Qianming Xu

A Railway Traction Power Conditioner Using Modular Multilevel Converter and Its Control Strategy for High-Speed Railway System

With the development of high-speed and high-power electrified railway, power quality of traction power grid becomes more serious. In order to achieve three-phase balance of traction power grid, a modular multilevel converter (MMC)-based railway traction power conditioner (RTPC) is studied, which is characterized by modular multilevel cascaded structure. The RTPC consists of four H-Bridge links and output filter inductors, which can be connected to 27.5-kV traction feeders in co-phase supply system directly. This conditioner can be used for different kinds of traction system, especially suited to Scott traction system. According to the analysis of equivalent electrical model and power balance of RTPC, a hierarchical control strategy is put forward aiming to balance the submodule capacitor voltages, which ensures the safety and stability of system. Finally, the structure and its control strategy are verified by the simulation and experiment effectively.

Hierarchical Direct Power Control of Modular Multilevel Converter for Tundish Heating

In this paper, the full bridge-based modular multilevel converter (MMC) is discussed for the application of tundish induction heating power supply (TIHPS). TIHPS features three-phase to single-phase ac/ac conversion and variable output frequency, and has intense demand for the power transmission stability and output current control precision. MMC is suitable for this application and especially the bulky and costly transformer and ac filter in the conventional topology can be dispensable. Therefore, it is interesting to investigate the applicable control method for MMC-TIHPS. According to the instantaneous power theory, a hierarchical direct power control based on the improved deadbeat current control is presented to assure the energy balance inside and outside MMC-TIHPS and reduce the sensitiveness of the current controller to parameter uncertainties. The proposed control strategy dispenses with the phase-locked loop, and does not exist the coupling among control variables of internal and external currents. Then, parameters design of energy controller is studied for stability analysis. Finally, MMC-TIHPS and proposed control method are verified by experimental results of a down-scaled physical prototype.

Multilevel Power Conditioner and its Model Predictive Control for Railway Traction System

In the rapid development process of a high-speed electrified railway, power quality problems in traction power grid have become increasingly deteriorative. In order to ensure a three-phase balanced traction power grid, a modular multilevel converter based railway traction power conditioner (RTPC) is presented. The RTPC consists of four H-Bridge clusters and filter inductors, and it can be directly connected to traction feeders in a co-phase traction system without insulation transformers. According to the equivalent circuit analysis of RTPC system, it can be considered as four single-phase inversion systems. Based on the equivalent control model of each single-phase cluster, the relationship between the multilevel output voltage and current slopes in a control period is analyzed, and an improved model predictive control is proposed. Moreover, a linear combination of two different output levels is proposed to improve tracking performance of cluster current control, and enhance the waveform quality of ac current. Finally, both simulation and experiment results are presented to verify the effectiveness of the structure and its control method.

Analysis and Comparison of Modular Railway Power Conditioner for High-Speed Railway Traction System

With the rapid development of modern electrified railway, negative-sequence current minimization is one of the most important considerations in the high-speed railway traction system. In the past, many multiple or multilevel topologies with high compensation capacity have been introduced for railway power conditioner (RPC). This paper presents a simplified quantitative comparison of five previous modular RPC topologies for negative sequence compensation in V/V and SCOTT traction systems, aiming for an optimal selection of the compensators. Performance criteria such as transformer requirement, voltage stress and current stress of a power switch, numbers of the power switches and capacitor are derived by analytical methods. Moreover, the numerical comparison of operating controllers is completed for modular RPCs. In addition, power losses of five modular RPCs are obtained by theoretical analysis, IPOSIM calculation as well as PSIM simulation. These calculations are validated via simulations results in PSIM. The main conclusion is that presented modular RPCs can be divided into general purpose RPC and special purpose RPC in terms of the behavior and efficiency. It is helpful to choose the appropriate topology for specific applications.

Second Ripple Current Suppression by Two Bandpass Filters and Current Sharing Method for Energy Storage Converters in DC Microgrid

With the increase in ac loads injected into the dc microgrid (MG) through inverters, the second ripple current (SRC) in the front-end energy storage converter (ESC) and the circulating current among the ESCs in dc MG become more and more serious. In this paper, the SRC suppression method by introducing two bandpass filters (BPFs) into the output voltage and inductance current feedback of the ESC is proposed. Compared with the traditional dual-loop control method, the proposed method effectively reduces the SRC and improves the dynamic performance in case of a lower cutoff frequency in the outer voltage loop. Simultaneously, an adaptive droop control method by introducing the fine-tuning virtual resistances is adopted to reduce the output voltage deviation of parallel ESCs and improve the output current sharing among the ESCs. Considering the allowed range of deviation between the output voltage and the rated voltage for each ESC, the impacts of the line power loss and circulating current power loss caused by the introduced virtual resistances are analyzed in detail. While the sum of the line power loss and circulating current power loss reaches the minimum value, the appropriate control parameters are obtained. Simulation and experimental results verify the validity of the proposed method.

Reactive Power Strategy of Cascaded Delta-Connected STATCOM Under Asymmetrical Voltage Conditions

Cascaded static synchronous compensator (STATCOM) is an effective solution for reactive power support in middle/high voltage conditions, and it has been widely employed to control reactive power in photovoltaic (PV) plants, wind farms, and industrial occasions. In this paper, reactive power control strategy of cascaded delta-connected STATCOM under asymmetrical voltage conditions is investigated. A new phase current reference calculation method is proposed to support reactive power continually under abnormal voltage conditions considering cluster voltage balancing control and phase current limitation. Constrains between voltage and current phasors of STATCOM are deduced. Then, the analytical expressions of phase currents and circulating current references are solved. Therefore, reactive power support and the safe operation of STATCOM can be obtained simultaneously by limiting the peak value of the phase current references. Furthermore, the reactive power support capability of cascaded STATCOM under asymmetrical voltage conditions is explored and compared by combining the proposed references calculation method with the three generalized current references calculation strategies. Finally, simulation and experimental results have been given to verify the theoretical studies.

A circulating current suppressing method of MMC based STATCOM for negative-sequence compensation

As for the reactive power and negative sequence compensation caused by the unbalanced load, the modular multilevel converter based static synchronous compensator (MMC-STATCOM) is adopted and analyzed. To reduce the complexity of control system, the three-phase current error signal is transformed into αβ frame and proportional-resonant controller is adopted to implement the static current tracking. In order to reduce the operation performance of MMC-STATCOM, a system small-signal model is established and an efficient circulating current suppressing method is proposed. As the circulating current of each phase is controlled individually, this method can be widely applied in diverse MMCs. The simulation and experiment results have validated the feasibility of the proposed control method.

Analysis, Design, and Implementation of Passivity-Based Control for Multilevel Railway Power Conditioner

In recent years, railway power conditioner (RPC) has been used to improve the power quality of a traction power system. In engineering application of RPC, mismatches between parameters used in controller and actual values are inevitable, which will increase the difficulty of current controller design and deteriorate compensation performance. In this paper, a passivity-based control (PBC) system is studied for multilevel RPC to enhance its tolerance for mismatches. According to the topology of multilevel RPC, equivalent electrical and mathematical models are developed. To employ PBC, Euler–Lagrange system model of RPC is established by Park (αβ/dq) transformation, and the passivity of RPC and stability of PBC are proved. On this basis, the robustness of PBC is analyzed and criteria for damping selection are derived. Finally, simulation and experiments have been carried out to verify the structure and control method in the paper.

Research of single-carrier modulation strategy for modular multilevel converter

This paper proposed an improved carrier phase disposition modulation strategy for modular multilevel converter(MMC), namely a single-carrier modulation strategy. It uses a single triangular carrier to achieve multilevel modulation. It has advantages of both good harmonic characteristics for carrier phase disposition modulation and switch balanced distribution of carrier phase shifted modulation. In order to solve the problem of submodule capacitor voltage imbalance of the MMC, this paper presents a switching allocation strategy according to submodule capacitor voltage sorting. The proposed modulation strategy and switch balanced distribution strategy was verified by the PSIM simulation.