Grounded Stator Winding Sychronous Generator
Grounded Stator Winding Sychronous Generator
(OP)
In a cruise ship, 5 identical synchronous generators are connected in parallel sharing the total load.
One generator failed to ground. Preliminary testing, with Megger and feeding AC to the fault shows one grounded spot on phase 2,
Generator Name Plate Data: 14000 kVA, 6600 V, 1224.7 Amps, 60 Hz, 514.3 rpm, ODP, PF 0.80
Stator Winding: 126 slots, 126 coils lap, 42 groups of 3 coil each, 7 circuits Wye
TPC 21, coil span 1-7, six terminal leads.
Since removal of this generator from the ship is very expensive and time consuming, the owner wants to clear the ground and work the generator with the coil or coils grounded cut out of the winding (capped).
Is this possible? Assuming a single coil is grounded what is the best way to clear that ground fault?
Should it develop dangerous internal circulating currents?
Have any one solved a similar problem? Many thanks in advance for your comments.
One generator failed to ground. Preliminary testing, with Megger and feeding AC to the fault shows one grounded spot on phase 2,
Generator Name Plate Data: 14000 kVA, 6600 V, 1224.7 Amps, 60 Hz, 514.3 rpm, ODP, PF 0.80
Stator Winding: 126 slots, 126 coils lap, 42 groups of 3 coil each, 7 circuits Wye
TPC 21, coil span 1-7, six terminal leads.
Since removal of this generator from the ship is very expensive and time consuming, the owner wants to clear the ground and work the generator with the coil or coils grounded cut out of the winding (capped).
Is this possible? Assuming a single coil is grounded what is the best way to clear that ground fault?
Should it develop dangerous internal circulating currents?
Have any one solved a similar problem? Many thanks in advance for your comments.





RE: Grounded Stator Winding Sychronous Generator
It is fairly common as a short-term repair for motors. In that case you take a look at expected current unbalance and consider whether derating is required. Also I think in some cases it may cause vibration.
For generators in parallel, you have the added consideration that unbalanced voltage can create circulating currents which may require further derating.
=====================================
Eng-tips forums: The best place on the web for engineering discussions.
RE: Grounded Stator Winding Sychronous Generator
I do have some literature on this topic. I'll send you an email.
=====================================
Eng-tips forums: The best place on the web for engineering discussions.
RE: Grounded Stator Winding Sychronous Generator
We are thinking of taking out the whole series of six coils on that circuit. Since this being a generator I am afraid the uneven voltage connected in parallel with the remaining six circuits will generate a large circulating current due to the low internal impedance of the winding.
Then we will think about the magnetic un-balance created when the circuit is removed. As you mentioned I have done this in motors but not in generators.
RE: Grounded Stator Winding Sychronous Generator
Like Electricpete sayds, the first to take account when you capped a Coil is the Current umbalance in the faulty phase, for it is necessary to cut one coil in the others two phases in order to maintain the balance, you need the winding draw to analize the right point to do this(120 electrical degrees aparted)then you will have a winding balanced.Another important think is tp cut in halves each capped coil in order to avoid undesired generated voltages and current circulations.
The derrating or not will be product of wire CMA analysis.With quick Calculations 6600 V/1.732= 3810 Volts per phase. for 1224.7 Amps and 7 Wyes we have seven paralell circuits per phase and each circuit will drain: 175 Amps (22.11 Ohms per path) for 6 Coils in each Paralell Circuit each coil will have 3.685 Ohms per coil With out one coil the path resistance will be : 18.425 and the current by this path : 210 Amps(20% More).
Regards
Petronila
RE: Grounded Stator Winding Sychronous Generator
What you are describing is the impedance of a load with seven parallel circuits, not the generator internal impedance. If the winding internal impedance of one branch should had the proposed 22.11 Ohms, the internal voltage drop when 175 amps are provided will be 3.85*175 = 3868 Volts! Almost 100% Voltage drop!
Since we have a winding with seven parallel circuits, to balance all phases, 21 coils need to be capped, one per circuit per phase. The risk is to saturate the magnetic circuit and/or overheat the field winding, since the voltage has to match the bus voltage for parallel operation with the other 4 generators.
RE: Grounded Stator Winding Sychronous Generator
To get the internal synchronous impedance of a generator at least the “No load saturation” and the “Short Circuit” curves are required.
RE: Grounded Stator Winding Sychronous Generator
I have bypassed coils in motors but not (so far) in generators exactly for the reasons you have stated.
Have you tried to contact EASA ?
RE: Grounded Stator Winding Sychronous Generator
RE: Grounded Stator Winding Sychronous Generator
To add some information; the winding has 6 rings (U1, V1, W1, U2, V2, W2); one per line and each ring has 7 evenly spaced connections to one end of two groups of three coils. The six lines are external for differential protection, three are power lines and three are to form the neutral.
The ground has been located in one group of phase W.
We have to develop a repair plan for the next port arrival of the ship and our technicians will go in the trip. The idea is to test the generator on line after it is modified by whatever agreed solution that is found. The parallel synchronization and load tests of the modified generator will be made by the time they reach another port. The implications of safety and money are obviously huge.
RE: Grounded Stator Winding Sychronous Generator
This is very dangerous. In other way to isolate (disconect) a part of the winding like in DC rotors, is not posible cause you won´t get the same voltage internal on the machine as you got in the other machines resulting in a excesive flow of reactive power...
This all i said is without knowing the electrical thing of the power plant... for me the best solution is to rewind the generator or try to isolate the winding on site, I had done it a lot of times.... no VPI but it work for a long time... as I said you I´d done a lot of time....
Maybe If you feed a transformer, is very dificult to get on primary a secondary ground, and it´s a little safe to work with one ground point if you got the star open to ground... in any time and the damage will be greater.
RE: Grounded Stator Winding Sychronous Generator
I can see where that gives you a balanced winding electrically: 7 circuit wye, now with 5 coils per leg. Each leg of a wye contains two pole-phase groups: one has two coils and the other has three coils.
Also since one half of the total groups have 2 coils/gp and the other half have 3 coils per group, you should be able to arrange the cut coils physically so that one coil is cut out of every other group around the whole machine (2/3/2/3/2/3 etc).
One interesting thing, it seems this machine has a very small coil pitch: 1-6. This is a 14-pole machine with 126 slots, so one full pole pitch would be 1-10, right? This machine I think has a coil pitch of 5/9 of full pitch {(6-1)/(10-1)}. That's the lowest I've ever heard of. Does that sound right to you?
It does seem you should be prepared for possible noise and vibration. Fractional slot machines are notoriously noisy and you have created something similar to a fractional slot machine.
Flux density as you mentioned should go up by a factor of 7/6, which can heat the core or cause exciting harmonics. Just to mention a remote possibility - maybe the system can tolerate operating at slightly reduced voltage or slightly increased frequency to reduce this overexcitation? I'm guessing that would be a tough evaluation requiring someone familiar with the ship's electrical system and electrical loads.
=====================================
Eng-tips forums: The best place on the web for engineering discussions.
RE: Grounded Stator Winding Sychronous Generator
Excitation varies as the load varies. Also the reactive power is dependant on excitation.
I suggest limiting excitation current to the maximum rated excitation current.
I would expect two issues.
1> The excitation should be adequate for lower levels of output power. Maximum excitation is only required at maximum output.
2> With a parallel machine if the excitation is insufficient the reactive power will drop. This will through some VARs over onto the other machines. This will probably not be a problem.
Power output. I would consider reducing the dead-rack adjustment on the governor. This setting may be more familiar by other names. It is the governor limit on the maximum fuel delivered. It is normally set up on a new machine with a load bank.
If I was working on a machine where with no information available for the governor, I would not make any changes.
If I could definitely identify the "dead-rack" adjustment, I would adjust it.
I would parallel the machine and advance the governor to pick up the amount of load that I want to set the adjustment for.
I would turn down the dead rack adjustment until the power output starts to fall.
I would increase the governor setting. (At this point, the dead-rack setting should be over riding the governaor setting.)
I would make a final adjustment of the dead-rack to the exact point that I want the power output level to be.
I would return the governor setting to the normal operating setting.
I would want to be assured that the Automatic Voltage Regulator will not damage itself or the field by trying to supply excess excitation. Other than that, I would let the other machines supply the reactive current if required.
respectfully
RE: Grounded Stator Winding Sychronous Generator
RE: Grounded Stator Winding Sychronous Generator
=====================================
Eng-tips forums: The best place on the web for engineering discussions.
RE: Grounded Stator Winding Sychronous Generator
respectfully
RE: Grounded Stator Winding Sychronous Generator
With 14 poles, I am not sure you can get 6 circuits.
aolalde
If, as pete says, you are successful in reconfiguring for 6 circuits, would appreciate if you can let me know how.
pete
I think you have my mail id. Would appreciate if you can pass it to aolalde. Only my new domain is gmail.com
RE: Grounded Stator Winding Sychronous Generator
RE: Grounded Stator Winding Sychronous Generator
May be aolalde will give the good news.
RE: Grounded Stator Winding Sychronous Generator
I know it is hard to understand how you can get 14 poles with six circuits, a winder shall know that parallel circuits allowed are only whole results of the ratio P/n. For P=14 only 14, 7 & 1 are satisfactory. That is true when you are constructing a whole winding. In fact this original winding complies with that rule.
However, now a perfect balanced stator winding is being modified. For this particular case having a synchronous salient pole generator helps to easily understand why 14 poles are still there provided by the rotor excitation.
Each individual coil is a voltage cell with a certain induced value and phase angle “Volts per coil” (a vector). When the six coils in one circuit connected in series add (as vectors), the Voltage line to neutral is achieved. As far as the flux distribution in the air gap is kept, each circuit will produce the line to neutral voltage and will help to feed current to the load. In theory if I should have a way to add or remove these cells to feed my load, I could use 7,6, 5, 4 etc.
Since nothing is for free, the load current will produce an armature reaction flux that will distort the original balanced distribution around the 14 poles, because some coils have load current an some do not have any. The more coils are put out of work the higher the magnetic unbalance, that will be worst if it is left on a localized zone of the winding. That is why we have decided to remove a circuit from the other two phases at 120 and 240 geometrical grades. I will let you know the results next week.
RE: Grounded Stator Winding Sychronous Generator
In the mean time, I will try and see how to connect the 6 ckts for 14 poles.
RE: Grounded Stator Winding Sychronous Generator
See, if this sounds right.
For 6 parallel circuits and with 6 coils in series to produce the rated voltage, the next lowest coil configuration is 108. For each phase, then the coil grouping will be
2 2 2
2 2 2
4 2
4 2
4 2
4 2
This means
1. 18 coils are to be cut out.
2. All series connections and jumpers have to be undone and reconnected as above.
If this is a global VPI winding, I am not sure how easy it will be to do the above.
If it is a resin rich and the ground fault is in the top coil, I would lift the coil and reinsulate it in-situ.
RE: Grounded Stator Winding Sychronous Generator
2 2 2
2 2 2
3 3
3 3
3 3
3 3
RE: Grounded Stator Winding Sychronous Generator
=====================================
Eng-tips forums: The best place on the web for engineering discussions.
RE: Grounded Stator Winding Sychronous Generator
You obviously know your stuff. The best way to effect this repair is to remove one parallel circuit from each phase. This will give an electrically balanced winding and output will be limited to 6/7 of the normal rating.
The magnetic circuit will be unbalanced, so you are right in removing the other two parallels at 120 and 240 mechanical degrees from the orginal fault.
The good coils do not need to be removed, just cut the lead and cap the leads. The failed coil will need to be removed or cut to ensure no circulating currents are allowed to flow. Remember it will be sharing a slot with another coil which could be at rated volts.
RE: Grounded Stator Winding Sychronous Generator
We did exactly as posted by motorspert.
The results were 100% satisfactory. After testing at 85% of original full load (the machine capacity was de-rated), we did not notice any difference on the modified generator as compared to the others working in parallel. We got almost the same temperature rise, vibration and excitation, all into conservative figures.
RE: Grounded Stator Winding Sychronous Generator
I am sure you must have been working through the holidays but then you have started the new year a happy man with a very happy client.
Congratulations. And, thanks for the feedback.
(I tried to give you another LPS but the site doesn't allow it apparently)