Should motor contribution fault currents be considered in protection coordination studies? 1

[bennyvw]

I'm currently modeling my 11kV distribution network in ETAP with a number of lumped loads representing the large motor installation base at the site.

As part of my protection coordination study I've found that some of the feeder protection devices are tripping as a result of fault currents from motor contribution.

I'm unclear on how ETAP models motor contribution to short circuit fault current, does it assume a constant contribution for the duration of the fault? Or does it decay quickly as is the case in reality?

My trip times on these relays are in the order 1-2 seconds at these fault levels. I would have assumed at that point the motor contribution to short circuit fault current would be negligible. Can these trips be ignored? Or should I turn off motor contribution in ETAP?

 

[cranky108]

As always, the answer depends.

Some motors and applications should be modeled, but at 11kV I believe that very few devices would need that detail of modeling.

I recall a customer that had several 900 kVA synchronous motors, and it made a big difference in the fault currents. Even bigger when that installed VFD on them.
On the other hand, rarely would you need to model induction motors.

 

[Gr8blu]

bennyvw: The motor contribution to a fault will be maintained at the fault level as long as two conditions are met. These are 1) the coil is still turning with respect to a magnetic field (or the field is turning with respect to the coil), and 2) there is a path for the current to flow. I have had several incidents where the duration of the fault current is measured in excess of TEN MINUTES, due to the mechanical movement of the rotor (and its associated winding) within the magnetic field of the stator - even when power to the stator winding is turned off and all that's left is residual magnetism.

Converting energy to motion for more than half a century

 

[dpc]

In this specific situation, it's pretty unlikely that motors will still be contributing significant fault current after 1 second. But motor contributions are certainly real while they last. I've encountered a refinery with cogeneration where directional overcurrent relays mis-tripped for grid faults because the motor contributions were not considered.

 

[bennyvw]

Thanks everyone for the replies!

Gr8blu - Assuming that this fault is on a separate feeder to the lumped load, I assume the lumped load / motors would still be energised (although with a reduced bus voltage) and rotating. Is this how the sustained motor contribution occurs?

 

[Gr8blu]

bennyvw: I forgot to add a third criteria. The machine has to go into "generator" mode. If the machines connected to the bus are "motoring" (i.e., still driving their respective mechanical loads), then the contribution to the fault should be zero. If, however, the machine dips into generating mode (load is driving the machine), then yes, it will contribute as long as it rotates.

Converting energy to motion for more than half a century

 

[RRaghunath]

IEEE 399 (popularly known as Brown Book) has been a good reference when it comes to equipment modelling and power system studies.
"Table 7-2—Reactance values for first cycle and interrupting duty calculations" in IEEE 399 specifically indicates how the induction motors to be treated when it comes to contribution during faults.