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Sargardani (Electrical) (OP)
28 May 04 13:14
I would appreciate if some one could explain the theory behind the stress (in volts per mil) on coil insulation of an electric generator. What's its significance and why is it taken into account while designing an electric generator?

Any help in this regard would be highly appreciated.

Thank you.

Helpful Member!  gordonl (Electrical)
28 May 04 15:49
This is usually the voltage stress on the ground wall insulation, the insulation between the coil and the core iron.  The voltage stress between coil turns is lower than that between the coil and ground.  A typical value for this insulation is 55V/mil, so a machine designed to operate at 13.8KV has 8KV Line to Ground so groundwall insulation would be 8000/55=145mils or .145 inches thick.
edison123 (Electrical)
29 May 04 9:42
With modern insulation materials like samica, calmica etc., usually the following are the wall thicknesses (radial)

3.3 KV - 1.5 to 1.8 mm

6.6 KV - 2.0 to 2.2 mm

11 KV - 3.0 to 3.3 mm

13.2 KV - 3.8 to 4.2 mm
powerjunx (Electrical)
1 Jun 04 13:09
gordon and edison has all good points.. but what standard you had followed? is it a rule of thumb?
Helpful Member!  electricpete (Electrical)
1 Jun 04 13:34
On the original question, volts per mil has more significance than just volts because material breakdown depends on field strength = volts/distance, not directly on voltage.  We can compute the actual electrical field (VLG / distance) for a motor design and compare it to the breakdown strength of the materials used.  (how much field can the material stand before breakdwon is expected).

Air breakdown strength ~ 3kv / mm

One layer of mica paper tape is on the order of 0.2 mm thick.  A half-lapped layer is 0.4 mm thick   As a ballpark many designs and rewind specifications will require 1 half-lapped layer of mica tape per kv of VLL and possibly one additional layer.   This corresponds to a steady state stress on the order (1kv /sqrt3) / 0.4mm ~ 1.5 kv / mm. Obviously this provides a substantial margin since mica paper tape has higher dielectric strength than air (3kv/ mm).

But it is not as simple a matter as steady state stress. There are transients to consider and the ground wall must be coordinated with turn and strand insulation and constructability considerations.

It is a subject far above my head and I don’t see a lot of concrete literature on the subject. Most of what is out there is in the form of time-proven methods and thumbrules from experts.

As a user we typically do not get involved in specifying volts per mil.  We rely on the performance-related tests which are well defined by the standards: ac hi-pot for ground insulation and surge test for turn insulation.

The closest thing to a spec that I know of EPRI Rewind specification 1000897 which does not allow volt stress exceeding 50 volts per mil.

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electricpete (Electrical)
1 Jun 04 14:27
I’m sorry, I misquoted the EPRI document. Less than 75 volts per mil is “required” and  less than 50 volts per mil is “preferred”  (not sure what preferred means in a spec).

Since I have mixed my units, I should clarify "50 volts per mil" means 50 volts per 0.001 inch. That is on the order of 2kv per mm, 2/3 the strength of dry air.

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electricpete (Electrical)
1 Jun 04 15:13
I reviewed some notes and I see for solid insulation compared to air:
dielectric constant Epsilon is 4x as high as air
dielectric breakdown strength is 100x as high as air

So the breakdown strength of the solid dielectric insulation itself is not an issue. However in the real world there are expected a number of voids within the insulating material.  I believe that control of stress within these voids drives the volts per mil limits.

As a very gross approach we can estimate voltage stress within an air void using capacitive voltage divider approach with assumed dimensions of void etc

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edison123 (Electrical)
2 Jun 04 8:40
All MV stator windings are designed and manufactured to ultimately pass power frequency test at 2 rated voltage + 1 KV. The modern mica-epoxy-glass based insulation systems have minimum breakdown voltage of 20 KV/mm of insulation thickness.

Also, individual coils/bars, during manufacture, undergo hipots at still higher voltage (typically 1.3 times the above mentioned level)as an in-house standard. Hence, plenty of safety factor is built into any good insulation design/system to avoid premature coil/winding failures.

Voids within insulation, between coils and slot walls, are another issue affecting tan delta and partial discharges.

Sargardani (Electrical) (OP)
2 Jun 04 10:38
I appreciate the response of all who contributed in  clarifying the situation for me.
wilkin (Electrical)
2 Jun 04 15:40
Air is difficult to iliminate from insulation as it is in
tiny spaces within most materials.
at 50 volts per mil these spaces will not discharge and cause breakdown.
With special processing higher levels are achievable.
Typical breakdown figures for air
 mm        v/m
        
0.025        355.6
0.123        177.8
0.25        127
2.5        63.5
10        25.4
24.4        10.16

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