Harris Institute of Technical Training

Hello Brad,
We have a few 2400 volt 3wire ungrounded delta substations for a member that we meter with normal 5S 2 element metering. We will soon be rebuilding one of these subs and upgrading to a larger transformer. One of our engineers wants to know if we can meter this station with 3 vt's and 3ct's and if not I'm sure he wants to know why. I don't really have a good answer other than Blondel's Theorem. Can you explain further? Is there an alternate method of metering this service?
Thanks, Shannon

Hello Shannon,
Thanks for your question. Metering folks like adhering to Blondel’s Theorem in all aspects of the metering world. It is often a mantra that many of us recite by heart and focus our efforts in complying with. There have been many times when his theorem was overlooked and in many metering installations, it was truly ignored. In the past, finding a way to trick this theorem was often how we did metering. Doing it this way saved us a little money on the device that is supposedly the cash register of the company, which is not a very good way to keep your profits and losses accurate.

So, now for your answer: I am often asked this question and with what is being offered by your engineer it is not a Blondel’s Theorem solution and you are correct in saying “according to Blondel’s the three phase, three wire installation should have two CTs and two VTs and a two element meter”. As I said, we often tricked Blondel’s to save money on the meter and meter installation. By metering this 3 phase, 3 wire DELTA system with three CTs and three VTs is over-metered, but not out of the ordinary. It is truly fine to meter this with three VTs and three CTs connected as you would a four wire WYE installation. The VTs will need to be connected phase to neutral in the installation with the neutral connected securely to the substation ground grid. You will use a CT in each of the three conductors and with this connection the metering will be correct. Whatever the phase to phase voltage of the three phase, three wire system is now, you will need to divide it by the square root of three to determine what the primary voltage of the VTs will be.

I believe the engineer is looking for the information from all three current transformers for system requirements and measuring the three wire system with three CTs will allow this.
Hopefully this answer works for you. If you have further questions please let me know.
Brad