×
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!
  • Students Click Here

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here

Jobs

Differential protection settings for line-transformer block

Differential protection settings for line-transformer block

Differential protection settings for line-transformer block

(OP)
I have difficulties to determine relay protection settings for the following case:
We have block "110 kV cable - 110/20kV transformer". Cable is 240 sq. mm, 700 m long (I suppose it's impedance could be neglected). Transformer is 10 MVA, Uk=11%. CT's on supply side (110 kV) are 400/1 A, on remote end (20kV) are 800/1 A. Relay protection is RED670, interconnection via FO-cable, and it covers both line and trafo.

My problem is to deside about Id> and Id>> values. Nominal current of the transformer is very small ( only 52 A), compared with CT's. But CT's are chosen because of other considerations, mainly high short-circuit level in 110 kV switchgear - it is 19 kA, and necessary connection to existing electromechanical busbar differential protection. (BB-protection is on a separate core, but anyway it is difficult to obtain CT's with so different cores, especially when one of them to be with low CT-ratio and high saturation level.)
So, what to choose as a reference for tresholds calculation ? In Application Manual for this relay it is suggested to use as IBase (defined by ABB as "Base (refferent) current of the differential protection") the nominal value of CT's - in my case 400 A. But if I choose standard (and minimum possible for setting) value Id> = 0,2 IB, it will be 80 A primary, more than the nominal current of the transformer. Usually for standard 87T function we use 0,2 In trafo.

I hope somebody here already has faced similar problem - what is your advice?

------------------------
It may be like this in theory and practice, but in real life it is completely different.
The favourite sentence of my army sergeant
 

RE: Differential protection settings for line-transformer block

lz5pl.
I try check this issue for you. I understand your problem, Id>> isn't problem, but Id> with restraint.....0.2=80A more than nominal trafo current....

Get in touch.
Best Regards
Slava

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members!


Resources