Fei Liu

Bio: Fei Liu is an academic researcher from Wuhan University. The author has contributed to research in topics: Active filter & Control theory. The author has an hindex of 11, co-authored 32 publications receiving 1806 citations. Previous affiliations of Fei Liu include Huazhong University of Science and Technology.

A Variable Step Size INC MPPT Method for PV Systems

Abstract: Maximum power point tracking (MPPT) techniques are employed in photovoltaic (PV) systems to make full utilization of PV array output power which depends on solar irradiation and ambient temperature. Among all the MPPT strategies, the incremental conductance (INC) algorithm is widely used due to the high tracking accuracy at steady state and good adaptability to the rapidly changing atmospheric conditions. In this paper, a modified variable step size INC MPPT algorithm is proposed, which automatically adjusts the step size to track the PV array maximum power point. Compared with the conventional fixed step size method, the proposed approach can effectively improve the MPPT speed and accuracy simultaneously. Furthermore, it is simple and can be easily implemented in digital signal processors. A theoretical analysis and the design principle of the proposed method are provided and its feasibility is also verified by simulation and experimental results.

Parameter Design of a Two-Current-Loop Controller Used in a Grid-Connected Inverter System With LCL Filter

Abstract: LCL filters offer a better choice of attenuating switching frequency harmonics. However, in a grid-connected system, an LCL filter may cause resonance which is a disaster for the system's stability. In order to solve the problem, a two-current-loop control strategy, which includes grid-current outer loop and filter-capacitor-current inner loop, is adopted here. The implementation of this strategy is easy, but the tuning procedure is complex since the outer and inner controllers cannot cooperate well if the parameters of the controllers are not suitable. There is no literature which mentions a method to help give out accurate parameters of the controller. The difficulty of designing the controller is that only two feedbacks cannot provide complete information of a three-order LCL filter. A specific method is proposed in this paper to design the controller. The advantage of this method is to provide a way to maximize the utilization of the two feedbacks to get good system performance through parameter determination. The practicability of the method is tested by using bode diagram, and the antidistortion ability of the system is analyzed. Finally, experimental results verify the availability and correctness of the proposed method.

Analysis and Improvement of Maximum Power Point Tracking Algorithm Based on Incremental Conductance Method for Photovoltaic Array

Abstract: Photovoltaic (PV) array in the photovoltaic power system (PVPS) has the nonlinear current-voltage characteristic which is affected by the panels temperature and irradiance conditions To improve the energy converter efficiency of the PVPS, the maximum power point tracking algorithm should be adopted to maximize the yield of the PV array The incremental conductance (INC) method is widely used in the PVPS due to its succinctness and high tracking efficiency The conventional INC algorithm using a fixed iteration step-size is impossible to achieve rapid dynamic response and good steady tracking accuracy simultaneously, because if the step-size is increased for rapid dynamic response, the tracking accuracy is decreased and vice versa An improved algorithm with variable step- size is proposed to solve the problem in this paper The contrastive experimental results between the improved and the conventional INC algorithm are presented to demonstrate its effectiveness

Design and research on parameter of LCL filter in three-phase grid-connected inverter

Abstract: Since LCL filter has smaller inductance value comparing to L type filter with the same performance in harmonic suppression. it is gradually used in high-power and low-frequency current-source-controlled grid-connected converters. However design of LCL filter's parameter not only relates switch frequency ripple attenuation, but also impacts on performance of grid-connected current controller. This paper firstly introduced a harmonic model of LCL filter in grid-connected operation, then researched the variable relationship among LCL filter's parameter and resonance frequency and high-frequency ripple attenuation. Based on above analysis a reasonable design method was brought out in order to achieve optimal effect under the precondition of saving inductance magnetic core of LCL filter, at the same time guaranteeing the resonance frequency of LCL filter was not too small lest restrict current controller resign. Finally this design method was verified by the experimental results.

Solid-State Circuit Breaker Snubber Design for Transient Overvoltage Suppression at Bus Fault Interruption in Low-Voltage DC Microgrid

Abstract: Under a short-circuit fault in low-voltage dc microgrid, solid-state circuit breaker (SSCB) assumes the responsibility of the quick and effective isolation of the faulted area, while its own safety and reliability depends on the overvoltage suppression ability of its snubber. For SSCB snubber design, however, traditional method suited for converter switch snubber cannot be directly applied, because SSCB snubber stresses more on its overvoltage suppression and fault current withstanding instead of snubber loss reduction. Therefore, this paper strives to build a snubber design method specialized for SSCB overvoltage suppression at bus fault interruption. A comprehensive and target-focused set of measurements is proposed and applied to the comparison of three RCD snubber candidates: charge-discharge type, discharge-suppressing type I, and discharge-suppressing type II. The set of measurements includes six indicators with peak SSCB voltage and peak bus current as the main indexes. After parameters are determined, the indicators of the three snubbers are compared, through which discharge-suppressing type I is selected as the most appropriate SSCB snubber. Experiments of the three snubbers have also been conducted in a ±200-V/dc system to verify the correctness of comparison results and the validity of discharge-suppressing type I for SSCB overvoltage suppression at dc-bus fault.

Evaluation of the Main MPPT Techniques for Photovoltaic Applications

Abstract: This paper presents evaluations among the most usual maximum power point tracking (MPPT) techniques, doing meaningful comparisons with respect to the amount of energy extracted from the photovoltaic (PV) panel [tracking factor (TF)] in relation to the available power, PV voltage ripple, dynamic response, and use of sensors. Using MatLab/Simulink and dSPACE platforms, a digitally controlled boost dc-dc converter was implemented and connected to an Agilent Solar Array E4350B simulator in order to verify the analytical procedures. The main experimental results are presented for conventional MPPT algorithms and improved MPPT algorithms named IC based on proportional-integral (PI) and perturb and observe based on PI. Moreover, the dynamic response and the TF are also evaluated using a user-friendly interface, which is capable of online program power profiles and computes the TF. Finally, a typical daily insulation is used in order to verify the experimental results for the main PV MPPT methods.

A Hybrid AC/DC Microgrid and Its Coordination Control

Abstract: This paper proposes a hybrid ac/dc micro grid to reduce the processes of multiple dc-ac-dc or ac-dc-ac conversions in an individual ac or dc grid. The hybrid grid consists of both ac and dc networks connected together by multi-bidirectional converters. AC sources and loads are connected to the ac network whereas dc sources and loads are tied to the dc network. Energy storage systems can be connected to dc or ac links. The proposed hybrid grid can operate in a grid-tied or autonomous mode. The coordination control algorithms are proposed for smooth power transfer between ac and dc links and for stable system operation under various generation and load conditions. Uncertainty and intermittent characteristics of wind speed, solar irradiation level, ambient temperature, and load are also considered in system control and operation. A small hybrid grid has been modeled and simulated using the Simulink in the MATLAB. The simulation results show that the system can maintain stable operation under the proposed coordination control schemes when the grid is switched from one operating condition to another.

Simulation and Hardware Implementation of Incremental Conductance MPPT With Direct Control Method Using Cuk Converter

Abstract: This paper presents simulation and hardware implementation of incremental conductance (IncCond) maximum power point tracking (MPPT) used in solar array power systems with direct control method. The main difference of the proposed system to existing MPPT systems includes elimination of the proportional-integral control loop and investigation of the effect of simplifying the control circuit. Contributions are made in several aspects of the whole system, including converter design, system simulation, controller programming, and experimental setup. The resultant system is capable of tracking MPPs accurately and rapidly without steady-state oscillation, and also, its dynamic performance is satisfactory. The IncCond algorithm is used to track MPPs because it performs precise control under rapidly changing atmospheric conditions. MATLAB and Simulink were employed for simulation studies, and Code Composer Studio v3.1 was used to program a TMS320F2812 digital signal processor. The proposed system was developed and tested successfully on a photovoltaic solar panel in the laboratory. Experimental results indicate the feasibility and improved functionality of the system.

A Novel Improved Variable Step-Size Incremental-Resistance MPPT Method for PV Systems

Abstract: Maximum power point (MPP) tracking (MPPT) techniques are widely applied in photovoltaic (PV) systems to make PV array generate peak power which depends on solar irradiation. Among all the MPPT strategies, the incremental-conductance (INC) algorithm is widely employed due to easy implementation and high tracking accuracy. In this paper, a novel variable step-size incremental-resistance MPPT algorithm is introduced, which not only has the merits of INC but also automatically adjusts the step size to track the PV array MPP. Compared with the variable step-size INC method, the proposed scheme can greatly improve the MPPT response speed and accuracy at steady state simultaneously. Moreover, it is more suitable for practical operating conditions due to a wider operating range. This paper provides the theoretical analysis and the design principle of the proposed MPPT strategy. Simulation and experimental results verify its feasibility.

Filter-Based Active Damping of Voltage Source Converters With $LCL$ Filter

Abstract: Pulsewidth modulation (PWM) voltage source converters are becoming a popular interface to the power grid for many applications. Hence, issues related to the reduction of PWM harmonics injection in the power grid are becoming more relevant. The use of high-order filters like LCL filters is a standard solution to provide the proper attenuation of PWM carrier and sideband voltage harmonics. However, those grid filters introduce potentially unstable dynamics that should be properly damped either passively or actively. The second solution suffers from control and system complexity (a high number of sensors and a high-order controller), even if it is more attractive due to the absence of losses in the damping resistors and due to its flexibility. An interesting and straightforward active damping solution consists in plugging in, in cascade to the main controller, a filter that should damp the unstable dynamics. No more sensors are needed, but there are open issues such as preserving the bandwidth, robustness, and limited complexity. This paper provides a systematic approach to the design of filter-based active damping methods. The tuning procedures, performance, robustness, and limitations of the different solutions are discussed with theoretical analysis, selected simulation, and experimental results.