Abstract
THIS dissertation addresses the technical challenges associated with the operation and control of high-power modular multilevel converters. To improve the performance of modular multilevel converter (MMC), a generalized three-phase mathematical model with common-mode voltage (CMV) is proposed. By using the proposed mathematical model, the magnitude of circulating currents, capacitors voltage ripple, and the ripple in DC-link current during balanced and unbalanced operating conditions can be minimized. The modulation scheme and switching frequency are directly affected the output power quality and the performance of the converter and control method. In this dissertation, a novel sampled average and space vector modulation scheme is proposed. These modulation schemes are suitable to control the MMC with any number of submodules (without modifications), operates at low switching frequency, minimizes the ripple in output current and voltage harmonic distortion, and reduces the output filter size. For reliable operation of MMC, the voltage balancing among submodules is mandatory. This dissertation proposes a generalized single-stage balancing approach with reduced current sensors to control the MMC. The proposed balancing approach is suitable to implement with both phase-shifted and level-shifted pulse width modulation schemes. With the proposed approach, it is also possible to control the MMC with half-bridge and three level flying capacitor submodules. Also, an improved balancing approach often referred as the dual-stage balancing approach is proposed to minimize the voltage harmonic distortion and device power losses. This dissertation also proposes a direct model predictive control (D-MPC) approach to minimize the ripple in submodule capacitors voltage. To implement D-MPC approach, a discrete-time model of MMC with CMV is proposed. With the use of proposed model, the D-MPC approach does not require a cost function to minimize the circulating currents. The computational complexity is one of the major issues in the implementation of D-MPC approach for MMC. In this dissertation, a novel reduced computational MPC approaches named as dual-stage D-MPC and indirect model predictive control (I-MPC) approach are proposed. These approaches significantly minimize the computational complexity and, improve the voltage and current waveform quality while operating at the low switching frequency. Finally, the simulation and experimental studies are presented to validate the dynamic and steady-state performance of proposed methodologies. Index Terms • Modular Multilevel Converters. • Capacitors Voltage Balancing. • Pulse Width Modulation Schemes. • Circulating Currents. • Capacitors Voltage Ripple • Direct Model Predictive Control. • Dual-Stage Direct Model Predictive Control. • Indirect Model Predictive Control. • Total Harmonic Distortion.
Highlights
POWER converters are well known in industry and academia as one of the preferred choices for efficient power conversion systems [1]
The cascaded H-bridge (CHB) and cascaded neutral-point clamped (CNPC) converters are constructed with a cascade connection of low-power submodules with an isolated DC source in each phase
The results show that the LSC-pulse width modulation (PWM) scheme produces a large ripple in submodule capacitors voltage and large magnitude of circulating currents compared with the phase-shifted carrier pulse width modulation (PSC-PWM) scheme [52–54]
Summary
Superscript p Predicted quantity m Measured quantity k Submodule index number n Normalized quantity Extrapolated quantity ∗ Reference quantity → Vector quantity. Subscript h x ∈ {a, b, c} y ∈ {u, l} z e t dev d tc ton tof f dc dr Capacitor index number AC output quantities Arm side quantities Circulating current component Estimated quantity total value Device quantity Diode quantity Transistor conduction state Transistor on-state Transistor off-state Diode conduction state Diode recovery state xxvi. System Quantities ma Amplitude modulation index fo Fundamental frequency (Hz) fc Carrier frequency (Hz) fsw Switching frequency (Hz). Φ Phase angle between phases (rad) φc Phase angle between carriers (deg) φci Interleave angle between carriers (deg)
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