×
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

Zero Sequence / Earth Fault Setting

Zero Sequence / Earth Fault Setting

Zero Sequence / Earth Fault Setting

(OP)
Hi,
   We have an exisitng installation with a soft started compressor (motor f.l.c. 515A) with a P&B Golds digital motor protection relay type PBSJ 3E5 N11. There are three CTs (750/5) residually connected one per phase. One setting in the relay relates to the zero sequence or earth fault setting.  I understand the theory behing what it is but how is this value calculated?  All the manual says is the value is ajustable over the range 0.06 to 0.6 x In.

Regards   
 

RE: Zero Sequence / Earth Fault Setting

It's the phasor sum of the three phase currents.  This should be very close to zero under normal operating conditions.  

David Castor
www.cvoes.com

RE: Zero Sequence / Earth Fault Setting

(OP)
Hi thanks for that.  As I said I know the theory behind the what and why.  What I'm after is how to calculate what setting I need for the protection relay.

RE: Zero Sequence / Earth Fault Setting

It can be set quite low since it does not have to coordinate with anything downstream.  The main issue is nuisance tripping during motor starting due to inrush and locked rotor current.  If you have high quality relay CTs, you can set at minimum with a very short delay (1 or 2 cycles).  If it trips during starting, the settings can be increased.

There is no calculation.   

David Castor
www.cvoes.com

RE: Zero Sequence / Earth Fault Setting

(OP)
Thanks dpc - I did read somewhere where it said it is more of an art form than a science.  Must admit I'd prefer it if there were some calculation to be done, I suppose it's because I like things in black and white!  Not sure about the quality of the CTs as I say it's an old existing installation with a new motor.  Looks like we'll start low and then raise as and when necessary.  Thanks for your help.      

RE: Zero Sequence / Earth Fault Setting

adelaidepom:

It is not a matter of preference. There are competing goals while determining protection settings, such as degree of equipment protection, degree of availability ( avoiding nuisance trips), personal safety (e.g. arc flash hazard) and coordination with upstream and downstream devices.  So there is seldom one answer that can be reached by calculation. There could be a range of settings that all could be acceptable or not acceptable. That is where the 'art' comes in. Work with one of your senior engineers, familiar with your system design and objectives.

Also look up applicable local codes.

 

Rafiq Bulsara
http://www.srengineersct.com

RE: Zero Sequence / Earth Fault Setting

Since it didn't seem clear yet. Those CT's in theory should measure the ground current. However, due to small differences between each CT they may also falsely show some current too.

Is the power system solid or resistance grounded? If this is a solidly grounded system and a low voltage application, the main breaker could easily trip as quickly or even quicker than that ground fault relay would operate.

 

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