Hi Omid,

Pavel has made a couple of points that I will tend to agree with. Here is some food for thought - you may consider as to which of these fits the bill !

Both #3 and #4 are effects of aging, and the unbalances will "take time" to aggravate to levels that are detectable by protection systems. Both require shut down of the unit, servicing amounting to replacement (insulation or bearings), and reinsertion to service.

In both cases, when you take the machine out and do a simple OC test, the rms EMF recorded in the three phases will be different. In #3, additionally, with the rotor at standstill, the impedance recorded for the three phase windings will have significant differences.

Now since you know the history of your generator, you are the best judge as to how to proceed with the investigation, and what to suspect !! :-) :-)

Hope that lengthy posting was of some use...

-Sanjay

Can you give more details about the system

Unbalanced, phase load demand could be the main cause...

Unbalanced loads, damaged generator windings (typically winding shorts).

Six similar Genset connected to one bus. One of those Gensets have unbalanced current.

Is it possible its because of air gap of the failed phase?

What is solution for this problem?

The first step should be to disconnect this generator from the bus and record no-load voltages.

Dear Omid Syt 

Normally, generator output (voltage and current) is always balanced. Even if the load is unbalanced, the generator sets have to produce a balanced output. However, if a generator is producing unbalanced current then there must be some mechanical problem (hardware related problem).  Yes, problem with air-gap may be one of the cause. 

Any assymetry in the generator could result in unbalanced output - including unequal air gaps. For "normal generators", this should not play an important role as you typically have more than 1 pole pair.

But as you are repeatedly asking about air gap: you already know that the generator was damaged - due to rotor maintenance or similar ?

In this case it is not due to unbalanced load for sure. Problem is in generator itself. The reason may be asumed by analyzing significance of asymmetry. I don`t think that significant unbalance may be driven by not equally distributed air gap. If genset is of the same units (factory made) it is unlikely, that air gap differs that much. Winding short circuit is more probable reason. Although, protection should detect it.

Hi Omid,

Pavel has made a couple of points that I will tend to agree with. Here is some food for thought - you may consider as to which of these fits the bill !

Both #3 and #4 are effects of aging, and the unbalances will "take time" to aggravate to levels that are detectable by protection systems. Both require shut down of the unit, servicing amounting to replacement (insulation or bearings), and reinsertion to service.

In both cases, when you take the machine out and do a simple OC test, the rms EMF recorded in the three phases will be different. In #3, additionally, with the rotor at standstill, the impedance recorded for the three phase windings will have significant differences.

Now since you know the history of your generator, you are the best judge as to how to proceed with the investigation, and what to suspect !! :-) :-)

Hope that lengthy posting was of some use...

-Sanjay

Omid Syt:

Hi, the first question is:

- The generator is in OPEN in its terminals?, that is it is not connected to any electrical charge and ONLY it gives the tensions of empty?

- Then, what current is the one that is unbalance?

1. unbalanced loading

2. fault in the transmission lines,

3. inter turn fault

the reason can be anything

an inter-turn short within the same winding coil.

Bad bearing, could produced, un uniform air gap with vibration, between rotor and stator while in rotation, could produced unbalance of load current, due to produced unbalanced EMF in winding......

Problems

When the ratio of the voltage to frequency (volts/Hz) exceeds a set value for a given generator, severe overheating could occur due to saturation of the magnetic core of the generator and the subsequent inducement of stray flux in components not designed to carry flux.Such over-excitation most often occurs during start-up or shutdown while the unit is operating at reduced frequency, or during a complete load rejection, which leaves transmission lines connected to the generating station.

Unbalanced faults and other system conditions can cause unbalanced three phase currents in the generator. The negative sequence components of these currents cause double frequency currents in the rotor that can lead to overheating and damage.

Although a single field-ground fault will not affect the operation of a generator or produce any immediate damaging effects, the first ground fault establishes a ground reference, thereby making a second ground fault more likely. This will increase the stress to ground at other points in the field. A second ground fault will cause extensive damage by:

• Shorting out parts of the field winding
• Causing high unit vibrations
• Causing rotor heating from unbalanced currents
• Arc damage at the points of the fault

The reason of a  generator unbalance current  is theoretically of one of two alternatives below.   

 a) generator side unbalance( typically winding turn shortfault )

b) load side unbalance

Most probably b) must be the reason,  reason. because in case of a) ,you can find it soon as the inner fault is very unstabe . 

In case of (b) various serious problems may ne caused.

* fast temperature rise of the rotor surface because eddy current would be caused at the surfece by  negaticve sequence current, 

In this case it is quite dengerous because  the slot wedges would be quickly  overheated and annealed and the mechanical strength would be lost very quickly.   

  So  I2 must be strictly limited to be within a threshold value by the standard.  (negative sequence current withstanding values are specified by IEC or by other standards ).

:

All Dears

Thank you.

Here it is our power plant gens currents:

gen1:

R=1677

S=1596

T=1540

gen2:

R=1524

S=1362

T=1360

gen3:

R=1220

S=1394

T=1351

gen4=

R=1239

S=1188

T=1195

All bus voltages are 400v.

What do you think about this?

Hi Omid,

Thanks for the detail.

In a lighter vein, to me each of them seems to be suffering from some unbalance or the other ! :-) :-)

Just kidding...

Well, I recall having given you some tips in an earlier reply. Now I have only one additional suggestion.

Please check out your load side unbalances carefully before suspecting either of the generators - that's all.

Cheers !

-Sanjay

We cut the neutral cable and now R-S-T Currents are balance.

Our step up transformer connection is Y/delta.

What do you think?

Hi Omid,

Good to hear from you once again.

Further, nice to know that you have made a lot of progress with a single step.

If simply elimination of the neutral connection removes the unbalance, then my interpretation is relatively straightforward. It should also be easy for you to solve as well.

So here are some suggestions:

BTW, once you complete the balancing exercise, ground the neutral of exactly one of the parallel connected generators - not all !!

Hopefully that should take care of your troubles !!!

-Sanjay

Dear Sanjay

Thank you so much.

I have the same problem. My generator on each phase have a different current drawn.

V= 400V

R= 23

Y= 20

B= 0

Is this normal?

No Jesher,

You too seem to have the same problem as discussed above in my last reply to Omid - in fact, the problem is relatively serious in your case !

Your B-phase is totally unloaded - I guess your loads are mostly single phase, and someone progressively piled them between R and Y, perhaps simply because it did not pose any immediate problem ! But if you continue, the generator will eventually develop issues...

Please examine the connected loads carefully, and reconnect them appropriately for better balance between the three phases.

-Sanjay

Dear Sanjoy Roy,

Thanks for this clarification.

Much appreciated.

Jesher.