An integrated generator-rectifier system for high-power AC-DC conversion

High power ac-dc conversion plays an increasingly important role in energy conversion systems, such as wind turbines or electric ship grids. Conventional conversion architectures rely heavily on active rectifiers, which consist of fully-controlled power-electronics switches. These make the system bulky, lossy, and less reliable. This dissertation presents an alternative approach: integrating a multi-port permanent magnet synchronous generator (PMSG) with series-stacked power converters to create an integrated generator-rectifier system. An active rectifier process only a fraction of the total converted power while regulating the dc bus. Diode bridges process the remaining power, allowing substantial increases in overall power density, efficiency, and reliability. Filter capacitors are commonly connected to the output of passive rectifiers to reduce the dc-bus voltage ripple. These filters are the main contributor to the overall system size, weight, cost, and failure, as well as to the low power factor at the ac ports powering the passive rectifiers. These capacitors can be eliminated by implementing an appropriate phase shift between different ac ports. Alternatively, the filtering function can be integrated to the active rectifier through active control. A voltage opposite the passive-rectifier ripple component is synthesized at the active-rectifier dc-side by modulating the ac-side current. Compensation occurs due to the series connection of the rectifier dc outputs. Deployment of the integrated generator-rectifier systems in wind-energy applications requires maximum power point tracking (MPPT) capability, which seems to be challenging due to the presence of numerous uncontrolled passive rectifiers. Due to the series connection and a constant dc-bus voltage, the dc-side current of the active rectifier sets the output power of the passive rectifiers, and consequently the total generator output power. The dc-side current is dependent on the $d$-axis current, which can be commanded to follow a reference value. This reasoning lays the foundation for MPPT using the integrated generator-rectifier system. Alternatively, the $d$-axis current can be used to regulate the dc-bus voltage, opening up opportunities in dc grid-forming applications. Elimination of capacitors at the diode rectifiers’ output by appropriately phase-shifting the voltages of a multi-port generator further improves overall architecture's reliability. A generalized framework is developed to evaluate the interactions among the different generator ports, diode-bridge rectifiers, and the active rectifier used to control the power flow. This framework allows quantifying the effect of integration on the dc bus power ripple and power imbalance among different generator ports. An exemplary winding layout is proposed that ensures theoretically zero interaction between the passive ports though all the ports are mounted on a magnetic structure. Furthermore, the framework provides the guideline for generator designs to ensure successful integration with the rectifier system. Finally, per-unit generator inductance is shown to be the handshake parameter between the generator and the rectifiers. The generator and power electronics designs are paired together to form feasible systems. Annual energy production calculation based on multiple wind profiles shows higher energy yield by the integrated system than the conventional solutions. This result proves the integrated generator-rectifier system's suitability for offshore wind energy harvesting