Lang Huang

A Sliding-Mode Controller With Multiresonant Sliding Surface for Single-Phase Grid-Connected VSI With an LCL Filter

This paper presents a sliding-mode control (SMC) scheme via multiresonant sliding surface for single-phase grid-connected voltage source inverter with an LCL filter to eliminate the grid current tracking error as well as suppress its total harmonic distortion (THD). In general, the design of SMC leads to a sliding surface that is a linear combination of the system state variables and the generated references. The sliding surface would drift while the system parameters change or external disturbance exists, which affects the tracking error and THD of system output seriously. Moreover, for ac tracking system, integral SMC can reduce, but not fully alleviate the sinusoidal tracking error and has limited ability to suppress the grid current harmonics, especially for high-order harmonics. In order to fully eliminate the grid current tracking error and suppress its THD effectively, multiple resonant terms of the grid current error are added to the sliding function. It is used for the first time in the SMC. This method can be used for an arbitrary ac tracking system. Simulation and experimental results on a 5-kVA single-phase grid-connected inverter prototype show the effectiveness of the proposed control strategy. The tracking precision of the grid current is about 0.91% and the THD of the grid current is 0.76%.

A novel repetitive control scheme for three-phase grid-connected inverter with LCL filter

Grid voltage distortion gives rise to extra grid current harmonics in grid-connected inverter system. To damp these harmonics, this paper proposes a novel control strategy in which inverter output current is regulated by PI controller and grid current is controlled by repetitive controller (RC) for three-phase grid-connected voltage source inverter (VSI) with LCL filter under d-q rotating frame. This paper implements RC to regulate grid current based on the stability of the system under inverter output current control loop. This paper discusses the design of RC and analyzes the stability and effectiveness of the proposed control strategy in damping the grid current harmonics arising from grid voltage distortion. At last, proposed scheme is verified on a 100 kW three-phase grid-connected PV inverter prototype. Simulation and experimental results show that the proposed scheme has a good performance.

Dynamic Modeling Based on Coupled Modes for Wireless Power Transfer Systems

A novel dynamic modeling method based on the concept of coupled modes is proposed for wireless power transfer (WPT) systems which use magnetic resonant coupling. The proposed method aims on the dynamics of the overall WPT system, including the nonlinear inverter and rectifier. It uses the slowly varying amplitudes and phases of coupled modes rather than resonant currents and voltages to describe the coupled resonances. Three analytical models-averaged model, small signal model, and conductance network model are developed sequentially by using the proposed method. The orders of the developed models are equal to or lower than that of the discrete state space model. In contrast, the existing dynamic modeling methods for WPT systems and resonant converters have to transform the discrete state space model into a higher order model or use complex currents and voltages in order to adopt the averaging method and obtain an analytical model. Simulation and experimental results give a firm support to the proposed method and models. The concept employed in this paper provides a deeper insight into the dynamic behaviors of coupled resonances.

Coil structure optimization method for improving coupling coefficient of wireless power transfer

The low coupling coefficient is the main reason that results in difficulties to design a high performance mid-range wireless power transfer (WPT) system which uses magnetic resonant coupling. The advantages of improving the coupling coefficient are quantitatively discussed in this paper. It is highlighted that improving coupling coefficient is the only method to reduce the sensitivity to frequencies. Furthermore, this paper proposes a method that improves the coupling coefficient by optimizing the structure of the coils. The optimization of a pair of spiral coils is presented. The method has much lower computation cost than finite element analysis (FEA) and its accuracy is sufficient for design procedures.

A discrete-time integral sliding-mode controller with nonlinearity compensation for three-phase grid-connected photovoltaic inverter

This paper presents a novel control strategy that combines a discrete-time integral sliding-mode controller (DISMC) with nonlinearity compensation under d-q rotating frame for three-phase grid-connected photovoltaic inverter to suppress the current harmonics as well as improve the dynamic response and robustness. Because of the nonlinearity introduced by dead time and turn on/off delay of three-phase grid-connected inverter, the output voltage of inverter is distorted seriously. This distortion increases the THD of the grid current. In this paper, the nonlinearity of inverter output voltage is analyzed and the model of inverter with the nonlinearity effect is established accordingly. In addition, a discrete-time integral sliding-mode controller is designed according to the new model to regulate the grid current. The novel control strategy achieves both exact nonlinearity compensation and strong robustness control of the whole system. Finally the control scheme is verified in a 100 kW three-phase grid-connected photovoltaic inverter prototype. The simulation and experimental results show that the proposed control strategy has fast dynamic response, strong robustness and good current harmonics rejection.

Voltage control and fluctuation suppression of the three-phase SST with DC bus in dual rotating reference frames

The three-phase solid-state transformers(SST) with DC bus have been widely studied in micro-grid systems. The double line frequency component of AC power on DC-link in SST is unavoidable because of the inherent single phase structure of the HV-side cascaded H-Bridge(CHB) converter. However, the AC power at double line frequency of three phases can be mutually eliminated by enabling DAB converters with the ability to process the power. This paper develops a closed multi-loops method of voltage control and fluctuation suppression for the three phase SST system with DC-bus in dual rotating reference frames. Theoretical analysis and modeling are provided. The performance of the proposed control strategy is verified through simulation results and the experiment is under way.

Hierarchical model predictive control of modular multilevel matrix converter for low frequency AC transmission

The modular multilevel matrix converter (M3C) applied to a low frequency alternating current (LFAC) transmission system is investigated. Due to the complexity and tight coupling of the converter, a novel hierarchical model predictive control (MPC) scheme with a cascaded structure of power control, capacitor voltage averaging and balancing control for M3C is proposed. The terminal behavioral model of M3C is derived out and the fast separated space-vectors approximation method is realized. The proposed scheme clearly achieves the system-level multi-objective control without the empirical procedure of the weighting factor design and significantly reduces the computational cost. This is the first time to apply the MPC to a converter in a hierarchical structure. Simulation results of a 7-level M3C are provided to illustrate the systems performance.

A novel current dual-loop control strategy for three-phase grid-connected VSI with LCL filter

In grid-connected PV inverter system, grid voltage distortion gives rise to extra grid current harmonics. This paper first analyzes the shortage of inverter output current control scheme in suppressing the grid current harmonics caused by grid voltage distortion for grid-connected three-phase voltage source inverter (VSI) with LCL filter under d-q rotating frame. To damp these harmonics, we propose a novel dual-loop control scheme in which inverter output current inner loop is controlled by PI controller and grid current outer loop is regulated by the combination of PI and PR controller. PR controller yields a good performance in damping special order harmonics under this control scheme. Output admittance of the system is derived to analyze the impact of grid voltage distortion on grid current. Stability and effectiveness of this proposed strategy are verified with theoretical analysis as well as simulation and experimental results under 100kW PV inverter prototype.

A discrete time-domain model for fast simulation of MMC circuits

In this paper, a novel discrete time-domain MMC Sub Module (SM) model is proposed for fast simulation of MMC circuits. Firstly, by using Modified Nodal Approach (MNA), the switch in SM is replaced by a changeable voltage source in series with a resistor. Then the SM model is simplified in form and mathematics expression for fast simulation. Secondly, stability of these algorithms are discussed in detail. It is because applying various numerical integration algorithms to the SM model will get different difference expressions, which may result in stability problem. Finally, the efficiency and accuracy of the model are validated using a 20-level MMC circuit by comparison with commercial simulation software.

An exact discrete-time model considering dead-time nonlinearity for an H-bridge grid-connected inverter

With the development of the renewable energy, grid-connected H-bridge inverter plays a more important role than ever. However, the dead-time and the zero crossing clamping (ZCC) effect caused by dead-time can cause THD problem. As a consequence, more and more researches have focused on this issue. Most of the researches use the harmonics to replace the effect of dead-time, which ignores ZCC effect. Some other researches use the average current to distinguish the ZCC effect, which is not accurate enough. This paper analyzes the whole changing process of inductor current ripples during power cycle and a complete discrete-time model of the H-bridge grid-connected inverter with dead-time is established. On the basis of the model, the analysis of dynamic behavior is carried out, the result of which shows that the length of dead-time can cause nonlinear phenomenon and reduces the stability region of the controller parameters. Besides, this model is also useful to the nonlinearity compensation of the system.