Lijun Hang

Novel circulating current suppression strategy for MMC based on quasi-PR controller

According to the advantages and disadvantages of the traditional control strategy used in modular multilevel converter (MMC), an improved control method is discussed in this paper. In the proposed method, the outer loop of traditional control strategy is still in use to balance submodule capacitor voltage. However, the dc component extraction and quasi-proportional resonant controller (quasi-PR) controller are added to modify the inner loop to eliminate second-order circulating harmonic current. Besides, design principles of parameters related to the improved part are given in discrete domain and the stability analysis are also discussed. Finally, experiment results under traditional and improved control strategy are analyzed and compared, which indicate that the improved control method has the merit of capacitor voltage balance and performs well in suppressing circulating harmonic current.

Modeling and Analysis of a Dual-Active-Bridge-Isolated Bidirectional DC/DC Converter to Minimize RMS Current With Whole Operating Range

The triple phase shift (TPS) modulation scheme, which provides three control freedoms, is of great importance for the optimized operation of a dual active bridge (DAB) isolated bidirectional dc/dc converter. First of all, this paper introduces an accurate, universal model to describe the analytic expressions of the DAB converter under TPS control. Based on this, six operating modes of the DAB converter are further discussed. Afterwards, the concept of global optimal condition (GOC) equations is proposed to derive the closed form of analytic expressions of an optimal modulation scheme that makes the DAB converter operate with minimized root-mean-square (rms) current during whole power range with different operating modes. According to the GOC equations, the physical explanation of the proposed modulation scheme is further given in details, and the complex interaction among the control variables, the transferred power, and rms current is revealed. The real-time optimization process of the proposed method is also specified. Finally, the proposed methods are applied to a laboratory prototype. The experimental results confirm the theoretical analysis and practical feasibility of the proposed strategies.

Power flow and inductor current analysis of PWM control for Dual Active Bridge converter

PWM control method is an important method for the optimization operation of Dual Active Bridge (DAB) converter. The traditional way which is used to analyze PWM control is not precise and comprehensive. This paper gives the accurate calculation of power flow and RMS value of inductor current by proposing a novel method. Six modes are analyzed based on the control signal distributions. Finally, the simulations and experiments are used to verify the theoretic results.

A novel five-level inverter for solar system

This paper compares the advantages and disadvantages of I-type 5-level inverter and T-type 5-level inverter, involving the conduction loss, commutation path in reactive power and switching loss, especially at high frequency. Based on the analysis, this paper proposes a novel 5-level inverter for solar system, aiming at reducing both the conduction loss and switching loss at high frequency, and solving their common issue of unbalanced devices dissipation. A prototype has been built to verify the operation and performance of the proposed inverter.

Protected control method for Voltage Source Converter (VSC) in AC/DC hybrid grid under grid unbalanced faults

In AC/DC hybrid grid, the individual AC and DC grids exist. The power converters are the key power conversation interfaces between these two kinds of individual grids. Under unbalanced AC grids, the grid voltage is distorted and the harmonics is introduced. Therefore, the active and reactive power also give rise to the uncontrolled oscillations, which may damage the interface converters. In order to ensure the safety of the converters and the facilities in the grids, a proper control strategy is crucial for AC/DC hybrid grid. In this paper, the current harmonic for VSC is theoretically analyzed under unbalanced condition, furthermore methods to separate the positive- and negative- sequence components of grid voltages are summed up and analyzed. Based on the theoretical analysis, the different grid-connected converter protected control methods are analyzed and compared, and a new control method which can control the oscillations of active and reactive power by parameter k, has been proposed. Finally, the experimental results verified the validity and advantages of this proposed method.

An improved DTC controller for DFIG-based wind generation system

This paper presents an improved direct torque control (DTC) method for the wind power generation system based on the doubly-fed induction generator (DFIG). The basic principle of direct torque control based on DFIG system is analyzed. An improved strategy is presented by increasing the division accuracy of rotor flux position and the number of basic voltage vectors. The proposed method improves the performances of the system and reduces the rotor and torque ripples. By comparing the simulation results of the conventional and the improved DTC method applied in DFIG-based wind power generation system, the effectiveness of the proposed control method is validated.

Input impedance improvement by using digital compensation strategy for high power density rectifiers with wide load operation

In this paper, an improved control loop based on feedforward is proposed to reduce the total harmonic distortion of the input current (THDi), realizing power factor correction (PFC) in MCM. The frequency responses of the input impedance for a wide frequency range are improved and unified by adding feedforward duty-ratio to the output of the original current-loop. A 5 kW experimental prototype is built to verify the validity of this strategy. The experimental results show that the system has close to unity power factor (PF), and THDi is significantly reduced.