Civil Engineering Important indian Standard codes

Important indian Standard codes:books:

Important indian Standard codes:books::heart::books:
PKR ELECTRICAL ENGINNERING
* IS: 456 – code of practice for plain and reinforced concrete.
* IS: 383 – specifications for fine & coarse aggregate from natural sources for concrete.
* IS: 2386 – methods of tests for aggregate for concrete.
* IS: 2430 – methods of sampling.
* IS: 4082 – specifications for storage of materials.
* IS: 2116 – permissible clay, silt & fine dust contents in sand.
* IS: 2250 – compressive strength test for cement mortar cubes.
* IS: 269 – specifications for 33 grade OPC.
* IS: 8112 – specifications for 43 grade OPC.
* IS: 12269 – specifications for 53 grade OPC.
* IS: 455 – specifications for PSC (Portland slag cement).
* IS: 1489 – specifications for PPC (Portland pozzolana cement).
* IS: 6909 – specifications for SSC (super sulphated cement).
* IS: 8041 – specifications for RHPC (Rapid Hardening Portland cement).
* IS: 12330 – specifications for SRPC (sulphate resistant portland cement).
* IS: 6452 – specifications for HAC for structural use (high alumina cement).
* IS: 3466 – specifications for masonry cement.
* IS: 4031 – chemical analysis and tests on cement.
PKR ELECTRICAL ENGINEERING

* IS: 456; 10262; SP 23 – codes for designing concrete mixes.
* IS: 1199 – methods of sampling and analysis of concrete.
* IS: 516 – methods of test for strength of concrete.
* IS: 13311 – ultrasonic testing of concrete structures.
* IS: 4925 – specifications for concrete batching plant.
* IS: 3025 – tests on water samples.
* IS: 4990 – specifications for plywood formwork for concrete.
* IS: 9103 – specifications for concrete admixtures.
* IS: 12200 – specifications for PVC water bars.
* IS: 1077 – specifications for bricks for masonry work.
* IS: 5454 – methods of sampling of bricks for tests.
* IS: 3495 – methods of testing of bricks.
* IS: 1786 – cold-worked HYSD steel rebars (grade Fe415 & Fe500).
* IS: 432; 226; 2062 – mild steel of grade I.
* IS: 432; 1877 – mild steel of grade II.
* IS: 1566 – specifications for hard drawn steel wire fabric for reinforcing concrete.
* IS: 1785 – specifications for plain hard drawn steel wire fabric for prestressed concrete.
* IS: 2090 – specifications for high tensile strength steel bar for prestressed concrete.
* IS: 2062 – specifications for steel for general purposes.
* IS: 226 – specifications for rolled steel made from structural steel.
* IS: 2074 – specifications for prime coat for structural steel.
* IS: 2932 – specifications for synthetic enamel paint for structural steel.
* IS: 12118 – specifications for Polysulphide sealants


PKR ELECTRICAL ENGINEERING

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Principles of Radiation Protection: Question and answers

Principles of Radiation Protection

Practices:
a. Justification: No practice shall be adopted unless its introduction produces a
sufficient benefit to the exposed individual or to the society to offset the
radiation harm that it might cause.
b. Optimisation: All exposures shall be kept As Low As Reasonably Achievable
(ALARA) economic and social factors being taken into consideration.
c. Dose limitations: Individual exposures are limited by dose limits since the
dose above the dose limits are unacceptable
Intervention: The general principles of radiological protection for intervention are:
a. The reduction in dose should be sufficient to justify the harm and the costs to
the individual and the society due to the intervention.
b. The benefit of the reduction in dose less the cost of intervention should be As
Large As Reasonably Achievable.
c. Dose limits do not apply in case of intervention. However there will be some
projected dose levels above which intervention will be justified because of
serious deterministic effects.

Dose limits: Occupational Workers
a. For stochastic effects: The dose limit for uniform irradiation of the whole body
shall be 20 mSv (2 Rem) averaged over 5 years (January 1,1999 to December
31,2003) and shall not exceed 30 mSv (3 Rem) in a single year.
b. The average whole body dose for the occupational workers in the station
should normally not exceed 5 mSv (500 Rem).
c. For deterministic effects, the dose limit shall be 500 mSv (50 Rem) in a year
to Bone surface, Skin and for the lens of the eye, for which the limit shall be
150 mSv (15 Rem) in a year.
The whole body exposure level should Remain less 10 mSv (1 Rem) in any month
and 15 mSv (1.5 Rem) in any calendar quarter.
In case of intakes of radioactive material into the body, the total amount of activity
taken into the body in a calendar year shall not exceed one ALI (Annual Limit on
Intake).
Incase of exposure resulting from both external radiation and intake of radionuclides
in the body it shall be ensured that the sum of effective dose resulting from all such
exposures does not exceed the annual dose limits.
Whole body dose Ii -- + Σ -- < 1
0.02 Sv (ALI) I
Where Ii is the intake of the i th radio-nuclide and (ALI) i the ALI value for the i th
radio-nuclide.

Planned Exposure: Situation may occur in-frequently during normal operations when
it may be necessary to permit a few workers to receive dose in excess of the annual
whole body dose limit. In such circumstances, Station director may permit exposure
such that dose does not exceed 30 mSv in a single year and 20 mSv averaged over 5
years.

External Exposure Control: Any external whole body exposure that exceeds 5 mSv
(0.5 Rem) in any month is referred to as significant dose.

Internal Exposure Control
a. Proper ventilation of work areas and use of the recommended protective
equipment would avoid intake of radionuclides in the body.
b. In any case of actual or suspected high intakes HPU should be contacted for
advice and appropriate action.
KGS-Operating Manual on Radiation Protection Procedures
For assessment of internal exposure due to tritium, bioassay of urine will be taken as
the standard reference. For assessment of internal exposure by radionuclides other
than tritium bioassay and/ or whole body counting whichever is applicable will be
taken as standard reference. For control of intake of tritium the following procedures
shall be l Exposure Control followed:
Permissible Contamination levels
Air borne Contamination the levels of air borne contamination in working areas at
the station should be maintained below the Derived Air Concentration (DAC) values
DAC (Bq/m3) = ALI Bq/2400 m3
Investigation of Doses
Investigation levels Whole body dose: Committee (SDIC) shall investigate these
exposures
Dose Reference Levels for Investigation
Tissue/Organ Investigation Levels mSv (Rem)
Monthly Quarterly Yearly
Whole body 10 (1) 15 (1.5) 20 (2)
Skin 100 (10) 300 (30) 500 (50)
Lens of Eyes 30 (3) 80 (8) 150 (15)

The functions of SDIC:
a. To investigate fully the causes of the doses above the investigation levels and
to prepare a factual report.
b. To suggest Remedial measures to prevent recurrence of such doses.
c. To suggest further action in respect of work to be allocated to the exposed
person.
d. To recommend Remedial measure and medical follow up wherever necessary.
Exposure exceeding any of the limits stipulated below shall be regarded as
potentially serious:
Whole body dose: 100 mSv (10 Rem)
Exposure to Eye Lens: 300 mSv (30 Rem)
Such cases shall be referred to Head, Medical Group, BARC and Chairman,
SARCOP immediately
Head, Medical Group, BARC shall initiate appropriate medical investigation
Medical report shall be submitted to Chairman, SARCOP, within a week. Chairman,
SARCOP shall constitute a special committee for investigation of such exposures.
Tritium half life: Radiological =12.3 years, Biological = 7 days, tritium effective =
(TR *TB) / (TR +TB)
(12.3*365*7) / (12.3+365+7) = 7 Days
REVERSE SQUARE LAW: Dose at a rate form the point of source is inversely
proportional to the square of the distance. I is inversely proportional to L / d square
Technical specification Values: Fission products
Noble gases 14.8 TBq / day, Tritium = 13TBq/day, Ar 41 = 2.04 TBq / day,
I-131 = 185 MBq/day, Particulate = 1480 MBq / day
Liquids: Tritium = 1.295 TBq/day
RADIOACTIVE TRANSPORT INDEX: 1 meter from the source shield. Declaration
of radioactive material = 70 kiloBq / kg

Maintenance Performance Planning
1. Essential sequence of maintenance Jobs execution.
Identify maintenance jobs, asses safety, radiological precautions, plan the work,
do the pre-job briefing, take the safety and ALARA measures, carry out the
maintenance, test and normalise equipment or system, update records and history
cards, review maintenance performance and devise future strategy, achieve
excellence in maintenance through dedicated team work.
2. Maintenance performance indicator based on equipment & work control
Maintenance performance indicator (MPI) is the measure of performance of each
aspects of maintenance. These are established as convenient measures to evaluate
current performance levels against standard as well as an index to compare with
past performance

MPI base on equipment performance (EMPI)
a) Equivalent availability % (should be as high as possible)
Equipment operating time x 100
Equipment operating time + down time
b) Mean time between failures (MBTF) should be as high as possible
Number of operating hours
Number of failures/breakdowns
c) Meantime to repair (MTTR) as low as possible
Sum of repair time
Number of breakdowns
d) Number of plant outage caused due to equipment failure. (Objective should
be zero)
e) Number of respective failures during reporting period. (as low as possible)
f) Number of breakdowns during reporting period (As low as possible)

3. Maintenance Performance indicator based on work control

a. Work control indicator (WCI) should be near to unity
No. of DR received from control room per month
No. of PM jobs planned
4. Maintenance performance indicator based on maintenance man hours
1) Man hours spent on breakdown maintenance
2) Man hours spent on PM including implementation of ECN/FCN’s etc.
3) % man hours spent on breakdown maintenance
Man hours spent on = Breakdown maintenance 100
Total maintenance man hour available
4) % of man hours spent on PM
= Man hours spent on PM x 100

Total maintenance man hour available

• What is FME explaining with the examples?

Foreign material is defined as material that is not part of a system or component as
designed. This includes dirt, debris, broken or missing parts, slag, tools rags,
liquids/chemicals, lapping compounds, grinding particles and any other item that
would affect the intended operation of a system or component
All personnel shall assume responsibility for preventing the introduction of foreign
material into systems. This will minimize damage or harmful effects. Such as
corrosion, fuel damage, component malfunction, or failure, changes in chemistry.
Reduced heat transfer, increased radiation levels, changes in system flow
characteristic and improper contact operation.
Specific actions includes the following
Work packages will be planned using field walk downs to determine specific FME
recommendations
If temporary dams are installed which will not be readily visible upon system
closure, verification of removal shall be included in the checklist.

• What is the importance of communications?

Effective, open communication is essential for safe and efficient performance of
plant maintenance. Expressing concerns describing assignments, discussing
problems, are few aspects of maintenance of communication. Clear and
unambiguous communication is an integral part of procedure compliance and safe
work practice. The following additional communication practices will be followed.
a. Repeat back is used to ensure accurate communication, especially when portable
radios, headsets, or telephones are being used.
b. Upon completion of a task, technicians shall report job completions to their
supervisors and seek additional assignments.
c. To confirm to the principle of solving problems at the lowest possible level,
potential grievance issues are to be discussed with the first line supervisor.
d. Plant approved terminology, equipment identification and abbreviations are to be
used at all times.
e. 2-way communication is required at times!
f. Listen
g. Understand
h. Then reply or repeat message.

• Importance of self checking peer verification

a. STAR Principle

S- Stop pause before performing a task,
T- Think Understand exactly what is to be done before taking any action.
A- Act Touch the component without actuating it. Then do it.
R- Review, verifies that the actual responses is the expected response.
b. Self-checking is a self-verification step or action before it is performed. This
behavior is developed through constant checking to ensure the intended action is
correctly and positively performed on the right equipment. Consistently applied this
will minimize error by forming a barrier against complacency and over confidence.
All are responsible for conducting self-checking prior to manipulating a component
or devices, or altering equipment configuration. For examples relays, positioning
switches, breaker or valves, lifting/landing wires, connecting test equipment,
removing or installing fuses.
c. Any deficiency found in the field like labels, nameplate missing/tampered
should be intimated to the supervisor.
d. Do it right the first time.
e. Peer verification is achieved through the use of inspection points, these include
dual verification, independent verification, supervisory verification and quality
verification. Peer verification leads to a broader concept of checking other.
f. Questioning attitude should develop for continuously learning.

The distinction between predictive and periodic maintenance is presented below.
a. Use predictive maintenance results to trend and monitor equipment performance
so that needed corrective or preventive maintenance can be performed before
equipment failure.
b. Predictive maintenance actions are determined by the data required to monitor
equipment condition.
Examples are as follows:
Vibration analysis (includes spectral analysis and bearing temperature
monitoring) and lubrication oil and grease analysis are used to monitor rotating
equipment.
Infrared surveys (thermography) are performed on heat producing equipment
such as motors, circuit breakers, batteries, load centers, bus ducts, transformers
and insulated areas to monitor for high resistance or insulation breakdown.
Oil analyses are performed on lubrication for rotating equipment to identify
degrading equipment and chemical breakdown of lubricants.
Motor operated valves are diagnostically tested and analysed. Tests determine
parameters such as run current, valve stem thrust and torque switch and limit
switch actuation points.
c. Periodic maintenance is time based action taken on equipment to prevent
breakdown and involves servicing such as lubrication, filter changes, cleaning,
testing, adjustments, calibration and inspection. Periodic maintenance can also be
initiated because of the results of predictive maintenance, vendor
recommendation, or experience. Examples are as follows:
a. Scheduled valve re-packing to avoid leakage based on previous experience.
b. Replacement of bearings or pump realignment as indicated from vibration
analysis and/or lubricating oil analysis
c. Major or minor overhauls based on experience or vendor
recommendations.
d. Maintenance on equipment belonging to a redundant safety system if so
allowed by the Technical Specifications
d. Preventive Maintenance Programme Effectiveness
Continually review the preventive maintenance programme for effectiveness, and
change if necessary based on changes in plant design, operating conditions,
regulatory commitments and as found conditions. In addition, unexpected
equipment failures should result in a critical self-assessment to determine why the
previous maintenance activities were insufficient to maintain equipment
reliability. The primary objectives of the programme are to reduce future
component failures, optimize preventive maintenance tasks and use of resources,
identify programme scope and satisfy regulatory and utility concerns. Emphasize
obtaining accurate feedback on preventive maintenance tasks. Enhancement,
provide additional guidance on methods to determine preventive maintenance
effectiveness.

• What are the advantages and disadvantages of planned maintenance?

Advantages of Planned Maintenance.
1. As the name reflects maintenance jobs planned properly ie. manpower, tools,
tackles, required for maintenance is well planned and readily available or
reserved for planned job
2. Procedures for doing the job in well known before doing the job and job can
be performed as per procedure/checklist.
3. It saves the time and unplanned outages of equipment.
4. Common facilities/tools/tackles/ in the section in the section can be made
available at the right time as job and requirements for that are already well
planned.
5. Overtime to employees can be limited.
6. Job can be done systematically, accurately as quality job can be expected.

Disadvantages of Unplanned maintenance.
1. Unplanned job won’t have any expectations when to start, when to stop.
2. Man power/tools tackles were available/not available at the right time is not
ensured.
3. Job may have to do in hurry which can lead to mistaken or job can be done
leisurely (no sufficient work front for the available manpower.) so wastage of
man machine tools etc.
4. In NPP we cannot accept unplanned jobs, as all works are safety
related/important.

• What is pre-job briefing and post job briefing?

Pre-job briefing: Unit no, DR/WP/, USI/system/load, Job description, Eqpt history,
Scope of Job, Any special tool or equipment required, Safety/Alarm, Procedures,
expectation for the job, tech specifications requirements, communication, FME
requirements, environmental concepts, any abnormal conditions.
Post job briefing: Details of work done, difficulties faced, deficiency found, parts
replaced, experience to be communicated, review modification, review procedure,
any suggestions, drawing updating, updating of history card, completion of
checklist, any testing/logic checks required, clearance for surrendering permit.

• What are the activities by which performance of the station will be judged?

Activities of station by which performance is judged by public
a. Capacity factor.
b. Availability factor.
c. Radiation release (gas and liquid effluents).
d. Thermal release.
e. Man-rem.
f. Development programs.
g. Public awareness.
h. Usefulness of the plant product to the public.
i. Employment and other facilities provided to the local public.
j. The Basic amenities provided to the employees.
k. The standard of living of the employees.
l. The profit earned by the Plant.
m. The quality and cleanliness in and around the Plant.


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BATTERIES Question and answers

BATTERIES

• What are the disadvantages of the maintenance free battery?

The life of battery is only five years.
The state of charge of a battery not knowing by the specific gravity of a battery. We
can know by voltage only.

• What are the problems of hydrogen concentration?

If the concentration of hydrogen more than 4% and less than 74% explosion
problems will be there. Therefore the concentration of hydrogen is restricted to less
than 1% by air changer ventilation system.

• Why lead acid battery requires so much large initial charging?

Initially for a new battery, negative plate will be PbO instead of Pb. To convert all of
them back to Pb, we need so much prolonged initial charging.

• What are the protections adopted in UPS or PMCC supply?

LV (incomer) or UPS input
CTZM, Over current, Short circuit protections
PMCC S or T input or UPS output
Under voltage (27), 51RYB, 51N protections.

• Write chemical equation for lead acid cell.

PbO2 + H2 SO4 􀃆 PbSO4 +2 H2 (during discharge)
Pb + SO4 􀃆 PbSO4 (during charge)

• Is the chemical reaction of plante cells same as tubular lead acid cells?

No. Plante cells having both electrodes are lead (Pb) only.
During charging, H2O 􀃆 H2 + O2
O2 react with Pb to form PbO2 + (+ve plate)
During discharge, Pb + c 􀃆PbSO4 (on -ve plate)
While PbO2 􀃆 Pb + O2 (on +ve plate).
That is converted back to lead.
Therefore PbSO4 formed only on -ve plate. That is sulphation problem are reduced
by 50%

• What are the advantages of plante type batteries?

Plante plate type batteries have longer life and can with stand rapid discharge.

• Why battery room should be located separately in a power station?

Possibility of battery explosion
Corrosive atmosphere by acid spray.
Fire hazard.

• What are the disadvantages of nickel cadmium battery?

Status of charge not known
Number of cells are more
Cost very high
Environmental protection agency considers cadmium as a hazardous material,
difficult to discard at the end of life.

• What are the effects of temperature in lead acid battery?

Higher electrolyte temp - ah capacity increases but life reduces.
Lower electrolytic temp - ah capacity reduces since chemical reaction rate reduces.

• Why ventilation is essential for Ni-Cad also?

Gases evolved H2 O2 can form explosive mixture.

• How H2 O2 generated in lead acid battery?

At end of charge, when most of the Pb is converted. H2 O2 generated from H2O. O2
appears as gas at positive plate. H2 at negative plate, i.e. gassing starts.

• Why current reduced after gassing?

Excessive gassing shortens the life of battery by scouring the active materials at the
surface of the plates.

• Why aged battery consumes more water?


As aging increases, antimony migrates to negative plate 􀃆 secondary cell reaction.
Therefore more charging current require 􀃆 more water consumption.

• What happens after aging?

Shedding of active material during charging.

Shedding increases with overcharging, heavy discharge, batteries short ckt.

• What is the other effect of low temperature?

When specific gravity decreases, acid freezing point increases, soon reached at low
temperature, volume increases container cracks.

• Why temperature correction required?

As temperature increases, specific gravity decreases. The hydrometer immersion is
more, showing lower readings. Therefore 10ºC raises, 7 points to be added,
corrected to get 27 ºC reading.

• What is meant by sulphation of the cell?


During the discharge lead sulphate is produced and during the charging the same is
converted back into lead and lead peroxide. If the cell is left under charged, lead
sulphate would form which will not reverse back into lead and lead peroxide during
charging. Due to this the cell will loose its original capacity.

• What you meant by shedding?

During the charging and discharging the active materials will undergo volumetric
changes. Due to this some of the material may not be retained with the parent
material and will shed and collected at the bottom of the container. This loss of
active material is called shedding. Due to this the cell will loose its capacity.

• Acid should be poured to water. Why is it so?

When acid and water is mixed lot of heat is generated. Hence there is chances of
splashing of the liquid. If water is poured to acid will splash causing injuries. In the
other case splashing will be of water with concentration of acid, which will not
hazardous as the other one.

• What types of lighting fittings are used in the battery room?

Flame proof acid resistant

• Why ungrounded 250V DC system adopted in our system?

The 250V DC system is feeding to some of the vital loads such as breaker control
etc. Even if one ground has occurred then also these controls should be available.
Continuous monitoring of ground current is employed to eliminate the by passing of
logic due to double ground.

• Why battery capacity limits to 20 minute?

Battery cost is more.

It is better to restore class 3 faster by DG set then putting large battery.
20 min, is enough to shutdown the unit safely.

• What are the main parts of lead acid battery?

a. Container
b. Lead dioxide positive plates
c. Lead negative plates
d. Post strap and seal assemble
e. Separators and retainers
f. Sulphuric acid electrolyte
g. Inter cell connector (lead plated copper)

• What are the different types of charging?

(Normally always) Float charging – 2.15V per cell
It maintains the battery fully charged condition during standby operation by
delivering a small amount of current to cancel the effect of battery natural selfdischarge.
Equalizing charging (2.7V/cell) once in 3 months
Recharge a battery capacity through recovering all useable active materials in the
cell plates.
Boost charging
Boost charging is a quick charging process, which is generally required, if the battery
is drained to a large extent.

• What is the double sulphate reaction?

Pb O2 + Pb + 2H2 SO4 ↔ 2PbSO4 +2H2O
During charging oxygen at positive plate and hydrogen at negative plate are
releasing.

• What are the effects of over charging?

Gassing
Heating
Loosening of plate active material

• What you mean by Drooping characteristics of charger?

When the charger is connected to excess load of charger rating the charger should
able to supply the load with out over loaded by maintaining the terminal voltage
within limit without over load trip. This called a drooping output voltage
characteristics.

• What are the effects of under charging?

Sulphation
Buckling of plates

• What are the effects of high temperature?

Gassing of electrolyte and evaporation
Service life is halved for every 8 deg increase above 25 deg.

• What are the effects of low temperature?

Increased electrolyte viscosity.

• What type of thermometer is used for acid batteries?

Alcohol type thermometer.

• What are the tests for battery?

Conduct test – to check the capacity batteries
Impedance test – to check the utilization of active materials.

• What are the functions of charger?

1. For initial charging.
2. For float charging.
3. For battery equalizing charging.
4. For battery boost charging.
5. To supply normal DC loads.

• What is the rating of switchyard battery charger and batteries?

Switchyard is having Float cum boost charger of 282 V DC maximum and 100
Amps rated charger of six-pulse full wave thyristerised controlled rectifier.
Batteries are tubular 220 V DC. +ve plate is made up of low antimory lead selenium
(Pb) and –ve plate is made up of paste plate type (O2). Container or tube is made up
of polyester and glass fibre.
1. Momentary load 160 A / minute.
2. Continuous load 40 A / hour.
3. Cell voltage 1.8 V DC and total number of cells are 106 in battery bank.
4. Float voltage 2.16 V per cell to 2.18 V per cell.
5. Maximum system voltage is 106 * 2.18 = 242 V DC
Battery rated for 224 A for one minute or 80 A for 60 minutes.
Specific gravity 1110 ± 5 and specific gravity after 10 hrs discharge is 1150 ± 5.

• What are the protections provided in charger?


1. Over load (49).
2. Over voltage (59).
3. Short-circuit (3250 Amps).
4. Phase sequence and phase fail.
5. di/dt and dv/dt protection.


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CIRCUIT BREAKER Question and answers

CIRCUIT BREAKER

• Give circuit breaker nameplate details of 6.6 kV, and 415 V.

6.6 kV (SF6 circuit Breaker) 415 V (Air Circuit Breaker)
Type HPA12/1240C(Tr./ PM) 812 (MCC/UPS)
HPA12/2040C(Tr./ PM) 610 (Tie/PM/MCC)
3037 (B/c, Incomer)
Standard IEC 56
Rated voltage 12 kV (6.6 kV) 415 V
Insulation level 28 / 75 kV 660 V
Rated current 1250A/2000A 1600A/1000A/3750A
Breaking current 40 kA 50 kA (rms.)
Making current 100 kA 105 kA (peak)
Short ckt withstand 40 kA/sec 50 kA/sec
Closing time 52 milli sec 60 sec (III/IV) & 30 sec (II)
Opening time 75 milli sec 35 sec (III/IV) & 40 sec (II)
SF6 pressure 2.3 - 2.8 bar (2.2 alarm) at 20°C
Sliding contact Copper with silver of 10 microns

• What are the difference between DCCB and ACCB?

DCCB
Two poles seriesed for one side.
Breaker is adequately de-rated for use in dc circuits.
Only DINF, DIRS provided. DIT 5 will not work for dc.
In GFB, magnetic blowout coils used to increase the speed of rise of arc into the arc
chutes for effective quenching.

ACCB
The inherent current zero of sine wave helps arc quenching. For DCCB arc
quenching is difficult, since current zero is not existing naturally.

• What are the indications used in 415V and 6.6 kV breakers panel?

415 V Breaker 6.6 kV Breaker
Open Green Green
Close Red Red
Test White
Service Blue
Auto trip Yellow White
Spring charge Blue
Gas pressure Yellow
Voltage (RYB) Red

• What are the advantages and disadvantages of 415V English Electric breaker?

Advantages
1. Auto reclosing shutters.
2. Proven wiping contacts.
3. Sturdy rugged mechanism.
4. Reliable aux. Switches contacts.
5. Slow closing facility independent of closing spring.

Disadvantages
Bulky, more space, spring charge motor 5A. , Spring charging time 14 sec 4 sec at
timing, trip extends in test position also. No neutral bus bar link, we cannot finger
contact resistance since fixed on bus side.

• Type of closing spring is compression type.(415V)

• Why parallel operations of classIII buses are not permitted?

Fault on one side affects the other buses, switchgear fault level rating is exceeded.

• Why auto transfer is blocked for back up protection?

Because Backup protection operates normally for bus faults. All main protections are
generally operating for internal faults therefore there is no point in restoring the
power supply through auto transfer when there is a bus fault existing.

• What decides the control transformer VA rating?

Contactor coil VA rating.

• Can we use AC contactor in DC circuit?

Yes, but with adequate de-rating.

• Can we use ac coils in dc circuit?

Yes with economy resistor in series.

• Why shading rings provided in armature core of ac contactor. Why not for dc
contactor?

Because the force developed is not steady in ac therefore contacts will chatter but if
shading ring is used force developed becomes steady due to splitting of phases of
flux, therefore contacts becomes bounce free and humming sound reduces.

• Why copper contact are not used in contactor?

Because corrosion rate increases. Poor surface property , large closing force
required.

• Why pure silver is not used in contactor?

Affected by sulphur, mechanical or arcing damages (adv. Lower voltage drop)

• What are the types of contactors?

Type Contacts Rating Use
3 TB 41 2 NO + 2 NC 12 A 0 – 5.5 kW
3 TA 22 2 NO + 2 NC 30 A 7.5 – 11 kW
3 TA 13 2 NO + 2 NC 38 A 15 kW
3 TA 24 4 NO + 2 NC 70 A 18 – 38 kW
3 TA 16 4 NO + 2 NC 105 A 40 – 48 kW
3 TA 28 4 NO + 2 NC 170 A 55 – 80 kW
3 TA 28 3 NO + 3 NC 170 A 55 – 80 kW
3 TB 56 4 NO + 2 NC 400 A 93 kW for F/M supply PM.
3 TD 11 2 NO + 2 NC 12 A 0 – 1.5 kW for valve motors.
3 T I 22 2 NO + 2 NC 30 A 1.6 – 7.5 kW for valve motors.

• What material used for contactor?

Silver-nickel for <100A
Silver-cadmium oxide for large currents.

• Why pick up voltage is more than drop out voltage?

Initially air gap is more. Large force is required to overcome the high reluctance
initially. After closing air gap is reduced. Hence drop voltage is reduced.

• Can we file the pitted contacts of contactor?

No, use emery paper and etc.

• How the contact resistance can increase?

Humidity + salty air, dust, poor contact pressure

• Fusing current- the current at which the fuse element melts depends upon the
material, length and diameter.

• Fusing factor- fusing current /rated current (1.25 -1.75)

• Prospective fault current – first loop of fault current

• Cut off current - actual peak value of current reached due to interruption by fuse
blowing.

• What is rated current and short circuit current?

Rated current = VA / √3 * V Amps.
Short circuit current = VA * 100 / %Z * √3 * V Amps.

• What is the advantage of lower cut off current?


Less electromagnetic forces on contactor / CB’s

• How two fuses in-series are discriminated?

Total I2t of minor fuse should be less than pre-arcing I2t of major fuse.
Major fuse should be greater than 1.5 times the minor fuse.

• Why OLR time set high for belt driven fans?

Because acceleration time is high.

• What is the safety reason to keep the contactor / MCC remote from motor?

Contactor = sparking equipment. Not suiting for hazardous location.

• Where wound type CT used and where not used?

Used where low CT ratio req.

Not used where high short ckt. Current exists.

• What is advantage of cast resin CT’s?

Can withstand bursting forces under short ckt, protect damages against external
causes impervious to moisture.

• Why fuses with fusing factor more than 1.5 is not allowed in PVC cables?

Because PVC cables have low thermal capacity than paper cables. Full loading of
PVC only possible if it has close excess current protection (i.e.) Fusing Factor = 1.5

• What are the advantages of HRC fuses re-wirable fuses? What are the characteristics
of HRC fuse?

1. Consistent and stable characteristics for accuracy of discrimination. Capacity
to break at high and low current. It is inverse time characteristics, as the
current is high the time taken to break the circuit is less.
2. Arc quenching is reliable. Chemical action between quartz and arc gas gives
high resistance to the arc. Quartz does not produce more gas after observing
heat as its sand powder observes more heat of the arc.
3. Non deteriorating since it is sealed. No maintenance, Cheap and indication is
available.
Characteristics
1. I2t characteristics. This determines the energy that element can pass and to
determine the cut off characteristics.
105 Total I2t
104
103
102 Pre-arcing I2t
10
10 50 100 150 200

Fuse rating
2. Inverse time characteristics, which is useful for selection of the fuse for motor.
75
50

Current
20
10
0.2 0.5 0.7 0.9 1 sec

Time
Inverse time characteristics
Current
Fuse characteristics
Margin to avoid fuse operation
During starting
Minimum fusing current
Staring current
Running current

Time
Motor selection characteristics

• What is the advantage of CMM relay over normal Inv. O/C current relay?

Inv. O/C relay under protects at low current and over protects at high currents.
CMM: accounts for both +ve and –ve sequence currents i.e. Single phasing /
unbalance supply conditions and gives three times more weightage for the –ve phase
sequence current heating than + ve sequence current heating. i.e. Net rotor heating =
I1
2 + 3 I2
2.
Therefore CMM relay protection characteristic is closely matched to motor heating
characteristic. So it is better than thermal overload relay also.

• What is the purpose of anti-pumping relay?

When closing signal is continuously existing even after the closing of the breaker the
anti-pumping relay will be picked up and it will not allow the breaker to close back
in case of tripping of the breaker.

• Why breaker tripping is prohibited on very low pressure?

The efficiency with which the arc quenching is taking place in the breaker depends
on the air pressure. So if the air pressure is low effective arc quenching will not take
place which will result in damage of CB. Hence the tripping of the breaker at very
low air pressure is prevented.

• Why neutral breaker used in DG neutral grounding?

In case of high earth fault currents it is therefore normal practice to install a circuit
breaker in the neutral of the generator in order to reduce the total fault clearance
time.

• What are the protections used in Class III & Class IV 415 V LV side?

Class III 415 V LV side
1. 51 (inverse over current)
2. 50 (instantaneous over current)
3. 27 (under voltage)
4. 51N (earth fault)
5. 64 (REF)
Class IV 415 V LV side
1. 51 RYB (inverse over current)
2. 51N (earth fault)

• What are the protections used in 415 V Class II side?

LV to UPS
1. CTZM
2. 50 (instantaneous over current)
3. short circuit
UPS to PMCC S & T
1. 51 RYB (inverse over current)
2. 27 (under voltage)
3. 51N (earth fault)
From Class III to Class II tie
CTZM at Class III and 51, 51N at Class II.

• Define the followings.

Insulation level – it is the combination of rated voltage, the corresponding impulse
withstand voltage, which together characterize the insulation of the equipment as
regards its ability to withstand the electrical stresses.
Rated short circuit breaking current – it is the highest RMS value of short circuit
current which the circuit breaker is capable of breaking the circuit in safe.
Making current – it is the peak value of first loop of current of short circuit current
which the circuit breaker is capable of making at the rated voltage.
Rated making current = 2.5 times rated breaking current.
Short time rating – it is the RMS value of current that the circuit breaker can carry in
a fully closed position during a specified time.
Impulse withstand voltage – it is the amplitude of the standard voltage wave with the
insulation of equipment can withstand.
Power frequency withstand voltage – it is RMS value of alternating voltage wave of
power frequency (50 Hz) which the insulation of equipment should withstand.

• What is switchgear?

Equipment which is used for switching, controlling and protecting an electrical
circuit.

• Bus bar specifications of 6.6kV and 415 V.

415V
Aluminium 200 X 12mm
2 nos. per phase, 1 no. for neutral
Bus bar joints – silver plated to 5 micron thick (tightness 50 NM)
6.6kV
Copper
Silver coated joints
Insulation level – 27kV
Fault level estimated – 27kA Designed – 40kA

• What is the type of arc extinction in switchgears?

415V – resistance method (through arc splitter)
6.6kV – single puffer principle

• How Arc quenching is done?

When fault occurs depending on design element melts at one point and arc starts and
a transient current is super imposed on prospective current. When the sum of two is
zeroing the arc is quenching.

• What is lock out relay?

It is the relay to prevent the closing of circuit breaker after tripping (protection)
without attention of the operator.

• What is the significance of SF6 gas pressure in 6.6kV breakers?

Density gauge
Green – correct SF6 pressure (3 –3.5bar)
Yellow – pressure for breaking system fault current (refilling should be done)
Red – SF6 pressure less than 2 bar, which indicates leak in the system.

• What is the measure of atmospheric pressure and PSI?

1 Atmospheric pressure = 1.033 kg/cm2.
1 PSI = 0.07031 kg/cm2.

• What is the distribution of DC control supply in CL IV, III, II- 415V and 6.6 kV?

Closing coil and Trip coil 2 supply from one source.
Trip coil 1 supply from one source.
Protections supply from one source.

• Why 86.1 and 86.2 relays are used?

All electrical protection is wired to 86.1 and under voltage protection is wired to 86.2
relay for automatic restoration in EMTR.


• What is the difference between lockout of 6.6 kV and 415 V CB’s?

Voltage levels.
Manual / auto reset.
Lockout relays of 415 V breakers are all Electro-magnetic type. There is no
mechanical latch. But in 6.6 kV it is of mechanical latch type relays.

• If breaker contact resistance is more what action to be taken?

In 6.6 kV breakers 1250 A breaker contact resistance is <100 μς.
2000 A breaker contact resistance is <50 μς.
If contact resistance is found more than this value should be sent to the manufacturer
for repair.

• What are the interlocks between 415 V and 6.6 kV switch gear?

6.6 kV
Breaker closed cannot rack in or out.
Service lever cannot move while breaker closed.
Breaker cannot be closed in in-between position.
415 V
Breaker door cannot be opened when breaker is in service.
Breaker cannot be closed in in-between position.
Breaker closed cannot be rack in or out.

• Where are the provision of GR-A and GR-B tie possibilities?

Bus D1-D2 to E1-E2 (CL III 6.6 kV)
Bus X to Bus Y (CL III 415V)
Bus S to Bus T (CL II 415 V)

• What are the properties of SF6 gas?

This is inert gas. Odorless, non-toxic, colourless, stable, non-inflammable and
density is more hence high dielectric strength. The special property of this gas is
Electro negativity. This gas attracts electrons to form –ve ions and –ve ion are havier
than electrons and more slow in conduction, so that resistance in medium is increases
and get arc get extinguishes.


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CABLES Question and answers

CABLES

• What are the purposes of cable trays?

a. Avoid sagging of cables
b. Give mechanical support

• What are the disadvantages of paper insulation?

a. Absorbs moisture.
b. Cable termination/sealing problems.

• What are the advantages of XLPE?

a. Easy routing at heights
b. Easy maintenance
c. Large current (90 ºC)
d. No sheath (no fatigue)
e. No paper tape wrapping technique
f. High dielectric strength
g. Very little deformation even at high temp,
h. More rated current, overload, short ckt capacity
i. Low tan delta and hence suits long routes
j. Very light
k. Good mechanical properties.

• What is the specified cable life?

50 years.

• How armours /sheaths grounded?

1 core cables -- sheaths/shields/armours grounded at one end only, other end
insulated to prevent the circulating current through sheaths.
3 core cables -- grounded at both ends but not including core balance CT’s, since
even small induced current causes 50N operation.

• Why armouring done?

For mechanical strength, protect against damage by impact of an object.

• How the required conductor size can be reduced by use of HRC fuses?

HRC fuse limits the peak amplitude of fault current. HRC fuse melts at prospective
current but not allows circuit to pass their high rupturing capacity that is it’s kA.


• What are the types of cables used in 6.6 kV and 415 V system voltages?

In 6.6 kV system AC (unearthed) grade having stranded aluminium conductors
cables are used. Their insulation’s are as follows.
1. XLPE – Crossed linked polyethylene insulation.
2. FRLS PVC – Fire retardant low smoke insulation with PVC inner and outer
insulation. In RB copper conductor stranded cables are used.
3. FS – fire survival insulation.
4. HR PVC – heat resistant insulation.
In 415 V system 1100 V grade copper or aluminium stranded cables are used. Their
insulations are HR PVC and FS type.

• At what temperature cables are rated?

Normally cables are rated for 40°C
Maximum temp in °C Short time temp in °C
PVC 70 160
HRPVC 85 160
Fire survival 90 250
Silicon rubber 90 250
XLPE cable 90 250

• Why 1.1 kV grade cables used for 415V?

To take care of the both earthed / unearthed systems.

• While carrying out cable joints, why should we ensure the continuity of 1) metallised
paper for PILC, 2) sheath and armour.

Continuity of metallised paper ensures less voltage gradient, hence preventing
puncture of insulation.
Continuity of sheath / armour ensures that grounding is maintained, so no over
voltage is induced, and easy to detect earthfault in cables.

• Why bimetallic washers provided in aluminium copper transition joints?

To avoid galvanic corrosion failure.

• What is the type cable used in radiation areas?

Mineral insulated (MI) cables.


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EMTR AND AUTO TRANSFER Question and answers

EMTR AND AUTO TRANSFER

• What is meant by EMTR?

In case of normal supply failure to CL-III & CL-II the loads will be fed by the
alternative/standby sources. This changeover of supply is called Emergency
Transfer.

• How EMTR is initiated?

EMTR is initiated on sensing the CL-III & CL-II bus under voltage.

• What are the routes of EMTR?

CL-III bus under voltage. The DG’s will start. All the breakers connected to the
affected bus will trip. DG breaker will close on dead bus. Loads will be restored one
by one.
CL-II bus under voltage. The tie breaker of the affected bus will close.

• What does load-shedding mean?

When there is only one source to feed the two buses, the total loads can not be fed by
this single source. Hence some of the less important load will not be allowed to start
or it will be tripped if it is running.

• What does total load-shedding mean?

Even after the load shedding the is continue to deliver more than the rated power
sensed by overpower relay or running with under frequency sensed by the under
frequency relay the total load shedding will take place. In case of auxiliary
transformer is feeding the total load shedding will take place after 4 minutes.

• What does auto transfer mean?

If one of the sources is tripped on main protection its breaker will be tripped and the
tie-breaker will close. This transfer of supply from one source to other source is
called auto transfer. To restore the class IV whenever the UT or SUT is lost Auto
transfer is provided.

• Why auto transfer is prevented if the backup protection is operated?

The back up protection is supposed to operate in case of a bus fault. Hence the auto
transfer is prevented.

• What are the types Auto transfer scheme?

1. Fast transfer scheme to close the tie-breaker in less than 200 milli seconds. This
limits transient current and voltage dip in the bus and does quick acceleration of
the motors.


2. Slow transfer after 200 milli seconds.

• What are the conditions for Auto transfer scheme?

Backup protections are not allowed to initiate the Auto transfer. Because under
voltage may exist in the bus. Similarly protection in LV of the transformers are not
initiating the Auto transfer scheme. There are three metrics used in Auto transfer
scheme and 2/3 logic is adopted.
Conditions
1. Fault generator (86BG or 86A1).
2. Fault in UT.
3. Fault in GT.
4. Fault in 220 kV bus (SUT).
5. Fault in SUT (86M).

• What are the uses of EMTR scheme?

1. To restore class III when class IV supply fails.
2. To extend supply to class II when UPS fails.
3. In one DG condition to load restoration.
4. Sub sequent restoration of large motor loads.

• What are the sequences of motor load restoration in EMTR?

1. AHPPW –1001 - 4 Sec.
2. APWC – 1003 - 8 Sec.
3. APWC – 1004 - 12 Sec.
4. AHPPW –1002 - 16 Sec.
5. PPP – 1001 - 20 Sec.
6. MOD – 1002 - 24 Sec.
7. MOD – 1001 - 28 Sec.
8. Air Comp – 1002 - 32 Sec.
9. ABFP – 1006 - 36 Sec.
10. 7343 Exst Fan 1003 - 40 Sec.
11. ECCS PM 1001 - 44 Sec.
12. ECCS PM 1002 - 48 Sec.

• Why synchronizing scheme has been adopted? What are interlocks provided?

To check running and incoming buses, which are going to be interconnected are in
synchronism with each other.
Interlocks
1. Synch selector.
2. Only one breaker can operate at a time.
3. Master synch relay contact should available (bypass will bypass this synch
contact).
Bypass facility is provided to close the breaker on dead bus only.

• What are the settings provided in synchronizing scheme (SKE Relay)?

Voltage – V1- V2 = 10%
Frequency – t = 0.05 Sec (2.5 Hz)
% Slip – 0.45
Phase angle difference - 20°.

• What are the major losses in nuclear power generating system?

Condenser - 500 MW.
Moderator - 40 MW.
C/V and E/S system - 3.7 MW.

• Give the specification of DG and NGR of DG.

DG specification
Type HSPTL 12/653 synchronous generator.
KVA 2815
Volts 6600 V
Amps 246 A
Phase 3
Frequency 50 Hz
Insulation Class-F
PF 0.8
Duty S1
IC 01
IP 23
RPM 1000
Exciter 110 V, 3.2 A (Brush – less of permanent magnet, electronic
automatic voltage regulator)
Ambient temp 50°C
NGR specification
Resistance at 20°C 95.3Ω
Voltage 6.6 kV/√3
Insulation class 7.2 kV
Transient current 40 A / second.
Continuous rating 10 Amps.

• What is the operational requirement of DG’s?

1. Whenever class IV fails DG sets (2 + 1 standby) are started by EMTR and capable
of restoring class III loads within 30 Seconds. One example is given below.
Event Minutes Seconds Milli seconds Difference
Class IV fail 00 00 280 00:00:280
EMTR initiate 00 01 303 00:01:023
DG start (1, 2, 3) 00 01 336 00:00:033
Voltage, Speed reached 00 07 257 00:05:921
CB 351, 361, 370 Closed 00 07 491 00:00:234

Restoration will be done in 07 seconds and 491 milli seconds. After this to build up
power and frequency it takes about 1 minute 26 seconds and 852 milli seconds. Then
load restoration starts as per EMTR scheme.
2. DG’s are capable of paralleling with 6.6 kV class IV supplies.
3. DG’s are capable of paralleling with each other.

• What are the design criteria’s of DG’s?

1. 4000 starts and 4000 hrs run at full load for lifetime.
2. Operation at 45°C and high atmospheric condition.
3. Designed to run in earthquake and seismic condition.
4. Designed to start as per EMTR scheme and take load as per EMTR and load
shedding scheme in one-DG condition without drop in voltage or frequency 25%
and 5% respectively.
5. Designed for run in high speed and to build up voltage and speed within 10
seconds.
6. Designed to start and loading with external row water for cooling for 3 minutes.
7. Designed to start at normal and load condition temperature.
8. Designed to run at no load for 4 hrs in a 4 months with affecting the load and over
load.
9. Designed to supply power in one-DG condition.
10. Continuous supply is 2250 kW and can run at 2475 kW for 2 hrs in 24 hrs at 6.6
kV and 0.8 PF.
11. Designed to start and stop at 48 V DC supply and stop at 220 V DC in the case of
48 V DC is not available.
12. DG – 3 is physically separated for control and installed at adjacent unit. Because
in case of unit is not avail then for cooling water is available in other unit.
13. Monorail of 3 Ton is provided.
14. Provisions are made for filtered air and ventilation and combustion.
15. Co2 fire fighting system is provided for smoothening effect in case of fire.
16. Active process water from class III is provided.

• What are the auxiliaries required for DG?

1. Starting air system.
Components are compressor, air dryer, air receiver, solenoid valve for start
control, pneumatic starting air valve, air distributor and injection valve at each
cylinder.
This system operates at high pressure and also provided with soft start of 8-bar
pressure in testing of system periodically.
2. Lub oil system.
This is closed loop of having oil sump of capacity of 7 days at full load. This also
supplies oil to bearing lubrication, crankshaft, piston, and wiper.
This closed loop Circuit includes pre-lubricating circuit and normal lubricating
circuit. Pre- lubricating circuit is controlled by PLC, but at first commissioning
and overhauling pre-lub start immediately.
3. Water cooling system.
This is provided with closed loop fresh water for remove heat from lubricating oil
system, charge air cooler, engine components (cylinder lines, cylinder head etc).
Fresh water chemistry is controlled to avoid organic growth and corrosion. This
water is heated for normal operation to avoid thermal shock. This water is cooled
by active process water. One tank is provided to transfer fresh water to the
system. Before transferring fresh water chemical addition should be done.
4. Fuel oil system.
The engine driven fuel oil pump is flooded with fuel oil from the day tank by
gravity. Low-pressure fuel from fuel pump is supplied to individual injection
pump is injected to individual cylinders through injectors.
5. Combustion air and exhaust gas system.
The engine is supplied by compressed combustion air with the help of exhaust gas
driver Turbo – charger. Each bank cylinder is provided with a Turbo – charger.
Turbo – charger is provided with filters. The exhaust is passes through silencer.
6. Speed governing system.
Governer is hydraulic mechanical type. The governer is linked to the fuel racks.
The maximum work output of the UG – 8 governer is 8 lb – ft over the full 42°
travel. For full load 30°is sufficient and remaining for overloading.
Governer comprises
a. Speed droop setting.
b. Oil sight glass.
c. Load limiter
d. Compensation pointer and adjuster.
e. Local speed adjuster.
7. Two numbers of ventillation fans are provided in each DG building. One starts at
respective DG breaker close and other at >45°C. DG room is provided with 7
numbers of smoke detectors (ionized type) and 7 numbers of flame detectors
(photoelectric type).
8. Separate DG’s are having separate MCC for their auxiliaries power supply. For
DG – 1 auxiliaries MCC P1. For DG – 2 auxiliaries MCC Q1 and for DG – 3
MCC DG – 3 is provided.
9. Phase winding is provided with two RTD’s for hotspot measurement and bearing
provided with one RTD each.
10. LCP is provided one each for each DG.

• When DG’s start is not possible?


1. DG trip.
2. Turning gear engaged.
3. DG set stop push button pressed.

• What are stages of DG starting?

1. At start signal compressed air through solenoid valve passes over piston and
rotates the shaft.
2. At speed >60-rpm ignition starts.
3. Closing of excitation starts at >800-rpm.
4. At speed of > 900-rpm rated speed and rated voltage signal starts.
5. Closing of over speed is at >1150-rpm.

• What are the protections provided for DG?

1. Differential protection (87).
2. Over speed of engine.
3. Reverse power protection (in LOCA condition time delay).
4. Low lubricating oil pressure.
5. Cooling water temperature high.
6. 6.6 kV switchgear protections.
7. Excitation failure.
8. Emergency stops push button.
In LOCA condition 4 – 8 protections are not permitted to operate.


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Electronics Question and answers

Electronics

• What is Diode?

Diode is a two-layer semiconductor device, conducts only positive cycles when
applied to anode.

• What is Thyristor?

Thyristor is a four or more layer semiconductor device & having 3 or more
junctions. It is also called Silicon Controlled Rectifiers (SCR). A healthy SCR must
block in both the directions at least 1MΩ resistance, a fused SCR will conducts in
both directions.

• What is material used in making semiconductor?

Silicon & Germanium are the raw materials used for making semiconductor.
Semiconductors are located between conductors & insulators in the resistivity
spectrum & allow current to flow only under certain conditions.

• What is material used in making non-linear resistor & purpose of it in field discharge
resistor?

Silicon carbide materials used for making non-linear resistor. The purpose of this
resistor is to avoid surge voltage when field breaker opens. These resistors are
connected in parallel to the main field winding (Rotor).

• How over voltage is produced in Field breaker?

Over voltages appear if synchronous generators and motors fall out of step inducing
an AC voltage in the field system. Depending on the type of construction of the
machine and the slip this voltage can become un-permissibly high, for this purpose
over voltage protectors are provided in the field breaker cubicle.

• What is firing angle?

The angle in the AC cycle at which the thyristor starts conducting at the application
of positive voltage to gate is known as the firing angle (α)

• What is Inverter operation?

When firing angle a = 90º the positive & negative voltages areas are equal. With a
higher than 90º the negative areas are greater so the total voltage becomes negative.
This condition is termed as 'Inverter Operation".

• What is the purpose of RC network across thyristor?

RC network across each thyristor protects against Hole Storage Effect.

• Why reactors are provided in Thyristor bridges?


Reactors are provided to limit the rate of rise of current (di/dt) in the device, thereby
avoiding possible damage to the device. They also effect a proper sharing of load
among thyristor bridges when connected in parallel.

• What is purpose of connecting thyristor bridges in parallel?

Thyristor bridges are connected in parallel to improve current rating.

• What is purpose of Load angle limiter?

Load angle limiter, it either limits the angle between grid load center and the rotor
axis or generator terminal & rotor axis.

• What is purpose of Rotor angle limiter?

It limits generator voltage & rotor voltage.

• What is purpose of Rotor current limiter?

It limits overloading of rotor.

• What is purpose of Stator current limiter?

It limits the stator current.


• What is purpose of Slip stabilization?

It avoids oscillation of the AC machine (Rotor oscillations).

• What is purpose of reactive power (VAR)?

It is an energy required to built up magnetic field to drive the power.

• What is the advantage of Static Excitation?

Fast response time, high reliability, interchangeability of parts during operation, less
wear & tear due to static devices & less maintenance.

• What is the advantage of field forcing in the rotor?

Field forcing acts for 10 seconds to maintain the generator terminal voltage during
fault condition so as to operated the protection relays.

• When the negative sequence reactance arises?

Negative sequence arises whenever there is any unbalance present in the system.
Their effect is to set up a field rotating in opposite direction to the main field.

• When the Zero sequence reactance arises?

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


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Basic Electricity Question and answers

Basic Electricity

• What is Current?

The flow of electrons in a circuit is called current, it is measured in Amperes (I).

• What is Voltage?

Voltage is the difference in potential (charge) between two points or voltage is the
amount of driving force or pressure applied to a circuit, it is measured in Volts (V).

• What is Resistance?

The resistance of a circuit is the circuit's opposition to the movement of electrons. A
resistor restricts or limits the amount of current flowing in a electrical circuit, it is
measured in Ohm (Ω).
Series Resistor: When resistors are connected in series they have one point in
common. The total resistance is equal to the sum of the individual resistors.
R tot = R1 + R2 + R3
The current in a series circuit is the same in each component of the circuit because
the current must flow through each resistor in series to get to the next resistor.
I tot = I1 = I2 = I3
The applied voltage divides across each component in a circuit in proportion to the
resistance of the component. V tot = V1 + V2 + V3
Parallel Resistor: When resistors are connected in parallel, they have two points in
common. The total resistance of parallel resistors is equal to the reciprocal of the
sum of the reciprocal of the individual resistors. R tot of a parallel circuit is called the
equivalent resistance,
R eq = 1/1/R1+ 1/R2+1/R3

• Why color-coding is necessary for resistor?

A wide variety of resistors are physically large enough to have their resistance value
printed on them. However, carbon composition resistors are too small for this
method of identification so a color coding system is used. Four bands are printed on
one end of the resistor and are read from the band closest to the end of the resistor
toward the center. Each color represents a numerical value as indicated below.
0- Black 3- Orange 6- Blue 9- White ±5% - Gold } Tolerance
1- Brown 4- Yellow 7- Violet 0.1- Gold ±10% - Silver }
2- Red 5- Green 8- Grey 0.01- Silver
Suppose the color bands of a resistor are yellow, violet, red and gold. The resistance
value is determined as follows:
4 7 00 = 4700Ω
Yellow = 4
Violet = 7
Red = 2 (two zeros)
Gold = ±5%
4700+5% = 4935} The actual resistance should be between 4467 and 4935 ohms.
4700-5% = 4465}
Occasionally a fifth band is used to indicate the failure rate of the resistor:
Yellow 0.001% per 1000 hours
Orange 0.01% per 1000 hours
Red 0.1% per 1000 hours
Brown 1.0% per 1000 hours

• What does Ohm's Law states?

In a closed electrical circuit, current is directly proportional to voltage and inversely
proportional to resistance at constant temperature.
I= V/R. Where I= Current, V= Voltage & R= Resistance

• What does Kirchhoff's voltage law states?

Kirchhoff's voltage law states that " the algebraic sum of potential rises and drops
around a closed loop is zero." Σ􀀴V= 0

• What does Kirchhoff's current law states?

Kirchhoff's current law states that " the algebraic sum of current entering and leaving
a node is zero." (A node is a junction of two or more branches.)

• What is a Capacitor?

When two conductors are placed side by side, separated by a nonconductive
material, and connected across a battery, free electrons drift in the direction of the

driving voltage is called capacitor. Its unit is farad, normally in micro farad (μf) or
Pico farad (pf).

• What is Inductor?

Inductors use the ability of electrical current to create a magnetic field. If a voltage is
applied to a coil of wire, the current flowing in the coil will cause a magnetic field to
develop. The more times the wire is coiled and the more current there is in the coil,
the greater the strength of the magnetic field. Its unit is Henry, normally in milli
Henry (mH).

• What is Inductance?

The property of a coil that opposes a change in the current flow is called inductance.
The inductance of a coil depends on four factors:
1) The number of turns (windings) in the coil. Inductance is proportional to the
square of the number of turns in the coil.
2) The diameter of the coil. The larger the diameter of the coil, the higher the
inductance.
3) The permeability (ability to become magnetized) of the core material.
4) The length of the coil. The shorter the coil, the higher the inductance.

• What is Power?

Power is a rate of doing work, or works done per unit in time. The unit for measuring
power is the Watt (W). Power in watts is equal to the product of the applied voltage
and the current flowing. Stated algebraically, P = I E

• What is Alternating Current?

Continually changing amount and direction of the current and voltage is called
alternating current (AC). The components of an ac circuit causes a time period to be
introduced between current and voltage; that is, current and voltage are out of phase.

Few definitions and symbols used in alternating current
1) Amplitude or peak value: The maximum value reached by a waveform.
2) Capacitive reactance (XC): measured in ohms, is the opposition to a change in
current flow provided by a capacitor. Capacitive reactance causes current to lead
voltage by 90°: Xc = 1/2p f C.
3) Cycle: The portion of a waveform contained in one period of time.
4) Effective value: The value of voltage that occurs at 45° (0.7071 times the
maximum value).
5) Frequency: The number of cycles per second is called frequency and measured in
Hertz (Hz).
6) Impedance (Z): The opposition to current flow in an ac circuit. It is a combination
of resistance, Inductance and capacitance.
7) Inductive Reactance (XL): Inductive reactance, measured in ohms, is the
opposition to a change in current flow produced by a coil of wire. Inductive
reactance causes current to lag voltage by 90° : XL = 1/2π f L.
8) Instantaneous value: The magnitude of a waveform at any instant of time.
9) Period (T): The time interval between successive repetitions of a periodic
waveform.
10) Periodic waveform: A waveform that continually repeats itself after the same
time interval.
11) Resistance: the opposition of a circuit to the movement of electrons. Resistance
in an ac circuit acts the same as resistance in dc circuit.


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Measuring Instruments Question and answers

Measuring Instruments

• What is Voltmeter?

It is an instrument to measure voltage. It is always connected in parallel to the power
supply.

• What is Ammeter?

It is an instrument to measure the current flowing in a circuit. It is always connected
in series with the load.

• What is Megger?

The megohmmeter, commonly called a megger for short, is used to measure very
high resistance values. It is primarily used to test the insulation of conductors. To
measure high resistance values, a high voltage is applied, either by the use of a handcranked
generator or electronic power supply.

• What is Clamp-On Ammeter?

Clamp-on ammeter is used to check the current in a circuit, without being physically
connected in a circuit. They are convenient to use in the field since the circuit does
not have to be opened to take a current reading.

• What is Infrared or Thermal scanner?

Infrared or thermal scanners are used to measure temperature without contact with
the equipment. They produce an image of the component showing temperature
variations, this is effective in spotting worn or loose connections and components in
industrial circuits.

• What is Phase sequence indicator?

Phase sequence indicator used to indicate the 3-phase direction- comes in two styles;
Lights and meters. In the lighted variety, a sequence of light goes on for the phase
sequence being read, while the meter indicates which phase direction it is reading.

• What is Rotation tester?

This device is used during the installation of a motor to determine the direction of the
motor once it is installed. The shaft is mechanically rotated in the desired direction
and the meter indicates if that is the direction in which the motor will rotate.

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