Medium-voltage Multilevel Converters Research Articles

Development of a Modular Educational Kit for Research and Teaching on Power Electronics and Multilevel Converters

This paper presents the detailed project of a modular educational kit to be used in research and teaching activities in the area of power electronics - with emphasis on applications in medium voltage multilevel converters. The half bridge topology has been chosen for the submodules, as they can be arranged together in a multitude of other topologies. A system composed by a mother board and daughter cards is presented. A DSP control card and a FPGA System-On-Module are inserted in the mother board in order to control the half bridge submodules. The targets of this project are applications in medium voltage - hence, the communication between the submodules and the FPGA/DSP is performed through optical fibers. Also a self power system is presented for the submodules, in order for them to operate at floating potentials. Experimental results have been presented for the most usual topologies of converters, including a Modular Multilevel Converter operating at 2000V.

Design and Control of Medium-Voltage Multilevel Converter for Direct Grid Integration of Photovoltaic System

This paper deals with the design and control of a medium voltage cascaded H-bridge (CHB) multilevel converter for the direct grid integration of a large photovoltaic (PV) system. It presents the design considerations for selecting an optimum number of levels in the multilevel converter, which are to be both efficient and economical. This system is designed for medium voltage, 11 kV, and hence it can be directly interfaced with the grid without a step-up transformer. It contributes large cost savings in the installation, operation and maintenance of the large solar plant. With the increased number of levels, the converter output voltage waveform is a good approximation to the sine waveform. Therefore, the total harmonic distortions (THDs) of the output AC converter voltage and the grid current are reduced, which significantly improves the power quality of the output voltage waveform reducing the size and hence the cost of the filter. The effect of an increased number of levels on THDs is also studied. The control employed is a decoupled control with the synchronous reference frame—phase-locked loop (SRF-PLL), and a 19-level cascaded converter is operating at the low switching frequency. The separate improved perturb and observation (P&O) maximum power point tracking (MPPT) algorithm is used for tracking the maximum power from each array of the multistring solar photovoltaic (SPV) system. Both steady-state and dynamic performances of the system are simulated in MATLAB/ Simulink environment and validated in accordance with the IEEE-519 standard. A substantial reduction in THDs of the output AC converter voltage and the grid current, with increased number of levels, is observed in this system.

Control of High-Power Wind Energy Conversion System Fed by Multi-level Converters

The progress of Variable Speed Wind Turbine Systems (VS-WTS) is currently focused in reaching higher power ratings (8MW). Also, medium voltage operation is at this power level, particularly at electrical network, a required feature. Consequently, high power medium-voltage multilevel converters for VS-WTS have been attracting more and more attentions. Nevertheless, most modern high-power VS-WTS consist of a large number of Wind Turbine Generators (WTGs) connected and operating simultaneously to reach the multi-megawatt level. This study presents a control strategy for a grid-connected onshore wind farm based Permanent Magnet Synchronous Generator (PMSG), where cascaded multilevel converters are employed on the generators and grid side. The generator-side converters provide speed control of the turbine-generator units to implement Maximum Power Point Tracking. The grid side Neutral Point diode Clamped (NPC) converter system regulates reactive and real power flow to the electrical network. So, it regulates the dc bus voltage and the ac side power-factor. The modulation technique for the proposed topology is developed from Space Vector Modulation. The overall control approaches of a rectifiers and an NPC are also described. Nonlinear Sliding Mode technique (NL-SM) with speed control and Vector Oriented Control are developed. To validate the proposed approaches, simulations are carried out on a high-power wind energy conversion system with multi-level converters and MATLAB/Simulink software. The simulation results demonstrate a good performance in diverse scenarios.

A Transformerless PCB-Based Medium-Voltage Multilevel Power Converter With a DC Capacitor Balancing Circuit

This paper presents a new method of constructing a transformerless voltage-sourced medium-voltage multilevel converter using existing discrete power semiconductor devices and printed circuit board technology. While the approach is general, it is particularly well suited for medium-voltage converters and motor drives in the 4.16 kV 500–1000 kW range. A novel way of visualizing the power stage topology is developed, which allows simplified mechanical layouts, while managing the commutation paths. Using so many discrete devices typically drives cost and complexity of the gate-drive system including its control and isolation; a gate-drive circuit is presented to address this problem. As with most multilevel topologies, the dc-link voltages must be balanced during operation. This is accomplished using an auxiliary circuit made up of the same power stage and an associated control algorithm. Experimental results are presented for a 4.16-kV 746-kW five-level power converter prototype.

Medium-Voltage Multilevel Converters—State of the Art, Challenges, and Requirements in Industrial Applications

This paper gives an overview of medium-voltage (MV) multilevel converters with a focus on achieving minimum harmonic distortion and high efficiency at low switching frequency operation. Increasing the power rating by minimizing switching frequency while still maintaining reasonable power quality is an important requirement and a persistent challenge for the industry. Existing solutions are discussed and analyzed based on their topologies, limitations, and control techniques. As a preferred option for future research and application, an inverter configuration based on three-level building blocks to generate five-level voltage waveforms is suggested. This paper shows that such an inverter may be operated at a very low switching frequency to achieve minimum on-state and dynamic device losses for highly efficient MV drive applications while maintaining low harmonic distortion.