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LaymanÆs explanation importance of coil spacing on 13.8kv motor

LaymanÆs explanation importance of coil spacing on 13.8kv motor

LaymanÆs explanation importance of coil spacing on 13.8kv motor

(OP)
In 13.2kv machines, if there is not adequate air spacing between coils of differing voltage in the endwinding areas, surface partial discharge occurs at that location.  Unfortunately this is not intuitive to everyone. Some people think that if each coil insulation can withstand full voltage to ground (for example in slot section), then there should be no problem with coils in the endwinding regardless of their spacing because we have two lengths of insulation and less than twice the voltage... therefore they think airgap distance shouldn't matter.

There is a standard explanation I have seen using capacitance.  In words I would say that the dielectric properties of the insulation tend to minimize field within the insulation at the expense of increasing field within the air.    In math, I analyse simple parallel plate capacitive model of series air gap and insulation. Since no charge at the interface we have boundary condition:
EPS_air * Eair = EPS_ins * Eins  
where EPS = EPSilon = dielectric constant, E = field, ins = insulation, air=air
Using typical Eins ~ 4*Eair, we have
Eins = Eair/4  [equation 1]

The total voltage between phases is the integral of the electric fields with respect to distance:
Vtotal = Eair * Dair + Eins * Dins   [equation 2]
where D = Distance (thickness) of air and insulation in direction along the field

Substitue Eins from equation 1 into equation 2:
Vtotal = Eair * (Dair + Dins/4)
Eair = Vtot / (Dair + Dins/4)  
This Eair the same field we would see if we had the FULL voltage is applied accross the air gap distance plus another air distance equal 25% of insulation distance.    The insulation doesn't buy us much in terms of preventing air breakdown when there is a small airgap.

This explanation does not seem to resonate since not everyone wants to look at the equations. I am at a loss of how to explain the danger of the small air spacing between in an intuitive way without getting into equations.  Any suggestions?

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

RE: LaymanÆs explanation importance of coil spacing on 13.8kv motor

(OP)

Quote:

Using typical Eins ~ 4*Eair
should have been

Quote:

Using typical EPSins ~ 4*EPSair

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

RE: LaymanÆs explanation importance of coil spacing on 13.8kv motor

Tell 'em that real insulation is guaranteed to have pinholes, and lightning results.

 

Mike Halloran
Pembroke Pines, FL, USA

RE: LaymanÆs explanation importance of coil spacing on 13.8kv motor

That's not quite what Pete means. And there are no PD problems when it comes to DC - not as far as I know.

It is about a potential divider that consists of a piece of dielectric (insulation) some air and another piece of dielectric. Now, I think it is intuitively clear to anyone that knows about electricity that a capacitive current flows through this voltage divider.

I also think that most people with some electrical understanding accepts that voltage across each part of the voltage divider is inversely proportional to that part's capacitance (that is how a capacitive voltage divider works) and that the capacitance of the air part is (a lot) lower than the capacitance of the insulation (which has a high epsilon).

If you can make your audience follow you that far - and that is usually not so difficult - then you can end the discussion telling them that if voltage drop across the air part is high, then the electric field in that air is also high and partial discharge may occur.
 

Gunnar Englund
www.gke.org
--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

RE: LaymanÆs explanation importance of coil spacing on 13.8kv motor

Depends on the audience.

 

Mike Halloran
Pembroke Pines, FL, USA

RE: LaymanÆs explanation importance of coil spacing on 13.8kv motor

The end winding pd is not taken that very seriously as compared to slot pd because it takes years before this pd kills the winding. For me at least, spacing in end winding is important for cooling air/hydrogen flow path.

Muthu
www.edison.co.in

RE: LaymanÆs explanation importance of coil spacing on 13.8kv motor

(OP)
Gunnar – thanks. Circuit approach is probably a good compromise between equations and words.

Muthu - I agree there is different severity to assign to pd depending on where you see it. I use Greg Stone's "Electrical Insulation for Rotating Machines" as a guide in that respect.   

I agree small degree of variability of coil spacing coming from most shops a problem if that's what you're suggesting.  Would you let slide 1 leave you shop? I hope not, but someone did.

As it turns out, this one one family of 13.2kv motors (including the one shown attached) that seems prone to visual indications of partial discharge everywhere:
at the ties in the endwindings
between top/bottom coils in the slot
near the 1st bend outside the slot
burning of the interface between semi-conducting paint and grading paint
burning of semi-con visible in the vent ducts (slide 3)

Clearly it is not all one cause.  One asks the question:  What is it that ties all of these various different things together to make this family of motors a bad actor.  The best I can come up with: workmanship. Both in spacing and in the coatings (what else could it be... we have no unusual contaminants that I know of).

Attached slide 1 is a view of portion of endwinding.  There is no evidence of movement, just ridiculously uneven spacing.   It is tougher to show the differences close to the core, but that's where most of the visual pd symptoms area.  If you look at the axial distance from core to first bend outside the slot, it is very uneven.

This motor showed highly non-linear DC step voltage at 28kvdc on dc step voltage test (jumped from 3 microamps at 26kv to 14 microamps at 28kvdc).

Slots were mapped using corona probe with normal line-to-neutral voltage applied.   Even though the entire phase is at the same voltage during the test, the line-end coils had much higher corona probe readings han the neutral end coils.  Clearly that demonstrates electrically-induced aging (pd?) has gone on while the motor was in service.  The highest three readings on corona probe were 40, 50 70.  IEEE limit is 20.

The motor is 20 years old and has been refurbished once before at the 10 year point.

Nothing unusual shows up in our on-line partial discharge monitoring.
 

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

RE: LaymanÆs explanation importance of coil spacing on 13.8kv motor

Agree, but Pete used the winding heads as an example. The problem is also present in motor cables where PWM HF components force hich currents through air between core insulations. Attach picture with comments in an obscure language. The picture is valid also in your language.

Gunnar Englund
www.gke.org
--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

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