VT Primary Earthing in Ungrounded Systems

A few more years of schooling...?

_{Ahem. That's a little unfair. One never stops learning in this world, particularly on CR4.}

"The only stupid question is the one that no-one asks." - Anon

A1) Because ungrounded systems have no earth reference at the source of supply. Therefore there can be no significant earth fault current with such an arrangement.

What the VTs are doing is monitoring the phase voltage with respect to earth; if one of the phases drops with respect to earth then it can be inferred that there is less-than-perfect insulation between the phase and general earth potential. That constitutes a fault that needs to be eliminated, however it can be tolerated for a while, and that is why such arrangements are used were a loss of power could be extremely hazardous; hospitals sometimes use ungrounded systems, for example - one wouldn't want equipment that is sustaining life to go off when a pesky earth fault appears somewhere else on the distribution network!

See the Wikipedia page entitled Earthing Systems for a better understanding.

A2) Nothing.

Because a VT (Voltage Transformer) is a high impedance device, rendering it useless a grounding device which must be able to handle the available short circuit current without failing.

Take a look at zig-zag grounding transformers, which are designed to provide a source of ground fault current for an ungrounded system.

I have never seen them applied on an effectively ungrounded system, such as a Y sourced system at a great distance. There is much information on the pitfalls of operating power systems with no ground reference, a common practice in the 1950s & 1960s.

http://ecmweb.com/content/basics-zigzag-transformers

https://electricalnotes.wordpress.com/2012/05/04/zig-zag-connection-of-transformer/

http://apps.geindustrial.com/publibrary/checkout/Zig-Zag-Trans?TNR=White%20Papers|Zig-Zag-Trans|generic

There is a small fault current in ungrounded systems. Does this small current flow through the primary windings of the potential transformer?

1. What effect does this small current have on PT Primary winding if it flows?

2. How this small current affect the secondary voltage?

The small current is not considered fault current, except what is allowed by the PT winding if grounded, it is normally charging current for the distributed capacitance of each of the 3 phases to ground or earth potential.

1. If you have wye connected PTs also grounded, then one primary winding will show zero voltage

2. The secondary will transform the primary voltage.

The link below shows simplified operation of wye grounded PTs

http://www.electrotechnik.net/2009/03/using-broken-delta-protection-for-earth.html

Also below is more detail for your questions

http://www.i-gard.com/Downloads/techlib/Application%20Guides/C-400EA%20Application%20Guide.pdf

The charging current of the faulted phase goes to zero. The vector sum of the charging currents in the unfaulted phases is 3 times the normal values and flows through the ground.

Will this current flow to ground through the neutral of the primary winding of VT, if the primary neutral is grounded?

1. Read bottom of Page 2 in second reference given above.

2. You answered that in your first statement.

Not sure what you are driving at...