gadfly56
Senior Member
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- New Jersey
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- Professional Engineer, Fire & Life Safety
Primary Lightning Strike- Can someone explain whats going on here?
- Thread starter mbrooke
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mbrooke
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I've heard that the shortest path to ground is indeed applicable for lightning.
- Location
- Placerville, CA, USA
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- Retired PV System Designer
With occasional seemingly random side trips, just to prove its unpredictability.
mbrooke
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- United States
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RumRunner
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- SCV Ca, USA
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- Retired EE
So, saying that, you firmly believe on the old adage that “current takes the path of least resistance”.
This goes for current without regard -- whether that said current is through lightning strike or kitchen appliance.
Is that your position?
synchro
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- Chicago, IL
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- EE
Below is my take, for what it's worth, about your questions about Fig. 12.18 in section 12.5.3 of Electric Power Distribution Handbook. The quoted content is from that section.
"Lightning current into the neutral winding of the transformer (usually X2) induces
possibly damaging stresses in the high-voltage winding near the ground and line ends..."
So the author is mainly concerned here about over-voltage damage on the transformer primary that's caused by lightning current entering the neutral terminal of the secondary.
"Currents couple less to the high-voltage winding on transformers with an interlaced secondary winding,
which is used on core-type transformers and some shelltype transformers."
"For a balanced surge (equal in both windings), the inductance provided by the transformer is approximately the hot
leg to hot leg short-circuit reactance. Transformers with interlaced secondaries have significantly lower inductances (see Table 12.10).
Higher transformer inductance induces a higher voltage on the primary-side winding."
So the lightning current entering the neutral at the secondary center-tap splits into the 2 windings on either side (i.e., a common mode input), but the magnetic fields from the two currents cancel if the windings are tightly coupled to eachother by interlacing (leading to high mutual and low leakage inductance). So with ideally no magnetic field the inductance is minimized and so are any "loop voltages" induced on the secondary that would be transformed into a damaging high voltage spike on the primary.
Because the voltage across an inductor is L dI/dt, the large fast rising current pulse of a lightning strike can produce a damaging high voltage pulse, and the higher the inductance the higher the voltage produced.
"Current through the secondary neutral creates a voltage drop along the neutral. This voltage
drop will push current through the transformer when load is connected at the customer (Figure 12.18)"
I don't think they are necessarily saying that that no customer load makes the situation worse, but he is illustrating the consequences of both cases. In this section the main focus appears to be minimizing distribution transformer damage, and not customer side damage.
If the transformer does not have tight coupled secondary and so the inductance presented to the common-mode balanced current is high, then secondary protection becomes more necessary:
"Utilities may use secondary arresters or spark gaps to reduce
the transformer failure rate on noninterlaced units."
I believe you're correct that all L-L loads would not conduct the currents from this common mode excitation of the transformer because the induced "loop voltages" would ideally be the same. But I think if this happens the lightning current is going to find a path through the customer panel no matter what loads are connected there, because the voltage will rise until something breaks down.
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