Minimum spacing between bar end arms and crossover clearance.

[kh2]

This subject is about the minimum spacing between the bar end arms and the minimum clearance at the first crossover bar behind the top bar on a 13.8 kV rotating machine, 3 phase wye connection, NEMA Class F insulation, air cooled hydro generator. Some specifications for the stator rewind asked for a spacing and clearance not to be less that 5/8 inches (0.625 inches). The purpose for this minimum spacing (I was told) is to avoid the partial dicharge activities that might develop between two bars (coils) belonging to different phases during the 30 years life expectancy of the windings. In this case the voltage difference betweeen the bars would be the same as rated voltage (13.8 kV).

Please advise if there is a Stndard or a formula to be used on making the decision on the minimum healthy spacing and clearance since some existing (old) machines may not have the space available at the top and at the bottom to allow for the 3/8 inches clearnce.

Appreciate very much your input and sharing your experience. Many thanks.

 

[kh2]

There is an equation which is used to calculatate the voltage in the air spacing between the bars (coils) end arms or the bars crossover. From the voltage you calculate the airgap volt/mil and if it is less than 50 volts (rms) per mil, the spacing or the gap is okay. The 50 volts/mil is the electrical stress breakdown voltage for the air.

The equation is Va = V x [1/(1 + 2d/{ei x t})]
Va = voltage across the air spacing.
V = voltage difference between conductors (ph-ph is the max).
d = groundwall insulation (inches)
ei = relative permittivity of groundwall insulation = 4, the relative permittivity of air = 1
t = spacing or airgap (inches)

Example: d = 0.140 inches; t = 0.5 inches; and V = 13.8 kV.
Va = 12.1 kV & volt/mil = 12100/500 = 24 volts/mil which is less than 50 volts/mil and therefore, 0.5 inches spacing is adequate based on this equation. And you can use 0.25 inches and still below the 50 volts/mil.

The explaination for the 5/8" (0.625 inches) spacing that is given on many specifications for the winding of 13.8 kV machines could be that the air spacing with time is contaminated from moisture or oil mixed with dust/dirt and develop electric tracking along the end arms. This introduces a conductive path between the adjacent bars (coils) of different phases and slowly partial dischares can arise and attack the ground wall insualtion on the surface. Permanent phase-to-phase or phase-to-ground deterioration may occur and the groundwall failes. The farther away the endarms from each other the better because it slowers the development of the partial discharge and increase the life of the winding. It is a very slow failure and could take 10 years to develop.

 

[edison123]

kh2

Incidentally, I am also interested in the end winding gaps between coils/bars. See my post "coil to coil clearances ..." in electric motor forum.

The equation you have given sounds interesting. Where did you find it ? Is there a theory behind it ?

 

[kh2]

edison123,

The equation was given on a paper published on Doble Client Conference Minutes 1974 on Section 7 "Rotating Machinery" on page Sec.7-101. The title is "Corona Protection for High-Voltage Stator Winding".

The 50 volt/mil is the electrical stress for air gap breakdown voltage for gaps in the range of 20 to 5000 mils and atmospheric pressure. On the Partial Discharge Seminars on stator windings, it is stated that an arc, or a partial discharge will occur if the voltage across air-void (or air gap) diectric exceeds the electrical breakdown point for air (Eair = Vair/dair > 3 kV/mm). The 3 kV/mm is 76 volts/mil (Peak-Peak) = 76/1.414 = 53 (r.m.s.) volt/mil and that is why the not exceed 50 volt/mil was used. Regards, kh2