Chen and Wang 1 proposed a dual T-Type four-level converter with a variable-carrier voltage balancing method. The capacitor voltage imbalance mechanism was revealed in 1, Sec. III, with a conclusion ...that the overdischarge of the middle capacitor is the root cause of the capacitor voltage imbalance issue. This has been experimentally validated by the articles 1, 2. Although the conclusion is correct, the theoretical derivation of the average capacitor currents is incorrect as the input current and the duty cycle of every power switch were not considered in the previous version. As a correspondence, we would like to present the corrected equations of the average capacitor current calculation in Section III. In addition, there is one more correction to the capacitor voltage ripple description in Section V.
Reliability of battery diagnosis depends on accurate estimation of the state of charge (SOC) and battery characterizing parameters including maximum capacity, internal impedance, polarization ...coefficients, and their probabilistic characterizations. This paper develops a framework that employs real-time operating data to estimate jointly the SOC and parameters, performs statistical analysis to derive quantitative diagnostic procedures with error analysis. Convergence of the algorithms, asymptotic distributions, and diagnosis reliability analysis are performed rigorously by using stochastic differential equations, central limit theorems, and large deviations principles. Simulated case studies and experimental data are used to illustrate the diagnosis algorithms and their capabilities. Experimental studies are conducted to verify the results.
A high level of electricity price volatility increases financial risks and safety issues in power system operations. Storage devices as an efficient solution to mitigating price volatility have ...attracted extensive attention. In this paper, the mechanism of how storage devices impact price volatility is investigated at first. Through analyzing the connection between an economic dispatch problem and its Lagrange dual, we reveal that the capacity and charge/discharge power of a storage device installed at a node have an aggregate impact on the range of the price changes of that node. A model to calculate the sensitivity of price volatility to storage device parameters is proposed. To determine the optimal nodes to place storage devices and the optimal storage capacities to constrain the price change levels, this paper develops a chance-constrained optimization model, and a scenario-based approach is adopted to solve the problem. The IEEE 24-bus system is employed in the case studies, associated with Gurobi and Julia. Simulation studies show that if there is one node with the highest price change all the time, the price volatility must have the highest sensitivity to the storage device on that node. However, in the case where there are multiple nodes having the highest price changes at different times, a node with high price change does not necessarily mean high sensitivity. The simulation studies verify that the chance-constrained optimization model is effective to suppress excess price changes.
•The impact of energy storage devices on price volatility is revealed by Lagrange duality.•A model to calculate sensitivity of price volatility to storage parameters is established.•A chance-constrained optimization model is proposed to find the optimal nodes to place storage devices.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UL, UM, UPCLJ, UPUK, ZAGLJ, ZRSKP
This article investigates a drag-type vertical-axis wind turbine that is targeted for small-scale wind energy system applications. Based on aerodynamics models, the three-dimensional simulation ...studies have been carried out to obtain the force distributions along blades and eventually the torque and power coefficients for different vertical-axis wind turbine configurations. An optimal vertical-axis wind turbine configuration is chosen based on the comparative analysis, and a 2 kW prototype system has been implemented based on the design. The effectiveness of the three-dimensional models and simulation results has been verified by the measured data from the actual vertical-axis wind turbine system. The wake impacts to the vertical-axis wind turbine caused by nearby objects are also analyzed. The simulation results and the actual operation experiences show that the proposed system has the characteristics of low cut-in speed, high power density, and robustness to adjacent objects (such as buildings and other wind turbines), which make it suitable for small-scale wind energy systems in populated areas including urban environment.
This paper presents a coordinated control strategy for a hybrid wind farm with doubly-fed induction generator (DFIG)- and direct-driven permanent-magnet synchronous generator (PMSG)-based wind ...turbines under symmetrical grid faults. The proposed low-voltage ride-through (LVRT) strategy is based on a novel current allocation principle and is implemented for individual DFIG- or PMSG-based wind turbines. No communication equipment between different wind power generators is required. By monitoring the local voltages and active power outputs of the corresponding wind generators, the proposed control strategy can control the hybrid wind farm to provide the maximum reactive power to support the grid voltage during a symmetrical grid fault. As a result, the reduction in the active power output from the hybrid wind farm can be decreased, which also helps avoid generator over-speed issues and supply active power support for the power grid. In addition, the reactive current upper limits of DFIG- and PMSG-based sub-wind farms are investigated by considering different active power outputs and different grid voltage dip depths, and the feasible regions of the two types of sub-wind farms for meeting the LVRT requirements are further studied. Finally, the effectiveness of the proposed coordinated LVRT control strategy for the hybrid wind farm is validated by simulation and experimental results.
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IZUM, KILJ, NUK, PILJ, PNG, SAZU, UL, UM, UPUK
In a single-stage buck-boost quasi-switched boost inverter (qSBI), the shoot-through state insertion causes high amplitude common-mode voltage. Consequently, the qSBI becomes less attractive in ...transformerless photovoltaic (PV) systems. In this paper, a novel space vector pulse-width modulation for a modified qSBI is introduced to reduce the magnitude of common-mode voltage and push the modulation index up to 1. By properly choosing the shoot-through interval time, shoot-through states are considered to be inserted for boosting voltage and also reducing the THD value of the output voltage. The mathematical analysis and operating principles of the converter are discussed and verified through PSIM simulations. Finally, an experimental prototype is validated based on a TMS320F28335 DSP microcontroller and a DE0-Nano FPGA digital control platform.
The frequency components of an induction machine due to different types of internal faults and their combinations are studied in this paper. Instead of taking a look at the current and control ...signals directly, this paper studies the impact caused by the basic inner structure on the inductances and their frequency components. Modified winding function and equivalent air-gap length are computed such that the combinations of internal faults can be accommodated. Static and dynamic eccentricity reshape the air-gap in a direct manner, while saturation brings third order harmonics, which instead can be equivalently considered as the effect caused by the fluctuation of air-gap length. Thus eccentricity faults and saturation have combined impacts on the equivalent air-gap length. Modified winding function accounting for all space harmonics simplifies the computation of inductances, and is also adequate for the case with rotor asymmetry. By extracting frequency components, different internal fault features are revealed. ANSYS/Maxwell is employed to simulate different cases to verify the theoretical analysis.
This paper presents an effective approach for wind turbine (WT) condition assessment based on the data collected from wind farm supervisory control and data acquisition (SCADA) system.
Three types of ...assessment indices are determined based on the monitoring parameters obtained from the SCADA system. Neural Networks (NNs) are used to establish prediction models for the assessment indices that are dependent on environmental conditions such as ambient temperature and wind speed.
An abnormal level index (ALI) is defined to quantify the abnormal level of the proposed indices.
Prediction errors of the prediction models follow a normal distribution. Thus, the ALIs can be calculated based on the probability density function of normal distribution. For other assessment indices, the ALIs are calculated by the nonparametric estimation based cumulative probability density function. A Back-Propagation NN (BPNN) algorithm is used for the overall WT condition assessment.
The inputs to the BPNN are the ALIs of the proposed indices. The network structure and the number of nodes in the hidden layer are carefully chosen when the BPNN model is being trained. The condition assessment method has been used for real 1.5 MW WTs with doubly fed induction generators. Results show that the proposed assessment method could effectively predict the change of operating conditions prior to fault occurrences and provide early alarming of the developing faults of WTs. KCI Citation Count: 1
Control theories for discrete event systems modeled as finite state machines have been well developed to address various fundamental control issues. However, finite state machine model has long ...suffered from the problem of state explosion that renders it unsuitable for some practical applications. In an attempt to mitigate the state explosion problem, we propose an efficient representation that appends finite sets of variables to finite state machines in modeling discrete event systems. We also present the control synthesis techniques for such finite state machines with variables (FSMwV). We first present our notion and means of control under this representation. We next present our algorithms for both offline and online synthesis of safety control policies. We then apply these results to the control of electric power grids.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK
There has been no marginal emission information and/or marginal fuel mix data published by the regional transmission organizations (RTOs) or independent system operators (ISOs) in real-time. This ...paper presents a support vector machine (SVM) based method to estimate and predict hourly marginal emissions and marginal fuel mix in real-time in the PJM market area. Input to our SVM-based model includes a variety of publicly available data including the real-time locational marginal prices (LMPs), load demand, wind generation, historical marginal fuel data, and other information (such as day of the week and holidays). The results from the SVM are compared with real data from the years 2014 and 2015.
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