×
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

Grounding of the middle phase on VT secondaries
3

Grounding of the middle phase on VT secondaries

Grounding of the middle phase on VT secondaries

(OP)
Hi all,
Caould anyone explain why in countries such as Australia and probably England, the middle phase of the VT secondaries are earthed?
What is the benefit of this set up as opposed to grounding neutral?
Thanks,
Dean

RE: Grounding of the middle phase on VT secondaries

Dean

I think this pratice stems from Metering where the 2 watt meter method requires 2 CT's and 2 line voltages. With this arrangement the red and blue phases is now at line potential to the white and it saves the expense and bother of running a neutral conductor throughout the panels.

RCC

RE: Grounding of the middle phase on VT secondaries


The suggested connection is consistent with North-American ANSI/IEEE Std C57.13.3-1983 …Grounding of Instrument Transformer Secondary Circuits…

In protective relaying, an open-delta-primary / corner-grounded, open-delta secondary arrangement for VTs will block zero-sequence voltage, as will ungrounded-wye-primary / corner-grounded-wye secondary VT sets.  To pass zero-sequence voltage, the VTs must be configured grounded-wye primary / grounded-wye secondary.
  
In metering, the ungrounded/corner-grounded VT sets comply with Blondel’s theorem for 3-wire circuits, where grounded-wye primary / grounded-wye secondary VTs are needed to satisfy Blondel with 4-wire circuits.
 

RE: Grounding of the middle phase on VT secondaries

Suggestion to the previous posting: The stated circuits, namely 3-wire and 4-wire, have to be three phase circuits.

RE: Grounding of the middle phase on VT secondaries

Deansharafi,

I have read (somewhere) that earthing the secondary middle phase VT, for star-star VTs, is normal for systems where very limited earth fault current is available on the primary side (such as with unearthed or high resistance earthed systems).  The thought is that even on the secondary side of the VT, the most likley fault will be a phase to earth fault and by having one of the phases earthed, such faults automatically become phase to phase faults, for which there will be sufficient fault current available to blow fuses.

I can not lay my hands on the text to provide a positive reference but will look some more.

RE: Grounding of the middle phase on VT secondaries

Suggestion: Visit
http://www.ohiosemitronics.com/support/AppendixE.pdf
for:
Two-wattmeter method, Blondel's Theorem, etc.
http://www.themeterguy.com/Theory.htm
for: Blondel's theorem, and various metering details
http://www.usbr.gov/power/data/fist/fist3_10/3_10_4.htm
for: Figure 11, two-watthour meter connection (also known as Aaron's Connection). See
http://www.atal.nl/datasheets/PQA-acc.pdf
for rare references to the Aaron's two-clamp connection

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