Novel double-layer DC/AC railway traction power supply system with renewable integration

Back-to-back converter based railway traction power supply system (TPSS) can eliminate neutral sections in the traction side and improve power quality in the grid side, but it still has some drawbacks such as low reliability, difficulty in accepting large-capacity renewable energy, and power mismatches. In this study, a double-layer DC/AC TPSS with renewable integration is proposed to address these challenges and to improve system performance. The proposed topology breaks the limit of back-to-back structure and enables more flexible free energy flow. A top-down system design method is proposed in this study. Firstly, the characteristics of the proposed TPSS for integration with renewable power are described and compared with the traditional back-to-back topology. Secondly, a DC droop controller and a AC droop controller are designed for DC layer grid and AC layer grid, respectively, to control the power flow in each layer. The traditional AC droop control is based on the inductive transmission impedance, but the resistance of traction transmission line cannot be ignored. Thus, a modified droop control strategy with the consideration of line resistance is also proposed in this study. Subsequently, the voltage control strategy for the single modular multilevel converter is designed to track the reference signal from the upper droop controller. Finally, a general double-layer DC/AC TPSS is designed from bottom to top, and the simulation results confirm that the proposed TPSS with renewable integration is capable of delivering desirable performance

A Novel Circulating Current Suppression for Paralleled Current Source Converter Based on Virtual Impedance Concept

The circulating current is one of the important issues for parallel converters. It affects the system stable operation and degrades the power quality. In order to reduce the circulating current of the parallel converter and reduce the harmonic pollution to the power grid, a new circulating current suppression strategy is proposed for the parallel current source converter without any communication line. This strategy is able to realize the current sharing between parallel modules by changing the external characteristics of the parallel modules to thus suppress the circulating current among the parallel current source converters. The proposed control strategy adopts DC-side droop control and AC-side virtual impedance control. The DC-side droop control is used to generate the reference voltage of each parallel module, while the AC-side virtual impedance is used to the circulating current suppression. We performed a time domain test of the parallel converter, and the results show that the proposed control strategy reduced the RMS circulating current of the parallel converter by 50% and effectively reduced the grid-side current THD while ensuring the stable operation of the converter. The effectiveness of the proposed control strategy was, therefore, verified

Efficient Implementation of Multilevel inverter with new modulation scheme for Reducing THD

This paper proposed an improved phase disposition pulse width modulation (PDPWM) for a modular multilevel inverter which is used for Photovoltaic grid connection. This new modulation method is based on selective virtual loop mapping, to achieve dynamic capacitor voltage balance without the help of an extra compensation signal. The concept of virtual submodule (VSM) is first established, and by changing the loop mapping relationships between the VSMs and the real sub-modules, the voltages of the upper/lower arm?s capacitors can be well balanced. This method does not requiring sorting voltages from highest to lowest, and just identify the MIN and MAX capacitor voltage?s index which makes it suitable for a modular multilevel converter with a large number of sub-modules in one arm. Compared to carrier phase-shifted PWM (CPSPWM), this method is more easily to be realized in field-programmable gate array and has much stronger dynamic regulation ability, and is conducive to the control of circulating current and Power quality injected into the grid. The maximum power point tracking is achieved with a fuzzy logic controller. The validity of the proposed system is confirmed by simulations

A Control Scheme to Suppress Circulating Currents in Parallel-Connected Three-Phase Inverters

[EN] The parallel operation of inverters has many benefits, such as modularity and redundancy. However, the parallel connection of inverters produces circulating currents that may result in malfunctions of the system. In this work, a control technique for the elimination of the low-frequency components of the circulating currents in grid-connected inverters is presented. The proposed control structure contains n - 1 zero-sequence control loops, with n being the number of inverters connected in parallel. Simulation and experimental results have been carried out on a prototype composed of two 5 kW inverters connected in parallel. The results have been obtained by considering the following mismatches between both inverters: inductance values of the grid filters, unbalance of the delivered power, and the use of different modulation techniques.This research was funded by the Spanish "Ministerio de Asuntos Economicos y Transformacion Digital" and the European Regional Development Fund (ERDF), under grants RTI2018100732-B-C21 and PID2021-122835OB-C22.Liberos, M.; González-Medina, R.; Patrao Herrero, I.; Garcerá, G.; Figueres Amorós, E. (2022). A Control Scheme to Suppress Circulating Currents in Parallel-Connected Three-Phase Inverters. Electronics. 11(22):1-23. https://doi.org/10.3390/electronics11223720123112

Application of Modular Multilevel Converter for Interfacing Grid-Connected Photovoltaic Conversion Plants

This thesis investigates the applicability of the Modular Multilevel Converter (MMC) for interfacing grid connected photovoltaic conversion plants. A detailed three-phase 9-level simulation model is implemented in Simulink. Two control objectives are identified as distinctive for the MMC: Capacitor voltage balancing and suppression of circulating currents, both of which are included in the model. The MMC is controlled by a modified Level-Shifted Pulse Width Modulator. The model is verified by comparing its behaviour to that of the mathematical model of the MMC. The nature of photovoltaic power generation makes Maximum Power Point Tracking (MPPT) important to maximize the power yield from a pv module. All the pv modules connected to the same MPP tracker should have the same operating conditions. For largescale pv farms this is only feasible with multiple MPP trackers. Two pv inverter configurations are identified as suitable for grid connection of large-scale pv farms using the MMC: Cascaded dc-dc converters and multi-string inverter. With the former, the three phase legs share the same dc link voltage. With a multi-string topology, each submodule is fed by a separate pv string. Thus, power imbalance between the submodules are inevitable. This can be remedied by power imbalance compensation. For grid side control Synchronous Reference Frame Control (SRFC) and Model Predictive Control (MPC) is considered. MPC has the advantage of handling non-linear constraints on both states and variables. In addition it is reported to perform better than SRFC during dynamic conditions, which are likely to occur with power generation from pv modules. SRFC is implemented in the MMC simulation model. It synchronizes with the grid and delivers power at unity power factor

Study and evaluation of distributed power electronic converters in photovoltaic generation applications

This research project has proposed a new modulation technique called “Local Carrier Pulse Width Modulation” (LC-PWM) for MMCs with different cell voltages, taking into account the measured cell voltages to generate switching sequences with more accurate timing. It also adapts the modulator sampling period to improve the transitions from level to level, an important issue to reduce noise at the internal circulating currents. As a result, the new modulation LC-PWM technique reduces the output distortion in a wider range of voltage situations. Furthermore, it effectively eliminates unnecessary AC components of circulating currents, resulting in lower power losses and higher MMC efficiency.Departamento de Tecnología ElectrónicaDoctorado en Ingeniería Industria