Step-up Transformer Frequency?

[Franko]

I posted this in Electric Power Eng'r forum - no response:

We used a step-up voltage transformer for a test run with a variable-speed 60 Hz motor. There was an odd subsynchronous frequency near 9 Hz found in the converter output current signal. Could the frequency come from the step-up transformer?

 

[DickDV]

Was this converter an AC drive or some other device?

 

[Franko]

It's an LCI drive with a DC-link converter - we are thinking that the step-up transformer is affecting the electric circuit. Has anyone found that the effective resonance frequency could be as low as 8-9 Hz? This is a large motor with about 10 MW operated on a stable grid.

 

[DickDV]

Load Commutated Inverters and 10MW motors are out of my league. Some else needs to deal with this.

 

[GTstartup]

What's the configuration of the transformer? If the LCI is connected to the Y then the transformer should be left ungrounded (i.e the star point floating)to prevent harmonics.

 

[Franko]

I believe it is isolated WYE connection and has a secondary grounded connection. However, harmonics are not the issue - they were low - but there was the odd low-frequency 8 to 9 Hz.

 

[Bung]

Could it be an interaction between the voltage control on the inverter and the load of the motor? If there is a sudden change in conditions (eg motor switching on), the inverter compensates for added load, which affects motor maybe causing inverter to back off, so motor draws more current and so on back and forth. A mechanical analogy would be trying to push and pull a lump of concrete using a long spring - you can get an oscillatory condition between the lump and the pusher.


Bung
Life is non-linear...

 

[Franko]

Perhaps it could be an interaction - the frequency was shown to be constant with motor operating speed. It likely is a natural frequency of the electrical circuit. Has anyone ever seen one that low - i.e. indicative of a long time constant?
See:
quinn.mech.uakron.edu/me340/notes/me340sum.ps