Module 7
Power System Review Course
Protection & Coordination
By: Dr. Hamid Jaffari
Fuse
Transformer
Typical Distribution System
7500 KVA OA
9375 KVA(125%) FA
10,000 KVA (135%) FOA
7500 KVA OA
9375 KVA(125%) FA
10,000 KVA (135%) FOA
Protection & Coordination
 System Protection Instrumentaion
 Principals of Protection
 Protection Devices (Circuit breakers, Reclosers,
Fuses)
 Principals of Coordination
 Coordination Study
 Fuse-Fuse Coordination
 Recloser-Recloser Coordination
 Recloser-Fuse Coordination
 Relay-Recloser-Fuse Coordination
Power System Review
System Protection
Components
 Instrument Transformers
 PTs
 CTs
 Circuit Breakers & Relays
 Mechanical
 Digital
 Reclosers
 Fuses
Power System Review
Protective Devices Characteristics
 Breaker:
 Distribution Class 12 kA<Isymmetrical Rating<20 kA
 Transmission Class> 50 kA
 Extinguishing arc by means of:
 Oil, air blast, sulfur hexafluoride gas (sF6), vacuum, or simple arc
chutes
 Recloser:
 4 kA< Isymmetrical Rating<12 kA
 Interrupting occurs in Oil or vacuum
 Designed to “reclose” after fault is cleared
 Sectionalizers:
 interrupting capability<10KA
 Master minded by Reclosers
Power System Review
Protective Devices Characteristics
 Expulsion Fuses:
 Typically 8 kA to 16 kA
 Subject to X/R (interrupting capability decreases as ↑ X/R )
 Power Fuse are generally available for Operating Voltage <169KV
with 15KA<X/R<25KA
 Distribution Fuse are generally good for Operating Voltage<40K
with 5KA<X/R<15KA
 less accurate, inexpensive, but effective.
 Current limiting fuses (CLFs)
 As much as 50 kA
 May have minimum interrupting capability, additional
protection may be needed
 Oil switches:
 limited current interrupting capability.
Power System Review
Fault calculations
Note: I-fault drops off as inverse of Distance 1/d
Power System Review
Principals of Protective Devices
 Definition of Protective Devices: Protective
Devices have Time-Current, Time-Voltage, or
Time-Frequency Characteristics
 Protective Devices are responsible for
removing undesired conditions:
 Voltage
 Current
 Frequency
Power System Review
What is the purpose?
 Clear Temporary faults and restore power
when possible.
 Interrupt Permanent Faults and Lock Out
 Interrupt Faults in Proper Sequence
 Remove undesired power conditions to
maintain:
 Steady State
 Stability
Power System Review
EPRI Fault Study
Faults
Percentage
Phase-to-Ground
65%
Phase-to-Phase
11%
Phase-Phase to Ground
2%
Three Phase
2%
One Phase on the Ground
15%
Two Phase on the Ground
2%
Others
3%
Power System Review
Protection & Coordination
 What is Coordination?
Definition: Proper trip sequencing of protective devices
to isolate the fault and minimizing outage. This means
proper coordination between time and current curves
during power system abnormal conditions.
 Coordination is :
 1/3 Science
 1/3 Art
 1/3 Luck
Coordination
 Protective Devices with Time-Current characteristics are:
 Relays, Reclosers, Fault Interrupters, and Fuses.
 How about Sectionalizers??
Device
Relay
Recloser
FI
Fuse
Relay
Relay-Relay
Relay-Recloser
Relay-FI
Relay-Fuse
Recloser
Recloser-Relay
Recloser-Recloser
Recloser
Recloser-Fuse
FI
FI-Relay
FI-Recloser
FI-FI
FI-Fuse
Fuse
Fuse-Relay
Fuse-Recloser
Fuse-FI
Fuse-Fuse
Fuse-Fuse
Coordination
Fuse Selection Process
Fuse Type/Class
Expulsion(Dist, Substation, etc)
CLF
Voltage Class
Fuse Isymm interrupting capability rating
X/R ratio
Fuse continuous current rating
i.e. K(150%), T(150%), QA(100%)
Fuse Application
Fuses are generally:
 CLF-used for short-circuit protection
 Non-CLF or Expulsion Fuse is used for
Overload protection
 Selection Criteria:
 Non-CLF: 140% of full load
 CLF:
150% of full load
Expulsion Fuse
Fuse Clearing Time
@1/2 cycle
Zero Crossing
CT = MT+ Arcing Time
Transient Voltage
@
Clearing time
100
Isc  Ifull  load x
Z%
Current Limiting Fuse(CLF)
60,000 A
7,400 A
B
Curtsey of www.littlefuse.com
Fuse Types & Porperties
 Fuse has two TCC curves
 Minimum Melting
 Total Clearing
 Common fuse types through 27kV are:
 Slow: T
 Fast: K
 Avoid mixing different type fuses for better coordination
 Skip at least a size in each fuse class (K, T, H, C, etc)
for better coordination between two fuses(i.e. 20K, 40K,
65K, etc)
Power System Review
Fusing Philosophy
 Lateral tap fuse selection Criterion:
I
= 2x ILoad
 Cold load
 Daily/monthly/Seasonal cyclic Load
 Peak load
 Transformer Fuse Selection Criterion:
 Minimum Fuse Size= Irated x 1.2
 Cold Load
 Inrush Current
 Operational Limits
lateral
Power System Review
Fuse Speed & Continuous Rating
Fuse
Allowable
Continuous
current
Rating(%)
K-tin
150%
K-silver
100%
N
100%
T
150%
QA
100%
S
150%
Fuse speed from fast to slow → N>QA>K>T>S
Power System Review
Fusing Distribution Transformers
 Why? In order to protect the transformer against
internal faults, downstream bolted faults, high
impedance secondary faults.
 Fuse must withstand transient surge currents
caused by lightening, XFMR magnetizing inrush
current, and cold-load pickup. Therefore, fuse
must be capable of handling:
 Cold Load Pickup
 Inrush Current
Power System Review
Dist Transformer using…continued
 Steps to select XFMR fuse size:
1. Calculate Cold Load Pickup withstand level:
1. I
= 3 x I (full load for 10 seconds)
2. Calculate Inrush Current withstand limits:
1. I inrush= 12 x I (Full Load for 0.1 seconds)
2. I inrush= 25 x I (full load for .01 seconds)
3. Select the nearest primary fuse rating that:
1.
2.
cold-load
Starts with 120% of XFMR rated load: Minimum Fuse Size = IRated x 1.2
Meets the Cold load & Inrush Requirements (Steps 1 &2)
4. Select the Fuse type (K, T,H, QA,etc) and coordinate it
with upstream & downstream fuses in service.
Note: Using EEI-NEMA type K, T, and T Fuses Provides
protection between 200% to 300% of Rated Load
Power System Review
Transformer Fusing…continued
 Example: Determine the minimum size fuse for
a 300 KVA, 13.8kV/277/480 volts XFMR using
“K” type fuse?
Ifull  load 
300 KVA
3 x 13.8kV

300
 12.55 Amp
23.9
ICold  load @ 10Sec  3x12.55  37.65 Amp
IInrush@ 0 .1Sec  12 x12.55  150.6 Amp
Iinrush@ .01Sec  25x12.55  313.75 Amp
MinimumFuse : 15K (120%) ? OR 20K (160%) ?
Power System Review
Transformer Fusing…continued
Problem Area
Power System Review
Substation Transformer Protection
 Plot XFMR operational limits:
 Thermal & Mechanical limits (Damage Curve).
 Use FA Rating for XFMR Damage Curve Plot
 XFMR Inrush &Cold-Load pickup.
 Apply applicable NEMA & IEEE Standards:
 IEEE C57.109-1993 oil immersed XFMR
 IEEE C57.12.59-2001 dry-type XFMR
 Example:
22.86 kV / 4.16 kV
Z  6%
7500 KVA OA
9375 KVA(125%) FA
10,000 KVA (135%) FOA
Power System Review
Transformer Operating Limitations
t(sec)
FLA
200
Thermal
I2t = 1250
(D-D LL) 0.87
Infrequent Fault
(D-R LG) 0.58
Frequent Fault
Mechanical
2
K=(1/Z)2t
Inrush
2.5
Isc
I (pu)
25
Power System Review
Devices’ Damage Curves
t
Time
2
It
I2t
I2t
I2 2 t
Motor
Xfmr
Cable
Gen
I2t
I-Current
=Let-through Energy
Power System Review
Transformer Prim Fuse Protection
Capacitor Fusing
 Capacitor fuse must be between 135% to 165% of its
full load current rating depending on manufacturer.
Capacitor Fuse  Full  Load x 1.35
or
Capacitor Fuse  Full  Load x 1.65
 Example: Find appropriate fuse size for a 1800 KVAR
cap bank installed on a 22.86kV line.
Ifull  load 
1800 KVAR
1800

 45.5 Amp
3 x 22.86kV 39.6
Note: Check fuse continuous/overload capability (100%-150%)
Capacitor Fuse *1.5  45.5 *1.35
Fuse  40.5 Amp
Select 50 K or 60QA Fuse
Fuse-Fuse Coordination
Time
A
Source
B
Fuse Fuse
CTof A
Fault
MT of A
Load
CT of B
Fuse A (75% of MT)
Current
coordination limit
 Fuse-fuse coordination must follow the following
rule:
CT (down stream fuse)
Time Ratio of
 75%
MT (upstream fuse)
 Desirable coordination: Time Ratio of two fuses
Must Not Exceed the 75% Ratio
tCT ( downstream fuse)  tMT (upstream) * 0.75
Power System Review
Fuse-Fuse Coordination
 Example: What is the minimum size fuse that
coordinates with 50K lateral tap fuse if calculated
fault current is 1000Amp at point B?
 MT(A Fuse)=0.051 sec for Fault Current @ 1000
Amp
CT (down stream fuse B)
 0.75
MT (upstream fuse A)
50 K
A
CT (down stream fuse B)
 0.75  CT  0.75 x0.051
.051
CT  0.038 sec
Power System Review
B
?K
26 amp
I  G  1000 amp
50K Fuse-20K Fuse Coordination
50 K
A
B
20 K
26 amp
I  G  1000 amp
Coordination Limit
Fuse-Fuse Coordination
 Example: Select a fuse size at point C That can
achieve proper coordination with upstream fuses.
Ifault  Max  2000 amp
A
90 amp
100 K
65K
80 amp
B
Ifault  Max  1500 amp
C
35 amp
? KI
fault  Max
Power System Review
 1000 amp
Fuse-Fuse coordination
Ifault  Max  2000 amp
A
90 amp
100 K
65K
80 amp
B
Ifault  Max  1500 amp
C
35 amp
? KI
fault  Max
 1000 amp
Recloser-Fuse
Coordination
Recloser
 Defined in ANSI/IEEE C37.60
 Settings Require Selecting:
 Minimum Pick up or Coil size
 Curve Selection:
 Fast Curve
 Slow Curve
 Operating Sequence:
 Number of Fast Curve shots
 Number of Slow Curve shots
 Shots to lockout
 Reset time
Power System Review
Recloser
 Two Types:
 Hydraulic
 Minimum Pickup is done by selecting appropriate rating for
Series Coil inside the tank
 TCC Curve Selection & Settings are done inside the tank
 Electronics
 Minimum Pickup: Trip Resistors
 TCC Curves and Timing plugs are done at the front panel
 Control
 Hydraulic
 Solid State
 Microprocessor
Power System Review
Electronic Recloser Settings
 TCC Curve
 Curve Selection/Type
 Settings:
 Min Trip Setting
 Phase & Ground
 Instantaneous Trip Setting
 Phase & ground
 Constant Time Adder
 Amp Multiplier
 Reclosing Operation Setting
 Typically Two Fast/Two Delay
 0.5 sec<Reclosing intervals<60 sec
Power System Review
Electronic Recloser Settings
 Phase Trip Setting
 Minimum Trip= (Range of 2 to 2.5) x Max Load Current
 This facilitates cold load pick up & load growth
 Ground Trip Setting
 Minimum Trip= (Range of 0.3 to 0.5)x Phase Minimum Trip
 Min trip setting helps to protect against high impedance faults
 Instantaneous Setting
 Trip Setting= (Range of 4 to 16)X Minimum Trip
Power System Review
Hydraulic Recloser Phase Trip-Setting
 Estimate the Peak load
 Determine the Coil Size:
 Inrush Current dictates the coil rating selection
 Coil Size(Amp)=1.25 x Peak Load
 Calculate the Minimum Phase Trip Setting:
 Minimum Trip(Amp)=2 x Peak-Load
 Some utilities use factor of 2.5 or 3.0
 Example (W type Hydraulic Recloser; coil sizes are 100, 140, 200, 280,
400, and 560):
 Assume Peak Load= 150A
 Coil Size= 1.25 x 150= 187.5→ Thus select 200 A
 Minimum Trip= 2x150= 300A→ Select 400A Min Trip Level
(Note: 200A Coil has Minimum Trip Rating of 400A)
Power System Review
Hydraulic Recloser Ground Trip-Setting
 Steps:
 1. Calculate the Normal Load Unbalance
Iground  MinTrip  I 1( Normal Load  Unbalance)  I 2( Load UnbalanceCreated bythe Largest Single  Phase Device)
I 1  INormal Load  Unbalance  10% of Peak  Load
I 2  ILargest Load  Unbalance  Largets Tap Fuse(Amp)
 2. Calculate the end-of-line minimum fault current level.
The Iground-Trip must be bellow Imin-Fault.
Iground  MinTrip  IEnd  of
 Line Min fault
 3. Estimate the Ground Minimum Trip Level.
IUnbalance Load( I 1  I 2)  Iground  Min Trip  IEnd  of  Line Min fault
Power System Review
TCC Coordination Time Margins
 Recloser-Reloser
 Hydrolic Reclosers:
 Min Trip and continuous current are both dependent of the coil size
 Reclosing intervals are 1, 1.5, and 2 seconds
 Small Reclosers have Series coil : H, VH, L, and E series
TCC Curve Separation >12 Cycles (0.2 sec); typically 0.25-0.30 sec
 Large Reclosers have High-Voltage solenoid : D, V, W, VW series
TCC Curve Separation > 8 Cycles(0.133 sec); typically 0.2 sec
 Electronic Reclosers:
 Unlike Hydraulic recloses, Min Trip is independent of the Recloser’s
continuous rating
 Reclosing intervals are 2, 5, and 15 seconds
 Min Trip selection must allow for the cold-load pick up & load growth
TCC Curve Separation > 0.30 Sec
0.30 Sec=0.22 sec (CT saturation& errors)+0.08 sec (Breaker Opening time)
TCC Coordination Time Margins
 Electronic Recloser to Hydraulic Recloser:
 TCC Curve Separation > 0.2 Sec; typically 0.25 sec
 Recloser-Fuse
 Methode#1: Use K factor for Recsloser: Range of
1.25<K<1.8
 Methode#2: ADD Recloser Cumulative Times
Add the cumulative reclose interval for a 2A-2C recloser sequence and coordinate with Fuse
Minimum Clearing curve x 0.75
Power System Review
Recloser-Fuse Coordination
Recloser Cumulative Time Method
R
Time
Fuse
'
'
b
a
 <Coordination Limit <
B'  2 A  2B
 Temp Fault:
Recloser operates; Fuse
is saved.
 Permanent Fault:
Recloser operates first,
then fuse blows
B delay
A'  2 A
A(Fast)
a' a b' b
Current
75% of fuse MT curve (fuse damage curve)
Power System Review
Recloser-Fuse Coordination
K-Factor Method
'
 a <Coordination Limit< b '
 Refer to Manufacturer’s
Time
Supplied Tables
 Extract applicable K-Factor
 Multiply Curves by K-Factor
 Note: K-factor is a t-Multiplier
B  K(Time Multiplier) * Curve B
B delay
A  K(Time Multiplier) * Curve A
A(Fast)
a' a b' b
Current
75% of fuse MT curve (fuse damage curve)
Power System Review
Cooper Reclosers K-Factor
Table Below -Curtsey of Cooper Power Systems
•What is K-Factor?
•K-Factors are Time
multiplying factors for various
Reclosing Intervals. K-Factor
shifts the curves up
increasing the time value by
K-Factor for the same current
value.
Power System Review
Source Fuse-Recloser Coordination
 Phase Trip Setting Steps:
FUSE
5000 KVA
Z  6%
R
 1. Calculate Full Load Iprim & Isec
Iprim 
5000
22.86 3
 126.3 A
I sec 
5000
4.16 3
 694 A
R
22.86 kV / 4.16 kV
3 - Fault  4500A
 - G Fault  3800A
I  400A(Peak)
RX Recloser
  2 A & 2C
g  1A2 & 1E
 2. Select an Appropriate Fuse size
 Fuse Size: 1.5x126.3A=189.5A → Select 200K
 3. Select an Appropriate Recloser Coil Size:
End  of  Line FAULT
 Calculate Coil Size: 1.25x400=500A
3 - Fault  1500A
  G Fault  175 A
 Select Coil size: 560A
 4. Desired Minimum Phase Trip= 2.5 x Peak Load to
override the Inrush IPhase  2.5x 400  1000 A
Note: Cooper W & RX type Recloser Ratings:
Coil Size
Min Trip Rating Interrupting Rating
560 A
1120 A
10,000 A
Source Fuse-Recloser Coordination
 Ground Trip Setting Steps:
 1. Estimate Normal Load unbalance: 10% of Peak-Load
 Example: Iunbalance= 10% of Peak Load
Iunbalance-Normal= 0.1 x 400=40A
 2. Estimate the load unbalance created by the largest
single-phase device:
 Example: Assume the largest single phase Load is 90A
Fuse.
Iunbalance-Load=90A
 3. Calculate Unbalanced downline Ground Current:
IGround  Iunbalance Normal  Iunbalance Load
IGround  90  40  130 A
 4. Select Minimum Ground Trip:
Ig Min  Trip  140 A
IG  Unbalanced (130A)  IGround  Setting (140A)  IMin  Fault (175A)
Source Side Fuse-Recloser Coordination
Recloser-Fuse (Load Side) Coordination
•Determine an appropriate Fuse size @ Point B
•Which fuse coordinates the best (100K or 140K?)
•Answer
140K; Why?
FUSE
5000 KVA
Z  6%
R
R
22.86 kV / 4.16 kV
400 Amp
RX Recloser
2 A2C
B
90 Amp
FAULT
Power System Review
  G  1900 A
Min Fault  600 A
Recloser-Fuse (Load Side) Coordination
FUSE
5000 KVA
Z  6%
R
3 - Fault  4500A
 - G Fault  3800A
R
22.86 kV / 4.16 kV
I  400A(Peak)
RX Recloser
  2 A & 2C
g  1A2 & 1E
B
End  of  Line FAULT
3 - Fault  1500A
  G Fault  175 A
3 - Fault  1500A
  G Fault  175 A
Fuse
100 K
140 K
Coordination Limit
1112 A
2380 A
Relays
Mechanical
&
Digital
Protection System Elements
 Protective relays
 Circuit breakers
 Current and voltage transducers
 Communications channels
 DC supply system
 Control cables
Three-Phase Diagram of the Protection Team
CB
+
CTs
Protected
Equipment
Control
SI
DC Station
Battery
Relay
Relay
Contact
SI
Relay
52a
52
TC
VTs
–
Circuit
Breaker
Red
Lamp
Most Common Protective Relays
Protection Principles for Transmission &
Distribution Lines:
Overcurrent (50, 51, 50N, 51N)
Directional Overcurrent (67, 67N)
Distance (21, 21N)
Differential (87)
Circuit Breaker Selection
 Relay (The Brain)
 CT Ratio
 PT or VT Ratio
 Interrupting Cycle
 Voltage Class
 K
=(VMAX/Vmin)
 BIL rating
rating
Power System Review
Relay-Circuit Breaker Operation
A
B
C
Ground relay
Phase relays
Circuit
Breaker
In
CTs
Ia
IbIb
Ic
In  Ia  Ib  Ic
Ic
Load
LOAD
Load
Ib
Power System Review
Ia
Induction-Type Relays
Power System Review
Relay Classification
 Overcurrent
 Overvoltage
 Undervoltage
 Differential
 Directional
 Under Frequency
 Distance
Power System Review
Relays for Phase Faults
Time overcurrent
51
Instantaneous & time overcurrent
Directional Time Overcurrent
50/51
67
Instantaneous & directional time over
current
50/67
Directional Instantaneous Overcurrent
67
Step Time Overcurrent
51
Directional Instantaneous and directional
67
Zone Distance
21
Power System Review
Relays for Ground Faults
Time Overcurrent
51N
Instantaneous & Time Overcurrent
50N/51N
Product Overcurrent
67N
Instantaneous and Product Overcurrent
67N/50N
Directional time overcurrent
67N
Instantaneous and directional time overcurrent 67N
Directional Instantaneous Overcurrent
67N
Three-zone distance system
21N
Power System Review
Transformer Protection
Open-Phase Condition
CTRatio  120
Ia  3.3330, Ib  0, Ic  3.333  120
In  Ia  Ib  Ic  Ia  Ic
In  3.333  1.667  j 2.886
In  1.666  j 2.886
In  3.33360
A
B
C
400 a
0 a 400 a
In  3.33360
In
CTs=600/5
3.30
Ia
0
Ib
3.3  120
Ic
In  Ia  0  Ic
In
Ic
Ia
Open
Ground Relay Could Pickup
Single-Phase to Ground
Fault
A
CTRatio  120
B
Ia  500, Ib  3.333  120 Ic  3.333  120
C
In  Ia  Ib  Ic
400
a
400 a
In  50  1.667  1.667
ISC
In  46.6660
In
In  46.70
CTs=600/5
Ia  500
Short
ISC
6,000a
3.3  120
3.3  120
Ic
In
Ib
Ground & Phase Relays both Pickup
Ia
Line-to-Line to Ground Fault
ISC
A
B
C
400 a
ISC
CTRatio  120
Ia  500, Ib  50  120 Ic  3.333  120
In  Ia  Ib  Ic
In  50  25  j 43.301  1.667  j 2.886
In  23.333  j 40.415
In  46.67  60
In  46.7  60
In
CTs=600/5
Ia  500
Short
Ia
50  120
Ib
3.3  120
IC
6,000a
Ic
Ia
Ib
Ground & Phase Relays both Pickup
In
Three-Phase to Ground
Fault
A
B
C
Ia  12,000
ISC
Ia  12,000
CTRatio  120
Ia  1000, Ib  100  120 Ic  100  120
In  Ia  Ib  Ic
In  0
Ia  12,000
ISC
ISC
In  0
In
CTs=600/5
Ia  1000
Ia  12,000
Ib  100a 2
Ib  12,000
Ic  12,000
Ic  100a
Ic
Ib
Short
Ic
Ia
Ib
Only Phase Relays Pickup
Ia
Relay Settings
Two Settings:
1. Time Overcurrent
2. Instantaneous
 Instantaneous Setting
 Time Overcurrent pickup
must be capable of
& time setting must be
handling:
capable of handling:
 XFMR Inrush
 Peak Load
 Capacitor Inrush
 Cold-Load Pickup
 Asymmetrical Faults
 Motor starting
Safety factor=1.2xSymm Fault
Power System Review
Relay Settings
 Phase Time Over Current (TOC) Setting
 Phase pick up:
 Method 1: 2xImax < I Pick up < I Min (phase-phase Fault current)
Note: Ensure I Min-Fault ≥ 2 x I Peak- Load
I Min=Iphase-to-Phase=0.866 x I Three-Phase fault
 Method 2: 25% Margin ; IPickup= Ifull-load/0.80
 Ground Time Over Current (TOC) Setting
 Ground Pick up:
 Method 1: 2xI Normal ground Current < I Pick up< I Min Ground Fault Current
Where; Normal Ground Current Range: 10% to 20% of Load Current
 Method 2: Ipickup=(0.40 to 0.75)x Ipeak-Load
Power System Review
Instantaneous Relays Pickup
Settings
 Instantaneous Pickup
 Range:
2 x IPhase  Pickup  Iins tan tan eous  10 x IPhase  Pickup
 Typical Instantaneous Phase & Ground
 Pick up= 2x Time Over Current relay pick ups
Power System Review
How to determine Pickup &
Time Dial?
 Step 1: Calculate Short Circuit Current @ each Bus
( usually Phase-Phase Fault)
 Step 2: Identify CT Ratio & Breaker Interrupting Cycles at
each Bus
 Step 3: Calculate Relay Minimum Pickup for each Device
 Step 3: Starts with the last relay and apply Time Margin of
0.3 to 0.4 sec (ANSI/IEEE Std-242 )between Relays:
o CB’s operating time (5 cycles):
o Relay Over travel time:
o Safety factor (CT saturation, Errors):
Total
Power System Review
0.08 sec
0.1 sec
0.22 sec
0.4 Seconds
Relay-Relay Setting
0.4 Sec
Power System Review
TCC Coordination Time
Margins
 Relay-Fuse TCC Curve Separation Rule:
 Mechanical Relay requires minimum time margin of 0.3 sec
time interval
 Digital Relay requires minimum time margin of 0.2 sec time
interval
 Relay-Relay (Mechanical) TCC Curve Separation Rule:
 According to ANSI/IEEE Std-242:1986, the rime interval
between two relays in series must be 0.3 to 0.4 seconds.
This time interval components are:
 Circuit Breaker Operating Time(5cycle): 0.08 sec
 Relay Overtravel Time: 0.1 sec
 Safety factor for CT Saturation & errors: 0.22 sec
Power System Review
TCC Coordination
Time Margins
 Relay-Relay (Digital)TCC Curve Separation:
 Time margin between series Relays must be minimum of
0.25 sec. This time separation consists of the following:
 5 Cycle Breaker (0.08 sec)
 Relay Accuracy (0.04 sec)
 Safety factor & CT Ration (0.13 sec)
 Relay-Recloser
 Time Margin between Mechanical Relay & Hydraulic
Recloser must be minimum of 0.28-0.30 sec
 Time Margin between Mechanical Relay & Electronic
Recloser must be minimum of 0.25 sec
Power System Review
Relay-Recloser-Fuse
Coordination
 In the following CKT coordinate Breaker B1, Cooper
Form-4C Recloser, ABB PCD2000 Reclsoer, and 100K
Tap Fuse.
B2
Load  231A
2
3
Cooper
Form  4C
25MVA
Load  400A
115kV / 22.86kV
115 kV
BUS
ABB
PCD2000
B1
Load  300A
R
Load  200A
R3
1
100K
FAULT @ Bus3
FAULT @ Bus2
FAULT @ Bus1
3  6000A
Φ  N  2000A
3  4000A
Min Fault  1000A
3  3000 A
Min Fault  443 A
L1
Power System Review
L2
L3
Relay-Recloser-Fuse Coordination
 Relay settings:
 Phase setting:
PU=2.4 x 400=960A
 Ground setting: PU= 960/2=480 A
 Cooper Recloser Form 4C settings:
 Phase setting:
PU=2x300=600A
 Ground setting: 160A<PU=600/3=200A<443A
 ABB Recloser Model PCD2000 settings:
 Phase setting:
PU=2x200=400A
 Ground setting: 140A<(PU=150 A)<443
Power System Review
Relay-Recloser-Fuse Coordination
Over Voltage Protection
 Insulation Voltage Class
 Basic Impulse Level (BIL)
 Nameplate Rating
 Surge arrestors.
Power System Review
Overvoltage Protection
 Sources of Overvoltage:
 Ferroresonance
 Low Order Hormonics
 Voltage Regulation (XFMR LTC Malfunction)
 Transients caused by:
 Lightning surge
 Switching operations
 Line-to-Ground faults
 Capacitor Bank Switching
 Protection methods:
 Surge Arresters(ANSI C62.1-1981)
 Static Wires
Useful IEEE/ANSI Standards
 Graph of Curves can be found in ANSI/IEEE Standard








C37.91-1985, “Guide for Protective Relay Applications
to Power Transformers,”
ANSI/IEEE C57.109-1993, “Guide for Transformer
Through-Fault Current Duration.”
IEEE/ANSI Standards 141&242
IEEE Std 242 – Buff Book
IEEE Std 141 – Red Book
IEEE Std 399 – Brown Book
•IEEE C37.90 – Relays
IEEE C37.91 – Transformer Protection
IEEE C37.102 – Guide for AC Generator Protection
References
1. J.D. Golver, M.S. Sarma, Power System Analysis and design,
4th ed., (Thomson Crop, 2008).
2. M.S. Sarma, Electric Machines, 2nd ed., (West Publishing Company,
1985).
3. A.E. Fitzgerald, C. Kingsley, and S. Umans, Electric
Machinery, 4th ed. (New York: McGraw-Hill, 1983).
4. P.M. Anderson, Analysis of Faulted Power systems(Ames, IA: Iowa
Satate university Press, 1973).
5.W.D. Stevenson, Jr., Elements of Power System Analysis, 4th
ed. (New York: McGraw-Hill, 1982).
Solution
Break
Time !!!!!
 Answer: 37.5 KVA