CRITICAL REVIEW OF 1:'OD.ERN TRENDS IN DESIGN OF LARGE CAPACITY A. C - GENERATORS WITH S,)ECIAL REFERENCE TO HYDRO-GENERATORS

Abstract

In recent years there have been many notable advances in the construction of large water wheel generators and Steam turbo generators. Moreover increasing consumption of electrical energy and long term planning of extensive power supply systems have re- sulted in manufacture of large capacity single units which even 10-12 years ago would have appeared scarcely possible. At present the largest of hydro generating units are built in U.5.S.R.(viz) 500 MW units for Kharosnoyarsk Power Station. The largest turbo generators are built in U.S.A. (viz.) 1000 MW unit built by Allischalmers for Ravenswood New York, 900 MW units built by G.E.C. and Westinghouse. The design of Water Wheel generators has progressed in many respects as a result of advances in tenchniques and available materials. The experimental research work both on models and full scale has led to better utilisation of active materials of magnetic circuit and the windings. Improvements in electrical Sheet Steels and winding insulations have been significant and the use of better steels for highly stressed rotor parts is also of Importance. The performance of a Synchronous generators depends to a large extent on various characterstic design factors. The factors have to be judiciously considered before evolving economic design of a generator. The characterstics of water turbine influence the design of generator is many ways (viz) its low speed necessitates many poles, the overspeed imposes centrifugal force limitations on diameter, the critical speed must be above over speed, thrust bearings are needed to resist hydraulic thrust and the weight of rotor etc. The generator is designed for 15% overload capacity. Higher power factor is preferred. With the high speed voltage regulators lower short circuit ratio and higher synchronous reactance are adopted to reduce capital costs. Higher reactance would mean that rupturing capacity of breakers could be less since short circuit currents are reduced. Optimum fly wheel effect is provided as against previous designs with higher Inertia constant, a short circuit ratio of 1.0 - 1.1 for hydro generators and 0.5 - 0.8 for turbo generators is nowadays adopted. Some manufacturers are adopting higher specific speed to reduce eott. The recent designs have resulted in high efficiency figures of 96-98%. Higher generation voltages of 16-18 KV for hydro genera-tors and upto 24/27 KV for turbo generators are adopted. The vertical shaft arrangement is widely used for medium and large capacity hydro generators. The turbo generators are only of horizontal shaft type. The umbrella type hydro generators are preferred by many manufacturers since the main advantages are less costs and simpler lubrication arrangements. The stator frame is divided into parts due to transport restrictions. The practice of assembling the stator core at site from laminations is adopted. Stator windings in recent years have been of double layer type using either single turn bars or multi-turn coils. Roebel bar construction is used. New synthetic insulation systems are developed for coil insulation. Heat cycle tests are conducted for stator coils. Solid disc rotors are employed for high speed machines. But many of the large capacity units are nowadays of punched segmental rotor construction. The poles are dovetail jointed to the rotor ring. The rotor with poles must withstand the run-avay speed. For large capacity generators closed circuit ventilation with Air coolers is adopted. The excitation is normally by main and pilot exciters. Recently for large capacity sets, a main exciter and a thyristor converter system is employed. Recently D.C. supply from rectifier system is also used. The shaft is normally hollow. In U.S.S.R., welded tubular shafts are used for large capacity sets. The stator must be designed to withstand short circuit stresses. The thrust bearing is of Kingsbury pad type or spring type. Electric brakes are recently used. The cylindrical rotor turbo generator is a horizontal shaft machine which must be run at the highest possible speed. Revolutionary developments have taken place in design of turbo --'~~ generators (viz) Hydrogen cooling, direct cooling of stator and rotor, special rotor forging techniques etc. With the same physical dimensions, higher capacity units are evolved due these recent developments. The rotor is solid shaft type and is designed for centrifugal stresses. The stator is shipped in parts. Separate exciters are used or direct connected exciters with gear box are employed. Research work by various design Engineers and scientists is expected to result in larger capacity units and economical construction. Complex design problems are being solved on COMPUTER and economic designs are worked out.