motor diversity 1

[wonderchi]

I've entered a group of synchronous motors in SKM and tries to compare the fault currents at the buses when load (around 22kW) is at 60% compared to 100%.

The results I obtained is the same, is that normal. I believe not!!

 

[waross]

I suggest that you recheck your motor sizes. 22kw is very small for one synchronous motor, let alone a group.
The motor contributions to fault current may be a very small percentage of the total fault current on a stiff system.
respectfully

 

[davidbeach]

Motor contribution to fault current is not a function of the load on the motor, but is a function of the motor parameters.

 

[wonderchi]

Thx for your replies

Each load is about 22kW but the total load in each bus is less than 100kW.

Just to confirm wht davidbeach is saying about the motor contribution to the fault current, if the load is running at 100% and its x/r ratio is 3, and if now if the load is running at 60% is the x/r ratio different?

 

[davidbeach]

The motor parameters that determine the contribution of the motor to a fault are independent of load. It is only necessary that the motor be running at the time of fault initiation. Obviously any motor that isn't running won't contribute to the fault, so if you are reducing your load by turning off motors, then you will have a lower fault current. But if you can ever have all the motors energized, that is the fault current contribution to use.

 

[wonderchi]

But consider this, just prior to the fault, your synchronous motor is running at 100% and therefore it draws current I, when you reduce the load to 60% capacity, the current is now less that I, and when the fault occurs, load at 100% is drawing more than 60% therefore more current go to the fault point giving a bigger fault current.

True?

 

[davidbeach]

Fault current flows in the opposite direction from load current. So, if there is any affect, fault current can build faster if there was no load current pre-fault.

 

[PHovnanian]

The fault current contributed by a synchronous moter will depend on the field current change between the 60% and 100% load cases. If the field remains unchanged, so will the fault currents.

That said, as was previously pointed out, for a stiff source, the motor contributions might not be very much.