Short circuit current contribution

Hello,

when calculating short circuit currents on supply side of drive, is it necessary to take into account contribution from S120 and G120 Basic/Active/Smart Line Modules driven motors?

Is contribution affected by activation of regenerative feedback capability or not?

Kind regards,

Nikola

Short answer?  No.

Longer answer?  No.  SS capacity does not depend on connected loads.

Thanks Jeff!

Than I understand well, when it comes to SC calculations on main 0.4 kV busbars (AC), I will ignore all vfd driven motors.

Kind regards,

Nikola

Now I'm a bit confused.

If I have a asynchronous motor connected to main 0.4 kV AC bus, when calculating short cirucuit currents at that bus motor will contribute to the fault with approximately starting current.

If the motor is powered via S120 or G120 Basic/Active/Smart Line Modules driven motors with activatied/deactivated regenerative feedback capability-than there is no motor contribution to short circuit current at main 0.4 kV AC bus?

You are confusing two things, short circuit calculation and effect line impedance on a load.

Calculation of short circuit current depends on line impedances upstream of the place of fault.  The connected load is not relevant.

If you are wondering about the effect of online starts, then connected load does matter, again because of the effect of upstream impedances which can be determined from the SSC.  But SSC is not affected by connected load.

To answer your second question, when an inverter is applied to motor, there is no more inrush current as the current is now controlled, so the maximum disturbance would me the current limit set by the drive.

About short current calculation upstream of the drive there are no confusions, and the effect of the "static" load, as well as the asyncronus motor alone. Now, the nature of the motor itself changes when drive is instaled.What interests me is "limit set by the drive" when contributing to short circuit current by the motor and the drive when short circuit occurs at point of connection of the drive, for S120 or G120 Basic/Active/Smart Line Modules, and effects of activatied/deactivated regenerative feedback capability.

Kind regards,

Nikola

Jeff Casagrande

You are confusing two things, short circuit calculation and effect line impedance on a load.

Calculation of short circuit current depends on line impedances upstream of the place of fault.  The connected load is not relevant.

If you are wondering about the effect of online starts, then connected load does matter, again because of the effect of upstream impedances which can be determined from the SSC.  But SSC is not affected by connected load.

To answer your second question, when an inverter is applied to motor, there is no more inrush current as the current is now controlled, so the maximum disturbance would me the current limit set by the drive.

 This statement is completely incorrect. The potential short circuit current at any point in an electrical system depends on several factors:

• Short circuit current contribution from all utility sources
• Quantity, size, transient and sub-transient reactances of all motors, generators and solid state power conversion equipment connected to the system regardless of whether that device is conventionally considered a load or a source.
• Impedance of the distribution system from all sources as described above to the fault
• The nature of the fault itself.
• Presence of impedance grounding systems if solving for a line-ground or line-line-ground fault.

Now, to answer the original question. If your motor is connected via a VFD, it will not contribute to the available fault current at any point in the system other than between the VFD output terminals and the input terminals of your motor.