Megger (Insulation Resistance test) on 150 kV OH TL procedure?

[energymonkey]

Hello everyone,

I'm working on 150 kV project that nearly finish. My friend told me that before we 'energize' (the first time that the line powered with 150 kV) the line, we need to megger it. Then it give me some question :

1. is it really necessary? after all, we had have visualy line check to make sure that there were no things that attached on the wire that will cause a short circuited to ground. And the distance of the TL are 40 KM. ain't that ineffective?

2. on the substation, the line were connected to surge arrester. were that okay to do the megger without releasing the connection?

3. are there any international procedure for it (IEC, NETA, etc..)?

thanks..

 

[energymonkey]

well, it seem should be 13 replies.. i am a newbie to this forum anyway..

 

[FPelec]

energymonkey,
energymonkey,
in the utility in which I work we do not perform such an insulation test on overhead lines (from 132 to 400 kV) before its first energization. Generally the line is left energized for a short period of time in order to check for insulation withstand.
Of course visual inspection of clearances is performed on entire line lenght before its energization.
P.S. 150 kV is not a very usual voltage level; just for curiosity, can you say in which state/country your line is located?

Si duri puer ingeni videtur,
preconem facias vel architectum.

 

[defreitas]

Energymonkey, in the utility which I work at, we usually try to do a I/R test (megger) on 132kV and 275kV lines, prior to initial energisation, but we mostly find that it can be hard to identify any issues...

On a 40km OH line you are probably going to find a "fairly low" value of I/R as the number of insulators that you are going to have in parallel is reasonably high, driving you only to more confusion...

It really comes down to preference and experience when the subject is I/R on long lines...

About your surge arresters, they should not get damaged with an I/R test, but you may find that dipending on the constraction of it, it may show a "short" when you put a megger across it, as it's designed to AC and not DC, in which case, you would have to disconnect them from your line and test the line by itself...

The bottom line is that if you are commissioning a line, even you not having tested I/R on the line, your protection will operate, should the line be faulted...

An other procedure used, but more commonly on cables is to "soak the cable" for 24 hours before putting any load on it...

The idea of that is: to test I/R before energising, energise for 24 hours , then de-energise it and test again. What the 24 hours of AC on the cable will do is to drive all moisture off the insulators and therefore your second reading of I/R will show to be better than the initial one...

I hope that helps in anyway...



Dan de Freitas

 

[rasevskii]

150Kv is a voltage used in Switzerland, notably Zurich. perhaps that is the location of the OP.

It is very unsafe to connect test equipment directly to OH lines at this voltage level, even if unenergized. The line must be earthed by
an earthing switch or approved line equipment. Open circuit lines can build up to dangerous voltage levels due induction or capacitive coupling.

rasevskii

 

[energymonkey]

sory everyone, last few days I was on the site. because the OH line pass through some hill and forest.

@ Fpelec, I'm currently work in Indonesia. we already use this volt since log time ago, in fact that some OH line was a heritage from Netherlands. Yes I have read some paper that there are also countries that use 132 kV till 400 kV. On our SOP, we used to megger the OH, but I'm kinda freshman on my company. And I only read it from article that some OH line need to megger before energize and some doesn't. I was just curious, that teoritically whether I conduct the I/R test on OH line or not, same like defreitas said, the protection will operate. and the line will faulted.

@ defreitas, Yes, I though you have the same SOP like mine. after we try to I/R test, we energize the line for 24 hour and, de-energize it, then we conduct an I/R test again. And yes, I got a fairly low I/R value. On the other project, the value of the I/R only 12 Mohm. Even after 24 hour energize, the value does not so improved. that's made me curious. is that because the distance, or because the LA/SA. I have read that some LA/SA not to be subject to DC. and we put the I/R test instrument before the SA on outdoor substation. I ever think that could be the problem, I will check again.


How often do you use "soak the cable" procedure? and on which condition? and sorry, I never hear that.

Now, I meet a condition that we have to I/R test on OH that the line were induced by 150 kV (single V line),is there any other ways that can be used other than rolling blackouts?

@ ravenskii, that why I need a blackout.