Inverter Control Strategies and High Power-Density Implementation

  • Author / Creator
    Ebrahimi, Mohammad
  • As the key components of ac distributed generation (DG) units, inverters significantly affect the reliability, efficiency, and power quality of the system. DG inverters are required to i) operate in both grid-connected (GC) and standalone (SA) and seamlessly transfer between these modes, ii) support the grid during transients, and iii) provide advanced grid functionalities such as fault ride through and reactive power regulation. Addressing all of these requirements while achieving the desired efficiency and power density is the focus of this thesis. New control structures for fast and robust operation of inverters are proposed in this thesis. A synchronous (or DQ) reference frame controller to regulate the output power of single-phase grid-connected inverters is proposed. Improved characteristics of the proposed controller provide advanced grid functionalities for the inverter. To further improve the inverters' dynamic performance, hysteresis and peak current controllers are taken into consideration. A fixed switching frequency peak current controller is proposed to mimic the switching pattern of pulse width modulation (PWM) schemes. The proposed controller maintains the PWM switching harmonic spectrum at the inverter's output. Current controlled inverters normally do not provide grid supporting features and they are known as a source of instability in weak grid conditions. Therefore, virtual synchronous machines (VSM) have been investigated to address such concerns. A VSM controller with an improved damping is proposed in this thesis. Implementation of a VSM controller for double stage inverters with limited storage or dc bus capacitor is also studied. Replacing Electrolytic capacitors with durable Film types, improves reliability of a single-phase inverter at the expense of degrading its power density. A method to compensate the double-frequency ripple for single-phase double-stage inverters is proposed that optimizes the size of the required Film capacitor and further improve the power density. Using this method, a 5kVA single-phase inverter with effective switching frequency of 240kHz is implemented based on Gallium Nitride (GaN) power switches. This inverter does not require any cooling fans, which further improve its reliability. The proposed controllers are validated on this inverter and details of this prototype are also presented in this thesis.

  • Subjects / Keywords
  • Graduation date
    Spring 2020
  • Type of Item
  • Degree
    Doctor of Philosophy
  • DOI
  • License
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