Grounding of the middle phase on VT secondaries 3

[Deansharafi]

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

 

[RalphChristie]

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

 

[busbar]

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.

 

[jbartos]

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

 

[bigamp]

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.

 

[jbartos]

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