Question about dielectric charge

[Lee1987]

Anyone know what is the dielectric charge in an overhead line?

and what the dielectric charge is in a three phase underground cable?

Also what measures can be used to remove the unwanted charge in both cases?

Thanks Lee

 

[Knobhead]

Basically when you test an OH or UG line you are charging a big capacitor. If you just dropped your Megger on to 5KM of cable the first off reading would indicate a shorted cable. As the test continues the IR readings rises, as the cable charges the IR reading will rise. Most MV Megger’s have a time reading in the display. A standard test for a LV cable could be “1KV for 2 minutes”. In that time the reading should have stabilised to a consistent reading.

If you are testing large distribution system you should have a schedule of testing requirements.

A word of warning. After the tests the cable should be discharged between cores and earth. Forget to do it and it bites you back! go on to 11KV cables with HiPot testing the discharge can be fatal, but the testing equipment will normaly have discharge gear incorporated in to it. but don’t rely on it.


Just as an aside. I tested a 200KVAr PF capacitor bank @ 500V each capacitor was tested separately. Even so I got through 20 sets of batteries.

 

[Silly Sausage]

You in a good mood tonight?
With an answer like that.

 

[Knobhead]

It’s Sunday, I’ve had a few down the pub, cooked a nice leg of lamb with my favourite vegetables.
I’m entitled to be in a good mood.

Then some Lancastrian grouch comes along to spoil it!

 

[Silly Sausage]

It’s Sunday, I’ve had a few down the pub, cooked a nice leg of lamb with my favourite vegetables.
I’m entitled to be in a good mood.

Then some Lancastrian grouch comes along to spoil it!

Anyway, I'm Leeds born & bred.