Harris Institute of Technical Training

I have a customer with 480Y/277V transformer secondary and has installed Form 35S meter with 2CT's and no PT's. Meter is Elster A3 solid-state. The equipment end use is a PV inverter which is 3-wire, but the transformer is 4-wire, so I feel this should be 9S and not 35S. In addition they grounded the Vref at the bottom of the meter socket, which I believe is incorrect. Should the voltage Vref at the bottom of the socket be connected to the phase without the CT's and not grounded? They feel it is a delta installation but I feel it is 4-wire wye. The meter is showing about 5% error to another meter. Any thoughts?

Dave,
Thanks for the question. Your thinking is correct. If the customer has access to all four wires from the 480Y/277 volt three phase transformer secondary, then the metering needs to be a four wire meter (a 9S meter is the correct Blondel’s solution). If this is the case, then the only way to guarantee that any and all the loads will be metered correctly is with the 9S meter and three CTs.

If the customer has access to all four wires, then the present metering scheme could be and would be correct, if they only have a three phase, three wire load connected at 480 volts. However, if there are phase to neutral loads, then the metering is guaranteed to be incorrect and the customer billing will either be too high or too low, but it will not be accurate. It may be that the customer has only a three phase, three wire load today, but what might be the requirements or future plans for this installation?

If the customer only has access to the three phase conductors from the four wire system, then the metering should be correct for a three phase, three wire load. However, if they create a neutral point and connect any phase to neutral loads then there will again be incorrect metering and billing.

What you are describing here as Vref, I am thinking is a reference to the neutral conductor. If that is the case, then this grounded conductor cannot be connected to the phase conductor without a CT.

As to why you are seeing a 5% error on this installation, I wish I could help you more with this issue. A 5% error doesn’t make much sense to me with this type of metering set up.

If the decision is to leave the 35S meter in service, then make sure that you follow Blondel’s Theorem. Make a common connection to the conductor without a CT or current coil to both potential coils of the meter (this means that you would be using the third energized phase conductor and not the neutral conductor).
Brad

It is possible to meter a 4 wire wye system with a Form 35S meter. It is a non-blondel solution and is subject to the same errors as any other non-blondel solutions (i.e. Forms 2S,3S, 4S, 6S, 14S, 15S). See page 30 of the ABB All-in- One Book for the correct wiring.

http://www.support.tikaenergy.com/tikaDocs/Elster_ABB_All_in_One_Book.pdf

Just a note of interest, the Form 5S meter was once known as the universal meter because it could be used for so many metering applications!