Changsong Chen

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.

Optimal Design Methodology for LLC Resonant Converter in Battery Charging Applications Based on Time-Weighted Average Efficiency

In this paper, an LLC resonant converter design methodology for battery charging applications is proposed aiming at achieving high efficiency. Compared with traditional resistive or constant power load applications, the battery voltage and current are nonlinear and vary with the charging profiles, making the optimal design of battery charger more difficult and complicated. Based on the characteristics of the battery charging profiles, a new time-weighted average efficiency (TWAE) index is proposed, which represents the average weight of conversion efficiency during battery charging period. Converters losses are calculated based on the standard loss models using the current and voltage information derived from the LLC steady-state model, which is solved utilizing the numerical non-linear programming techniques. In addition, the TWAE is achieved serving as the objective function to optimize the converter parameters. To reduce the search space and speed up the search algorithm, a variable-step exhaustive search algorithm is applied considering the constraints of operation and variable range. Finally, a 3 kW prototype LLC converter, which converts 220 VAC from the grid to a wide output voltage range from 60 to 100 V is built and a TWAE of 94.74% and a peak efficiency of 95.19% are achieved, validating the effectiveness of the presented method.

An LCC-Compensated Resonant Converter Optimized for Robust Reaction to Large Coupling Variation in Dynamic Wireless Power Transfer

In this paper, a reformed compensation network based on traditional LCL topology is proposed to realize robust reaction to large coupling variation that is common in dynamic wireless charging application. From the perspective of smoothing, the power characteristic against coupling factor and to achieve high efficiency simultaneously, extra reactive element is inserted into the resonant tank to provide higher degree of design freedom, forming the improved LCC type compensated inductive power transfer (IPT) system that features with distinct parameter design approach. Comparing to existing compensation topologies, the optimized LCC compensation topology shows more robust power characteristic against variation of coupling factor and, thus, minimizes the need for complex control which is usually undesired in dynamic charging scheme, thereby helping maintain effective power transfer in dynamic charging application and largely enhancing the systematic controllability. Finally, the effectiveness of the proposed method is experimentally verified. The power transfer profile is smooth against coupling factor to realize high tolerance to position as the power drop is no more than 20% within almost 200% of coupling factor variation.

A General Design Method of Primary Compensation Network for Dynamic WPT System Maintaining Stable Transmission Power

Dynamic wireless power transmission (DWPT) is considered as a solution to the problems encountered in the development of electrical vehicle (EV), such as range anxiety resulted from battery bottleneck and the difficulty of convenient charging. However, the requirement of DWPT, maintaining almost constant transmission power with constant voltage load in effective movement process, is quite different from Stationary WPT. The power fluctuation is easily brought by the coupling coils over large misalignment in movement. A general design method of primary compensation network from the perspective of DWPT is presented in this paper. Under the premise of ensuring high transmission efficiency and soft switching, a novel T-type compensation network for DWPT is proposed, which maintains a stable output characteristic over a wide misalignment, companied with an inherent current limiting ability under no-load operation. A WPT prototype with a fixed frequency operation based on the T-type compensation network is built. The output power is kept almost stable even though magnetic coupling coefficient varies twice showing the effectiveness of the design method.

Low frequency ripple propagation analysis in LLC resonant converter based on signal modulation-demodulation theory and reduction based on PIR control strategy

Two-stage AC-DC-DC converters are widely used in battery charge systems. While in a single-phase two-stage AC-DC-DC converter system, the pulsating power which is twice of the grids fundamental frequency will affect the DC bus voltage, which will eventually deteriorate the quality of output voltage. This paper analyzed the propagation of low frequency voltage ripple in LLC DC-DC resonant converter based on signal modulation-demodulation theory and quantitative relationship between the amplitude of input and output low frequency ripple voltage was obtained. A PIR controller was designed to reduce the output low frequency voltage ripple. Experimental results with a 1kW prototype verified the accuracy of the obtained quantitative expression and the designed PIR controller can reduce 76.7% 100Hz ripple voltage at output side of LLC resonant converter.

Real-valued MUSIC algorithm for power harmonics and interharmonics estimation

Recently, advanced spectrum estimation methods, including the MUSIC (Multiple Signal Classification) algorithms, are being gradually employed for high-resolution power harmonics analysis. However, most of them are proposed to detect frequencies of complex-valued signals, so that any real-valued signal should be transformed into complex form. This data pre-treatment may lead to additional computation burden. In addition, the picket-fence effects also exist as in the FFT algorithm and cause poor frequency resolution. To overcome these drawbacks, a real-valued MUSIC algorithm is proposed for power harmonics analysis in this paper. The algorithm is based on the subspace decomposition theory and the computation of pseudospectrum is also provided. Additionally, to improve the measuring precision, the Newton–Raphson algorithm is adopted to optimize the harmonic frequencies significantly. Simulation results show that, in the real-valued MUSIC pseudospectrum, the spectral peaks of actual harmonic components can be more easily distinguished from the false peaks caused by noise, and the computational complexity is notably lower than that of the classic complex MUSIC, as well as the detecting accuracy is close to that of root-MUSIC algorithm which is quite time consuming. Experimental results prove that the proposed strategy is more suitable for high-resolution power harmonics estimation. Copyright

Techno-economical analysis of Vanadium redox and Lead-acid batteries in stand-alone photovoltaic systems

Sizing and techno-economical analysis of a stand-alone photovoltaic generation system with different energy storage is presented in this paper. The method adopted is based on the Loss of power supply probability and levelized cost of energy concepts. A new energy storage technology of Vanadium redox battery was applied in photovoltaic power system, which was compared to traditional Lead-acid battery from their physical characteristics and economical efficiency. And the mathematical models of photovoltaic, Vanadium redox battery, Lead-acid battery are developed. Simulation results show that the techno-economical of Vanadium redox battery was prior and fit for long-term energy storage.

A Variable Duty Cycle Soft Startup Strategy for LLC Series Resonant Converter Based on Optimal Current-Limiting Curve

For the LLC series resonant converter (LLC-SRC), the resonant current inrush during the startup is a threat to the safe operation of the main switches, and frequency-decreasing startup method is usually adopted. The very high initial start-up frequency can suppress this inrush effectively, but increases the output current demand of drive ICs. In this paper, a variable duty cycle soft startup strategy based on optimal current-limiting curve is presented, which can realize the suppression of the resonant current inrush under low startup frequency. Due to the nonlinearity of the LLC-SRC, the mode transition conditions are illustrated and a generalized numerical model of the LLC-SRC is established, providing a highly accurate prediction on resonant variables and output during the startup process. Based on the numerical model, the relationship between the startup current, initial startup frequency, and the duty cycle is revealed to quantify the startup process and achieve optimal initial startup parameters. A numerical algorithm is presented to obtain the optimal current-limiting curve during the startup. Two variable duty cycle startup methods are applied to verify the effectiveness of the proposed strategy, namely the PWM startup method and the phase-shift startup method. The experimental results are presented to verify the proposed soft startup strategy.

An Improved Start-up Method for LLC Series Resonant Converter Based on State-plane Analysis

In order to limit the resonant current, the LLC resonant converter usually starts up with a frequency which is much higher than the resonant frequency. However, if the start-up frequency is too high, the build-up time of the output voltage will be very long. Moreover, the start-up frequency is also limited by the hardware condition such as drive circuit. In this paper, the start-up process for the LLC resonant converter is investigated based on state-plane method. The expression of start-up current in the resonant tank is derived. The relationship between start-up current and start-up frequency is discussed. According to the analysis, some rules are given about how to select the start-up frequency. An improved start-up method is proposed to limit the start-up current at a low start-up frequency. Experimental results are presented to verify the conclusions.

Centralized control of parallel connected power conditioning system for battery energy storage system in charge-discharge-storage power station

This paper presents a centralized control system that coordinates parallel operations of power conditioning system (PCS) for battery energy storage system (BESS) in charge-discharge-storage power station. An overall energy management system is implemented to optimize power flow among different battery energy storage systems during both grid-connected and islanded operations. In islanded mode, the traditional load sharing strategy will result in battery aging and increased circulation if the state of charge (SOC) of each battery is different. Therefore, a power allocation strategy based on SOC and module capacity is designed, which can make full use of the total capacity of the parallel system. The design concept of the proposed centralized control system is evaluated through simulation and experiment studies under different test scenarios.