Insulator Flashover,Backflash Rate

[roy9]

Dear all,

Do you know what is the acceptable rate of failure on a 33kv line due to lightning.

 

[MikeDB]

I am not aware of any "standards" for this. What might be acceptable for a rural distribution feed might be very unacceptable for an industrial plant with a lot of electronic controls. Perhaps one of the power quality folks can provide guidelines, but I would guess on the order of 4 to 6 outages per year per 100 miles of line. There are several things that can be done to improve this. What kind of failures are you experiencing?

 

[RRaghunath]

May be you can check the CIGRE survey reports. CIGRE conduct regular surveys to establish failure rates of various equipment and publish reports. I have seen one for SF6 outdoor circuit breakers some time back.

 

[RalphChristie]

Try and find a copy of the Westinghouse T&D Reference Book. Chapter 17 (pg 578 - pg 609) is dedicated to "Line design based upon direct strokes"

The design of a transmission line agains lightning for a desired performance, is practically independent of operating voltage. The main consideration is how to obtain a protection level for the desired performance. The basic principles underlaying the design of a line based on the direct-stroke theory are:
1) Ground wires with sufficient mechanical strengh must be located to shield the line conductors adequately from direct strokes.
2) Adequate clearance from the line conductor to the tower or to ground must be maintained so that the full effectiveness of the insulation structure can be obtained.
3) Adequate clearance from ground wires to conductor must be maintained, especially at the midspan, to prevent flashover to the conductors up to the protective voltage level used for the line design.
4) Last, but equally as important, tower-footing resistances as low as are economically justified must be secured

 

[cuky2000]

As a rough indication of the outage rates normally expected and considered acceptable, the following figures are quoted:

11-66 kV rural lines : 0.7 - 3 outages / 100 km years