Design of a New Generator

1. INTRODUCTION : Gas Turbine-Generator system has proximity to zero level due to absence of condenser. Therefore, Generator should have Terminal Bushings at the top of the Stator Frame. This avoids digging of tunnel / trench for routing Bus Duct and also avoids accumulation of gases from safety point of view. It was decided to develop a module THRI 108/44 for application with Gas Turbine. 2. DESIGN CHALLENGES : Since, bar type design of THRI 108/44 module for GTG application is not available. Hence the same had to be designed afresh taking the following major design challenges into consideration: . 1. Shifting Terminal bushings for tapping power from bottom to top. 2. 2. Location, number and size of coolers and cooler ducts in stator Frame to accommodate connections between Bus Bar and Terminal Bushings. 2. 3. Proven ventilation scheme inside stator frame for cooling of stator core, windings and its overhangs. 2. 4. Provision of static excitation system by providing slip ring shaft. 2. 5. Position of Barring Gear on Exciter End (non drive end) requiring Slip Ring shaft rotor having a matching coupling with generator rotor on one side and barring gear on other side. 2. 6. Provision of routing of piping emanating from bottom of stator and end shield in the foundation. 2. 7. Routing of connections between bus bar and terminal bushings maintaining required electrical connections 2. 8. Use of existing components as far as possible for inventory and variety reduction. 2. 9. Mechanical and Electrical calculations for soundness of design. 3. ACTION PLAN : 3. 1. Identification of assemblies for fresh designing – 34 numbers design groups out of 125 groups were identified to be designed afresh. (Annexure-1) 3. 2. Carrying out Exhaustive Electromagnetic, Mechanical, Ventilation & Heat Transfer calculations. . 3. Development of detailed design documents, scrutiny by technology, incorporating the changes suggested and release of documents (drawings and CBOM). 3. 4. Verification of design by Internal design groups – Internal design groups of experts were formed and Changes suggested by different groups were incorporated in the documents. 3. 5. Strengt h of stator frame under various load conditions, its natural frequency and rotor dynamics to be carried out by Corporate R & D to validate the design. 4. INNOVATIVE SOLUTIONS AND ITS DETAILS : 4. 1. Electromagnetic calculations were carried out and design data sheet issued for preparation of design documents. 4. 2. Since the Terminal Bushings are to be mounted at the top, there was no space available to accommodate 4 Nos. Hydrogen cooler ducts (400 x 570 mm), as provided in conventional THRI design (ref. Fig. 1). Therefore, it was decided to use two nos Twin-coolers (ref Fig. 2&3). This concept has 2 nos. twin coolers placed in two separate cooler ducts. Operating conditions of one cooler out of operation apply here also as in case of conventional 4 Nos. separate coolers. 4. 3. Changes in size of cooler and cooler ducts (550 x 650 mm) and duct location resulted in increase of width of stator body to 4200 mm from 4000 mm and height 4550 mm. Accordingly, stator body was redesigned completely maintaining routings of ribs, plates etc to follow the ventilation scheme of existing machine. Full length Foundation Support has been provided on stator in line with conventional GTGs. 4. 4. Due to the above mentioned increased dimensions of stator body, the generator is not suitable for rail transport (limit up to 4040 mm). Thus the generator stator shall have to be transported by road only. A dragging fixture has been designed and shall be welded to the bottom of stator frame to place it directly on the road trailer without requiring any additional fixture. It will also help placing the stator at any location without any support or can be dragged at power station for erection, if required. 4. 5. It was proposed to use common Core Assembly for GTG as well as STG for standardization and variety reduction. STG design required a power output of 261 MW. To enhance the MCR rating it was decided to optimize the ventilation flow paths in the stator core. Number of ventilation ducts has been increased from 85 to 98 without any increase in the total core length. Width of the ventilation ducts was earlier a combination of 5 mm & 10 mm, which has now been changed to 5 mm, 8 mm & 10 mm (refer Fig. 4). In order to optimize electromagnetic performance of the machine, net iron length of core is maintained same. Modified ventilation circuit vis-a-vis existing one is depicted in Figs. 5 & 6. 4. 6. Thickness of core ETS segments (Electro Technical Steel segments) has been increased from 0. 5 mm to 0. 65 mm without any loss in quality. This will enhance the rigidity of core end zone packets. It will also increase the productivity and reduce core assembly time in shop by about 30%. 4. 3. Shifting of Terminal Bushings to the top of Stator Frame, required redesign of Connecting Bus-bars and Arrangement of Terminal Bushing Connection Assemblies maintaining the air gap clearances(Fig-7). 4. 4. End Ring is introduced in this design for providing better rigidity in stator winding overhang. 4. 5. To make the generator suitable for static excitation system, new Slip Ring Shaft has been designed. Couplings of Slip Ring Shaft have been redesigned to suit TG rotor (EE) at one end and Barring gear at the other end. 4. 6. In view of relocation of terminal bushings, drawings related to temperature and pressure monitoring circuits were prepared afresh. 4. 7. There is no availability of overhead cranes in Gas Turbine hall over Turbogenerator due to low ceiling of the building. This poses a major challenge for insertion of rotor into stator. Rotor weighs around 42 tons. So, a new Trailer with Prime-Mover has been designed for Rotor Insertion in to Generator for GTG application. 4. 8. Design Documents (drawings and CBOMs) were sent to Corporate R & D for to carry out following calculations :- (1)Static Analysis – (a) Lifting of stator with 4 lugs b) Short Circuit Torque (c) Hydraulic test at 10 bar (2) Dynamic and Harmonic Analysis of Stator Frame with Core, Windings and rotor (3) Rotor Dynamics 4. 9. Generator Outline diagram for GTG is enclosed as Fig. 8. 5. RESULTS OF MECHANICAL CALCULATIONS : 5. 1. Static Analysis – Load ConditionsMaximum Stresses(N/mm? )Reference Lifting of Stator with 4 lugs110 Fig-9 Short Circuit Torque with 3. 5 bar internal pressure of Hydrogen238 Fig-10 Deformations of side wall due to Hydraulic Pressure of 10 bar (fig-11) Locations on side wallDeformations (mm) TEEE 1700 mm above centerline1. 040. 46 1700 mm below centerline0. 460. 40 5. 2. Dynamic and Harmonic Analysis of stator with core & windings DirectionFrequency PeakReference Axial32. 3 HzFig-12,13 Vertical82 HzFig-14 Horizontal136 HzFig-15 6. TECHNOLOGICAL GAINS : 8. 1. THRI bar type Turbogenerator has been developed for the first time for application with Gas Turbine. 8. 2. New design Stator can be placed directly on the trailer during transportation by road. It can be dragged at site during erection, if required. 8. 3. The new design of core is suitable for THRI STG design also due to improved ventilation. This will result in lower temperature rise leading to more reliable operation and enhanced life of the machine. . 4. Rationalisation and standardization of components has been taken into consideration to develop this design. Assemblies like Rotor, End Shield, Winding bars, Terminal Bushings, Shaft Seals, Oil Catchers etc will be used from that of existing design variant. 8. 5. Existing major tooling like those for fabrication & machining of Stator Frame, ass embly of core, Hydraulic and Pneumatic testing of Stator Frame will be used. 7. CONCLUSION : THRI bar type Turbogenerator has been developed in-house for the application with Gas Turbine. With the concerted efforts of the team the design work was completed by 31st August, 2005. This new design has been approved by an external review team comprising of experts from IIT- Roorkee, Corp. R&D and RC Puram besides Haridwar experts from Technology, Quality & Engineering. Discussions were also held at PEM Delhi along with R. C. Puram for erection and maintenance of various Generator components like coolers, End Shields, Insert Covers, Rotor, Slip Ring Shaft Assembly and Bearings etc. New design features were explained and these were taken in to account for development of Power Plant layout.