Voltage Transformer Wye Ungrounded Pri - Wye Grounded Sec Question

[E_engineer]

Hello, I am working on a design for a 4160V system which is high impedance grounded through an NGR. I'm looking at the design for a voltage transformer which is 4200/120V, wye/wye connected. One of the engineers said they typically leave the primary side of the VT ungrounded, but they ground the secondary side going to the relaying/metering. My concern with this is that if there is a ground fault on the primary side of the VT, the primary side fuse may not blow and you will have higher voltages on the non-faulted phases of the VT which could damage insulation.

 

[davidbeach]

Primary should be grounded. If it isn’t there’s no way to detect a 4160 ground fault.

When one this sentence into the German to translate wanted, would one the fact exploit, that the word order and the punctuation already with the German conventions agree.

-- Douglas Hofstadter, Jan 1982

 

[E_engineer]

Thanks, that's my thought was as well. I just could not find any information online to support it.

 

[waross]

Grounding the 4160 Volt source is the responsibility of the owner of the 4160 Volt system.
If the primary wye point is not connected to the system neutral (usually by grounding) then the secondary voltages may be unbalanced in response to unbalanced secondary loads.
This may not be as bad as a simple analysis may indicate as the effect may be limited by transformer saturation due to over-voltages caused by secondary faults or unbalanced secondary loading..

--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[RRaghunath]

In case of High Resistance Grounded (HRG) system, there is a possibility of primary side ground fault protection picking up for a VT secondary ground fault, in case the neutrals of both primary and secondary of the VT are grounded and in case the ground fault protection is set sensitive.
This could be the reason why VT primary star is not grounded, probably.
In Generating stations with HRG of Generator neutral, IEEE recommends grounding of y-phase instead of neutral on VT secondary. The primary neutral is grounded. With this arrangement, secondary ground fault will not be seen as ground fault on primary side of VT.

R Raghunath

 

[wcaseyharman]

General Electrical large gas and steam turbine generators almost exclusively use open delta PTs on the medium voltage system to address the issue of PT secondary faults tripping the NGR ground detection. I looked back and found open delta PTs as far back as 1935 on a GE hydro generator up to the newest gas turbine we own, built in 2001.