Abstract
Modern distributed generation (DG)-grid interfacing voltage source converters (VSCs) are required to assure system stability under different grid conditions, while high-order output filters, such as LCL filter, are used to minimize their physical size. They are necessary to deal with many intrinsic and extrinsic challenges, such as oscillation due to filter resonance, uncertainty of the grid condition, disturbances at the point of common coupling (PCC), etc. This thesis focuses on finding advanced digital control strategies, exhibiting high operational stability under wide parametric variations and superior dynamic behaviors, for DG-grid interfacing converters with high-order output filter.Among many prior arts in meeting the abovementioned requirements, active damping techniques are widely preferred to mitigate the filter resonance and ensure the system stability. To provide a more insightful understanding of the active damping characteristics, a capacitor-voltage-feedforward-based active damping mechanism is investigated to mitigate both the filter resonance and harmonic distortion of the grid current. Many performance and design tradeoffs are found to be considered in the controller design.
With the recent advances in microelectronics and digital controllers, digital nonlinear control methods would be favorable in exhibiting high operational stability under wide parametric variations for DG-grid interfacing converters with high-order output filters. This thesis formulates the small-signal model of the fast boundary control with second-order switching surface (BC2) and studies its characteristics. It is found that the BC2 eliminates the filter resonance in the transfer characteristics. Accordingly, a BC2 cascaded with a deadbeat controller (DBC) is attempted in controlling single-phase grid-connected inverter with LCL filter. Such a cascaded BC2-DBC control scheme eliminates the filter resonance and advances the performance characteristics of the inverter under different grid conditions. It is also extended to a virtually-grounded three-phase four-wire grid-connected inverter and modified with an intra-cycle information recovery mechanism and reduced current sensors. Moreover, switching saturation and the output current distortion caused by the potential dc bus splitting voltage unbalance are avoided by using the average intra-cycle information of the duty cycle of the switching network.
A cascaded BC2-DBC controller can ensure system stability for single-phase grid-connected inverter with LCL filter operating under different grid conditions. However, apart from requiring numerous sensors and being challenged by unbalanced filter parameters, the system will also be in variable switching frequency operation, due to coupling circuit variables amongst phases, when controlling a three-phase three-wire inverter. Thus, a boundary controller that utilizes second-order switching surface to directly track the output current of the three-phase three-wire grid-connected inverter with an LCL filter is proposed. By applying the 60-degree (60ยบ) discontinuous pulse-width modulation (DPWM) scheme for a fictitious decoupled dual-buck structure in each operation sector, two separate sets of switching criteria with reduced number of current sensors, fixed frequency operation, and indirect filter capacitor current feedforward for dictating the states of the switches of two half-bridge legs are formulated. This technique can avoid dealing with the challenges caused by the interactions among three independent current regulators.
| Date of Award | 23 Dec 2016 |
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| Original language | English |
| Awarding Institution |
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| Supervisor | Shu Hung Henry CHUNG (Supervisor) |
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