CAT Generator Current Unbalance Question 2

[SparkOmatix]

During testing CAT noticed a small load unbalance on our generator and I wanted to get thoughts on this. The CAT technician seemed to think it might cause premature wear on the generator eventually. They said typically they did not get too concerned unless the imbalance was over one hundred amps. The readings were as follows:

Phase A- 865-870Amps
Phase B- 815-820Amps
Phase C- 800-805Amps

Anything to be concerned with? A penny for your thoughts!

 

[dpc]

Unbalanced currents in a generator produce a negative sequence current in the machine. This can cause overheating of the rotor. I would contact CAT engineering and have them provide you with the maximum level of imbalance that the unit is designed to handle.

If you do a search for generator negative sequence current, you will probably find more info.

David Castor

http://www.cvoes.com

 

[rbulsara]

Could be the function of the load. What type of load? What were the voltage readings?

I generally would not worry about the imbalance as long as the amps are within the gen rating.

Rafiq Bulsara

http://www.srengineersct.com

 

[catserveng]

Typical maximum negative sequence current for CAT SR4 and SR4B generators is 10%, unless it is a custom unit. Your selling dealer can provide you the TMI (Technical Marketing Information) for you specific generator.

Frankly I don't see a problem in the numbers provided. The imbalance could be a function of the load, but can also be from the generator itself depending on the frame size and if it has what we called a "square lam" stator. Square lamination stators were built to reduce lamination steel waste, mostly in generators used for standby service. They are fairly easy to identify, the no load rated voltage varies as much as 2% phase to phase. The only time it has caused us any problems is in some parallel island mode applications we had some circulating current issues.

Most CAT tail ends have stator RTD's or TC's installed, if the dealer or you continues to be concerned, monitor the stator temps. The limits are also published in the TMI data sheet for the specific generator.

Hope that helps. Mike L.

 

[waross]

We had a pair of 1.2 MW Cat units and a load that we couldn't balance. At a loading similar to yours, we would often have 200 amps difference. After 6 or 7 years service, routine testing indicated that the rotor insulation was dropping. The rotors were pulled and showed signs of heating. The rotors were refurbished and put back into service with the same loads.
While this may indicate the severity of less than ideal conditions, beware. It seems that every year machines are built with less iron and copper for a given rating. A new machine may not stand abuse as well as a similar ten year old machine.

Bill
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"Why not the best?"
Jimmy Carter

 

[ScottyUK]

Don't rely on stator RTDs to protect you from high NPS currents. By the time the rotor is developing enough losses to show up in the stator RTDs you will be in the market for a rotor rewind. A properly set NPS relay is the only practical means of protecting the machine, and with modern integrated relays the NPS function is often available without additional cost.


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If we learn from our mistakes I'm getting a great education!

 

[SparkOmatix]

I thank you all for your time and effort in commenting on my post. All of you have provided great insight!