Imbalanced current on 3 phase system 1

[cjhut]

I have a 3 phase 480v 2000a main lug only switchboard being fed from a wye secondary of a 1500KVA three phase oil filled transformer. The primary voltage is 2.3kv. There are three fusible disconnects on the switchboard feeding three soft starters each driving a three phase squirrel cage motor. The switchboard is about 6ft away from the transformer secondary and is fed by parallelled conductors in wire trough conecting to the 2000amp bus.The parallel conductors all appear to be of the same length,termination,etc. The soft starts are fed from the disconnects on the switchboard through cable tray using 1 single conductor per phase.Everything is relatively new including the transformer. I am having a noticable differnce in current on all three phases on all three of these motors. Example: if I measure the voltage with a load I read phaseA-B volts 480, phase B-c volts 472, and phase A-C 475, each phase to ground around 277. So my voltage looks good. What I am having trouble understanding is, my current readings look like this A-360 B-320 C-311amps.These readings were taken from a 300 HP motor running a screw compressor that was about 85% loaded from an air demand standpoint. The FLA of this motor is 345amps, so A phase is running above that.I am seeing this same realtionship on the two other identical machine/motors. This problem has been here since
the installation, so this is nothing new. I was wondering if this might have to do with the impedence of the windings of the transformer being different? We are not using 3 single phase transformers it is one unit. It is causing me some problems because if the machines have to run at full load for an extended period of time the soft starter will trip out on phase A overload. I understand the voltage is slightly different from phase to phase, but the difference between volts/amps isn't proportional. If impedence is an issue how can I prove it?

 

[cjhut]

To some a of the above questions, I did use as true RMS meter. In fact, I used two fluke true rms meters for both the voltage and amperage and got the same readings. I also did not notice a change in the voltage with the loading of this transformer. I haven't taken it down to no load, but I did think of stepping up the load and taking readings, which didn't change (voltage).