unbalance in cable 1

[mhkawi]

we have 15 cables (5cables per phase) from transformer to switcg gear MV 6.6KV we have great unbalance btween the same phase example for phase R (324,98,87,124,120) i wonder where is this great difference coming from
may be from CT or may be from cable arrangement but i don`t know why this big difference

 

[SparkyLarks]

There are two possibilites
1) The current in the cables are the same and there is a CT issue
2) The current in the cables is different and the CT is working ok.

A few questions
1) Is a similar unbalance seen in the S&T Phase
2) Is there a neutral on the system
3) What cables are installed
4) What is the expected load?
5) Have you a CT on each cable? or are the figures from a clamp on meter
6) How are the cables installed,
a)Cable Tray, conduit, Direct burried, Duct
b)spacing between cables
c)Trefoil or flat
d)Are there other cables installed along the same run
e)Are the cables earthed at both ends or just one?
f) What is the cable Length
g) How are the cables terminated together(


If you are using a CT installed on each cable then I would suggest checking with a clamp on ammeter.


possible issues may be
1) Current flowing in the screen/armour
2) currents in the screen being measured onone CT and not on others
3) Current imbalance due to cable arranbement
4) Poor teerminations on some cables

 

[7anoter4]

I agree with SparkyLarks but I think one of the issues [no.3 from the SparkyLarks list] it is more possible:
"Current imbalance due to cable arrangement"
I did a roughly translation from Spanish and I send a sketch from Prysmian[Cable Manufacturer] "Frequent errors in cable selection and lay-out". Here is a recommendation how to run parallel multi-single core cables per phase in order to equilibrate the current per cable. But for 5 cables per phase it is not possible a symmetrical arrangement. Have to be 4 or 6 cables per phase

 

[7anoter4]

I'll try an improved image.

 

[mhkawi]

R(280,120,87,97,188)
S(300,275,95,85,185)
T(320,264,99,180,85)
The current in the cables is different and the CT is working ok.

A few questions :
1) Is a similar unbalance seen in the S&T Phase
yes
2) Is there a neutral on the system
No 3 winding XFMR (19/6.6KV 32MVA)and 2 sec winding going to 2 SWGR incomers each incomers have 15 cables (5 per phase each is 300mm`2
3) What cables are installed
1x300mm`2 6.6KV cu with cu screen XLPE /PVC
4) What is the expected load?
16MVA
5) Have you a CT on each cable? or are the figures from a clamp on meter
we have one CT for each phase and 3 part of bus bar from CT and 5 cables connecting to 3 part of BB
6) How are the cables installed,
a)Cable Tray
b)spacing between cables(don`t know)
c)flat
d)Are there other cables installed along the same run
yes (30cables MV 15 per each incomer)
e)Are the cables earthed at both ends or just one?
just once
f) What is the cable Length
170m


If you are using a CT installed on each cable
no


possible issues may be
1) Current flowing in the screen/armour may be
2) currents in the screen being measured onone CT and not on others
3) Current imbalance due to cable arranbement i suggest this but i wonder of this big difference
4) Poor teerminations on some cables we repear termination with torque 85NM for bolt12

Thank SparkyLarks
for this valuable post!