Differential CT grounding 1

[ctpt]

Hi everybody!

It is recommended that for a differential relay connection, the CT’s should be grounded at one end only and not at both ends. I will really appreciate if someone explains me in detail what problem is encountered if the CT’s are grounded at both ends. Is the relay going to trip due to an external fault (short circuit as well as ground fault)?

Thanks in advance.

 

[jbartos]

Suggestion: The reason why the ground should be made at only one point is to avoid improper relay operation and damage to the CT interconnections. If the grounds are made at two or more locations, circulating currents may be caused to flow in the differential circuit because of differences of potential between the grounding points, owing to the flow of large ground currents during ground faults.

 

[busbar]

See recommendations in ANSI/IEEE Std C57.13.3-1983 …Guide for the Grounding of Instrument Transformer Secondary Circuits and Cases Figures 11-15 pertain to differential relaying applications.

 

[dpc]

http://www.powellelectric.com/pdf/ptb/booklet 1-91.pdf

See PTB #87

 

[ctpt]

Thanks everybody. I got the answer. I am little confused though from the GE DGP relay connection diagram which shows that the CTs (either side of a generator) are to be grounded at both ends separately. These CTs are of course common for diff. and other protections. Am I missing something there?

 

[jbartos]

Suggestion: Visit

http://www.geindustrial.com/pm/notes/artsci/art12.pdf

for theoretical background and references:
"Grounding the Secondaries of Differentially Connected CT's"

 

[DiscoP]

Every set of CT secondaries should be earthed at one point.

The CT's on either side of the generator do not have a common connection point when using the DGP relay, so they are treated as separate circuits, and earthed separately.

Generator Differential schemes for older protection relays had the CT star points connected together and could be treated as one set of CT secondaries, so only one earthing connection is used.