High-voltage pulse generator based on sequentially charged MMC-SMs operating in a voltage-boost mode

Pulse forming networks and Marx generators are the classical rectangular waveform pulse generators (PGs). They are inflexible and their capacitors must be fully charged to the required voltage from 0V before delivering each high-voltage (HV) pulse. They are only able to generate unipolar pulses; if bipolar pulses are sought another generator fed from a negative supply voltage is added. Recently, several power electronics based PGs have been proposed. This paper presents an HV power electronics based PG, which is based on Half-Bridge Modular Multilevel Converter (HB-MMC) sub-modules (SMs) charged sequentially in a voltage boost mode. Each SM capacitor and main switch form a boost converter with the charging input supply and inductor. As a result, all SM capacitors are charged to a voltage greater than the input. During the discharging process the SM capacitors are connected in series, producing a rectangular HV pulse across the load. The proposed charging method allows a reduction in the converter footprint in comparison with recently proposed MMC sequentially charged PG topologies. Although only rectangular pulse waveforms are sought in this paper, a SM capacitor voltage balance method allows multilevel pulse generation. The viability of the proposed converter is confirmed by MATLAB/Simulink simulation and scaled-down experimentation

A HVDC shunt tap based on unidirectional hybrid modular DC-DC converter with simultaneous charging and sequential discharging of capacitors

In this paper, a new HVDC shunt tap is proposed. The proposed configuration consists of a unidirectional hybrid modular DC-DC converter followed by a voltage source converter for DC-AC conversion to feed a local AC network connected to the tap output terminals. The proposed DC-DC converter consists of a high-voltage valve, and series-connected unidirectional half-bridge Sub-Modules (SMs). Unlike Marx generator circuit concept, the DC-DC conversion in the proposed configuration is achieved by enabling simultaneous charging of series-connected capacitors (i.e. SMs capacitors), and sequential discharging of capacitors. Compared to Marx-generator based switched capacitor DC-DC converters, the proposed configuration has a lower number of semiconductor devices, which affects positively the system cost, and reduces the control complexity. Detailed illustration, design, and control of the proposed approach are presented. Simulation results are presented to validate the proposed approach.This publication was made possible by NPRP grant NPRP ( 9-092-2-045 ) from the Qatar National Research Fund (a member of Qatar Foundation). The statements made herein are solely the responsibility of the authors

A Grid-Connected Capacitor-Tapped Multimodule Converter for HVDC Applications: Operational Concept and Control

In this paper, a dc-ac buck converter is proposed as a grid-side converter in high voltage dc (HVdc) transmission systems for high-power renewable energy source grid integration. The proposed architecture consists of multimodules of half-bridge voltage source converters (HB-VSCs). The dc terminals of the HB-VSCs are connected in series across the entire dc link (i.e., capacitor tapped), whereas their ac outputs are connected to multiwinding transformers to provide the three-phase terminals for ac grid integration. The proposed grid-connected capacitor-tapped multimodule converter (CT-MC) architecture, inspired from the HVdc shunt tap proposed by ABB, provides a dc-ac conversion with a relatively moderate voltage rating of semiconductor devices. It also provides operation with a lower number of semiconductor devices, gate driver circuits, voltage sensors, and lower total MVA rating of semiconductor devices (67%) compared with the conventional three-phase HB modular multilevel converter, which positively affects the system cost and reduces the computational burden of the employed controller. In this paper, the operational concept and control of the CT-MC are presented along with a capacitor voltage balancing approach. Simulation results are presented during normal and abnormal conditions to show the viability of the proposed architecture. Finally, a scaled down prototype for the CT-MC is employed to validate the converter operation.This work was supported by the National Priorities Research Program (NPRP) from the Qatar National Research Fund (a member of Qatar Foundation) under Grant 9-092-2-04

Power angle control of grid-connected capacitor-tapped multi-module voltage source converter

HVDC transmission suits transmitting large amounts of renewable energy power over long distances with low transmission losses. The Capacitor-Tapped Multi-module Converter (CT-MC) can be employed effectively to integrate the transmitted power through the HVDC lines into a medium voltage AC grid. In this paper, the power angle control of grid-connected the CT-MC is introduced. The power-angle control is simple and can be implemented easily, where the active power is generally controlled via controlling the phase-angle shift between the Voltage Source Converter (VSC) and the AC system, while the reactive power is controlled via varying the VSC output voltage magnitude. The operational concept of CT-MC along with its power angle control is Prasented. Simulation results for the system during normal as well as abnormal operating conditions are Prasented for validation using Matlab/Simulink platform.Scopu

Investigation of three-phase capacitor-tapped multi-module voltage source converter with selective harmonic elimination

In conventional Two-Level Voltage Source Converters (2L-VSCs), connecting between high/medium dc voltage and medium/low ac voltage requires semiconductor devices rated at the high/medium dc-voltage, which negatively affects the converter cost. Alternatively, the Capacitor-Tapped Multi-module VSC (CT-MC) can be used, where it has a good bucking capability. The CT-MC consists of three (or multiple of three) Half-Bridge Inverter (HBI) modules, where the dc terminals of these modules are connected in series across the high/medium dc-link voltage, i.e. semiconductor devices with a lower voltage rating can be employed. The ac outputs of the involved HBIs are integrated to the ac gird through proper transformer(s). In this paper, the performance of the CT-MC is investigated, when Selective Harmonic Elimination (SHE) method is employed for dc-ac conversion to reduce the converter switching losses, while keeping the Total Harmonic Distortion (THD) of the ac currents within the recommended range. Simulation results are Prasented to show the performance of CT-MC when SHE is applied.Scopu

Power control of grid-connected high-gain boost full-bridge modular multilevel converter

Conventional Modular Multilevel Converter (MMC) with half-bridge submodules (HB-SMs) can be used for gird-integration of renewable energy sources, but with limited voltage gain. To elevate the generated ac output voltage level, a bulky low-frequency step-up transformer can be employed at the converter ac side. Alternatively, single-stage dc-ac Boost FullBridge MMC (BFB-MMC) can be used effectively. In this paper, the performance of high-gain grid-connected BFB-MMC is investigated under active and reactive power control. The BFB-MMC can generate an ac output voltage with magnitude of kVdc where k is a positive integer number, and Vdc is the input dc voltage level. Each arm in the BFB-MMC contains (2k+1) Full-bridge submodules (FB-SMs) each rated at 0.5 Vdc. Each arm is controlled to generate bipolar sinusoidal stepped voltage ranged from (0.5+k) Vdc to (0.5-k)Vdc with steps of 0.5 Vdc while keeping the sum of upper and lower arm voltages in the same leg equals the input dc voltage. A Detailed illustration for BFB-MMC operational concept, capacitor voltage balancing technique, and active and reactive power controller is presented. Finally, simulation model for the grid-connected BFB-MMC has been built. The simulation results show the effectiveness of BFB-MMC in the grid-connected applications. - 2017 IEEE

Conceptual study of AC-powered switched non-polarized-capacitors based solid-state bipolar Marx pulse generator

The conventional unipolar Marx generator can be employed for bipolar pulsed electric field by adding a high-voltage H-bridge at the load side which necessitates stacked switches. Alternatively, bipolar Marx generator with low-voltage switches can be employed. The existing bipolar Marx generators are typically fed from dc supplies, where polarized capacitors are employed. The capacitance of involved capacitors is selected such that the total energy stored in the generator's capacitors is higher than the bipolar pulse energy. As a result, polarized capacitors with relatively high capacitances are required. In addition, a high number of controlled switches per stage are required (at least four switches per stage). In this paper, a new solid-state bipolar Marx pulse generator with only three controlled switches per stage is proposed. The proposed generator is fed from sinusoidal/square ac voltage and non-polarized capacitors with low capacitance are employed. The proposed bipolar Marx-generator can be used effectively with resistive loads for exponential pulse generation. A detailed illustration of the proposed approach is presented and a discussion, to elucidate the differences between the proposed pulse generator and the other existing ones, are presented. Simulation results for the proposed pulse generator are presented to validate the concept. Finally, experimental results have been obtained from a low-voltage model of the proposed generator.Qatar Foundation; Qatar National Research FundScopu

A Full-Bridge Submodule-Based Modular Unipolar/Bipolar High-Voltage Pulse Generator with Sequential Charging of Capacitors

Repetitive high pulsed electric field (PEF) is an effective method to kill microorganisms and bacteria in water treatment applications. The PEF can be generated by applying high-power electromagnetic pulse across the sample to be treated. There are two main types of high-voltage pulse generators, namely, unipolar and bipolar. In this paper, a full-bridge submodule-based modular high-voltage pulse generator, having the ability to generate unipolar and bipolar high-voltage pulses with different shapes from a relatively low-voltage input dc supply, is proposed. In the proposed configuration, relatively low-voltage insulated gate bipolar transistors (IGBTs) are required to generate the high-voltage bipolar pulses. A thyristor rated at the level of the pulsed output voltage is required in the proposed configuration to bypass the load during the charging process of capacitors. In the proposed approach, a thyristor is used instead of the self-commutated high-voltage switch (e.g., series-connected IGBTs), as thyristors are available with high-voltage ratings and possess inherent reverse voltage blocking capability. A detailed illustration of the proposed configuration and its operational concept are introduced in this paper. Simulation and experimental results are presented to validate the proposed approach.Qatar National Research FundScopu

Effect of stator winding connection of five-phase induction machines on torque ripples under open line condition

One of the main challenges in designing high-power drive systems is the quality of the developed torque. High-torque-ripple magnitudes result in serious vibration and acoustic noise problems that affect the lifetime of a drive train. Multiphase machines intrinsically offer the torque with higher quality and are also being promoted for their high fault tolerant capability when compared with their three-phase counterparts. During open-phase conditions, the induced nonfundamental sequence current components have a detrimental effect on the machine performance especially under open-loop control. Although optimal current control is usually employed to ensure certain optimization criterion, such as minimizing torque ripples, optimizing flux distribution, or minimizing copper loss, are met, it usually entails a sophisticated current controller. This paper studies the effect of a stator winding connection of a five-phase induction machine on the induced torque ripples. Two possible connections, namely, star and pentagon connections are compared under healthy as well as fault conditions with one-line open. The comparison is conducted using both finite-element simulation and experimental results using a 1.5-Hp five-phase prototype induction machine. The comparison shows that the pentagon connection reduces the machine-induced torque ripples and improves the overall machine performance under fault conditions.Scopu

A Self-Balanced Bidirectional Medium-/High-Voltage Hybrid Modular DC-DC Converter with Low-Voltage Common DC-Link and Sequential Charging/Discharging of Submodules Capacitors

This paper proposes a medium-/high-voltage high-power hybrid modular DC-DC converter to interconnect two different dc-voltage levels in medium-/high-voltage dc grids. The converter consists of half-bridge submodules (SMs) rated at a moderate voltage level, i.e., series connection of semiconductor devices is avoided while connecting two high dc-voltage levels. The proposed architecture provides self-balancing operation, i.e., unlike conventional modular converters, no need for capacitor voltage measurement, which enhances the system reliability and simplicity. Sequential charging/discharging of SMs capacitors is adopted in the proposed approach to ensure a self-balanced operation. The proposed architecture can provide a high conversion ratio as well as power transfer in both directions, i.e., bidirectional DC-DC converter. Detailed illustration of the proposed approach operational concept, design, and overall control system is presented. Simulation and experimental results are presented to show the viability of the proposed configuration and validate the claims.Qatar National Research Fund, Qatar FoundationScopu