2.3 Steam Turbine Generators
A proven design of steam turbine generator that is already in service in China was chosen. Each steam turbine generator consists of a 3,000 rpm three cylinders reheat steam turbine solidly coupled to a water/hydrogen cooled two pole generator, and will be equipped with a comprehensive set of auxiliary systems, instrumentation and control to ensure safe and efficient operation under all operating conditions. The steam turbine generator unit has a generator rated at 739.6 MVA at 20 kV, 50 Hz. The net generator output at 20 kV and 0.85 power factor lagging is 626 MW. The boiler/steam turbine are in combination capable of achieving a maximum continuous rating equivalent to 658 MW (gross), thus providing an adequate margin to cater for foreseeable adverse operating conditions, including un-recoverable through life degradation of Power Plant performance.
The turbine has a combined High Pressure (“HP”) and Intermediate Pressure (“IP”) turbine and two double flow Low Pressure (“LP”) cylinders. Steam leaving the HP cylinder is returned to the boiler for further heating before expansion through the IP cylinder. Steam leaving the LP cylinders at a very low sub-atmospheric pressure will be condensed by passing through a condenser that has cooling water tubes. The condensate will then be returned to the boiler for reuse via a polishing plant and a series of heaters to a deaerator.
Each turbine generator has a dedicated cooling water system consisting of condenser, natural draft cooling tower and 2 x 50% duty cooling water pumps, as well as a set of equipment and instrumentation to control the quality of water and hydrogen used and to ensure safety of the machine. The cooling towers are circular hyperbolic reinforced concrete structures 157 m high.
2.4 Electrical Systems
Power is generated at 20 kV and the voltage is stepped up to 500 kV by means of generator transformers. Power will be supplied to the Thai Grid via twin 500 kV overhead power lines. A double circuit 115 kV overhead power line will supply power to the Lao Grid. All necessary switchgear, controls, protection equipment, metering and maintenance isolators are included to permit safe and efficient operation of the Power Plant. The switchyard is an integral part of the Power Plant and is located within the boundary of the Power Plant. It is located at the western side of the Power Plant to suit the direction of the 500 kV power lines leaving the Hongsa Valley. The 500 kV switchyard is of the open terminal double bus bar type, and in accordance with normal good utility practice is enclosed within a safety fence to restrict access.
Unit transformers supply power from the generators to the auxiliary power system. Power to start up the Power Plant can be drawn from either the Thai or the Lao Grid. Large auxiliaries are supplied with power at 10 kV, and all other supplies will be provided at 400 Volts.
In case of an emergency, 2 x 1,500 kW 400 V diesel generators will be installed to ensure that the Power Plant can be shut down safely, and essential supplies are maintained until connection to the Grid is re-established.