Distribution Transformers DC Megger Test

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eric1985

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Hello all,
I have often heard the term " dc megger test " for a distribution transformer; so is anyone can explain to me the detailed processes/procedures of this testing and its purpose doing that for the transformer? Also, I know how the "megger testing" work in theory, however, I don't have any pratical experience for it; so I would appreciate it if some industrial professionals can also explain to me in details. Thank you very much!!!
 
eric1985:

Is your transformer liquid or dry type?

A dry type transformer is more vulnerable to over voltage breakdown.

A DC high voltage test is 'gentler' on it than the corresponding AC high voltage test and so to be preferred.
 
Haji said:
eric1985:

Is your transformer liquid or dry type?

A dry type transformer is more vulnerable to over voltage breakdown.

A DC high voltage test is 'gentler' on it than the corresponding AC high voltage test and so to be preferred.
Click to expand...

Thanks Haji,
The transformer is overhead, dry type; so for the DC voltages, are you using the 1kV or 5 kV or something else? As well as for the winding resistances, what values are we expecting? Also, do you know are there any NEC/IEEE regulations involved with this test? Thanks!
 
Megger Manual

Megger Manual

http://www.biddlemegger.com/biddle/Stitch-new.pdf
eric1985 said:
Hello all,
I have often heard the term " dc megger test " for a distribution transformer; so is anyone can explain to me the detailed processes/procedures of this testing and its purpose doing that for the transformer? Also, I know how the "megger testing" work in theory, however, I don't have any pratical experience for it; so I would appreciate it if some industrial professionals can also explain to me in details. Thank you very much!!!
Click to expand...

http://www.biddlemegger.com/biddle/Stitch-new.pdf
 
From NETA

Perform insulation-resistance tests winding-to-winding and each winding-to-ground. Apply
voltage in accordance with manufacturer’s published data. In the absence of manufacturer’s
published data, use Table 100.5.
Calculate the dielectric absorption ratio and/or polarization index*1, as appropriate.

You should record temperature and humidity, correcting readings accordingly

For 600 VAC equipment 1000 VDC with a minimum reading of 100 Megohms

*1-http://www.predictivemaintenance.co.za/polarizationindex.php


Licensed to
 
Last edited:
brian john said:
From NETA

Perform insulation-resistance tests winding-to-winding and each winding-to-ground. Apply
voltage in accordance with manufacturer?s published data. In the absence of manufacturer?s
published data, use Table 100.5.
Calculate the dielectric absorption ratio and/or polarization index*1, as appropriate.

You should record temperature and humidity, correcting readings accordingly

For 600 VAC equipment 1000 VDC with a minimum reading of 100 Megohms

*1-http://www.predictivemaintenance.co.za/polarizationindex.php


Licensed to
Click to expand...

Thank you for your useful info Brian! It seems you are the expert in power engineering,,,,,,, one more think to be cleared " why do we consider temperature and humidity"?
 
eric1985 said:
Thank you for your useful info Brian! It seems you are the expert in power engineering,,,,,,, one more think to be cleared " why do we consider temperature and humidity"?
Click to expand...

Both will have an effect on your measured resistance, not much you can do about humidity besides testing under similar conditions. Tempature has a huge effect and all results need to be corrected to have any meaning, specifically when it comes to trend analysis. Most equipment is corrected to 20C.

View attachment Temp correction 20C.doc
 
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