Power System Protection

1. Power System Protection:
Dr. O. P. Rahi
Electrical Engineering Department,
National Institute of Technology Hamirpur (H.P), India.

2. Contents
 Recap: Fault Analysis, Symmetrical & unsymmetrical
 Abnormal operation condition
 Protective system and its attributes
 System transducers
 Various principle of power system protection
 OCP Principle
 Differential Protection Principle
 Distance Protection Principle
 Directional Protection Principle
 Conclusion

3. Abnormal Operation Conditions
 Over current
 Over voltage
 Under voltage
 Under current
 Under frequency
 Over frequency
 High temperature
 Insulation failure

4. Protective System and its
Attributes
 Selectivity
 Reliability
 Dependability
 Security
 Speed
 Discrimination
 Simplicity
 Sensitivity
 Economy
4

5. Selectivity
 Selectivity implies an ability of a protective relay to
identify faulty system. A relay should not confuse
some peculiarities of an apparatus with a fault.
 For example, transformer when energized can draw
up to 20 times rated current (inrush current) which can
confuse, both overcurrent and transformer differential
protection.
 Typically, inrush currents are characterized by large
second harmonic content.
5

6. Reliability
 A relaying system has to be reliable. Reliability can be achieved by
redundancy i.e. duplicating the relaying system. Obviously redundancy
can be a costly proposition. Another way to improve reliability is to ask
an existing relay say, protecting an apparatus A to backup protection of
apparatus B.
 Both the approaches are used (simultaneously) in practice. However, it
is important to realize that back-up protection must be provided for safe
operation of relaying system. Redundancy in protection also depends
upon the criticality of the power apparatus.
 For example, a 400 kV transmission line will have independent
(duplicated) protection using same or a different philosophy; on the other
hand, a distribution system will not have such local back-up. A
quantitative measure for reliability is defined as follows:
6

7. Dependability
 A relay is said to be dependable if it trips only when it is expected
to trip. This happens either when the fault is in it's primary
jurisdiction or when it is called upon to provide the back-up
protection. However, false tripping of relays or tripping for faults
that is either not within it's jurisdiction, or within it's purview.
 Power system may get unnecessarily stressed or else there can
be loss of service. Dependability is the degree of certainty that
the relay will operate correctly:
 Dependability can be improved by increasing the sensitivity of
the relaying system.
7

8. Reliability
 Note that number of desired trips can be greater than or equal to
number of correct trips. A desired trip may not happen for various
reasons like, the fault level being below the relaying sensitivity,
stuck circuit breaker, incorrect setting of relays poor maintenance
of circuit breaker etc.
8

9. Sensitivity
 For simplicity, consider the case of overcurrent protection.
The protective system must have ability to detect the
smallest possible fault current. The smaller the current that
it can detect, the more sensitive it is.
 One way to improve sensitivity is to determine
characteristic signature of a fault. It is unique to the fault
type and it does not occur in the normal operation. For
example, earth faults involve zero sequence current.
 This provide a very sensitive method to detect earth faults.
Once, this signature is seen, abnormality is rightly
classified and hence appropriate action is initialized.
9

10. Security
 On the other hand, security is a property used to
characterize false tripping on the relays. A relay is
said to be secure if it does not trip when it is not
expected to trip. It is the degree of certainty that the
relay will not operate incorrectly:

10

11. Speed of Relaying
 The protective scheme must disconnect the faulty part of the
system as soon as possible. If the faulty part takes more time
to disconnect,
 It may damage the component which is carrying faulty current.
 To maximize safety, and minimize equipment damage and
system instability, a fault should be cleared as quickly as
possible.
 This implies that relay should quickly arrive at a decision and
circuit breaker operation should be fast enough. Typically, a
fast circuit breaker would operate in about two cycles.
 A reasonable time estimate for ascertaining presence of fault
is one cycle. This implies approximately three cycle fault
clearing time for primary protection.
11

12. Discrimination
 Discrimination is the ability of the protective
scheme to identify between the normal
conditions and abnormal conditions. The
relay scheme must operate under abnormal
conditions.
12

13. Simplicity
 To maintain and install easy, the protective
scheme must be simple in operation. The
complex systems are difficult while
maintenance and installation.
 The simpler the system it will more reliable.
Therefore, the protective scheme must simple.
13

14. Economy
 The important quality of any system is the cost. The
cost of a system is as low as possible. Sometimes, to
make the system more secure, the cost of a protective
scheme is more than the cost of the system which we
want to secure.
 So, to avoid this situation, the cost of a protective
scheme is not more than 5 % of the total cost of
equipment.
14

15. Example
 The performance of an overcurrent relay
was monitored over a period of one year. It
was found that the relay operated 14 times,
out of which 12 were correct trips. If the
relay failed to issue trip decision on 3
occasions, compute dependability, security
and reliability of the relay.
15

16. Solution
 Number of correct trips = 12
 Number of desired trips = 12 + 3 = 15
16

17. Contents of the Chapter
 Introduction to power system protection
 Abnormal operation condition,
 Protective system and its attributes
 System transducer
 Various principle of power system
protection.
17

18. System Transducers
 CT’s & PT’s
 Main CT’s & PT’s
 Auxiliary CT’s & PT’s
 Difference in Protection & Meas. CT’s & PT’s
 Types of CT’s & PT’s
 Causes of Errors in CT’s & PT’s
 Ways to Minimize the errors
 Saturation of CT’s & PT’s

19. Instrument Transformers
 Significance of fault Current
 Saturation of CTs
 Errors in CTs
19

20. Basic Principles of Protection
 Overcurrent Protection
 Differential Protection
 Distance Protection
 Directional Protection
20

21. Principle of Over-current
Protection
 Fault Current
 Pick up value
 Instantaneous OCP
 Time Graded OCP
 Current Graded OCP
 DMT
 IDMT
 PSM
 TSM
21

22. Principle of Differential Protection
 Differential Protection
 Merz Price Differential Protection
 Biased Differential Protection
 Circulating Current Type Differential Protection
 Opposed Voltage Type Differential Protection
22

23. 23

24. 24

25. Principle of Distance Protection
 Distance Protection
25

26. Distance Relays
 Distance relays are the protecting equipment that functions
based on the distance of the fault point on the transmission
line.
 The basic principle of distance relay is it functions
depending on the distance of fault in the transmission line.
Its operation depends on the impedance between the point
of fault and the point where the relay is installed. The
impedance between the point of fault and the location of
the relay is calculated based on the voltage and currents of
the transmission line during the fault case. The impedance
at that particular point becomes the operating quantity for
the relay.
26

27.  Types of Distance Relays
 Impedance Relays: This type of relay will operate
for the impedance of the line.
 Reactance Relays: This type of relay will not
operate for the resistance of the line. It will operate
only for reactance.
 Mho or Admittance Relays:
 Applications of Distance Relays
27

28. Principle of Directional Protection
 Directional Relays
 Relay Comparators
 Amplitude Comparators
 Phase Comparators
 Hybrid Comparators
 Applications of Directional Relays
28

29. Directional Static Overcurrent
Relay
29

30. Conclusion
 Recap:
 Abnormal operating conditions
 Protective system and its attributes
 System transducers
 Basic principles of protection
 Next:
 Protection of transmission lines

31. Thank you