Lecture 1. INTRODUCTION TO BASIC PROTECTION AND RELAYING SCHEMES.pptx
1. Agenda
Why protection is needed
Principles and elements of the
protection system
Basic protection schemes
Digital relay advantages and
enhancements
2. 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
3. Power System Protection
Operation during severe disturbances:
System element protection
System protection
Automatic reclosing
Automatic transfer to alternate power
supplies
Automatic synchronization
6. 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”
7. 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
8. Short Circuits Produce High
Currents
Fault
Substation
a
b
c
I
I
Wire
Three-Phase Line
Thousands of Amps
9. 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.
10. 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
13. Protection System Elements
Protective relays
Circuit breakers
Current and voltage transducers
Communications channels
DC supply system
Control cables
14. 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.
15. How relays are differentiated?
Can be differentiated based on:
* Functional categories
* Input quantities
*Operating Principles
* Performance Characteristics.
16. What are various design
criteria?
* Dependability/Reliability
* Security
* Selectivity
*Speed
* Simplicity/flexibility
*Stability
*Performance Vs. Economy
17. What are various technique
used?
* Electromechanical
*Solid state/Static
* Microprocessor/Numerical
18. 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.
19. 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.
20. Types of relays
As per function:
Main
Auxiliary
Signal
As per actuating quantity
Overrelays
Underrelays
21. Types…
As per connection
Primary
Secondary(common)
As per action on CB
Direct acting
Indirect acting
As per construction
Electromagnetic
22. Types..
Static
Numerical
As per comparator types
Single input comparator
Two input comparator
Multiple input comparator
23. 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
30. Examples of Relay Panels
Old Electromechanical
Microprocessor-
Based Relay
31. 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 under
voltage, or a combination of both
Many other detection principles determine the design
of protective relays
37. 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
44. 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 ,
,
48. 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
49. Differential Protection Principle
No Relay Operation if CTs Are Considered Ideal
External
Fault
IDIF = 0
CT CT
50
Balanced CT Ratio
Protected
Equipment
51. 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
52. Possible Scheme – Percentage
Differential Protection Principle
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
53. Differential Protection
Applications
Bus protection
Transformer protection
Generator protection
Line protection
Large motor protection
Reactor protection
Capacitor bank protection
Compound equipment protection
54. 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
55. 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
56. 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…
