Using wye PT's on 3-wire system

[Dumbo2929]

We have a 23kV 3-phase 3-wire system to be metered. The only available PT's are single bushing. Would it be accurate to install these in an un-grounded wye connection on the primary side and a grounded wye connection on the secondary side and use a three-phase 4-wire metering set-up? I know it is not preferred, the question is will it work?

 

[scottf]

Single-bushing VTs should be used on grounded-wye systems only. No exceptions (that I can think of).

Frankly, accuracy is the least of worries (not that the accuracy wouldn't be wrong too).

Single-bushing VTs, typically have the H2 (IEEE) or P2 (IEC) bonded/tied to the base plate/terminal box. That plate/terminal box is going to be grounded...either directly by the connection of a ground conductor to the VT's ground pad or indirectly by mounting the VT on a metallic structure.

I don't time/space to list all of the problems that would come from an install you describe...just don't do it.

 

[LionelHutz]

The primary side would need the case floated and connected to the ungrounded neutral of the system. First, it doesn't sound like the neutral exists and secondly this creates a big issue on the secondary side since that would put the primary system neutral voltage onto the secondary windings making the secondary wiring and meter unsafe.

 

[PHovnanian]

As others have pointed out, not grounding the high side PT neutrals is both a safety issue and a practical one. Since the PT H2 connections are brought out to the case, the only way to connect them ungrounded would be to mount them on an insulated support. The safety issues are too numerous to mention.

That said, if the PT and metering voltage ratings are sufficient to withstand line to line voltages, then it is possible to use these for metering your system. The PT high side neutrals (H2) will be grounded through their mounting structure to the metering equipment grounding system. It is only necessary to measure the phase currents and phase voltages to one common point of connection of the metering voltage elements (Blondel's Theorem). That common point can be the metering system ground point as long as the metering voltage elements (PTs and metering) can withstand the possible unbalanced system voltages.

 

[VTer]

Is this a 23kV 3-wire solidly grounded wye system?

"Throughout space there is energy. Is this energy static or kinetic! If static our hopes are in vain; if kinetic — and this we know it is, for certain — then it is a mere question of time when men will succeed in attaching their machinery to the very wheelwork of nature". – Nikola Tesla