Stator Bar of 800 MW Turbogenerator

MANUFACTURING PROCESS OF BARS

1. Insulation Check

For the manufacturing of stator bars insulated copper conductors are brought from Bangalore, it’s of two type’s i.e. solid copper conductor & hollow copper conductors which are used in water cooled stator layer glass insulation.

·         Insulating materials and the conductors are ensured to be certified.

·         If required the insulation is checked by the respective agencies.

 2. Conductor cutting & End Cleaning

This process is done by automatic CNC machine. In this process the pre-insulated copper conductor is cut into number of pieces of required length (length given in drawing as per design) insulation is removed from both ends of the copper conductor. 800MW bar contains 60 strands 50 solid Cu strands and 10 hollow stainless steel strands.

3. Transposition

Transposition means changing/shifting of position of each conductor in active core (slot) part. After cutting the required number of conductors, the conductors are arranged on the 12 comb in staggered manner and then bends are given to the conductors with the help of bending die at required distance. Then the conductors are taken out from the comb and die and placed with their ends in a line and transposition is carried out. This process is repeated for making another half of the bar which would be mirror image of the first half. The two halves of the bar are overlapped over each other and a spacer is placed between the two halves.

Transposition is done in two ways:

·         Half Pitch Transposition: - In this transposition, the first conductor at one end of the bar becomes the last conductor at the other end of the bar.

·         Full Pitch Transposition: - In this method of transposition, the first conductor at one end of the bar becomes the last conductor in the mid of the bar and then again becomes the first conductor at the other end of the bar.

Need of Transpostion

1.      To reduce eddy current losses.

2.      Equalize the voltage generator.

3.      To minimize skin effect of ac current, small cross section of conductor is used.

4. Crossover insulation

The pre insulation of the copper conductor may get damaged due to mechanical bending in die during transposition, hence the insulating spacers are provided at the crossover portion of the conductors. A filler material (insulating putty of molding micanite) is provided along the height of the bar to maintain the rectangular shape and to cover the difference of level of conductors. To eliminate inter turn short at bends during edge wise bending and leveling of bars in slots portion for proper stacking.

5. Stack Pressing

This process is basically done to consolidate the solid as well as the hollow conductors into a single bar, the insulating material provide at crossover positions which has gluing properties melts and helps in the proper consolidation of the bar. In this process the bar is pressed both vertically and horizontally in the Pressing Machine. The pressing machines are also provided with the heaters Each bar is loaded on each tier of heating plate and it is then provided with an initial pressure of 35±5 kg per cm2 and an initial temperature of 100-110° C. then the temperature is allowed to raise to the maximum of 160°C, this temperature is kept for at least one hour with a final pressure of 80±10 kg per cm2 vertically and 70±10 kg per cm2 horizontally. After pressing the bar at specified pressure and maximum temperature for one hour, the bars are then cooled placing them on cooling plates through which water is allowed to follow below 50°C. The cooled bars are checked for their heights and widths at pressed portion of the bar. This complete process performed above is for one half of the bar. The same process is then repeated for the other half of the bar similarly and then checked for bar’s height and width at pressed portions as done before. Then the ends of both the bars are cleaned with rectified spirit and sand paper for the inter strand test.

6. Inter Strand Short test

The consolidation bar stack is tested for the short between any two conductors in the bar, if found then it has to be rectified. This is done to ensure that no local current is flowing due to short circuit between conductors. (300V A/C supply). For this test all the bare conductors at both the ends are separated from each other so that they do not short circuit. Then a live wire is connected to a conductor and received from it consecutive conductor to light a lamp. Hence if the lamp lights up it shows short circuit between the two conductors due to improper insulation between them. . It shows insulation failure between the conductors, these conductors are then replaced and bar is followed through all the previous processes. Similarly all the conductors are checked for any short circuit. After passing both the half bars from I.S.T. the bars are combined together and pressed into a single bar. A sheet of mica glass insulation is provided between the two halves as the insulation also has some gluing property to consolidate the two halves into a single bar. All the steps of pressing are again followed for the complete bar and then it is allowed to cool.

7. Forming

In heavy generators each stator coil is very large in size and hence its manufacturing is difficult as it will be very difficult to handle and process the complete coil. So each coil is manufactured in two halves i.e. Upper half and Lower half. The bars which are processed till this process are straight and hence they are required to be formed into the upper half or the lower half. Hence this process of forming is very important as in this process the bars are bending from both the ends of the bar i.e. at exciter side and turbine side at specific angles and shape as per design. This process is done manually. First the former is set according to the design making different angles at different positions. Each bar of a variant is bent or formed at same angle. Once the former is set, the bar is mounted on it and formed manually by melleting it from both the ends. After forming the formed portion of the bars are assumed to get loose creating air gaps between the conductors due to such a heavy melleting. Thus the formed portion of the bar are clamped at different places and provided with heaters to reconsolidate the formed portion. After few hours the bar is then allowed to cool at room temperature.

8. End Conductor Cutting

Due to heavy malting of the bar for forming it from both the side i.e. exciter side and turbine side, the conductors of the bar are produced at the ends of the bar. Therefore to bring all the conductors of the bar in a plane End Conductor Cutting is required to be done. Both the solid and hollow conductors are cut and from both the sides of the bar. Also this process of cutting of the hollow conductors helps in opening of vents of hollow conductors from both the ends of the bar.

9. Pickling Process

After the forming and cutting process of the bar and due to the transportation of the bar, there accumulate very minute particles of dust on the ends of the bar. These particles may cause hindrances at the time of brazing and may create some air gaps. So, in order to remove these unwanted particles, basically the oil particles pickling process is performed. The basic solution used for this process is called pickling solution.

The constituents of the pickling solution are:

·         Sulphuric acid (H2SO4)-10%

·         Phosphoric acid (H3PO4)-5%

·          Hydrogen peroxide (H2O2)-5%

·         Water (H2O)

There are many other solutions used in this process, these are:

·         Ammonia

·         Thinner

·         Water

·         Rectified spirit

·         Nitrogen

In this process the following steps are involved:

·         The end of the bar is dipped in thinner to remove the dust or oil particles.

·         The end is then dried with the help of N2 gas.

·         Then the bar end is dipped in pickling solution for 5 to 10 minutes.

·         This is then followed with dipping in water.

·         As there is acid in the pickling solution, so accordingly to neutralize the acidic effect of the acid it has to be treated with some base, so the bar is then dipped in Ammonia (10%).

·         It is then followed with dipping in water.

·         As water takes a lot of time to evaporate, it may let the tiny chemical particles on the bar to react with it and leave spots on the bar, this may weaken the insulation at those particular points and thus damage it, further it may lead to poor brazing. So in order to avoid such a damage the bar is dipped in rectified spirit to make the water contents evaporate as soon as possible.

·         Then the bar is dried with N2 gas.

·         Finally it is wrapped with cotton cloth to avoid any manual touch with may deposit any dust particle on it until it is ready for brazing. The same process of pickling then performed on the other end of the bar.

10. Contact Sleeve Mounting

After the pickling process is complete, contact sleeve and water box bottom parts are mounted on both the ends of the bar. Contact sleeve, as clear from the name itself, it is a rectangular sleeve which is used to make contact between upper and lower bars when laid out in the generator to form a complete coil. All the solid conductors are cut from the ends of the bar after the contact sleeve except the outer solid conductors. None of the conductor is cut before the contact sleeve so that to make proper contact between the upper and the lower bars. The water box is mounted in two halves; first the water box bottom part is mounted after the contact sleeve keeping some distance between them. Both the ends of the bar are then again pickled along with contact sleeve and water box bottom part following the same procedure of pickling.

11. Brazing of coil lugs

For water cooled generator bars, the electrical connection contact and water box for inlet and outlet of water are brazed. The contact sleeves, which are used to make contact between the upper bar and the lower bar to form a complete coil and the water box bottom part, both together are called Coil Lug, so the brazing of these two parts at the end of a bar is called Coil Lug Brazing. Brazing is basically the process of fixation of both the contact sleeve and water box bottom part or we can say both are properly consolidated using brazing material at high temperatures. Consolidation is properly done so that no air gap is left which would adversely affect the conducting properties of the contact sleeve and the bar.

The two types of brazing processes are:

·         Thermal brazing (using LPG)

·         Induction brazing :

 As induction brazing is more beneficial than thermal brazing as it does not requires any inflammable gas for heating. So now a day’s Induction brazing process is used. For brazing as we use very high temperature so that is possible due to carbon strips fixed on the brazer. Carbon has very high resistance and when the current is allowed to pass through it due to induction the material to be brazed i.e. Contact Sleeve gets super heated. Induction heating is hence faster and even more efficient than thermal heating.

When the contact sleeve becomes red hot at 750ºC, the brazing material is then pushed to fill the air gaps and to consolidate the contact sleeve on the bar. It is then allowed to cool.

The types of brazing materials used are:

·         LAG 15P ( Alloy Silver 15 Phosphorous)

·         LAG 40Cd ( Alloy Silver 40 Cadmium)

Same process of brazing is then repeated for the water box bottom part. After the brazing of water box bottom part the bar is then allowed to cool at room temperature.

·         We also use teal wielding in 800 MW stator bar.

12. End Conductor Cutting

As is clear from the name itself this is the process of cutting of extra conductors i.e. the hollow conductors left after the water box bottom part are cut from both the ends of the bar. These conductors are required to be cut so that water box top part with a nozzle is to be mounted on the water box bottom part. The vents of the hollow conductors are visible from the ends of the water box bottom part. The openings of the vents of the hollow conductors are then checked to be uniform. After cutting of extra conductors from both the ends of the bar, pickling process is done. Same pickling process as mentioned before is then done at both the ends of the bar for removing the dust particles, oil particles and other unwanted impurities which we assume to be there after the brazing process of the contact sleeve and water box bottom part. After pickling the bar ends are then wrapped in cotton cloth until the water test is performed.

13. Water Flow Test

Water flow test is performed so that there not any blockage in the vents and also to check the flow of water. Flow should be uniform i.e. all the vents should have same water flow. In this test water is supplied from one side of the bar and is collected from the other side of the bar in a tank and is supplied back to the first end. Flow of the water is checked by measuring the distance of the flow of water. If there is any kind of blockage inside the hollow conductors of the bar then it is indicated by flow of water i.e. flow of water will be not uniform and the bar is assumed to be chocked. Then cocked conductors are changed or repaired and the bar is reformed and water test is again performed to check the flow of water.

14. Brazing Of  Water Box Top Part

Brazing of water box top part is done with water box bottom part in brazing section of Block-4.For this it should be kept in mind that water box bottom part should have same material as that of water box top part. The difference in previous brazing and this brazing is that, now flux is also introduced with brazing material and also flux is mixed with copper chips. Copper chips are added to make the electrical contact between the top and bottom part without the copper chips this will be not possible. In this brazing material is formed in the shape as that of water box side where it has to be brazed. Brazing material is formed in square shape for top and bottom part and in ring shape for the brazing of nozzle for water flow. Then both the parts are heated with induction brazer as done earlier. It is heated until they become red hot then the brazing material melts due to its low melting temperature then the melting temperature of the water box top and bottom part. Brazing material is also introduced at the boundaries until it is completely consolidated that is the entire air gap is completely filled. After the brazing of water box top part the bar is then allowed to cool at room temperature.

15. Nitrogen Leak Test

As the name suggests Nitrogen gas is the main constituent of the test. This test is similar to puncture test that is inflated tube is dipped in water for any leakage, the air bubbles gives us the exact position of the puncture. Similarly on the same steps this test is performed. Basically this test is performed to check any leakage in bar i.e. to check the brazing. In this test nitrogen at a pressure of 10Kg/Cm2 is forced from one end of the bar and other end of the bar is blocked then the pressure of 10Kg/Cm2 is maintained and both the ends of the bar are dipped in water. The bubbles if there any tells us that brazing is weak or not done properly. So if bubbles appear then brazing is done again and same test is performed for the proper brazing of water box top and bottom part. Then both the ends of the bar wrapped in cotton cloth so that it is not disturbed i.e. to 18 avoid any manual touch which may deposit any dust particle on it until it is ready for brazing.

17. Internal Pickling

After various test performed on the bar the pickling process i.e. removal of any dust particles lubricants etc is done only on outer surface of the bar (ends). But from inside it is assumed that there must be some impurities, foreign material must be present. For this purpose inside pickling is done.

As mentioned earlier same process is repeated but from inside of the hollow conductors this time.

In this process the following steps are involved:

·         The thinner is passed through the bar to remove the dust or oil particles.

·         The dried with the help of N2 gas.

·         Then the pickling solution is passed for 5 to 10 minutes.

·         Then water is flowed in the bar.

·         As there is acid in the pickling solution, so accordingly to neutralize the acidic effect of the acid it has to be treated with some base, so the bar is filled with Ammonia (10%).

·         It is then followed flowing of water again.

·         As water takes a lot of time to evaporate, it may let the tiny chemical particles in the bar to react with it and leave spots in the bar, this may weaken the insulation at those particular points and thus damage it, further it may lead to poor brazing. So in order to avoid such a damage the rectified spirit is flowed in the bar to make the water contents evaporate as soon as possible.

·         The bar is then dried with N2 gas.

·         Finally it is wrapped with cotton cloth to avoid any manual touch which may deposit any dust particle on it until it is ready for brazing.

18. Baroscopic Test

This test is performed to check the impurities or foreign material inside the bar. To look inside with naked eye is not possible so we use baroscopic for this purpose. So after internal pickling we use a baroscopic to look inside the bar. Baroscopic is having a probe having a LED (light emitting diode). LED is used to provide light inside the bar. The probe of the baroscopic is inserted from one end of the bar and seen from a magnifying lens. The probe gives a light which is reflected by the sides of the bar (inside) and with the help of magnifying lens we get a clear image that what is there in the bar. If impurities are present then these are removed then only after the bar is send to next operation.

19. Water Test

Here again water test is performed. In earlier water test water flow was checked without the water box top part. But now the top part is brazed to the bottom part and to check its flow this test is performed. Also to check the proper flow from the nozzle this test is performed. In this test water is forced or pushed from one end of the bar and flow of water is checked from other end of the bar. This test tells us the presence of any choke vents of water box top part. If any choke vent is present then it is repaired or whole of the water box top part is changed with new one

20. Thermal Shock Test

Thermal shock test is performed to check whether the bar can withstand extreme conditions i.e. when the generator is working or running at about 3000 Rpm. This is achieved by thermal shock test. Thermal shock test consist of series of hot water & cold water cycles alternatively. So firstly hot water at temperature of about 90ºC is passed through the bar and then cold water at about room temperature 27ºC approx. is flowed inside the bar alternatively. This process is repeated again and again. This process is repeated for about 26 times i.e. it has 26 cycles of hot and cold water. So this test tells us the strength of the bar so that it can withstand the working conditions of the generator. Temperature of the bar is taken in account according to the working temperature of the generator. The cycles of hot (800C) and cold (300C) water are flown through the bar to ensure the thermal expansion and contraction of the joints.

21. Helium Leakage Test

Helium test is the most important test of all, because helium test is performed to check any minute leakage within the bar and at the brazed portion. Any minute leakage which couldn’t be checked by water test can easily be observed by helium test because helium is one of the lightest gases. So this is also the customers check point, before performing this test customer is by called and they see themselves the test being performed and if They find the test all right then only the bar is send to next process. In helium test, whole of the bar is wrapped in the polythene excluding the end points. The helium gas at pressure of 11Kg/Cm2 is passed through the bar and a probe connected to the gauge is inserted inside the polythene at different places. The gauge will show deflection if there is any helium atom present. Gauge will show reading even if 1 helium atom in 100000 atoms is present. This test checks the strength of brazing and insulation.

22. Reforming

After all the previous processes which had undergone on the bar it is assumed that the shape of the bar is deformed from its original shape due to handling of the bar from one place to another for different processes. So to keep the shape of the bar as per design, the bar is checked if it is found distorted then it is reformed in the previous former machine by placing the bar on the former and malleting.

23. Insulation

The bar is insulated with the given number of  layers to build the wall thickness of insulation subjected to the generating voltage of the machine. Insulation is basically done to prevent any kind of short circuit between the bar and the stator core when the bar is assembled in the stator of the machine. The stator bars are insulated with Micalastic (trade name) insulation. High quality mica, selected epoxy resins and a matching vacuum impregnation process are the characteristic features of the micalastic insulation for large turbo generators.

The bar is provided with 14 layers of insulation. Out of these fourteen layers, 12 layers are provided by using a machine called CNC TAPPING MACHINE manufactured by a company named MICAMATION. The rest of the two layers are provided manually.

The insulating material varies as according to the rating of the machines as follows:

·         For 800 MW : Mica Splitting Tape

·         For THRI : Mica Glass

During machine taping in the slot portion the transition from machine to hand taping is adjusted at the start of the bend for a tape width for every layer from bar center. After about half the number of layers, the taping is to be started further at the bend. The mica tape consists of a thin high strength baking material to which the mica is bounded by synthetic resin. The number of layers i.e. the thickness of insulation is determined by the voltage of the machine.

24. Impregnation & Baking

A.      Thermo reactive System: In case of rich resin insulation the bar is pressed in closed box in heated condition and baked under pressure and temperature as per requirement for a given period.

B.      Micalastic System : In case of poor resin system the insulated bars are heated under vacuum and the impregnated (dipped) in heated resin so that all the air gaps are filled, layer by layer, with resin. Then extra resin is drained out and bars are heated and baked under pressed condition in closed box fixture.

C.      VPI Micalastic System: - The bars already lay in closed fixture and full fixture is impregnated (dipped) in resin and then fixture with box is baked under given temperature for given duration.

D.     VIP Micalastic System: - The individual (Separate) bar is heated in vacuum and impregnated in resin. Then bar is taken out and pressed in closed box fixture and then baked at given temperature for given duration.

25. Micalastic Insulation and Impregnation

For insulation with micalastic, the conductor strands are arranged together to form a compact assembly and set to the required shape. This assembly is then baked with epoxy resin to give it mechanical strength required for further processing. The bars are then dried under vacuum and impregnated with synthetic resin, which by reason of its low viscosity penetrates the insulation thoroughly and eliminates all voids. After impregnation under vacuum, the bars are subjected to pressure, with nitrogen being used as pressurizing medium. The impregnated bars with direct conductor cooling are then brought to the required dimensions in moulds and cured in an oven at a high temperature. This complete process involves pressing of the bars, drying by heating, impregnation in resin and finally curing. These processes are discussed briefly below:

·         Pressing: The bars are pressed to bring them back to the original dimensions.

·         Drying: The stator windings are to be dried under vacuum 0.1 m bar at (60±5) °C for 15 hours, minimum. The drying temperature is to be increased to (65±2) °C if the initial viscosity of the impregnating resin mixture is high. The temperature distribution should be as uniform as possible. The drying under vacuum can be stopped if the pressure rises, 10 minutes after closing of vacuum valve is less than 0.06 m bar.

·         Impregnation: The impregnation resin mixture is to be heated in the working tank, to (60±3) °C or in case of higher initial viscosity. At a temperature of 50ºC, the impregnating resin mixture is to be degassed with 1-5 m bar vacuum. Subsequently the stator windings are to be dipped continuously in resin hardener mix such that the highest locations of the windings are at least 100 mm below the resin level. After 10 minutes of resin stabilization, pressure is increased by application of nitrogen. Pressure is to be gradually increased in uniform stages within 80 minutes to 4 bars and to be maintained for 120 minutes in the impregnation tank. The impregnation of the stator winding is to be monitored continuously. Further it is to be decided whether to increase the pressure or to stop the impregnation process, however the total period of nitrogen pressure cycle shall in any case not exceed 4 hours. The impregnation tank during shut down is to be closed and kept either filled with nitrogen (1.1bar) or low vacuum.

·         Curing: To prevent heating in the overhang portion, the curing of the impregnated stator winding is to be done with a maximum 160°C hot air. The curing period is extended for such a long time till the measurement positions in the core indicate (140±5) ºC for minimum 8 hours.

26. Finishing

The baked and dimensionally correct bars are sanded - off to smoothen the edges and the surface is calibrated, if required, for the dimension.

27. Conducting varnish coating:

1.      OCP (Outer Corona Protection) Coating:- The black semi-conducting varnish coating is applied on the bar surface on the core length.

2.      ECP (End Corona Protection) Coating:- The grey semi-conducting varnish is applied at the bend outside core end of bars in gradient to prevent from discharge and minimize the end corona.

Resin System:-

a)      Rich resin or Thermo reactive insulation system:- In this type of insulation system the bond content in resin is 35-37%. The raw materials are ready to use and require preservation and working on temperature 20-250C. Its shelf life is one year when kept at temperature 200C which could be increased when kept at temperature of 50C.

b)      Poor resin or Micalastic insulation system:- In this type of insulation the bond content in the resin is 5-7% and insulating material is prepared with accelerator treatment. The temperature control need not required. The insulating material is applied on job and then the same is impregnated (fully dipped) in the resin.

Testing:-

When the bar is dried it is wrapped with aluminum sheet to make outer surface of the bar conducting so that tan δ and H.V. (High voltage test) can be performed. After these tests the coating of red gel is applied at both the ends of the bar.

 28 .TanΔ Test

This Test is performed to find the capacitance of the bar because bar will act as capacitor when it is laid in the stator of the generator. In this test Schearing Bridge which works on the principle of wheat-stone bridge is used to find the unknown capacitance of the bar. For good efficiency, capacitance of the bar should be high but we never approach pure capacitance.

In this test also same bar which is wrapped with aluminum is used to make the bar conductive throughout the outer surface of the bar. The two conducting material i.e. the aluminum sheet and the conductors of the bar act as two plates of the capacitor and the insulation on the bar act as dielectric medium for the capacitor. The capacitance of the bar is found and of angle of deviation due to impurity in the insulation is obtained from the formula: C4 * R4 * 10-4 = tan δ For different ratings of the bar the acceptable value of tan δ is fixed according to their insulation requirement as per design considerations.

29. H.V. (High Voltage) Testing

This test is also known as insulation test because this test is performed to check the insulation of the bar. In this test the bar which is already wrapped with aluminum is used. High voltage is applied to the bar using auto-transformer and it is increased in steps according to the working voltage of the generator. If the insulation is weak the bar will puncture at the place of weaker insulation. For 800 MW the working voltage of the bar is 82.5 KV. If any of the bar fails this test i.e. bar is punctured at any point then the bar is sent back for re-insulation and all the processes are repeated again.

 30. Dispatched For Winding

The bars preserved with polythene sleeve to protect from dust, oil , dirt , rain etc. Then the bars are send to Block-1 for winding of turbo and hydro g