3 phase faults
3 phase faults
(OP)
My question is very basic.
We had a big discussion last night with my work mates about 3 phase faults. One engineer said that ALL 3 PHASE FAULTS ARE SYMETRICAL. He even explained that a fault in 3 phases would cause all currents to increase symmetrically and the reverse for voltages. I feel saying all 3phase faults are symmetrical is allitle bit radical. My reasoning is that if the fault occurs in all the phases, at the same point, and at the same instant then the fault will be symmetrical….may I call that ‘a perfect fault’? An example to this would be dropping a metal piece at a perfect position on busbars or cutting all the phases at the same point. So I want to believe that it is possible to have 3 phase faults that are not symmetrical because a perfect fault is almost an impossible happening. I would think that the origin of a 3-phase fault is actually between 2 phases. Then the 3rd phase get involved (to make it a 3 phase fault) due to the aftermath of the initial fault i.e. ionisation of air, flash, insulation breakdown, heat, rupture etc. Any input or correction about this?
We had a big discussion last night with my work mates about 3 phase faults. One engineer said that ALL 3 PHASE FAULTS ARE SYMETRICAL. He even explained that a fault in 3 phases would cause all currents to increase symmetrically and the reverse for voltages. I feel saying all 3phase faults are symmetrical is allitle bit radical. My reasoning is that if the fault occurs in all the phases, at the same point, and at the same instant then the fault will be symmetrical….may I call that ‘a perfect fault’? An example to this would be dropping a metal piece at a perfect position on busbars or cutting all the phases at the same point. So I want to believe that it is possible to have 3 phase faults that are not symmetrical because a perfect fault is almost an impossible happening. I would think that the origin of a 3-phase fault is actually between 2 phases. Then the 3rd phase get involved (to make it a 3 phase fault) due to the aftermath of the initial fault i.e. ionisation of air, flash, insulation breakdown, heat, rupture etc. Any input or correction about this?






RE: 3 phase faults
An asymetric fault refers to the fault current having a DC component and not being a symetrical sine wave. The term "asymetrical fault current" is not related to the equality or inequality of the currents in the three phases. It refers to the fact that most fault currents have a large DC component and the resulting fault current wave-form is not a symetrical sine wave.
Given that the magnitude of the DC component, or offset, in a fault current, depends on the point in the sine wave that the fault occurs, it is doubtful if a bolted three phase fault will result in equal asymetrical currents, even if the phase to phase faults occur simultaneously, so refering to a bolted three phase fault as symetrical is probably not symantically correct either.
The magnitude of the asymetric current is often greater than the symetric current, and should not be ignored.
yours
RE: 3 phase faults
RE: 3 phase faults
RE: 3 phase faults
Ungrounded and balanced three phase faults will not have any zero or negative sequence current, so they are symmetrical by one definition. Two or more phases will have a decaying exponential term, so the unfiltered current will have an asymmetry factor. Confusing enough?
RE: 3 phase faults
I think the term "symmetrical" in this context means all three phases balanced, as in symmetrical components.
RE: 3 phase faults
1stThe impedance we are talking about is it line impedance or load. Because I hear that in normal fault analyses you ignore the load impedance? The faults I am referring to are ones that may happen up-stream....supply side. Therefore making the load impedance immaterial.
2nd question is: - If you will always have neutral currents, then it looks like there are no symmetrical faults in real world. What are symmetrical faults then?
3rd And by the way you are talking about neutral currents. What if its delta connected system? I assume the other side of supply transformer (wherever it is) will be star connected hence neutral and neutral currents. Are we together or em I missing something? I hope you are seeing it my way.
RE: 3 phase faults
RE: 3 phase faults
If the fault does not involve ground or neutral, you will not have a neutral current. I just haven't seen records of such a fault.
RE: 3 phase faults
Symmetrical 3 phase fault currents are referring to the symmetry of the AC sine wave and not the balance between three phases. 3 phase faults are assumed to be 'balanced' for the purpose of anyalysis and specifying short circuit rating of equipment.
RE: 3 phase faults
RE: 3 phase faults
I'm looking at record of such a fault right now, no discernable rise in residual current. Although I'm not yet sure of the cause in this case, these faults are common enough. A branch breaks free in the wind and lands across three phases. Of course 2 phases go high before the third joins them, but the fault eventually becomes a symmetrical three phase one.
If you want a fault that evolves less, try closing into a line with the personal safety grounds left on.
RE: 3 phase faults
"1stThe impedance we are talking about is it line impedance or load. Because I hear that in normal fault analyses you ignore the load impedance? The faults I am referring to are ones that may happen up-stream....supply side. Therefore making the load impedance immaterial."
In the theoretical 3-phase bolted fault, there is zero impedance at the fault connection, and the upstream sequence impedance is symmetrical.
In an actual fault, the fault current depends on the actual impedance at the fault, i.e., the impedance of the 'metal piece' you dropped and it's connections to the buses, along with the true upstream system impedance, which is never truly symmetrical.
"2nd question is: - If you will always have neutral currents, then it looks like there are no symmetrical faults in real world. What are symmetrical faults then?"
3-phase symmetrical faults are used to model the 'worst-case' fault in a short circuit study to determine the adequacy of the connected equipment.
"3rd And by the way you are talking about neutral currents. What if its delta connected system? I assume the other side of supply transformer (wherever it is) will be star connected hence neutral and neutral currents. Are we together or em I missing something? I hope you are seeing it my way."
Neutral currents are not modeled in a 3-phase symmetrical fault, whether the system is 3-wire or 4-wire.
RE: 3 phase faults
To the original question: I do not believe the three phase fault that evolved from a line to line fault is somehow less symmetrical than one that involved all three phases initially. But the concept of the balanced three phase fault does rely on some engineering approximations such as balanced in magnitude and phase source voltage, balanced source impedance (make sure the lines are transposed and that the fault occurs only at the end of a complete transposition) and balanced fault impedance. None of these things are achievable to perfection in the real world making the perfect symmetrical fault also unachievable. We can get close enough for all practical purposes though.
RE: 3 phase faults
RE: 3 phase faults
The OP is obvioulsy confused in the meaning of Symmetrical and 'balanced' in this case.
RE: 3 phase faults
'..... The only difference is the impedances'
My question about the impedance was triggered by Rocketir’s post otherwise I was not confused. We are all on the same page now.
Thanks to you all for the valid posts and contributions. I am glad it’s all ending with some humour from 'jghrist'
RE: 3 phase faults
1. Symmetrical Faults:
This simply means 3 phase ‘bolted’ faults…hence in the analysis of such a fault you don’t need to worry about breaking the system down into symmetrical components b/c the system is symmetrical (theoretically at least). We know what in the real world this is never really true…
2. Asymmetrical Faults:
All other faults! L-L, L-G, L-L-G…To perform the analysis on this type of network you must break the system down into symmetrical components (because the faulted system is no inherently symmetrical) to perform analysis.
BOTH 1 and 2 above have:
a. Asymmetrical region:
This happens @ the beginning of the fault. This region generally consists of a sinusoid of sorts riding an exponentially decaying dc offset.
b. Symmetrical region
This region follows the decaying dc offset. Once the dc offset decays to zero and the faulted current follows a sinusoidal pattern which is (more or less) symmetric) about the x-axis.
Anything else is just semantics…It's more important to understand what's 'going-on'.
RE: 3 phase faults
I would refer to item 2as unsymmetrical faults. This is how Stevenson refers to them in "Elements of Power System Analysis". I'd leave the asymmetrical nomenclature to refer to the dc offset.
RE: 3 phase faults
RE: 3 phase faults
I generally agree with the fact that a "pure 3-phase fault with relative amount of fault resistance" does not exist. It is the evolution of a 2-phase fault into the third phase, asnd from relay point of view we have to pick.up already the 2-phase fault.
So usually the fault resistance in an impedance protection relay is calculated for the 2-phase faults and not for the three-phase ones.
A three phase fault is usually the load. That's why the fault resistance should be limited by the minimum load impedance (today's distance relas usually have the phase-phase loops responsible for "both" 3-phase and 2-phase faults).
Anyway I saw some of them, reorded by the relays:
1) fire below the overhead line. It looks th eionisation managed to be homogeneous and it was recorded as "pure" 3-phase fault with a quite symmetrical fault resistance.
2) 2-phase fault. Trip. Autorecloser. The other line-side did not trip and the fault evolves . When A/R recloses, it recloses on a "pure 3-phase fault with symmetrical fault resistance". Nice job for switch onto fault.
my regards