Mi Dong

A Three-Level T-Type Indirect Matrix Converter Based on the Third-Harmonic Injection Technique

An indirect matrix converter (IMC) is a direct ac-ac power converter. As an improvement of the output waveform quality, several types of three-level IMC are proposed and investigated. However, two major problems of these converters still exist. First, in some specific situations, such as wind energy conversion systems and flexible ac transmission systems, these converters’ capabilities of generating input reactive power must be improved greatly. Second, neutral-point voltage balancing is a key challenge to the normal operation of multilevel IMCs, but the control of balancing the neutral-point voltage for multilevel IMCs has not been investigated so far. To address these issues, a three-level T-type indirect MC (3LT2IMC) topology as well as a carrier-based modulation method is proposed. In addition to inheriting the advantages of the third-harmonic injection two-stage MC such as extended input reactive power control range and no need for synchronization in modulation, 3LT2IMC could provide an improved output power quality. Besides, a closed-loop control algorithm for balancing the neutral-point voltage is developed. The presented modulation strategy and control algorithm are generalized and can be extended to other three-level IMCs. Finally, the proposed topology and method are verified by simulation and experimental results.

First-order Generalized Integrator Based Frequency Locked Loop and Synchronization for Three-Phase Grid-connected Converters under Adverse Grid Conditions

This paper presents an alternative frequency adaptive grid synchronization technique named HDN-FLL, which can accurately extract the fundamental positive- and negative-sequence components and interested harmonics in adverse three-phase grid voltage. The HDN-FLL is based on the harmonic decoupling network (HDN) consisting of multiple first order complex vector filters (FOCVF) with a frequency-locked loop (FLL), which makes the system frequency adaptive. The stability of the proposed FLL is strictly verified to be global asymptotically stable. In addition, the linearization and parameters tuning of the FLL is also discussed. The structure of the HDN has been widely used as a prefilter in grid synchronization techniques. However, the stability of the general HDN is seldom discussed. In this paper, the transfer function expression of the general HDN is deduced and its stability is verified by the root locus method. To show the advantages of the HDN-FLL, a simulation comparison with other gird synchronization methods is carried out. Experimental results verify the excellent performance of the proposed synchronization method.

A Simple Open-Circuit Detection Strategy for a Single-Phase Grid-Connected PV Inverter Fed From Power Optimizers

A simple and real-time open-circuit fault (OCF) detection method is proposed for a single-phase grid-connected photovoltaic inverter fed by series-connected power optimizers (POs). To implement the proposed method, PO controllers periodically monitor POs’ output voltage variations and calculate output voltage residual errors. Then, on the PO side, the OCF identification rule for the inverter is developed, which is based on the amount of voltage variation and the improved judgment criterion derived from voltage residual error and power. The proposed OCF detection method is of low cost and is real time, needing no extra hardware and no communication between the inverter and POs. Generally speaking, the method provides a reliable and fast solution to detect OCFs in the series-connected distributed dc power or energy storage systems. Its effectiveness and reliability are verified by experimental results.

A power electronic transformer based on a 4-plate compact capacitive coupler

This paper proposes a power electronic transformer based on a capacitive coupler. The coupler consists of four plates which are parallel-placed very closely. And there is a dielectric layer between each two plates to increase the capacitance value. The two plates on the same side are placed close to each other to maintain a large capacitance. The system topology is symmetrical to achieve bi-directional power transfer as both sides are composed of a full bridge inverter and a LCL circuit. The LCL circuit is used for filtering waves and forming resonance with the coupler. The coupler is modeled to simplify the analysis of the circuit. The whole system is verified by theory and simulation.

Design criteria for parallel connected-buck converters in DC microgrid loaded by CPLs

Power electronic converters and electric motor drives in advanced automotive systems, when they are tightly regulated, behave as constant power loads (CPLs) at the input terminals and usually cause negative impedance instability problems. Especially in a DC microgrid system with multiple parallel-connected buck converters, which is loaded by CPLs, the improper parameter settings of the buck converters may trigger oscillation and collapse. This paper analyzes the relationships of the parameters in a mathematical angle and proposes a design criteria to place the design constraints on the buck converters to prevent instabilities. Simulation results based on MATLAB/Simulink verify that the proposed criteria could insure the systems stability.

Novel active synchronization strategy for multi-bus microgrid with distributed cooperation control

This paper presents a novel active synchronization control strategy of multi-bus microgrid based on distributed cooperation technology. The method can reconnect microgrid back to utility grid (UG) seamlessly with sparse communication channels. Through the cross product of voltage vectors in the synchronizer, the frequency and phase of microgrid can track the main grid simultaneously. The voltage differences of UG and point of common coupling (PCC) are fed back to subsequent PI controllers to generate synchronization correction signals, which are only sent to the leader distributed generators (DG). Meanwhile, each DG exchanges information with its neighbors. The voltage of PCC can follow and synchronize with the main grid for a seamless transition and all DGs achieve the consensus behavior. Compared with traditional synchronization methods, the proposed method obviates complex communication network and improves flexibility and redundancy. Simulation results are presented to verify the effectiveness of the proposed method.

A harmonic and reactive power sharing control strategy for islanded microgrids

For an islanded microgrid with local linear loads and public nonlinear loads, this paper proposes a control strategy to achieve better reactive power sharing and harmonic current sharing based on second-order general integrator (SOGI) model and virtual impedance. First, the SOGI alternative structure filters out fundamental current and harmonic current respectively. Then, virtual inductance and virtual resistance are introduced to fundamental current and harmonic current, respectively, which meet the traditional frequency/voltage droop control application conditions and improve the reactive power sharing accuracy. In short, not only the public harmonic current can be shared also power quality for point of common coupling (pCC) is improved. The feasibility and effectiveness of the proposed strategy are validated by using Matlab simulation.

The Optimization Design of Main Circuit for Hybrid Active Power Filter

The performance of hybrid active power filter (HAPF) with different components and parameters is analyzed, a multi-objective optimization algorithm based on improved particle swarm optimization (PSO) is proposed. To reduce the influence of engineering experiences and redundant constraints, a unified description of performance for HAPF is proposed on the basis of theoretical analysis, and redundant constraints are removed from the optimization algorithm; To avoid premature of PSO algorithm, non-linear time-varying parameters are introduced, and a new global optimal particle selection operator is employed to improve solution diversity. The experimental results show that the proposed optimal design of main circuit for HAPF is of correctness and effectiveness.

A New Maximum Power Point Tracking Control Scheme in Photovoltaic Systems for Energy Saving

Maximum power point tracking (MPPT) control could significantly increase the overall reliability and conversion efficiency of grid-connected photovoltaic (PV) systems, so it can reduce energy loss in the photoelectric conversion. In this paper, a new MPPT (Zero Average Incremental Conductance) control algorithm is proposed. This method has the advantages of traditional IncCond MPPT. Moreover, it is stable against interference and not sensitive to system models. Therefore the proposed algorithm could be available and widely used in practical MPPT systems. A 200W grid-connected PV system prototype is built in lab. Experiment results verify the effectiveness of the proposed MPPT method.