Power System Protection Fundamentals: Teaching Students the Basics

Power System Protection
Fundamentals

What should we teach students
about power system protection?

Copyright © SEL 2008

Agenda

 Why protection is needed
 Principles and elements of the protection
system
 Basic protection schemes
 Digital relay advantages and enhancements


Disturbances: Light or Severe
 The power system must maintain acceptable
operation 24 hours a day
 Voltage and frequency must stay within certain
limits

 Small disturbances
 The control system can handle these
 Example: variation in transformer or generator load

 Severe disturbances require a protection
system
 They can jeopardize the entire power system
 They cannot be overcome by a control system Copyright © SEL 2008

Power System Protection

Operation during severe disturbances:
 System element protection
 System protection
 Automatic reclosing
 Automatic transfer to alternate power
supplies
 Automatic synchronization


Electric Power System Exposure to
External Agents


Damage to Main Equipment


Protection System

A series of devices whose main purpose
is to protect persons and primary electric
power equipment from the effects of faults

The “Sentinels”


Blackouts

Characteristics Main Causes
 Loss of service in a  Overreaction of the
large area or protection system
population region
 Bad design of the
 Hazard to human life protection system
 May result in
enormous economic
losses


Short Circuits Produce High
Currents

Three-Phase Line
a
b
c
I

Substation Fault

Thousands of Amps I
Wire


Electrical Equipment Thermal Damage
t

Damage Damage Curve
Time

I
In Imd Short-Circuit
Rated Value
Current

Mechanical Damage During
Short Circuits
 Very destructive in busbars, isolators, supports,
transformers, and machines
 Damage is instantaneous
Mechanical
Forces
f1 f2
i1
i2

Rigid Conductors f1(t) = k i1(t) i2(t)

The Fuse

Fuse

Transformer


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


DC Tripping Circuit


Circuit Breakers


Current Transformers

Very High Voltage CT
Medium-Voltage CT


Voltage Transformers

Medium Voltage

Note: Voltage transformers
are also known as potential
High Voltage transformers

Protective Relays


Examples of Relay Panels

Microprocessor-
Based Relay

Old Electromechanical


How Do Relays Detect Faults?
 When a fault takes place, the current, voltage,
frequency, and other electrical variables
behave in a peculiar way. For example:
 Current suddenly increases
 Voltage suddenly decreases

 Relays can measure the currents and the
voltages and detect that there is an
overcurrent, or an undervoltage, or a
combination of both
 Many other detection principles determine the
design of protective relays

Main Protection Requirements
 Reliability
 Dependability
 Security
 Selectivity
 Speed
 System stability
 Equipment damage
 Power quality
 Sensitivity
 High-impedance faults
 Dispersed generation Copyright © SEL 2008

Primary Protection


Primary Protection Zone Overlapping
Protection
Zone A
52 Protection
Zone B
To Zone A
Relays
To Zone B
Relays

Protection
Zone A
52 Protection
Zone B
To Zone A
Relays To Zone B
Relays

Backup Protection

Breaker 5
Fails
C D
A E

1 2 5 6 11 12

T
B F

3 4 7 8 9 10


Typical Short-Circuit Type
Distribution

Single-Phase-Ground: 70–80%
Phase-Phase-Ground: 17–10%
Phase-Phase: 10–8%
Three-Phase: 3–2%


Balanced vs.
Unbalanced Conditions
Ia
Ic
Ic

Ia

Ib
Ib
Balanced System Unbalanced System

Decomposition of an Unbalanced
System


Power Line Protection Principles

 Overcurrent (50, 51, 50N, 51N)
 Directional Overcurrent (67, 67N)
 Distance (21, 21N)
 Differential (87)


Application of Inverse-Type
Relays
Relay t
Operation
Time

I
Radial Line

Fault Load


Inverse-Time Relay Coordination

I

Distance
t

} ∆T } ∆T } ∆T
Distance

Addition of Instantaneous OC
Element

Relay t
Operation
Time

I
Radial Line

Fault Load


50/51 Relay Coordination

I

Distance
t

} ∆T } ∆T } ∆T
Distance

Directional Overcurrent Protection
Basic Applications

K

L


Directional Overcurrent Protection
Basic Principle
V I

F2 F1

Relay

Reverse Fault (F2) Forward Fault (F1)
I
V

V I

Overcurrent Relay Problem

E
I SETTING ≈
Z S1 + (0.8) Z L1

 Relay operates when the following condition
holds:
I FAULT = I a > I SETTING

 As Z s1
changes, the relay’s “reach” will change,
since setting is fixed
E
I FAULT ( LIMIT ) =
′
Z S1 + (0.8) Z L1

Distance Relay Principle
L
d
I a , Ib , Ic

Radial
21 Three-Phase
Va ,Vb ,Vc Line
Solid Fault

Suppose Relay Is Designed to Operate
When:
| Va |≤ (0.8) | Z L1 || I a |

The Impedance Relay Characteristic

R 2 + X 2 ≤ Z r21

X Plain Impedance Relay
Operation Zone

Z ≤ Z r1 Radius Zr1
Zr1

R


Need for Directionality

F2 F1
1 2 3 4 5 6

RELAY 3 X
Operation Zone
F1

F2 R
Nonselective
Relay Operation


Directionality Improvement
F2 F1
1 2 3 4 5 6

RELAY 3 X
Operation Zone Directional Impedance
F1 Relay Characteristic

F2 R
The Relay Will
Not Operate for
This Fault

Mho Element Characteristic
(Directional Impedance Relay)

Operates when: V ≤ I Z M cos( ϕ − ϕ MT )
Z ≤ Z M cos( ϕ − ϕ MT )


Three-Zone Distance Protection
Time

Zone 3
Zone 2
Zone 1

1 2 3 4 5 6

Time
Zone 1 Is Instantaneous

Line Protection With Mho Elements
X

C

B

A
R
D

E

Circular Distance Relay Characteristics
X X
PLAIN OFFSET
IMPEDANCE MHO (2)

R

R
X
X
LENS
MHO (RESTRICTED MHO 1)

R R

X X

OFFSET TOMATO
MHO (1) (RESTRICTED MHO 2)

R R


Semi-Plane Type Characteristics
X X
DIRECTIONAL
RESTRICTED
DIRECTIONAL

R

R
X X

REACTANCE RESTRICTED
REACTANCE

R R

X X

OHM
QUADRILATERAL

R
R


Distance Protection
Summary

 Current and voltage information
 Phase elements: more sensitive than 67
elements
 Ground elements: less sensitive than
67N elements
 Application: looped and parallel lines


Directional Comparison
Pilot Protection Systems


Permissive Overreaching
Transfer Trip


Basic POTT Logic

Key XMTR

Zone 2 Elements
AND Trip
RCVR


Directional Comparison
Blocking Scheme


Basic DCB Logic

Zone 3 Key XMTR

Carrier Coordination

Time Delay
CC
Zone 2 0
Trip
RCVR


Differential Protection Principle

Balanced CT Ratio

CT CT
Protected
Equipment External
Fault

50 IDIF = 0

No Relay Operation if CTs Are Considered Ideal


CTR CTR
Protected
Equipment
Internal
Fault

50 IDIF > ISETTING

Relay Operates

Problem of Unequal CT Performance
CT CT
Protected
Equipment External
Fault

50 IDIF ¹ 0

 False differential current can occur if a CT
saturates during a through-fault
 Use some measure of through-current to
desensitize the relay when high currents are
present Copyright © SEL 2008

Differential Protection Applications
 Bus protection
 Transformer protection
 Generator protection
 Line protection
 Large motor protection
 Reactor protection
 Capacitor bank protection
 Compound equipment protection

Differential Protection
Summary
 The overcurrent differential scheme is simple
and economical, but it does not respond well to
unequal current transformer performance
 The percentage differential scheme responds
better to CT saturation
 Percentage differential protection can be
analyzed in the relay and the alpha plane
 Differential protection is the best alternative
selectivity/speed with present technology

Multiple Input Differential Schemes
Examples

Differential Protection Zone
ĪSP ĪRP

ĪT

I1 I2 I3 I4
OP

Bus Differential: Several Inputs
Three-Winding Transformer
Differential: Three Inputs


Advantages of Digital Relays

Compatibility with
Low maintenance
Multifunctional digital integrated
(self-supervision)
systems

Highly sensitive,
Highly reliable
secure, and Adaptive
(self-supervision)
selective

Reduced burden
Programmable
on Low Cost
Versatile
CTs and VTs


Synchrophasors Provide a
“Snapshot” of the Power System


The Future
 Improvements in computer-based
protection
 Highly reliable and viable communication
systems (satellite, optical fiber, etc.)
 Integration of control, command,
protection, and communication
 Improvements to human-machine
interface
 Much more Copyright © SEL 2008

Power System Protection Fundamentals: Teaching Students the Basics

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