Stress on coil insulation volts per mil
Stress on coil insulation volts per mil
(OP)
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.
Any help in this regard would be highly appreciated.
Thank you.






RE: Stress on coil insulation volts per mil
RE: Stress on coil insulation volts per mil
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
RE: Stress on coil insulation volts per mil
RE: Stress on coil insulation volts per mil
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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RE: Stress on coil insulation volts per mil
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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RE: Stress on coil insulation volts per mil
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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RE: Stress on coil insulation volts per mil
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.
RE: Stress on coil insulation volts per mil
RE: Stress on coil insulation volts per mil
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