In a liberated electricity market with a steadily growing percentage of fluctuating renewables the load related requirements of modern steam power plants are noticeably changing. Whereas the past has seen mainly coal-fired units being operated in base load now highly efficient part load behavior becomes more and more important as well as quickly responding frequency support at minimized investment costs. …In the article various approaches will be identified, discussed and evaluated under economical criteria focused on the above described challenges for future power generating technologies. One central idea is to shift the pure sliding pressure mode down to an intermediate load range where the upper limit is reached at around 70% and optimize the blade path efficiency according to this point. Along with this strategy concepts are presented which allow frequency support from primary to hour- reserve of maximized load steps. Moreover it shall be explained how it is principally possible to use the same cycle conditions for load steps and increasing part load efficiency at the same time. Another idea is to improve the plant performance at lower load ranges by raising the main and reheat steam temperature accompanied by special maintenance concepts. The ideas presented in this article are mainly derived from a steam turbine point of view. Nevertheless some requirements and effects on the overall plant are taken into account additionally. The presented approaches can be applied for new apparatus as for the upgrade of existing units. As the drivers for a more flexible operation of steam plants are especially strong in markets which do not guarantee an attractive utilization of the plant in produced MWhrs/year, investment decisions for new plants have been delayed or cancelled due to the difficult market conditions. Therefore special attention will be paid in this paper to the application of the new flexibility features in power plants which are already in operating and which have been designed originally with the main focus on highest efficiencies in base load operation. The difficulties and limitations given with the existing plant design will most likely be compensated by the economical advantages of the more flexible plant operation after the modernization.

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Environmental aspects have increased the pressure on the fossil power generation industry to reduce carbon dioxide (CO2) emissions. One way to achieve this is by increasing the overall plant efficiency, which also fosters an economical plant operation. How can the efficiency of a next generation coal fired ultra super critical (USC) steam power plant (SPP) be increased significantly in the nearest future while maintaining its familiar reliability and availability at the same time? In China’s national USC SPP demonstration project, Pingshan Phase II, this challenge is met by a double reheat cross compound turboset with one elevated and one conventional turbine layout, together with increased steam parameters of up to 325 bar and steam temperatures of up to 630°C. The nominal electrical capacity of the plant will be 1350 megawatts (MW). With this set up, a ‘half-net’ efficiency of more than 52.2 percent is expected [‘half-net’ = gross efficiency with generator power reduced by boiler feed water pump power consumption]. The first, elevated turbine train consists of two high-pressure modules having different pressure stages and one generator and it is located close to the main headers of the boiler at a height of appr. 83 meters. This unique turbine arrangement allows the expensive high-temperature pipes to be shortened, leading to substantially reduced pipe pressure losses and costs. The second turbine train will be installed on a conventional turbine deck at a height of appr. 17 meters and consists of two intermediate pressure and three low pressure turbine modules as well as a second generator. In this paper, the advanced steam turbine technology for this power plant concept is presented and discussed in detail. To achieve the next level of efficiency with an SPP today, the application of the 700°C material class is not possible to due to the slow progress of the associated technology development. It is more expedient to exploit the limits of the 600°C material class to the highest possible extent in USC conditions i.e. to the pressures and temperatures mentioned above. Design concept studies have shown that 52.2% ‘half-net’ efficiency cannot be achieved with a single reheat layout, so a double reheat (DRH) layout has been chosen. In addition, 1350 MW cannot be achieved with one turbine train (tandem compound), but only with two turbine trains (cross compound). In order to achieve the highest reliability possible, proven turbine design topologies and features have been used. The major change to the Siemens barrel type VHP turbine was a material change from 10% Chromium steels to FB2 and CB2. The HP turbine received increased wall thicknesses as well as a similar material change compared to a standard USC design. In order to control the oxidation at these elevated temperatures, oxidation protection measures have been applied where required. The startup procedure has been tailored specifically to the needs of a double reheat cross compound configuration.

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