What is the harm if we install primary side 11 KV VCB at a
distance of one Km from the 11/3.3KV 12.5 MVA transformer
through 11 KV cable.
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Answer Posted / khor
No harm at all,but finding cable is a problem.11kv cable
supply for 12.5MVA must carry 650A per phase which is too
big for most standard cables.Standard VCB are 630A,1250A &
2000A,2500A.The more commonly available large size 3-core
300sqmm 11kv copper(450Amp) or alluminium conductor
cables,which is still flexible and can be transported &
coiled in drums of 250 to 300 metre lengths.There are
500sqmm 11kv 1-core xlpe/awa/pe cables available.
If you use only one transformer which is more
economical,the 3.3 KV side will be more than 2187Amp(plus
additional amp on losses and fans).While you can find 2500A
VCB,you will need more than 600sqmm copper conductors which
is not flexible/practical.
For the better/optimum design,seperate the final 415V and
3.3KV loads required and divide them into 2 groups and look
at the MVA and size them in seperate circuits.If 12.5 MVA
is all in 3.3Kv then employ 2 Main transformers where you
are not putting all the eggs in one basket.You need to
remember to look at the short circuit capacity( in KA 1 sec
or 3sec ) of the cables in circuit where small MVA is
involved.For e.g.1 MVA needs 52A @11KV but you cannot use
35sqmm cables,as it will blow like a fuse during fault
situation.
1 km of cables will have quite enough impedence to
limit/lower the fault level of the system.Protection of
this cable can be done with Pilot protection relays on
sending and rereving end,operated by class X CTs mounted in
the 11KV VCB panels.1 KM of Pilot wires of 2.5 sqmm 7-cores
alomg this cable is requird.So far I am assuming that it is
uderground cable is used.For Ariel Bundle Cable,3-core 185
sqmm aluminium is available.
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