132kV Transmission line induction - pre-commissioning tests

[acog]

I am performing some basic pre-commissioning tests on a 132kV transmission line shortly. The line is around 40km in length and is earthed at both ends with portable earths. Several medium voltage and high voltage lines run parallel or intersect this transmission line over the route.

I am concerned about induction on the line during testing (particularly when the earths are removed). Is there a method to check the extent of induced voltage on the line prior to connecting a test instrument?

Note: no voltage transformers are connected to the line.

I am guessing I could use a clamp style ammeter (also known as 'tong') or a rogowski coil to measure the current flowing through the portable earth prior to removal. The measured current may give me an indication of the prospective induced voltage on the phase conductor after portable earth removal.

Is there an 'industry best practice' procedure to do test for induction?

Also, I am using a typical Megger 5kV insulation tester for the insulation resistance and continuity tests. I am simply going to lift the earths off one phase, perform a 1 minute test at 5kV on the isolated phase to the other phases and ground to confirm insulation resistance.

Then for confirmation of continuity, I plan to install an earth at the remote end on the phase under test and test again which should give me a low resistance (voltage collapse). Is this the industry standard procedure?

Appreciate your comments/thoughts.

 

[isquaredr]

Acog
Be very careful with induced voltages they can and do KILL, your megger probably wont like then either. I would test all cable sections either with vlf or series resonance at the appropliate voltage to satisfy your cop's and visually inspect overhead line sections. If your protection is also new make sure you have some previously proven protection with reduced settings applied behind your new circuit.

 

[acog]

Thanks isquaredr

We don't interact with any lines unless they are locally earthed. Earths are applied via insulated live line sticks, test leads are connected while the line is earthed. Safety is of course paramount. No personnel will be within flashover proximity to the line unless it is earthed.

Understood that the Megger won't like induced voltages, that is why I'm looking to verify the open circuit voltage somehow before I remove the earths so the instrument is not subject to a voltage in excess of its withstand capabilities. I can work out the withstand capabilities, but how do you safely determine the induced voltage on the line?

I expect there is a method prior to removal of earths via a measurement of the current flowing in the portable earths?

This same problem exists with any other instrument (VLF etc.) and surely all utilities around the world

The question is, how is this done when a voltage transformer is not connected?

 

[Marmite]

There's absolutely no point in Meggering a 40km 132kV overhead line with a 5kV Megger. If you manage to survive to tell the tale (which is doubtful) the results will be meaningless.
Regards
Marmite

 

[inpran]

acog, if you are trying to gauge the healthiness of a line via 5 kV Megger, then, as mentioned by Marmite, it is futile. I have used offline fault locator before commissioning of lines for checking line healthiness and diagnosis. (you can google for "offline fault locator")
There is no safe means to measure the induction voltage without voltage transformer.

 

[davidbeach]

There are hot stick voltmeters that can be used to measure the voltage with out the use of a VT.

 

[isquaredr]

acog
I have never attempted to perform the test you propose based on risk of personal injury, I have tested isolated cable sections using series resonance as mentioned earlier. I also have a 132kV circuit to commission shortly and plan for a lineworker to visit each support and varify (sign) that conductors are free from earth throughout the route.

 

[acog]

Thanks for the replies.

I measured approximately 70V induced voltage on two phases (with respect to to ground at one end of the line when all earths on the line were removed), and 135V on the other phase. Unfortunately this 'noise' was too high for the modern digital insulation resistance meter to reject and the instrument would not test.

I completely agree that insulation testing is not an overly useful functional test for transmission lines of this voltage and length. We are doing it because the client requested it. It could pose safety risks (if performed in an un-safe manner); it is important to note that these risks are also present when constructing the line in the first place where it is required to clamp-in conductors and use conductive draw wires which may be a few kilometers in length. Earthing and live line tools are required to mitigate the risks.

Now, let's assume that we all agree that insulation testing is not required. Then how do we perform other pre-commissioning tests on long isolated lines where induction is present? For example,

+ Line impedance measurements (to verify sequence impedances for confirmation of protection settings)?
+ Continuity tests (to verify phasing is correct from one end to the other)?

 

[isquaredr]

Acog
Line impeadance can be gained by conductor data, zo or k factor is more tricky as multiple return paths are usually possible (ground and return conductor). Omicron supply equipment to take on site accurate measurements CPC100 + CPCU1

https://www.omicron.at/en/products/all/primary-testing-diagnostics/cp-cu1/

Phasing and continuity can be visually traced, although real phasing checks are required if connecting to an existing system.

 

[stevenal]

To check phasing, close in one end and check with phasing sticks across the open switch at the other.

http://www.megger.com/common/documents/DETEX_DS_en_V12.pdf

 

[stevenal]

Not sure why Megger doesn't go higher in voltage. Here's a better link.

http://www.hubbellpowersystems.com/catalogs/lineman/2450.pdf

 

[racookpe1978]

Well, don't forget the basics too: Anything that can be built, can be built wrong.

Fly the line (or drive it with a partner so somebody is looking at each tower), each connection, each kilometer of new cable. Excessive (or too little) sag, clearances, loose parts, un-connected parts, simple criss-crossed wires, missing lightening wires, bad lights, etc.

 

[itsmoked]

Find a guy with a quad-copter. Don't pay him, since that would not be allowed, just 'help him with costs'. Get a little cigarette-lighter inverter and plug a 24inch screen into it that the video feed can be patched into.

​

Keith Cress
kcress -

http://www.flaminsystems.com

 

[Hipoter]

You might be able to do a DC Hipot or AC Hipot with a larger unit (like a 100kV set) as long as it has enough current (say above 1A). If you know the capacitance/inductance model of the line, it helps you to know how much is the test current for a known test voltage (100kV/5min).

 

[dfdt]

You can calculate induced voltage by multiplying zero seq mutual impedance with prevailing unbalance current