL

Short circuit:
circuit breaker current 3 CB
trip elements

Fuses 3 FU
Undervoltage:
inherent with integral
control supply and — —
three-wire control
circuit
Ground fault (when
specified): ground
relay with toroidal CT — —

© 2017-2021 ETAP/Operation Technology, Inc. All Rights Reserved. Protection and Coordination
 NEMA Sizes of Motor Starter System

FOR A 50 % CURRENT CAPACITY

FUSE SIZE
MAXIMUM CONDUCTOR LENGTH FOR ABOVE AND

CLASS J
FUSE
BELOW GROUND CONDUIT SYSTEMS. ABOVE GROUND CIRCUIT BREAKER
SYSTEMS HAVE DIRECT SOLAR EXPOSURE. 750 C
SIZE
CONDUCTOR TEMPERATURE, 450 C AMBIENT

460V NEC FLC

CONDUCTOR
GROUNDING
MOTOR HP

STARTER

MINIMUM
SIZE

SIZE

LARGER GROUND

LENGTH FOR 1%
LENGTH FOR 1%

LARGER WIRE
CONDUCTOR

DROP WITH
USE NEXT
MAXIMUM

MAXIMUM
VOLTAGE

VOLTAGE
LARGEST
MINIMUM
250% 200% 150%

DROP

NEXT
WIRE

WIRE
SIZE

SIZE
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

© 2017-2021 ETAP/Operation Technology, Inc. All Rights Reserved. Protection and Coordination
 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

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

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 Thermal Limit Curve

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 Thermal Limit Curve
Typical Curve

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 Cable Protection

© 2017-2021 ETAP/Operation Technology, Inc. All Rights Reserved. Protection and Coordination
 Cable Protection
• Standards & References
– 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

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 Cable Protection
The actual temperature rise of a cable when exposed to
a short circuit current for a known time is calculated by:
2 t
A=
 T2 + 234 
0.0297log  
 T1 + 234 
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)

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

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 Shielded Cable

The normal tape

width is 1½
inches

© 2017-2021 ETAP/Operation Technology, Inc. All Rights Reserved. Protection and Coordination
 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).

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 Transformer Protection

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 Transformer Protection
• Standards & References
– 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

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 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

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 Transformer Categories I, II

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 Transformer Category III

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 Transformer Category IV

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 Transformer Damage Curves
FLA

200
Thermal

t I2t = 1250
(D-D LL) 0.87
(sec)
Infrequent Fault
(D-R LG) 0.58

Frequent Fault
2
Mechanical
K=(1/Z)2t
Inrush

2.5 Isc 25 I (pu)

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 Transformer Protection (NEC)
MAXIMUM RATING OR SETTING FOR OVERCURRENT DEVICE
PRIMARY SECONDARY
Over 600 Volts Over 600 Volts 600 Volts or Below

Transformer Circuit Fuse Circuit Fuse Circuit Breaker

Rated Breaker Rating Breaker Rating Setting or Fuse
Impedance Setting Setting Rating

Not more than 600 % 300 % 300 % 250% 125%

6% (250% supervised)

More than 6% 400 % 300 % 250% 225% 125%

and not more (250% supervised)
than 10%
Table 450-3(A) source: NEC

© 2017-2021 ETAP/Operation Technology, Inc. All Rights Reserved. Protection and Coordination
 Suggested Transformer Protection
Winding and/or power system Winding and/or power system
Protective system grounded neutral grounded neutral ungrounded
Above
Up to 10 MVA Up to 10 MVA
Above 10 MVA 10 MVA

Differential - √ - √

Time over current √ √ √ √

Instantaneous restricted
ground fault √ √ - -

Time delayed ground fault √ √ - -

√ -
√
Gas detection

Over excitation -
√ √ √
Overheating -
√ -
√

© 2017-2021 ETAP/Operation Technology, Inc. All Rights Reserved. Protection and Coordination
 Question
What is ANSI Transformer Curve Shift?

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 Transformer Shift Factor

© 2017-2021 ETAP/Operation Technology, Inc. All Rights Reserved. Protection and Coordination
 Dyg Transformer Through Fault

© 2017-2021 ETAP/Operation Technology, Inc. All Rights Reserved. Protection and Coordination
 Question
What is meant by Frequent and Infrequent Faults
in transformers?

© 2017-2021 ETAP/Operation Technology, Inc. All Rights Reserved. Protection and Coordination
 Frequent and Infrequent Faults
Source

Transformer primary-side protective device

(fuses, relayed circuit breakers, etc.) may be
selected by reference to the infrequent-fault-
incidence protection curve
Infrequent-Fault
Incidence Zone* 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

Fault will be cleared by transformer primary-side

protective device or by optional main secondary-
side protection device

Feeder protective device

Frequent-Fault Fault will be cleared by

Incidence Zone* feeder protective device

Feeders
* 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: IEEE C57

© 2017-2021 ETAP/Operation Technology, Inc. All Rights Reserved. Protection and Coordination
 Zone Viewer
• Display of protection zones in Zone Viewer
– Path, Zone, or Component tracking on OLV
– Filtering capability based on zone type or voltage
– User selection of Zones or Elements
– Creation of Star TCC Views
• Individual TCC per selected zone and elements
• Single TCC for all selected zones and elements

© 2017-2021 ETAP/Operation Technology, Inc. All Rights Reserved. Protection and Coordination
 Zone Selection Tools
• Zone Selection Tools
– Extend to source
– Extend between
– Extend bus levels

© 2017-2021 ETAP/Operation Technology, Inc. All Rights Reserved. Protection and Coordination
 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

© 2017-2021 ETAP/Operation Technology, Inc. All Rights Reserved. Protection and Coordination
 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

© 2017-2021 ETAP/Operation Technology, Inc. All Rights Reserved. Protection and Coordination
 Ground Fault Detection
• Direct (Ground, 50G/51G)
– Grounded-phase (3I0) current is detected directly with a
current transformer installed in the grounded neutral
conductor.

© 2017-2021 ETAP/Operation Technology, Inc. All Rights Reserved. Protection and Coordination
 Ground Fault Detection
• 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.

© 2017-2021 ETAP/Operation Technology, Inc. All Rights Reserved. Protection and Coordination
 Ground Fault Detection
• Residual
– Grounded-phase current is detected as the unbalance in
the current produced by the phase current transformers

© 2017-2021 ETAP/Operation Technology, Inc. All Rights Reserved. Protection and Coordination
 ETAP Terminology
• Relay Ground Function (50/51G)
– Externally measured residual current (2 inputs)
– Indicates relay has Ground CT source
• Relay Neutral Function (50/51N)
– Relay internally measured residual current (6 inputs)
– Indicates relay internally calculates residual (3Io) current from Phase CTS.

© 2017-2021 ETAP/Operation Technology, Inc. All Rights Reserved. Protection and Coordination
 Relay Ground Inputs

© 2017-2021 ETAP/Operation Technology, Inc. All Rights Reserved. Protection and Coordination
 Relay Sensitive Ground Inputs

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 Relay Neutral Inputs

© 2017-2021 ETAP/Operation Technology, Inc. All Rights Reserved. Protection and Coordination
 Relay Function Diagram

© 2017-2021 ETAP/Operation Technology, Inc. All Rights Reserved. Protection and Coordination
 ETAP Star Auto-Evaluation
• Automated and intelligent detection of protection zones
• Overcurrent Protection and Coordination Evaluation
• Customized evaluation criteria based on Rule Book
• Evaluation and Fault Current Type selection in Study Case
• Tabular and graphical display of evaluation results
• Graphical snapshot and evaluation report

© 2017-2021 ETAP/Operation Technology, Inc. All Rights Reserved. Protection and Coordination
 Protection Zone Detection
• Intelligent detection of protection zones
– Source (Generator)
– Bus (single, multiple sections, ties)
– Branch (transformer, cable, line)
– Load (synchronous/induction motor, static/lump load)
– Extended Zone (source and branch, cascaded branches,
branch and bus, bus and load)

© 2017-2021 ETAP/Operation Technology, Inc. All Rights Reserved. Protection and Coordination
 Evaluation Rule Book
• Protection Rules
– Default rules based on industry practice
– Adjustable evaluation criteria based on standards or user
defined
– Support of NEC overload protection such as:
• NEC 240.101 for HV cable protection
• NEC 430.32 for motor protection
• NEC 450.3 for transformer protection

© 2017-2021 ETAP/Operation Technology, Inc. All Rights Reserved. Protection and Coordination
 Evaluation Rule Book
• Coordination Rules
– Delay time margins
– Instantaneous minimum current ratio
– Global or individual consideration of switching time for
HV/MV breakers
– Optional coordination between main and tie

© 2017-2021 ETAP/Operation Technology, Inc. All Rights Reserved. Protection and Coordination
 • Selection of rules • Fault Type
• Evaluation – Phase
– Protection – Ground (Coordination)
– Coordination – Phase and Ground
– Protection & Coordination • Evaluation Messages
• Fault Type • Zone Selective Interlock
– Phase • Arc Flash Evaluation
– Ground (Coordination)
– Phase and Ground

© 2017-2021 ETAP/Operation Technology, Inc. All Rights Reserved. Protection and Coordination
 Results Viewer
• Tabular and graphical evaluation results
• Max through fault current
• Colorful status indication
• Extensive messages
• On the spot editing capability
• Evaluation update after modification
– Dynamic
– Static
• Rule Book View
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 Results Viewer
• Graphical evaluation (Temp Star View)
– Dynamic update of view based on through fault
– Correlation between selected elements on graphical and
tabular view
– Graphical adjustment of settings
– Graphical tools
– Screen capture

© 2017-2021 ETAP/Operation Technology, Inc. All Rights Reserved. Protection and Coordination
 Evaluation Report
• Excel
– Study assumptions
– Separate sheet for each protection zone
– Coordination sheet
– Extensive messages
– Option to have passed messages

© 2017-2021 ETAP/Operation Technology, Inc. All Rights Reserved. Protection and Coordination