1/2 Cycle Momentary Symmetrical vs Asymmetrical Fault Current can you clear up the confusion? 2

[bdn2004]

I thought the first 1/2 cycle is analyzed because there are asymmetrical currents and that's what drives up that value in that short time span.

The short circuit diagram on the left is the symmetrical 1/2 cycle momentary current selection. The one on right is the same circuits but using the asymmetrical currents - which are way higher.

Obviously I'm confused. Can someone explain this simply?

 

[waross]

DECAY.
Your are not considering the decay of the DC component of the asymmetrical current.
The DC component will decay and after a time the current will become symmetrical.
The peak asymmetrical current is important in design as the magnetic forces are proportional to the square of the peak current.
In the very early days of industrial electrification there were instances of bus bars being ripped loose from their mounts by the magnetic forces developed by a short circuit.

SYMMETRICAL SHORT CIRCUIT CURRENT.
Typically called Available Short Circuit Current or ASCC.
A component rated at a given kA ASCC will be able to withstand the Asymmetrical current associated with an Available Short Circuit Current at a specified X/R ratio. It is an artificial rating that facilitates the use of the supply %-impedance to select suitable down-stream interrupting equipment.
This rating system is suitable for typical distribution systems.

If you find yourself designing transmission systems, you may encounter a transformer with a very high X/R ratio that may develop greater peak asymmetrical current than the current used to determine the ASCC of interrupting devices.
In this case, higher rated interrupting devices must be used.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[Kiribanda]

Asym (rms)=Sym (rms) + DC component (X/R ratio).
The actual short circuit current wave is asymmetrical due to the X/R ratio at the fault location.
Higher the X/R, the peaky the asymmetrical wave.
Using THEVENIN's eqt cct we always calculate the initial sym. rms sc current at the fault location.
and also the X/R ratio. Knowing the sym(rms) sc current & the X/R ratio, one can calculate
Asym(rms) and Asym(pk)values at the fault location.
Therefore,

1)Calculated Sym. rms sc currents are shown in the left image.
2)Depending on the display provisions in the software you have used,
calculated Asym(rms) currents or Asym(pk) are displayed in the right side image.

 

[dpc]

The answer is in the names. Asymmetrical includes the dc offset current that may be present. Symmetrical does not. Both are rms values. But you're right that the maximum for both occurs in the first 1/2 cycle. Other factors that make the 1/2 cycle higher - decaying motor contributions and generator contributions.

 

[bdn2004]

Ok...so the 1/2 cycle asymmetrical current is [dc component of the total current in the system at the fault point that is discharging into the fault + the symmetrical fault current]. The higher the X/R ratio at that point the higher the asymmetrical current will be and the longer it will take to decay to the symmetrical fault current. Got it. I think.

I guess another part of my confusion is a 3-phase symmetrical fault is calculated for maximum fault Amps. But in the case of a line to line fault the currents in the phases are no longer “symmetrical”, as the magnitudes are not the same in all three phases. And this is also referred to as asymmetrical. Am I wrong?

Since there is this dc component that exists and the X/R ratio is given by the Utility at the starting point of the short circuit calculations - then why is everything based on symmetrical currents? Seems it would be easier to use one number.

One reason I’m asking this question is that when you look at that circuit - all the symmetrical numbers say that that MCCBs are sufficiently dutied. But doing an equipment evaluation with the software says that far right MCCB is under dutied. It’s confusing to me why that’s true.

 

[waross]

Seems it would be easier to use one number.

Yes, you are correct.
The Available Short Circuit Current is that number.
While a breaker interrupting capacity may be listed as 10 kA, it will be tested and suitable for interrupting the ASYMMETRICAL current associated with a 10 kA symmetrical current.
Yes, the fault current may be different in the various phases, but a high fault current in one phase must return in the other phases, so not that much different,
and
we are concerned with the worst case current.
If a breaker and the mechanical bus bar supports in an MCC or in switchgear is adequate for the worst case current in one phase then lessor currents in other phases are not an issue.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[stevenal]

I see the the confusion. We have symmetrical (3P) and asymmetrical (L-L, and L-G) faults, and we we have the symmetrical and DC components of asymmetrical fault current. These are not the same concept. Perhaps balanced and unbalanced are better terms for fault types when discussing both concepts together.

 

[cuky2000]

Allow me to offer the following comments:
1) Any circuit breaker shall be able to withstand the force and heat generated during a short circuit.
2) Circuit breaker is tested at rms symmetrical value and a given X/R ratio prescribed in the standard.
3)The circuit breaker shall be capable of closing and latching any power frequency making current whose maximum peak current.
4)Circuit breaker requires closing and latching current under fault.
• [sub]The Peak current for closing and latching is determined by multiplying the symmetrical interrupting rating current on the nameplate (k[sub]Arms[/sub]) by 2.6 (older std x 2.7) for 60 Hz or 2.5 for 50 Hz.
• The momentarily current can be obtained by multiplying the symmetrical interrupting rating current on the nameplate (kA[sub]rms[/sub]) by 1.55 (older std x 1.60) for 60 Hz.[/sub]
5) Relaying needs to ensure that the backup breaker is tripped before the front-line breaker exceeds its short-time rating.

Below is an illustration of the SC. The 1/2 cycle region is indicated with the peak and momentarily multiplication values. I hope this contributes to clarifying the subject.

 

[dpc]

"Symmetrical/asymmetrical fault current", at least in the US, universally (in my experience)refers to the symmetry of the current wave about the zero axis. 3-phase faults are "balanced" faults and L-L and L-G are "unbalanced". Could just be a terminology issue or difference in usage.

 

[bdn2004]

Cuky2000,

Nice diagram that explains it pretty well. What does the term closing and latching mean? In a short circuit we want the circuit breaker opening. Is that the same thing?

I understand the circuit breaker is tested at a certain X/R ratio and if your system is at higher X/R ratio at that point, the circuit breaker has to be de-rated. The software flags for that.

5) So the upstream breaker relay has to be set so that it interrupts the downstream circuit breaker before it exceeds its short time ratings?...is the short time the same as the 1/2 cycle ratings?

So say there is a main in the switchgear and a bunch of feeder breakers. The backup is the Main. Is this the situation referred to? Seems like we need a circuit breaker damage curve.

 

[cuky2000]

bdn2004,

Below are the couple definitions requested:

[sub]The close and latch capability is the ability of the breaker to be closed onto a faulted system and stay closed to allow the fault to be cleared downstream. This is also referred to in the IEC marketplace as the Rated short-circuit making current witch is the maximum peak current the circuit breaker shall be able to close and latch against.[/sub]

[sub]The rated short-time withstand current is the maximum current (expressed in kA rms) which the equipment shall be able to carry in the closed position for a specified time duration. The rated short-time withstand current is equal to the rated short circuit breaking current. Standard values for duration are 1 or 3 s.[/sub]

Yes, the example provided having the main breaker as the upstream device is applicable to the situation discussed above.
In the event of a short circuit on the feeder, the main breaker should be capable to withstand the mechanical forces and the heat generated without open or damage.

 

[stevenal]

dpc,

My studies were in the USA, and I learned these definitions (although not from this school.) I prefer balanced/unbalanced to avoid confusion, but as usual they failed to ask me.

 

[wattyeng]

Sub transient component xd” of AC short circuit current is the initial short circuit period - the X/R ratio of system defines the dc component, hence severity of asymmetry and decay - sub transient value xd”, is used to correctly specify switchgear ratings

I.Eng MIMMM MIET MIPowerE AIOSH

 

[waross]

Are we confusing asymmetrical faults with asymmetrical fault current?
A symmetrical fault may result in asymmetrical fault current.
Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[wattyeng]

Symmetrical and asymmetrical are quite different - a symmetrical fault, with no dc component maintains symmetry around the x axis - an asymmetrical fault, with dc component due to X/R, has highest peak during sub transient period (first half cycle) with asymmetry decay in accordance with system X/R (higher X/R, longer decay to symmetrical steady state period)

Hence why switchgear etc should be evaluated and rated for worst case asymmetrical sub transient peak - breaker OEMs test breakers at industry standard X/R


I.Eng MIMMM MIET MIPowerE AIOSH

 

[waross]

wattyeng, please look at stevenal's link and then re-read my post.
According to the definition in the link, a symmetrical fault, involves all three phases.
An asymmetrical fault involves less than three phases and possibly a neutral.
Either type of fault may have symmetrical fault current, asymmtrical fault current or a combination of both symmetrical and asymmetrical fault currents depending on the point on wave of the closure.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[wattyeng]

Symmetrical faults have no asymmetry - they are the most onerous RMS currents but in practice are rare
Asymmetrical faults are far more common (system fault imbalance) and produce the highest peak values (during the sub transient period) - the extent of asymmetry decay is a product of system X/R (dc component) - peak of asymmetry depends upon voltage waveform at instant of fault and dc component
An asymmetrical fault will return to symmetrical once the dc component has fully decayed (steady state period)

I.Eng MIMMM MIET MIPowerE AIOSH

 

[bdn2004]

I want to follow up on Cuky2000 post....

"The rated short-time withstand current is the maximum current (expressed in kA rms) which the equipment shall be able to carry in the closed position for a specified time duration. The rated short-time withstand current is equal to the rated short circuit breaking current. Standard values for duration are 1 or 3 s."

In original post the short circuit symmetrical in the bus that this breaker is connected to is 26.55 kA. Asymmetrical is 35.931 kA. Per this data sheet out of the Sq D catalog the HD breaker is rated 18kA. Obviously this is too low for this application. But does that 18kA rating mean that this breaker will withstand 18kA for 1 to three seconds?

 

[bdn2004]

And this is the 1200 Amp Main and the 1200 Amp Feeder Breakers that are under the short circuit ratings. The feeder breakers are set to trip at 240 Amps.

 

[dpc]

In original post the short circuit symmetrical in the bus that this breaker is connected to is 26.55 kA. Asymmetrical is 35.931 kA. Per this data sheet out of the Sq D catalog the HD breaker is rated 18kA. Obviously this is too low for this application. But does that 18kA rating mean that this breaker will withstand 18kA for 1 to three seconds?

The short circuit requirements for circuit breakers depends on the type of breaker and the voltage level - as well as the standards used.

For low voltage molded case breakers such as the Square D HD, it has no short circuit withstand rating at all. Only the 1/2 cycle momentary current is relevant. Molded case breakers must have an instantaneous trip, and the short circuit ratings are based solely on its ability to interrupt the 1/2 cycle momentary fault current. The short circuit rating listed is based on symmetrical amps, but the standards require short circuit testing at a maximum X/R ratio that varies depending on the breaker type, size and standard. From this test X/R ratio, you can derive the maximum asymmetrical current it was tested at. If the max asymmetrical current in your application exceeds this value, you essentially have to derate the breaker.

Low voltage power circuit breakers do have a 30 cycle withstand rating. High voltage breakers have both a 1/2 cycle withstand or close and latch rating plus an interrupting rating.

And there are differences in the ANSI and IEC ratings, test procedures and calculations. Many new low voltage breakers have dual ANSI and IEC ratings as you can see in your table.

Cheers,

Dave