Power loss management for modular multilevel converters

• Main Author: Qiu, Huan
• Other Authors: Tang Yi
• Format: Thesis-Doctor of Philosophy
• Language: English
• Published: Nanyang Technological University 2021
• Subjects: Engineering::Electrical and electronic engineering::Power electronics
• Online Access: https://hdl.handle.net/10356/152413
• Institution: Nanyang Technological University

Due to the increasing requirements for reduction of carbon footprint and energy conservation, more voltage source converters (VSCs) are implemented in modern power systems, such as the interface to integrate renewable energy sources (RES), and variable speed electric motor drives. Also, in the applications such as high voltage direct current (HVDC) systems and flexible alternating current transmission systems (FACTS), VSCs are widely utilized on account of the superior features of the VSCs, such as independent active and reactive power control, unity power factor operation, and fast dynamic response due to high switching frequency which are attractive and necessary in these applications. Initially, the prevailing VSC topology is the two-level converter which is still preferred in low voltage applications. For medium/high voltage and high-power applications, two-level converters are not suitable because of the limitations of power electronic devices. Thus, a transition to multilevel converters is necessary to meet the medium-/high-voltage and high-power power conversion demands. There are many promising commercialized multilevel converter topologies implemented in power systems, such as Neutral Point Clamped (NPC) Converters, Cascaded H-bridge (CHB) Converters, and Modular Multilevel Converters (MMCs). Among them, the MMC is the most attractive solution due to its prominent features, e.g. modularity, scalability, transform-less configuration, superior harmonic performance, low power device stress, and high reliability from redundancy. However, one of the major burdens for the wide application of the MMC is its reliability/lifetime. The most vulnerable component in MMC is the semiconductor switching device which has a high failure rate due to the severe thermal stresses caused by the large power losses. MMC utilizes more semiconductor switching devices than other topologies, leading to a considerable amount of power losses which will reduce the efficiency and result in reduced reliability/lifetime of the system. Thus, the power loss management for MMCs is indispensable to improve the system reliability and efficiency for further lifetime extension and application of MMCs, which is the focus of this thesis. There is a significant uneven loss distribution between the top and bottom semiconductor devices within one submodule (SM) of the MMC owing to the DC component in the arm currents. The stress difference caused by the uneven loss distribution between top and bottom semiconductor modules will lead to the lifetime difference of the semiconductor devices which shortens the lifetime of the whole MMC system. To extend the lifetime of MMCs without modification of system hardware, a device-level loss balancing control method is proposed to reduce the large loss unbalance within each SM in this thesis. In the proposed method, the accurate device loss models are developed to feedback real-time device losses into the loss balancing controller. The controller (by regulating the SM capacitor voltages) adjusts the semiconductor conducting duty cycles to even the loss distribution without affecting the performance of AC output and capacitor voltage balancing. The loss distributions between the upper arm and the lower arm of one phase leg in the MMC differs from each other under asymmetric arm impedance condition. It will result in a very large difference in thermal stresses among SMs in the upper arm and the SMs in the lower arm causing a large variance of arm lifetime. To even the loss distribution under asymmetric arm impedance conditions, an arm-level loss balancing control is proposed in this thesis. The averaged power losses of upper arm SMs and lower arm SMs are added into the control loop to balance the losses between the upper arm and the lower arm. Moreover, the large fundamental-frequency harmonic in the circulating current caused by the asymmetric arm impedance is well suppressed, and the divergence between the capacitor voltages of the SMs in the upper arm and those of the SMs in the lower arms are eliminated with the proposed control method. The discontinuous pulse width modulation (DPWM) is advantageous in terms of efficiency, which can reduce the switching loss and thus increasing the system reliability. A generalized DPWM with on-line optimization ability is proposed in this thesis and its principle is discussed in detail. The generalized DPWM is capable to produce various zero-sequence components according to different requirements with two control variables. Moreover, the switching loss model of the MMC is developed, based on which the optimized zero-sequence component for the MMC with the generalized DPWM to minimize the switching loss is identified. The circulating current control signals and the voltage balancing control signals are considered in the generalized DPWM when generating the zero-sequence component so that no additional SMs are needed to ensure the normal operation of the MMC. Furthermore, the impact of the injection of the zero-sequence component in the MMC on the circulating current is analyzed and a modified circulating current controller is proposed to further suppress the circulating current. The efficiency and the reliability of the MMC can be greatly improved with the proposed DPWM. This thesis consists of three parts. The first part introduces the basic structure and operation principles of MMCs. Also, a comprehensive steady-state analysis of the MMC is conducted and the mathematical model based on the equivalent circuits of the MMC is developed. Based on the analysis and model of the MMC, the control strategies for the MMC are introduced. The commonly used cascaded control method for MMCs, which includes output current control, capacitor voltage control, and the circulating current suppression control, is discussed in this part. The general modulation techniques for MMCs have been briefly summarized in the first part, and the commonly used phase-shifted-carrier pulse-width-modulation (PSC-PWM) is reviewed in detail and utilized in this thesis. The second part investigates the three aforementioned power loss issues. The effectiveness and the feasibility of the proposed methods are validated on a scale-down single-phase MMC prototype and the experimental results are provided in this part. The last part concludes the thesis and recommends some future works.