Harris Institute of Technical Training

I just want to make sure I am right. We are getting a new 46KV metering point on our system. When I spoke to the engineers they said “Our 46 kV system is operated as an ungrounded wye scheme, therefore we will only be extending the three phase conductors to the demarcation point. We establish our grounding system for the next lower voltage at the substation where it is stepped down.” I was hoping to get a grounded wye scheme so I could use 3 VTs and 3 CTs with a 9S meter, but being it is ungrounded, I believe I have to treat it like a 3phase 3wire delta and meter it with 2VTs and 2CTs and a 5S meter. I just want to make sure I am correct on this being it is the first ungrounded wye scheme we have on our system.

Jon,
You are totally correct in your thought process when it comes to metering the ungrounded WYE system with a 2 VT, 2CT and a 5S meter installation. The customer will only be given the three energized phases from the WYE system and the three wires (from Blondel’s Theorem) will be metered by a two element metering set-up. The two VTs will share the un-metered voltage (no CT) for the common connection to both. This common voltage is the phase that will not have a CT connected to it. There are several diagrams in your Meterman’s Bible (book 3) that show how to put this connection together. I like Drawing 93 on page 249 from the Meterman's bible to use as an example connection. If you have any questions about this, or any other of the diagrams in the books, please let me know. To help build your trust and confidence in Blondel’s Theorem, this metering set-up will correctly measure any load connected to it, whether it is single phase, phase to phase or a straight three phase load.
One thing to remember is because the primary voltage is above 30000 volts (46kV) then you will use 115 volts as the secondary voltage which means your VT ratio will be 400:1.
Like I say, if you have any questions give me a call or return e-mail.
Take care and Be Safe All Ways,
Brad
1-503-901-6132