An Advanced Three-Level Active Neutral-Point-Clamped Converter With Improved Fault-Tolerant Capabilities

A resilient fault-tolerant silicon carbide (SiC) three-level power converter topology is introduced based on the traditional active neutral-point-clamped converter. This novel converter topology incorporates a redundant leg to provide fault tolerance during switch open-circuit faults and short-circuit faults. Additionally, the topology is capable of maintaining full output voltage and maximum modulation index in the presence of switch open and short-circuit faults. Moreover, the redundant leg can be employed to share load current with other phase legs to balance thermal stress among semiconductor switches during normal operation. A 25-kW prototype of the novel topology was designed and constructed utilizing 1.2-kV SiC metal-oxide-semiconductor field-effect transistors. Experimental results confirm the anticipated theoretical capabilities of this new three-level converter topology

Medium Voltage Generation System with Five-level NPC Converters for Kite Tidal Power

Offshore power generation has emerged as a prominent source of energy and the installed capacity of new plants has been steadily increasing in recent years. Tidal power specifically is a promising renewable energy source which has not been highly exploited yet, despite its distinctive advantages of being predictable and independent of weather conditions. The main objective of this Licentiate thesis is to analyze and propose solutions for two common problems in offshore power production, which are the power variations due to the non-steady speed profile of the water speed flowing through the turbine and the efficient transportation of the produced power to the shore.The tidal power application utilized in this thesis is the subsea kite, which is a recently developed tidal energy conversion technology that can increase the generated power compared to a traditional static tidal turbine. A turbine is mounted on a submerged kite and the kite moves inside the sea following a predefined trajectory and generating electric power from the tidal currents. The speed and torque of the turbine varies periodically due to the periodic movement of the kite in the sea and, therefore, the control of the generator needs to be able to handle this variable generated power. The kite studied in this thesis has rated active power of 500 kW.In the first part of the thesis, the power generation system of the subsea kite is modelled and the profile of the generated power is extracted given a specific tidal current and turbine geometry. The control of the power converters is described and tested for the specific profile of the generated power. The speed of the generator is controlled by a properly designed Maximum Power Point Tracking algorithm, which ensures that the generator extracts the maximum power from the tidal stream. Experimental verification of the model of this innovative system is also conducted on a laboratory 35 kVA emulator of the tidal power generator.The second part of the thesis deals with the design of a medium voltage generator\ua0drive. The use of medium voltage in the power generation system is highly advantageous for the tidal kite application, since it can reduce the current flowing through the undersea cables connecting the tidal plant to the local grid. Therefore, the size of the cables can be reduced. The drive proposed here uses two 5-level Neutral Point Clamped (NPC) converters connected back-to-back. The 5-level NPC converters can operate with high voltage, while using multiple low-voltagerated power switches. Contrarily, the typical 2-Level converters have limited voltage capability, since they would require more expensive high-voltage-rated power switches. The increased operating voltage of the power conversion system results to lower current and losses in the cables. Another advantage of the NPC converter is the low harmonics at the ac side, which reduces the requirements for passive grid filters. However, the voltage balancing of the dc-link capacitors in this converter topology is a challenge which has not been effectively solved in previous studies. Therefore, a novel voltage balancing strategy is proposed here that uses advanced Space-Vector-Modulation techniques and hardware-based voltage balancing schemes with reduced number of components and lower power losses. Finally, a laboratory prototype of the NPC-converter-based power conversion system is developed with rated power 50 kVA. SiC MOSFETs are used on theconverters to further increase the system’s efficiency and voltage capability.This thesis presents the model, control and laboratory emulator of a kite-based tidal power generator. The experimental set-up can be utilized for conducting research on other renewable sources, such as wind power, that have similar performance. Also, the developed multilevel drive is suitable for various applications where medium voltage grid-connected drives are used and particularly in distributed renewable power generation

Model Predictive Controlled Active NPC Inverter for Voltage Stress Balancing among the Semiconductor Power Switches

© Published under licence by IOP Publishing Ltd. This paper presents a model predictive controlled three-level three-phase active neutral-point-clamped (ANPC) inverter for distributing the voltage stress among the semiconductor power switches as well as balancing the neutral-point voltage. The model predictive control (MPC) concept uses the discrete variables and effectively operates the ANPC inverter by avoiding any linear controller or modulation techniques. A 4.0 kW three-level three-phase ANPC inverter is developed in the MATLAB/Simulink environment to verify the effectiveness of the proposed MPC scheme. The results confirm that the proposed model predictive controlled ANPC inverter equally distributes the voltage across all the semiconductor power switches and provides lowest THD (0.99%) compared with the traditional NPC inverter. Moreover, the neutral-point voltage balancing is accurately maintained by the proposed MPC algorithm. Furthermore, this MPC concept shows the robustness capability against the parameter uncertainties of the system which is also analyzed by MATLAB/Simulink