Why would Motor Contributions Not Affect Fault Levels?

[jgoebel]

This conclusion is one that apparently a couple people have reached, as they have said this in another thread. I would like to gain more intuitive understanding of this.

I understand perfectly the idea that a motor will 'feed back' on the distribution system and act like a generator when power is disconnected (until the motor slows and finally stops). Why is this any different from when a motor is being powered and running ?

I ask this question because I was specifically asked to include motor contribution in our fault study by a very senior and smart engineer. Unless a couple people know something that he doesn't I'm not sure I can agree with what I've read on here. Also, SKM software spits out the higher fault levels when you model a motor as a motor and not as a bus. If the feeder could only see the fault current from the distribution system OR the fault current from the motor, then SKM wouldn't do this.

Right?

 

[dpc]

It would really help if we knew what thread you were referring to so it could be put into context.

First, you have to distinguish between induction and synchronous motors and then the motor horsepower and speed.

Your statement that motors will act like generators until the motor stops is not correct.

Induction motors rely on the system voltage for excitation. During a fault, they will contribute fault current, but only for a very brief time - a few cycles at best.

Because synchronous motors have an external source of excitation, they can contribute fault current for a somewhat longer period of time.

So, yes motors do contribute short circuit current, but not for very long.

 

[jraef]

Assuming you meant this thread? thread238-192141

To expand on dpc's response, induction motors can contribute to the SC current level, but not substantially to the energy, so the destructive power behind their contribution is minimal if even measurable. But you have to consider the point at which you are measuring SC in the first place.

Once disconnected from the supply, i.e. a main breaker opens, the only contribution comes as a result of residual magnetism in the motor itself. Even then, an induction motor will only generate when it is running super-synchronous with regards to the supply, otherwise it is still a load. When a main opens ahead of it the supply frequency becomes zero, meaning the motor is technically super synchronous. So it will regenerate as long as the residual magnetism is there, but that's typically only a few cycles.

If a fault occurs on a bus that is still connected to the supply, i.e. before something clears it upstream such as the main breaker, motors still on that bus can continue to contribute as long as the supply is still there to excite them (still speaking induction here). But once again, a motor does not continue to regenerate even if still connected unless it is also running super-synchronous with regards to the supply. So a motor connected to a load that can overhaul after a fault MAY contribute continuously, but the concept is not universal to all motors all of the time.

I've had people with more expertise in fault analysis tell me that because of the worst case scenario of the way a fault can occur, sometimes the motors on-line at the time may be overexcited for a few cycles and that can allow them to regenerate longer. That is the scenario behind needing to calculate their contribution in a fault study. But don't take my word on that part. Fault analysis is not my forte. I'm more of an observer.

 

[davidbeach]

Given the title of the thread and the way the question is phrased, I'm not sure I know what is being asked, but...

Motor contribution is real and a concern for the first few cycles as stated. Motor contribution may be sufficient to push a fuse into its current limiting range where it wouldn't have been without the motor contribution. Motor contribution may cause a breaker to exceed its withstand rating even if it doesn't begin to open until after the motor contribution dies out. If a fault study provides only one value, that value will be the initial fault current, including all motors. If you dig further into the SKM fault reports you will see (if you turn on the option) fault values at a couple of subsequent times. You will see that these later fault values will be lower than the initial fault current. That's the way fault studies are done.

 

[txtim]

Allow me to add my two cents. I used SKM to do an analysis several years ago so I'm drawing from memory.

When you model your motors, you will input all of the nameplate data including sub-transient and transient reactances (xd' and xd''). When running a breaker duty study, the software will likely use the sub-transient reactance. If you are runing a fault analysis and relay coordination, you may be looking at transient reactance when fault current availablity is given.

The diference of course being duration of fault where breaker rating is nearly instantaneous and worst case (as well as asymmetry/DC offset) and coordination is more likely longer than the sub-transient time period of a couple of cycles with less DC offset.

Not an expert, just done a little analysis and offering my opinion.

 

[electricpete]

Those comments all make sense to me. One nit-pick: I don't like the terminology "residual magnetism" in this context. Residual magnetism is the magnetism that remains in a core after you have removed all exciting currents (corresponds to Br on the B vs H curve). But the flux that decays during the first few cycles according to the "open circuit time constant" is primarily associated with currents and linked fluxes that are decaying but not yet zero. The core residual magnetism is much smaller.

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[electricpete]

Whoops... "open circuit time constant" should have been "short circuit time constant".

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[WDeanN]

jgoebel, my question on the forum

http://www.eng-tips.com/viewthread.cfm?qid=192141

regarded specifically motor contribution to arc flash after the recommended 6-8 cycles for a fault study. Because the recommendation for arc flash is a two second maximum, (and some assume longer) it appears that the motor contribution will decay before that time. Arc flash studies that I have seen and performed up to this point assumed interrupting current (including motor contribution) was available for the entire time period.
It is there initially, however, and is a concern, as already stated for breaker interrupting studies.

 

[WDeanN]

By the way, I'm still looking for any help on this issue. You can reply at the above thread.

I know that arc flash calculations are touchy, but I am trying to get the calculated values as close to reality as I can at this point. I think it undermines the whole arc flash calculations when posted values have no basis in reality, or what has actually occured, and been observed on a bus. Getting a buy in from electricians and others in the field is hard enough as it is.

 

[jgoebel]

Thanks guys,

This does shed some light on the situation.

Motor contributions will have an impact on calculated upstream fault current values for a short (few cycles) duration is the consensus I am hearing.

I need to pick up my college motor/power circuits book and review the different kind of motors, before I can understand that difference that dpc noted between induction and synchronous motors..for fault current stuff.

How do you breakdown the motor contribution on a time or cycle basis in SKM like a couple people have noted?

 

[txtim]

I believe SKM will automatically use the subtransient reactance for a duty rating study and transient reactance for a fault current analysis.

As I stated in an earlier post, my experience is limited but as I understand it, the synchronous machines (remember they can be utilized motors or generators at any time) just have a larger, longer decrement curve due to the fact that they are externally excited and are usually being driven by an outside prime mover, not just rotor inertia (if being used as a generator). therefore, they will typically contribute more substantial currents for longer periods of time than their induction counterparts.

Also, the larger the machine HP induction or synchronous), the lower the reactances tend to be and therefore, the higher the current contribution.

 

[dpc]

Per ANSI C37, there are two breaker ratings of interest: Momentary (Close & Latch) and Interrupting. The networks used are slightly different for each, as specified in the ANSI standards. If you are doing an ANSI breaker duty calculation in SKM or any other software, the program will use different motor contribution values for the momentary and interrupting calculations, depending on motor horsepower.

 

[ijl]

jgoebel, you asked "How do you breakdown the motor contribution on a time or cycle basis in SKM like a couple people have noted?"

I do not know about SKM, but see

http://pp.kpnet.fi/ijl

for an idea how the contributions from different sources (motors, generators) decay with time. Please note the simplifying assumption, that the rotating speeds stay constant.

 

[QBplanner]

Even, induction motors, contribute fault current or not also depends on system pre fault condition and post fault conditions. Not necessary always contribute fault current even in a short period say a few ms. If the post fault motor terminal voltage is even higher than internal field voltage, induction motor will not contribute any current to system.

 

[QBplanner]

By the way, I got a lot of such "senior or smart engineers" (I am in Canada) who always claim 30 years experiences. They even don't know how to size the 500kV shunt reactor. Always trust yourself!

 

[electricpete]

QBplanner - under what circumstances would a fault increase the motor terminal voltage?

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[QBplanner]

Electricpete:

Don't misunderstand,I am just remind don't always say induction motor contribute "fault current" even in a short period. Be professional. during open phase(s) fault, Induction motor not necessary contribue anything to system.

 

[sslobodan]

I run trough posts so I am sorry If I am only repeating what someone said, but you need to understand the basic functioning of current. See the EE system is trying to be in balance all the time so you have generators that are generating exactly the same amount of power as consumption. When you have imbalance system is trying to compensate. Imbalance can be different in nature. For instance if you have short circuit all the current from entire network will rush to that point so it can equalize imbalance (that is why we turn off the faulty line).
Now you need to know how the process is running in engines. When they are powered up they magnetisase their coils and start to turn in a way opposite than rotation of generator. That is the way they make grid in balance. We use that power for mechanical work like pumping water. When the motor is disconnected from the grid. due to the same principle of getting the network system balanced he will return the remaining (magnetic) energy to the network acting as a generator to the rest of the power network until there is no magnetic energy in him that can produce current flow.

I hope this answer is good enough for you to understand the issue