Seconday Current during Power Transformer Energization 2

[adeschamps]

Hi,
We recently energized a 166MVA power transformer (230kV / 34.5kV --Grounded Wye / Grounded Wye thru resistance). We recorded currents on the primary and secondary side. The primary currents were as spected (~2000Amps) but I was surprised to see current on the secondary side, these currents aren't big in magnitude ~2-3Amps. Do these currents come from the capacitance of the secondary bus? The seconday CB was open so there was only about 15mts of insulated cable.

Cheers.

 

[dpc]

There is a fair amount of capacitance within the transformer itself.

 

[constantlylearning]

If by "primary currents" you mean currents on the 230 kV side, I'd be shocked if you saw 2000 Amps. For a 166 MVA transformer, rated current on the 230 kV side is 416 Amps.

With the exception of exciting current (which is very small, usually less than 3% of rated current), you shouldn't see any current on the primary if there's no current on the secondary.

Excluding exciting current, the following equation should hold true:

Vprimary*Ipimary = Vsecondary*Isecondary

 

[living2learn]

Doesn't transformer inrush range from 5-20X the full load currents? 2000/416=4.8 so seeing 2000A for a moment on the primary side wouldn't be that far off

 

[constantlylearning]

Yeah. You're right. But, inrush is going to have a really strange shape when compared to fundamental current. I'm not sure why I assumed he was referring to fundamental current, but that was my mistake.

 

[jghrist]

The charging current for 15 m of cable would be about 0.07 A. Charging current for the transformer capacitance would not appear in the secondary leads.

 

[collies99]

Do you have a one-line you can post ?

Is this a constant 2-3 amps in question ? Or was it just momentary as per inrush current (12 cycles duration there about) ?

 

[slavag]

Some noise.

 

[pscheshki]

In my opinion this is inrush current in the primery side of transformer. This is 2-nd or 5-th harmonic, but it's important to know how many periods is this 2000Amps?

 

[marks1080]

Hard to comment without knowing the test/measurment set up...

The 2000A primary sounds right for short time inrush. If you have oscillography, the waveform is interesting to look at.

2 - 3 amps secondary sounds off to me... Check to make sure the primary and secondary measurment leads you are using aren't close together or coiled around one another... possible its just induction you are reading. Was the measurment taking in a live switchyard?