PT Primary Neutral Earthed

If you forget to earth the primary Neutral, the zero sequence component flow is interrupted.

The zero sequence component is due to an earth fault hence phase to earth voltage of system having zero sequence component deviate from the normal value.

Problems with ground fault protections, problems with voltage restrained O.C.

Please confirm whether it is three single phase units or Three phase unit

Dear sir

It is three phase unit.

Thank u sir

Yours sincerely

G.Govarthinam

Chennai

Nothing will happen.

What you must earth is the secondary neutral point if you have earth leakage detecting devices and protections on the secondary circuits.

The Primary is 110KV you said. If it is Start connected as the secondary, there is no need to connect the start point to earth unless required by the supplying grid for any reason (normally, the earthing is done somwhere else upstream.

I am just giving you an answer for your info. You must consult the local regulation and the power supplier to that PT Primary!

The Voltage Transformer you describe would probably invert & the resulting high voltages would cause mag losses & heating which ultimately would burn the VT out.

I suggest you go to the forum discussion on "Tarrif Metering..." & study Diagram 2 for understanding. This unearthed condition is much the same as an isolated earthing system where the earth return connection path is interrupted.

For further information the 1931 AIEE Vol50 "Physical Nature of Neutral Instability" is recommended.

This discussion raises "LOST ART" aspects which we should all guard against.

"The Voltage Transformer you describe would probably invert & the resulting high voltages would cause mag losses & heating which ultimately would burn the VT out."

Dear sir

I understand from ur statement that "primary voltage will appear in secondary winding when primary neutral not earthed."This will cause core saturation and will result in core heating.

My doubt is whether this concept will apply for both three phase PT unit and 3*single phase PT unit,both connected in star-star connection.

Thank you very much sir for educating the beginners like me.

Sincerely

G.Govarthinam

Chennai

Your understanding or my explanation is incorrect.

You are from India & I expect that the IEC & BS standards mainly apply there.

If that is so then the voltage factor of the VT in question is 1.5/30s or 1.9/30s or 1.9/8H & it is an earthed VT as defined in the above specs & the application is defined in table 1(or 2) dependent on the SYSTEM EARTHING.

You said the Primary voltage was 110kV & you mention star connections. Therefore the VT can only be one of the above VF/s as unearthed 3 Phase VTs are limited to 52kV as per BS7729. I expect on a 110kV transmission line the VF would be 1.5/30s as limiting the earth fault current is expensive & generally unnecessary. So your definition of 3 phase or single phase is not relevant.

With inversion( displacement of the neutral) both Primary & secondary volts rise as per the diagram2 I referred you to.

Safety earthing the secondary is very important but it is not your question as I understand it.