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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100 × 1.05 × C.T.Primary Current (Amps)

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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