Medium voltage direct current (MVDC) converter for pebble bed modular reactor (PBMR)
Pretorius, Hendrik de Villiers
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Nuclear and renewable energy systems will probably be used more and more extensively in future due to high environmental demands regarding pollution and exhaustion of the world's gas and coal reserves. Because most types of renewable energy systems do not supply electric power at line frequency and voltage a converter is used to connect these sources to the existing power system. The Pebble Bed Modular Reactor (PBMR) is a nuclear power plant currently using a 50 Hz synchronous generator. A high-speed generator is now being investigated as an alternative to the conventional 3000 rpm synchronous generator. This option is considered, because the turbines in the thermo hydraulic system will run more efficiently at high speed and can be smaller in diameter. It also implies a smaller generator and possible reduction in the number of turbine shah. By using a 150 Hz generator implies that the generated electrical power frequency is also not that of the grid. This situation is similar to that of some renewable energy systems like wind farms. Although the frequency of wind farm generated power is in most cases lower than 50 Hz (nominal grid frequency in South Africa), the same technique of converting the generated power to direct current (DC) can be used. Direct current converters are also used to connect asynchronous networks, oil platforms, 2 limiting flicker mitigation andl6- Hz railway systems to national grids. 3 These applications are therefore used in this thesis as a starting point for discussing the reason why a Medium Voltage Direct Current (MVDC) converter was chosen over a High Voltage Direct Current w C )for the PBMR system. MVDC converters can be used to start-up the PBMR without the Start-up Blower System and Static Frequency converter. The converter can also control the active and reactive power flow and the frequency control allows a standard PBMR design for 50 Hz and 60 Hz systems. A high-speed induction generator was found to be a good combination for use in cooperation with the converter. The construction of a 180 MW high-speed generator is however currently not possible. MVDC and especially IGBT technology are new technologies and are therefore expensive at this stage.
- Engineering