Efficient Generation of Power In Medium Voltage Direct Current Systems: Variable Speed Operation and Rectifier Considerations

By investigating variable speed operation of an engine and different types of ac-dc rectifier topologies, this dissertation provides procedures to develop and implement efficient Medium Voltage Direct Current (MVDC) power systems. The research reported here offered solutions to the following issues: 1) Efficiency improvement of a gas turbine by running it in variable speed operation mode. 2) The advantages and disadvantages of different ac-dc rectifier topologies considering both the efficiency and rectifier performance (dc bus voltage regulation, weight/size/volume of passive components, number of semi-conductor devices, displacement power factor, complexity of control strategy, etc.) 3) Implementation of variable speed MVDC power system if variable speed operation brings great advantages for fuel saving. A semi-theoretical analysis was developed to address issue 1). This analysis revealed the optimal efficiency and the corresponding optimal speed as a function of shaft load for both single-shaft and twin-shaft gas turbines. These semi-theoretical results were confirmed by detailed simulations that show it is possible to achieve as much as 15% reduction in fuel consumption when variable speed operation is used instead of fixed speed, for a generating plant to electrically propel a ship having powering characteristics similar to those of a DDG-51. A series of boost type rectifiers for applications in MVDC power systems were compared to address issues 2). The comparison included only boost rectifiers since higher distribution voltage leads to higher efficiency. The boost type rectifiers are diode rectifier with boost chopper (diode rectifier), three-level diode clamped voltage source converter (VSC), and modular multilevel converter (MMC). The comparison metrics include complexity of control strategy, number of switching devices, number/value/weight/volume of inductors and capacitors, the dc bus voltage performance (peak-peak voltage ripple, overshoot, and settling time), the total harmonic distortion (THD) of the ac side input current and voltage, displacement power factor (DPF), power efficiency for different load conditions, and performance under variable ac frequency operation. The comparison shows that MMC is the most efficient and provides the best dc bus performance, by adding more capacitors (both number of capacitors and total weight), and more complex control schemes into the system. VSC provides a relative high efficiency and dc bus performance with less passive components, which indicates VSC is a good choice considering both performance and cost. A procedure to implement an integrated variable speed MVDC power system was proposed to address issues 3). For each component of dc power generation chains, gas turbine, synchronous generator, and ac-dc rectifier, their steady-state efficiencies and system efficiency vs. dc load were analyzed. The relative system efficiency improvement (an index of fuel reduction) can be up to 35% for 10% loading. The relative efficiency improvement decreases to 5% as the dc load increases to 45% loading. Following the procedure, a variable speed MVDC power system simulation model was developed. The dynamic response shows that the system can track the speed command for different load demands while yielding the highest efficiency and good system stability