White phase earthed VT's

[JY]

Hi,
On the plant at which I work we have the convention that on voltage transformers the white phase is tied to earth and the neutral left floating. I believe this arose from the installation in the 50's (Reyrolle gear) and has continued since.

My question is does anyone know of the reason this is used as against grounding the neutral?

Thanks

Jason

 

[BJC]

I'm not sure what "white phase" is.
It is not uncommon to use an open delta for potential voltage systems. The secondary is a three wire system with no neutral. The systems work quite well and have for some time.

 

[gordonl]

This is just a thought,

Assuming white phase is just one of the three secondary phases, perhaps one phase is grounded instead of the neutral so that changes in the ground potetial don't change the phase to neutral voltages measured by the PT's. Grounding the white phase would then provide a connection to ground for ground reference measurements versus neutral reference measurements. I've never seen an installation like this, but my experience isn't extensive.

Please feel free to cut this up.

 

[busbar]

JY, could you give us a location for this system? There are a lot of regional differences and corresponding jargon, and the internet sort of masks that. Also, what is the PT-winding configuration? In North America, grounded-wye primary, grounded-wye secondary potential transformers are common for solidly-grounded or low-resistance-grounded distribution, and ungrounded open-delta primary, corner-grounded open-delta secondary are often used for high-resistance-grounded (or ungrounded) distribution. Occasionally, there are also (fairly rare, in my experience) corner-grounded electrical systems. In the US, the ‘white’ conductor often refers to the grounded-neutral conductor [typically the common-wye point of a system, or ‘XO’ bushing], so I hope you can appreciate the confusion in trying to assist you. Having a wye-connected distribution system with one phase grounded and a separate neutral conductor is probably rare, but it sounds like that’s what you’re describing. Can you clarify this a bit? In your description, “…white phase is tied to earth and the neutral left floating.” are you referring to the ‘white phase’ as on the PT primaries and ‘neutral’ on the PT secondaries?

 

[JY]

Thanks for the responses, hopefully I can clarify the lack of information.

The location is in Australia, what we have is an 11kV substation with an impedence earthed neutral bus (6.05ohm) which ties the star points of the two incoming 22kV/11kV Delta/Star transformers.

The PTs are on every OCB, in Wye/Wye configuration with one secondary phase tied to earth. The neutral (secondary PT star point) is left floating. In this instance the neutral has not been taken out of the VT terminal box and the two ungrounded phases are typically used on MWHr & Volts indicators.

On all other sub-stations on site this convention has also been used. The company has UK origins and the design is possibly an old UK convention?

Hopefully that clears up a few points, its really just a nagging thing in my mind as to what if any advantage this provides?

Thanks