A New Modular Marx Derived Multilevel Converter

A new Modular Marx Multilevel Converter, M(3)C, is presented. The M(3)C topology was developed based on the Marx Generator concept and can contribute to technological innovation for sustainability by enabling wind energy off-shore modular multilevel power switching converters with an arbitrary number of levels. This paper solves both the DC capacitor voltage balancing problem and modularity problems of multilevel converters, using a modified cell of a solid-state Marx modulator, previously developed by authors for high voltage pulsed power applications. The paper details the structure and operation of the M(3)C modules, and their assembling to obtain multilevel converters. Sliding mode control is applied to a M(3)C leg and the vector leading to automatic capacitor voltage equalization is chosen. Simulation results are presented to show the effectiveness of the proposed M(3)C topology

A Compact DC-DC Converter for Offshore Wind Farm Application

A DC-DC converter suitable for the grid integration of windfarms through a DC grid is presented. The operation is based on the Marx principle where charged capacitors are connected in series and parallel in turn to achieve the voltage transformation. The two inductors at the two ends of the converter are designed to resonate with the capacitors to create resonance forcing current zeros to enable zero current switching thereby reducing switching losses. The design of a 50 MW, 6kV/30kV DC-DC converter was carried out by analysis and simulatio

Bidirectional Marx DC-DC Converter for Offshore Wind Farm Application

The bidirectional DC-DC converter explained in this paper is based on the Marx principle, and is capable of achieving step-up and step-down voltage transformations at kV level and is able to handle MW level power transfers in both directions. The main features of this topology is the absence of a high frequency transformer, reduced weight, volume, and soft switching to reduce the switching losses. In the boost mode, five capacitors are charged in parallel and discharged in series to achieve the step-up action, and in the buck mode the converse action takes place. The operating principle is explained, and the steady-state analysis of the converter is given. Matlab/Simulink simulation of a 50MW converter, interfacing 6kV, and 30kV systems supports and validates the theoretical analysis, and enables positive supporting conclusions to be made

High-voltage pulse generators incorporating modular multilevel converter sub-modules

Recent research established the effectiveness of applying a pulsed electric field to deactivate harmful microorganisms (such as bacteria and E. coli). Successful deactivation is achieved by lethal electroporation; a process that produces electric pores in the biological cell membrane of the harmful microorganisms when subjected to high-voltage (HV) pulses. The HV pulses are designed to create pores beyond a critical size at which the biological cell can reseal.;In contrast when applying non-lethal electroporation, the cell-membrane survives after the electroporation process. This is required, for example, when inserting protein cells in the cell-membrane. In both lethal and non-lethal electroporation, HV pulses in the kilo-Volt range (1-100 kV) with durations ranging between nanoseconds and milliseconds are required.;This thesis proposes nine pulse generator (PG) topologies based on power electronic devices and modular multilevel converter sub-modules. The proposed topologies are divided into two main groups namely: PGs fed from a HV DC supply and PGs fed from an LV DC supply. The first group presents a new family of HV DC fed topologies that improve the performance of existing HV DC fed PGs, such as flexible pulse-waveform generation and full utilisation of the DC link voltage.;The second group is dedicated to a new family of LV DC fed PG topologies which have flexible pulse-waveform generation, controlled operation efficiency, and high voltage gain.;All the proposed PG topologies share the important aspect in the newly developed HV PGs, that is modularity, which offers redundancy and robust pulse generation operation.;The presented PG topologies are supported by theoretical analysis, simulations, and experimentation.Recent research established the effectiveness of applying a pulsed electric field to deactivate harmful microorganisms (such as bacteria and E. coli). Successful deactivation is achieved by lethal electroporation; a process that produces electric pores in the biological cell membrane of the harmful microorganisms when subjected to high-voltage (HV) pulses. The HV pulses are designed to create pores beyond a critical size at which the biological cell can reseal.;In contrast when applying non-lethal electroporation, the cell-membrane survives after the electroporation process. This is required, for example, when inserting protein cells in the cell-membrane. In both lethal and non-lethal electroporation, HV pulses in the kilo-Volt range (1-100 kV) with durations ranging between nanoseconds and milliseconds are required.;This thesis proposes nine pulse generator (PG) topologies based on power electronic devices and modular multilevel converter sub-modules. The proposed topologies are divided into two main groups namely: PGs fed from a HV DC supply and PGs fed from an LV DC supply. The first group presents a new family of HV DC fed topologies that improve the performance of existing HV DC fed PGs, such as flexible pulse-waveform generation and full utilisation of the DC link voltage.;The second group is dedicated to a new family of LV DC fed PG topologies which have flexible pulse-waveform generation, controlled operation efficiency, and high voltage gain.;All the proposed PG topologies share the important aspect in the newly developed HV PGs, that is modularity, which offers redundancy and robust pulse generation operation.;The presented PG topologies are supported by theoretical analysis, simulations, and experimentation

DC-DC CONVERTER FOR POWER COLLECTION IN WIND FARMS

Offshore wind farms have grown rapidly in number in recent years. Several large-scale offshore wind farms are planned to be built at further than 100 km from the United Kingdom coast. While high-voltage high-power installations have addressed the technical issues associated with reactive power flow in AC transmission, reactive power can be avoided by using High-Voltage Direct Current transmission (HVDC). Reactive power causes problems when transmission distances are long, therefore, HVDC transmission is now being considered for wind farm grid connection. However, as wind farms constitute weak systems Line Commutated Converter (LCC) based HVDC is not viable and newer Modular Multilevel Converter (MMC) based Voltage Source Converters (VSC) are needed for the AC-DC conversion. One of the key components in such systems is the DC-DC converter, which is required to act as the interface between the generation, transmission, and distribution voltage levels, and reduces the power conversion stages, avoiding transformers typically used in AC grid integration systems. In addition, there is no high-power Medium-Voltage MV DC-DC converter available for offshore wind farm energy systems at present. The specification requirements of high-power MV DC-DC converters can be set once the output characteristics of the wind turbine generators have been reviewed. An offshore wind farm with MVDC-grid collection does not exist today, but it is a promising alternative, although specification analysis of high power MV DC-DC converters is necessary. The work reported in this thesis aims to introduce two types of high power MV DC-DC converter topologies, for offshore wind farm energy systems, termed single-stage, and multi-stage converters. Ways of reducing losses by soft switching and reduction in the number of components are considered. Both topologies are based on the Marx principle where capacitors are charged in parallel and discharged in series to achieve the step-up voltage transformation. During doldrums, light and calm wind, and for maintenance work, it is necessary to supply the offshore wind farm with auxiliary power. This thesis proposes a novel Bidirectional Modular DC-DC converter (BMDC) and evaluates its performance. The simulation results show that the proposed BMDC allows up to 5% of the wind farm’s power rating to be drown from the onshore substation. This means that the proposed DC-DC converter is capable to provide bidirectional power flow. For offshore wind farm application, BMDC can be inserted between the offshore wind farm and onshore substation. The studies, in this thesis, are based on an input DC collection at 6 kV with the DC to DC converter stepping up the voltage to 30 kV. The proposed system is integrated and simulated with the DC offshore wind farm and a Voltage Source Converter (VSC) in the onshore station. The steady-state simulation results, to transmit the power between two different voltage levels, and the dynamic performance of the proposed converter were investigated. The advantages of the proposed converter include its simple design and that it does not require an AC transformer; hence can easily be implemented in an offshore wind farm since it requires less weight and size on the platform in the sea, which ultimately results in minimal cost. Furthermore, the proposed converter can ride through a fault which complies with the UK Grid code. However, in this case, it is necessary to provide protection systems such as a large chopper resistor for energy absorption or de-loading the wind turbine. Finally, the proposed integrated BMDC converter showed its suitability for offshore wind farms as well as improving their reliability

High voltage cascaded step-up DC-DC Marx converter for offshore wind energy systems

This paper presents an improved cascaded DC-DC resonant converter for offshore windfarms. The improvements are reduced losses and the number of components. The topology is based on the Marx principle where charged capacitors are charged in parallel and discharged in series to achieve the voltage transformation. The four inductors of the converter are designed to resonate with the capacitors to create resonance forcing current zeros to enable zero current switching thereby reducing switching losses. The operating principles and design considerations of the proposed converter are discussed and the design equations are presented. In order to evaluate the operation of 50 MW converter aimed at connecting a 30 kV DC Busbar in a wind power collection system to a 360 kV high voltage DC bus for transmission to the onshore grid was simulated and the results are presente

A Compact DC-DC Converter for Offshore Wind Farm Application

A DC-DC converter suitable for the grid integration of windfarms through a DC grid is presented. The operation is based on the Marx principle where charged capacitors are connected in series and parallel in turn to achieve the voltage transformation. The two inductors at the two ends of the converter are designed to resonate with the capacitors to create resonance forcing current zeros to enable zero current switching thereby reducing switching losses. The design of a 50 MW, 6kV/30kV DC-DC converter was carried out by analysis and simulatio

Cascaded Converters For Integration And Management Of Grid Level Energy Storage Systems

ABSTRACT CASCADED CONVERTERS FOR INTEGRATION AND MANAGEMENT OF GRID-LEVEL ENERGY STORAGE SYSTEMS by ZUHAIR ALAAS December 2017 Advisor: Dr. Caisheng Wang Major: ELECTRICAL ENGINEERING Degree: Doctor of Philosophy This research work proposes two cascaded multilevel inverter structures for BESS. The gating and switching control of switching devices in both inverter typologies are done by using a phase-shifted PWM scheme. The first proposed isolated multilevel inverter is made up of three-phase six-switch inverter blocks with a reduced number of power components compared with traditional isolated CHB. The suggested isolated converter has only one battery string for three-phase system that can be used for high voltage and high power applications such as grid connected BESS and alternative energy systems. The isolated inverter enables dq frame based simple control and eliminates the issues of single-phase pulsating power, which can cause detrimental impacts on certain dc sources. Simulation studies have been carried out to compare the proposed isolated multi-level inverter with an H-bridge cascaded transformer inverter. The simulation results verified the performance of the isolated inverter. The second proposed topology is a Hierarchal Cascaded Multilevel Converter (HCMC) with phase to phase SOC balancing capability which also for high voltage and high power battery energy storage systems. The HCMC has a hybrid structure of half-bridge converters and H-bridge inverters and the voltage can be hierarchically cascaded to reach the desired value at the half-bridge and the H-bridge levels. The uniform SOC battery management is achieved by controlling the half-bridge converters that are connected to individual battery modules/cells. Simulation studies and experimental results have been carried on a large scale battery system under different operating conditions to verify the effectiveness of the proposed inverters. Moreover, this dissertation presents a new three-phase SOC equalizing circuit, called six-switch energy-level balancing circuit (SSBC), which can be used to realize uniform SOC operation for full utilization of the battery capacity in proposed HCMC or any CMI inverter while keeping balanced three-phase operation. A sinusoidal PWM modulation technique is used to control power transferring between phases. Simulation results have been carried out to verify the performance of the proposed SSBC circuit of uniform three-phase SOC balancing

Bidirectional marx DC–DC converter for offshore wind farm application

The bidirectional DC–DC converter explained here is based on the Marx principle and is capable of achieving step-up and step-down voltage transformations at kV level and is able to handle MW-level power transfers in both directions. The main features of this topology are the absence of a high-frequency transformer, reduced weight, volume, and soft switching to reduce the switching losses. In the boost mode, five capacitors are charged in parallel and discharged in series to achieve the step-up action, and in the buck mode, the converse action takes place. The operating principle is explained, and the steady-state analysis of the converter is given. Matlab/Simulink simulation of a 50 MW converter, interfacing 6 kV, and 30 kV systems supports and validates the theoretical analysis, enables positive supporting the conclusions to be made

Transient voltage stresses in MMC–HVDC links – impulse analysis and novel proposals for synthetic laboratory generation

To evaluate and optimise insulation coordination concepts for state of the art high-voltage direct current (HVDC) transmission systems, appropriate test voltage shapes are required for laboratory imitation of occurring stresses. While especially transient voltages in the monopolar modular multilevel converter (MMC)–HVDC links show an extensive deviation from commonly applied switching impulse shapes, this study focusses on the analysis of over-voltages subsequent to direct current pole to ground faults. Additionally, novel methods for synthetic laboratory test voltage generation are proposed. Based on simulated transients occurring during fault scenarios in different symmetrical monopolar ±320 kV MMC–HVDC schemes, curve fitting, and related analysis techniques are used in order to compare simulated over-voltages with standard test voltage shapes. Moreover, these techniques further allow the identification of novel relevant impulse characteristics. Subsequently, design considerations for the generation of non-standard impulses based on single-stage circuits are derived and discussed. Those synthetically generated voltages may, later on, provide the basis for future investigations on related dielectric effects caused by those non-normative over-voltages