DC Machines Question and answers

DC Machines

Conversion of one form of energy into another enables us to use natural power
sources as well as manufactured power sources to produce our electrical power
supply. Although electricity can be produced by friction, pressure, heat, light,
chemical action and magnetism, the most common method used by large power
producers is magnetism.

• What is Electric Generator?

Electric generators are called a dynamo that converts mechanical energy into
electrical energy. A dynamo consists of two basic parts- the stationary part and the
rotating part.

• How electromotive force is created in a generator?

When a conductor cuts the magnetic lines of forces, an Electro motive force (emf) is
generated.
The magnitude of the generated voltage is directly proportional to the rate of change
at which a conductor cuts the magnetic lines of force.

• What is DC motor?

An electric motor converts electrical energy in to mechanical energy.

• How many types of DC motors are there?

DC shunt motor: shunt motor speed varies slightly from no load to full load.
DC series motor: series motor speed varies greatly as load changes.
DC compound motor: the compound motor contains both a shunt field and a series
field and therefore has characteristics between the shunt and the series motors. This
motor has the good starting torque characteristics provided by the series field, while
the shunt field provides for a relatively constant speed.

Maintenance & Troubleshooting

• Troubleshooting is a field if repair work that usually tells how well the student
has learned the lessons. The principles involved in control functions, components
and circuit analysis, along with the basic laws of electricity.

• Your best tool when troubleshooting is your ability of think. Don't jump to
conclusions. Have confidence in your ability. Learn how the equipment in your
area is supposed to operate both electrically and mechanically.

• Observe all plant rules and regulations. Electricity can be dangerous. In addition
to the hazards of electrical shock and electrocution, burns from an electrical flash
can be devastating. Be careful when opening the circuit. The inductive kick that
can occur when a circuit opens produces a voltage that is many times the voltage
applied to the system.

• No matter how complex or expensive an electrical control system is, the
components of the system begin to deteriorate as soon as they are installed and
failure of some components in the system will ultimately result.

• Blown fuses, overload contacts, open contacts, short circuits, burned out coils and
grounds are responsible for most electrical circuit failures.

• Troubleshooting can be generalized in 3 steps:
1) Determine the symptoms; that is, find out how it acts. (When equipment is
operating properly, you should find out how it is supposed to function.)
2) Decide by logical reasoning what might be wrong. (Try to isolate the problem
to a section of the control.)
3) Determine what has to be done to correct the problem.

• If we are troubleshooting an existing circuit, one that has been in service and
operated properly, we can eliminate the possibility of fault installations or design.

• The first step- determine the symptoms- can best be accomplished by working
with the machine operator and following the machine through its sequence to the
point of failure.

• Remember that no matter how complex, control circuit are made up of only two
things. Contacts that open and close a circuit and coils that operate the contacts,
keeping in mind the control voltage.

• Probably the single most important rule in trouble shooting is to remember to
change only one thing at a time.

• Remember the operator knows the machine operation and can be an asset to you
in your troubleshooting. Question the operator but don't challenge his operating
ability.


• Anyone attempting to troubleshoot without a drawing and a meter is wasting the
time.

• Instead of random checking the circuit; start from the source to the machine or
from the machine to the source.

• Finally take time to think.

Radiation Protection
Fission reaction
92U235
0n1
54Xe144 + 38Sr90 + neutron + radiation + Energy
Tritium formation
1H2
0n1
1H3
1 Seivert = 100 Rem
Annual Dose Limit (ADL) = 20 mSev or 2 Rem for Employees.
Annual Dose Limit (ADL) = 1 5 mSev or 1.5 Rem for Contractor.
Annual Dose Limit (ADL) = 1 mSev or 100 mRem for Public.
5 Years = 100 mSev or 10 Rem
DAC (Derived Air concentration)
>10 DAC use tritium bottles
10-15 DAC use airline
>50 DAC use ventilated plastic suite (VP suite)
1 DAC for 1 hour = 0.01 mSev or 1 mRem.

• Why no entry for Moderator room & Pump room during operating condition?
Due to the presence of N16 & O17, which are high gamma emitter, their field is
around 7 mev.

• What are the gases discharged to the stack?

Argon-41, Tritium, fission products, noble gases & Iodine particulates.

• What are the emergencies provided in the plant?

Plant Emergency: Excessive release of radioactive material or high radiation fields in
a section of the plant
Site Emergency: Uncontrolled release of radioactive material or high radiation fields
with in the site boundary
Off- Site Emergency: High release of radioactive material from the plant resulting in
significantly increased radiation fields and/or contamination levels extending to
areas outside the site
Emergency Planning Zones (EPZ): Emergency planning zone, defined around the
plant up to 16 km, provides a basic geographic frame work for decision making
on implementing measures as part of a graded response in the event of an
emergency. The area around the Kaiga generating station is divided into the
following Zones up to 16 km radius.
Exclusion Zone: The exclusion Zone extends up to a distance of 1.6 km around the
central plant zone of 0.7 km where no public habitation is permitted. This zone is
physically isolated from out side areas by plant fencing and is under the control of
Kaiga Generating Station.
Sterilised Zone: Sterilised zone is an area where no new growth of population is
permitted. Natural growth is however allowed in this Zone. This are extends up to
a radius of 5 km from the central plant Zone. This Zone is defined to restrict the
population to an easily transportable number in case of an emergency.
Primary Zone: The primary Zone extends up to 8 km from central part Zone where
protective measures like evacuation and sheltering are required against possible
plume exposures during an Emergency.
Secondary Zone: The secondary Zone extends up to 16 km from central plant Zone
protective measures like sheltering control on food stuff are required against
possible exposure from ingestion of radioactivity.


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CLASSIFICATION OF EMERGENCIES Question and answers

CLASSIFICATION OF EMERGENCIES

Emergencies are classified on the basis of the nature and severity of the incident. The
effect of the emergency may be restricted either to a small area of the plant or a few
individuals or it may pose damage to the installation staff. Emergencies of more
severe nature could result in unacceptably enhanced release of radioactive materials
or toxic/noxious substance from the plant of resulting in hazard in the surrounding
public domain. Accordingly the emergencies are classified into:

1. Plant emergency
2. Site emergency
3. Off-site emergency

Plant Emergency
This type of emergency is classified in to
a) Personal emergency
b) Emergency Alert
c) plant emergency

Personal Emergency: This involves accidents or incidents in any of the plant areas,
which call for emergency treatment of personal. The situation may result from
high radiation exposure or significant contamination or abnormal intake of
radioactivity by personal. The examples of personal emergencies are listed in

Annexure-I.
Emergency Alert/Emergency Standby: This involves abnormal conditions, which
have a potential to proliferate in to a more serious situation but still provide time
for pre-cautionary and constructive steps to prevent an emergency situation or
migrate its consequences. The examples of emergency Alert are listed in

Annexure-II.
Plant emergency This involves excessive release of radioactive materials or high
radiation fields in a section of the plant requiring operator action and/or automatic
operation of the safety system. Although positive isolation or restriction on
occupancy of the affected areas might be enforced, evacuation of personal might
be required if it is suspected that the doses to personal or likely to exceed the
intervention levels. The examples of plant emergency conditions or listed in

Annexure-III.
Site Emergency
This class of emergency arises due to situation, which seriously affect plant
operation involving high radiation fields in accessible areas and release of
radioactive materials extending beyond the plant up to the site environment. The
protective measures such as incorporation of stable Iodine, sheltering and evacuation
of personal from plant areas other than control room to areas designated to be
habitable under the site emergency conditions and evacuation of non-essential
persons from the site may be considered. The examples of site-emergency condition
are listed in

Annexure –IV.

Off-site Emergency
An Off site emergency situation results when the release of radioactive materials
from the plant is of a magnitude necessitating protective action to be taken for
members of the public in the neighborhood of the plant.

EMERGENCY DECLARATION AND NOTIFICATIONS:

Declaration of Emergencies shall be made by the Duty SCE / PED based on the
information from the plant or as per the advice from Kaiga emergency Committee
(KGEC).
Declaration of Emergency: Siren will be sounded as described below for declaring
emergency. Following the Siren, there should be an announcement.
Siren: Short intermittent siren 5 seconds on, 5 seconds off for a period of two
minutes.
Emergency Announcement:
The announcement shall be made as follows;

"ATTENTION ALL PERSONNEL - THERE IS PLANT EMERGENCY"
THE INCIDENT AREA IS …………………………………….
THE ASSEMBLY AREA IS ……………………………………
THE EMERGENCY CONTROL CENTRE IS……………...…..
PERSONS PRESENT AT …………… SHOULD AVOID GOING TO ………...
This announcement shall be repeated thrice in English, Hindi and Kannada.
Evacuation: Evacuation if necessary will be made by announcement on Public
Address (PA) system.
Termination of Emergency: A continuous Siren is sounded for 2 minutes. Following
the emergency Siren, there shall be an announcement in English, Hindi and Kannada
on public address system terminating the emergency.
Notification Codes:
The messages for notification of start/termination of on site and off-site emergencies
are indicated as follows. These should be disseminated to various agencies. The
codes for notification of commencement or termination of various types of
emergencies are:

a) External radiation exposure (mSv) DAC-hr (HTO) DAC-hr(I-131)
DAC-hr (I-131) ------(≤ 1)
(For meeting iodine thyroid dose limit of 50 mSv)
The explanatory notes for these guidelines are given in

Annexure-IX.
Countermeasures during a radiation emergency: Following countermeasures have
been identified for control of exposures during a radiological emergency within the
plant site areas and in the public domain.
1. Sheltering
2. Administration of Stable Iodine
3. Evacuation.
4. Relocation.
5. Control of Access.
6. Control of Food and Water
7. Decontamination of Affected Areas and Buildings.
DOMAIN:
Domain 1 = 0.1 mSv/hr
Domain 2 = 0.01 mSv/hr
Domain 3 = less than 0.01 mSv/hr
Stochastic and Deterministic effects.
Stochastic effects: Stochastic effects are those for which the probability of an effect
occurring, rather than its severity, is regarded as a function of dose, without
threshold. Example: Cancer.
Deterministic effects: Deterministic effects are those for which the severity of the
effect varies with the dose, and for which a threshold may, therefore, occur.
Examples Cataract, permanent or temporary sterility.
Practices: Any human activity, which increases the overall exposure to radiation, is
a "Practice" such as operation of nuclear power stations.
Intervention: Any human action intended to reduce or avert exposures to sources
which are not part of controlled practices or which are out of control as a
consequence of an accident is "Intervention".
Objectives of Radiation Protection: Prevent deterministic effects and to limit the
stochastic effects to levels deemed to be acceptable.

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TRANSFORMERS Question and answers

TRANSFORMERS

• Give transformer nameplate details of GT, SUT, UAT, SET, 415 V Aux transformer
and Lighting transformer.

GT SUT UAT SET 415V Aux trans. L Trans.
USI 5210 5120 5220 4120 5242 5231
Make Telk Telk BHEL BHEL EE Square
Automation
STD IS – 2026 IS– 2026
Type WFOC Oil immersed Oil immersed DRY RESIN
CAST DRY RESIN CAST DRY RESIN CAST
Cooling OFWF ONAF / ONAN ONAF / ONAN AN AN AN
VA 260/260 MVA 35/20/20/12 MVA
24.5/14/14 MVA
35/20/20 MVA
24.5/14/14 MVA
3150 kVA 1800/1200 kVA 250 kVA
Volts 235/16.5 kV 220/6.9/6.9/11
kV
16.5/6.9/6.9
Kv
16500/575
V 6600/435 V 415/415V
Amps 639/9098 A 64/1172/440 A
91/1675/629 A
1266/858A
1676/1172 A 157.5/2400 A 630/250,125A
No of φ 3 3 3 3 3 3
Frequency 50 Hz 50 Hz 50 Hz 50 Hz 50 Hz 50 Hz
Impedance 13.13 (14) % 9.75% / 18.82% 10 ±10% HV
22 ±10% LV
Vector YNd11 Yn yno yno
(d1)
D yn1 yn1 Dyn Dyn11 Dyn11
Oil 42000 Lt. 25260 Lt. 19750 Lt.
Tap change Off load ON load HV ON load HV
Tapchange% 10 steps of 2.5 % ϒ12% in 1.5% steps ϒ12% in 1.5% steps

• What is the use of Tertiary winding?

Star connected circuit, which has an isolated neutral there can be no zero sequence
components. Since the zero sequence components are by definition in time phase
with another their sum can not be zero at the junction point as per kirchoff’s law. It
follows that there are limitations upon the phase loading of a bank of transformers
connected in star – star unless the neutral points are connected to the source of power
in such a manner that the zero sequence components of current have a return path or
unless the transformer are provided with tertiary winding.

• What is E/F current limit for SUT and UT?

400a limited by 10 ohms resistor.

• What is the coverage of differential protection for SUT?

Covers from 230 kV bushing to 6.6 kV breaker end.

• What are the advantages of dry type transformer?

No fire hazard.
It can be mounted indoor.


• During unit operation, can we parallel 2 SUT?

No, due to switchgear limitation.


• Why 6.6 kV transformer is resistance grounded by 10 ohms and current limited to
400 A?

a) To reduce burning and melting in faulted switchgear or machine.
b) To reduce mechanical stresses in equipment.
c) To reduce the electrical hazards by stray ground fault currents in the ground
return path.
d) To reduce momentary line voltage dip due to ground fault.
e) The current is limited to 400a, that is ¼ th of the load current to reduce the size of
the screen in 6.6 kV XLPE (cross-linked polyethylene). Therefore the cost of the
cable decreases.

• During unit operation can we have one UT feeding both unit 6.6 kV loads?

No, logically prevented.

• During unit operation, can we parallel UT & SUT continuously?

No, due to switchgear limitation.

• What is the design basis of 6.6 kV aluminium bus bars?

a) Temperature rises not exceed 90 ºC.
b) Withstand short ckt stresses.
c) Take care of thermal expansion.

• Why 2 types of earth fault relays in 6.6 kV side of transformers?

I – Trips 6.6 kV breakers only. It gives primary protection for 6.6 kV bus bars.
I1 – Trips the both HT and LT breakers. It acts as a backup to ref and also acts as
backup to bus bar earthfault relay.

• Why core balance CT is preferred over residual connected CT’s to sense earth fault
in 6.6 kV feeders?

a) To avoid relay mal-operation due to CT saturation
b) Better sensitivity is got.
c) High pickup and TMS avoided in IDMT earth fault relay.

• How selection of cooling fluid in GT done?

a) There are 5 factors are there.
b) Density
c) Coefficient of thermal expansion
d) Viscosity
e) Specific heat
f) Thermal conductivity.

• What are the ranges in which each type is effective?


ONAN – Natural cooling – up to 15 MVA.
ONAF – Air forced radiators cooling – 10 to 100 MVA depending on availability of
area.
OFWF = oil forced and water forced used in more than 100 MVA.

• Why off load tap changer was chosen for GT?

Because our plant works on base load always.

• What are the advantages of OFWF?

Ensure the differential temperature between top and bottom of transformer is
minimum and Effect of ambient air temperature is minimum.

• What is the type of lightning arrestor for GT?

Zn O (zinc oxide) types.

• What is the purpose of header breaker in water circuit?

The header breaker ensures oil pressure greater than that of water pressure always.
Therefore there is no leak of water into oil.

• Why thermosyphon filter required?

To keep required dryness/improve dryness of the transformer insulation, internal part
of transformer. When transformer operates, due to pressure head between top and
bottom small quantity of oil flows through filters (absorbent material activated
alumina grade g-80 removes the moisture from oil). Absorbent material remove
slag, acids, peroxides, ionic impurities from oil, which otherwise accelerate
against of oil. Absorbent unit is reactivated at regular intervals.

• What is the purpose of pronol conservator (KAPP)?

Flexible separator avoids direct contact with atmosphere. Efficient barrier between
oil and air. Ensures the protection against water vapour, suppression of gas bubbles
formation in the oil.

• Why main generator/UT is not provided with separate overfluxing protection?

Since GT is provided with overfluxing protection, it is adequate to protect main
generator / UT also. Main generator can withstand higher degree of overfluxing. If a
generator CB is used, separate overfluxing protection is essential for main generator.

• What is the advantage of Pressure relief device in TELK type GT over explosion
vent of BHEL, even though in both cases oil will be expelled out during sudden
pressure rise?

During internal fault, the internal pressure rise is relieved by the expelling out of oil
through Pressure relief device /explosion vent. However the Pressure relief device
closes back when the pressure drops. Hence the oil exposure to atmosphere is
minimised, thus saving large quantity of costly transformer oil from oxidation and
moisture absorption. Fire hazard due to transformer oil does not exist after the
closure of Pressure relief device.


• To reduce tower-footing resistance, which are better to use a) chemical, b) ground
rods, c) counter poise?

B & C

• Why tap changer is kept at neutral end?

a) To reduce insulation cost of tap changer.
b) But reactance changeover the tap range increases.

• Why guard connection is given for megger?

For true measurement of IR value of HV to earth of a transformer, connect line to
HV, earth to transformer tank and guard to LV. Therefore leakage current from HV
to LV is not included.

• Why lighting isolation transformer is req.?

a) 3 wire to 4 wire conversion, since neutral is required for lighting load.
b) Prevents transfer of E/F currents
c) Reduces the fault level on secondary side and permits use of small sized cables /
CB’s / fuses.

• Why neutrals are solid grounded above 33 kV?

a) Less transient over voltage due to arcing grounds.
b) Voltage of phases are limited to phase to ground voltage. (No neutral shifting)
c) Allows graded insulation of transformer (low cost)
d) Fast E/F protection.

• Why SET is chosen as Dyn 11?

To have smooth commutation in generation in between stator and rotor.

• Why all 415V transformers are chosen Dyn 11? What are the protections provided
for the 415V transformers?

a) To facilitate interchange.
b) To have momentary parallel during changeover.
Protections
a) Door interlock to trip HT and LT breakers.
b) LT breaker can on only after HT breaker is in on position.
c) Instantaneous O/C and inverse O/C (50 + 51).
d) Instantaneous E/F (50N).
e) IDMT E/F and restricted E/F (51N + 64).
f) Winding temp high trip (140°C trip and 130°C alarm)

• What is the instrument name used for thermograph?

Infrared camera.

• Why neutrals are solid grounded below 600v?


Human safety

Permits enough E/F current because ground resistance is large in less than 415v,
hence fast fault clearance,
Equipment safety against over voltage.

• What are the advantages of ungrounded system?

Supply is maintained even with fault on one line
Less interference to communication lines because of absence of zero sequence
currents.

• Why resistance grounding preferred for less than 33 kV and more than 415 V?

a) To limit the earth fault current for equipment safety else, high short ckt forces
dislocate in windings/bus bars etc,
b) Over voltage due to arcing ground reduced
c) Permits earth fault protection (not possible in ungrounded system)

• What is meant by tan-delta measurement?

It is the tan of the angle between the capacitive current and the total current.
Ir
Ic Ic - capacitive current
I Ir - resistive current
I - total current
As the value of tan delta increases the resistive component of the current in
increasing. Hence it shows a weak insulation.

• What is the vector group of GT, UT, SUT?

Yd11
Dy1
Yy0

• Why all the transformers are having different vector group?

UT and SUT are getting paralleled at 6.6 kV bus. Hence they should have voltage of
same phase relationship. This is achieved by assigning different vector group to the
transformers.

• What are the built in protections for transformers?

a) Buchholz relay
b) Explosion vent or relief valve
c) Gas operated relay for on load tap changers.

• Why water pressure is kept below the oil pressure? How it is maintained?

Incase of a heat exchanger tube failure the water should not go inside the
transformer. For this purpose the oil pressure is kept above the water pressure.

• What is the requirement oil in a transformer?

Oil is used removal of heat produced in the transformer and also as insulating
medium.


• What is meant by over fluxing of transformer?

When the voltage is increased and the frequency is reduced the transformer will draw
high magnetising current. This will result in higher core loss and subsequent heating
of core and ultimate failure of transformer. Hence over fluxing protection is provided
for the transformer.

• What type oil pumps are used?

Canned rotor pumps.

• What is oil reclaiming and reconditioning?

In reclaiming process the oil treated to remove all its impurities like acidity, sludge,
sediments, moisture etc. The treated oil will be in par with the new oil. In
reconditioning process (filtering of oil) only moisture and suspended impurities and
sediments are removed.

• Why there is no mixing of oil of tap changer and transformer?

When the tap changing takes place arc is struck between the contacts. Due to this the
oil inside the tap changer will be highly carbonised. If both oil get mixed up the
quality of transformer tank oil will come down. This is not advisable. Hence both
oils are kept separately.

• Why the tap changers are always connected to HV side of the transformer?

During tap changing action the load current has to be shifted from one tap to another
tap. In case HV wining the load current will be less. Hence lesser arcing will take
place.

• What is the purpose of conservator?

To accommodate the change in volume of oil during increase in temperature.

• Why the neutral is earthed through earthing resistance in case of UT and SUT?

This is done to limit the earth fault current.

• Why REF is provided in the LV side of SUT and UT?

The LV sides of the two transformers are earthed through the resistance. This will
limit the flow of current in case of LV earth fault. Hence the differential protection
may not act for a LV earth fault. Hence ref protection is provided.

• Why twin secondary SUT?

As per is, the rating single secondary power transformer is limited to 25MVA (6.6
kV) or 40 MVA (11 kV), in order to limit the 3 phase symmetrical fault level with in
26-40 kA (contribution from grid and local machines)

• How 6.6 kV-bus supply was chosen?

11 kV was rejected in view of the high insulation cost with 11 kV motors.
3.3 kV was rejected, since max motor size with 3.3 kV bus is limited to 2 MW. But
we are having the motors having rating more than 2 MW, which cannot suit to 3.3
kV bus. 6.6 kV bus we can start upto 5 mw size motor.

• Why oil transformers are out door?

Oil fire point = 170 ºC easy catching of fire.

• What are the I.S used in transformers?

IS – 1866 FOR MAINTENANCE AND SUPERVISION OF OIL
IS – 10593 FOR GAS ANALYSIS
IS – 1886 FOR INSTALLATION AND MAINTENANCE

• When oil filteration is required?

On reweaving oil test results.
Draining of oil for maintenance
Topping up of transformer oil

• Why oil filteration is required?

To remove water, sediments, sludge etc.

• What are the types of oil used for in transformer for cooling?

Paraffin based and naphtha based (in INDIA)

• What are the types of bushing used in transformer?

Condenser type bushing
Porcelain type bushing

• What are the precautions to be taken while terminating the bushings?

Contact surfaces with intermediate plates,
Mating surfaces should be identical.

• How bushings are terminated inside the transformers?

By grooving method or by binding wire method.

• Why ICT are used?(INTERPOSING CT)

To correct the system primary CT errors in case of high current faults out side CT
zone (ICT’s primary CT is 800/1, but in fault current may go to thousands of amps.
This ICT will take care of those errors.
a) Matching the ratios.
b) Matching the phase angle differences.

• How CT is connecting in ckt?

If the primary of CT is delta connected load the CT will be in star connection and
vice versa. This is because to have square root 3 time compensation.

• What type of gasket and adhesive are used in transformer?

Gasket – Neoprene based rubberised cork type RC70-C. (IS4253)
Adhesive –Dunlop adhesive S-758
These are recommended by TELK


• What are the precautions to store the Gasket?

a) Stress free storage
b) No folding
c) No reuse
d) Replace with same thickness

• What is the in built protection for transformer?

PRV to protect from over pressurization of tank due to the release of gases, oil etc.
This is the replacement for the explosion vent.

• Why UT, SUT secondary is rated for 6.9kV where as bus voltage is 6.6kV?

The no load secondary 6.9kV voltage level adequately takes into account voltage
drop during loaded condition to cater station buses at 6.6kV level.

• Why our GT having off load tap changer?

Because our station is base load station.

• Why vector group of SUT is chosen as Yn-Yo-Yo?

To facilitate momentary paralleling of SUT with UT on 6.6kV buses.

• Grounding of various transformers.

GT HV solidly grounded
LV (delta)
UT HV (delta)
LV cast stainless steel 9.95 ohms 400A for 10 seconds.
SUT HV solidly grounded
LV cast stainless steel 9.95 ohms 400A for 10 seconds.

• What are the protections for GENERATOR TRANSFORMER?

a) Differential protection
b) Restricted earthfault protection
c) Backup earthfault protection
d) GT phase back up protection
e) Overfluxing protection
f) Oil surge (gas) protection
g) High winding temperature and oil temperature protection.

• What are the protections for SUT?

a) Over current protection for phase and earth fault
b) Differential protection
c) HV and LV restricted earthfault protection
d) HV side directional back up over current protection for phase and earth fault.
e) LV back up over current and earth fault protection
f) Over fluxing protection
g) Buchholz and high oil, winding temperature protection.


• What are the protections for UT?

a) Differential protection
b) LV restricted earthfault protection
c) LV back up earthfault and over current protection
d) Buchholz and high oil, winding temperature protection.

• What is the purpose of carona ring?

To minimize the arcing current during switching operations of disconnecting
switches.

• What are the various tests on transformers?

a) Tan delta and capacitance dissipation factor
b) Tests on cooling fans
c) Tests on OLTC
d) Vector group test
e) Short circuit test
f) Open circuit test
g) Insulation resistance test
h) Turns ratio test
i) Winding resistance test.

• Why input transformer of PUPS module 1 is delta-delta and module 2 is delta-star?

With the help of this arrangement, combined DC output from both chargers is
equivalent to that from a 12-pulse rectifier. Advantage of 12-pulse rectifier is that the
mains current is fairly close to sine wave. Harmonics injected into system by rectifier
are low. The phase angle difference 30-degree between module 1 output and module
2 output give 12-pulse output.

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CT’s, PT’s and PROTECTION Question and answers

CT’s, PT’s and PROTECTION

• What is the inrush current peak of the transformer?

6 to 8 time that of full load current.

• Why REF is now is used for HV side also in GT/SUT?

The E/F setting of differential is poor.

• Why IDMT over current relay is always used as backup?

Because setting has to be 200% to emergency loading and TMS be large to grade
with feeder. Therefore very slow for internal faults/terminal faults/uncleared LV
faults.

• Purpose of standby E/F protection in SUT/UT?

Back up for LV winding, LV neutral CT- CDG 12 – resistance earthing – relay set
high time delay to discriminate with LV feeder and trip transformer if sustained E/F,
also protects neutral earthing resistor.

• Why do we use O/C & E/F protection on both sides of transformer?

Power in feed exists on both ends.

• Why in DG E/F protection, we do not open class IV CB’s or supply CB’s?

Delta of aux. Transformer prevents E/F currents from grid into DG neutral.

• Why 100% winding protection is felt essential for main generator stator E/F

protection? (Used in NAPS onwards?)

At MAPS 4% of winding is not protected. Earlier felt that the Electro magnetic stress
due high external fault currents near 4% of neutral may not be high to cause E/F
here. But now felt that the mechanical stress can leads to E/F.

• How 100% winding protection is given there?

a) Inject sub harmonic AC current into generator neutral. Monitor its amplitude. E/F
impedance reduces so current drawn increases and trips (Not used).
b) 3rd harmonic voltage monitored on neutral, fault near neutral upto 25% winding.
3rd harmonic voltage reduces to zero. Above this 3rd harmonic voltage increases,
so combined both 3rd harmonic and zero sequence relays for 100% covering, no
blind zone.

• What is the basic purpose of class-B protection?

Class-B avoids load rejection. For modern machines, the inertia is less and easily
gets damaged on overload. Therefore trip only for internal faults.
Low forward interlock prevents the risk of run away if a CIES valve fails to close.

• What are the effects of GT over fluxing?

a. Eddy current circulation.
b. Magnetising current increases
c. Winding temp increases
d. Transformer noise/vibration increases.
e. Over heating of non laminated metal parts (affected by stray fluxes)

• Why stabilising resistor in REF or residual E/F scheme?

Required against CT saturation under heavy through fault currents.

• Why in transformer the LV CB also be tripped along with HV CB for a primary side
fault?

Auxiliary transformer 415v delta star transformer, if HV CB alone tripped then back
feeding from LV side (say DG runs parallel with transformer)—arcing voltage at the
fault on primary—fault fed for more time – more damage.

• Why high impedance circulating current differential?

Under through faults, CT’s of different phases saturates differently. Net spill current
will operate low impedance CAG relay, so high impedance scheme with CAG
relay and stabilising resistor used.

• How to reduce the CT error?

Error reduces if load increases.

• What is the advantage of housing CT’s with in bushings?

Bushing acts as a primary insulator for the CT.

• Why the earthing transformer primary voltage is 16.5 kV rated in main generator

even though actual voltage during the E/F is root 3 times less?

The transformer should not saturate during E/F otherwise it will cause
ferroresonance with the GT winding capacitance. Dangerous O/V and neutral
shifting will occur. During loss of load or field forcing conditions, the transformer
voltage increases to cause saturation. Saturation can also occur due to point on wave
of application causing flux doubling.

• Where are the following relays used?

a) Very inverse b) extremely inverse relays c) definite time O/C Relay d)
instantaneous O/C Relay.
a) Very Inverse – Used where inverse protection reduces substantially as distance from
source increases, operating time doubles for a fault current reduction from 7 in to 4 in,
used where the short ckt current is independent of generating conditions.

b) Extremely inverse – Used for feeders subjected to peak making currents. Grade with
HRC fuses, e.g. Refrigerator, pumps.
c) Definite time O/C Relay – Where neutral is resistance earthed- fixed ground current.
d) Instantaneous O/C Relay – Used along with inverse O/C relay – to get higher grading
margin. Disadvantage – Under minimum generation it may not operate.

• Why delta – delta CT’s are used for star – star transformer differential protection?

Say primary neutral is not solidly earthed. Then for any earthfault on secondary
terminal, the primary current distribution is so for external fault, the differential is
likely to operate if sequence current from flowing into relay. The 2:1:1 distribution is
possibly only for core type or delta tertiary.

• Show the CT characteristics.

Knee point region (Protection characteristics)
Saturation region
Peak flux density
Linear region
Ankle point
(Measuring CT characteristics)
RMS amp turns

• What is knee point?

Knee point is the region, where 10% increase in flux causes 50% increase in exciting
ampere-turns.

• When will you say that the CT is saturated?

When checking the CT with the secondary injection method a 10% increases in the
voltage causes a 50% increase in the current the CT is said to be saturated.

• What is the problem anticipated due to CT saturation?

The CT will not be able to drive the current through the circuit causing nonoperation
of relays. In some other case when the currents in the two phases are
compared for relay operation the relay may operate due to unbalance.

• How can you de-saturate the CT?

Pass ac current through the primary and vary the current from zero to maximum with
secondary in shorted condition.

Pass dc current in the secondary and vary it from zero to maximum.

• Why CT should not be open circuited?

Very high voltage will be induced in secondary due to less back emf resulting in the
failure of the insulation.

• What precaution should be taken while removing a current operated relay when the
equipment is in service?


Ensure that the CT is not getting opened by shorting the appropriate terminals.
(Eventhough the terminals are automatically shorted once relay is removed the above
point may carried out to ensure the same)

• What do 10p15 mean?

When the current passed through the CT is 15 times the rated current then the
secondary current will have a composite error of 10%

• Where core balance CT’s are used?

In earth fault protection used. It senses the zero sequence current.

• What are the specifications of CT?

Protection CT - Error. Alf. KpV.
Metering CT - Error. Burden.
Differential CT - Class PS.
Core balance or E/F CT - 5-p type.
Primary current -
Rating of CT - 1. 15 ( full load current )
Short time rating - 1 sec.

• Why differential protection for PHT motors?

For more than 2500 kW motors it is required to provide differential protection. It is
biased Relay against internal phase fault or earth fault very fast. Insensitive to
starting current and stalling current.

• What are the errors of the following CT’ s 5p. 10p. 15p. At rated current?

5p - 1 % Ratio error ± 60 min phase error
10p - 3 % Ratio error ± 60 min phase error
15p - 5 % Ratio error ± 60 min phase error

• What is the operating point in the Magnetising characteristic of protection CT &
measuring CT?

Protection CT – Operation at ankle point only.
Measuring CT – Operation from ankle to knee point

• What is over voltage interturn test for CT?

With secondary open, pass rated current in primary for 1 min. Then check secondary
for insulation.

• A CT has 2 – secondary windings. If we use only one secondary winding can we
keep the unused secondary winding short circuited?

No. If it is short-circuited then the ratio will not get correctly. The turns of primary
winding will be shared between 2 secondary windings. So the unused secondary
winding should kept open.

• But is it advisable to keep the secondary of CT in open conditions? Will not induce
very high voltage?


If the CT has only one secondary winding, we should keep it always short cktd for
safety, but if the CT has multiple secondary, then if one secondary voltage is kept
limited by suitable loading, then the other secondary voltage is eventually limited
proportionately.

• Why PT fuse fails protection?

Mho relays will mal-operate if PT voltage is lost to the relay, so tripping blocked by
sensing PT fuse failure.

• What is the 2 stage stalling protection for PHT motor?

Because locked rotors withstand time of motor is less than starting time of motor
under reduced voltage conditions.
Stage 1 = 350% 6 sec for starting at rated voltage
(Because starting time = 6 sec + hot stall time = 7 sec)
Stage 2 = 175% 15 sec to permit 14 seconds starting time under reduced voltage
condition
(Since starting current is less, stage 1 will not operate)

• Purpose of start up protection? Is it always in service?

Trips the generator. If generator is excited with internal fault the over current 50s trip
the generator to prevent major damage. The earth fault relay 64c also. The relays are
polarised dc armature type, sensitive to all frequencies, since the frequency need not
to be 50 Hz initially during start up. Start up protection is cutout as soon as generator
CB is closed.

• What is the standard CT polarity?

Primary current enters at P1 and secondary current leaves at S2.

• Does over load relay give 100% guarantee against the single phasing?

No. It depends on the motor load and the motor winding (star or delta).

• What are the effects of single phasing?

a. Current will increase √3 times.
b. More heat in stator and rotor parts.
c. Insulation failure and short circuit & Ground fault may occur.

• What is the purpose of CT & PT?

For transformation of current, voltage to a lower level for the purpose of
Measurement, Protection and Control.

• Where CT secondary of 1A we are using?

For long distance current transmission, to reduce the IR drop.

• What is the nomenclature of English electric relay?

a) First letter-operating quantity
b) Second letter-movement
c) Third letter-application
d) Fourth letter-special variation.

• Define knee point voltage.

The voltage applied to secondary of CT keeping the primary open at which
10% increase in voltage causes 50% increase in excitation current.

• What is differential protection?

It is the current balance type protection, in which vector difference between current
entering the winding is used for relay operation.


• What are the checks on CT & PT?

a) Polarity checks
b) Insulation checks
c) Ratio checks
d) Knee point voltage (only for PS class CT)- magnetising characteristic test.

• What is Local Breaker Back up protection?

In case of local breaker fails to operate during fault due to mechanical failure this
protection will protect the system from sever damage. It will trip all the other
breakers in that bus after time delay.


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RELAYS & General Description of Relays Question and answers

RELAYS

• Write down the relay numbers and their designation.

1 MASTER ELEMENT 51 AC TIME OVER CURRENT RELAY

2 TIME DELAY STARTING OR CLEARING 52 AC CIRCUIT BREAKER

3 CHECK OR INTERPOSING RELAY 53 EXCITER OR DC GENERATOR

4 MASTER CONTACTOR 54 SPARE

5 STOPPING DEVICE 55 POWER FACTOR RELAY

6 STARTING CIRCUIT BREAKER 56 FIELD APPLICATION RELAY

7 ANODE CIRCUIT BREAKER 57 SHORT CIRCUITING DEVICE

8 CONTROL POWER DISCONNECT DEVICE 58 RECTIFICATION FAILURE RELAY

9 REVERSING DEVICE 59 OVER VOLTAGE RELAY

10 UNIT SEQUENCE RELAY 60 VOLTAGE OR CURRENT BALANCE RELAY

11 SPARE 61 SPARE

12 OVER SPEED RELAY 62 TIME DELAY STOPPING OR OPENING DEVICE

13 SYNCHRONISING SPEED DEVICE 63 LIQUID OR GAS OR VACCUM RELAY

14 UNDER SPEED DEVICE 64 GROUND PROTECTION RELAY

15 SPEED OR FREQUENCY MATCHING DEVICE 65 GOVERNOR

16 SPARE 66 NOTCHING OR JOGGING RELAY

17 SHUNTING OR DISCHARGE SWITCH 67 AC DIRECTIONAL OVER CURRENT RELAY

18 ACCELERATING OR DE-ACCELERATING DEVICE 68 BLOCKING RELAY

19 STARTING OR RUNNING TRANSITION DEVICE 69 PERMISSIVE CONTACT DEVICE

20 ELECTRICALLY OPERATED VALVE 70 RHEOSTAT, ELECTRICALLY OPERATED

21 DISTANCE PROTECTION RELAY 71 LIQUID OR GAS LEVEL RELAY

22 EQUALIZER CIRCUIT BREAKER 72 DC CIRCUIT BREAKER

23 TEMPERATURE CONTROL DEVICE 73 LOAD RESISTOR CONTACTOR

24 SPARE 74 ALARM RELAY

25 SYNCHRONISING DEVICE 75 POSITION MECHANISM

26 APPARATUS THERMAL DEVICE 76 DC OVER CURRENT RELAY

27 UNDER VOLTAGE RELAY 77 PULSE TRANSMITTER

28 FLAME DETECTOR 78 PHASE ANGLE OR OUT OF STEP RELAY

29 ISOLATING CONTACTOR 79 AC RECLOSING RELAY

30 ANNUNCIATER RELAY 80 SUPPLY FAIL

31 SEPARATE EXCITATION DEVICE 81 FREQUENCY RELAY

32 DIRECTIONAL POWER RELAY 82 DC RECLOSING RELAY

33 POSITION SWITCH 83 AUTOMATIC SELECTION

34 MASTER SEQUENCE DEVICE 84 OPERATING MECHANISM

35 SLIP RING SHORT CIRCUIT DEVICE 85 CARRIER OR PILOT WIRE RECEIVER RELAY

36 POLARITY OR POLARIZING VOLTAGE DEVICE 86 LOCK OUT RELAY

37 UNDER CURRENT OR UNDER POWER RELAY 87 DIFFERENTIAL PROTECTION RELAY

38 BEARING PROTECTIVE DEVICE 88 AUXILIARY MOTOR OR MOTOR GENERATOR

39 MECHANICAL CONDITION MONITOR 89 LINE SWITCH

40 FIELD RELAY 90 REGULATING DEVICE

41 FIELD CIRCUIT BREAKER 91 VOLTAGE DIRECTIONAL RELAY

42 RUNNING CIRCUIT BREAKER 92 VOLTAGE & POWER DIRECTIONAL RELAY

43 MANUAL TRANSFER OR SELECTOR DEVICE 93 FIELD CHANGING RELAY

44 UNIT SEQUENCE STARTING RELAY 94 TRIPPING OR TRIP FREE RELAY

45 ATMOSPHERIC CONDITION MONITOR 95 SUPERVISION RELAY

46 CURRENT UNBALANCE RELAY 96 SPECIAL APPLICATION

47 POLE DISCREPANCY 97 FUSE FAIL RELAY

48 INCOMPLETE SEQUENCE RELAY 98 SPECIAL APPLICATION

49 THERMAL OVER LOAD RELAY 99 OVER FLUXING RELAY

50 INSTANTANEOUS OVER CURRENT RELAY 100 SPECIAL APPLICATION


General Description of Relays

NOMENCLATURE FOR ENGLISH ELECTRIC RELAY

FIRST LETTER – OPERATING QUANTITY

A- PHASE ANGLE COMPARISON SECOND LETTER – MOVEMENT
B- BALANCED CURRENT A ATTRACTED ARMATURE
C- CURRENT B BUCHHOLZ
D -DIFFERENTIAL C INDUCTION CUP
E- DIRECTION D INDUCTION DISC
F- FREQUENCY G GALVANOMETER (MOVING COIL)
I- DIRECTIONAL CURRENT T TRANSISTOR
K- RATE OF RISE OF CURRENT
N- MANUAL
O- OIL PRESSURE
P- POLY PHASE VA
R- REACTIVE VA
S- SLIP FREQUENCY
T- TEMPERATURE
V- POTENTIAL
W- WATTS (POWER)
Y- ADMITTANCE
Z- IMPEDANCE

THIRD LETTER – APPLICATION

A- AUXILIARY R RE CLOSING
B- TESTING S SYNCHRONISING
C- CARRIER (COUNTING) T TIMER OR TRANSFORMER
D- DIRECTIONAL U DEFINITE TIME
E- EARTH (GROUND) V VOLTAGE TIME
F- FLAG & ALARM INDICATOR W PILOT WIRE
G- GENERAL OR GENERATOR WA INTERPOSING
H- HARMONIC RESTRAINT WJ INTER TRIPPING
I- INTERLOCK OR INDUSTRIAL X SUPERVISORY
J- TRIPPING Y FLASH BACK (BACK FIRE)
JE- TRIPPING (ELECT. RESET) Z SPECIAL APPLICATION
JH- TRIPPING (HAND RESET) ZS ZERO SEQUENCE
JS- TRIPPING (SELF RESET)
JC- CONTROL
K- CHECK ALARM
L- LIMITING
M- SEMAPHORE OR MOTOR
N- NEGATIVE SEQUENCE
O- OUT OF STEP
P- POTENTIAL FAILURE
Q- ALARM

FOURTH LETTER

M – SPECIAL VARIATION
Sl. No. E E Relay Application
1 CTM Motor protection
2 CTU Locked rotor. Thermal alarm
3 CDG I.D.M.T. over current or earth fault of transformer
4 CAG Instantaneous over current or earth faults.
5 VAGM Under voltage
6 WDG Under /Over power for DG set
7 FTG Under frequency
8 VAPM Fuse failure


• What is Knee point voltage?

EMF applied to secondary of current transformer (CT) which, when increased by
10% voltage causes the excitation current to increase by 50%.

• What is I.D.M.T?

Inverse time relay with definite minimum time is called IDMT.

• What is Negative sequence reactance?

Negative sequence can arise whenever there is any unbalance present in the system.
Their effect is to setup a field rotating in opposite direction to the main field.

• What is Zero sequence reactance?

If a machine is operating with an earthed neutral, a system earth fault will give rise to
zero sequence current in the machine.

• Purpose of over current relay (Inverse); type- CDG

It is a self powered inverse time over current and earth fault relay, used for selective
phase and earth fault protection in time graded systems for A.C. machines,
transformers, feeders etc. A non-directional heavily damped induction disc relay,
which has an adjustable inverse time/current characteristic with a definite minimum
time. The relay has a high torque movement combined with low burden and low
overshoot. The relay disc is so shaped that as it rotates the driving torque increases
and offsets the changing restraining torque of the control spring.

• Purpose of Directional inverse Over current & earth fault relay; type- CDD

Directional phase or earth fault protection of ring-mains, parallel transformers,
transformer feeders, parallel feeders etc., employing the time graded principle.; This
relay comprises an inductive disc over current unit with wound shading coils and a
directional high speed induction cup unit. The cup-unit contact is wired across the
shading coils so that no torque is exerted on the disc of the over current unit until the
cup unit contact closes. The inductive disc unit is thus directionally controlled and it
operates only when the current flows in the tripping direction. The directional unit is
a high speed, low inertia four pole induction cup movement designed to give a high,
steady and non-vibrating torque. its current coil is connected in series with the
operating coil of the induction disc unit. The directional unit is normally provided
with voltage polarising coils.

• Purpose of Over current & earth fault relay; type- CAG

This relays are designed for instantaneous phase or earth fault protection and
instantaneous high set over current protection.; A standard hinged-armature unit
forms the basic movement for this relay. It consists an operating coil mounted on a
cylindrical iron core bolted to a frame at one end. This frame extends along the side

of the coil, with its end forming a knife-edge on which the armature is pivotally
mounted. The armature is 'L' shaped and pivoted at its corner so that one arm can be
attracted to the end of the core while the other arm to operate a set of contacts.

• Purpose of Local breaker back-up relay; type -CTIG

To safe guard against the drastic consequences of failure to clear faults rapidly, many
power supply authorities install 2 independent systems of protection for major
transmission lines. There remains however the possibility of the circuit breaker itself
failing to operate and this hazard is traditionally covered by remote breaker back-up.;
CTIG relay is a 3 phase or 2 phase and earth fault instantaneous over current unit
intended for use with a time delay to give back-up protection in the event of a circuit
breaker failure. A particular feature of the CTIG relay is a fast reset, which enables
the time delay to be set closer to the breaker trip-time.

• Purpose of Battery earth fault relay; type- CAEM-21

The battery earth fault relay is used to detect earth faults and deterioration of wiring
insulation in either pole of battery. The scheme consists of a centre tapped resistor, a
measuring relay, plug setting bridge, auxiliary relay and rectifier bridge to provide
unidirectional supply to the measuring relay coil. For different battery voltages
different values of centre tapped resistors are used. Variable sensitivities are
provided by means of the tapped coil whose taps are connected to the plug setting
bridge. The centre tap of resistor is brought to one terminals of the relay and this
terminal is either directly earthed or earthed through a centre zero milli
ammeter. Under healthy condition no current flows through the measuring relay coil
and in any pole of the battery or wiring insulation failure, current flows through the
measuring relay coil and the relay operates.

• Purpose of Rotor earth fault relay (type- CAEM-33)

When a single E/F is detected in the DC field circuit of a machine, the machine has
to be taken out of service at the first opportunity. This is because, if allowed to run
with an E/F on the rotor, a subsequent second E/F can cause severe damage to the
machine. However, a relay like CAEM-33 which can detect such a second E/F and
trip out the machine can make it possible to run the machine even with a single E/F,
without any such risks, thus helping to preserve the generation capacity. The start of
the second rotor earth fault detection scheme is a very sensitivity transductor
element. The AC winding of the transductor is connected in series with a rectified
AC voltage relay. The Dc winding of the transductor on the other hand is connected
in series with the rotor E/F circuit. Under normal conditions- i.e. with no DC
flowing, the AC wining of the transductor presents a high impedance, and the AC
voltage applied is mostly dropped across this winding. Hence the relay remains deenergised.
When a second rotor E/F occurs, a DC current flows through the
transductor dc winding which causes the impedance of the AC winding to reduce
considerably by driving the transductor core into saturation. Hence, the applied
voltage is fully available across the relay and it operates.

• Purpose of Sensitive earth fault relay (type - CTUM-15 & CTIGM-15

It may not be always possible to detect high resistance faults by convectional earth
fault relaying. In such cases a very sensitive current relay will be required for this
purpose. It can be connected residually since it has an adjustable definite time delay
provided to take care of transient spills in the residual circuit due to CT mismatch.
Also, its low burden enables it to be used with existing CT's/ Relays without
affecting the performance.; The incoming current is stepped down by an internal
current transformer and converted to a voltage by a variable resistor network. The
signal is compared with an internal reference. When this reference level is exceeded,
a time delay is initiated, after the time delay has elapsed, a relay operates.

• Purpose of Negative phase sequence current relay; Type- CTN/CTNM

Negative phase sequence current in the stator of a generator, caused due to
unbalanced loads or faults, it induces double frequency eddy current in the rotor.
These currents, if allowed to persist, can cause serious overheating and the purpose
of this relay is to disconnect the machine before such excess temperature is reached.
The inputs from the current transformers, which are connected in each phase of the
generator supply, are fed to a negative sequence filter which gives an AC output
voltage proportional to the negative sequence current. This voltage is rectified and
smoothed and fed into the squaring circuit of the main measuring element, the
definite time delay circuit and the alarm element. The output from the squaring
circuit is proportional to the square of the input voltage and is applied directly to the
main timing circuit to give the required relationship between I2
2t and the relay
operates time t.

• Purpose of definite time Over current & earth fault relay: Type-CTU

This relay can be used for definite time over current protection against phase and
Earth faults on medium and low voltage distribution systems. The definite time relay
offers a considerable advantage over inverse time relays in instances where there ia a
wide variation in line impedance. Another application is in the field of stalling
protection of motors. When the thermal overload relay does not provide protection
against stalling, separate definite time O/C relay type CTU can be used to provide
the same. This relay comes in following nomenclature: CTU-12/22/32/52/62/15.
CTU relay combines the advantage of complete static measurements with
characteristic of the robust, well proved attracted armature unit. These relays
measure current and time accurately, imposes low burden on CT's. Each phase
comprises a static overload detector and timer, which is accurate over a 10:1 time
setting range. When the positive peak of the input signal exceeds the reference level,
the time delay circuit starts and after a preset time, drives the output relay.
Instantaneous high set unit when fitted uses alternate half cycle for measurement and
through a separate level detector drives a separate output relay.

• Purpose of Motor protection relay: Type- CTMM/CTMFM

This relay contains all the protection factors to protect the motor, from Thermal
overload (Ith), Instantaneous over current (I1), Instantaneous or time delayed
unbalance element, Earth fault Element (I0) & Stalling protection (I1(t))

• Purpose of Overfluxing Relay: Type-GTTM

Transformers need protection against the risk of damage, which may be caused when
the transformers are operated at flux density levels significantly greater than the
design values. The overfluxing withstand time is generally found to be varying
inversely with the working flux density in the core, having higher withstand times
during extreme overfluxing conditions.
The overfluxing condition can occur during system over voltage or under frequency
conditions.
The basic operating principle is to produce an ac voltage, which depends upon the
ratio between AC input voltage and the frequency. The AC input voltage is fed to a
step-down transformer, which also provides isolation and the stepped down voltage
is fed to a V/F ratio detector circuit. This circuit is a simple operational amplifier
integrator with the provision for V/F pickup adjustment. The AC voltage is rectified
by true RMS. to dc converter. This circuit gives a frequency output and this
frequency increases rapidly with the increase in voltage. The frequency output is
given to a curve shaping circuit, which involves counter and comparators. The
counter counts the frequency output and the number of counts required for final trip
condition is changed by the comparator circuits to get the required timing
characteristic.

• Purpose of Biased Differential Relay: Type-MBCH

This relay is suitable for protection of two or three winding power transformers, auto
transformers or generators transformer units.
The differential transformer protection measuring circuit is based on the well-known
Merz-price circulating principle.

• Purpose of Digital frequency relay: Type-MFVUM

This relay is used to monitor the frequency of an electrical system. The relay are
suitable for any application in industrial plants and to generators where definite time
under or over frequency protection is required.
The operating principle of the relay is the comparison of the time interval of the
incoming frequency with that of a preset time derived from an accurate oscillator
within the relay. The incoming frequency signal is connected to an INPUT
CIRCUIT, which then drives an IMPULSE GENERATOR to produce a pulse at the
beginning of each period of the input voltage. The preset time interval is obtained
from an OSCILLATOR and COUNTER and adjustment is achieved using
SELECTOR switches, which drive a DECODER circuit. A COMPARATOR
compares the two-time interval and this triggers an adjustable TIMER which then
operates the output relay and latched light emitting diode (LED) glows.

• Purpose of Stator Earthfault Relay: Type-PVMM


A 100% stator earthfault protection is designed to detect earthfault occurring in the
regions of machine winding close to the neutral end. This relay is a composite
modular relay that gives 100% stator earthfault protection for machines, whose
neutral are not directly earthed. It works on the principle involving monitoring of the
neutral side and line-side components of the third harmonic voltages produced by
AC generators in service.

• Purpose of Voltage regulating relay and line drop compensator: Type-VTJCM & CIJC.

This relay is used with on load transformer tap changers and induction regulators to
provide close and accurate automatic voltage regulation on power systems of any
voltage.
When the regulated voltage moves outside a dead band, set by the sensitivity control,
the volts high or volts low circuits are initiated and after a time delay, determined by
the response characteristic, the appropriate tap changer control auxiliary relay closes
its contact to initiate a tap change.

• Purpose of Directional power relay: Type-MWTU.

This relay setting ranges from 0.25% to 18.56% of rated power. This makes the relay
suitable for sensitive reverse power applications. For example with turbo-generator,
where the detection of 1% or 2% reverse power is necessary to prevent the
synchronous machine from motoring in the event of the power from the prime mover
becoming too low. It is also suitable for low forward power interlock and under
power protection.

• Purpose of Check synchronising relay: Type-SKD/SKE.

This relay is used to prevent interconnection of badly synchronised supplies. Type
SKD relay are used for auto reclosing sequence, type SKE relay are used to
safeguard manual synchronising of generators. Phase measurement is achieved by
algebraically subtracting the 2 supply voltage waveforms and comparing the
resultant modulated beat waveform envelope with a Dc reference voltage. The DC
reference is proportional to the sum of the peaks of the 2 supply voltages to provide
phase measurement independent of supply voltage variation.

• Purpose of Static distance protection: Type-SHPM.

This relay (QUADRAMHO) is a static distance protection relay specially designed
for comprehensive high-speed protection of HV & EHV distribution/transmission
lines. 3 zones of protection are included, each employing separate measuring
elements, one element each for 3 phase-to-phase and 3 phase-to-earth faults. Thus a
total; of 18 elements are provided thereby increasing the reliability of the protection.
Poly phase measuring elements are not used in QUADRAMHO as in some of the
contemporary schemes. The relay is suitable for both three poles & single-and-threepole
tripping of the circuit breaker.

• Purpose of Static offset MHO relay: Type- YTGM.

This relay is a static single phase, single step, and distance protection with MHO
offset MHO characteristic. With suitable current/voltage input selection, the relay
can be made to have the required characteristic in the R-X plane for various
applications such as Generator Field failure protection, Generator backup impedance

protection and as offset MHO relay for use in conjunction with generator pole
slipping protection.

• Purpose of sensitive power relay: Type-WCD.

This power relay is a sensitive Poly phase induction cup unit, providing under power,
reverse power and over power protection. This relay detects a reversal of current
flow, caused by insufficient driving power from the prime mover, preventing the
generator operating as a synchronous motor. The electrical quantities energize
windings on an eight pole laminated stator. The moving contact is operated by a cup
shaped Aluminium rotor, which turns on jewelled bearings in an air gap between the
stator and a fixed center core. Only a small arc of rotation is needed to cause contact
closer. Low rotor inertia and very high driving torque ensures a high speed
operations.

• Purpose of pole slipping relay: Type-ZTO.

This pole slipping relay has been designed to protect synchronous Generators against
the possibility of the machine running in the unstable region of the power angle
curve which would result in power oscillations and pole slip. The relay consists
basically of one directional relay and one blinder relay operating in conjunction with
a 40-80 milli seconds static timer. Intended primarily for installation between the
generator and associated transformer (preferably on the generator terminals)

• Purpose of fuse failure relay: Type - VAPM

This relay detects a failure or inadvertent removal of voltage transformer secondary
fuses and prevention of incorrect tripping of circuit breakers. This relay consists of a
rectified AC voltage operated hinged armature unit. Three coils for the three phases
are wound over a single core producing in effect a common relay for the three
phases. Each coil is connected across one of the voltage transformer secondary fuses
and under healthy conditions, this coil is short circuited by the fuse and cannot be
energized. When one or more fuses or are removed the appropriate coil(s) is
energized under relay operates immediately to open the trip circuit.

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TRANSFORMER PROTECTIONS Question and answers

TRANSFORMER PROTECTIONS

a) OVERALL DIFFERENTIAL PROTECTION (87 GT)
This protection which is used as the differential protection of the transformer, also covers
the generator and unit transformer. The differential transformer protection measuring
circuit is based on the well-known MERZ-PRICE circulating current principle.
Fig-1 shows the relay functional block diagram. The output from each bias resistance
transformer T3 to T5 proportional to the appropriate primary line currents, are rectified
and summed to produce a bias resistance voltage. Any resulting difference current is
circulated through the transformer T1 & T2. The output from T1 is rectified and combined
with the bias voltage to produce a signal, which is applied to the amplitude comparator.
The comparator output is in the form of pulses which vary in width depending on the
amplitude of the combined bias and difference voltages where the measurement of the
interval between these pulse indicate less than a preset time, an internal fault is indicated
and a trip signal initiated after a short time delay (1/f sec), level set by the bias.
An unrestrained high set circuit, which monitors the differential current, will over ride the
amplitude comparator circuit and operate the relay output element when the difference
current is above the high set settings.
Fig-2 shows the basic circuit diagram of the differential protection and fig-3 shows the
current direction of the restraint/differential transformers in the relay. The currents I1, I2,
& I3 are the output of generator CT, UT CT and GT CT respectively. These currents is
passing through the star connected restraint transformer, the algebraic sum of vector
(I1+I2+I3 = I4) is passing through the differential transformer, which will give the output
for operating the relay (87).

b) OVER FLUXING PROTECTION (59V/F)
This is designed to protect the transformer from damages caused by the flux density in the
core exceeds the designed value. The excessive flux can cause serious overheating of
metallic parts and in extreme case can cause localized rapid melting of generator and
transformer core laminations. Over fluxing can be caused by regulator failure, load
reduction or excessive excitation with generator off-line it can also result from decreasing
speed while the regulator or the operator attempts to maintain rated stator voltage. Its
main application is to protect the transformers where, unless considerable care is taken,
the flux density can become excessive during the running up or running down sequence.
The flux density in the core depends on the ratio of terminal voltage (V) divided by the
frequency (f). Normally the over fluxing withstand characteristics of the transformer are
120% over fluxing for 2 minutes
135% over fluxing for 1 minutes
140% over fluxing for 5 seconds.
Whenever the v/f ratio of the transformer exceeds the pre-set time, the relay will operate
and initiate
• Running down the AVR if the machine is off the bus bar.
• Tripping the GT breaker.

OPERATING PRINCIPLE:

The basic principle of the relay is to produce an alternating voltage, which is proportional
to the ratio of voltage & frequency, and to compare this with a fixed voltage. When the
peak of the alternating voltage exceeds the fixed dc reference, the first timer is started. At
the end of the fixed timer cycle the second adjustable timer is initiated.
To obtain the correct measuring quantity the applied voltage V is converted to a current
by means of a resistor R. This V/R is arranged to flow through a capacitor C to produce
an output voltage
V/2 π f RC.
Over fluxing relay which consists of Voltage/Frequency measuring circuit, which output
is given to a comparator, compares with dc reference and to give an output after a fixed
time delay of 0.5 to 1.0 seconds. After the end of fixed time delay, the 2nd variable timer
initiates. The fixed time auxiliary has one of its two pairs of contact wired out which is
normally arranged to operate a follower.

c) GENERATOR TRANSFORMER RESTRICTED EARTH FAULT PROTECTION (64)
In addition to overall differential protection, a restricted earth fault protection covering
the transformer HV winding only is provided. The zone of protection extends from CT
provided on the transformer neutral end to the CT provided on the transformer bushings.
The relay is high impedance type and high speed of operation. A non-linear resistance is
connected across the relay terminal to limit the voltage developed during serial internal
fault. This protection energizes Class-A trip of the turbo-generator.
REF relay
Transformer

SCHEME OF RESTRICTED E/F PROTECTION

d) GENERATOR TRANSFORMER BACK-UP OVER CURRENT PROTECTION FOR PHASE FAULT (51)
This protection consists of a 3 phase over current relay. The relay is 3-pole version of
very inverse time over current relay plus high set instantaneous over current relay. This
will act as the back up protection for the transformer fault due to the fault current flowing
from system side. This may also serve limited back up protection function for fault
external to the transformer. This will energize Class-A trip.
R

e) GENERATOR TRANSFORMER BACK UP EARTH FAULT PROTECTION (51N)
This is a simple inverse type over current relay connected to the neutral CT of
transformer. This relay provided back up function for fault both internal and external to
the transformer, This protection energizes Class-A trip.

f) OVER LOAD MONITORING (49)
Measuring oil temperature and winding temperature indirectly monitors the loading of the
generator transformer. The oil and winding temperature indicators are provided with
contacts for initiating alarms as a first stage and tripping as the second stage whenever the
oil and winding temperature limits are exceeded. The oil temperature /winding
temperature trips are routed through Class-C trip.

g) GAS PROTECTION (63)
A Buchholz relay is supplied along with the transformer. The relay has two contacts one
closes on slow gas formation and initiate alarm. The second contact closes of sudden
surge of oil flow in case of severe internal fault and this contact is wired for tripping the
unit in Class-A trip.
The relay consists of two float switches contained in a closed housing, which is located in
the pipe from transformer to conservator tank. Any internal fault in the transformer
comes, the oil decomposes and the generating gases which passes up the pipe towards the
conservator and is trapped in the relay. In this two float relay the top float responds the
slow accumulation of gas due to mild and incipient fault, the lower float being deflected
by the oil surges caused by a major fault. The float control contacts, in the first stage give
an alarm and second case to isolate the transformer.

• What are the set values of generator protection?

TYPE OF PROTECTION AND ITS SET VALUES
No. Type of Protection Set Values CT/PT Ratio Time Delay Class
1. Generator differential 0.5A(10%) 10000/5 Inst. Class-A1
2. Generator Inter turn 0.5A(10%) 5000/5 Inst. Class-A1
3 Generator reverse power 0.5% 10000/5 5 sec Class-A1
(stage 2 Tx trip)
4. 100% Stator Earth Fault ND = 5V(3r
harmonic 70
100%)
16.5 kV/110V 1.0 sec Class-A1
5. 2nd Rotor Earth Fault 1.0 mA --- --- Class-A1
6. Over Frequency 51.5 Hz 16.5 kV/110V 0.1 sec 86 BG
7. Over Voltage 120% 16.5 kV/110V 2.0 sec Class-A
8. Overall Differential 1.0A 10000/5A Inst. Class-A1
9. GT Restricted E/F 0.1A 800/1A Inst. Class-A1
10. GT Gas Protection --- ---- Inst. Class-A1
11. GT Fire --- --- Inst. Class-A1
12. GT Over Current PSM-1.0
Inst. – 8.0
800/1 A TMS=0.4 Class-A1
13. GT Earth Fault PSM-0.2
TMS-0.52
800/1A 2.0 sec Class-A1
14. Impedance Protection
Stage-1
--- 10000/5A 2.0 sec Class-A2
15. Generator Over Current
During Starting
50 mA 10000/5A Inst. Class-A2
16. Generator Back-up Earth
Fault
PSM-5.4V 16.5 kV/110V TMS =0.3 Class-A2
17. Stator Earth Fault During Starting 100 mA 300/1A Inst. Class-A2
18. Low Forward Power 0.5% of FP 10000/5A 2.5 sec Class-B1
19. GT Over Fluxing Stage-1 120% --- 2 min Class-B
20. GT Over Fluxing Stage-2 135% --- 1 min Class-B
21. Negative Sequence 5% 10000/5A Inverse Class-B
22. Field Failure --- 10000/5A Inverse Class-A2
23. Under Frequency 47.77 Hz 16.5 kV/110V 4 sec Class-C
24. GT Winding Temp. High 130O C --- --- Class-C
25. GT Oil Temp. High 90O C --- --- Class-C

• What is arc and what is spark?

Spark - the heat produced that ignites, due to the rubbing of two metals is called the
spark.

Arc – the electrical discharge between two electrodes is called the arc. Arc is the
self-sustained discharge of electricity between electrodes in a gas or vapour, which
has a high voltage discharge at the cathode.

• What precautions should be taken while meggering main generator?

All PT’s are racked out.
Earthing transformer grounding terminals disconnected.
Barring gear shall be kept off.
Stator water shall be drained fully and hot air blown through conductors.
Generator flexible lines shall be disconnected to isolate GT/UT.

• What is the speed equation for AC machine?

N = 120 f / P
N – Speed in RPM
f - Frequency in Hz
P – Number of poles

• What is emf equation of alternator?

Emf = 4.44 kc kd f ∅ T volts.
Kd = Distribution factor = sin m β/2
m sin β/2
kc/kp = Coil span factor /Pitch factor = cos α/2
kf = Form factor = 1.11
∴Average emf induced / Cycle = ∅ N P/ 60
= ∅ P ∗120 f
60 * P
=2 f ∅ volt
If Z is the number of conductors = 2T (T = two sides of conductor)
emf induced = 2 f ∅ Z =2 f ∅ 2T = 4 f ∅ T
∴ RMS value of emf induced = form factor * emf
= 1.11 * 4 f ∅ T
= 4.44 f ∅ T volts.

• What is the emf equation for DC generator?

P * ∅ * Z * N
60 * A
A = number of parallel paths. That is for lap winding it is equal to Z and for wave
winding it equal to 2.

• What are the factors, which varies terminal voltage of generator?

a) Voltage drop due to resistance (Ra drop). This is negligible.
b) Voltage drops due to leakage reactance (XL).
c) Voltage drops due to armature reaction.

• What is meant by Armature reaction?

The effect of armature flux on the main field flux is called Armature reaction, where
armature flux weakens the main field flux. In Alternator power factor contributes
more importance in Armature reaction.
a) In Unity power factor field strength is average and effect is distortional. So
voltage variation will not be too much.
b) In lagging power factor armature flux is directly opposite to the main field flux.
That is armature flux is lagging 90ο by main field flux. So the result is
demagnetizing the field. Due to less field flux less voltage at the alternator
terminals and excitation required is more.
c) In leading power factor armature flux is leading by 90ο to the main field flux. The
result is additive and main field strength is more and excitation has to be reduced.
Otherwise end parts or overhang portion of the generator will heat.

• What is meant by voltage regulation?

If there is a change in load, there is a change in terminal voltage. This change not
only depends upon the load but also on power factor. The voltage regulation is
termed as the rise in voltage when full load is removed divided by rated terminal
voltage (Excitation and speed remains constant).
∴ Regulation in % = E0 – V
V
In case of leading power factor terminal voltage will fall and regulation is negative.
PF leading
Terminal
Voltage PF unity
PF lagging
Load current
Generator voltage characteristics

• Why double squirrel cage motor used in barring gear? Why?

To have high starting torque.
In AC motors torque is directly proportional to φ (flux), I2 and cosφ2.
i.e T ;φ (flux* I2 * cos φ2.
∴ T = k * φ (flux)* I2 * cos φ2.
Rotor at standstill E2;φ (flux)
∴ T = k * E2 * I2 * cos φ2.

In double squirrel cage motor inner cage is low resistive and high inductive. The
outer cage is high resistive and low inductive. In case of inner cage Z (impedance) is
less (XL = 2􀀟 f L). If the rotor is having high inductance at starting I2 will lag E2 by
large and cos φ2 (Rotor PF = R2 / Z2) is very less. So torque
is less.
At staring rotor torque is proportional to the rotor
resistance. At starting inductance is high and the Z is--
--------

• What are the logics adopted to close the field breaker?

a) Turbine speed 2880 rpm.
b) Class A, B and BG trip reset.
c) Auto/manual reference minimum.
d) Auto/manual channels supply normal.
e) FB closing circuit healthy.

• What you mean by positive sequence, negative sequence and zero sequence of
voltage?

Positive phase sequence
A system of vectors is said to have positive sequence if they are all of equal
magnitude and are displaced by 120° with same time interval to arrive at fixed axis
of reference as that of generated voltage. The positive phase sequence is represented
below and the vectors arrive along X-axis in order 1, 2, 3 and conscript P has been
used to designate as positive sequence.
E3P
Anti clock direction
120°
E3P
E3P
Negative phase sequence
A system of vectors is said to have a negative phase sequence if they are of equal
magnitude displaced at an angle of 120° but arrive at the axis of reference at the
regular interval same as that of positive phase sequence but in order of 1, 3, 2. That
is the order is reversed.
E3N

Clock direction
120°
E3N
E3N

Zero sequence
A system of vectors in a phase system is said to have zero phase sequence if all the
three vectors are not displaced from each other and there will be no phase sequence

in such cases. The current or voltages in the 3-phase circuit vary simultaneously in
all the 3- phases. Such phase sequence is zero phase sequence.
E1O
E2O
E3O

• What is rotor and stator resistance values?

Rotor resistance = 98.1 m􀀍
Stator winding resistance’s
R φ = 3.1􀀍/3.1􀀍
Y φ = 3.1􀀍/3.1􀀍
B φ = 3.1􀀍/3.1􀀍

• What is the rating of generator PT fuse?

24 kV, 3.15 Amps.

• What is the wearing rate of generator Slipring?

Generator Slipring wearing rate is 0.025 mm /1000 hrs.

• What is the brush pressure on Slipring?

Recommended brush pressure in the Slipring is 150 to 200 gms/cm2 (0.9 to 1 kg).

• What are the properties of hydrogen and DM water?

Hydrogen
a. Windage losses are less. Hence efficiency increased.
b. Heat transfer is more. Hence output per volume is increased.
c. No corona discharge, which makes insulation life long.
d. Lesser denser and penetration and cooling more.
e. No fire risk at purity 4% to 74%.

DM Water
a. Non toxic and low viscosity.
b. High thermal conductivity.
c. Low conductivity.
d. Freedom from fire risk.
e. External heat exchanger used.

• What are the chemical tests on hydrogen and DM water?

Hydrogen
a) Hydrogen purity in % (volume/volume).
b) Relative humidity in % (30% is nominal).

DM water
a) PH of DM water (less than 6.5 is acidic and more than 7 is alkaline where oxygen
is not forming). PH is also called IP (isotopic purity).
b) Conductivity.
c) Copper traces.
d) Dissolved oxygen (to trace corrosion and 1.2% is more).


• What are the logic’s adopted in barring gear motor?

For start permission
a) Local or remote start.
b) JOP is running.
c) Motor hand barring is permissive.
d) 42 contactor in MCC is off.
e) Turbine speed is <100 rpm.

Start permission (42S of MCC)
a) All above
b) Bearing oil pressure is >0.35 kg/ cm2.
c) No thermal over load of 42S.
d) No one-DG condition.

Start permission (42 of MCC)
a) Start permissive of 42S.
b) Barring gear engage or motor speed reached to 1475 rpm.
c) Turbine speed is <100 rpm.
d) Bearing oil pressure is >0.35 kg/ cm2.
e) No thermal over load of 42.
f) No one-DG condition.

• What is the equation for resistance measurement of PT 100 thermocouple?

°C = (R-100) / 0.39

• What are the requirements for synchronization and setting for generator?

a) Same phase sequence.
b) Voltage should in-phase and angle should not be more than 10°.
c) Voltage value must be same and difference of 5% is allowed.
d) Frequency should be same and difference of 0.1% i.e. 0.05 Hz is allowed.

• What is the recommended IR value for generator?

Main generator is class B insulated machine. Without stator water recommended
insulation value for the generator is R60 = kV + 1 MΩ
R60 – minimum recommended IR in MΩ of entire winding at 40°C of 60 Sec.
kV – rated voltage.
For the IR measurement 1 kV megger should be used.

• What is the type turbine installed in KGS?

Tandem compounded to expansion of steam, impulse reaction type.

• State HP & LP turbine steam values.

HP LP

Pressure Flow Temp Pressure Flow Temp
I/L 40 kg/cm2 1333 t/h 250°C I/L 5.664 kg/cm2 232.9°C
O/L 6.02 kg/cm2 O/L
Wetness (I/L) 0.26% Wetness (I/L)
Wetness (O/L) 11.058% Wetness (O/L)

• State turbine governor setting.

On 2560 rpm turbine governor becomes effective and on 2760 rpm is turbine
governor take over speed.

• What is requirement of speeder gear assembly?

To bring the turbine to synchronous speed and get tight lock with grid by grid
frequency. BPC signal is given in Auto mode to the speeder gear motor.

• What is the purpose of LLG?

To ensure that the turbine load never exceed the reactor output, to incorporate
turbine follow reactor feature governing system.

• What is the purpose of OSLG?

This gear mainly used to control the steam flow so as to limit the machine from over
speeding. On following occasions the over speed limiting gear acts.
a) When the flow of steam corresponds to load is 2/3 and
b) Electrical power on generator falls 1/3 of full load.

• What is the logic in lubrication oil pump system?

Normally main oil pump (MOP) will feed the required lub oil to turbine governor
and lubrication. If the pressure drops to 5.3 kg/cm2 6.6 kV 373 kW Aux. Oil Pump
will start. If further pressure falls to <0.65 kg/cm2 Flushing Oil Pump will start. If
further pressure drops <0.35 kg/cm2 Emergency Oil Pump will start.
Lubricating oil inlet temperature will be 40°C and outlet temperature will be 70°C.

• What is the purpose of TOPP (turbine oil purification plant)?

The purpose of TOPP is to remove the water ingress in turbine oil system from the
gland leaks, cooler leakage, and solid metal particles, which are produced due to rust,
wear of bearings and to normalize the low quality oil.
The remove capacity of TOPP is, for solids – 5 microns and for water – 300 to 500
parts per milli.


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