New modulation scheme for bidirectional quasi z-source modular multilevel converters

This paper proposes a dedicated modulation scheme for a bidirectional quasi Z-source modular multilevel converter (BqZS-MMC). The operation principle and a suitable PWM method are proposed. The relation between the modulation index and shoot-through duty ratio is derived. A formula for calculating the required value of quasi Z-source capacitance is given. The simulation results presented in the paper validate the operation and the performance of the proposed topology

Experimental Validation of a quasi Z-Source Modular Multilevel Converter with DC Fault Blocking Capability

This paper considers the design methodology and the modulation of the quasi Z-source modular multilevel converter (qZS-MMC) with half bridge sub-modules and evaluates its performance in voltage boosting mode for medium voltage applications. The qZS-MMC consists of two quasi Z-source networks inserted between the two terminals of the DC input source and the DC-link terminals of a modular multilevel converter (MMC), which allows the generation of an output voltage larger than the input DC voltage. Two modulation schemes have been analysed based on a mathematical derivation for the converter internal voltages, currents, and stored energy. The quasi Z-source circuit is proven to provide the qZS-MMC with half bridge sub-modules to deal with DC-faults. The experimental results validate the performance of the proposed modulation schemes and the DC-fault blocking capability of the qZS MMC. Finally, the losses of the qZS-MMC is compared against a standard MMC using full bridge sub-modules that can also provide DC fault capability. The range in which the qZS-MMC is more efficient has been identified. Furthermore, the qZS-MMC can provide a significant reduction in number of semiconductor power devices with the same performance

New modulation scheme for bidirectional qZS modular multi-level converters

This study proposes a dedicated modulation scheme for a bidirectional quasi-Z-source (qZS) modular multi-level converter. The operation principle and a suitable pulse-width modulation method are proposed. The relation between the modulation index and shoot-through duty ratio is derived. A formula for calculating the required value of qZS capacitance is given. The simulation results presented in the study validate the operation and the performance of the proposed topology

New modulation scheme for bidirectional quasi z-source modular multilevel converters

This paper proposes a dedicated modulation scheme for a bidirectional quasi Z-source modular multilevel converter (BqZS-MMC). The operation principle and a suitable PWM method are proposed. The relation between the modulation index and shoot-through duty ratio is derived. A formula for calculating the required value of quasi Z-source capacitance is given. The simulation results presented in the paper validate the operation and the performance of the proposed topology

Modeling and Experimental Evaluation of Z-Source Modular Multilevel Converter Using Reduced Inserted Cells Technique

The integration of a Z-source network with a modular multilevel converter (MMC) to provide voltage step-up function is proposed in this paper. The proposed Z-source modular multilevel converter (ZS-MMC) uses a Z-source network connected between the DC source and the DC-link terminals of the MMC. The operation principle of the ZS-MMC is presented utilising the reduced inserted cells (RICs) modulation technique. Compared to the quasi ZS-MMC previously developed by the authors, the ZS-MMC has small fundamental frequency component in the ZS inductor current which requires a smaller inductor size for the Z-source network and is also more efficient. The ZS-MMC is compared to other topologies that can accomplish buck and boost capabilities, such as the quasi Z-source MMC, the quasi Z-source cascaded multilevel converter and the full-bridge based MMC, to validate the viability of the proposed converter. The operation of the ZS-MMC employing the RICs technique is confirmed experimentally

Analysis, design and evaluation of quasi Z-Source modular multilevel converter

The deployment of renewable energy production capability needs to increase significantly in the near future to be able to meaningfully reduce emissions caused by current electrical power generation. Also, the electrical energy and power generation will have to increase due to the need to decarbonize the transport which currently runs on fossil fuels. Recently, newly installed renewable energy production capabilities such as wind turbines and photovoltaics outpaced newly built fossil fuels generation capabilities. The voltage generated by these renewable energy sources often fluctuates in accordance with the weather conditions. Therefore, power converters are required to regulate the output voltage and maximize the available generated power. Technically speaking, available power converter solutions with both voltage step down and step-up capability are indispensable to maximise the capturing of renewable power. For this reason, the development of novel power converter topologies that could bring additional advantages to existing solutions is still under the scope. A new breed of multilevel converters, which is modular multilevel converter (MMC) which is able to be connected directly in medium and high voltage DC and AC networks, has been proposed and gained much attention. The MMC based on half-bridge sub-modules (HBSMs) is the simplest configuration but provides only voltage step-down conversion which is not sufficient for effective interfacing with many renewable energy sources. This thesis proposes the integration of the impedance networks especially quasi Z-source (qZS) network with the HBSM based modular multilevel converter (MMC). This integration has not previously been reported. The proposed quasi Z source modular multilevel converter (qZS-MMC) has the advantages of not only performing the commonly used voltage step down function but also voltage step up function. In addition, it was found that the proposed converter has an inherent capability to block the fault current during a DC side fault. The proposed qZS-MMC consists of two quasi Z-source networks inserted between the terminals of the input DC source and the DC-link terminals of a modular multilevel converter. The operation of the qZS-network requires the introduction of a short circuit at its output terminals in order to increase the energy stored in the qZS network inductors. This increase in the stored energy provides the converter’s voltage boosting capability. To provide the short circuit current path, each qZS-network is connected to a chain-link of series connected switches at its end terminals. By gating only one of these chain-links, the generated output voltage will be highly distorted. Therefore, two modulation techniques named simultaneously shorted (SS) and reduced inserted cells (RICs), are proposed to avoid any distortion in the output voltage. These techniques are analysed and compared based on a mathematical derivation for the converter internal voltages and currents. Based on these, guidelines for sizing the different passive components and a procedure for estimating the semiconductor losses are presented for each modulation technique. The DC-fault blocking capability of the proposed converter is investigated, where the converter behavior is illustrated for pole-to-pole and pole-to-ground DC side faults. The proposed qZS-MMC is compared with the full-bridge based MMC (FB-MMC) and quasi Z-source cascaded multilevel converter (qZS CMI) in order to validate its feasibility. The comparison accounts for the required number of the passive and active components, voltage and current stresses, the semiconductor power losses and output voltage waveform quality. The performance of the proposed converter is evaluated through simulation using PLECs software and different test cases were considered. The simulation results demonstrate the ability of the converter to perform buck-boost operation using the proposed modulation techniques and to block the DC-fault current. A laboratory scaled prototype is built and is used to experimentally validate the operation of the converter and the DC-fault blocking capability

Analysis, design and evaluation of quasi Z-Source modular multilevel converter

The deployment of renewable energy production capability needs to increase significantly in the near future to be able to meaningfully reduce emissions caused by current electrical power generation. Also, the electrical energy and power generation will have to increase due to the need to decarbonize the transport which currently runs on fossil fuels. Recently, newly installed renewable energy production capabilities such as wind turbines and photovoltaics outpaced newly built fossil fuels generation capabilities. The voltage generated by these renewable energy sources often fluctuates in accordance with the weather conditions. Therefore, power converters are required to regulate the output voltage and maximize the available generated power. Technically speaking, available power converter solutions with both voltage step down and step-up capability are indispensable to maximise the capturing of renewable power. For this reason, the development of novel power converter topologies that could bring additional advantages to existing solutions is still under the scope. A new breed of multilevel converters, which is modular multilevel converter (MMC) which is able to be connected directly in medium and high voltage DC and AC networks, has been proposed and gained much attention. The MMC based on half-bridge sub-modules (HBSMs) is the simplest configuration but provides only voltage step-down conversion which is not sufficient for effective interfacing with many renewable energy sources. This thesis proposes the integration of the impedance networks especially quasi Z-source (qZS) network with the HBSM based modular multilevel converter (MMC). This integration has not previously been reported. The proposed quasi Z source modular multilevel converter (qZS-MMC) has the advantages of not only performing the commonly used voltage step down function but also voltage step up function. In addition, it was found that the proposed converter has an inherent capability to block the fault current during a DC side fault. The proposed qZS-MMC consists of two quasi Z-source networks inserted between the terminals of the input DC source and the DC-link terminals of a modular multilevel converter. The operation of the qZS-network requires the introduction of a short circuit at its output terminals in order to increase the energy stored in the qZS network inductors. This increase in the stored energy provides the converter’s voltage boosting capability. To provide the short circuit current path, each qZS-network is connected to a chain-link of series connected switches at its end terminals. By gating only one of these chain-links, the generated output voltage will be highly distorted. Therefore, two modulation techniques named simultaneously shorted (SS) and reduced inserted cells (RICs), are proposed to avoid any distortion in the output voltage. These techniques are analysed and compared based on a mathematical derivation for the converter internal voltages and currents. Based on these, guidelines for sizing the different passive components and a procedure for estimating the semiconductor losses are presented for each modulation technique. The DC-fault blocking capability of the proposed converter is investigated, where the converter behavior is illustrated for pole-to-pole and pole-to-ground DC side faults. The proposed qZS-MMC is compared with the full-bridge based MMC (FB-MMC) and quasi Z-source cascaded multilevel converter (qZS CMI) in order to validate its feasibility. The comparison accounts for the required number of the passive and active components, voltage and current stresses, the semiconductor power losses and output voltage waveform quality. The performance of the proposed converter is evaluated through simulation using PLECs software and different test cases were considered. The simulation results demonstrate the ability of the converter to perform buck-boost operation using the proposed modulation techniques and to block the DC-fault current. A laboratory scaled prototype is built and is used to experimentally validate the operation of the converter and the DC-fault blocking capability