Variations in stator currents on AC generator

[powergage]

We recently ran an URGE test on a steam turbine generator. We didn't experience any problems, but after reviewing some data, we realized that there is some variation in the stator currents for the different phases. We measured up to about a 3% difference between two of the phases. My question is: Is this a problem? I am not sure if this is typical of steam turbine generators or if this is something to be addressed and researched. Any input appreciated. Thanks!!

 

[radomir]

It is obviously a problem of the electrical generator. Something is wrong with it: either OEM has forgotten to do some necessary tuning between phases (e.g. adjust resistance) or maybe you were supposed to do some fine adjustments. Those are only wild guesses of a mechanical engineer but definitely problem is electrical.

Steam turbine (ST) is only a powering device, which has to run the generator at constant speed (3600 rpm in NAFTA, 3000 in EU) up to a certain level of load – electrical power consumption. If that load level is exceeded, max ST’s power is exceeded, ST will not be able to maintain constant rpm and power outage will be somewhere around. This is just to remind you electrical guys that every powering device has its limit as well as your generators.

 

[RAMConsult]

powergage, the short answer is that they should be better balanced, within 0.5%. The best place for this question is the Electric Power Engineering forum, you'll find answers to similar queries.

 

[powergage]

radomir-
Thanks for the reminder


RAM-
Thanks for the advice. I have read further on similar postings in the Electric Power Forum and am looking into some possible causes.

 

[JamesZero]

october 24
did you find and answer to your question re phase unbalance. If not perhaps I can help. Most likely this is not a problem in the Generator. need to know settings for Generator Protection devices 46(Negative Sequence), 27G (Neutral/Ground undervoltage)in isolated neutral system, 51G(ground overcurrent in solidly grounded or resistance grounded systems. Basically if your protection is set correctly then you can just leave it do its job.IF IT WORKS - DON'T FIX IT!

 

[Bize]

The rotors of most synchronous generators are designed to operate in a relatively unchanging magnetic field. (i.e. it has one or more pairs of fixed north and south poles that spin synchronously within a similar set of magnetic fields spinning because of the current through the stator.)

If the current through the coils in the stator are unbalanced in a certain way and by a certain amount, its magnetic field will vary enough to induce surface currents on the generator rotor iron. These can cause local heating on the rotor and even cracking of the metal. This varying magnetic field can also cause torsional oscillation of the rotor train.

The measure of the damaging unbalance component is called "negative phase sequence" currents (EE's just love impressing people with big words.) Most specs state that rotors are supposed take up to 25% of the rated current as negative phase sequence currents, but sometimes rotors can be damaged with much less (especially if they are near torsional resonance and there are other system unbalances.)

Assuming the measurement system is well balanced, three percent phase variation does not seem like much, but to accurately measure your unbalance you need a special meter and relay protection designed to measure negative phase sequence currents.