Phase to Phase Fault?

[EEC]

Whats happening with the flow of electrons when there is a phase to phase fault on a 3 phase system? I observed this happening when two overhead cables came in contact with each other three times. There was an explosion that occured three times. Can someone explain in theory what happened?

 

[haskindm]

That is a true "short circuit". Most of what we call short circuits are actually "ground faults" where a "hot" conductor comes in contact with a grounded surface. In the phase to phase fault conductors that are supposed to carry current are, but without going through the high resistance "load" so it is an extremely high amperage event. Because it is not a solid or "bolted" short, there is arcing, which is the "explosion" that you saw. The only resistance in the circuit is the resisitance of the conductors, which will usually be much less than 1-ohm. Using Ohms law, if you have a 240 volt phase to phase fault with 1/2 ohm resistance you will see 480 amps flowing in the circuit until the overcurrent device (if any) opens.

 

[kingpb]

At the time of the fault, assuming fault is between line b and c, the fault current flowing into the fault is;

Ifa+Ifb+Ifc = 0

Since line a is not involved, Ifa = 0, therefore; Ifb= - Ifc

and Vb = Vc

Then the solution to determine the various values becomes an exercise in symmetrical components whereby:

Va1 = Va2 and

Ia1 = Vf/(Z1 + Z2)

indicating that the positive and negative sequence networks should be connected in parallel at the fault point in order to simulate/calculate a line-line fault.

As far as seeing the "explosion" 3 times, could have been the conductors touching, then the arc caused them to separate, but the momentum then caused them to touch again, etc., and the level of fault current did not meet the threshold on the relaying so it did not trip, or more likely there was a fault (first explosion) the line relaying tripped to clear the fault, and tried to automatic reclose, but fault was still present (2nd explosion) and tripped again, the line relaying tried a 2nd automatic reclose, but fault was still there (3rd explosion) and tripped again and stayed tripped, i.e. 3 explosions.

This type of event/with two reclosure attempts is very common for utilities.

 

[kwired]

I vote for the 3 explosions being caused by circuit recloser operating three times then remaining open.

 

[G._S._Ohm]

Assuming a Y connected load, source voltage 10 kV, load resistance much greater than source resistance and a line to line short close to the destination end.

Leaving out some details and using a website that converts polar to rectangular and back again,
the 10 kV at 0 degrees fights with the 10 kV at 120 degrees and so at one end of the remaining load resistor you get
(10000 +j0) - (5000 + j8660)= 5000 - j8660 = 10000/300 divided by 2 = 5000/300 [call it V1] = 2500 -j4330

At the other end of the load resistor you get
10000/240 [call it V2] = -5000 - j8660.

The load current is now (V2-V1)/R. If R = 1 ohm then it's just V2 -V1.

[-5000 - j8660] - [2500 -j4330]
=-7500 -j4330
=8660/210

Before the fault the voltages and currents for a balanced system with known resistive loads is easy to calculate.

 

[LarryFine]

As far as the electrons are concerned, it doesn't matter whether a fault is line to line or line to ground. The line-to-line fault is usually of higher current due to higher voltage and simple Ohm's Law, and a line-to-ground fault (if to other than a conductor, conduit, or enclosure) may have enough resistance to interfere with OCPD operation.