k.prakashchandra


{ City } karkala
< Country > india
* Profession * rtd e.e.
User No # 54717
Total Questions Posted # 0
Total Answers Posted # 521

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Answers / { k.prakashchandra }

Question { 18847 }

On load tap changer (OLTC) explode?
Transformer
Vector group: Ynd11
Rated Power: 353MVA
Rated Current: 693A
Voltage:294/20KV

OLTC
Type: T III Y1000 110/c 12193w
Max Operation Current: 295A
Step Voltage: 550V
Arrangement in Tx windings: Y Connection

Problem: Damage of Unit 1 Generator Transformer On Load Tap
Changer (OLTC)

Description of Incident:
9.00 a.m. :- Generator No Load test on excitation system
started

10.30 a.m :- Carried out AVR change over
test

11.30 a.m :- Preparation for Generator Manual Synchronizing
test


12.00 a.m :- Manual Synshronizing test started.
Observed incoming generator voltage higher than running voltage.
CECR operator operated Generator Transformer OLTC to bring down
the incoming generator voltage

12.03 a.m :-Generator Transformer abnormal alarm operated at
Gen.Control Desk (CECR)
CECR operater at Generator transformer bay informed that the
transformer
oil splashed out from OLTC.
CECR operater informed that he heard explosion inside OLTC.
Observed Oil level & Oil Flow alaram operated at Tranformer
local annunciator.

12.05 am :- CECR operator 'Open Gen Field Breaker”

12.10 a.m. :- Elec1 team inspected the OLTC and found the
OLTC rupture disk was broken
and oil spilled at OLTC area.
- Elec1 found the OLTC top cover bulging.

Do anyone know why and possibility cause happen in this OLTC.


Answer

Generator voltage is decided by excitation and not by OLTC which is provided on HV side . If the voltage on Generator side ( LV ) is reduced with OLTC ,then the voltage difference will be more resulting in more current due to poor lagging pf . AVR maintains the set voltage if apfc mode is not selected . This happened due to wrong operation of OLTC and loose contacts in OLTC . You could have equalized the voltage operating AVR voltage control .

Is This Answer Correct ?    1 Yes 1 No

Question { L&T, 16806 }

if we use fuse in neutral what will happen?


Answer

Additionally , if neutral fuse blows off , all single phase appliances are affected

Is This Answer Correct ?    1 Yes 0 No


Question { 6127 }

I am getting differential earth fault in my transformer
realy , i have checked all its CT & control wiring but do
not get fault, Please help


Answer

I think it is REFR which picks up only if there is earth
fault whithin the transformer .i.e. between LT an HT
CTs.and an NCT is to be provided

Is This Answer Correct ?    1 Yes 0 No

Question { Enercon, 19866 }

what is the ideal earth resistance


Answer

Ideal earth resistance is zero, which is almost
impossible.The recommended max values in ohms for stations
are 1.33KV-5 2.66-2 3.110 to 400 KV -1.0 4.generating-0.5

Is This Answer Correct ?    0 Yes 0 No

Question { Navrachana University, 92473 }

What is the working of AVR in AC generators?


Answer

AVR maintains the set voltage at the same
value,irrespective of the speed

Is This Answer Correct ?    6 Yes 7 No

Question { L&T, 3496 }

How to calculate cable size between battery charger to
Battery ?


Answer

It should be able to carry the maximum charging current continuously

Is This Answer Correct ?    0 Yes 0 No

Question { Essar, 3141 }

two transformer connected in parallel.tap change are
different.if i want to on the buscoupler what happen?


Answer

Current sharing will be in the ratio of voltages i.e. more
the voltage higher the current

Is This Answer Correct ?    1 Yes 2 No

Question { 3514 }

what is load balancing. what maximum load diffence is
allowed between phases.


Answer

Making currents in all phases equal so that there is no
current in neutral.Maximum imbalance allowed depends on
equipment and type. For capacitor bank 1% ,transmission
line 10% and distribution lines 20%.The relay settings are
at the %ges of full load currents.

Is This Answer Correct ?    1 Yes 4 No

Question { 4634 }

if we want ot select cable size with reference to load
current it take than what is the formula and it is same for
aluminium or copper?


Answer

In case of transformers, for copper cable 2.5A/sq mm and Al
i.5A/sq mm ,for generators 1.5A/sq mm and 1 A/sq mm
respectively.For others 4A & 2.5A per sq mm

Is This Answer Correct ?    2 Yes 0 No

Question { 2093 }

whenever any squirrel or a crow falls on the transformer
input line then there is a big explosion and 440 volts of
power is being discharged through the neutral line due to
which appliances in off mode are also burned.we have
earthing for the transformer as well as the bus bar
chamber.why is it happening?


Answer

440 V side neutral and Tr body are connected to earth closely

Is This Answer Correct ?    0 Yes 0 No

Question { 3112 }

In case of power/distribution transformer what is percent
impedence?


Answer

% impedance = (rated current * impedance *100)/rated voltage

Is This Answer Correct ?    1 Yes 1 No

Question { 2410 }

whenever a squirrel or a crow gets tangled near the out put
of the feild transformer there is a discharge of 440 volts
of current through the neutral due to which my appliances
blow up causing a huge bang.will the RCCB help in these
conditions


Answer

No ,as the event has taken place outside its protection zone

Is This Answer Correct ?    1 Yes 1 No

Question { 2588 }

what could be the possible reasons for copper cable to have
low insualation?


Answer

You mean thinner insulation ?

Is This Answer Correct ?    3 Yes 1 No

Question { 2604 }

wht d.c produces corrosion


Answer

Fixed polarities

Is This Answer Correct ?    1 Yes 0 No

Question { 1968 }

Please give me a ansver for why currant transforme noe
point should erthing


Answer

To remove the leakage voltage from primary due to mutual capacitance

Is This Answer Correct ?    1 Yes 0 No

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