There are three main aspects of power system protection: normal operation, prevention of electrical failures, and mitigation of failure effects. Protective relaying is one feature aimed at minimizing equipment damage and service interruptions when failures occur. Relays have evolved from electromechanical to static and digital types. Overcurrent protection is used for phase and earth faults, with inverse-time curves allowing coordination between relays. Adequate grading margins must be provided between relay operations.

2. DISTRIBUTION PROTECTION
R.Panneer Selvam B,E.,M.I.E.,
Former Superintending Engineer
Protection and Communication
CHENNAI

3. There are three aspects of a power system
These aspects are as follows:
A. Normal operation
B. Prevention of electrical failure.
C. Mitigation of the effects of electrical failure.

4. Normal Operation

The term normal operation assumes no failures of equipment, no mistakes of
personnel, nor Acts of God. It involves the minimum requirements for supplying
the existing load and a certain amount of anticipated future load.
A. Choice between hydro, steam, or other sources of power.
B. Location of generating stations.
C. Transmission of power to the load.
D. Study of the load characteristics and planning for its future growth.
E. Metering
F. Voltage and frequency regulation.
G. System operation.
E. Normal maintenance

5. Outages

Electrical equipment failures would cause intolerable outages.

There must be additional provisions to minimize damage to
equipment and interruptions to the service when failures occur.

Two recourses are open:
(1) to incorporate features of design aimed at preventing
failures, and
(2) to include provisions for mitigating the effects of failure
when it occurs

6. Prevention of Electrical Failure

The type of electrical failure that causes greatest concern is the short
circuit, or “fault” as it is usually called, but there are other abnormal
operating conditions peculiar to certain elements of the system that
also require attention. Some of the features of design and operation
aimed at preventing electrical failure are:
A. Provision of adequate insulation.
B. Coordination of insulation strength with the capabilities of lightning arresters.
C. Use of overhead ground wires and low tower-footing resistance.
D. Design for mechanical strength to reduce exposure, and to minimize the likelihood
of failure causable by animals, birds, insects, dirt, etc.
E. Proper operation and maintenance practices .

7. Design And Operation For Mitigating The
Effects Of Failure

Some of the features of design and operation for mitigating the effects of failure are:
A. Features that mitigate the immediate effects of an electrical failure.
1. Design to limit the magnitude of short-circuit current.1
a. By avoiding too large concentrations of generating capacity.
b. By using current-limiting impedance.
2. Design to withstand mechanical stresses and heating owing to short-circuit currents.
3. Time-delay under-voltage devices on circuit breakers to prevent dropping loads
during momentary voltage dips.
4. Ground-fault neutralizers (Petersen coils).
B. Features for promptly disconnecting the faulty element.
1. Protective relaying.
2. Circuit breakers with sufficient interrupting capacity.
3. Fuses.
C. Features that mitigate the loss of the faulty element.
1. Alternate circuits.
2. Reserve generator and transformer capacity.
3. Automatic reclosing.

8. Protective Relaying



Thus, protective relaying is one of several features of system design
concerned with minimizing damage to equipment and interruptions to
service when electrical failures occur.
When we say that relays “protect,” we mean that, together with other
equipment, the relays help to minimize damage and improve service.
Therefore, the capabilities and the application requirements of
protective-relaying equipments should be considered
concurrently with the other features.

9. Protection Technology
 The
last thirty years have seen enormous
changes in relay technology.
 The electromechanical relay in all of its
different forms has been replaced
successively by static, digital and numerical
relays

10. ELECTROMECHANICAL RELAYS


The mechanical force is generated through current
flow in one or more windings on a magnetic core or
cores, hence the term electromechanical relay
The principle advantage of such relays is that
–
–
they provide galvanic isolation between the inputs and
outputs in a simple, cheap and reliable form
therefore for simple on/off switching functions where the
output contacts have to carry substantial currents, they are
still used.

11. Philosophy of over Current Protection
 Over
current protection is directed entirely to
the clearance of faults, although with the
settings usually adopted some measure of
overload protection may be obtained.

12. Attracted Armature Relays


These generally
consist of an ironcored electromagnet that attracts a
hinged armature when
energized.
A restoring force is
provided by means of
a spring or gravity so
that the armature will
return to its original
position when the
electromagnet is deenergized.

13. Typical attracted armature relays


The contacts can be
made quite robust and
hence able to make,
carry and break
relatively large
currents under quite
onerous conditions
(highly inductive
circuits).
This is still a significant
advantage of this type
of relay that ensures
its continued use.

15. The basic connections of a protective relay

16. Torque production in an induction
relay.

17. Shaded-pole structure
.
Watthour-meter
structure.

18. Induction Cup Unit
• These two structures
are shown in Fig
• They most closely
resemble an induction
motor, except that the
rotor iron is stationary,
• Only the rotorconductor portion
being free to rotate.

19. Induction-Cup and Double-Induction-Loop
Structures
Double-induction-loop structure. Single-induction-loop structure.

20. The data required for a relay
setting study
i. A single-line diagram of the power system involved, showing the
type and rating of the protection devices and their associated
current transformers
ii. The impedances in ohms, per cent or per unit, of all power
transformers, rotating machine and feeder circuits
iii. The maximum and minimum values of short circuit currents
that are expected to flow through each protection device
iv. The maximum load current through protection devices
v. The starting current requirements of motors and the starting and
locked rotor/stalling times of induction motors
vi. The transformer inrush, thermal withstand and damage
characteristics
vii. Decrement curves showing the rate of decay of the fault current
supplied by the generators
viii.Performance curves of the current transformers.

21. Over current Protection
for Phase and Earth Faults

24. PRINCIPLES OF
TIME/CURRENT GRADING

26. Fault withstand levels
• The typical duration of external short circuits that a transformer
can sustain without damage if the current is limited only by the
self-reactance is shown in Table
• Maximum mechanical stress on windings occurs during the first
cycle of the fault.
• Avoidance of damage is a matter of transformer design.

27. Over current and Earth Fault Relay
•
•
•
•
Starting current
103 - 110% of current setting.
Closing current
130% of current setting.
Resetting current
The maximum current up to which
the disc will completely reset is 90%
of current setting.
Time settings
0 - 3s at 10 times current setting.
0 - 1.3s at 10 times current setting.
•
•
Resetting time
With the time multiplier set at 1.0 the
resetting times of the above are 9s and 4s
respectively.
Overshoot
On removal of a current equal to 20 times
current setting the overshoot times of the
above are 0.065s and 0.04s respectively.

30. Inverse-time curves.

33. Inverse-time curves.

36. Operating characteristic of over current relays

:-

characteristic


RI curves
operating time
1
t =  x K
- 0.236/ I




Normal inverse

0.14
t =  x K
0.02
I
very inverse
13.5
t =  x K
I–1
Extremely inverse
80
t =  x K
I2 -1








–1
Long time stand by earth fault

120
t =  x K
I-1

Logarithmic inverse

where t = relay operating time
K = scale constant or TMS according to curve
I = multiple of set current Is or PSM = Fault current / CT ratio x PSM


t = 5.8 - 1.35 log n (I / I n)

41. Grading Margin
 The time interval that must be allowed between the
operation of two adjacent relays in order to achieve
correct discrimination between them is called the
grading margin.
 The grading margin depends on a number of factors:
i. the fault current interrupting time of the circuit
breaker
ii. relay timing errors
iii. the overshoot time of the relay
iv. CT errors
v. final margin on completion of operation

42. Grading Margin

At one time 0.5s was a normal grading
margin. With faster modern circuit
breakers and a lower relay overshoot time,
0.4s is reasonable, while under the best
conditions even lower intervals may be
practical.