Mitsubishi's first Model 701G gas turbine is scheduled to enter commercial operation in July 1999 at Tohoku Electric Power Company's 1610 MWe Higashi Niigata unit 4 combined cycle power plant. Higashi Niigata is also the home of Japan's first large combined cycle plant, which has been operating since 1984. The site therefore provides a good illustration of extraordinary progress made in combined cycle technology over the past 15 years or so.

The 701G gas turbine is the scaled- up 50 Hz version of Mitsubishi’s 230 MWe 501G gas turbine (the 60 Hz machine was described in the May 1997 issue of Modern Power Systems). The 501G was the first turbine of its kind to be designed for 1500°C turbine inlet conditions.

The first 501G was installed for operating trials at a specially built combined cycle power plant constructed in Mitsubishi Heavy Industries’ Takasago works. Testing of this machine started in February 1997 and full power was achieved on 7 April. The trials were completed in 3 months and the plant received its official inspection in June 1997 and began long term reliability proving tests.

The 501/701G gas turbines have evolved from MHI’s extensive experience of developing and manufacturing large heavy duty combustion turbines, initially in collaboration with Westinghouse, now Siemens Westinghouse. In 1981, development work was completed on the MW 701D series gas turbines. These had a turbine inlet temperature of 1150°C and were used successfully in Japan’s first large scale combined cycle plant, which was also built at Tohoku Electric Company’s Higashi Niigata site (Higashi Niigata Unit 3).

In 1985 the development of the F-series began and shop tests were completed in 1989. The F-series has a turbine inlet gas temperature of 1350°C. The technology has proved to be very reliable and, to date, 43 units have been ordered for combined cycle plants both in Japan and for export. Altogether, about 130 units of the D and F series have been sold.

To obtain still higher efficiencies, MHI started to develop the G-series in 1993 and the turbine inlet temperature was increased to 1500°C. The 501G development programme culminated in the long term operating tests currently underway at the Takasago factory. The basic design of the 701G is the same as the 501G but the physical dimensions have been increased, giving it a larger output. Four units have been ordered for the Higashi Niigata 4 station. These are designated 701G1 machines, rated at 271 MWe each, although the 701G is capable of producing up to 308 MWe.

501G prototype tests

The combined cycle plant at Takasago on which the 501G prototype tests are being carried out has an installed capacity of 330 MWe. It consists of a 225 MWe 501G gas turbine, an unfired heat recovery steam generator and a 105 MWe steam turbine.

The fuel is city gas 13A, which is vaporised LNG. During the initial trials, measurements of temperature, pressure and vibration were taken at 1800 points. The tests showed that the compressor characteristics were excellent and that vibration levels were all well within the allowable design values.

Temperature measurements of hot parts, including the combustor, turbine blades and vanes were all below the allowable design temperatures during operation with a turbine inlet gas temperature of 1500°C.

Other parameters such as rotor expansion and blade tip clearances were also confirmed to have adequate margins for all operating and start up conditions. In the year following commercial operation, the plant operated for a total of 3500 hours and was started up 250 times.

701G development and design

The 50 Hz 701G uses essentially the same combustor and turbine first and second stage blades and vanes as the 501G, thus avoiding the need for further development testing. The design also benefited from the test results on the 501G prototype. The first 701G was assembled and tested in the Takasago factory in March 1998 before being sent to the Higashi Niigata site for installation. After on-site trials, the first block (two gas turbines and one steam turbine) will go into commercial operation in July 1999.

The 701G1 gas turbine is structurally similar to the 701F design. It is a two bearing single shaft machine with the generator coupled to the compressor end, an arrangement which permits an axial turbine exhaust suitable for combined cycle applications.

The first three stages of turbine vanes are cooled by air taken from different stages of the compressor while the turbine rotor and blades are cooled by air taken from the compressor discharge and then passed through an external cooler and filter.

Some key features of the 701G can be summarised as follows:

  • Compressor. A 17-stage axial flow compressor is used to give a pressure ratio of 19 (compared to 16 for the 701F). Multiple circular arc airfoils and controlled diffusion airfoils are used to achieve a high flow rate and a high efficiency. Variable pitch inlet guide vanes are provided to improve performance at part loads. Bled air is taken from the 6th, 11th and 14th stages.

  • Combustor. The combustor must provide 1500°C turbine inlet conditions with the same NOx limit specification that is applied to the current 701F machine. This means that the distribution of temperatures in the combustor must be carefully controlled to keep them in the 1500-1600°C range. For this reason, closed type steam cooling is used for the walls of the combustor to avoid the use of cooling air. Tests have shown that this design of combustor can operate with NOx levels of 25 ppm or better. Heat from the cooling steam can be recovered in the steam cycle.

  • Turbine. The turbine is a four-stage reaction turbine. The first two stages have free standing, thermal barrier coated blades, while the third and fourth stages have shrouded blades similar to those that are used on the 701F. Fully three-dimensional contoured airfoils, which are obtained by stacking airfoils along a curve in the radial direction, are employed.

    Air cooling is used for the blades and vanes of the first three stages, with full coverage film cooling. The cooling air for the first stage vanes comes from the compressor outlet whilst that for the second and third stage vanes is bled air from the 14th and 11th stages of the compressor. As mentioned earlier, the turbine rotor and blades are cooled with externally filtered and cooled air from the compressor outlet.

    Even though the inlet temperature has been increased, the metal temperatures are no higher than those of earlier gas turbines.

    Two new nickel based alloys were developed by MHI, jointly with Mitsubishi Steel Manufacturing Company and Mitsubishi Materials Corporation. MGA 2400 is used for the turbine vanes and is derived from the IN 939 Ni-based alloy but has improved welding characteristics. MGA 1400 is used for the turbine blades and has a higher creep strength than the popularly used IN 738 LC alloy. In the case of the first and second stage blades, the creep strength is further improved by using directionally solidified MGA 1400 material.

  • Efficiency. The design features of the 701G1 as briefly outlined above, result in a thermal efficiency of 38.7 per cent compared with 37 per cent for the 701F. When used in combined cycle plants, the 701G1 will give overall efficiencies of 58 per cent or better.

    The Niigata plant

    The utility which has purchased the first 701G1 gas turbines, Tohoku Electric Power Company, serves the northern part of the main Japanese island. This part of Japan is relatively undeveloped but there are several large population centres, one of which is the city of Niigata on the west coast, about 230 km north west of Tokyo.

    The utility has an installed capacity of some 12 500 MWe. Of this, 2434 MWe comes from a network of 212 hydro units, some of which are quite small. The utility also operates two nuclear units, Onagawa 1 and 2 and has two more under construction (Onagawa 3 and Higashidori 1). In addition the utility has five geothermal stations but the main generation comes from seven fossil fuel thermal power stations. Two of these are located close to Niigata.

    The original Niigata plant has two steam turbine units in operation producing 500 MWe. These burn heavy and crude oil together with some natural gas which is produced locally. The Higashi (East) Niigata station consists of two 350 MWe steam turbine units, referred to as Minato units 1 and 2, two 600 MWe steam turbine units, Higashi 1 and 2, and the original combined cycle plant, unit 3, rated at 1090 MWe. The total output of the power station is currently 2990 MWe.

    The Higashi Niigata 3 combined cycle plant has six MW701D gas turbines and began operating in 1984. Natural gas and imported LNG are used as fuels.

    Higashi Niigata unit 4, the new combined cycle plant, which is nearing completion, is located alongside the existing unit 3. It will have a total output of 1610 MWe and consists of two blocks each of which has two 701G1 gas turbines, two unfired heat recovery steam generators and a single 265 MWe steam turbine. The first block will go into commercial operation in July 1999 and the second in July 2000.

    According to Tohoku Electric, the 701G1 gas turbines will be conservatively rated at 270 MWe and will be operated at a turbine inlet temperature of 1400°C. However Tohuku says a figure of 50 per cent for what it calls overall “plant thermal efficiency” is still expected which it claims will be a new record. Some indication of the progress in combined cycle technology can be obtained by comparing units 3 and 4. Unit 3 has six gas turbines and produces 1060 MWe with, according to Tohoku, a “plant thermal efficiency” of 44 per cent whereas in unit 4 four gas turbines will produce 1610 MWe at a “plant thermal efficiency” rate of 50 per cent. The improvement in efficiency will result in a saving of around 180 000 tons of LNG per year and lower CO2 emissions per kWh.

    Improvements have also been made in the steam cycle. Heat transfer in the superheaters and reheaters has been improved by using small diameter tubes and a reheat temperature of 566°C has been adopted to improve the thermal efficiency.

    Other new features include the use of “integrated transformers”, in which the main transformer and house supply transformer are housed in a single tank. The start-up system uses thyristors that are able to provide a variable frequency supply so that the generators can be brought up to speed using the generators as synchronous motors. This method provides cost savings without affecting the reliability.

    Comparison of 701F and 701G performance (both 50 Hz)