Substation Insulator Flashover Point

[wbd]

In a situation such as a 115kV substation the post insulators are approximately 38 inches high. Now the dielectric breakdown in air is approximately 10kV per inch or for 115kV about 11.5". So the insulators are approximatly 3 times larger than the dielectric breakdown distance.

My questions:
1. Would or is it possible for a flashover to occur over the entire insulator which would result in an arc length of 38 inches?
2. Or would the flashover only occur when the distance was 11.5 inches meaning the insulator had failed in some way?

 

[Power0020]

The flashover can occur during overvoltages but there is no direct link between the applied voltage and the BD since the relation is non-linear.

The 10% BD probability is when the applied voltage equals to BIL, i.e. roughly 450H (H height in meters, BIL in kV).

at 50% BD probability, the applied voltage is called CFO (Critical Flash Over),

The relationship between BIL & CFO depends on statistical analysis of BD curve.

 

[crshears]

Contamination can affect this profoundly...

If a substation is located in a northern climate near a busy highway where in the winter the spreading of salt on roadways is employed, under just the right [or is it wrong?] meteorological conditions, such as during a night-time fall of wet snow, the entire switchyard can start to light up and sizzle like something out of a Frankenstein movie; it can be quite awe-inspiring to behold...although you might not want to be looking when one of those arcs finally becomes a full-blown flashover that takes out an entire bus by relay protection; the sound and fury of the flashover itself combined with the operation of a half-dozen air-blast circuit breakers interrupting fault current can be quite unnerving.

At locations like this, idle insulators are deliberately placed on wooden skids at strategic spots in the switchyard, and over the course of the winter periodic measurements are taken so that prophylactic 'power washing' [spraying of insulators with clean demineralized water] can be undertaken as needed to avoid getting into this pickle in the first place...unless the good Lord looks upon us with undeserved favour and sends a heavy rain along with a driving wind - - and there've been times that that's exactly what we've yearned for!


CR

"As iron sharpens iron, so one person sharpens another." [Proverbs 27:17, NIV]

 

[prc]

I thought we cannot apply directly the air flashover distances for application over the porcelain surface. One is the non uniformity in the distribution of equipotential lines over the porcelain surface ( ie voltage stress concentration) In a condenser bushing where the voltage lines are made uniform axially, the flashover distance can be much less than in a post insulator. Another is the effect of pollution and humidity over the porcelain surface.40-50 years back it was thought that flashover value under polluted conditions can be maintained by providing more protected ( ie not exposed to rain) creepage over the porcelain ( by providing anti-fog sheds ie deep, under corrugated sheds at close pitches) and specifications were particular to mention that protected creepage length of porcelain shall be more than 50 % of the total creepage length. Later research ( specially by NGK) revealed that this is quite against reality and for better performance under pollution, the protected distance shall be reduced with wide spaced short sheds( and today's spec insist creepage length to be less than 50%) The dry band under the deep sheds were getting wet under humidity and causing discharge and flashover under pollution with first rains. Today's 'easy clean' sheds ( long /short tapering sheds instead of earlier horizontal sheds) allow rains to wash out the deposits thereby avoiding a flashover. These findings were incorporated long back in IEC standards 60071-2, 60815-1,2,3 and CIGRE reports 158 &361 with regard to selection of insulators for polluted environments.

1990's saw the development of RTV (room temperature vulcanizing) silicone coatings (mainly from Canada) which made a quantum jump in pollution performance. Refer CIGRE Reports 442,478.

observations from crshears is quite interesting, something unknown to me. a tropical man. One interesting feature seen in India-only certain areas the seashore salt deposit on porcelain is causing problem while other areas it is not an issue at all.