Question { 4699 }

I want to know what must the CT ratio be on a load of 300A
is a 3PH nutral to earh installation?

Answer

Load current is 300A. The equipment shall withstand the
starting current -which is around -5-8xIn.It is preferable
to select 1ooo-1200A/1A OR 5A.
WE ADOPT THIS THUMB RULE for our protection setting& co-
ordination.

Question { 4514 }

WHAT HAPPEN IF LT SUPPLY 415 V FED TO THE HT 3.3KV MOTOR

Answer

please note that insulation of the motor is rated for
3.3kv.,which is a medium voltage.
Now ,when we apply a low voltage 415v, the motor will crawl
and draws heavy current.so the upstream device or the
circuit breaker trips.
We have also experienced that cubile arcing fault takes
places and burns the cabinet metal clad cabinets.If it a
old module,then the arc chute of the CB FEEDER burns/smokes.

Question { 3696 }

What is the difference between Earth Fault protection &
Neutral Protection

Answer

as you are aware, in electrical engineering power
transformers or motor winding are connected in star-
undrounded or grounded,Delta connected.

Earth fault protection may be due to capacitaive
circulating current,flowing through the body of equipment
to ground. In large generator,motor it may be 5-10A
unbalanced current flowing through Neutral to ground, and
also fault current sigle phase to ground,double phase to
ground,or three phase to ground.these are severe
faluts.damages equipment severly.

Question { 2543 }

how to select the surge suppresor for the 33 KV supply and
what will be the rating, selection guide

Answer

we use :33kv power supply-1.2x33/1,732kv voltage rating,for
current -5ka or 10kA , the nearest value as per selection
guide bymanufacturer

Question { 4617 }

Will it be OK to connect a 5MVA 33/6.6kV Ynd11 Transformer
to a Star connected Motor. Or on other words connecting a
Delta to a Star.

I have another 30MVA 33/11 Dyn11 and would it be practical
to convert it to Dd0 consequently converting the Voltage
Ratio to 33/6.35kV Dd0.

Answer

yes. what is the capacity of motor?
In your case , grid voltage is 33kv, Yn connected.Hence
power transformer has neutral connection brought
out.Neutral CT is installed to detect the ground faults.
secondary of 5MVA tranformer is 6.6kv,
This is Feeder bus voltage.6.6kv Feeder bus feeds Motor (
star connected). and Feeder bus is not interconnected to
another source. It is independent.This 6.6kv rotates at 30
lagging to primary3kv system.
Motor must be use inverse & instantaneous time
overcurrent and earth fault ( CDG61+cdg12) or equivalent
manufacturer protection
we use delt to star as weelas star 6.6motor.
No problems experienced.
In the othercase to change a 30MVA,33/6.35KV IS
REDESIGN/MANUFATURING. iT IS EXPENSIVE.No use at all and
advantage. You should change 6.6kv sw.gr also.

Question { 2962 }

why ligthing is not available in switch yards?

Answer

Generally all sw yds are designed with outdoor lighting
system with normal and emergency power source.
the main reason may be security,power saving.management
decisions