Interpretation of Loads at the primary of a three-phase, delta-wye transformer

[cdw238]

I currently had the primary feeder of a 225-kVA delta-wye transformer metered. The metering report indicates 480-volts at all three of the phase measurements (L1-L2, L2-L3, L3-L1) but indicates amp readings at only two of the phase measurements (L1-L2 and L3-L1). The amp readings at
L2-L3 were reported as zero amps.

Does this seem correct?

Additionally, is the overall measured load (amps) for this system just determined by adding together the recorded values that were present at the L1-L2 and L3-L1 readings?

Something doesn't seem correct here but I may just be second guessing myself.

 

[charlie b]

First of all, welcome to the forum. Secondly, I fear that I do not understand the question. Specifically, I do not know what you mean be describing an amp reading as ?L1-L2.? Amp readings are taken by placing a clamp on ammeter (or permanently installed current transformer) around a single wire, not between two wires. Also, you did not give the loading conditions of the transformer. What, if anything, is connected to the secondary? Is this transformer in service, or is it being tested at the manufacturer?s facility before being shipped to the site? Please clarify the situation.

 

[cdw238]

The transformer serves branch circuit panels used for lab equipment at a tenant space.

The meter report reported the amps under the three phases. Phase A recorded approximately 50 amps, Phase B recorded 0 amps, and Phase C recorded approximately 50 amps.

I was trying to get a better understanding of 0 amps being present at Phase B.

 

[brian john]

If I understand you, you have a permanently installed meter?

If so once you have resolved what was noted above then there is an issue with

The meter, the CT's or the wiring between the CT's and meter, this could be as simple as a shorting jumper on the CT.

Measure current on the secondaries of the CTs. measure current on the primary conductors.

 

[BJ Conner]

Have you checked the internals of the transformer? You could have and open lead(s) in B phase.

 

[broadgage]

Is the transformer stepping down from utility voltage to 480 volts ?
If so the readings on the 480 volt side may be correct, if the loads are all single phase, and no loads are connected to one phase, or these loads were not in use when the readings taken.

Is the transformer stepping down from 480 volts, to some lower voltage ?
If so somthing is wrong, presuming that the current readings are on the 480 volt primary. If the installation is working correctly then I suspect an error in taking readings with a portable instrument, or defective/miswired ammeters if the readings from installed instruments.

Alternatively if the transformer is 480 volt delta to 120/208 volt wye, then perhaps the current readings are on the 120/208 volt side ?
The readings could then be correct, if only two phases are loaded.

A three phase transformer should allways have some current in each primary connection, even with no load on the output.

 

[rcwilson]

Where are you reading amps? If you are looking at a multi-function meter also used to measure kw and kwh usage, you may have a two or 2-1/2 element meter.

Three phase, three wire power can be accurately measured using a watt-hour element for each phase with three phase-neutral voltages and three current signals a (3-element meter) or by just measuring current in two of the phases in a two-elemnt meter. (Search for Blondel's theorem to read up on this.) A 2-1/2 element meter uses three voltages and two currents, usually A & C currents. For 3-phase, 3-wire loads Ia + Ic = -Ib. The meter is wired internally to add A+C to get B current.

The currents may be correct but you are reading the wrong type of meter.

If it is a programmmable meter like an Electro-Industries Shark , the setup programming might be wrong. If it is programmed for 2 elelment or 2-1/2 element, the phase B current will read 0. Reprogram it for three-element.