different types of motor protection schemes - protection and switchgear -

1. Protection & Switchgear
UNIT III - APPARATUS PROTECTION

2. Motor protection

3. Protection circuit
for induction
motor

4. Protection circuit
for single phasing
of induction motor

5. Principle of merz - price system for the protection of power transformer -
problem encountered in differential protection of transformer
1- The differential relay has at least two actuating quantities say I1, I2
2- The two or more quantities should be similar i.e. current/current.
3- The relay responds to the vector difference between the two i.e. to I1-I2, which
includes magnitude and/or phase angle difference.
Difficulties of differential protection
a) Difference in pilot wire lengths.
b) CT's ratio error during short circuits.
c) Saturation of CT magnetic circuit during short circuit condition.
d) Magnetizing current inrush in transformer while switching in.

6. Principle of merz - price system for the protection of power transformer

7. IFETCE/EEE/IV year/VII
Sem/EE2402/PSG/unit-

8. Earth Fault or Leakage Protection

9. Combined Leakage and Overload Protection

10. buchholz relay for transformer protection

12. Advantages
1. Normally a protective relay does not indicate the appearance of the fault.
It operates when fault occurs. But Buchholz relay gives an indication of the
fault at very early stage, by anticipating the fault and operating the alarm
circuit. Thus the transformer can be taken out of service before any type of
serious damage occurs.
2. It is the simplest protection in case of transformers.
Limitations
1. Can be used only for oil immersed transformers having conservator tanks.
2. Only faults below oil level are detected.
3. Setting of the mercury switches cannot be kept too sensitive otherwise
the relay can operate due to bubbles, vibration, earthquakes mechanical
shocks etc.
4. The relay is slow to operate having minimum operating time of 0.1
seconds and average time of 0.2 seconds.

13. Busbar protection

17. Protection employed for transmission line.
The probability of faults occurring on the lines is much more due to their greater length
and exposure to atmospheric conditions
requirements of line protection
(i) In the event of short-circuit, the circuit breaker which is near to the fault
should open, all other circuit breakers remaining in a closed position.
(ii) In case the nearest breaker to the fault fails to open, back-up protection
should be provided by the adjacent circuit breakers.
(iii) The relay operating time should be just as short as possible in order to
preserve system stability, without unnecessary tripping of circuits.
The common methods of line protection are :
(i) Time-graded over current protection
(ii) Differential protection
(iii) Distance protection

18. Time-Graded Over current Protection

19. Ring main system
Parallel feeders.

21. Carrier current protection

23. Coupling capacitor Line trap unit

27. Differential Protection of Alternator

28. Modified Differential Protection for Alternator

32. Generator Capability Curve
Synchronous generators are rated in terms of the maximum MVA output at a
specified voltage and power factor (usually 0.85 or 0.9 lagging) which they can
carry continuously without overheating
There are two powers
1. Real power
2. Reactive power
Real power is limited by prime mover capability.
Reactive power is limited by three parameters.
Armature current limit
Field current limit
End region heat limit

33. Complex power, S= P+jQ
S=
S=
Graph is drawn between real and reactive power.
Where, P= real power,
Q= reactive power,
= phasor angle,
r= radius of the power angle
]
sin
[cos
|
||
| 
 i
Vt
It 
Vt
It*

In the P-Q plane the armature current
limit, as shown in Fig, appears as a
circle with centre at the origin and
radius equal to the MVA rating

34. 34
POWER DIAGRAM (CAPABILITY DIAGRAM):
• ASSUMPTION: I.R. drop is negligible. CASE-I:
In Δ ABC, BC=E Sinδ
In Δ BCD, BC=IXd CosФ
E Sinδ = IXd CosФ
Multiply both sides by V
Xd
EV Sinδ = VI CosФ = REAL
Xd POWER
At δ=90°, We get the maximum power i.e. the theoritical stability line.
• CASE-I I: In Δ ABC, CD=AC – AD; In Δ BCD, CD=IXd SinФ
In Δ ABC, AC=E Cosδ & AD = V
IXd SinФ = E Cosδ - V ; Multiply both sides by V , We get
Xd
EV Cos δ – V2 = VI Sin Ф = REACTIVE POWER
Xd Xd
E
V
MW
MVAR
I
Φ
Φ
A
B
C
D

35. END REGION HEAT LIMIT:
End region heat can be limited only during the under excited condition.

36. Circuit Breakers

37. Circuit Breakers Definition:
A circuit breaker can make or break a circuit either manually or
automatically under all conditions viz., no-load, full-load and short-circuit
conditions.
A Circuit Breakers Definition is a piece of equipment which can
 makeor breaka circuit either
manually or by remote conditions
 break a circuit automatically under fault
conditions
control under
normal
 make a circuit either manually or by remote control under
faith conditions

38. Arc Phenomenon:
When a short-circuit occurs, a heavy current flows through the
contacts of the ‘circuit breaker, before they are opened by the protective
system. Arc is formed when the contacts are open.
The arc resistance depends upon the following factors :
Degree of ionisation – the arc resistance increases with the decrease
in the number of ionised particles between the contacts.
Length of the arc – the arc resistance increases with the length of the arc
i.e., separation of contacts.
Cross-section of arc – the arc resistance increases with the decrease in
area of X-section of the arc.

39. Principles of Arc Extinction:
Before discussing the methods of arc extinction, it is necessary to examine the
factors responsible for the maintenance of arc between the contacts. These are :
p.d. between the contacts
ionised particles between contacts
Methods of Arc Extinction:
There are two methods of extinguishing the arc in Circuit Breakers Definition viz
1.High resistance method.
2.Low resistance or current zero method
1.High resistance method: The resistance of the arc may be increased by :
 Lengthening the arc: The resistance of the arc is directly proportional to its
length. The length of the arc can be increased by increasing the gap between
contacts.

40.  Cooling the arc: Cooling helps in the deionisation of the medium between the
contacts. This increases the arc resistance. Efficient cooling may be
obtained by a gas blast directed along the arc.
 Reducing X-section of the arc: If the area of X-section of the arc is
reduced, the voltage necessary to maintain the arc is increased. In other
words, the resistance of the arc path is The cross-section of the arc can be
reduced by letting the arc pass through a narrow opening or by having smaller
area of contacts.
 Splitting the are: The resistance of the arc can be increased by splitting the
arc into a number of smaller arcs in series. Each one of these arcs
experiences the effect of lengthening and cooling.

41. 2. Low resistance or Current zero method
The rapid increase of dielectric strength of the medium near current zero
can be achieved by :
 causing the ionised particles in the space between contacts to
recombine into neutral molecules.
 sweeping the ionieed particles away and replacing them by un-
ionised particles
Therefore, the real problem in a.c. arc interruption is to rapidly deionise the
medium between contacts as soon as the current becomes zero so that the
rising contact voltage or restriking

42. voltage cannot breakdown the space between contacts. The de-ionisation of
the medium can be achieved by:
Lengthening of the Gap: The dielectric strength of the medium is proportional to
the length of the gap between contacts. Therefore, by opening the contacts rapidly,
higher dielectric strength of the medium can be achieved.
high Pressure: If the pressure in the vicinity of the arc is increased, the density of
the
particles constituting the discharge also increases. The increased density of
particles causes higher rate of de-ionisation and consequently the dielectric
strength of the medium between contacts is increased.
cooling: Natural combination of ionised particles takes place more rapidly if they
are al- lowed to cool. Therefore, dielectric strength of the medium between the
contacts can be increased by cooling the arc.
blast effect: If the ionised particles between the contacts are swept away and
replaced
by unionised particles, the dielectric strength of the medium can be
increased considerably. This may be achieved by a gas blast directed along the
discharge or by forcing oil into the contact space.

43. Important Points to be Remember:
The following are the important terms much used in the circuit breaker analysis:
1.Arc voltage: It is the voltage that appears across the contacts of the circuit breaker
during the arcing period.
2.Restriking Voltage: It is the transient voltage that appears across the contacts at or
near current zero during arcing period.
3.Recovery voltage: It is the normal frequency (50 Hz) r.m.s. voltage that
appears across the contacts of the circuit breaker after final arc extinction. It is
approximately equal to the system voltage.

55. Energy Recovery Theory

64. Testing of circuit
breaker

65.  Necessary of Testing of circuit
breaker:-
Why "Testing of Circuit Breaker" is Necessary?
 A Circuit Breaker should be capable of carrying, making, and breaking under
normal and abnormal conditions. In any power system circuit breaker has
to withstand power frequency over voltages and transient over voltages
due to switching and lightning.
 The performance of a circuit breaker under normal and abnormal
conditions can be verified by performing different type of tests on circuit
breakers. The main purpose of testing of circuit breakers is to confirm if
circuit breaker is able to work on particular voltage and current ratings or
not.

66. There are mainly two tests classified:
1) Type test
2) Routine Test
1) Type Tests:
The purpose of type tests is to prove design features and the
quality of circuit breaker. Type tests are not conducted on
each circuit breaker. This is done to prove the capabilities
and to confirm the rated characteristics of the circuit
breakers.

67. 2)Routine Tests:
Routine test is performed before circuit breaker
dispatch to ensure the product. This gives result
about defects in materials and construction of
circuit breaker. We can check quality of material
of circuit breaker by performing Routine Test.

68.  Mechanical endurance tests
 Thermal tests
 Dielectric tests
 Measurement of resistance of the main circuits
 Short Circuit tests

69. In this test, the C.B.. is open and closed 500 times or other value as
agreed to between the purchaser and the supplier.the test are carried
out without current through the main circuit of the C.B.Out of the
total number of tests, 10% should be closed-open operation,that is
with the tripping,mechanism energized by the closing of main
contacts.During the tests,occasional lubrication,but no mechanical
adjustments are permissible.after the tests,all parts including
contacts should be in good condition and there should be no
permanent distortion and undue wear of the parts.

70. This test determines the maximum normal current that the circuit
breaker can carry without exceeding the maximum allowable
temperature rise.In this test the rated normal current of normal
frequency is passed through the current carrying parts of circuit
breaker.
Method are recognized by Indian standards for measuring
temperature rise of parts:-
1) Thermometer method
2) Thermocouple method
3) Self resistance method

71. 1) Breaking capacity Test:-
• Sequence of performing this tests is as follows:-
 First of all,the master circuit breaker (MB)and the breaker under
test (TB)are closed.
 The s.c.current is passed by closing the make switch.
 The circuit breaker under test(TB) is opened to interrupt the
s.c.current at desired moment.
• The following measurements related to the breaking
capacity performance are taken from the oscillogram
during the test:-
 Symmetrical breaking current
 Asymmetrical breaking current
 Amplitude factor
 Natural frequency of oscillations and RRRV(RATE OF RISE OF
RISTRIKING VOLTAGE)

72. Sequence of Performing this test :-
First of all,the master circuit breaker (MB)and the make
switch(MS) are closed.
Then,the short circuit current is initiated by closing the test
breaker (TB).
The rated short circuit making current i.e.the peak value of the
first major loop of the short circuit current envelope is
measured from the oscillorgram.

73. 3) Short Time Withstand Current Capacity
•In this test, the rated short-time withstand current
is applied to the circuit breaker under test for the
specified duration of the time.
•The rated short time withstand current is equal to
be rated short circuit breaking current and standard
value of rated duration of short circuit current is 1
second or 3 seconds.
•The current is measured by taking an
oscillograph of the short circuit current wave. After
the test, there should be no mechanical or insulation
damage and any contact welding.

74. SHORT CIRCUIT TEST IN LABORATORIES

75. SYNTYHETIC TESTS

77. Comparison of Fuse and Circuit Breaker

78. Fuse

79. Types of fuse
• Low voltage fuse
a)Semi-enclosed re-wireable fuse
b)High rupturing capacity (HRC) catridge fuse
c) HRC fuse with tripping device
Fuse upto 400 V with breaking capacity of
about 4000A
• High voltage fuse
Fuse upto 33KV with breaking capacity of
about 8700A

80. High rupturing capacity (HRC)
catridge fuse
• Chalk
• Dust of marbles
• Quartz dust
• Plaster of paris

81. HRC fuse with tripping device

82. HRC fuse

83. MCBs or Miniature Circuit
Breakers
• Electromechanical devices which protect
an electrical circuit from an overcurrent.
• The overcurrent, in an electrical circuit,
may result from short circuit, overload or
faulty design.
• An MCB is a better alternative to a Fuse
since it does not require replacement once
an overload is detected

84. MCBs or Miniature Circuit
Breakers

85. MCBs or Miniature Circuit Breakers