×
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

Query on High-Impedance Bus Differential Relay Operation

Query on High-Impedance Bus Differential Relay Operation

Query on High-Impedance Bus Differential Relay Operation

(OP)
Dear All,

Good day!!! Hope everyone doing well now.

I have a query regarding the effect of power cable shield ground which is covered by the high-impedance bus differential relay with a window type CT as current analog input.

Our bus is having 6 distribution feeders each with breaker and power cable length to about 10meters as take off and distributed through overhead lines. The zone of the bus protection is covered by window type CTs which is partially extended to the power cable length. The power cable shield is being grounded only at one, near the bus, and the other end (towards overhead line) left hang.

My question is, does will affect the operation of the relay when external faults (fault at overhead line) occur? Considering our power cable shield is grounded that way.

Please advise.

Thanks & Regards,

GRAEE

RE: Query on High-Impedance Bus Differential Relay Operation

If the shield is only grounded once, there is no current through the shield; with no current through the shield it can't have much impact on the differential.

RE: Query on High-Impedance Bus Differential Relay Operation

The cable shield ground needs to be connected below the CT, or if it is above the CT then the connection needs to be brought back out through the window of the CT so that the CT only measures the summated currents in the phase conductors. Otherwise a fault out on the network with stray current returning back via some of the cable sheaths could cause operation of the busbar protection.
Regards
Marmite

RE: Query on High-Impedance Bus Differential Relay Operation

(OP)
Dear davidbeach & Marmite,

Thanks for the enlightenment.

The cable shield ground is connected above the CT but only in one end, the other end is not grounded. As explained by davidbeach, we expect no return current for external fault.

Regards,

GRAEE

RE: Query on High-Impedance Bus Differential Relay Operation

As long as your cable metallic shield remains isolated from the ground surrounding it then you what you say is true. I don't know what type of cable you are using or the nature of the installation whether ducted or direct buried, but if there is a possibility of the shield being in direct contact with ground due to a damaged oversheath etc then there is a possibility of stray currents returning via the sheath to the substation, which are not measured and cancelled out by the CT on the same circuit, and hence operate the bus bar protection. An example might be a fault on an adjacent circuit such as a downed conductor.
Regards
Marmite

RE: Query on High-Impedance Bus Differential Relay Operation

(OP)
Dear Marmite,

We're using single core unarmoured XLPE Insulated PVC Oversheathed Cable and this cable is lay down on the trench.

What's the best practice?..Is it to connect one end of cable shield above the CT and brought back out through the window type CT and connect to ground? Or we left both ends of cable shield floating and ungrounded?

Regards,

GRAEE

RE: Query on High-Impedance Bus Differential Relay Operation

Will the jacket withstand possible transient shield to earth voltages during through faults? As far as I know, sheathing is for physical protection and is not tested for its dielectric strength. I would bring the shield back through the window.

RE: Query on High-Impedance Bus Differential Relay Operation

Bring the shield back through the window to ground. If a cable fault occurs, the fault current will flow in the shield to the ground connection. If it the connection is above the CT, the current will trip the differential. This may be OK if the intent is to include the cables in the differential zone of protection.

If you don't want to trip the bus differential on a cable fault, route the shield ground back through the CT.

RE: Query on High-Impedance Bus Differential Relay Operation

(OP)

Ok, we have to route back out the shield to the CT window.

I have another query on bus differential relay performance where it covers the whole length of the cable. The shield is within/between the zone of the bus differential and one end of the shield is grounded. Would that also affect the operation of the relay?

Thanks

RE: Query on High-Impedance Bus Differential Relay Operation

You would still route the cable shield ground back through the CT. Consider a ground fault between the conductor and the shield, with all of the return current flowing through the shield. If you grounded the shield on the source side of the CT, then no net current would flow through the CT and the differential would not 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