Lightning & transformers

[Tim Cummings]

I am watching your DVD on grounding versus bonding and during the discussion reagarding lightning striking a power line and inducing an extremly high voltage through a transformer a couple of questions came to me.

1. If lightning is DC power can it actually induce voltage across a transformer....this seems to defy basic electrical theory.

2. Is it more likely that the lighting is destroying the insulation of the transformer and passing through to the secondary?

 

[bphgravity]

1. Lightning has dynamic electrical properties that exhibit attributes to both DC and AC.

2. That is one possiblity.

 

[don_resqcapt19]

Tim,

1. If lightning is DC power can it actually induce voltage across a transformer....this seems to defy basic electrical theory.

Not really. Ask any "oldtimer" about testing transformers with a flash light continuity tester. If you still were touching both leads of the transformer as you disconnected the test leads, you got a jolt. It is a single pulse caused by the collapsing DC field.
Don

 

[jtester]

I have cut up numerous utility pole mounted transformers that failed due to lightning and found, in general, that the high voltage coils failed within a few windings of where the lightning entered the transformer. The failures were almost always holes in the insulation within a few turns of the end of the coil. The lightning didn't go thru the transformer, it got stopped almost immediately. I found holes on the HV lead side, incoming side, as well as the neutral lead side, grounded side, of the HV winding.

If you think about it, since lightning posesses all frequencies, most will find the transformer impedance too great to pass thru. The transformer is a very high impedance.

I believe most customer failures related to lightning strikes come from lightning being on either the neutral or ground and burning things up from there, not being on a phase wire and going to ground.

Lots of people disagree with me, but I've seen too many transformers that wouldn't pass the strike through to the secondary.

Jim T

 

[mpross]

inductance/capacitance of transformers

inductance/capacitance of transformers

jtester,

i also agree that lightning has infinite frequency (T=0; 1/0=inf) and that the impulse voltage from lightning will not be able to make it through the high impedance windings (Xl = wL).

...but what about the capacitance associated with the transformer (Xc = -1/(wC) this would be zero impedance right)?

i don't think this is talked about much because it isn't needed for the current ratings of the device, but it does affect the transient conditions associated with a lightning strike.

~mp

 

[jtester]

mp

I suspect that the capacitance is much less than the inductance, given the large number of turns. The other factor would be that the inductance is series and the capacitance is shunt. They wouldn't tend to offset each other.

Jim T

 

[sparky59]

lightning can do anything it wants to do.

 

[williewatt]

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[williewatt]

Looking for a specific indent tool!

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Williewatt