Power Outage...Power Restored...Damage!

[wirenut1980]

Now consider a transformer. If the primary line is broken at the instant of maximum magnetizing current in the primary, then we have the potential of a very large voltage generated at the primary. This is reflected to the secondary and any load on the secondary. Any effective impedances in the transformer, the magnitude of the load, and the stored energy in the transformer at this interruption will determine the transient voltage subjected to the load.

The df/dt is the source of the transient voltage from the transformer core.

So is the maximum magnetizing current in any one cycle in the transformer when the current is at the peak of the waveform? And maximum magnetizing current determines maximum stored energy in the transformer? And am I correct if I say magnetizing current increases as the load current of the transformer increases?

 

[gar]

080504-1511 EST

wirenut1980:

It is not the load current as reflected to the primary, but primarily the current to magnitize the core and stray field. This going to be approximately the peak current of the unloaded transformer which occurs at the voltage zero crossing. Somehow we may need to take into account load current in the leakage inductance of the transformer to get a more precise model, but as a first approximation to answer your original question it is sufficient to only look at the unloaded condition.

If at that current peak, the source voltage zero crossing, we open the circuit very quickly, then a large df/dt voltage occurs.

When I was talking about the ignition coil I failed to point out that the primary voltage rises to to a large value compared to the 6 to 12 V excitation voltage when the breaker points open. If my memory is correct this may be in the 300 to 600 V range on the primary. This peak primary voltage is a function of the inductances and the breaker points capacitor and secondary capacitance.

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[ELA]

080504-1511 EST



If at that current peak, the source voltage zero crossing, we open the circuit very quickly, then a large df/dt voltage occurs.

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is that the same as V= L (di/dt) ?
Isn't it that abrupt the di/dt causes a a large voltage spike? This spike may indeed be broadband, and could be thought of as a df/dt after the fact?

 

[hillbilly]

080403-1301 EST

In the case of a distribution transformer it is a step down transformer so the inductively induced voltage from the primary to secondary will be reduced, but the voltage on the primary from an inductive kick can be very large relative to the normal input primary voltage so we still can get a very large secondary voltage relative to the nominal value.
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What would you estimate that secondary voltage (spike) to be on a average residential transformer with a 7200V primary?

steve

 

[gar]

080505-1205 EST

ELA:

Yes.

Before the switch is opened the forcing function is the source line voltage so the current and flux have to match the incoming voltage. On interruption of the circuit the rate of decline of current will define the voltage thru the rate of change of flux. Basically on closing the switch we have a voltage driven circuit and on opening the switch is is a current driven circuit. v = N df/dt is always functioning. It is simply a matter of what is controlling the flux.

hillbilly:

The worst case peak transient voltage will be highly dependent on the load, the impedance from the transformer to the load, and any transient limiters. Without too much load maybe 20 to 50 times the normal line voltage. Thus, for the 120 V side maybe 6000 V.

If on my work bench I insert and remove a plug to an 8 ft Slimline I can see 5000 V transients at that plug. Obviously as I move away from the plug to another location the transients are diminished.

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[gar]

080514-2036 EST

A couple of interesting references but very long.

http://www.cis.state.mi.us/mpsc/orders/alj/2007/u14888notpfd.1907.pdf

Reading thru all the legal discussion it appears the service drop neutral burned open near the transformer and that a hot line shorted or arced to the neutral to the house, and thus put 240 or at least something higher than 120 to part of the house. Could it have been that lightning may have actually hit the neutral at the rupture point.

Assuming the pole transformer center tap was grounded with a ground rod at the pole I do not see how the ground connection at the house, even if it had been good, would have made any difference.

I can assume the resistance from the pole ground rod to the house ground would be unlikely to be less than 10 ohms. This would never blow the pole fuse on the transformer primary. Even 1 ohm, 120 A, would not blow the primary fuse because this is a 200 A service, and even if the input was fused at 5 A a 120 A secondary load would never blow the fuse. The primary is 23,200 V so the turns ratio is 193 to 1, and the reflected primary current is 120/193 = 0.62 A.

To moderately quicky blow the 25 A primary fuse we might asume 50 A. At this level we need 50*193 = 9650 A on a 120 V side to achieve this. At 120 V the loop impedance including transformer series impedance needs to be below 120/9650 = 0.012 ohms. No way will you get an impedance lower than this from one ground rod to another at even 10 ft.

I would also believe that the transient limiter in the office should have a series link that would open before burning up. No mention was made that the device that caused the fire might have had this kind of oversight in its design.


This one discusses Michigan's events relative to the great eastern blackoput.

http://www.michigan.gov/documents/mpsc_blackout_77423_7.pdf

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