Advances in Modular and Flying-capacitor Multilevel Converters


December 2022


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In the era of growing awareness and concern for environmental changes, power-electronic engineers are leading the transition away from fossil fuels to renewable energy sources. In general, renewable energy plants, such as wind farms and solar PV farms, are located far from the consumption centers. Therefore, an efficient transfer of energy over long distances or via submerged cables is needed at higher voltages. This was made possible by rapid advancements in high-power converter technology. The voltage source modular multilevel converter(VSC-MMC) is the workhorse of modern HVDC systems. A major portion of this thesis is devoted to improvements in the existing MMC technology. The submodule capacitor is a crucial component in an MMC system that determines its size, weight, and cost. For offshore installations, the footprint of the converter is an important deciding factor. Therefore, the submodule capacitor sizing is pivotal, whose value is determined by the peak-to-peak voltage ripple it handles during converter operation. In this thesis, various techniques were explored to address the issue of voltage ripple in submodule capacitors. A new submodule topology (SHBSM) was proposed to approach this problem from a structural point of view. It has been shown with the help of simulation results that the proposed SHBSM has 34% lower peak-peak ripple compared to the commonly used HBSMs at low switching frequency operation. In addition, the proposed SHBSM has fewer device losses compared to HBSM, making it a strong contender. A control approach was explored to further reduce the submodule capacitor voltage ripple. The higher modulation index(Ma > 1) was investigated in addition to the injection of second-harmonic circulating current. A wide range of modulation indexes(Ma ∈ [0.6, 2]) and SHCC indexes(a ∈ [−1, 2]) were considered to find the optimal combinations. It has been identified that at Ma = 2/√3, a = 1, the ripple is at its minimum, which is one-fifth of the standard operating condition(Ma = 1, a = 0) ripple. In addition to overmodulation and SHCC injection, the third-harmonic common-mode voltage(THV) injection technique was explored for further reduction in the voltage ripple. In the presence of THV injection, Ma = 1.23, a = 0.81 was identified as the overall optimal point which results in the least voltage ripple in the submodule capacitors. At this point, the ripple is a mere 13% of the standard operating condition ripple voltage. Detailed mathematical derivations are provided along with the simulation results to validate the ripple reduction at the identified optimal operating points. Submodule capacitor voltages need to be maintained close to their nominal voltages. In the case of identical and fewer submodules in an MMC arm, a simple PS-PWM technique ensures this voltage balancing. The required mathematical expressions are derived and presented to adapt this technique for different types of submodules with different voltage levels. Moreover, the required modifications to use for overmodulation MMC systems are presented. The other method, which is suitable for energy balancing in a large number of submodules with mismatched capacitance, is the sorting-selecting-based technique. In this thesis, this method was extended to HBSM/FBSM-based hybrid MMCs and DCSM-based MMCs. To black-start an MMC from a completely de-energized state, the submodule capacitors need to be precharged before converter operation. Although various precharging techniques exist in the literature, they have not considered MMC operating in the over-modulation range, which requires precharging of submodule capacitors to a higher voltage value. The precharging technique presented in this thesis addresses this issue. Moreover, precharging of different types of submodules was considered in this thesis, and detailed derivations and implementation for each type were detailed. In addition, to eliminate the need for a large precharging resistor and its corresponding dissipation losses, a modification using a smaller auxiliary source was also presented. With the help of eight different case studies, the proposed precharging technique was explained and validated using simulation and real-time hardware- in-loop results. Flying-capacitor multilevel converter(FCMC) is another topology that was focused on in this thesis. Although this multilevel converter was first proposed almost four decades ago, with the improvement in the high switching frequency capabilities of wide-band-pap devices, it is grabbing the attention of researchers once again. At a high switching frequency, the power density of FCMC is very high because of the smaller size requirement for flying capacitors. However, this topology suffers from the precharging issue. All its flying capacitors need to be charged to appropriate voltage levels before the converter operates. The conventional PS-PWM technique, which is popular for its natural voltage balancing capability during steady-state operation, fails to precharge all its flying capacitors in a single-phase odd-level FCMC. This issue was resolved with the help of the proposed mPS-PWM. Firstly, the reason for PS-PWM failure is investigated with the help of the state-space averaged model derived for 5L-FCMC. Later, the rationale behind the successful precharging using the mPS-PWM is explained mathematically and validated using extensive simulation analysis. With the help of a 9L-FCMC hardware prototype designed and built in the lab, the proposed mPS-PWM is validated for 5L, 7L, and 9L-FCMCs by demonstrating successful precharging. Later, the proposed mPS-PWM technique was extended to three-phase odd-level FCMCs.



Engineering, Electronics and Electrical