Energizing transformer- Why the spark? 2

[Mbrooke]

I can't say I have ever seen my POCO's transformers do this. Then again I've never watched one switched on at night in the dark. But for the curious me, why do orange sparks shoot out of the sides at 5:08 and 6:47 when they are switched on? Is this even supposed to happen?

https://www.youtube.com/watch?v=nwopW_SmZ8E

 

[ScottyUK]

Core saturation and magnetic flux escaping into the tank and structure where it induces currents in places where they aren't normally intended to flow. I've heard of this happening but first time I have seen it. You can hear the noise from the core in saturation.

 

[Mbrooke]

But I still think that wouldn't cause a spark because the tank would remain at uniform potential through out... but I must admit my understanding on the subject is thin.

 

[edison123]

Loose grounding connection of the tank?

Muthu

http://www.edison.co.in

 

[ScottyUK]

A lot of current passing through a joint with a relatively high resistance connection would do it - probably a bolted connection between steel components on the tank or something mounted off the main tank.

I think it's quite likely that the camera made it look a lot more energetic than it really was. I filmed some arc-flash tests at IPH in Berlin using my still camera on movie setting and it looked a whole lot more vivid than it did on IPH's high speed movie camera set up to record the tests for the official test records. Both the persistence and colour of the sparks / flames were exaggerated by my camera into Hollywood-style drama.

 

[prc]

When a transformer is switched on,maximum inrush current is when switching is done at zero on voltage wave.In that condition the flux density in core gets doubled to keep up voltage rise.The core gets super saturated and the flux cannot be confined to core and overflows out of it,mainly through the gap between core and innermost winding. The flux path is closed through tank cover and sides.This induces heavy current in metal for a couple of cycles. The current will meet a discontinuity at tank band joint(gasketed) at top cover or bottom of tank,in case of bell tank.Even though equaliser connections between cover and lower tank are provided at two positions,the current will be too high to take this circuitous path,but jumps across, creating a flash in air. Normally operators do not notice it as in day light difficult to notice and every time it may not happen as it depends on the inrush current value or the point of switching on voltage wave. The arc intensity depends on the MVA rating and nothing to do with voltage rating.

The worst situation that I met with was in a 570 MVA 132kV 3 phase GSU (tank length nearly 11 metres) where the paint on connecting bolts got burnt due to this arc and current flow. Problem was solved by adding more equalising connections between cover and tank.

Similar arc across tank band can be seen when ever transformers are short circuit tested (applying rated voltage on HV with LV shorted) at High Power labs. Here over current(5 to 25 times full load current,depending on percentage impedance)boosts up the leakage flux between HV and LV windings (depending on MVA and % impedance) that closes through tank cover and tank. Intensity of arc will be more than in case of switching on and will be there always.

 

[Mbrooke]

Thanks PRC, excellent explanation. Any idea on the voltage between the arc points?

 

[prc]

No idea about the voltage build up. Must be a few kV as it is has to flash over nearly across 5mm.See attached paper for more details.

 

[HamburgerHelper]

Wow. It makes sense but I would have never guessed it would have been to the degree that it was visible.

 

[Mbrooke]

For all we know it might be visible half the time- but I am in the camp that rarely sees anything switched on at night.


How often does it become visible?

 

[marks1080]

Video of substation work without people wearing hardhats?

You may want to remove that from youtube...

 

[Mbrooke]

Is a hard hat really necessary in a completed substation?

 

[marks1080]

It is absolutely necessary within the company I work for. Along with appropriate PPE such as safety glasses, FR clothing and work boots.

 

[HamburgerHelper]

Anything beyond a helmet is luxury at some places.

 

[Mbrooke]

That would make sense in terms of company policy. The glasses and clothing I agree with though, but to each his own.

 

[marks1080]

I agree, follow company policy for PPE.

However, would you really want another utility with more stringent (better) safety standards using your video as an example of what NOT to do?

I'm just not sure I would post that publicly for others to take advantage of.