Motor feeding change over 1

Is there is any effect on motor running if it is started from a source (Transformer 6/0.380 kV ,Yyn6) and then changed over to another source (Transformer 15/0.380 kV ,Dyn1).

The change over is to switch of the first source and then directly switch on the other source while the motor is still running.

 

[electricpete]

If sources are in-phase and transfer is make-before break, then no problem.

If sources are out-of-phase or there is a momentary interruption in power, then you need to look at it closer to avoid possibility of applying a voltage which is out-of-phase from motor residual voltage. There has been lots of discussion on this board on the subject. (try searching for coastdown or fast transfer reclose or slow transfer reclose). If you still have questions or can provide more details let us know.

 

[jbartos]

Suggestion: The transformers do not have the same vector group. Therefore, chances that the transformer secondaries will be in-phase are small. Even, if the transformers happen to have the same vector group, their secondaries may still be out of phase if their primary sources are different and unsynchronized.

 

[rhatcher]

It looks like jbartos is right. A star for him. The starting transformer is a wye-wye and the run is a delta-wye. Without more information, it is safe to assume that the secondaries of these transformers are not in phase.

It is possible for them to be (almost) in phase. Consider the following case:
common 15kV source --->15/6kV D-Y --->6/0.380 Y-Y --> start
common 15kV source ------------------->15/0.380 D-Y --> run

My question is...Even if the start and run circuits are in phase, why would you want to feed them from different transformers? The secondary voltages are the same, so it is not for reduced voltage starting. And, if starting is the reason, then wouldn't some form of reduced voltage starter be more practical than a separate transformer?

If Elhouni would elaborate on his reason for wanting this, perhaps someone in the forum could offer a more practical alternative.

Also, I am not familiar with the following : Transformer 6/0.380 kV ,Yyn6 and Transformer 15/0.380 kV ,Dyn1
Can anyone explain 'n6' and 'n1'?

 

[ScottyUK]

Hi rhatcher,

The 'n' indicates that the star point of the secondary winding is made available for grounding.

The number relates to the relative phase shift between primary phase 'A' and secondary phase 'a'. The numbers correspond to the hours on a clock face, e.g: 6 indicates 180° shift, 1 indicates +30° shift.


Hope this helps.

 

[rhatcher]

Thanks ScottyUK. I've never seen that before. The 30 degree shift for the D-Y is obvious but the 180 degree shift for the Y-Y would not be without that designation. For Y-Y or D-D with primary and secondary in phase, would the value be 0 or would it be left blank? Also, would there be a type 7 for a D-Y with 180+30 shift? If so, then for the example I gave to be true (start and run in phase) it would have to be:

common 15kV source -->15/6kV Dyn7--->6/0.380 Yyn6 --> start
common 15kV source ------------------>15/0.380 Dyn1 --> run

Is that how it works?

 

[ScottyUK]

Hi rhatcher,

Yes, your example is correct, although I haven't encountered a Dyn7 configuration before (UK / Europe). No doubt they are out there on someone's system.

 

[electricpete]

I don't think the original poster said anything about start and run circuits or reduced voltage starting. He just said he wanted to transfer sources at some point after starting while the motor is still running.

My response addressed the possibility that the two transformer secondaries may be in-phase or out of phase.

Is it possible in phase even though the immediately upstream transformers have different connections? Of course, in similar manner to what rhatcher described. We have at our plant an analogous situation. 13.8kv bus can be fed through a single 345 kv/13.8kv y/y transformer. It can also be fed from 25kv/13.8kv delta/wye transformer (plant aux transformer) which in turn can be fed from 345kv/25kv wye/delta transformer (generator stepup transformer). The 345kv comes from same system (in-phase) and the 13.8 from both paths are in-phase and paralleled during transfer.

Thx for your replay jbartos it was as useful as it was prompt.