Using Meter Form 35S on 4-wire service 1

[Dumbo2929]

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?

 

[tesladog58]

is this a 200 amp service, that makes a differences in meter types. we'll start with this question first to get
the right answer

 

[Dumbo2929]

It is a CT installation with a Class 20 meter. The feed is actually 800A at 480V

 

[tesladog58]

an easy phone call can solve this, to me

 

[waross]

I would not assume that any electronic meter will meter PWM inverter outputs accurately. There is the possibility of sampling errors depending on a combination of the sampling rate and the inverter pulse frequency. A few years ago one of our Gurus ran into gross meter errors with a family of electronic meters and a fairly simple load that was chopped in some manner.
Sampling errors aside;
The meter should meter delta properly. With a three wire inverter feed, and no other power connections from the inverter, I believe that the delta metering should be a good solution.
The wye connection of this meter assumes that the voltages are equal and will be in error if the voltages to the neutral are not equal.
The meter has only two elements.
The B phase current may be resolved into an A phase component and a C phase component.
The B phase current is passed through both the A phase and the C phase meter coils.
The A phase circuit in the meter only responds to the A phase component of the B phase current under the assumption that B phase voltage is equal to A phase voltage.
The C phase circuit in the meter only responds to the C phase component of the B phase current under the assumption that B phase voltage is equal to C phase voltage.
B phase metering errors will be proportional to any voltage differences.
Anecdote. We used this connection for wye services for quite a few years. Then we lost a submarine cable. The replacement cable was offset from the existing two phases with a resulting voltage difference that introduced errors into our metering.
We changed the meters out for full three element meters.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[waross]

Ground location is a matter of safety and convention. The location of the ground connection should not affect the metering accuracy.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[stevenal]

Blondel says count the wires and subtract one to find the number of meter elements needed. I get three. I understand the inverter is three wire, but what is to prevent your customer from adding a 277 line to neutral load in the future? I would insist the proper 9S meter be used.