PWM and Control Strategies for Modular CSC based High Power Application

  • Author / Creator
    Ding, Li
  • Current source converter (CSC) enjoys the features such as four quadrant operation, low dv/dt outputs, inherent short circuit protection as well as voltage boost capacity, which made it widely adopted in high voltage direct current (HVDC) system, industry high power medium voltage (MV) motor drives and sustainable energy conversations such as photovoltaic (PV) and wind turbine. Modular CSC has the benefits of improved reliability, extended power range as well as better harmonic performance, which attracted increased attentions these years. The parallel CSC topologies can be divided into two categories: independent DC-link and shared DC-link. One of the major concerns for CSC-fed MV drives is the common-mode voltage (CMV), which can cause motor winding isolation failure. To suppress the CMV, transformerless configuration with integrated common-mode (CM) and differential choke can effectively reduce the system size and cost compared with the bulky isolation transformer. Improved modulation strategies with CMV reduction is desired to further reduce the CM choke size. Another essential issue is the circulating current (CC) flowing through the parallel modules. Moreover, the DC current balance is the priority task for shared DC-link structure since only the total DC current can be regulated. For series CSC, the DC voltages of each module are added up and DC voltage quality improvement is the main target. Therefore, the modulation strategies should be designed specifically for different modular structures. Meanwhile, the low switching frequency of CSC based high power application puts great challenge for the controller and observer design.
    The purposes of this research work are to develop the improved modulation strategies as well as the control methods to deal with these challenges. The space vector modulation (SVM) based CMV average value reduction (AVR) methods are proposed for independent DC-link parallel configuration to suppress the CMV and CC while achieving multilevel output. A multilevel SVM based DC current balance and CMV reduction method is proposed for shared DC-link parallel structure to simultaneously balance the DC current and suppress CMV. To achieve easy-implement for N-CSC (N≥3) system, an interleaved carrier-based sinusoidal pulse-width modulation (SPWM) is proposed and it can be further extended into mixed series-parallel CSC system, where better AC current and DC voltage can be achieved simultaneously. Besides, a typical CSC-fed drive application with long cable application: electrical submersible pump (ESP) is also studied. A multi-loop controller is developed to improve the system dynamic performance and attenuate the LC resonance. Moreover, an adaptive discrete-time sliding mode observer (SMO) based sensorless method is proposed to estimate the rotor speed and position. Such modulation and control development can effectively overcome the aforementioned challenges and guarantee good performance for practical applications.

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