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Apparatus Protection Using Relays for Engineering Students
1. UNIT-3
APPARATUS PROTECTION
Current transformers and Potential transformers and their
applications in protection schemes - Protection of transformer,
generator, motor, bus bars and transmission line
2. 1. CT and PT are collectively known as transducers and
instrument transformer.
3. Current transformers and Potential transformers and their
applications in protection schemes
1. Current or voltage instrument transformers are necessary for
isolating the protection, control and measurement equipment
from the high voltages of a power system, and for supplying the
equipment with the appropriate values of current and voltage -
generally these are 1A or 5Α for the current coils, and 110 V for
the voltage coils.
2. The behaviour of current and voltage transformers during and
after the occurrence of a fault is critical in electrical protection
since errors in the signal from a transformer can cause
maloperation of the relays.
7. Current Transformer
It performs two tasks
1. Step down the heavy power system currents to low
values that are suitable for relays and other measuring
instruments (meters).
2. It isolate the relays and meters circuits from high
voltages of the power system.
3. The standard current ratings of secondary is 1 A to 5 A.
4. Depending on the applications, CTs are classified into
two categories
Measuring CTs
Protective CTs
8. Difference between measuring CT and protective CTs
Measuring CT
1. Measuring CT’s are required to give high accuracy for all load
currents upto 125 % of the rated current.
2. These CT’s may have a significant errors during fault
conditions.
3. These Measuring CT’s get saturated at about 1.25 times the full
load current.
Protective CT
1. Protective CT’s should not saturate upto 20 to 50 times full load
current.
2. The DC offset should also be considered while designing the
protective CT.
3. The fault current for a secondary of 5 A rating could be 100 to
250 A. Therefore CT secondary having a continuous rating of 5
A should have a short time current rating of 100 A to 250 A , so
that the same is not damaged.
10. CT Burden
• It is defined as the load connected across the
secondary of the CT. Which is usually expressed in
VA. It can also be expressed in terms of impedance at
the rated secondary current at a given power factor,
usually 0.7 lagging.
• If the rated burden at rated secondary current Is
amperes, the ohmic impedance of the burden can be
calculated as follows:
Calculate the VA output required for CT of 5 A rated secondary
current when burden consists of relay requiring 7.5 VA at 5 A and
connecting lead resistance of 0.08 ohm. Suggest the choices of the CT.
11. • When the relay is set to operate at current different from the
rated secondary current of the CT. The effective burden of the
relay can be calculated as follows.
Where , Pe = Effective CT Burden
Pr = VA burden of Relay
Is = rated secondary current
Ir = current setting of relay
The rated VA output of the CT selected should be higher than
standard value nearest to the calculated value.
12. For example:
The rated secondary current of a CT is 5A, The plug setting of
a relay is 3.75 A. The power consumption of the relay is 4 VA.
Calculate the effective VA burden on the CT.
CT error – magnitude or ratio error & phase angle error
Phase angle Error:
The phase difference between the primary and secondary
phasor is zero. But actual transformer there is always a
difference in phase between the two phasor. The phase
difference between the primary current phasor and secondary
current phasor is termed as phase angle error.
14. To minimize CT error:
For achieving minimum error in current transformer, one can
follow the following,
1. Using a core of high permeability and low hysteresis loss
magnetic materials.
2. Keeping the rated burden to the nearer value of the actual
burden.
3. Ensuring minimum length of flux path and increasing cross-
sectional area of the core, minimizing joint of the core.
4. Lowering the secondary internal impedance.
15. Classification of CTs
• Based on technology
– Electromagnetic CTs
– Opto electronic CTs (optical current sensors)
– Rogowski Coil
• Based on Location
– Indoor CTs
– Outdoor CTs
Based on Application
– Measuring or metering CTs
– Protective CTs
20. Why secondary of CTs should not be kept open ?
Secondary of CT should not be kept open. Either it should
be shorted or must be connected in series with the low R
coil or ammeters.
If it is left open, then current through secondary becomes
zero hence ampere turns produced by secondary which
generally oppose the primary ampere turns becomes zero.
This cause high flux in the core, so heavy emf will be
induced in the secondary side. This may damage the
insulation of the winding.
21. Potential Transformer PT
Voltage transformers (VT), also called potential
transformers (PT), It performs two task:
1. It is used to reduce the power system
voltages to standard lower values.
2. It isolate the relays and other instruments
from the high voltages of the power system.
They are designed to present negligible load to the supply
being measured and have an accurate voltage ratio and phase
relationship to enable accurate secondary connected metering.
The standard voltage rating of the secondary winding of
PTs used in practice is 110 V line to line or 110 / √3 line to
neutral.
22.
23. Phasor diagram
Is - Secondary current.
Es - Secondary induced emf.
Vs - Secondary terminal voltage.
Ip - Primary current.
Ep - Primary induced emf.
Vp - Primary terminal voltage
KT - Turns ratio = Numbers of primary
turns/number of secondary turns.
I0 - Excitation current.
Im - Magnetizing component of I0.
Iw - Core loss component of I0.
Φm - Main flux. β - Phase angle error.
24. PT error
Voltage error or Ratio error:
The difference between the ideal value Vp/KT and actual value Vs is
the voltage error or ratio error in a potential transformer, it can be
expressed as, Normally 2 % or 5%
Phase Error or Phase Angle Error in Potential or Voltage
Transformer
The angle ′β′ between the primary system voltage Vp and the reversed
secondary voltage vectors KT.Vs is the phase error.
Normally 2 or 3 degrees
To Minimize the overall error:
1. The internal resistance and reactance to an appropriate magnitude.
2. Minimum magnetising and loss components of exciting current
required by the core
25. Types of PT’s
• Electromagnetic type PT
– It is used upto 132 kV.
– It is similar to coventional wound type with additional
features to minimize errors.
• Coupling capacitor type PT
– At higher voltages above 132 kV, Electromagnetic type
becomes very expensive.
– Capacitor voltage divider is used. CCVT
• Opto electronic type PT’s
– Opto electronics send a circular polarized light beam that
travels through an optical fiber.
28. Comparison between CT and PT
CT PT
Connected in series with power
circuit.
Connected in parallel with power
circuit.
Secondary works almost in short
circuited condition.
Secondary works almost in open
circuit condition.
Primary current depends on
power circuit current.
Primary current depends on
secondary burden.
Secondary is never be open
circuited.
Secondary can be used in open
circuit condition.
29. Advantages of Instrument Transformers
• The normal range voltmeter and ammeter can be
used along with these transformers to measure high
voltage and currents
• The rating of low range meter can be fixed
irrespective of the value of high voltage or current to
be measured
• These transformers isolate the measurement from
high voltage and current circuits. This ensures safety
of the operator and makes the handling of the
equipments very easy and safe.
• These can be used for operating many types of
protecting devices such as relays or pilot lights.
• Several instruments can be fed economically by
single transformer.
30. Disadvantages of Instrument Transformers
• The only disadvantage of these instrument
transformers is that they can be used only for
a.c. circuits and not for d.c. circuits.
31. Applications of Instrument Transformers
• Circulating current differential protection.
• Over current phase fault protection.
• Distance protection
• Intermediate CTs for feeding protective
devices, measuring systems, relays etc.
32. Transformer Protection
Small size distribution transformers (up to 100 kVA , 11
kV/400 V) only high voltage fuses are used as main protective
device.
A 250 MVA, 15 kV/400 kV generator transformer may be
provided with very elaborate protection. This may consist of
percentage differential protection, a protection against
incipient faults and protection against over fluxing as
primary protection. These will be backed up by over current
protection.
33. Types of fault encountered in transformers
1. External faults or through faults.
1. Over load, Lightning, external short circuit unsymmetrical
faults, etc
2. Internal faults
1. Short circuit in transformer windings and connections
It is serious in nature, it includes LG, LL and inter turn
faults on HV and LV windings.
2. Incipient faults.
It is minor in nature but slowly develop into major faults. It
includes poor winding connections, core faults, failure of
coolant, regulator faults and bad load sharing between
transformers.
34. Possible Transformer Faults
• Overheating
• Winding faults
– Phase to phase faults
– Earth faults
– Interturn faults
• Open circuits (heating)
• Through faults (external faults)
• Over fluxing (V/f)
35. Transformer Protection
• Over current Protection.
• Percentage differential Protection (or) Merz-
Prize Protection.
• Protection against Magnetic inrush current.
• Earth Fault protection.
36. Over current Protection of Transformer
The pick up value of phase fault over current unit is set such that they
do not pick up on minimum permissible overload, but are sensitive to
smallest phase fault.
The pick up value of earth fault relay on the other hand is independent
of the loading. It is so sensitive. Normally time graded over current
relays are provided for backup protection.
37. Connection between Power
Transformer and CTs
Power Transformer Connections C. T Connections
Primary Secondary Primary Secondary
Star Delta Delta Star
Star Star Delta Delta
Delta Star Star Delta
Delta Delta Star Star
41. The basic requirements of the differential relay are,
1. The differential relay must not operate on load or
external faults.
2. It must operate on severe internal faults.
3. The neutrals of C.T. star and power transformer
stars are grounded.
42. Problems encountered in differential protection
• Unmatched characteristics of CTs
• Ratio change due to tap change
• Difference in lengths of pilot wires
• Magnetizing current inrush
44. 1. When a unloaded transformer is switched on, it draws
initial magnetising current which may be several times the rated
current of the transformer.
2. This initial magnetising current is called the magnetising
inrush current.
3. As the inrush current flows only in primary winding, the
differential protection will see this inrush current as an internal
fault.
4. The inrush current contents are 40 to 60 % dc component,
second harmonic 30 to 70 %, third harmonic 10 to 30 %.
5. This feature can be utilize to distinguish between inrush
current and fault current
45. 6. The third harmonic do not appear in CT leads as these
harmonics circulate in the delta winding or delta connected CT.
7. Tuned circuit Xc, XL allows only fundamental frequency
current to operating coil.
8. Second harmonic currents are diverted into the restraining
coil
46. Earth fault protection
Earth-fault usually involves a partial breakdown of winding
insulation to earth and the vector sum 3-phase is no longer zero. The
resultant current sets up flux in the core of the CT which induces e.m.f.
in the secondary winding.
47. Buchholz relay
• Gas operated relay, it is used to detect incipient faults which are initially
minor faults but may cause major fault in due course of time.
• It is slow actuating device, the , the minimum operating time is 0.1 sec, the
average time 0.2 sec.
48. Advantages of Buchholz Relay
• 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.
• It is the simplest protection in case of
transformers.
49. Disadvantages of Buchholz Relay
• Can be used only for oil immersed transformers
having conservator tanks
• Only faults below oil level are detected
• Setting of the mercury switches can not be kept
too sensitive otherwise the relay can operate
due to bubbles, vibration, earthquakes
mechanical shocks etc.
• The relay is sloe to operate having minimum
operating time of 0.1 seconds and average time
of 0.2 seconds.
50. Applications of Buchholz Relay
The following type of transformers fault can be
protected by the Buchholz relay and are indicated by
alarm:
• Local overheating
• Entrance of air bubbles in oil
• Core bolt insulation failure
• Short circuited laminations
• Loss of oil and reduction in oil level due to leakage
• Bad and loose electrical contacts
• Short circuit between phases
• Winding short circuit
• Bushing puncture
• Winding earth faults
51. Generator Protection
• Generator is accompanied by prime mover, excitation
system, voltage regulator, cooling system, etc its
protection becomes very complex and elaborate.
• The protection scheme for generator involves the
consideration of more possible failures or abnormal
operating conditions than any other system equipment.
Possible faults in generator.
• Stator fault.
– Phase to earth fault (occur in armature slots)
– Phase to phase fault (S.C. two phase winding, melting Cu)
– Inter turn fault (multiturn coil, due to high voltage, single turn)
• Rotor fault.
– Field ground fault (mechanical and thermal stresses on field winding)
– Rotor over heating
– Loss of excitation
52. • Abnormal running condition.
– Over loading (temperature rise)
– Over speed (sudden loss of load, turbogoverner)
– Unbalanced loading (-ve sequence current, rotor heating,
unsymmetrical faults, failure of C.B)
– Over Voltage (due to over speed, faulty operation of volt.
Regulators)
– Failure of prime mover (motoring action, draw active power,
reverse power protection by directional relay)
– Loss of excitation (fault in exciter, increase in speed, work as
induction gen., draw reactive power, overheating stator winding)
– Cooling system failure (overheating, insulation failure)
– Excessive vibration
– Difference in expansion between rotor and stator parts
– Loss of synchronism
53. Generator Protection schemes
• Stator protection
– Percentage differential protection
– Protection against stator inter turn faults
– Stator over heating protection
• Rotor protection
– Field ground fault protection
– Loss of excitation protection
– Protection against rotor overheating
• Miscellaneous protection
– Over voltage protection
– Over speed protection
– Protection against motoring
54. – Protection against vibration
– Bearing overheating protection
– Protection against voltage regulator failure.
55. Generator Protection
Stator protection:
Percentage Differential protection.
Earth fault protection.
Rotor protection.
Rotor Earth Fault protection
Protection against Loss of excitation
Protection against Loss of prime mover
58. Percentage differential protection of generator
or
Merz price protection of generator
Star connected Generator
This diagram is same for star connected motor
59. Percentage differential protection of generator
or
Merz price protection of generator
Delta connected Generator
This diagram is same for delta connected motor
60. Advantages of Merz-Price Scheme
• Very high speed operation with operating time
of about 15 msec.
• It allows low fault setting which ensures
maximum protection of machine windings
• In ensures complete stability under the most
severe through and external faults
• It does not require current transformers with
air gap or special balancing features
61. Earth Fault protection
Usually neutral is grounded to protect the complete
winding against ground faults.
When the neutral is solidly grounded, it is possible to
provide protection to complete winding of the generator
However, the neutral is
grounded through resistance to
limit the ground fault currents.
But it is not possible to protect
the complete winding against
ground faults.
63. The usual practice is to protect 80 to 85 % of the
generator winding against ground fault. The remaining 15-
20 % from neutral end is left unprotected.
64. Let, p % of the winding from the neutral remains
unprotected. Then (100-p) % of the winding is protected.
The ground fault current If is given by
Where V is the line to neutral and Rn is the neutral
grounding resistance.
75. Motor Protection
• Possible fault
The types of faults in motors are similar to those of generators
1. Stator Faults
1. Phase fault
2. Ground fault
3. Inter turn fault
2. Rotor Faults
Number of induction motors are being used in the industry, It is not
possible to make any general statements about the protection of
induction motor, since the protection scheme depends on the size
(horsepower/kW rating) of the motor and its importance in the system.
3. Prolonged Over load
4. Unbalanced supply voltages
including single phasing.
5. Under voltage
6. Reverse of phases.
7. Rotor jam’
8. Failure of bearings
76. S.
No.
Abnormal Condition Choice of Protection Circuit to be employed
1 Mechanical overload Overload release, thermal overload relay, over
current relay, MCB with built-in trip coil
2 Stalling or prolonged starting
of motor
Thermal relays, instantaneous overcurrent
relay
3 Under voltage Under voltage release, under voltage relay
4 Unbalanced voltage Negative phase sequence relays
5 Reverse phase sequence Phase reversal relay
6 Phase to phase fault or
phase to earth
HRC fuse, instantaneous overcurrent relays. For
large motors, differential protection may be
employed for economy
7 Single phasing Thermal overloading relays, single phase
preventer
78. Different types of Motor Protection Schemes
• Percentage differential protection of Motor (or) Merz Price
Protection
• Phase fault and ground fault protection of motor.
• Protection of motor against unbalance in supply voltage.
• Protection against overheating
84. Phase fault protection (short circuit protection) of motor
Protection against phase faults as well as ground faults can be
provided using either fuses , or over-current relays depending upon the
voltage rating and size of the motor. (fault current 8 -10 times, attracted
armature relay setting is 4 -5 times)
85. Protection of motor against unbalance in supply voltage.
Large induction motors are very sensitive to unbalances in
supply voltage. The negative sequence component, which comes
into picture because of the unbalance in the supply, is particularly
troublesome
86. Motor protection against over heating using RTD Relays
These relays operate from one or more RTDs that monitor the
temperature of the machine winding, motor or load bearings or
load case. They are usually applied to large motors of 1500 HP
and above.
87. Bus bar protection
The word bus is derived from the Latin word omnibus which
means common for all. Bus bars are the nerve-centres of the
power system where various circuits are connected together.
Busbars are located in switchyards, substations,
• Weakening of insulation because of ageing, corrosion because
of salty water,
• Breakdown of insulation because of overvoltages,
foreign objects, and so on.
For example, lizards and snakes are known to have caused bus
bar faults in remote unmanned substations.
The causes of faults experienced on busbars are:
88. Busbar Faults
• Failure of insulation due to material deterioration
• Failure of circuit breaker
• Earth fault due to failure of support insulator
• Flashover due to sustained excessive over voltages
• Errors in the operation and maintenance of
switchgear
• Earthquake and mechanical damage
• Accidents due to foreign bodies falling across the
busbars
• Flashover due to heavily polluted insulator
89. There is a large concentration of short-circuit capacity at the bus
bars. A fault on the bus bar, though rare, causes enormous
damage. When protective relays operate to isolate the bus bar
from the system,
What type of protection is best suited for busbar ?
Differential protection will suit this situation best because the
ends (terminals) of the system are physically near to each other.
Thus, by installing CTs on the two sides, we can simply
compare the current entering the busbar with that leaving it. Any
discrepancy between the two will immediately signal an internal
fault.
90. Bus-Bar Protection
• Percentage differential protection of bus bar
• Frame leakage protection
• High Impedance relaying scheme.
91. Percentage differential protection of bus bar
The algebraic sum of all the currents entering and leaving the
bus bar zone must be zero. Unless there is a fault therin. The
relay is connected to trip all the circuit breakers. In case of a bus
fault the algebraic sum of currents will not be zero and relay will
operate.
94. Difficulties in Busbar Protection
• Current levels for different circuits are different
• Large number of circuits to be protected
• Saturation of cores of current transformers due to
d.c. component in short circuit current is possible
which produces ratio error
• Due to various bus sections, the scheme becomes
complicated
• With large load changes, relay settings need to be
changed
95. Transmission line protection
Possible faults in a transmission line:
1. Symmetrical Faults.
2. Unsymmetrical Faults.
3. Lightning Faults.
Protection Schemes:
1. Over current Protection of transmission line
2. Distance protection of transmission line
3. Pilot relaying protection scheme of transmission line
97. Over current protection scheme of transmission line.
1.Current grading scheme
2. Time grading scheme
3. Combined current ant time grading scheme
98.
99. Distance Protection scheme of transmission Line
Distance protection schemes are commonly employed for providing the
primary or main protection and backup protection for AC transmission line and
distribution line against three phase faults, phase-to-phase faults, and phase-to-
ground faults. The distance relay of impedance relay or reactance relay or mho
relay can be employed in this protection scheme.
Distance Relay
Transmission line
100. Pilot relaying protection scheme
1. Pilot relaying schemes are used for the protection of
transmission lines. They fall into the category of unit
protection.
2. In these scheme some electrical quantities at the two ends
of the transmission line are compared and hence they
require some sort of interconnected channel (pilot) over
which information can be transmitted from one end t other.
Different types of pilot scheme based on the channels are
1. Wire pilot protection (distance upto 30 km)
2. Translay Scheme of protection
3. Carrier current pilot (high freq signal 50kHz to 700 kHz)
4. Micro wave pilot. (very high freq signal 450 MHz to
10000MHz)
101. Wire pilot protection
The pilot wires are used to connect the relays. Under normal
working condition, the two currents at both ends are equal and pilot
wires do not carry any current, keeping relays inoperative. Under
fault conditions, The two currents at two ends are no longer same,
this causes circulating current flow through pilot wires
102. Translay scheme
This system is the modified form of voltage-balance system. In the event of
fault on the protected feeder, current leaving the feeder will differ from the
current entering the feeder. Consequently, unequal voltages will be induced in
the secondary windings of the relays and current will circulate between the two
windings, causing the torque to be exerted on the disc of each relay.
103. Carrier Current protection scheme
This type of protection is used for protection of transmission
lines Carrier currents of the frequency range 30 to 200 kc/s in
USA and 80 to 500 kc/s (kHz) in UK are transmitted and
received through the transmission lines for the purpose of
protection.
104. Coupling capacitor:
coupling capacitors allows carrier frequency signals to
enter the carrier equipment but does not allow 50 Hz power
frequency currents to enter the carrier equipment.
It offers low reactance (1/ωC) to carrier frequency but
high reactance power frequency.
Line trap
It has a low impedance (less than 0.1 ohm) to 50 Hz and high
impedance to carrier frequencies. This unit prevents high
frequency signals from entering the neighbouring line, This unit
prevents high frequency signals from entering the neighbouring
line, and the carrier currents flow only in the protected line.
105. Microwave pilot protection scheme
1. Microwave scheme uses space as the channel.
2. It uses Ultra transmitter receiver system high frequency (450
MHz to 10000 MHz) for connecting the relaying equipment at
the terminals of the protected line.
106. Summary of transmission line protection
• Lines or feeders can be protected by several methods. Each
method has some advantages and some limitations. The classes
of protective relays used for line protection ; roughly in
ascending order of cost and complexity are :
Instantaneous overcurrent -
Time-overcurrent
Directional overcurrent
Distance
Pilot (pilot wire, power line carrier, or microwave).
