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Quiz: highest turn-to-turn voltage stress location due to surge
2

Quiz: highest turn-to-turn voltage stress location due to surge

Quiz: highest turn-to-turn voltage stress location due to surge

(OP)
Let's say I have a steep (0.1 usec rise-time) surge in the line-to-ground voltage entering the line lead of a form-wound motor.  What will be the location of the highest resulting turn-to-turn voltage stress:

A - first turn of the first coil
B - last turn of the first coil
C - something else?

("first" means closest to line end)
 

=====================================
(2B)+(2B)'  ?

RE: Quiz: highest turn-to-turn voltage stress location due to surge

Based in my experience wth MV motors, the highest would be on the first coil.

Mac

RE: Quiz: highest turn-to-turn voltage stress location due to surge

A - first turn of the first coil

View Clyde's profile on LinkedIn

RE: Quiz: highest turn-to-turn voltage stress location due to surge

2
(OP)
We have a vote from Clyde.

Mac - first turn or last turn in the first coil?
 

=====================================
(2B)+(2B)'  ?

RE: Quiz: highest turn-to-turn voltage stress location due to surge

I tend to agree with Clyde, although I sense there may be a little more to this question than first meets the eye.
  

----------------------------------
  
If we learn from our mistakes I'm getting a great education!
 

RE: Quiz: highest turn-to-turn voltage stress location due to surge

(OP)
2 Votes for A.
More than meets the eye... you're probably right. My quiz questions are usually the result of something that I learned that I wouldn't have thunk before I learned it (no grammar police please, you know what I meant).

=====================================
(2B)+(2B)'  ?

RE: Quiz: highest turn-to-turn voltage stress location due to surge

First turn in theory is most stressed. But in reality, I have seen surge failures happen across the winding and in some cases, even at the neutral end coils. I guess the weakest turn insulation would be the first to fail.

Muthu
www.edison.co.in

RE: Quiz: highest turn-to-turn voltage stress location due to surge

(OP)
I guess there are a lot of ways to address the question and field experience is one.  It may be difficult to separate the turn failures that are caused by mechanical effects, electrical stress effects, localized defect in the insulation material, or some combination.

The "answer" I was looking for was B (last turn of first coil).
This is based on my reading of EPRI 5826 Volume 1 "Turn Insulation Capability of Large AC Motors", with contributions from Gupta, Pillai, Stone, Greenwood and others.  An excerpt is below:

Quote (EPRI):

In propagating through successive coils, the surge front was thought to be flattened out because of internal winding capacitances. As a result, the voltage developed across successive coils was smaller in magnitude. For the same reason, the highest interturn voltage was thought to appear across the first turn in the line-end coil. Other authors argued that strong coupling between turns would make voltage distribution within the coil nearly uniform. Both these conjectures were subsequently refuted by experiment. which showed that interturn voltage distribution was highly nonuniform at fast surgefronts, and that the highest interturn voltage appeared across the last turn in the coil

The report describes detailed measurements on 11 different large form-wound motors, which all supported this conclusion (among other conclusions).  The measurements involved modifying a coil to provide access to voltage measurement at one point on each coil, then applying low-voltage steep-front wave and measuring the resulting voltages vs time at that one location in each turn.  There are a variety of test configurations used but they all support the conclusion of the highest stress across the last turn of the first coil.

There are other distinguishing features of the waveshapes that led the authors to develop a "simplified" model (that matches the measurements very well (not quite as well as their detailed EMTP models, but pretty close).  The simplified model is that the dominant mode of wave propagation within the coil is the "transmission line" formed by two adjacent turns.  In an N turn coil, the wave travels between turns  1 / 2,  then 2/ 3 etc then N-1/N... for a total of N-1 trips around the coil.    At the end of that N-1th trip it encounters a chance in surge environment since it now sees a series jumper which has a much higher wave impedance, so a large portion of the wave is reflected backwards at this interface (causing that large stress where the incoming/reflected wave overlap on the last turn of first coil).   The wave goes all the way back to the beginning of the coil, and then reflects forward again.  The total waveform can be constructed from lattice diagram accounting for several reflections (they provide a pretty detailed cookbook/recipe for that calculation... I might try it).   But due to the decaying nature of the wave it is really the first internal (within-coil) reflection that creates the highest stress. And that occurs at the end of the first coil.

Interesting that the transition from slot section to endturn section doesn't generate any reflections by their measurements and model.  The authors justify this by mentioning that the  the travel time accross a single slot section or coil end  section is short in relation to the wavelength associated with the highest frequencies present.  I don't completely follow how that supports the conclusion, but I'm happy to just picture that the presence of core permeability and ground plane does not significantly affect a transmission mode which is turn to turn involving primarily the capacitance of the turn insulation between two adjacent conductors.  

=====================================
(2B)+(2B)'  ?

RE: Quiz: highest turn-to-turn voltage stress location due to surge

(OP)
Correction in bold:
"EPRI 5826 Volume 1
should've been
"EPRI 5862 Volume 1

=====================================
(2B)+(2B)'  ?

RE: Quiz: highest turn-to-turn voltage stress location due to surge

This is going back a ways during my xfmr repair days.

So why is the 1st few (start) sections on a Power transformer reverse wound due to the inrush?  This was done on 10-15kva oil filled.  Is it because xfmrs are the opposite type of machine versus the motor winding?

RE: Quiz: highest turn-to-turn voltage stress location due to surge

(OP)
Let's say we had something resembling an infinite winding (many many turns with no impedance changes... no reflections). Then clearly the first few turns see the highest voltage accross them because there is no reflection phenomenon.. the only thing going on would be that an initially sharp pulse gets filtered to be smoother and smoother as it goes through the winding so the turns see less and less turn-to-turn voltage.

The motor has a unique feature that the windings are grouped into separate coils, so there is an impedance change between coils which causes a reflection at the end of the first coil that results in the highest turn voltage at that location.

A transformer certainly doesn't have the same separate coils that a motor does.  I pictured it acting more like the infinite winding I mentioned. I'm not familiar with the reversed turns that you're talking about (because I'm not familiar with the intimate details of transformer windings).

=====================================
(2B)+(2B)'  ?

RE: Quiz: highest turn-to-turn voltage stress location due to surge

Thx pete for that interesting EPRI report. I have actually seen only coil series jumpers cut off.  

Muthu
www.edison.co.in

RE: Quiz: highest turn-to-turn voltage stress location due to surge

ePete,

That makes a certain amount of sense - I was toying with impedance discontinuity as a possible reason why the conventional answer would be 'wrong' when I answered. Thanks for the additional insight.
  

----------------------------------
  
If we learn from our mistakes I'm getting a great education!
 

RE: Quiz: highest turn-to-turn voltage stress location due to surge

Pete  - meant ot say first turn of the first coil.  

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