Transformer connection

[Fernando]

We connected 3-75kVA transformers (%Z = 1.5) in a Delta (Primary - 12kV) and Delta (Secondary -480V). When we measured the voltage on the secondary, we read 500V Line to Line; but the line to ground voltage readings were not what I expected (230V, 240V, and 430V). I was expecting to read 277V on all three phases. Any clue of why the variation in line to ground voltages?

 

[infinity]

Do you have a grounded 4 wire Delta secondary?

 

[Fernando]

No, it's an ungrounded 3-wire delta.

 

[Fernando]

One thing to note is that, we tried connecting the transformer in a grounded Wye secondary. The Line to ground voltage all read 500V, but my line to line voltage now reads 840V.

 

[don_resqcapt19]

but the line to ground voltage readings were not what I expected (230V, 240V, and 430V). I was expecting to read 277V on all three phases. Any clue of why the variation in line to ground voltages?

If you are connected to a wiring system, the capacitive coupling of the ungrounded conductors causes this. It is not a problem. The voltage to ground will float around on an ungrounded delta system.

One thing to note is that, we tried connecting the transformer in a grounded Wye secondary. The Line to ground voltage all read 500V, but my line to line voltage now reads 840V.

If each coil is 480 volts, then you will get 1.73 times that if you connect the coils in a wye. As far as the 500 volts, if you have taps, you should change them to get the secondary voltage down to 480.
Don

 

[kingpb]

You may not need the taps right away, if the voltage you are reading is no-load off the transformer. You can overvoltage the bus by 5%, up to 10% so connecting your loads to the 500V should not be problem. You can adjust the taps after you know what the actual voltage is going to stabalize at.

 

[zog]

Have you tested the transformer? TTR test?

 

[jim dungar]

You have three single phase transformers, each with 480V low voltage windings.

There is no connection possible that will provide 277V line to ground.

 

[don_resqcapt19]

Jim,

You have three single phase transformers, each with 480V low voltage windings.
There is no connection possible that will provide 277V line to ground.

When these transformers are connected in delta, you will often read close to 277 phase to ground. This happens when everything on all three phases is in balance. If you have differning loads and wiring on the phases, you can get voltage readings like the original poster did. These readings do not indicate a problem.
Don

 

[jim dungar]

Jim,

When these transformers are connected in delta, you will often read close to 277 phase to ground. This happens when everything on all three phases is in balance.
Don

The original poster said he expected to see 277V. My point is, this is an unreasonable expectation based on his transformers only having 480V on the LV winding.

When dealing with ungrounded 480V systems, you are more likely to not get a voltage reading of 277V to ground than you are to get one. 277V will only show on a high impedance meter when all of the load conditions are perfectly balanced. Any difference in the coupling capacitance of the line conductors to ground will change your readings, the higher the impedance of the meter the more it will "respond".