Line to ground readings on ungrounded delta secondary

[winnie]

It seems strange to me that the primary neutral is left floating. Wouldn't that cause load-dependent voltage shifts?

This is true for wye:wye transformers.

With a wye:delta transformer, circulating current on the delta acts to create a low impedance neutral. This means that a wye:delta transformer will try to hold the wye point at the neutral of the phases.

If you connect a wye:delta transformer to a 4 wire system, then the transformer will try to force the neutral wire to the neutral of the phases. This may be desirable (eg. 'effective grounding'), but will also mean lots of circulating current in the transformer.

This is why you are _not_ supposed to connect the neutral when a delta:wye transformer is used in reverse.

-Jon

 

[jim dungar]

Never seen 480 volt lamps ....two 240 volt lamps in series on each leg of the wye...total of six lamps...these were all on 480 volt systems. Most every older industrial with ungrounded systems did this around here.

I stopped being surprised when I saw 5 instead of 6 lights as many maintenance workers did not understand their purpose.
Voltage divider style units could offer some neutralizing effect on systems with large coupling capacitance.

 

[paulengr]

Ungrounded deltas tend to be all over the place phase to ground wise but they are never zero unless there is a ground somewhere. It is the reason phase lights work.

 

[LarryFine]

This is why you are _not_ supposed to connect the neutral when a delta:wye transformer is used in reverse.

Right. I was looking at the diagram as utility transformers, and I forgot that I have even given the same advice.

 

[powerpete69]

Ok, so utility company comes out with 2 pretty bright experienced guys, nice to see.
Main breaker is OFF for the following tests.
The check amps thru cables with transformer energized and main off. They are all zero except one cable has .02 amps thru it.
They then de-engergize transformer, remove all the secondary cables, isolate them and re-energize.
They check the voltages on the secondary of the transformer again:
Line to ground:
A phase 255 V
B phase 14 V
C phase 13 V

Alright, not what we expected, but ok.

They then put the cables back and re-energize everything and check voltage again line to ground.
A phase 480 ish V
B phase near zero
C phase 480 ish V
Same as before.

Check amps again thru cables with it energized and main breaker open.....ZERO amps thru the 12 cables.

Clear as mud. Any ideas?

 

[winnie]

Ok, so utility company comes out with 2 pretty bright experienced guys, nice to see.
Main breaker is OFF for the following tests.
The check amps thru cables with transformer energized and main off. They are all zero except one cable has .02 amps thru it.
They then de-engergize transformer, remove all the secondary cables, isolate them and re-energize.
They check the voltages on the secondary of the transformer again:
Line to ground:
A phase 255 V
B phase 14 V
C phase 13 V

Alright, not what we expected, but ok.

That is the sort of reading that I expect for a small 'floating' system.

The meter itself has impedance to ground. It is placed in series with the transformer coils and any incidental leakage to ground. Because you are moving the meter from phase to phase the meter impedance is moving from phase to phase, changing the voltages.

If the meter was a 'low impedance' unit such as a 'wiggy', then I would expect 0V for each of the values.

They then put the cables back and re-energize everything and check voltage again line to ground.
A phase 480 ish V
B phase near zero
C phase 480 ish V
Same as before.

Check amps again thru cables with it energized and main breaker open.....ZERO amps thru the 12 cables.

Clear as mud. Any ideas?

Phase B has connectivity to ground, but possibly of relatively high impedance.

The current was probably being measured using a clamp on meter which would be hard pressed to actually see a couple of mA of current, but that small current is sufficient to 'drag' phase B to 0V.

-Jon

 

[kwired]

If you placed a 480 volt single phase load of some sort between A or C and ground and it can sustain that load without significantly dropping voltage, then there has to be a rather solid ground connection to B someplace.

If having trouble finding a single phase 480 volt load, use two identical 240 volt loads and place them in series, heating elements of some sort is probably the best thing to use. Could also use four 120 volt incandescent lamps in series.

 

[synchro]

I agree with kwired about testing it with a L-G single phase load that can work at 480V.
In addition to the loads he suggested, another option would be a single phase 480V /120V transformer with a load suitable for 120V.

 

[kwired]

I agree with kwired about testing it with a L-G single phase load that can work at 480V.
In addition to the loads he suggested, another option would be a single phase 480V /120V transformer with a load suitable for 120V.

Which very well may already be some those on site if service voltage is 480. Either control transformers in machines or some for general use lighting and receptacle loads.