How to Repair a Partial Core Stator Generator

By looking at the picture I would say that this generator is beyond repair. The cost of removing it and having it redone would be more than the purchase of a new one. The length of time to repair and return to service would be longer than the purchase of a replacement.

The problem with this type of fault is that it goes through the whole device and if you miss just one little thing your back to below start. Time and money wasted. Have a factory rep. visit and give an estimate new vs repair c/w with time factors.

Good Luck

Good guidance and excellent explanation.

It appears that you have been involved in this sort of repair procedure.

Lets hope it'll help Rita to weigh the situation and act accordingly.

Dear Mr. Siswanto,

It was great going through your post regarding "How to repair a partial core stator generator" threaded by Rita.

Well i am having same trouble but in my case a aluminium block(part of rotor) got dislodged and trapped between rotor and stator during machine's mechanical spinning at its rated speed(375rmp). Almost 2kg of aluminium were rubbed off and particles got deposited between stator laminations(2-3mm inside) and few core laminations were also dented. Almost 40mm height of stator throughout peripherially, core laminations are shorted to each other because of this deposition.

We megger test on the windings and found that it is intact. Now we have to remove these aluminium deposits, re-insulate damaged and aluminium deposited laminations and repair the dented core without overhauling the entire rotor. Currently we have removed one pole and started with cleaning and chipping of these depositions and repairing of these dents. But, trouble is, it is very difficult to remove these deposition and repair the dents without loosening the core. We dont want to overhaul the rotor or stator or even loosen the core because it will take lot of time. Our machine supplier are saying that EL CID test is not required for such damage because the machine is only 6 years old. But i am bit worried.

As it seems that you have experience regarding core repairing, can you suggest me how to go about with our work. Moreover, is it necessary to perform the EL CID test in our case? Any special insulation paint that can go inside the laminations by itself by just applying on the core tooth?

Our generators' info:

170MW-hydrogenr, stator dia:5000mm, air gap:29mm, 16 nos. salient poles, Core height: 3000mm, 252slots, wave winding, core packets:60no laminations, 0.5mm thick laminations.

Mr sonam_wangdi

Pls visit my blog: www.sispowergeneration.blogspot.com

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http://sispowergeneration.blogspot.com/p/stator-detail-inspection-for-high.html

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http://sispowergeneration.blogspot.com/p/rotor-thermal-sensitivity-phenomena-on.html

Welcome to CR4 Forum and thank you to contacting me

Its little bit difficult to removing the Aluminum when rubbing in the air gap, but it will very difficult to remove it from core surface when foreign material was melted.

Core damaged can occurring on the surface only and or inside of interlamination of core.

Pencil grinding can employ to remove the metal melting on the core surface but not inside or side of wall.

Fig1. Foreign material melt on the stator bore

Fig 2: close up foreign material melting on the core surface

Fig.3 after grinding

Fig.4 Stator core damaged due to rotor touching to stator core

Repair:

Fig, 1,2,3,. Stator core surface can be repaired with grinding to remove foreign material and separate the interlamination core with thin stainless plate and finishing with insulation coating.

Fig 4, Core damaged due to high friction between rotor to stator core, separation of interlamination core to repair of interlamination short.

When damaged of core at inside or side of wall is very difficult to remove without loosening core or re stacking.

Fig.5 Foreign material deposit on side wall

Fig.6 Your generator core

Fig 7. Rotor V Block

Repair:

Fig 5, repair core can be done with re stacking ,to remove the foreign material

Fig 6, repair can be done with combine grinding and separation of core lamination (see fig, 4), when separate the interlaminations, do with extra carefully since the stator winding still in place.

Inspection,

Core should be inspected by using core analysis, such as ELCID or Ring Flux, it mean to identify core short interlamination.

Visual inspection is not possible to identify the core short, ring flux can identify short interlamination with represent of "heat / temperature" but it can not to identify where is the problems location, ELCID can analysis the actual location and more accurate the threshold.

ELCID or Ring Flux should be repeated by several time, before repair and after.

Fig 8. ELCID Visualization.

Do spraying entire core with insulation painting, if you found broken or heavy dent on the teeth of core, apply RTV on this parts to avoid vibration and touching between interlamination during machine in operation.

Your quote: Our machine supplier are saying that EL CID test is not required for such damage because the machine is only 6 years old. But i am bit worried.

"Yes ELCID is normal to analysis of core when machine more than 10 years old (for normal operation / no accident was occurred)", your machine was touching to rotor with foreign material involving, and may some of those foreign materials was also melt and entry to the interlamination, and may also your interlamination was damaged due to high friction, such as dent / broken, etc.

Core inspection ( by ELCID or Ring Flux, depend on the site requirement and budgeting) should be done, since we can't detect the short interlamination by visual.

Remove rotor from stator bore to inspect the core in detail, you can save more time and much more efficient comparing to total time consumption when you are repairing improper way and finally you need more time to shutdown for repairing.

Good Luck

My Best Regards

Siswanto

Specialist Engineer

Power Generation

Emails: sis_cahya@yahoo.com

siscahyabhuwana@gmail.com

Blog : www.sispowergeneration.blpgspot.com

Dear Mr. Siswanto,
Thank you for your advice and suggestions. It was of great help for us. And thank you very much for offering your service. I will discuss with my superiors.
And you are right that the aluminum thing is our V block. What we think what happend was that the stud holding the V block got sheared and both broken stud and V block came between stator and rotor.

Currently we have been chipping out these aluminum deposits and separating the blocked air ducts. It is observed that these deposits have formed layers on the surface and got few millimeters inside the laminations. From 100% deposits we are able to remove only 80-90%. After each exercise we cleaned the core with white spirit (ethanol) and for your information, we have observed oil inside the laminations.
While checking the continuity for these laminations, we have noticed that all the core laminations are shorted to each other and to ground, even the healthy laminations. To confirm further we even tested the continuity between core laminations in other healthy generators and found the same result. Well, what i had in my mind was that all the laminations should be insulated from each other and even from ground, so, these findings were quite alarming. So, can you help me to understand how this is possible? If these laminations are shorted through some means then i feel it should not be a problem with some aluminum deposits at surface of core. Or is it really necessary to have all the laminations insulated from each other (100%)?, is there any tolerances?
After cleaning these core packets, we are planning to apply varnish over it and then carry out the test run. As we have only 4 nos of RTDs to monitor the whole Core tooth, do you suggest any idea on how to maximize our stator core temperature monitoring system? or is there any means to measure temporarily ?
As i know, our main concern is that there should not be any hotspot in the cores, so, is this hotspot instantaneous or gradual process because during test run we will be loading the machine in steps till110% load and monitor the temperature from above RTDS. For your information, we have also got RTDs for stator windings/slots. IF incase there should be any development of hotspots, will it be instantaneous? And i hope it won't melt the winding?

And still we haven't thought anything about the core analysis. Our generator expert still thinks that core analysis is not required. Moreover we don't have ELCID test or flux test equipments.
i will be Looking forward for your suggestions,
thank you

Dear Mr Sonam

1. While checking the continuity for these laminations, we have noticed that all the core laminations are shorted to each other and to ground, even the healthy laminations.

2. what i had in my mind was that all the laminations should be insulated from each other and even from ground

3. If these laminations are shorted through some means then i feel it should not be a problem with some aluminum deposits at surface of core

ð Its tested by megger or DC resistance ?

ð Core lamination has been laminated by micro film insulation coating (in micron thickness), and at core back-off to be shorted together (some of them using by welding) and clamping to body support, found problem shorted areas will resulting over fluxing.

This is SIEMENS Generator 16 poles, rotor was touching to the stator core, damaged core was found on the surface only, we test by Ring Flux, with 50% flux from the nominal total flux required was induced on this core, before repair at 1 minute, temperature at the core was reached 125 degrees Celsius, after repair, in 2 hours, temperature was come down to 45 degrees, with ambient temperature 32 degrees Celsius.

When found short interlamination on the coils area (slot) and tooth core, its will resulting high circulating current in the slot and or surface depend where is the short location.

4. As i know, our main concern is that there should not be any hotspot in the cores

Hot spot is caused by short inter lamination due to insulation fail on the magnetic flux areas (slot area and surface).

To identify the hot spot should using ELCID test instrument or ring flux, when using ring flux will be used, its should capable induced current in the core at least 60 – 70% from total number of rating flux required for stator. When using ELCID its need only 4% from the total flux required.

Visual inspection is not possible to identify the hot spot..!!!

When hot spot is not to solved, its will resulting over heating on the coil winding and insulation breakdown can occurred prematurely, finally replace of coil with new and repair the core should be planned..!

My recommendation.

Never operate this generator without any inspection for the hot spot with proper test and method.

- When using ELCID 100 mA is maximum threshold for the hot spot on the core

- When test by ring flux, if your stator is class F, maximum temperature on the core is 100 degrees Celsius, and different temperature at the adjacent core should not 10 degrees Celsius, and test must be done up to temperature reached equilibrium (may between 8 – 12 hrs) depend on the stator size and supply of current test.

Dear Mr. Siswanto,

these are some of the photos of our affacted core:

Rita check out this wed site is has lots of information on generators, good read.

http://www.gepower.com/prod_serv/products/tech_docs/en/downloads/ger4212.pdf

I don't think that repairs are possible as it will put the rotor in an out of balance situation.

Cheers Robert

It's a bleak explanation to her concern.

Dear mayt2u and Rita

Yes you right mayt, the best choice is replace the damaged core even partially.

The website as provide is to analysis of rotor or field, yes of course improper repair on the rotor winding will impact to the balance and vibration., we are also using this paper as reference when we are performing rewinding rotor GE 237.5 MVA / 3000 RPM

Based on our experiences the core (based on the photo) is still possible to repair, in my opinion its will take round 3 months including replace of damaged stator coil. The best choice is replace the damaged core lamination.

If you need to repair this core you must have good experiences and equipments, never do it if you do not have experience or not supported by the expert.

The core damage may caused by entry of foreign material inside when machine in operation, or loose of finger plate, because the old design of hydro generator, the finger plate is not welded.

Note:

Its just talking about our experiences,,,,,,

Rgds

Siswanto