Synchronous Motors Driving Gas Compressors, with load sharing problems

[skeandhu]

We have 3 large Elliot Gas Compressors, driven by 13.8 KV:14MW. Alstom Synchronous Motors. For some unknown reason one of the Compressors gives a higher Output Flow, with consequent increase of output power than the other two. We have the latest Compressor Controls System gear for anti-surge and load sharing etc. All the instrumentation has been checked several times, there appears to be no problem. So is it possible the excitation on the motors, if incorrectly adjusted could cause this load sharing problem. For some unknown reason the excitation controls aren't in anyway connected to the compressor control system?

 

[Skogsgurra]

I cannot imagine that excitation has anything to do with the compressor volumetric flow.

As long as the motors are sychronized to the mains, they run at exactly the same speeds and since speed is the only variable affecting flow from the drive side, it cannot be a motor problem - or anything electrical.

If your power factor differs between the motors, then the excitation should be checked. But not if the difference is in gas flow.

Gunnar Englund

http://www.gke.org

 

[waross]

Exitation will not affect the power but it will affect the current. Are you actually measuring the power or the current of the motors? Over exitation will increase current as the motor supplies leading vars to the system.

 

[UKpete]

skeandhu, is the motor on a variable speed drive?

The company I work for at the moment supplies magnetic bearings for a Siemens 26MW synchronous motor, this is on a VSD (a large one!). Co-incidentally, these are used on gas-field compressors.

 

[waross]

Sorry, I misunderstood your original information. Is this a problem that has developed or has it existed from new? Is your piping arranged so that this compressor has less resistance to flow than the other compressors? Reduced reduction to flow will often result in an increase in loading. More flow times the same pressure drop means more work done.
yours