Current Unbalance during start up 4

[Deansharafi]

Hi all,
I have a client with some unbalance current problem during start up of their HV motors.
They have three Siemens 6.6KV induction motors having a current unbalance @ starting of 15 - 30% for 1-1.5 seconds. They run a 6.6KV HV switchboard with 2 x 5 Mwatt steam turbines as base load and 5 x 3.8 Mwatt gas turbines as feed to the HV board. The board has an A & B sides, which is spilt by a fault limiter. From this they run 3 Compressors 6.6KV motors. 2 x Export compressor motors @ 757Amps FLC and 1 x Recycle compressor motor @ 426Amps FLC, Siemens rate these motors @ 3.5 times FLC at starting.
All 3 compressor motors run all day every day. The facility has been up and running since nine years ago and the motors have been in place since then with no previous current unbalance issues, but lately one of the compressors motor tripped during starting on high current unbalance of more than 20%.
This scenario happened to the other two motors with the smaller one having a less unbalance around 15%.
Some chart recordings from the secondaries of the protection CT's on motors were carried out which showed a very unusual waveform on Red Phase during start up.
Readings from the VT's on either side of the bus indicate that there is no voltage drop issue during normal operation of the plant.
Another fact is that once the compressor motors are up and running they run at a 1 - 2% unbalance indicating that the unbalance issue is only during high starting currents.
Since there are no major single phase loads in the system (unevenly distributed loads= voltage unbalance), and motors are connected to the board without any transformer being in between (which eliminates the possibility of having different taps = voltage unbalance), I am focusing on resistances in the system which may cause this sort of unbalance, as the three motors are behaving in the same manner and they started this from a specific time.
Have you guys experienced similar problems before and what is the line of your thinking in this regard? Thanks for your input.

 

[Deansharafi]

alehman, The fault limitor is one of the CLiP current limiting devices and consists of two chambers, one works as a reactor and the other one is a fuseable element. It is rated for 3000A and I am thinking that it might have developed a bigger resistance across Red phase.

 

[jbartos]

Comment: I agree with the previous posting that the fault limiter may experience fatique such that there is the high imbalance. Troubleshooting should be performed way upstream including the source.

 

[DanDel]

Just another comment; some older MV motors had reactor-start systems which placed reactance in two out of the three phases during startup. I don't know if this applies to your situatuon, but it would cause an inbalance during start-up.