Synchronous motor & Out of step protection type MFOS

[RRaghunath]

Synchronous motor of 11.5MW, 11kV rating drives a compressor. This is provided with Alstom make Out of step protection relay type MFOS. The relay has only current setting which is set at 120% of the motor fl rating of 615A.
The relay is seen operating and isolating the motor when ever there is a transient voltage dip due to a fault elsewhere in the system.
Any experiences, suggestions!

Thanks.

 

[plc123]

Hi,
it looks like during transient voltage dip due to a fault elsewhere in the system moment from compressor remains the same, but angle between magnetic vectors of rotor and stator grows due to "M prop V power 2". The same time
Stator current is propotional to the angle between magnetic vectors. So after voltage recovery you have current spike.
I am not sure about your case - may be reactors will help. But obviosly voltage drops too much. This may be the problem.
Way to fix are well known.
Thanks,
Vladimir.

 

[DougMSOE]

Out of step protection may be done 2 (or more) ways. The best method that I found is to use the rotor circuit of the synchronous motor. Far more dependable and more discriminating than the stator method.
Once the motor has synced, any transient in the rotor circuit, other than for power factor correction will most likely be out of step. Further, if this is occuring, you may want to check for shorted turns in the rotor field. In short a lack of MMF where the magnetic lock needs to be.

 

[xabproject]

Dear RRN,
I have heard of similar problems in a parasitic plant on the SPIC PetroChemicals Factory in Tuticorin, TN.
They introduced a short time delay, and reduced the O/C setting - well studied to override the transient periods that were recognised using a fast chart - OMNIACE or something of Japanese make. Is your trip instantaneous?
In any case, if you have closed this thread, let us all know what did you implement?
Best regards

 

[electricpete]

Sounds like good comments from others. I don't know much about the subject but a couple of thoughts.

1 - It seems to me that reactor suggested put you closer to an actual out-of-step condition.

2 - If your machine has been isolated during a power system transient, are you really sure that that trip was not necessary to protect the machine? Maybe the protection was doing exactly what it was supposed to.

 

[RRaghunath]

The MFOS relay operates only after sensing that the PF has gone down below 0.7 (in addition to the current magnitude) and it appears the trip is necessary to protect the motor (Thanks Electripete, Alstom also have explained it the same way).
We need to see how the voltage dip, that is excessive at the moment (for faults in adjacent feeders), can be minimised.

Thanks everyone for the responses.

 

[jbartos]

Suggestion to the previous posting: Voltage dips and other power supply quality variables are often treated by power line conditioners.

 

[swgrmfg]

I always add a 0.25 Second delay time to the PF trip. It helps ride over adverse conditons as those you are experiencing.

Gh

 

[jbartos]

Suggestion: When it comes to the 11.5MW synchronous motor load, .25 seconds in delay will need to be evaluated with respect to nature of the motor load.