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Topic Title: Corner grounding - Why?
Topic Summary: What's the purpose of "corner" or "leg" grounding/earth reference in 3 phase metering???
Created On: 12 October 2016 04:44 PM
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 12 October 2016 04:44 PM
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solaral

Posts: 45
Joined: 04 November 2006

I'm currently trying to integrate a power quality meter into a 3 phase metering system that has the yellow, "leg" gounded/earthed. For clarity, it is wired in star (wye) with the neutral connected to the star point and the earth reference connected to the yellow "phase" or "leg" connection.

What is the reasoning/purpose behind this can anyone tell me please?

Thanks for reading.

Solaral.
 14 October 2016 01:24 AM
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mapj1

Posts: 10455
Joined: 22 July 2004

I think you are mistaken. Normally one grounds and neutrals to the same place, or there is likely another problem. What country are you in ?

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 15 October 2016 10:45 AM
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solaral

Posts: 45
Joined: 04 November 2006

Definitely no mistake ~ yellow phase earthed on the secondary. Neutral floating but used as a reference for the three phase voltages. This is in the UK. Not sure where the switchgear originated from...
 15 October 2016 10:12 PM
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mlee1066

Posts: 32
Joined: 03 December 2007

It was very common, almost the norm in the UK. If I recall correctly, one reason that was given to me many years ago was that if the star point was earthed on the secondary, then an earth fault on the secondary would show as an earth fault on the primary. This may cause problems with sensitive earth fault settings.
 16 October 2016 09:23 AM
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ToniSM

Posts: 390
Joined: 21 November 2006

Corner grounded Y is used for metering VT's. Confused the hell out of me the first time I came across it.

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 17 October 2016 03:55 PM
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solaral

Posts: 45
Joined: 04 November 2006

Thanks for all the responses.

After some consideration, I have my doubts about the earth fault transfer. The secondary circuit is galvanically isolated from the primary so an earth fault on the secondary would just "look" like another load to be suuplied by the primary circuit. Saying that, I don't really know which is why I'm posting here.
 17 October 2016 05:56 PM
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mlee1066

Posts: 32
Joined: 03 December 2007

As the primary winding of the VT is connected to earth, current flowing through one phase of the primary, which is not balanced by current in the other phases, will be seen as an earth fault.


q]Originally posted by: solaral

Thanks for all the responses.



After some consideration, I have my doubts about the earth fault transfer. The secondary circuit is galvanically isolated from the primary so an earth fault on the secondary would just "look" like another load to be suuplied by the primary circuit. Saying that, I don't really know which is why I'm posting here.
 18 October 2016 09:15 AM
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solaral

Posts: 45
Joined: 04 November 2006

Thanks for the response mlee1066. I get what you're saying about the unbalanced load looking like an earth fault. Consider this though, the secondary neutral is now at phase (63.5V) voltage. What does an earth fault on that look like to the primary?

I've seen references that mention blocking zero sequence voltages but I'm still looking to find a comprehensive explaination for this configuration. The reason is mainly driven by "change control". If I'm to change the configuration to a star-point earthed secondary (to make the new meter work) why was it configured as a corner earth in the first place?
 19 October 2016 11:41 AM
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solaral

Posts: 45
Joined: 04 November 2006

Originally posted by: mlee1066

It was very common, almost the norm in the UK. If I recall correctly, one reason that was given to me many years ago was that if the star point was earthed on the secondary, then an earth fault on the secondary would show as an earth fault on the primary. This may cause problems with sensitive earth fault settings.


I've just found a link that supports this...

See: http://www.eng-tips.com/viewthread.cfm?qid=200371
 19 October 2016 03:51 PM
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solaral

Posts: 45
Joined: 04 November 2006

mlee1066... Thanks for your pointer.

I did some more digging and found the following text in, " The Art & Science of Protective Relaying," by GE. Chapter 10 (page199).

"If grounded-neutral wye-wye potential transformers are connected to the generator leads,
it may be impossible to get complete selectivity between the relay and the PT (potentialtransformer)
fuses for certain ground faults on the low-voltage side of the PTs, depending
on the fuse ratings and on the relay sensitivity. In other words, the relay may sometimes
operate when there is not enough fault current to blow a fuse. Such lack of coordination
might be considered an advantage; the relay will protect the PTs from thermal damage for
which the fuses could not protect. To make the relay insensitive enough so that it would
not operate for low-voltage ground faults would sacrifice too much sensitivity for generator
faults. Of course, if the relay has time delay, it will not operate for a momentary short
circuit such as might be caused inadvertently during testing. If the relay is used only to
sound an alarm, some selectivity may be sacrificed in the interests of sensitivity; the relay
will not operate frequently enough to be a nuisance."

In addition to this, operating with the Yellow earthed means that line voltage is available to blow a fuse rather than phase votlage which may not be enough to blow the PT fuses.
 14 November 2016 12:41 PM
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ArthurHall

Posts: 752
Joined: 25 July 2008

Earthing the yellow phase and fusing the neutral was the traditional way of connecting VT secondaries in the UK. I always understood the reason was to make earth faults have full phase to phase voltage across them so there is a better chance of the fuse blowing given the high impedance of VT secondary winding's. Modern practice is to earth the neutral in line with European practice.
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