Induction motor contribution to fault level 5

[Deansharafi]

Hi all,
In establishing the induction motor contribution to a fault level of a system, what criteria can be considered?
Are contributions of small LV (415V)motors considered significant to a fault level on an HV switchboard? These small motors normally don't have a considerable inertia and are connected via transformers and cables that have impedances.
Your thoughts please.

 

[rbulsara]

You can account for contribution from a group of small motors as one entity of HP equalling total of the small motors' individual HP.

You can assume transient reactance of the group motors to be 0.25, if you do not have the exact data.

This amounts to Isc from motors to be 4 times the total FLA.

Refer to IEEE buff book and/or red book for further reference.

 

[cuky2000]

Several SC application guide suggests to lumped all small motor together and assumed to be running. The rational behind this is that small motors are turned off and on frequently, so it is virtually impossible to predict which ones will be on the line when SC occurs.
At LV, the cable significantly damp the amount of SC current added by motor contribution. A practical approach suggested grouping all small motors and assuming to be connected at the transformer, MCC, SWGR or panelboard.
For industrial application greater than 400V up to 600 V, the secondary bus and have reactance of 25% on a kVA rating equal to 100% of the transformer rating. Motors connected to MCC or SWGR are treated similarly with reactance of 25% on the kVA rating equal to the connected motor horsepower.
On the other hand, for commercial application in 208 V or 240 V systems, the reactance also is considered 25% on the kVA rating equal to 50% of the transformer rating.

 

[peebee]

I agree with all above posts.

Re "Are contributions of small LV (415V)motors considered significant to a fault level on an HV switchboard?" -- Depends on how many small motors you have connected, and on the impedance of the upstream HV system. If you have a small HV system, and lots of 415v motors, well then yes, they could be significant.

 

[Kiribanda]

Hi Deansharafi,

I presume that your question is based on ANSI/ IEEE short circuit calculation method and not on IEC 60909 method.

If so, then my understanding is, if the HP is rating is less than 40 HP you can even neglect them for both half cycle and 1.5-4 cycle interrupting calculations.

I advise you to refer the Table 7.2 of IEEE STD-399-Brown book (which is also the guidance for all software programs), which gives information you are looking for.

Good luck!

Regards!
Kiribanda

 

[peebee]

Strange, my 1986 copy of the Red Book mentions that ANSI/IEEE C37.010 & C37.5 permit LV motors under 50hp be neglected on HV systems, but then the Red Book goes on to recommend that they be INCLUDED in the calcs at 3.6xFLA.

 

[sykimk]

In addition to above posts, you need to consider design tolerance of transformer impedance if you are in the design stage (e.g. -7.5% or -10% depends on your region).

For me, I usually use the 5 times FLA of motor lumped as a SC contribution currents without damping by cable impedance for conservative design regardless of applicable codes because SC contirbution of motors is less damping than I taught nevertheless considering cable impedance. (approx. lengh per motor is 100-150 meters)
I know my design concept is a little bit too conservative than ANSI/IEEE method but be comply with IEC method.

 

[mc5w]

You also need to check the starting code letter of the motor. American electric motors have a letter that gives the KVA per horsepower of locked rotor current. The fault current contribution will never exceed the locked rotor current. If the motor code letter is around G then your 25% impedance factor might be correct.

Split phase motors are around code letters L, M, or N and 4 inch (100 mm) diameter submersible well pump motors (say 1/2) hersepower) are more like code S. In these instances the motor would contribute more than its fair share of fault current.

In the case of 3 phase design C or D motors that have extra high starting torque you could also be dealing with a large code letter and contribution to fault current.

Mike Cole, mc5w@earthlink.net