Core for Stator Winding

If the class F insulation is thicker than the older class B, you might have some trouble cramming the new windings in. Why that particular change; were the old windings running too hot, and if so, why? Other than that, I don't why the old cores would have a problem, unless some other issue has arisen. But I'm just thinking out loud, and would suggest contacting the turbine manufacturer.

actually winding thickness is same.The unit experienced several faults in winding and quite a few instance of flash over.So we are replacing winding.The manufacturer wants to replace the core.But we are insisting on replacement of the damaged core

Replacement of stator winding to upgrade insulation from Class B to Class F is as meaningless as considering to replace the core.

A good quality electrical grade varnish coat on the winding is what actually is required to improve the quality of the existing insulation.

If you have to improve the performance of the generator you actually need to consider improvement in cooling.

actually winding thickness is same.The unit experienced several faults in winding and quite a few instance of flash over.So we are replacing winding.The manufacturer wants to replace the core.But we are insisting on replacement of the damaged core

Hi,

the core of your machine may need replacement if you want to use higher currents.

The winding will have some current in and this current is generating the magnetic field in the core structure. The magnetic field and related magnetic flux closes on itself by first coming out of one pole of the stator, traversing the air-gap traversing the rotor, once more one air-gap and back to the corresponding stator pole.

With rewiring and upgrading from class B to F you will push up the temperature limit.

But your stator and rotor iron has its different limits: saturation of magnetisation and losses by eddy-currents and magnetic hysteresis.

To judge about saturation it would necessitate knowledge of the material and its magnetisation curve and where in your stator is the maximum flux density. And to resolve: any more flux density allowed or with the existing design too near to saturation so any more current will only generate more heat and only marginally more flux density.

To judge about eddy currents and hysteresis loss the existing situation has to be checked: how much heat generated at rated power and can the machine dissipate more?

Think about the magnetic circuit similar as a hydraulic or electric circulating flow.

But in contrary to hydraulics or electrics the "resistance" of the magnetic structure changes rapidly to its end of usefulness if approaching saturation-flux-density.

And in addition to typical loss in hydraulics: flow resistance or in electric: electric resistance there are in magnetics two power losses: a.: eddy currents (that would go down if a better (higher electrical resistance) material is used, and b.: hysteresis-loss that would go down with a material of lower hysteresis.

So in any generator or motor the stator is made from laminated sheet metal. To lower eddy current limits either a material of lower electrical conductivity or finer laminations are needed.

To lower hysteresis loss a better material is needed.

Try to get the actual data of your machine if you want to draw more power from it: material, regions of highest flux-density, hysteresis loss, eddy-current loss.

Use the old stator if you are content with the existing power or if at higher power the data are still within acceptable limits.

RHABE

Thank you for the advice.We will not be increasing the power out put.The stator conductor size is the same as before.Also exciter remains the same.After removing the stator bars we were able to clean the core ducts so the cooling will be more effective.

You will have to change insulation in the core slots. You have to insert fibre glass insulation, winding varnish should be suitable for class 'F' insulation.I hope this job is carried out by qualified winder.You are lucky that old winding has lasted for 40 years as windings have natural tendency to die out after 10-15 years depending on the usage.

Class 'F' insulation is recommended for higher ambient temp.and higher temp. rise due to application.If that is situation then you should change over to class 'F' insulation

If there is no damage to the iron/core/laminations and no change in operating parameters I don't understand why there would be a recommendation to replace it. Even if there is some minor damage to the iron you can spread and re insulate the laminations.

The usual question in this type of a rewind is whether to replace the copper in the windings.

Insulation Class is thermal classification of insulation

Class B is equal 130 degree Celsius

Class F is equal 155 degree Celsius

Both classes are not voltage classification

Before rewinding or coil re fitting, you should test the interlamination of core, you can test with inducing by "ring flux or loop" or by MCT (Motor core tester) or ELCID (Electromagnetic Core Imperfection Detector)

Short interlamination can resulting "hot-spot" in the core and can cause local over heating on the winding.

Please check the following settlement for core inspection and repair.

· Forum Thread: How to Repair a Partial Core Stator Generator

http://cr4.globalspec.com/thread/32530

www.sispowergeneration.blogspot.com

Rgds

Siswanto

It would have been better if the thickness of new insulation is reduced. Class F insulation can withstand higher voltage for the thickness of class B insulation. So if thickness of class F insulation is reduced then the copper size can be increased and either copper loss can be reduced or higher current can be passed for the same temperature rise.

Class F insulation coils are much harder than class B insulation coils. Therefore a ripple spring would be required to be inserted between the coils to contain bar bouncing forces. In absence of such a spring core gets hammered and hot spots are created after some time.

If the core is healthy, (you like to check by core loss test or by ELCID) then core is not required to be changed. You may also like to check core tightness, broken ventilating duct spacer etc. In case manufacturer is insisting on core repair, get full justification for the same and then ask then to increase the permissible loading (active load in case there is margin in turbine, boiler etc. otherwise reactive loading.)

You may like to get opinion of some consultant in the this field.

If your slot space is sufficient to accommodate the ripple, is better to modified the stator wedge using radial spring, its used to reduced the coil movement during in operation due to coil shrinkage.

When stator coil has movement in the slot, coil insulation will come fail due to friction between coil insulation and core.

Normally, for generator more than 150 MW, stator coil is provide by side conductive ripple.

The radial spring can directly installed on the top coil or using the filler to reduce slot space or to increasing the spring compression, as the ripple spring manufacture recommendation the effective ripple compression is 85 - 100 % compressed

Coil arrangement in the slot, please see the figure as below:

On Toshiba Generator250 MVA / 15.5 kV

Coil arrangement in the slot (Copyright ISOVOLTA)

Radial Springs Class F (Copyright ISOVOLTA)

Rgds

Siswanto

Email: sis_cahya@yahoo.com

www.sispowergeneration.blogspot.com

its good to improve the winding from B class to F,

Prior to rewind, pls check the quality of the stator core using ELCID, to analyze of core lamination.

Replace or no replace for the stator core is depend on the core lamination quality. when found interlamination short will produce hot spot on those areas and can cause winding failure due to local overheating, see explanation before for core test

can some one describe as to how the core can be tested if the equipment needed is not readily available i mean can simple equipment like potentiometer,cables can be used.Also is the core testing a destructive method/testin.I was told it is like hipot but i do not find any thing destructive in it

EL CID is the only non-destructive test for core. Other test like core flux (or core loss test as some call it) is destructive test in case a core fault exists at a location where there is no temperature detector or which is deep inside core iron. In this test a power source and a cable is required. The cable is wound around core yoke through the bore and excited to generate near rated flux which causes iron loss and hence some temperature rise. This temperature rise is measured with the help of temperature detector or thermo-vision camera. This test could be destructive in case there is some shorted core and loss at that location is enough to raise its temperature at very fast rate. Then higher temperature would result in burning of core insulation. It would result in higher uninsulated solid mass thereby increasing iron loss and hence higher temperature. This would further increase the rate of burning the core insulation. This process triggers chain reaction and results in heavy damage to the core. Therefore this test can be considered destructive.

thank you for information.we have the unit/generating operating for last thirty years.No problem of heating but are replacing the class b with class f.is it essential to replace the core.what is the frequency of core replacement in other coutries.I mean is core repalced every time the winding is replaced

No. It is not essential to replace core during rewinding. But it is essential to thoroughly check the core before starting the winding process. An unwound core gives much better approach to inside the slot for any damage, looseness, etc. Whole core is hardly replaced with new one. It is only re-assembled if looseness is on the higher side and is beyond repair.