1050-208/120GRdY 3

[tulum]

Are there any concerns with unloaded xfmrs'?

We have a special application 3phase, 5kva, 1050-208/120GRdY transformer (hope designation is right) feeding a GFI receptacle on 1 phase. The other 2 phases are for future use.

For all intents and purposes this arrangement is the same as opening all breakers in my house except 1 receptacle, and that is obviously OK.

I guess my question is at what size do unloaded xfmr's start to pose a problem (if they do at all)?

At no-load is the transformer primary current not all excitation current?... and since the excitation current is largerly made up of magnetizing current would the primary current not be 90deg out of phase? What about ferroresenance? How long do the secondary cables have to be to produce enough line capacitance to produce ferro?... and does any of this really matter (I don't know that is why I am asking)?

These are just my initial thoughts (that I have put together from previous post). Instead of me guessing some more, what do you guys think?

Thanks

 

[busbar]

IEEE Standard C57.105-1978 Application of Transformer Connections in Three Phase Distribution Systems §7 Ferroresonance raises a few points. It suggests sthat ferroresonance is not much of a problem at less than 12kV, that there must be significant primary shunt capacitance, and that 1ø switching on a 3ø primary circuit must initiate the resonant-overvoltage scenario.

Based on those conditions, ferroresonance in the described instance should not be a significant problem.

 

[jbartos]

Suggestions to tulum (Industrial) Feb 20, 2004 marked ///\\Are there any concerns with unloaded xfmrs'?
///Generally, yes there are concerns.\\We have a special application 3phase, 5kva, 1050-208/120GRdY transformer (hope designation is right) feeding a GFI receptacle on 1 phase. The other 2 phases are for future use.
///What is the kVA size of the transformer? Are there taps?\\For all intents and purposes this arrangement is the same as opening all breakers in my house except 1 receptacle, and that is obviously OK.

I guess my question is at what size do unloaded xfmr's start to pose a problem (if they do at all)?
///There is a combination of factors that can cause a problem at a relatively small size of transformers. Specifically, overvoltages can pose concerns; especially, if there is not on line tap changer, i.e. fixed taps are being used to compensate for voltage drops. Higher voltages on the unloaded secondary side of the transformer can cause malfunctions of some loads, e.g. motors, ballasts, electronic devices, etc.\\At no-load is the transformer primary current not all excitation current?
///Yes, approximately so.\\... and since the excitation current is largely made up of magnetizing current would the primary current not be 90deg out of phase?
///Yes, it is approximately 90° inductive/lagging.\\ What about ferroresenance?
///The ferroresonance occurs in nonlinear circuits. The transformer has the parallel branch in its electrical equivalent circuit nonlinear, which may produce ferroresonance under certain conditions, e.g. the secondary is connected to capacitive load/transmissions.\\ How long do the secondary cables have to be to produce enough line capacitance to produce ferro?
///This would have to be calculated or simulated on PC.\\... and does any of this really matter (I don't know that is why I am asking)?
///It depends on the transformer size, power distribution configuration, loads, etc.\\These are just my initial thoughts (that I have put together from previous post). Instead of me guessing some more, what do you guys think?
///These concerns are more urgent from medium voltages and up.\\

 

[jghrist]

There should be no problem having two of the phases unloaded. The magnetizing current is inductive, so there will be a very low power factor, but the load is so low it won't make any difference. If the transformer has a delta primary and the primary feed is UG cable, there will be a possibility of ferroresonance with remote single phase switching of the primary. If this situation exists, ferroresonance will be more likely if the transformer is unloaded. There is no ferroresonance problem with secondary cables.

 

[busbar]

Does the subject transformer have wye or delta primary windings? In mining, a 1040Y system has 600 volts to ground.

 

[tulum]

Thanks everyone for the replies.

Jbartos,

The KVA size is 5kva with no taps (sorry about the previous poor wording.)

jghrist,

Why is ferroresenance more likely if the primary cables are UG cable? Is it solely because the capacitance UG is larger than in air?


Busbar,

This is a mining application xfmr. The transformer is delta primary (ungrounded) and grounded Y secondary. I believe the designation is 1050-208/120GRdY...

Regards,

 

[jghrist]

tulum wrote:

Why is ferroresenance more likely if the primary cables are UG cable? Is it solely because the capacitance UG is larger than in air?

yes.