Bangyin Liu

A Variable Step Size INC MPPT Method for PV Systems

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.

Optimal Allocation and Economic Analysis of Energy Storage System in Microgrids

This paper presents a methodology for the optimal allocation and economic analysis of energy storage system (ESS) in microgrids (MGs) on the basis of net present value (NPV). As the performance of an MG strongly depends on the allocation and arrangement of its ESS, optimal allocation methods and economic operation strategies of the ESS devices are required for the MG. To optimize the operation strategies and capacities of ESS in MGs, the financial benefit and dynamic models of ESS are discussed. And then, a matrix real-coded genetic algorithm is applied to find maximal NPV, in which each GA chromosome consists of a 2-D real number matrix representing the generation schedule of ESS and distributed generation sources. This paper is to suggest, among those available ESS, the optimal sizes and types of them and their optimal arrangement, such that the total NPV achieved during the system operational lifetime period is maximized. Finally, some computational simulation results are presented to verify the effectiveness of the proposed method.

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

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 systems 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.

Photovoltaic DC-Building-Module-Based BIPV System—Concept and Design Considerations

The photovoltaic (PV) modules used in the building-integrated PV (BIPV) system, generally, can be installed in different orientations and angles. Moreover, performance of the PV modules is easy to be affected by partial shadows and mismatch of their electrical parameters. Consequently, the conventional power configurations are difficult to obtain higher energy efficiency and reliability. Some improved power configurations of BIPV system have been presented to solve these problems. The objective of the paper is to give an overview of the existing solutions, and then, proposes an efficient and cost-effective power configuration of BIPV system from the energy conversion and control point of view. The proposed power configuration consists of plenty of PV dc-building module (PV-DCBM) and a centralized inverter. Each PV-DCBM includes a high step-up dc-dc converter integrated with a PV module into an individual electrical device. The PV-DCBMs are parallel connected, and then, connected to a common dc bus. The centralized inverter is connected to the grid. The design criterions, optimum design considerations, and design procedure of the proposed PV-DCBM-based BIPV system are proposed in this paper. The experimental results are presented to verify the validity and feasibility of the novel concept.

A Novel DC Capacitor Voltage Balance Control Method for Cascade Multilevel STATCOM

This paper presents a novel dc capacitor voltage balance control method for cascade multilevel static synchronous compensator (STATCOM) and a general analytical method for balance control strategy. Considering that the imbalance of dc capacitor voltage is caused by the inconsistency of active power absorbed and consumed by chain, a balance control strategy based on active voltage vector superposition is proposed, in which an active voltage component is superposed to chains output voltage to change its absorbed active power. A general analytical method based on vector analysis is also presented, by which the performance of balance control strategy can be analyzed, including stability and regulation capacity. To find out the most appropriate balance control strategy, a comparison still based on vector analysis among the proposed and other two commonly used methods is provided, from which it can be known that the proposed balance control strategy has the advantage of good stability and strong regulation capacity, and simulations are performed to prove it. The effectiveness of proposed control scheme has been verified by experimental results based on a three-phase 36-chain cascade multilevel STATCOM laboratory prototype.

Stability Analysis of Grid-Connected Inverter With LCL Filter Adopting a Digital Single-Loop Controller With Inherent Damping Characteristic

The stability problem of the grid-connected inverter with LCL filter adopting a digital single-loop controller without extra damping resistance is analyzed. The digital single-loop control based on the grid-side current feedback can attenuate the resonance introduced by LCL filters due to the inherent damping characteristic embedded in the control loop. The stability analysis is performed by means of a Nyquist diagram in the discrete domain and to provide a general design direction for filter and controller. The quantitative relationship of ratios of LCL resonance angular frequency to control frequency and controller parameters on the system stability is investigated. It is derived that the system would obtain preferable stability margin when the ratio is located at a certain range. The theoretical analyses are validated by experimental tests based on a 5-kW single-phase inverter laboratory prototype.

Influence of Delay on System Stability and Delay Optimization of Grid-Connected Inverters With LCL Filter

Delay is inevitable in digital-controlled system and the delay will change system phase-frequency characteristic, thus affecting system stability. The system stability of inductor-capacitor-inductor (LCL)-type grid-connected inverter with single-loop control based on inverter-side current and single-loop control based on grid-side current is analyzed both in continuous and discrete domains. The influence of delay time on system stability is systematically studied. A delay time control method that is capable of adjusting delay time is further proposed to improve system stability. The proposed delay time control method is applied in the experiment, making an unstable system to be stable, and verifies the analysis result and proposed method.

An Active Low-Frequency Ripple Control Method Based on the Virtual Capacitor Concept for BIPV Systems

This paper proposes an active low-frequency ripple control device (ALFRCD) based on virtual capacitor concept for building-integrated photovoltaic (BIPV) systems. With this method, the power quality of the BIPV system can be enhanced, and the lifetime and reliability are superior by reducing electrolytic capacitors. The low-frequency ripple current caused by the single-phase inverter can be compensated promptly by introducing a current integrator instead of unit feedback in the control strategy of the ALFRCD. With this current integrator, the device is unable to output dc current, which is similar to the function of a capacitor. Compared with the conventional method with high-pass filter, the presented method can achieve better compensation efficiency at most of the low frequencies and the response speed is superior as well. In addition, only one more integrator is needed in the control loop, which indicates the conciseness of the method. The performance is analyzed and the design algorithm is introduced in detail. Simulations and experimental results verified the proposed methods reliability and superiority in comparison with the method with high-pass filter and a proportional-integral controller.

An Islanding Detection Method Based on Dual-Frequency Harmonic Current Injection Under Grid Impedance Unbalanced Condition

This paper proposes a three-phase grid impedance detection method based on dual-frequency harmonic current injection for islanding detection. Grid impedance detection based on single harmonic current injection is reliable in three-phase impedance balanced grid. However, under impedance unbalanced condition, the harmonic voltage caused by the injected symmetric harmonic current is asymmetric, which affects the calculation of grid impedances and even leads to failed detection. The method based on dual-frequency harmonic current injection is injecting two non-characteristic symmetric harmonic currents, and then according to the different harmonic voltages caused by different frequency harmonic currents, all three-phase impedances can be calculated accurately. The implementing algorithm of the presented method is derived and its performance is analyzed in detail. Simulation and experiments are carried out under grid impedance balanced and unbalanced conditions. Theoretical analysis and experiment results proved that the proposed method is feasible.

Eliminating DC Current Injection in Current-Transformer-Sensed STATCOMs

Grid-connected power electronics converters/inverters usually have certain amount of dc component at their ac terminal. They are likely to inject unwanted direct current into the power grid, unless a line-frequency transformer is employed. This study investigates this dc injection problem for static synchronous compensator (STATCOM) and connects the dc injection with current transformer (CT), a current sensing element widely used in high-voltage converters. By introducing CTs model into the STATCOMs model, this study found that using CT for current feedback control can cause large dc injection, if no extra means is taken. Expressions of the dc injection for different controllers were derived. Then, a dc injection elimination method was proposed. This method eliminates the dc component of the output current by building an indirect dc feedback loop; therefore, it can prevent injecting the direct current even when it cannot be sensed. Experimental results taken on a 25-level cascaded multilevel STATCOM prototype verifies the analysis and the proposed method.