Question { 3531 }

This is about the recent problem that we have faced in our
project site.
We have 6/7.5 MVA. 66/11kV three phase star - star power
transformer. The neutral was solidly connected to the
earthing mat. But because of high resistance of earthing
mat (130 ohm on LV and 180 ohm on HV), one day LV side
neutral earthing conductor melted and we had a huge damage.

Can any one plz tell me what might happen when we connect
power transformer neutral to the earthing mat which has
high resistance (>100 ohm ) ?????

Answer

Have you measured IR properly, as the values are very high
?The IR should not exceed 2 ohms for a 66 KV MUSS .I do not
know whether primary neutral is to be earthed in case of
star/star tr.It is stated that LV side IR is 130 ohms ,and
hence the maximum earth fault current will be 50A.For 7.5
MVA tr the rated LV line current is about 400 A and EFR is
likely to be set at 80A.It appears that the earth wire
melted due to continuous earth fault current.If there is
conductor to ground fault at a distance of 100 m from tr
earthing ,there will be a pd of 63.5 V /m along the line
between the fault point and the neutral earth. If anybody
stands barefooted with feet 1 m apart along this line, he is
likely to be electrocuted as the power line does not
trip.Further the ground around the earthing also may be at
nearly phase voltage during earth fault

Question { 3963 }

why we are connecting the alternator to a steam turbine of
higher RPM then the rated RPM of alternator?
For example:( connecting 1500 rpm alternator to 7600 rpm
steam turbine). we can directly connect the same rpm
turbine to alternator know??? is there any special reason ???

Answer

Whenever there is sudden 'load throw off' ,the rpm of the
turbine increases considerably ,it may even attain run away
speed which may be 5 times the normal(working) rpm

Question { 2098 }

Why are we not getting shock while touching a metal which
is being welded?

Answer

As both are on the ground ( earth ) ,there is no potential difference

Question { Apollo, 4539 }

how to calculate the ac and dc wire losses with calculations
plz tell me

Answer

I^2R

Question { 3066 }

What happens if we give the supply to the rotor of induction
motor and make its terminal shorted?

Answer

As the back emf of the rotor can not sustain the applied
voltage ,the motor will draw very heavy current

Question { 5545 }

all 3 current transformers (C.T) are burnt in a L.T Panel -
what are the possible reasons

Answer

If measuring cts,may be due to defective rotary phase
selection switch- CTs open circuited

Question { DMRC, 4874 }

why we changed the
polarity of DC generator ?

Answer

To remove the residual magnetism in poles

Question { 3914 }

I have an enquiry of develop solar power plant, 0.69/11kV,
1500kV/750/750kVA, Dy11y11 Transformer. I want to know what
does 1500kV/750/750kVA means? Sorry if this is a noob
question.

Answer

It is 1500 KVA not KV,the total output via two secondaries
of 750 KVA each

Question { L&T, 10390 }

why Copper loss is represented as I square R ?

Answer

Pd across a resistor id I(amp) x R (ohm) volts ,power is V X
I = I^2 R watt

Question { L&T, 10390 }

why Copper loss is represented as I square R ?

Answer

Read above as ' Pd V across a resistor= I(amp) x R (ohm)
volt.Power = V x I = I^2 R watt' ( As pd is measured across
the resistor ,the pf is unity)

Question { 2928 }

why we use low kv lightining arrester in line for example 9
kv using in 11kv

Answer

The voltage between any conductor and the earth is only
11/1.732 = 6.35 KV

Question { 2930 }

how df/dt relay operates & how it works?

Answer

The relay measures rate of drop in frequency ,which is due sudden increase in load or decrease in generating capacity . When df/dt exceeds the set value ,the relay closes the trip circuit.

Question { TCS, 4929 }

why high current produce in case of transformer when turns are
internally gets shorted.

Answer

Impedance and back emf are reduced

Question { 1760 }

an amazing situation was faced when an alive distribution panel with 415 volts and carrying almost 200Amps has become short and this metal cubical showing 220 volts against neutral and earth. to redress any confusion i checked the panel with help of single 200watts test lamp which also glowing with max.intensity b/w neutral & metal body of DB. amazing situation is that i hold the DB firmly more than once with necked foot means without insulated shoes but not received any electrical shock. whereas our all running gen sets neutral points also proper grounded and this short DB was completely earthed , this is beyond my understanding . any body explain how is it possible . our running gen set neutral also grounded firmly

Answer

Measure voltage between neutral and phases , it appears that neutral is connected to a phase , being isolated

Question { 6584 }

what is the highest voltage for transmitting electrical
power in india

Answer

765 KV between C.garh and M.P.