Conesville 6 Generator Stator Core Replacement using Bonded Core Packs Authors James A. Cook, Siemens Power Generation Mechanical Engineer James R. Michalec, American Electric Power Staff Engineer, IEEE Fellow Presented At CIGRE Study Committee A1 and EPFL Joint Colloquium On Large Electrical Machines, Lausanne, Switzerland, September 7, 2005 INTRODUCTION: Siemens Power Generation has developed the technology to replace generator stator cores at the power plant site using bonded core packs (commonly called “donuts”). Installation of the new core is done with the frame in the horizontal position, without lifting the frame off the foundation, instead of stacking individual laminations where the frame is typically lifted into the vertical position. A typical method for building a stator core is to stack laminations individually. The stator core is stacked in the vertical position. Power plant facilities normally do not have the capability to lift a generator stator, and special riggers have to be hired to lift the stator into the vertical position. At American Electric Power’s, 493 MVA, Conesville Unit 6, the stator core was replaced using bonded core packs for the first time at the power plant site. The outage was in April and May 2005; the total time required to replace the core iron was 16 days. On Conesville 5, a sister unit of Conesville 6, the entire stator core was replaced in 2003 by hand stacking individual laminations. The total time required to replace the core iron was 54 days. (Photo 1) This new method for stator core replacement has several benefits during the outage including: 1) shorter outage duration, 2) no need to disconnect the stator auxiliaries like iso-phase bus, 3) no need to lift the stator frame from the foundation, 4) no need to realign stator frame, 5) no cost associated with heavy lifting, 6) less internal labor costs, and 7) no safety concerns associated with lifting the stator frame.
MANUFACTURING BONDED CORE PACKS: The bonded core packs (donuts) for the new Conesville 6 stator core were made using a patented process at a Siemens PG facility in Erfurt, Germany. The innovative process and resulting consolidated donut enables assembly in the field. This process has been used for new stator cores up to 514 MVA for approximately 10 years, but until now has not been applied to replacement of existing cores in the field. In Erfurt, all laminations are punched, deburred, and coated with Rembrantin® interlaminar insulation. The laminations are stacked on specially designed press plates. The donut stack height is typically between 600 mm and 850 mm, but many different heights are possible. (Photo 2) The vent plates for radial cooling are also made at Erfurt, and have laminations spot-welded to both sides of the vent finger. All components of the core, including the vent plates for radial cooling, laminations with insulated layers, and crown compensation are built-in during the bonded core pack stacking process. Once the laminations have been stacked to the desired donut height, another press plate is assembled on top of the stack. Threaded studs are evenly spaced around the outer diameter and inner diameter of the press plates. Another fixture tightens the studs evenly and simultaneously using a hydraulic system. (Photo 3) The pressure applied to the stack of laminations is approximately the same as the axial clamping pressure on the completed stator core. Next the donut is put into an oven and heat is applied. After cooling, the press-plates are tightened again. Then, the donut is vacuum pressure impregnated (VPI). This process fills much of the voids between laminations and bonds all the laminations together to form a bonded core pack that can be handled as one piece. After impregnation, the resin is cured in an oven. When curing is complete and the donut has cooled down, the upper press-plate is removed. At this point the donut is referred to as a “manufacturing donut”. For donuts that are to be assembled in the field, the completed manufacturing donut is now separated into 4 or 5 separate segments called “assembly donuts”. The smaller donuts are necessary for field assembly with the stator frame in the horizontal position. The assembly donuts are packed into special shipping containers which are specifically designed to support the weight of the donut in the horizontal and vertical orientations. The donuts are packed in a sequence that allows the field personnel to remove the donuts from each shipping container in the order they are assembled into the stator core.
FRAME PREPARATION: After the generator comes offline, the frame is opened and the unit is disassembled according to standard practice. At this time the bearing brackets, air-gap baffle, blower, cooler, and rotor are removed. The new stator core iron, assembled using bonded core packs, reuses the existing stator core support structure in the frame. This includes spring plates which isolate core vibration from the frame, and circumferential bore-rings which provide radial clamping. Also, the building bolts are reused. Building bolts are long studs spanning the entire core length and structurally connect the stator core iron to the support structure. The building bolts additionally provide axial clamping and electrical grounding. These components are the framework upon which the new stator core is built. Before the existing stator core is disassembled, bracing is applied to maintain the position of these components. The stator winding and stator core iron are removed using typical methods. At Conesville 6, the disassembly of the winding and stator core lasted approximately 5 days. Before the new stator core is assembled, the frame is cleaned and inspected. The cleaning is done to remove all oil, dirt, and debris from the frame. In addition, the building bolts are lightly buffed to remove any rust or roughness. Photo 4 shows the empty frame during the cleaning process. The building bolt welds are inspected at this time. Any necessary frame repairs are completed before the stator core assembly begins.
STACKING THE NEW STATOR CORE: The stator core is assembled starting at the turbine end of the generator. First, the turbine end endplate is accurately positioned. Once positioned, the endplate is firmly secured in place to prevent movement during the stacking process. This establishes the start point for stacking the bonded core packs. Fingerplates are stacked between the endplate and the first donut during installation of the first donut. Before the donuts can be assembled into the machine, the shipping container with donuts must be lifted and rotated using a crane from the horizontal shipping position to vertical. The shipping containers are designed to support the load during the lift. After the donuts and shipping container are turned up, the packaging and extra bracing is removed to provide access to the donuts. Photo 5 shows the shipping container after turning up when the donuts are ready to be installed. A special lifting fixture was designed for picking up the individual donuts in the vertical orientation. The saddle of the lifting fixture supports the donut ID at the tooth tops and bottom of the slot. The donut is lifted out of the shipping container and onto a trolley for installation into the frame. The saddle of the lifting fixture connects to the trolley and provides support until the donut is in position. Photo 6 is a picture taken while the donut was moved from the shipping container to the trolley. The trolley was designed to carry the donuts down the frame and stack them into position. (Photo 7) The wheels of the trolley ride on building bolts at the bottom of the frame. Building bolt extensions allow the trolley to roll outside of the unit. The trolley has a special set of wheels to roll on top of the exciter bedplate when outside of the frame. The donut sits on a fine positioning table which can adjust the donut position and orientation in all translations and rotations. Once the donut is on the trolley, it is ready to be moved to the other end of the frame. The rewind technicians push the trolley and donut carefully down the length of the frame. The donut is designed to have sufficient clearance at the outer diameter to avoid damage due to contact with the frame or building bolts. The first donut is pushed firmly against the fingerplates, and the remaining donuts are pushed firmly against the donuts already in place. Next, the donut is aligned to the previous donuts in the horizontal direction. The donut is set down onto solid fillers on top of the two building bolts at approximately the 3:00 and 9:00 positions. (Photo 8) Once the donut is in position, the trolley is backed away from the donut and is situated outside the frame to receive the next donut. At this point, the installed donut is supported only by the fillers at these two locations until the remaining pieces of the core to frame attachment are assembled. Five assembly donuts are set onto the fillers before they are permanently attached to the frame. The five donuts were all manufactured together and make up one manufacturing donut. Essentially, the donuts are reassembled into the frame in the same sequence and with the same donuts they were made. The core to frame attachment (patent pending) securely connects the stator core iron (donuts) to the building bolts of the stator frame. The three piece assembly has a filler of constant thickness, and a pair of opposing wedges. All of the parts span the length of 5 donuts. The wedges are adjusted to tightly fill the space between the building bolts and the donuts. There are two of these assemblies at each building bolt location for a total of 54 assemblies all around. Photos 8 and 9 show the installed core to frame attachments. After all of the core to frame attachments are in place, that group of bonded core packs is completely installed. The remaining donuts are installed in a similar manner until all of the main-body iron is assembled. For Conesville 6, all of the donuts were installed over a period of 7 days. After this is complete the exciter end fingerplates and endplates are installed. Laminated flux shunts and core support plates are stacked onto both ends. The core thru-bolts are installed, and hardware for the thrubolts and building bolts is assembled.
TIMELINE: April 20 – AEP finishes disassembly and turns over unit to Siemens PG April 22 – Finished stripping winding and start removing original core iron April 25 – Finished removing core iron April 26 – Set turbine end endplate and fingerplates. Installed step-iron donut. April 27 – Began installation of main-body donuts 1-5 April 28 – Finished installation of main-body donuts 1-5 April 29 – Finished installation of main-body donuts 6-10 April 30 – Finished installation of main-body donuts 11-15 May 01 – Finished installation of main-body donuts 16-20 and 21-25 May 02 – Finished installation of main-body donuts 26-30 May 03 – Finished installation of main-body donuts 31-35 and exciter end step-iron donut. Removed donut tooling, installed fingerplates, exciter end endplate, and thru-bolts. May 04 – Installed exciter end end-shield and core support plates. May 05 – Finished stacking turbine end end-shield and core support plates. On average, 5 donuts were installed with core to frame attachment wedges each day (2 – 12hr shifts)
CONCLUSION: In conclusion, Siemens Power Generation has developed a process that allows the use of bonded core pack technology to replace stator cores in the field without lifting the generator frame into the vertical position. The bonded core pack is a VPI process, practically eliminating all voids. Instead of the 110,000 individual lamination stacking interfaces; Conesville 6 has only 35 bonded core pack stacking interfaces. This technique is significantly quicker than conventional methods and does not involve expensive heavy lifts. The core replacement at American Electric Power’s 493 MVA, Conesville Unit 6, was the first application of the use of bonded core packs to replace the generator core iron at the power plant site. The new core is made from the latest materials and processes, which are designed to reduce core relaxation and increase the life cycle of the generator.
PICTURES:
Photo 1 Conesville 5 Stator Vertical Lift
Photo 3 Donut with Hydraulic Press
Photo 5 Donuts in Shipping Container and One Passing By
Photo 2 Stacked Donut with Press Plates
Photo 4 Cleaning the Stator Frame (Note the Building Bolts – 27 in this case)
Photo 8 Solid filler at 3:00 & 9:00 positions
Photo 6 Lifting Assembly Donut onto Trolley
Photo 9 Three Piece Wedge at Building Bolts
Photo 7 Donut loaded on Trolley
