This document discusses basic protection and relaying schemes used in power systems. It begins by explaining why protection systems are needed to handle severe disturbances that could jeopardize the power system. The key elements of a protection system are then introduced, including protective relays, circuit breakers, and current/voltage transducers. Common protection schemes like overcurrent, directional overcurrent, distance, and differential protection are described at a high level. The advantages of digital relays over electromechanical relays are also briefly mentioned. Overall, the document provides a high-level overview of protection systems and some of the basic schemes used to protect different components in a power grid.

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

2. Agenda
 Why protection is needed
 Principles and elements of the protection
system
 Basic protection schemes
 Digital relay advantages and enhancements

3. 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

4. Power System Protection
Operation during severe disturbances:
System element protection
System protection
Automatic reclosing
Automatic transfer to alternate power supplies
Automatic synchronization

5. Electric Power System Exposure to External
Agents

6. Damage to Main Equipment

7. 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”

8. 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

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

10. 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.

11. 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)

12. The Fuse
Fuse
Transformer

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

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

15. 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.

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

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

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

19. 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.

20. 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.

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

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

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

24. 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

25. Three-Phase Diagram of the Protection Team

26. DC Tripping Circuit

27. Circuit Breakers

28. Current Transformers
Very High Voltage CT
Medium-Voltage CT

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

30. Protective Relays

31. Examples of Relay Panels
Microprocessor-
Based Relay
Old Electromechanical

32. How Dofault takes place, the current,
 When a
Relays Detect Faults?
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

33. Primary Protection

34. 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

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

36. Typical Short-Circuit Type
Distribution
Single-Phase-Ground: 70–80%
Phase-Phase-Ground: 17–10%
Phase-Phase: 10–8%
Three-Phase: 3–2%

37. Balanced vs.
Unbalanced Conditions
Ia
Ic
Ic
Ia
Ib
Ib
Balanced System Unbalanced System

38. Decomposition of an Unbalanced
System

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

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

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

42. Inverse-Time Relay Coordination
I
Distance
t
} ∆T } ∆T } ∆T
Distance

43. 50/51 Relay Coordination
I
Distance
t
} ∆T } ∆T } ∆T
Distance

44. Directional Overcurrent Protection
Basic Applications
K
L

45. 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 |

46. The Impedance Relay
Characteristic
R 2 + X 2 ≤ Z r21
X Plain Impedance Relay
Operation Zone
Z ≤ Z r1 Radius Zr1
Zr1
R

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

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

49. 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

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

51. Differential Protection Principle
CTR CTR
Protected
Equipment
Internal
Fault
50 IDIF > ISETTING
Relay Operates

52. Problem of Unequal CT
Performance
CT CT
Protected
Equipment External
Fault
50 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

53. Possible Scheme – Percentage
Differential Protection Principle
ĪSP ĪRP
CTR Protected CTR
Equipment
ĪS ĪR
Relay
(87)
Compares: I OP = I S + I R
| IS | + | IR |
k ×I RT =k×
2

54. Differential Protection
Applications
 Bus protection
 Transformer protection
 Generator protection
 Line protection
 Large motor protection
 Reactor protection
 Capacitor bank protection
 Compound equipment protection

55. 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

56. 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

57. 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…

58. THANK YOU