Arc Flash Motor Contribution

[WDeanN]

Here at our facility we have several large synchronous motors. The question of motor contribution to our arc flash hazard has recently come up, especially with a software upgrade to SKM Power Tools for Windows, version 6.0.2.1, which allows the user to specify synchronous motor contributions over a time period. On one bus in particular, because there was no protective device on the motor that would clear the fault quickly, (we are using a two second maximum for our calculations, unless other conditions warrant otherwise) the motor contribution to the arc flash hazard was very significant. The calculations revealed 17 cal/cm2 with all 6 synchronous machines running, but only 2 cal/cm2 with no motors running.

While trying to research our options, I came across an article by J. C. Das, "Design Aspects of Industrial Distribution Systems to Limit Arc Flash Hazard" in the Nov/Dec 2005 issue of IEEE Transactions on Industry Applications. Mr. Das states that "The synchronous motor short-circuit contribution can be neglected in six to eight cycles."
Although he doesn't reference it directly, I found support for this in the 1997 edition of IEEE 399, the IEEE Brown Book. Chapter seven addresses short circuit studies.

Section 7.4.1 states:
Time delayed currents are the short-circuit currents that exist beyond 6 cycles (and up to 30 cycles) from the fault initiation. They are useful in determining currents sensed by time delayed relays and in assessing the sensitivity of overcurrent relays. These currents are assumed to contain no dc offset. Induction and synchronous motor contributions are neglected, and the contributing generators are assumed to have attained transient or higher value reactances (see Table 7-1).

This can also be found in the 1993 edition of IEEE 141, the IEEE Red Book. Chapter four addresses short circuit studies. Section 4.5.4.3 states:
For an application of time delay relays beyond six cycles, the equivalent system network representation will include only generators and passive elements, such as transformers and cables between the generators and the point of short circuit. The generators are represented by transient impedance or a larger impedance related to the magnitude of decaying generator short-circuit current at the specified calculation time. All motor contributions are omitted.

This is for breaker duty calculations, so the question remains: Can this be applied to arc flash studies? Otherwise, what is a reasonable time to assume for the motor contribution to an arc flash event at the upstream bus?

 

[ijl]

I do not know much about arc flash, but I have understood that it is like a short circuit with an impedance at the fault. If that is the case, then the decay of the current is similar to the decay of a short circuit current.

When the fault is at or near to the terminals of an sychnronous machine, the decay of the short circuit current depends on three main factors: the subtransient and transient time constants, and the decrease of the rotating speed. The time constants are parameters of the machine, and do not depend on the load. But the rate of decay depends on the fault impedance, but only indirectly.

The time constants can be given in two ways: either as so called open circuit time constants, or as short circuit time constants. The open circuit time constant gives the rate at which an excess current in the rotor decays, when the stator current is zero. It is essentially the time constant of the rotor winding. The short circuit time constant gives the rate of decay when the stator windings are shorted. The time constants are related, one can be calculated from the other. The short circuit time constant is smaller than the open circuit time constant, because the short circuit current (somehow) "consumes" the magnetic field of the rotor. When there is an impedance at the fault, the stator windings are neither open nor shorted, so that the rate of decay is something between the open circuit and short circuit time constants.

The time constants are typically so short, that the machine speed does not change much in the time frame of the constants. However, they are much longer than a couple of cycles, so that the contribution from the machines should be taken into account in the arc flash (and short circuit) calculations, I think.

It is difficult to say anything detailed about the decrease of the rotating speed without calculations or simulations. The decrease in the speed depends on the mechanical inertia of the rotating masses, on the braking effect of the load, and on the braking effect of the short circuit. When shorted, the machine acts as a generator, converting the mechanical energy into electrical energy, which is converted to heat in the resistances of the affected components (machine, arc, etc.). However, when so much time has passed that the decrease in the rotating speed becomes important, the short circuit current has reached the steady state value. This is typically much smaller than the initial current, so that the contribution from the motors to the arch flash may not any more be important in this phase.

 

[WDeanN]

ijl -
Thank you, that is just the type of information I am looking for. Now, if I could just narrow down a reasonable time limit, or value for the time constants to perform my own calculations...

Of course it will depend on the resistance of the arc, which is highly variable. I am trying to get best and worse case scenarios, though for a good idea of what is going on.

As far as loading, I understand what you mean by the time not really being important when compared to the short circuit time. Do you have any equations or simulations I could run to verify this?

 

[ijl]

Because the equations are pretty complicated, it might be easiest to simulate the transient with ATP/EMTP or similar. You may also get some idea of the decay of the currents with the program at

http://pp.kpnet.fi/ijl

But note, only AC currents are considered, and the rotating speed is assumed to be constant.

 

[Electic]

Not sure about SKM but with ETAP we can assign a clearing time arbitrarily, for arc flash calculations.

I suppose one could perform calculations three times:

1) calculate incident energy for a fault with all motors connected and fault clearing time set at motor contribution duration (i.e.: 6 cycles)

2) same calculation as above with all motors off line.

3) calculate the incident energy for a fault with motors off-line, but with actual device clearing times. Add the difference (in calories) between calculations 1 and 2 above.

Comments:
a) fault current in calculation 3 will be less than actual that could affect device clearing time. Doing as proposed should result in a conservative calculation, but not so conservative as leaving the motor contribution on for the entire fault duration.

b) This would incorrectly assume linear motor contribution rather than decaying as expected.

Just thinking out loud.....

 

[WDeanN]

Electic –
That was one of the things I didn’t like about ETAP – the inability to assign a broad maximum clearing time. You are forced to perform multiple calculations. I did find it more user friendly then SKM, however, and liked that the database was MS Access compatible, so that you could quickly open the database and see what was missing, or wrong, without going through every device.
The use of the macros in ETAP makes performing multiple calculations easier, however, and the results can be exported to MS Excel.

SKM also assumes linear motor contribution, but allows you to set the duration and magnitude for the one time step calculation. You would think that with the input of motor load, x’’, x’, and everything else, they could perform a better analysis. They already to it to a degree for motor starting studies. The two modules do not talk, apparently.

For now, I have settled on reducing motor contribution to 100% of rated (full load) current after 6 cycles. Any thoughts?