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BASIC PROTECTION AND
RELAYING SCHEMES
Submitted by-
Somali ajal Das
0901106068
Guided by-
 Dr. Abhimanyu
Mohapatra
 Dr. Ranjan Ku. Jena

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

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

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

Short Circuits Produce High Currents
Fault
Substation
a
b
c
I
I
Wire
Three-Phase Line
Thousands of Amps

FAULTS ON POWER SYSTEMS RISK :
Severe damage to the faulted equipment :
 Excessive current may flow;
 Causes burning of conductors or equipment
windings;
 Arcing - energy dissipation;
 Risk of explosions for oil - filled switchgear, or
when in hazardous environments.
Damage to adjacent plant :
 As the fault evolves, if not cleared quickly;
 Due to the voltage depression / loss of supply.

Mechanical Damage During
Short Circuits
 Very destructive in busbars, isolators,
supports, transformers, and machines
 Damage is instantaneous
i1
i2
f1 f2
Rigid Conductors f1(t) = k i1(t) i2(t)
Mechanical
Forces

Essential qualities of protection:
 Reliability
 Selectivity-
Absolute or relative
 Fastness
 Discrimination

Protection System Elements
 Protective relays
 Circuit breakers
 Current and voltage transducers
 Communications channels
 DC supply system
 Control cables

Protective relays:
 A device which detect intolerable or
unwanted conditions within the assigned
area.
 * A watchman or watchdog for the
equipment/area
 * Silent sentinels to power system.

How relays are differentiated?
 Can be differentiated based on:
 * Functional categories
 * Input quantities
 *Operating Principles
 * Performance Characteristics.

What are various design criteria?
 * Dependability/Reliability
 * Security
 * Selectivity
 *Speed
 * Simplicity/flexibility
 *Stability
 *Performance Vs. Economy

What are various technique used?
 * Electromechanical
 *Solid state/Static
 * Microprocessor/Numerical

Non-Unit, or Unrestricted Protection :
No specific point downstream up to which
protection will protect
 Will operate for faults on the protected
equipment;
 May also operate for faults on downstream
equipment, which has its own protection;
 Need for discrimination with downstream
protection, usually by means of time grading.

Unit, or Restricted Protection :
Has an accurately defined zone of
protection
 An item of power system plant is
protected as a unit;
 Will not operate for out of zone
faults, thus no back-up protection
for downstream faults.

Types of relays
As per function:
 Main
 Auxiliary
 Signal
As per actuating quantity
 Overrelays
 Underrelays

Types…
As per connection
 Primary
 Secondary(common)
As per action on CB
 Direct acting
 Indirect acting
As per construction
 Electromagnetic

Types..
 Static
 Numerical
As per comparator types
 Single input comparator
 Two input comparator
 Multiple input comparator

Methods of disciminations:
 To locate fault
by time
by current grading
by time and direction
by distance
by time, current and distance
by current balance
by power direction comparison
 Type of fault

Three-Phase Diagram of the Protection Team
CTs
VTs
Relay
CB
Control
Protected
Equipment

DC Tripping Circuit
SI
52
TC
DC Station
Battery
SI
Relay
Contact
Relay
Circuit
Breaker
52a
+
–
Red
Lamp

Current Transformers
Very High Voltage CT
Medium-Voltage CT

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

Examples of Relay Panels
Old Electromechanical
Microprocessor-
Based Relay

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

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

Backup Protection
A
C D
E
Breaker 5
Fails
1 2 5 6 11 12
T
3 4 7 8 9 10
B F

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
Balanced System Unbalanced System
c
I
a
I
b
I
a
I
c
I
b
I

Decomposition of an Unbalanced
System
Positive-Sequence
Balanced Balanced
Negative-Sequence
1
b
I
1
c
I
1
a
I
2
b
I
2
a
I
2
c
I
0
a
I
0
b
I
0
c
I
a
I
c
I
b
I
Zero-Sequence
Single-Phase

Power Line Protection Principles
 Overcurrent (50, 51, 50N, 51N)
 Directional Overcurrent (67, 67N)
 Distance (21, 21N)
 Differential (87)

Characteristics of overcurrent relays:
 Definite time
 IDMT- inverse definite minimum time
 Very inverse
 Extremely inverse

Application of Inverse-Type Relays
t
Relay
Operation
Time
I
Fault Load
Radial Line

Distance
Distance
t
I
  
T

Inverse-Time Relay Coordination
T
 T


50/51 Relay Coordination
Distance
Distance
t
I
  
T
 T
 T


Directional Overcurrent Protection
Basic Applications
K
L

Distance Relay Principle
Three-Phase
Solid Fault
d
L
Radial
Line
21
Suppose Relay Is Designed to Operate
When:
|
||
|
)
8
.
0
(
|
| 1 a
L
a I
Z
V 
c
b
a I
I
I ,
,
c
b
a V
V
V ,
,

The Impedance Relay
Characteristic
2
1
2
2
r
Z
X
R 

R
X Plain Impedance Relay
Operation Zone
Zr1
Radius Zr1
1
r
Z
Z 

Need for Directionality
1 2 3 4 5 6
F1
F2
R
X
RELAY 3
Operation Zone
F1
F2
Nonselective
Relay Operation

Three-Zone Distance Protection
1 2 3 4 5 6
Zone 1
Zone 2
Zone 3
Time
Time
Zone 1 Is Instantaneous

Circular Distance Relay Characteristics
MHO
OFFSET
MHO (1)
PLAIN
IMPEDANCE
R
X
R
X
R
X
OFFSET
MHO (2)
R
X
LENS
(RESTRICTED MHO 1)
TOMATO
(RESTRICTED MHO 2)
R
X
R
X

Differential Protection Principle
No Relay Operation if CTs Are Considered Ideal
External
Fault
IDIF = 0
CT CT
50
Balanced CT Ratio
Protected
Equipment

Internal
Fault
IDIF > ISETTING
CTR CTR
50
Relay Operates
Protected
Equipment

Problem of Unequal CT
Performance
 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
External
Fault
Protected
Equipment
IDIF  0
CT CT
50

Possible Scheme – Percentage
Protected
Equipment
ĪR
ĪS
CTR CTR
Compares:
Relay
(87)
OP S R
I I I
 
| | | |
2
S R
RT
I I
k I k

  
ĪRP
ĪSP

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

Advantages of Digital Relays
Multifunctional
Compatibility with
digital integrated
systems
Low maintenance
(self-supervision)
Highly sensitive,
secure, and
selective
Adaptive
Highly reliable
(self-supervision)
Reduced burden
on
CTs and VTs
Programmable
Versatile
Low Cost

Why study this protection
scheme??
 Protection scheme plays a vital & important role
for the normal operation or the steady state
operation of different components of power system
network, which must be reliable, fast and efficient.
 In order to achieve all these features, it is essential
that these should be proper care in designing and
choosing an appropriate and efficient protection
scheme.
 The protective relays functions as the brain
behind the whole schemes…

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