Production of the World’s Largest and Higher Efficiency 600MW Class Indirectly -Cooled Turbo-generator

D. Murata, M. Kakiuchi, H.Ito Toshiba Corporation, Japan

Abstract Large-scale 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 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 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 are increasingly applied. A gas 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 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

transfer Heat 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. For the hydrogen gas sealing, the same sealing system was adopted as with the proven generator having the hydrogen pressure of 520 kPa (g).

5 high

low

Fig.10 Stress analysis of stator frame

5. Results of factory test The developed generator was tested at the factory in January 2009. Fig.11 shows the generator while testing. The results of factory test are described below.

Fig.11 Generator on the testing bed

5.1 Characteristics During the tests, the no-load saturation curve as well as the short- circuit characteristic curve was measured. Field currents at no load rated voltage and short circuit rated current were within 5% of the design values. The short circuit ratio satisfied the specification of more than 0.58 %.

5.2 Losses and efficiency The losses were measured and the efficiency was calculated according to the Japanese national standard, which is equivalent to IEC60034. Fig.12 shows the comparison of the developed generator and conventional generator having water-cooled stator and diagonal flow rotor of the same capacity. The cross section area of the conductor of an indirectly cooled stator coil becomes larger than a directly cooled coil because the indirectly cooled coil is composed of only solid conductors. The radial flow rotor has the advantage to reduce the field copper loss because 6 ventilation holes are smaller than the holes of diagonal flow cooling. As a result, resistance loss of stator and rotor can be reduced. Calculated efficiency is more than 99.1% and exceeds that of conventional generators by 0.2%

Resistance loss of rotor

Resistance loss of stator

Stray load loss

Core loss

Windage loss

Friction loss

Excitation system loss 0 Developed generator Conventional generator

Fig.12 Comparison of losses

5.3 Temperatures The temperature of stator coils, rotor coils and the stator core was measured during heat-run tests. The difference of measured temperature and calculated temperature of each part was less than a few kelvin. The temperature rise of each part at rated condition was confirmed to meet the values regulated in the standard.

6. Comparison of improved and conventional generator Both improved and conventional generators are compared concerning dimension, weight and efficiency. Although the improved generator dimensions and total weights were increased by approximately 10 % compared to conventional one, the efficiency was remarkably improved from 98.9 % to 99.1 % as shown in Table 2. According to calculations, the efficiency increase saves CO2 emissions of 8,000 ton/year and coal consumption by 3,100 ton/year for a 600MW class coal-fired thermal power plant [3].

7 Table 2 Comparison of improved and conventional generator

Developed generator Conventional generator 670MVA 670MVA Length p.u. 1.02 1.00 Width p.u. 1.13 1.00 Height p.u. 1.01 1.00 Stator weight p.u. 1.08 1.00 Rotor weight p.u. 1.06 1.00 Total weight p.u. 1.08 1.00 Efficiency % 99.1 98.9

Consequently, indirectly hydrogen-cooled generators have the advantages over directly water-cooled generators in the life cycle cost, because of lower operation cost including operability and maintainability.

7. Conclusion Instigated by increasingly stringent environmental and economic boundary conditions, the authors developed and manufactured a 2P-60Hz-670MVA indirectly hydrogen-cooled generator that has the world largest capacity and higher efficiency. Employed technologies have been verified by analyses, tests and experiences in other many power generation projects. The efficiency of the developed generators is over 99.1% and the authors believe that the efficiency will meet all customers’ demands. However, further improvement will be possible in capacity and efficiency of indirectly hydrogen-cooled generator in the near future. Authors will pursue these improvements and provide high performance generators for the world.

BIBLIOGRAPHY [1] Working Group SC 11-01 CIGRE. “Impacts on turbine generator design by the application of increased thermal conducting stator insulation” (2002) [2] Working Group SC 11-01 CIGRE. “The world’s largest capacity turbine generators with indirect hydrogen-cooling” (2004) [3] H.Tomiki et. al., “60Hz Large Capacity Indirectly Hydrogen-Cooled Generator”, Toshiba Review Feb. 2010

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