Backing of Use of 300 KVA transformer 1

[Ron11]

Hello,

I am trying to calculate the % voltage drop and figure out the best approach to the scenario below

I have a SCE Overhead line 16.5KV feeding a transformer which steps the voltage down to 480V which then supplies a control panel about a 1000 feet away. From the panel, we have a firewater pump(150P) connected about 30 feet away. The FLA is 180 and LRA is 1085. Starting power factor is 30 percent and also we assuming normal impedance for the transformer at 5.75%.

I am having a hard time finding the voltage drop given the conditions. Also, I need to know if the 300KVA transformer is viable for the application taking into account the percent of nameplate min is 85%. Please can someone help me figure this out. Currently, I have found applications showing calculations without taking into account the distance. However, this is not accurate considering the 1000'. Please share your comments. Appreciate any help provide

Note: I need to justify backing use of the 300KVA transformer. SCE believes a 150KVA transformer should work fine but I feel this may not be the case considering a distance of a 1000'

 

[dpc]

I'm guessing the transformer impedance will be less the 5.75%. This standard impedance only applies above 500 kVA. Typical impedance range would be 2% to 5% for a 300 kVA.

See attached. Looks like about 79% voltage at the motor - this will depend on the feeder size. I assumed 2-250 kcmil per phase (copper).

 

[Ron11]

dpc,

Thank You. Is the 1000' ft incorporated in the load flow diagram? Is there a way to do these calculations by hand. I would appreciate it if you could share that.

 

[dpc]

The 1000' is included. You can see the voltage difference between the source end and load end of the cable.

This can be done by hand. You need to create an impedance diagram including:

Utility impedance
transformer impedance
cable impedance
impedance of motor at locked-rotor.

It's much easier if you use the per-unit system and put everything on the same impedance base. The IEEE Red Book should cover this pretty well.

Voltage drop calculations are, by nature, iterative. The current into the motor depends on the voltage at the motor which depends on the current. When doing it by hand, some trial and error is required.

I'd guess there are some on-line resources that can help with this calc at no cost.