Protective Schemes for Transformer And
Transcript of Protective Schemes for Transformer And
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PROTECTIVE SCHEMES FOR TRANSFORMER AND MOTOR
A Mini project submitted in partial fulfillment of the requirements forThe award of the degree of
BACHELOR OF TECHNOLOGYIn
ELECTRICAL AND ELECTRONICS ENGINEERING
SubmittedBy
MOUNIKA. P (07491A0211)NARENDRA. B (07491A0239)NAVEEN. U (07491A0241)ANIL KUMAR. P (07491A0223)PAVANI. K (07491A0212)
Under the guidance of
Prof. J. KRISHNA KISHORE, M.Tech.
Associate Professor, HOD.
Q.I.S COLLEGE OF ENGINEERING & TECHNOLOGY(Approved by A. I. C. T. E., Affiliated to J. N. T. University, KAKINADA.)
An ISO 9001:2000 Certified & An NBA accredited CollegeVENGAMUKKAPALEM - 523272, ONGOLE.
PRAKASAM (DIST.), A.P.2007-2011
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Q.I.S COLLEGE OF ENGINEERING & TECHNOLOGY(Approved by A.I.C.T.E., Affiliated to J. N. T. University, KAKINADA)
An ISO 9001:2008 Certified CollegeVENGAMUKKAPALEM-523272, ONGOLE
PRAKASAM (DIST.), A.P.2007-2011
ELECTRICAL & ELECTRONICS ENGINEERING DEPARTMENT
CERTIFICATEThis is to certify that the mini project report titled
“PROTECTIVE SCHEMES FOR TRANSFORMER AND MOTOR” Is the bonafied work carried out by
MOUNIKA.P (07491A0211) NARENDRA.B (07491A0239) NAVEEN.U (07491A0241) ANIL KUMAR.P (07491A0223) PAVANI.K (07491A0212)
Of B. Tech in partial fulfillment of the requirements for the award of bachelor of technology (B. Tech) in Electrical & Electronics Engineering (EEE) by J. N. T. University during the academic year 2010-2011
Sri. J.KRISHNA KISHORE, M.Tech Prof. J. KRISHNA KISHORE M.Tech.Associate Professor, HOD Head of the Department Project Guide
Principal External Examiner
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ACKNOWLEDGEMENT
We would like to express our profound sense of gratitude and indebtedness to our project guide J. KRISHNA KISHORE, HEAD OF THE DEPARTMENT, department of ELECTRICAL AND ELECTRONICS ENGINEERING for his valuable guidance, cooperation at each and every phase of mini project work and suggestions all the way through our work.
We would also express our sincere gratitude and thanks to the ASST. DIVISIONAL ENGINEER of VIJAYAWADA THERMAL POWER STATION (V.T.P.S) for allowing us to accomplish our work and other staff members of the plant VIJAYAWADA for their valuable guidance and support in every part of our work.
We would like to express our thanks to all the faculty members, staff of department of electrical and electronics engineering, who have rendered valuable help in making this project a successful one.
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PROTECTIVE SCHEMES FOR VARIOUS ELECTRICAL EQUIPMENT
CONTENTS:-
1. ABSTRACT
2. INTRODUCTION
3. POWER TRANSFORMER PROTECTION
3.1 CLASSIFICATION OF TRANSFORMERS
3.2 PROTECTION BY FUSE
3.3 PRIMARY BACK-UP PROTECTION
3.4 BUCHHOLTZ RELAY
3.5 DIFFERENTIAL PROTECTION 3.6 OVER CURRENT PROTECTION
3.8 COMBINED EARTH FAULT AND PHASE FAULT PROTECTION
3.9 RESTRICTED EARTH FAULT PROTECTION
4. MOTOR PROTECTION
4.1 PROTECTION OF SMALL MOTORS
4.2 PROTECTION OF LARGE MOTORS
4.3 OVERLOAD PROTECTION OF MOTORS
4.4 THERMAL OVERLOADPROTECTION 4.5 PROTECTION AGAINST UNBALANCE
4.6 PHASE TO PHASE SHORT CIRCUIT PROTECTION
5. MODERN TRENDS IN TRANSFORMER PROTECTION
6. CONCLUSION
7. REFERENCES & BIBLIOGRAPHY
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1. ABSTRACT
Protective systems have been undergoing improvements/modifications keep in step
with the requirements of larger & larger generating stations and complexity of interactions.
Protective systems are the heart of any power system. They play a very important role
in controlling and protecting various equipment in power system.
Therefore for reliable operation of any plant, protective systems are very important.
Keeping in phase with the development of advanced electronics, the shape and size of
protective systems are also getting major changes. Static & microprocessor based relays
came into existence which precisely control & protect the system from spurious faults.
Therefore in our project we studied various protective schemes that are employed for
Transformer & Motor.
2. INTRODUCTION
Protective relaying is an integral part of any electrical power system. The
fundamental objective of system protection is to quickly isolate a problem so that the
unaffected portions of the system can continue to function. The flip side of this objective
is that the protection system should not interrupt power for acceptable operating
conditions, including tolerable transients.
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The choice of protection depends upon several aspects such as type, rating of the
protected equipment, its important location, probable abnormal conditions, costs etc.
A fault in electrical equipment is defined as a defect in its electrical circuit due to
which the flow of current is diverted the intended.
Faults can be minimized by improving system design, improving quality of
component, better and adequate protective relaying, better operation and maintenance;
however the fault can’t be entirely eliminated.
The protective relay senses the abnormal condition in a part of power system and
given an alarm or isolate that part from the healthy system.
When abnormal conditions occur three basic objectives must always be met:
All endangered equipment must be protected from damage
The faulted components must be isolated and if not damaged, reenergized as rapidly
as possible.
Service interruption must be minimized.
3. POWER TRANSFERORMER PROTECTION
A power transformer constitutes an important and expensive component in a power
system. It is, therefore essential to provide an efficient protective relay scheme to protect the
transformer from any severe damage which might likely to be caused by short-circuited
faults with in the equipment itself or any sustained overload or fault conditions in the power
systems.
Protective relaying is necessary for every power transformer. The choice of protection
depends upon several aspects such as type, rating of transformer, its location, its importance,
probable abnormal conditions, cost etc. There are several transformers of various
ratings.Each needs certain adequate protection.
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The protective relaying senses the abnormal conditions give an alarm or isolate that
part from the healthy system. The relaying are compact, self contained devices which
respond to abnormal condition. The relay distinguishes the normal and abnormal conditions.
When an abnormal condition occurs relay closes its contacts there by trip circuit breaker
opens and faulty part is disconnected from the supply. The entire process is automatic and
fast.
Circuit breakers are switching devices which can interrupt normal and abnormal
currents. Besides relays and circuit breaker there are several other important components in
the protective relaying scheme. These include protective current transformer, voltage
transformers, protective relays, time delay relays, Auxiliary relays, trip circuits, secondary
circuits, auxiliary and accessories etc.
3.1 CLASSIFICATION OF TRANSFORMERS
Classification of power transformer for purpose of protective gear application would
be to take into account.
The voltage class
The M.V.A rating
Type of connections and number of windings.
Method of grounding the Y-connected neutrals.
The function it has to perform.
The schematic layout adopted
3.2 PROTECTION BY FUSE:
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Power transformer up to a limited capacity rating and voltage level can be protected
by means of high rupturing capacity fuses provided on the primary side. While this method is
simple and cheapest since no other costly switch gear equipment is needed, it has many draw
backs.
A fuse cannot detect the low current transformer earth faults. Besides, the fuse is
incapable of distinguishing faults currents from the transients magnetizing in rush currents
and normal load currents. A fuse would operate whether the fault is in the transformer zone
or outside the transformer zone. It is also not possible to accomplish simultaneous
interruption of all three phases in the event of a fault in any one of the phases. In view of
these limitations, the fuse protection of transformers has a limited application and is
generally employed where some relaxation could be made in the degree of supply continuity
and the amount of unbalanced loading.
PROTECTIVE RELAYS:
Functions of Protective Relays:
To sound an alarm or close the trip circuit breaker so as to disconnect a
transformer during abnormal conditions such as over-load, under voltage, temperature
rise, unbalanced load, reverse power, under frequency, short-circuit, etc.
To disconnect the abnormally operating transformer so as to prevent the
subsequent faults. E.g., over-load protection protects the transformer and prevents
insulation failure.
To localize the effect of fault by disconnecting the fault part from the healthy
part, causing least disturbance to the healthy system.
PROTECTIVE ZONE:
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A part of the system protected by a certain protective schemes is called protective
zone or zone of protection. The entire power-system is covered by a several protective zones
and no part of the system is left unprotected.
The boundary of a protective zone is determined by the location of current
transformer. Hence the current transformer is located such that the circuit breaker is covered
in the protective zones. The zone can be precisely identified in unit systems. Unit system is
one in which the protection responds to faults in the protected zone alone, and it does not
responds to faults beyond the protected zone. Each zone has certain protective scheme each
protection do not have exact zone boundary.\
3.3 PRIMARY BACK-UP PROTECTION:
Primary protection (Main Protection) is the essential protection provided for
protecting an equivalent machine.
ABNORMAL CONDITIONS AND STRESSES:
Power transformers are used in high voltage systems for transfering large loads. They
are subjected to voltage stresses, current stresses, thermal stresses and electromagnetic
stresses during their operation.
Voltage stresses are caused by normal voltage, power frequency over voltage,
impulse over voltages. They effect the internal and external insulation.
Current stresses are caused by normal current and short circuit currents flowing
through the transformer windings. The current stresses result in:
Temperature rise
Electromagnetic forces.
Environmental effects are caused by alternative variation in the ambient temperature,
atmospheric dust and pollution.
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Mechanical stress: during short circuit winding and bushings are subjected to dynamic
forces. Hence transformers are to be protected from all the above abnormalities.
3.4 BUCHHOLTZ RELAY:
The incipient faults in the transformer tank below oil level actuate buchholtz relay to
given an alarm. The arc produced due to fault causes decomposition of transformer oil. The
product of transformer oil decomposition contains more than 70% of hydrogen gas. This
hydrogen gas being light rises upwards and tries to go into the conservator. The buchholtz
relay is fitted in the pipe leading to the conservator. The gas gets collected in the upper
portion of the buchholtz relay.
There by the oil level in the buchholtz relay drops down. The float, floating in the oil
tilts down with lowering oil level while doing so the mercury switch attached to the float is
closed and the mercury switch closes the alarm circuit. There by the operator know that there
is some incipient fault in the transformer. The transformer is disconnected as early as
possible and the gas sample is tested. The testing of gas gives clue regarding the type of
insulation failure. Buchholtz relay gives an alarm so that the transformer can be disconnected
before the incipient fault grows into the serious one.
When a serious short circuit occurs in the transformer, the pressure in the tank
increases. The oil rushes towards the conservator and it passes through Buchholtz relay. The
plates in the buchholtz relay get presses by rushing oil. There by they close another switch
which inturn closes the trip circuit of circuit breaker. There after the transformer is removed.
The decomposition of transformer oil starts at about 3500C. The gas accumulated in
the upper portion of the relay can be trapped. The gas is tested for color, combustibility,
chemical test etc.
For faults above the oil level Buchholtz relay is inactive.
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FIG: 3.4.1 Buchholz Relay
Fig 3.4.2 connection diagram of buchholz relay
LIMITATIONS OF BUCHHOLTZ RELAY:
Only faults below oil levels are detected.
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Setting the mercury switch cannot be too sensitive otherwise there can be a false
operation by vibrations, earth quakes and mechanical shocks though the pipe line etc.
The relay is slow, minimum operating time is 0.1 sec and average time is 0.2 sec.
such a slow relay is unsatisfactory. However it is an excellent relay to bring to notice
incipient faults. Sparking in magnetic circuit is also detected.
Buchholtz relays are not provided for transformer below 500 K.V.A. A separate
buchholtz relay is provided with a tap changer to detect incipient faults in tap
changer. This does not respond to small arcing.
3.5 DIFFERENTIAL PROTECTION:
The differential protection responds to the vector difference between two similar
quantities .In protection of transformer C.T.’s are connected at each end of the transformer.
The CT.’s secondaries are connected in star or delta and the pilot wires are connected
between C.T’s of each end.The C.T connections and C.T ratios are such that the current fed
into the pilot wires from both the ends are equal during normal conditions and it varies
during fault conditions. During the internal faults such as phase to phase or phase to ground
the balance is disturbed.The out of balance current I1-I2 flows through the relay operating
coils. To avoid unwanted operation restraining bias coils are provided in series with pilot
wires. The ampere turns provide by the bias coil or restraining coil is proportional to average
of I1 and I2.
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Fig 3.5.1 Differential Protection of Transformer
DIFFICULTIES IN DIFFERENTIAL PROTECTION:
The differential protection may operate wrongly due to the following causes even
when there is no internal fault in the transformers.
The difference in pilot wire lengths:
The current transformer and machine to be protected are located at different sites and
normally it is not possible to connect relay coil to equi potential points. The difficulty is over
come by connecting adjustable resistors in series with the pilot wires .These are adjusted on
size to obtain the equi potential points.
C.T. Ratio Errors During short circuits:
The current transformers may have almost equal ratio at normal currents. But during
short circuit conditions the primary current are unduly large .the ratio errors of C.T.’.S on
either side differs during these conditions.
Saturation of C.T. magnetic circuit during short-circuits conditions:
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Due to these causes the relay may operate even for external faults. The relay may
lose its stability through these faults.
Magnetizing currents in rush in Transformer while switching in Tap Change:
When the transformer is connected to supply a large current in rush takes
place (6 to 10 times the full load current). This certainly causes operational of differential
relay though there is no fault in the transformer .To avoid this difficulty harmonic restraint is
provided for the differential relay.
TAP CHANGING:
The tap changing causes change in the transformation of the transformer. There by
the C.T.Ratio do not match with the new tap setting resulting current in pilot wires even
during healthy conditions. This aspect is taken care of by biased differential relay.
C.T.CONNECTIONS:
The percentage differential relay for 3 phase transformer has 3 operating coils and 3
restarting coils. These are connected to pilot wires on the secondary of the current
transformer. The connections are such that, for each phase, the differential current (I1-12)
flows through the operating coil.
In both the cases three current transformers are required at each side of the protected
transformer. The connections of the C.T.’s secondary are such that during normal conditions
and for external faults, no current should flow through the relay operating coil. The
differential protection provides the instantaneous protection (less than 1.0 seconds and no
internal time delay) within the protective zone.
Phase to phase faults.
Phase to ground faults.
It does not detect the faults and a high speed over relay is required for this purpose.
3.6 OVER CURRENT PROTECTION:
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Differential protection is uneconomical for power transformer below 5MVA.In such
cases over current protection is employed as main protection against phase faults. For
transformers above 5MVA over current protection is used in addition to differential
protection because the latest can not respond to through faults and if this through faults
persists for longer duration it creates stresses in the transformer. For small distribution
transformers over current protection is provided by means of fuse on HV side.
HARMONIC RESTRAINT:
In this method the predominant harmonic currents present in the inrush current are
filtered out from the operating coil circuit by means of tuning and utilize for applying a
blocking feature to the differential current relays at the time of transformer energization.Its
limitations are the danger of relay failure during internal faults when harmonic components
and dc of sets could also be generated due to CT saturation arcing at the point of fault.
3.7 RESTRICTED OVER CURRENT AND EARTH FAULT PROTECTION:
Over current and earth fault protection is provided as main protection for medium
transformers where differential protection is not provided.
Differential protection is generally uneconomical for the power a transformer below 1
M.V.A. in such case over current protection is employed as main protection against phase
faults. For the transformer above 1 M.V.A., if differential protection is used as main
protection over current protection is used in addition as backup for sustained through faults.
Earth fault protection is provided in addition to phase fault protection.
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Fig 3.7.1 Combined Restricted & Earth fault protection
3.8 COMBINED EARTH FAULT AND PHASE FAULT PROTECTION:
It is convenient to incorporate phase fault and earth fault relay in a combined phase
fault and earth fault protection. The increase in current of phases causes corresponding
increase in respective secondary currents. The secondary current flows through respective
relay unit. Very often only two phase relays are provided instead of three because in case
phase faults current in any at least 2 phases must increase. Hence, two relay units are enough.
The earth fault relay is residually connected.
3.9 RESTRICTED EARTH FAULT PROTECTION:
Earth fault relays connected in residual circuit of line C.T’s give protection against
earth faults on the delta or unearthed star connected winding of transformer. Earth faults on
secondary side are not reflected on the primary side, when primary winding is delta
connected or has unearthed star point. In such cases an earth fault relay connected in residual
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circuit of 3 C.T’.s on primary side operates on internal earth faults on secondary side do not
produce zero sequence currents on primary side.
Restricted earth fault protection may be use high speed tripping for faults on star
connected earthed secondary winding of power transformer protection.
When fault occurs very near to the neutral point of the transformer the voltage
available for driving earth fault current is small. Hence faults current would be low. If the
relay is to sense such faults, it has too sensitive and would therefore, operative spurious
signals, external faults and switching surges. Hence the practice is to set the relay such that it
operates for earth fault current of the order of the 15% of the rated winding current. Such
setting protects restricted portion of the winding.
Fig 3.9.1: Restricted earth fault protection scheme
OVER LOAD PROTECTION:The permissible over load and their duration depends upon the type of cooling and
insulation class of transformer. Higher over loads are permissible for a shorter duration.
Permissible duration of over Load
Over load % : 125 150 175 200 300
Duration : 125 45 15 10 1
(In minutes)
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Hence for substation transformers over load protection is generally arranged to
initiate alarm in unattended stations, over load protection is arranged to trip the breaker after
request time delay.
The transformer with utility equipment is prone to sudden over loads. (Furnace
transformer, Motor transformer). The over load protection for such transformer is also given
the requisite time delay. While selecting the over current protections of transformer, the
following aspects need consideration.
Magnetizing current inrush: inverse relays are not affected by the current inrush as they
have enough time lag. Instantaneous over current relays should high set to avoid mal
operation. The fault current on primary side and secondary side of power transformer are
different for phase-phase faults. Lower value should be selected for setting over current
relays. Primary full load current should be considered while setting the over current relay.
The setting of inverse over current relay is generally 125% of transformer rating to
take care of normal over loads. Enough time delay should be provided as for the application.
The setting of instantaneous over current relay on primary side should be more than
asymmetrical value of fault current for 3 phase fault on secondary side of transformer. This
setting is generally adequate to take care of magnetizing current inrush.
Same set of current transformers should not be used for differential protection and
over current protection.
THERMAL OVER HEATING PROTECTION OF LARGE TRANSORMERS:
Thermo couples or resistor temperature detectors are kept near each winding. These
are connected to a bridge circuit. When temperature increases above safe value, an alarm is
sounded. If measures are not taken, the circuit breaker is tripped after a certain temperature.
Some typical settings for oil temperatures are as follows
Switch of fans : 60oc
Alarm : 95oc
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Trip : 120oc
Oil temperature indicated by a thermometer. An oil thermometer, which is similar for
all oil filled transformer, can be consider as a partially effective protective device when
equipped with alarm contacts connected to give remote warning of abnormally high
temperature. Its location is such that it naturally monitors the hottest fluid that exists in the
transformers. The same thermometer is often used to start fan motors on transformers
equipped with automatic air blast to increase the name plate KVA rating.
Alarm contacts used in conjunction with an oil thermometer are adjustable but are
typically set in a sequence that brings fans at liquid temperature of 600c and actuate a switch
contact should the temperature reach 90oc. For a typical design at 300c ambient, the fans are
brought in to operation at about 90 percent rated load where as the alarm is given at about
130% rated load. These percentages will vary for each design and are dependent upon the
actual ambient above 30oc and higher at ambient under 30oc.Switches are usually capable of
readjustment through a range of 10oc.
HOT SPOT THERMOMETER (Winding Temperature Device):
The thermometer bulb is located in a pocket near the winding. The bulb is also heated
by a small heater connected across CT secondary. There by the heat given to the bulb is a
function of load current as well as the temperature of oil near winding. The device is matched
with heating curve of the transformer winding.
The reading of hot spot thermometer is related to actual thermal condition of
transformer than that of oil temperature indicator
LEAKAGGE –TO-FRAME PROTECTION:
Leakage-to –frame protection is a very simple system suitable for small power
transformers .It consists of a current transformer slipped over the earthen-connection of the
power transformer casing with single-pole over current relay connected directly across the
secondary through a setting –resistor. The power –transformer is mounted on concrete are
similar base so that it is lightly insulated from earth .an earth fault in either winding of the
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power transformer causes current to flow through the main earthling connection ,thus
energizing the current transformer and operating the relay
Fig 3.9.2 frame leakage protection scheme
OVER-FLUXING PROTECTION (High magnetic flux protection):
Increase in power frequency voltage causes increase in working magnetic flux, there
by increase the iron loss and magnetizing current. The core and core bolts get heated and the
lamination insulation is effected. Over fluxing protection is provided for generator-
transformers and feeder transformers where it is a possibility of over-fluxing due to sustained
over voltage. Over-fluxing causes over heating of core and insulation failure.
The resistance and capacitance are connected to secondary of VT. The voltage drop
across the resistance is a function of V/F, where V is the line to earth voltage and F is the
frequency. This voltage is fed to the volts ‘per hertz’ relay.
The magnetic flux in the transformer core is a function of V/F, hence the relay senses
magnetic flux condition. Over fluxing relay is provided with enough time lag.
The flux density (B) in the transformer core is proportional to the volts/HZ of supply
voltage, i.e. B is proportional to V/F.
SAFETY DEVICES AND FITMENTS WITH POWER TRANSFORMERS:
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The electrical protection systems can sense the abnormal condition by measuring
current/voltage. Besides electrical relays, a power transformer can be provided with the
following safety and monitoring devices.
1) Fluid level gauge
2) Vacuum gauge
3) Pressure / Vacuum switch
4) Sudden Pressure Relay
5) Pressure Relief Value.
6) Fluid Temperature Indicator.
7) Hot Spot Temperature Indicator
8) Gas Temperature Indicator
9) Combustible limit relays
10) Conservator
11) Breather.
PROTECTION OF TRANSFORMER IN PARALLEL:
The following protections are necessary in case of transformers operating in parallel.
1) Over current protection
2) Earth-fault protection
3) Direction over current and directional earth fault relays on secondary side to prevent
the healthy section feeding in to faulty section.
The feedback is prevented by operation of directional over current relay of faster setting. By
operation of this directional over current relay, the corresponding C.B is quickly tripped and
the feed back from the healthy section is prevented. The current coils of O.C relay and o.c.
relay on secondary side may be connected in series.
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PROTECTION OF GROUNDING TRANSFORMER:
The C.T secondary is data connected. An over current relay with time lag is inserted
in the delta. The zero sequence currents circulate in this delta. The time settings of this relay
are selected to coordinate with thermal rating of the earthing resistor (if used) or with time
setting of other fault relays. The earthing transformer is disconnected by opening the circuit
breaker, on persistent earth fault.
The other three relays provide protection against faults in the grounding transformer.
The job instantaneous relays set between 25-50% of continuous current rating of grounding
transformer. Buchholz relay is also is used. Earth fault protection is provided by residually
connected relay.
LOW OIL LEVEL-FLUID LEVEL GUAGE:
Low oil level is a harmful condition because internal insulation clearance, creepages
etc. between loads, bushing and tanks are left in air when the oil drops below the specified
level. Low oil could result from 1) initial mistake to full sufficient oil up to the 2) leakage of
the oil through the tank.
If the cooling tubes are partially cooled or nearly at ambient temperature, it is an
indication that the oil is not circulating in the cooling tubes and oil level has dropped below
the desired level. The cooling tubes are warned and level indicator gives an alarm, it may be
a false alarm and level indicator needs checking. Its position may be improper.
The level indicator has a float ended arm. The float is suspended in the oil. When the
oil level drops down, the float tilts the arm there by closing the alarm contacts. Both low and
high level alarm contacts are provided.
THE DELAY RELAYS:
Here an intentional time delay 5 to 8 on 50 Hz basis is introduced in the relay
operating time to over side the short time inrush current. This scheme while being simple
can’t be generally recommended for the large transformers as the time delay can result in
severe damage to the transformer during internal faults.
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UNDER VOLTAGE RELAY ACROSS THE RELAY OPERATING COIL:
The under voltage relay contacts which are closed when the transformer is de
energized, open out after the transformer is reconnected to the supply. A tripping suppressor
device is connected in the circuit for the relay to operate when there is a fault while
energizing. For this the under voltage contacts are connected in series with differential relay
operating contacts.
TRIPPING SUPPRESSOR DEVICE:
Here the under-voltage relay contacts are connected in series with the differential
relay operating contacts and they open out if the transformer is healthy at the time of
energeization. The main limitation with the scheme is the possible delay by the timer for the
10 current internal faults which affects the voltage, but slightly.
PRESSURE RELIEF AND PRESSURE RELAY:
This is different from rate of rise of pressure relay. Pressure relay and pressure relief
device is mounted on transformer tank. It releases gas pressure to the atmospheric during
1) High overload peaks
2) Prolonged overloads
3) Arcing faults within oil.
The pressure relief valve is spring loaded and has a seal-seat. When the pressure is
inside the tank increases above a certain value, the force on movable sub-assembly exceeds
the spring force and the valve operates. The alarm contacts are closed. After release of
pressure the valve may be manually reset.
RATE-OF-RISE PRESSURE RELAY:
Rate of rise pressure relay doesn’t respond to static pressure. It responds only to rate
of rise pressure resulting from internal arcing. The main pressure sensing element is a
pressure actuated micro-switch mounted inside a metallic bellow. Static pressure doesn’t
squeeze the bellow. The dynamic pressure squeezes the bellow and operates the micro-
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switch. In some designs, oil pressure itself squeezes the bellow filled with special oil. Rate of
rise of pressure relay is generally arranged to trip the transformer. It can mount on the tank.
Abnormal condition Protection RemarksIncipient fault below oil level resulting in decomposition of oil, faults between phases & earth.
Buchholtz relay sounds alarm (gas actuated relay)
Buchholtz relay used for transformers of rating 500 KVA and above.
Large internal faults:Phase to phasePhase to ground,Below oil level
Buchholtz relay Trips the circuit Breaker
Buchholtz relay slowAnd less sensitive.Buchholtz relay for tap change also
Fault in tap changer 1)percentage differential protection2) High speed high set over current relay.
Percentage differential protection used for transformers of rating above 5 MVA
Saturation of Magnetic circuit 1)over fluxing protection2)over voltage protection
For important generator transformer with bus bar protection
Earth faults 1) differential protection
2) Earth fault relay.
For transformers of and above 5 MVAa)instantaneous restricted E.F.Relayb) Time lag E.F.relay
Through faults (external faults)
1) grade time lag over current relay
2) 2) HRC fuses
Protection of distribution transformers
Small distribution transformers up to 500 KVA
Over loads 1) thermal over load relays
2) Temperature relay
Generally temperature indicators are provided on the transformersTemperature increase is indicated on control board also. Fans started at certain temperature
High voltage surges due to lighting, switching
1) Horn Gaps Not favored for important transformers.
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Lightning arresters
In addition to LA for incoming lines.
Protection of Generator Transformer Together
Generator- Transformer over all differential Protection
Generator protection Generator differential protection Stator earth fault protection
Negative phase sequence Protection Against unbalanced loading
Interturn faultReverse power protectionField Failure Protection.Rotor earth fault protectionTemperature sensors in slotsOver current relays in stator and rotor circuits Lightening arresters generator over voltage protection
Protection of Unit Auxiliary Transformer
Differential protectionRestricted earth Fault protectionBuchooltz RelayOver current protectionWinding and oil temperature sensorsOver flux protection
Protection of main transformer
H.V over current protectionH.V Restricted Earth Fault protectionBuchholtz RelayWinding and oil temperature sensorsLightening arresters on H.V.sideOver fluxing protection
Preventive measures- Sound alarm on Control panel
Continuous monitoring of outlet temperature of gaseous of liquid coolantsFlow monitorsLow boiler Pressure alarm /tripLubrication oil failureEmergency oil failureEmergency tripLow vacuum
4. MOTOR PROTECTION
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Every thermal power station has a number of motors to perform various functions
for running of the plant. If the motors are left unprotected it leads to loss of generation. So
the motor in the plant also should be protected from faults. Generally induction motors are
used. The induction motor rating in a thermal plant of 210MW capacities starts from few KW
to 40000KW.protection of the induction motors depends not only upon the rating and voltage
but also others such as power rating, speed, motor thermal rating, and type of load.
The two basic protections provided for every motor are:
1) Thermal Overload Protection
2) Short Circuit Protection
Switch gear used for motor protection are:
1) Contactor starters with HRC fuses and thermal over current relays-for small motors
below 150HP
2) Circuit breakers and associated relays for large motors
ABNORMAL CONDITIONS:
Typically the following abnormal conditions may be observed in an induction
motors:
1) Overloads: sustained or momentary overload
2) Supply failure including loss of all the 3-phase or 1-phase
3) Motor internal faults: phase to phase faults, phase to earth faults or open
Circuit faults.
4) Starting failures: prolonged starting of the motor
5) Rotor failures: blocked rotor
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1. EXTERNAL FAULTS FROM SUPPLY SYSTEM :
A) Unbalanced supply voltage
B) Under voltage
C) Single phasing
D) Reversed phase sequence and loss of synchronism
2) INTERNAL FAULTS:
A) Stator winding faults
B) Motor earth faults
C) Winding failure due to over loads due to the faults in the driven equipment.
D) Bearing failures.
E) Loss of synchronism due to over loads in case of synchronous motors.
F) Rotor earth faults in case of synchronous motors.
4.1 PROTECTION OF SMALL MOTORS:
Up to 30 H.P. rated motors, the motors are protected by H.R.C fuses, over current
trips or bimetallic thermal relays and under voltage relays are in-corporate in the stator
contactor circuits.
In the system single phasing is the worst situation during which the protection
arrangement fails to recognize the situation. Since the substantial back emf is available on the
faulted phase terminal to prevent the dropping of the voltage relay.
The motor is connected to 3-phase supply through fuse, isolating switch, themal relay
and contactor. When the contactor is closed through the closing circuit the motor gets 3-
phase supply and motor starts.
During overloads the thermal relay operates and there by control circuit is
disconnected and the contactor opens the contacts. There by supply to the motor cut off.
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HRC fuses provided repaid short circuit protection. Current is cut off even before it reaches
peak values.
Fig 4.1.1 protective scheme of small motor
4.2 PROTECTION OF LARGE MOTORS:
Bearings:
Ball and Roller grease and lubricated bearings are used to the motors of about 500
K.W. range. Sleeve bearings with oil lubricators are used for motors above 500K.W.range
Failure of ball or roller bearings results in over loading of the motor due to motor coming to a
stand still position on account of enormous friction in the bearings. The protection of the
motor can save the motor from destruction. The protection of bearings is not possible to this
type of protection. With special temperature detectors with the facility of measuring the rate
of rise in temperature, the bearings can be protected.
OVER HEATING OF MOTOR WINDINGS:
Over heating of winding can occur on mechanical over loading, unbalance supply
voltages and will be very severe during the operation of single phasing. The life of the
insulation drastically gets reduced i.e., reduced to 50 % of its life for every 8oc rise in
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temperature. Motor should not be allowed to operate above the 40 – 50 minutes, if it is over loaded up
to 110 %. Hot starts should be avoided unless the motor is specially designed to withstand
restarts when hot.
The protection scheme should also envisage against the following:
1) Stator and rotor protection against winding faults.
2) Reverse rotation.
3) Pull out protection in case of synchronous motors.
4) Field over load thermal protection.
5) Protection against sudden restoration of supply.
6) Under power and reverse power protection.
7) Over voltage and under frequency protection.
STALLING PROTECTION:
Motors can be protected against stalling of rotors either by thermal over load relays or
separate definite time over current relay.
LOSS OF SUPPLY:
When the supply is removed from and induction motor, its back emf will
decay exponentially and disappear in a few seconds. During that time interval there is a
decrease in speed so that the phase of back emf moves away from the position which
occupied before the supply of removal of the supply. Result is that the lower of the back emf
traces as spiral.
If the supply voltage is restored before 0.4 sec in 20 cycles, then the voltage
applied to the motor would be less than the system voltage because of the back emf and the
current would be less than the short circuit current. After 0.4 sec the voltage between the
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applied voltage and back emf is greater than the applied voltage and the short circuit current
would be correspondingly greater.
If the voltage was restored after 0.8 sec the short circuit would be one and half
times normal. This means, the mechanical forces exerted on the rotor would be over twice
the normal starting force and could damage the rotor structure.
For this reason under voltage relays are employed in large machines and are
disconnected when the loss of voltage exceeds say 0.8 seconds. By using either attracted
armature type relay with time delay features of induction type relay.
During the system fault, there is a loss of supply to all motors and the motors
will be contributing current to the fault and the back e.m.f. decays fast, less than 0.5 seconds.
4.3 OVERLOAD PROTECTION OF MOTORS:
The overload protection devices use over current sensing devices namely bimetal
relays, electromagnetic relays and static relays.
1) Overloads
2) Single phasing
3) Continuous overloads
The following conditions are sensed by embedded thermometers, thermostats etc.
1) Temperature due to high ambient temperature
2) Failure of cooling medium
The overload protection protects the motor against the overheating when running on load.
The principle is based on the fact that a thermal model of the motor is created in the relay, by
routing in a current that is proportional to the square of the load current of the motor.
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4.4 THERMAL OVERLOADPROTECTION:
The purpose of thermal overload protection is to protect the motor insulation from
excessive thermal stresses. During abnormal conditions the temperature of the winding
reaches excess of the safe limit and the life of insulation reduced. The embedded thermistors
give alarm when temperature of winding exceeds.
4.5 PROTECTION AGAINST UNBALANCE:
Unbalanced voltage by itself may not be harmful but the negative sequence currents
caused by it results in rotating magnetic field revolving in opposite direction. this field
induces double frequency current in the rotor body and conducts giving rise to heat due to
copper losses.
The unbalanced protection provided should prevent prolonged unbalanced conditions
but should not disconnect the motor for permissible unbalance of short duration which
depends on % of unbalance.
The unbalanced voltage protection an be based upon the following
1) Bimetallic relay for faster trip
2) phase unbalance relay
For small motors single phase prevention is provided and unbalance d current relays
are provided. The secondary currents of CTs are fed to negative phase sequence filter. the
output of the negative phase sequence filter is fed to an over current relay unit of a static
detector. The setting is passed on Z1/Z3 ratio and also permissible time for percentage
unbalance of the supply voltage.
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4.6 PHASE TO PHASE SHORT CIRCUIT PROTECTION:
Phase to phase faults short circuit in stator winding causes burn out of coils and
stampings. Hence the motor should be disconnected from supply very quickly. Fast over
current relays are provided for phase to phase short circuit protection.
The relays giving short circuit protection should not act during starting. The
setting of instantaneous over current relays should not be set below the starting current. The
setting of instantaneous over current relays should not be set below the starting current. The
setting should be just above the motor starting current.
For large motors such as boiler feed pump motors biased differential protection is
provided. It is most sensitive and quick protection for phase faults.
SYNCHRONOUS MOTORS:
The protection for synchronous motors i.e., the same as that for induction motors,but
with the addition of relay to detect the loss of synchronism and loss of supply.
Pole slipping occurs due to sudden severe over-loads and stator current increases and
the power factor decreases. During this time, out of slip relay will detect and has to trip out
the motor during the slip cycle.
The supply to synchronous motor is interrupted for more than say 0.3 sec, then there
is a danger that the supply is restored the motor may be out of step and therefore an under
voltage relay is required to the machine. Other protective devices are under-power and
reverse-power relays.
Instantaneous positive sequence (I1):
This unit gives protection against motor terminal faults and has a setting range of 6 to
12 times of the normal current.
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The setting is made by adjusting knurled knob against a calibrated scale.
Normally, this setting is kept at “6” assuming the motor starting current to be ‘5’
times the rates current. So this relay will not act for starting current.
Negative sequence unit (I2):
This unit gives protection against single phasing i.e., (single phase stalling) and has a
pick up setting range of 30 to 630 with time setting of 0.06 to 7.56 seconds.
A setting of half the normal 3 phase starting current will protect the motor against
single phase stalling and initial setting of one third of the starting current is recommended.
These settings may be limited by the following considerations.
A: Unbalanced in C. T. secondary currents due to saturation during motor
starting.
B: External unbalanced faults which may permit the motor to act as generator and
feed negative sequence current into the faults.
Earth fault unit (Io):
This unit gives protection against Earth Faults. The `Io` unit can be residually
connected or through a C.B.C.T. in case of 415 volts motors it is residually connected and for
6.6. K.V. motors, it is through C.B.C.T.
The element is provided with an external stabilizing resistor 0-27 ohm for 5 amps
rated relay and 5-500 ohms for 1 amp rated relay. When the element is connected to a
C.B.C.T. the pick up setting should be selected to suit the CBCT and the primary operation
current.
The current range is from 10 m. Amps to 160 m. Amps. For 1Amp C.T. and 50 m.A
to 800 m.A for 5 A C.T. Timers setting is 0.06 to 7.56 seconds. Since our 415 v system and
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6.6K.V. systems are unearthed systems, Io element is not connected to trip, but only to alarm
and the current setting is set at 20m. A.
Time delayed positive sequence unit I1(t):
This unit gives protection against stalling. A part of the output from the filter taken
for I1 is taken through a timer for obtaining time delay and given to the relay element I1 (t).
The current setting ranges from 1.5 to 6 times normal current. Timer setting ranges
from 0 to 60 seconds.
Normally the current is set at 1.5 times and the timer setting is kept depending upon
the motor to with stand the load characteristic. There is a mode switch either to include or
bypass the relay for some time or permanently.
The two positions of the switch are permanent and controlled. In the permanent
mode, the stailing protection is always service while in the controlled mode the stailing
protection is primed by an external device like a speed switch. The primary is done by a read
relay RR1 mounted inside the relay and the positive of the d.c voltage is extended to the
Read Relay through the contacts of the speed switch.
Controlled mode operation is generally used when the staling time characteristic of
the motor faults with the starting characteristics.
Thermal Unit (I th):
Pre-Trip Alarm Unit: A Pre Trip Alarm unit is provided on the relay which can be set
by rotary switch provided on the front panel. This unit is I th (a) can be set at 70 to 100% of
the Thermal Trip Setting.
An extreme position marked 0.1 can be used to prelude this facility. This unit has a
fixed time delay of 25, 50 or 75 seconds.
When the current in the motor exceeds the sorting on the rotary switch, the timing is
started and after the set time elapses a Read Relay is energized which can be used to initiate
alarm. This also lights up the pre-trip alarm L.E.D. (Light Emitting Diode)
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Thermal Setting:
Thermal Setting on the relay is provided by two potentiometers which are calibrat6ed
from 0.7 to 1.3 I. Both the potentiometers should be kept at the same position for the correct
operation of the relay.
1 : 4 sec
1a : 6 sec
2 : 8 sec
2a : 12 sec
3 : 16 sec
3a : 24 sec
There will be three characteristics in each relay and depending upon the motor
characteristics the curve can be chosen. Thermal reset push button is provided to reset the
thermal element. Thermal status is retained by the relay even with auxiliary supply failure for
several minutes.
The relay characteristic can be matched to a wide range of motor ratings, the relay
setting current is adjustable by means of two potentiometers. The thermal units begin to
operate when the current exceeds 105% of the setting current.
Ith setting Amps = I min × motor full load current × C.T.seconds (A)
I min is effective pick up current required.
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I min as % of motor full load current.
C.M.R motors 100 % full load.
Totally enclosed motors: 110 %
Open type motors: 125%
TESTING OF MOTOR PROTECTION RELAY:
1) UNDER Single phase conditions positive sequence:
I1 =negative sequence= I2
Line current IL = sqrt (3* I1) = sqrt (3* I2)
Thus for testing instantaneous I1 and I2 units the line current must be 3 times the required
unit currents.
2) The thermal unit equivalent operating current
I eq sqrt (sq (I1) + 6*sq (I2))
This reduces to sqrt (7/3)* IL
For an I eq = 5*setting current =sqrt (7/3)* I L
Or I L =sqrt (7/3) *5* I s
5. MODERN TRENDS IN TRANSFORMER
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PROTECTION
Micro-Processor Based Relays:
The increased growth of power systems both in size and complexity has brought
about the need for the fast and reliable relays to protect major equipment and to maintain
system stabilility. The electro magnetic relays has several drawbacks such as high burden on
the instrument transformers, high operating time, contact problems etc. though successfully
used the static relays suffer from a no. of disadvantages such as inflexibility, inadaptability to
changing the system conditions and complexity.
The concept of digital protection employing computers which show much promise in
providing improved performance, has evolved during past to decades. Digital computers can
easily fulfill the protection requirements of modern power system with out difficulties. But
their cost is 15 to 20 times more than that of conventional protective relaying schemes. The
cost of protective scheme should be about of 1% of the cost of the equipment to be protected.
The main features which have encouraged the design and development of micro
processor based relays there are economic compactness, reliability, flexibility and improved
performance over conventional relays. A no. of desired relaying characteristics such as over
current, directional, impedance, mho quadrilateral, elliptical etc can be obtained using the
same interface. Digital relays are user-friendly.
The primary protection for the A.C generator shall be an integrated digital protection
system including protection functions such control monitoring, diagnostic and
communication capabilities. A high degree of dependability and security shall be provided by
extensive self diagnostic routines and an optical redundant D.C supply. The protection
function shall operate over the range of 31-79 Hz with same accuracy at normal system
frequency.
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Digital system Electro Mechanical System
Comments
100 % Stator Protection
Complete over Excitation Protection
Unbalanced Armature currents Protection
Loss of excitation (two zones)
Reverse Power
IAV
Two set point over excitation protection
INC77
Generally one zone of protection
Lack of sensitivity for some applications
IAV protects 90-95% choice of 27TN and 64G in digital system for 100% stator ground protection.
Digital system better coordinates with transformer and generator capability curves.
More sensitive protection for negative sequence current condition.
Possibility of false trip with one zone protection during power swing.
Digital system offers better sensitivity.
Advantages of Micro Processor Relay:
Ability to combine a large no of protective and monitoring functions in single relay
unit.
Measured values of variables are processed digitally by micro processor.
High level of flexibility.
Increased reliability due to self checking.
User friendly yet higher capable.
High speed.
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6. CONCLUSION:
With the increase of demand day by day, the generation also increased. With the Increase of generation, the system stability has become a main problem. In order to maintain
The system stability and reliability, various protective schemes were introduced.
So, we have observed various protective schemes for major electrical equipment like
Transformer, Motor for reliable operation and survival of the equipment at VTPS.
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7. References & Bibliography:
www.googlesearch.com pictures
www.wikipedia.com history and briefing
B.L. Theraja theory
J.B.Gupta theory C.L.Wadwa theory
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