Insulation Testing Method/s Commonly Used for 69 kV Lines

[GRAEE]

Hello everybody, would like to solicit methods or practices used in testing the integrity of the insulation of 69 kV lines.

We have a Megger-MIT 520 (insulation tester) with maximum voltage of 5kV can be injected. Is this enough for testing the insulation of a 69 kV line of about 15-20 km long?

 

[Marmite]

It is not normal practice to test the insulation integrity of overhead lines, or to overvoltage test them either for that matter. Normally they are inspected visually either from the ground or by pole top inspection,or helicopter checking also that all working grounds/earths have been removed and accounted for, and then the controlling circuit breaker is closed to energise the line. Insulation testing will give meaningless results as it is completely weather dependent and greatly affected by fog, dew, pollution and other environmental factors.
Regards
Marmite

 

[Zogzog]

Yep, a 5kV megger test wont tell you much. The prefered method of testing these lines is corona scanning, usually helicopter based, we do a ton of these every summer.

 

[oldfieldguy]

I concur with Marmite and Zog. Hang the wire, do a visual, then close the switch.

Of course when I was in the high voltage field service business, I had a client that wanted his overhead 13.8 kV hipotted. I argued a bit, ever tactfully, then did it for the guy who was signing my timesheet.

He had us do it every two years. Historical data was all over the scale from the influences Marmite lists.

old field guy

 

[GRAEE]

Marmite,Zogzog & oldfieldguy,

We've done the visual inspection already. The line to be tested is a newly constructed line not yet energized. We have a little doubt to some of the insulators being used in the line. And we wanted to test the insulators installed in the line before it is being energized.

It takes time and not practical for us to test the insulators installed for this long line. We might experience flashover if we proceed the energization to some points in the line that are weak.

galen1119

 

[racobb]

May I ask why you have doubts about some of the insulators used?

Alan

 

[Marmite]

I echo Alan's question. Please provide more information as to why you have doubts over the insulators. Presumably you have a spec for the line and the insulators. The insulators will have been tested according to whatever the spec says by the manufacturer. Other than removing the ones you have doubts over and either testing them or swapping them out, you have little choice. Eventually you are going to have to energise the line for the first time. Paradoxically, the longer you leave it dead, the more likely it is to develop a fault. You might even find a few spans of conductor go missing!
Regards
Marmite

 

[Elmir]

I have performed dielectric withstand tests in the past in 69kV substations because it was required by the project engineer.

We performed the tests to NETA specifications. We used a stackable highpot. I believe we performed the test to approximately 120kV. The test only included some overhead bus, switches, and fuses.

 

[davidbeach]

You can hi pot bus, but I'd venture to say that you'll never successfully hi pot a line of any length as your hi pot equipment won't be able to supply the charging currents and maintain voltage.