BASIC PROTECTION AND
RELAYING SCHEMES
Submitted bySomali 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
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
Main Causes
Characteristics



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
Short Circuits Produce High Currents
Three-Phase Line
a
b
c
I
Fault
Substation
Thousands of Amps
I
Wire
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

Mechanical
Forces
f1
f2
i1
i2
Rigid Conductors
f1(t) = k i1(t) i2(t)
The Fuse
Fuse
Transformer
Essential qualities of protection:
Reliability
 SelectivityAbsolute 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
CB
CTs
Protected
Equipment
Control
Relay
VTs
DC Tripping Circuit
+
SI
DC Station
Battery
Relay
Contact
SI
52a
52
TC
–
Relay
Circuit
Breaker
Red
Lamp
Circuit Breakers
Current Transformers
Very High Voltage CT
Medium-Voltage CT
Voltage Transformers
Medium Voltage
High Voltage
Note: Voltage transformers
are also known as potential
transformers
Protective Relays
Examples of Relay Panels
MicroprocessorBased 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

Primary Protection
Primary Protection Zone Overlapping
Protection
Zone A
52
To Zone A
Relays
Protection
Zone B
To Zone B
Relays
Protection
Zone A
52
To Zone A
Relays
Protection
Zone B
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:
Phase-Phase-Ground:
Phase-Phase:
Three-Phase:
70–80%
17–10%
10–8%
3–2%
Balanced vs.
Unbalanced Conditions
Ia
Ic
Ic
Ia
Ib
Ib
Balanced System
Unbalanced System
Decomposition of an Unbalanced
System
Ia
Ic
Ib
I a1
I c1
Ia0
Ib0
Ic0
Ib 2
I b1
Ia2
Ic2
Zero-Sequence
Positive-Sequence
Negative-Sequence
Single-Phase
Balanced
Balanced
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
Relay
Operation
Time
t
I
Radial Line
Fault
Load
Inverse-Time Relay Coordination
I
Distance
t

T
 T
 T
Distance
50/51 Relay Coordination
I
Distance
t
 T
 T
 T
Distance
Directional Overcurrent Protection
Basic Applications
K
L
Distance Relay Principle
L
d
I a , Ib , Ic
Va ,Vb ,Vc
21
Three-Phase
Solid Fault
Suppose Relay Is Designed to Operate
When:
| Va | (0.8) | Z L1 || I a |
Radial
Line
The Impedance Relay
Characteristic
R 2  X 2  Z r21
X
Plain Impedance Relay
Operation Zone
Z  Z r1
Zr1
Radius Zr1
R
Need for Directionality
F1
F2
1
2
3
4
RELAY 3
Operation Zone
5
6
X
F1
F2
Nonselective
Relay Operation
R
Three-Zone Distance Protection
Time
Zone 3
Zone 2
Zone 1
1
2
3
4
5
6
Time
Zone 1 Is Instantaneous
Circular Distance Relay Characteristics
X
PLAIN
IMPEDANCE
X
OFFSET
MHO (2)
R
R
X
X
LENS
(RESTRICTED MHO 1)
MHO
R
R
X
X
OFFSET
MHO (1)
R
TOMATO
(RESTRICTED MHO 2)
R
Differential Protection Principle
Balanced CT Ratio
CT
CT
Protected
Equipment
50
External
Fault
IDIF = 0
No Relay Operation if CTs Are Considered Ideal
Differential Protection Principle
CTR
CTR
Protected
Equipment
Internal
Fault
50
IDIF > ISETTING
Relay Operates
Problem of Unequal CT
Performance
CT
Protected
Equipment
50
CT
External
Fault
IDIF  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

Possible Scheme – Percentage
Differential Protection Principle
CTR
ĪSP
ĪRP
Protected
Equipment
ĪS
CTR
ĪR
Relay
(87)
Compares:
IOP  I S  I R
k  I RT
| IS |  | IR |
k
2
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…
THANK YOU