Selective harmonic elimination methods for a cascaded H-bridge converter

In recent years there has been an increased demand for integration of renewable energy into the electricity grid. This has increased research into power converter solutions required to integrate renewable technology into the electricity supply. One such converter is a Cascaded H-Bridge (CHB) Multilevel Converter. Operation of such a topology requires strict control of power flow to ensure that energy is distributed equally across the converters energy storage components. For operation at high power levels, advanced modulation methods may be required to ensure that losses due to non-ideal semiconductor switching are minimised, whilst not compromising the quality of the voltage waveform being produced by the converter. This thesis presents several low switching frequency modulation methods based on Selective Harmonic Elimination (SHE) in order to address these two operational issues. The methods presented involve manipulating the H-Bridge cell voltages of the CHB converter to control power flow. Simulated results are supported by experimental verification from a seven level, single phase CHB converter

Geometry optimization and characterization of three-phase medium frequency transformer for 10kVA isolated DC-DC converter

Three-phase Dual Active Bridge converter is advisable for the High-power DC-DC conversion system. In the ac link, galvanically isolated transformer operated at a medium frequency range provides stepping up or down of the secondary bridge voltage. This paper provides a magnetic design optimization of the medium frequency transformer for maximizing its efficiency when excited by a non-sinusoidal waveform. In this paper, a mathematical design of a 10kVA non-sinusoidal transformer had been developed and validated using two-dimensional (2D) transient finite element analysis (FEA). The set of selected design variables is defined in order to enhance the power density and efficiency of the targeted transformer and an optimization is carried out. Finally, a 10kVA transformer is prototyped and the results of core losses for nonsinusoidal excitation is confirmed experimentally

An analytical modeling and estimating losses of power semiconductors in a three-phase dual active bridge converter for MVDC grids

Due to the increasing installation of renewable and decentralized power sources, Medium-voltage dc (MVDC) grids has been considered for an alternative application to medium-voltage ac (MVAC) application. Three-phase dual active bridge DC-DC (3DAB) converter is proposed as an attractive topology for MVDC grids due to its high power capability, smaller filtering parts, and galvanic isolation. In this paper, a first harmonic approximation (FHA) modeling of 3DAB converter is derived. Using the FHA modeling, a symmetrical modeling of switching devices is introduced and a 4MVA system for 40kV MVDC system has been validated in terms of conduction and switching losses. Experimental implementation of a 10kVA prototype and the results are presented

Design considerations for a high-power dual active bridge DC-DC converter with galvanically isolated transformer

Multi-megawatt scale isolated DC-DC converters are likely to become increasingly popular as means to interconnect the MVDC grids of different voltage levels. Threephase dual active bridge DC-DC (3DAB) converters operating with the zero-voltage switching (ZVS) is a promising candidate for the target multi-megawatt application. This paper presents a systematic approach of the design considerations for a 3DAB converter. Firstly, the use of snubber capacitors in medium voltage and medium frequency operating conditions is proposed. Snubber capacitor influence on turn-off current levels and ZVS operating range are introduced and analyzed. In addition, details of thermal management design are introduced. It is established through power loss analysis that the proposed design method reduces the semiconductor losses substantially at full load conditions. Finally, the proposed method has been validated from a 10kW simulation model using PLECS software package

Selective harmonic elimination methods for a cascaded H-bridge converter

In recent years there has been an increased demand for integration of renewable energy into the electricity grid. This has increased research into power converter solutions required to integrate renewable technology into the electricity supply. One such converter is a Cascaded H-Bridge (CHB) Multilevel Converter. Operation of such a topology requires strict control of power flow to ensure that energy is distributed equally across the converters energy storage components. For operation at high power levels, advanced modulation methods may be required to ensure that losses due to non-ideal semiconductor switching are minimised, whilst not compromising the quality of the voltage waveform being produced by the converter. This thesis presents several low switching frequency modulation methods based on Selective Harmonic Elimination (SHE) in order to address these two operational issues. The methods presented involve manipulating the H-Bridge cell voltages of the CHB converter to control power flow. Simulated results are supported by experimental verification from a seven level, single phase CHB converter

Robustness analysis and experimental validation of a fault detection and isolation method for the modular multilevel converter

This paper presents a fault detection and isolation (FDI) method for open-circuit faults of power semiconductor devices in a modular multilevel converter (MMC). The proposed FDI method is simple with only one sliding mode observer (SMO) equation and requires no additional transducers. The method is based on an SMO for the circulating current in an MMC. An open-circuit fault of power semiconductor device is detected when the observed circulating current diverges from the measured one. A fault is located by employing an assumption-verification process. To improve the robustness of the proposed FDI method, a new technique based on the observer injection term is introduced to estimate the value of the uncertainties and disturbances, this estimated value can be used to compensate the uncertainties and disturbances. As a result, the proposed FDI scheme can detect and locate an open-circuit fault in a power semiconductor device while ignoring parameter uncertainties, measurement error and other bounded disturbances. The FDI scheme has been implemented in a field programmable gate array (FPGA) using fixed point arithmetic and tested on a single phase MMC prototype. Experimental results under different load conditions show that an open-circuit faulty power semiconductor device in an MMC can be detected and located in less than 50ms

Design and implementation of magnetron power supply and emulator

The paper presents a novel resonant based high performance power converter solution for industrial magnetron systems. Based the characteristics of the magnetron, an emulator prototype is also proposed to represent the magnetron load behaviour in a laboratory environment. A detailed design and implementation procedure is presented, including the design and control of the resonant power converter, together with the magnetron emulator in practical aspects. Experimental results are provided in order to demonstrate the feasibility of the proposed converter and emulator

Fault detection for modular multilevel converters based on sliding mode observer

This letter presents a fault detection method for modular multilevel converters (MMC) which is capable of lo¬cating a faulty semiconductor switching device in the circuit. The proposed fault detection method is based on a sliding mode observer (SMO) and a switching model of a half-bridge, the approach taken is to conjecture the location of fault, modify the SMO accordingly and then compare the observed and measured states to verify, or otherwise, the assumption. This technique requires no additional measurement elements and can easily be implemented in a DSP or micro-controller. The operation and robustness of the fault detection technique are confirmed by simulation results for the fault condition of a semiconductor switching device appearing as an open-circuit

Arm balancing control and experimental validation of a grid connected MMC with pulsed DC load

This paper focuses on the operation of a grid connected Modular Multilevel Converter (MMC) supplying a pulsed DC load. The goal is to achieve minimum AC power fluctuation despite the high power fluctuation present on the DC side. The MMC has been selected for its inherent ability to decouple AC and DC current controllers. How¬ever, if no additional provisions are taken, the pulsed load causes imbalance of cell capacitor voltages between upper and lower arm in each phase. The paper presents the the-oretical analysis of the imbalance problem, and proposes a simple arm balancing controller to enable the operation of the converter under pulsed DC load. The effectiveness of the controller has been successfully verified on a 7 kW MMC experimental prototype with a 3 kA pulsed DC load

Quasi Z-source NPC inverter for PV application

This paper presents the operating principles and modified space vector modulation strategy for a three-phase quasi Z-source neutral point clamped inverter for solar photovoltaic applications. This topology combines the advantages of the neutral point clamped and quasi Z-source inverters. These advantages include single-stage buck-boost power conversion, continuous input current, and low voltage stress of switches. Simulation results are presented to verify the presented concepts