Production of the World’s Largest and Higher Efficiency 600MW Class Indirectly Hydrogen-Cooled Turbo-generator D. Murata, M. Kakiuchi, H.Ito Toshiba Corporation, Japan Abstract Large-scale electricity generation has to comply with increasingly stringent environmental and economic boundary conditions. That is the reason why the authors are developing generators with higher efficiency and larger capacity. Their new concept has an indirectly hydrogen-cooled stator coil to replace installations in which directly water-cooling has been inevitable up to now. The newly developed generator saves space and reduces complexity. The design eliminates the typical risk of water-cooled generators such as clogging of the stator coil cooling path and water leaks. Further, the indirectly hydrogen-cooled stator coil does not require a cooling path area in the cross section, which potentially results in a higher efficiency. The authors have developed the HTC (High Thermal Conductivity) insulation for the stator coil, and applied this to indirectly hydrogen-cooled generators since 2000. In 2009, the authors developed and manufactured a 2P-60Hz-670MVA generator, which is the world’s largest capacity as indirectly hydrogen-cooled generator, and have achieved a generator efficiency of over 99.1%. Hydrogen gas has less cooling ability than water. In order to enable adoption of indirectly hydrogen-cooling method to a 2P-60Hz-670MVA generator, , the technical challenges have been investigated and solved. Examples are the balancing between terminal voltage and stator current, optimization of stator and rotor cooling, verification of stator frame, hydrogen gas sealing, and stator coil end-support structure. The developed generators were tested according to the standard at the factory in January 2009. The developed generator proved to have the estimated performance and satisfied the specification. KEYWORDS Indirectly hydrogen-cooled generator, HTC (High Thermal Conductivity) stator insulation 1. Introduction In recent years, global environmental concern demands the reduction of CO2 emission in every activity. The electric power sector is a major energy consumer. To improve performance of the sector, efficient machinery such as combined cycle generation systems 1 with gas turbines are increasingly applied. A gas turbine is easy to install and operate, and also the generator applied should be simple and have high efficiency. In conventional thermal power plants, the efficient Ultra Super Critical steam turbine is going to be preferred, again with an adequate generator. On the basis of these backgrounds, the authors are developing generators with higher efficiency and larger capacity, having an indirectly hydrogen-cooled stator coil, to take place of capacity range in which directly water-cooling has been inevitable up to now. When increasing the power capacity of an indirectly hydrogen-cooled generator, a first requirement is to improve the cooling performance of stator coil. The authors have developed the HTC (High Thermal Conductivity) insulation for stator coil [1], and applied this to indirectly hydrogen-cooled generators since 2000. The HTC insulation has a thermal conductivity twice as high compared to normal insulation, due to high thermal conductive material applied to the mica insulation layers. The authors initially applied the HTC insulation to a 200MVA class generator, which was put into operation in 2001 and has been operating reliably for over 8 years. Since then, the authors have developed indirectly hydrogen-cooled generators with larger capacity [2]. In 2009, the authors developed and manufactured a 2P-60Hz-670MVA generator for a coal-fired thermal power plant. This generator has the world’s largest capacity as indirectly hydrogen-cooled generator, and achieved an efficiency of over 99.1%. In this paper, the applied technologies and shop test results are described. 2. Concept of indirectly hydrogen cooled generator The stator indirectly hydrogen-cooled generator needs neither the stator coil cooling water supplying unit nor its cooling water piping, and make it possible for saving space and simple layout, comparing with stator coil directly water-cooled generator. The generator itself does not use cooling water, then make it possible to eliminate the peculiar risk of water-cooled generator such as plugging of stator coil cooling path and water leaks, and then improves reliability. Further more, the stator coil indirectly hydrogen-cooled generator is composed of only solid conductors, and does not need to prepare the cooling path area in the cross section of the stator coil, then make it possible to secure more cupper conductor area and to reduce current density in the stator coil. So the stator coil indirectly hydrogen-cooled generators potentially realize the higher efficiency. In these points of view, authors conclude that indirectly hydrogen-cooled generator is advantageous to directly water-cooled generator in the life cycle cost, because of lower operation cost including operability and maintainability 3. Design Ratings of the developed generator are shown in Table 1. 2 Table 1 Ratings of the 2P-60Hz-670MVA generator Capacity (kVA) 670,000 Speed (min-1) 3,600 Voltage (V) 19,000 Current (A) 20,360 Power factor 0.9 Short circuit ratio More than 0.58 Hydrogen pressure (kPag) 520 Insulation class / Temperature rise F class / B rise Stator coil cooling / Indirectly hydrogen-cooling / Rotor coil cooling Directly hydrogen-cooling 4. Applied technologies 4.1 Balancing between terminal voltage and stator current From the viewpoint of indirectly-hydrogen cooling, thinner thickness of stator coil ground insulation is preferable. Lower current of a stator bar is preferable from the viewpoint of loss reduction. Multi parallel stator coil connection is applied to reduce terminal voltage and to reduce current of a stator bar. As a result, the terminal voltage and stator current of the developed 60Hz-670MVA generator is 19,000V and 20,360A, although the conventional water cooled generator has 22,000V and 17,583A. A larger surface area is preferable from the viewpoint of heat conduction. Fig.1 shows the stator bar cross section of the developed 60Hz-670MVA generator and of a conventional water-cooled 60Hz-670MVA generator. The stator coil cross section for indirectly hydrogen-cooled generator becomes larger than that of a directly water-cooled coil because the indirectly cooled coil is composed of only solid conductors. As a result, resistance loss of stator coil can be reduced. The developed generator The conventional generator Fig.1 Comparison of stator bar cross section 4.2 HTC stator insulation Heat generated in indirectly cooled stator bars is transferred through conductors, ground wall insulation, core and cooling gas as shown in Fig.2. In this heat transfer path, thermal conductivity of ground wall insulation is much lower than in the other components. Therefore, , the authors had developed HTC insulation, which has twice as high thermal 3 conductivity as conventional ground insulation and successfully applied this to hydrogen-cooled generators since 2000. This HTC insulation system is also applied to the developed hydrogen-cooled generator for the purpose of higher efficiency. Considering frequently start and stop operation for the above-mentioned generator, a thermal cycle test consisting of 3000 cycles was performed as shown in Fig. 3. This was in addition to a 1,000 cycles thermal cycle test performed previously [1]. The 3,000 cycles test condition is the same as the 1,000 cycles test, 40°C ↔ 155°C heated by AC current and 600 MVA class model slots. The voltage endurance characteristic after the 3,000 cycles test is shown in Fig. 4. Voltage endurance characteristics of fresh bars and test bars after 1,000cycles are described additionally. These three HTC insulation bars show equivalent characteristics. Conductor Heat transfer Ground insulation Core Fig.2 Heat transfer path of stator bar Fig.3 Thermal cycling test 2.0 1.5 after 3000 cycles 1.0 HTC(0 cycles) after 1000 cycles HTC(1000 cycles) 0.5 HTC(3000 cycles) Electrical stress (p.u.) stress Electrical 0.3 100 101 102 103 104 105 Voltage endurance (hrs.) Fig.4 Voltage endurance characteristic after 3,000 cycles 4.3 Optimized stator and rotor cooling The high hydrogen gas pressure of 520 kPa (g) has been chosen to enhance cooling ability. The number and section width of the stator ducts were optimized shown as in Fig.5, Fig.6 and Fig.7 to minimize temperature difference along stator bars; this results in reduced windage loss due to reduced gas flow. The radial flow cooling system has been optimised for effective cooling of rotor and stator coils in order to reduce windage loss. The area of sub-slot, radial duct pitch and ventilation path of the core end were optimized as shown in 4 Fig.6 and Fig.8 to minimize temperature difference along the rotor bars. The radial flow rotor shown in Fig.9 has the advantage to reduce the field copper loss because ventilation holes are smaller than the holes in case of diagonal flow cooling. Cooler Cooler Stator duct Stator Gas gap Rotor coil Rotor Axial Fan Fig.5 Cooling circuit of radial flow system Fig.6 Hydrogen gas temperature analysis Outlet Inlet Outlet Inlet Outlet Inlet Outlet Rotor Coil Temperature Rise [K] Rise Temperature Coil Rotor Stator Coil Temperature Rise(Ｋ) Core End Core Center Fig.7 Calculated stator coil temperature rise Fig.8 Calculated rotor coil temperature rise Wedge Creepage block Rotor coil Slot insulation Cooling gas flow Sub slot Fig.9 Radial flow cooling rotor 4.4 Stator frame and hydrogen gas sealing The stator frame and hydrogen gas sealing system are the main components to contain the pressurized hydrogen gas in the generator. Stator frame stress and deformation have been verified by finite element method (FEM) analysis. Fig.10 shows a result of the FEM stress analysis. The applied pressure is 800 kPa (g) according to IEC60034-3. Stresses in all parts of stator frame and bearing brackets are less than a half of permitted values in accordance with EN13445-3.