The overmodulation-based nonlinear voltage extension could increase the maximum output torque of the induction motor (IM) in the field-weakening (FW) region. Since the voltage distortion increases ...with the deepening of overmodulation degree, the extreme nonlinear voltage extension (six-step voltage) is only worthwhile in maximum torque demand conditions. However, the conventional overmodulation-FW scheme always adopts the six-step voltage regardless of torque demand, leading to severe torque ripple. To solve this problem, the minimum-nonlinear-voltage (MNV) method is proposed to satisfy the torque demand with minimum overmodulation-voltage nonlinearity. The difference between the limiting and the feedback values of the torque current is used for suppressing the nonlinear voltage extension during a nonmaximum torque demand state. The quantitative relation between the torque ripple and torque demand of the MNV method is calculated and compared with that of the conventional six-step-FW method. Analysis shows that the MNV method can eliminate the torque ripples of low and medium torque demand conditions completely and partially, respectively. The effectiveness of the proposed method is verified experimentally on a 3.7 kW IM platform.
The overmodulation canimprove the utilization of dc-link voltage in the induction motor (IM) field-weakening (FW) control, thereby extending the maximum output torque. Nevertheless, the high ...modulation index (MI) will cause severe torque ripple. To figure out the relation between the torque ripple and MI, the amplitude of torque ripple with different MI is calculated quantitatively. Then, a torque quality evaluation function (TQEF) is designed to evaluate the torque quality in overmodulation-FW region by considering both fundamental torque extension and torque ripple suppression. On this basis, a modulation index optimization module is proposed for the maximum TQEF value realization. The analysis shows that the studied method could achieve the optimal trade-off between the fundamental torque extension and the torque ripple suppression. The TQEF value of the studied method is higher than that of the conventional six-step voltage method and the inscribed circle voltage method, meaning that the torque quality is optimized. Finally, the effectiveness of the studied method is verified experimentally on an advanced RISC machines (ARM)-based 3.7 kW IM platform.
This article addresses the problem of optimizing voltage profiles in distribution networks. The voltage optimization is split into two stages; the former is performed offline and the latter online. ...First, the network is partitioned into several weakly coupled voltage control zones (VCZs) with pilot nodes (PNs). Then, the partitioning is used to optimize the voltage profiles of the distribution systems on the frame of a two time-scale-based coordinated approach. At the first level, a centralized voltage optimization problem (VOP), minimizing the distance of bus voltages at the PNs from their reference values and subject to linearized power flow equations, is solved to fix the positions of the on-load tap changer and of step-voltage regulators, and the reactive powers provided by capacitor banks. At the second level, the VOP is implemented according to a decentralized approach, in which the solution is obtained by applying a distributed algorithm based on the alternating direction method of multipliers. It optimizes in each VCZ the voltage at the PN by acting on the active and reactive powers provided by the distributed energy resources present in the VCZ; the VCZ solutions are driven to the global optimum of the whole distribution system by a limited data exchange between the PNs. The proposed approach reduces the complexity and computational burden typical when solving the VOP on a large scale system. The proposed strategy is tested on the modified IEEE 123-bus system; various load and generation scenarios are analyzed proving the effectiveness of the proposed approach in achieving the objective of voltage regulation.
In order to improve the reliability of vertical NAND (V-NAND) flash memory cells, a scheme using adaptive incremental step pulse programming (A-ISPP) and incremental step pulse erasing (ISPE) is ...proposed. Incremental step pulse programming (ISPP) with adaptive step voltage is used to precisely adjust <inline-formula> <tex-math notation="LaTeX">{V}_{\text {th}} </tex-math></inline-formula> to a low target value while rapidly increasing <inline-formula> <tex-math notation="LaTeX">{V}_{\text {th}} </tex-math></inline-formula> to a high target value. By applying ISPE after A-ISPP, an accurate <inline-formula> <tex-math notation="LaTeX">{V}_{\text {th}} </tex-math></inline-formula> with improved retention characteristics is obtained at a high target <inline-formula> <tex-math notation="LaTeX">{V}_{\text {th}} </tex-math></inline-formula> level. Compared to the conventional ISPP, the proposed scheme improves adjusted <inline-formula> <tex-math notation="LaTeX">{V}_{\text {th}} </tex-math></inline-formula> accuracy and <inline-formula> <tex-math notation="LaTeX">{V}_{\text {th}} </tex-math></inline-formula> dispersion by 60% using the same step voltage and a similar number of pulses. With the proposed scheme, the retention characteristics are also improved by ~43%, and the distribution of <inline-formula> <tex-math notation="LaTeX">\Delta {V}_{\text {th}} </tex-math></inline-formula> is narrowed by ~38%.
The commutation strategy is the important content of matrix converter industrial application, for it not only affects the converter safe operation, but also is closely related to the input and output ...performance. In this paper, the matrix converter and the three-level inverter are compared on the perspective of three-level, the relationship between them is studied, and the logical correspondence of switch tubes in two topologies is established. By analyzing the switching sequence of the three-level inverter and equivalent the clamping diode, the two-step voltage-controlled commutation strategy is obtained, which proves the equivalence between the matrix converter and three-level inverter.
The voltage regulation in distribution systems becomes more important these days due to the more complicated power flow profile caused by the integration of variable renewable energy sources such as ...photovoltaic (PV). Step voltage regulator (SVR) and load ratio controlled transformer (LRT) have been mainly used for the voltage regulation conventionally based on decentralized control methods using line drop compensation (LDC) logic for estimation of voltage drop at secondary side of the SVR. Here, it has been required for distribution system operator to determine LDC parameters properly to estimate the voltage drop based on locally measured information. Hence, in this paper, we proposed an advanced LDC logic in which the parameters can be flexibly switched based on the power flow conditions. The effectiveness of the proposed method was verified using 41-node distribution system model with PV.
In long distribution feeders, step voltage regulators (SVRs) with the line drop compensation have been widely implemented to control voltage profiles. After integration of photovoltaic (PV) systems, ...reactive power support from PV inverters can also be utilized in voltage regulation. Although both SVR and reactive power support can be effective to manage system voltage without coordination, problems such as large voltage variations and excessive SVR tap operations still exist in some strong PV power fluctuating days. In order to solve these issues, SVR and reactive power support should be assigned to different voltage regulation tasks according to their voltage regulation characteristics. Specifically, in a distribution system, an SVR should mainly deal with slowly changing quantities ( e.g. , load, upstream voltage), while the limited reactive power support should be used to counter fast fluctuating PV power. In this paper, the power factor droop parameters applied on PV inverters are optimally selected to achieve such coordination, so that voltage problems and excessive SVR tap operations can be successfully mitigated. The effectiveness of the proposed method is demonstrated via case studies. Future PV integration project in weak distribution systems can benefit from the innovative and practical methodology proposed in this paper.
Distribution system optimizations often lead to NP-hard programming problems. Although convex relaxations have proved effective for treating most of these problems, solution time increases ...drastically when discrete control devices ( dcd s), e.g., tap-changers, and capacitor banks, are involved. The models of dcd s are categorized in two categories. The models of the first category rely on auxiliary binary variables to achieve a set of convex constraints. The first proposed model in this paper expedites the models of the first category by upgrading the coding of variables. The second category includes the models that elude the computational burden of handling the auxiliary binary variables by relaxing the equations of dcd s and restricting the relaxation region through a sequential bound tightening. Though this approach expedites solving large problems, the risk of encountering a suboptimal/infeasible solution is considerable. The second propounded model resolves such issues while being kept computationally competitive, by designing a parameter-free stopping criterion, a systematic mechanism to reduce the step-size in bound tightening, and an error-cognizant convex model selection function. A wide range of studies demonstrates that the proposed models resolve the issues of those available. The paper concludes with broad recommendations on the suitability of these models.
•The analyses and reductions of backflow lightning overvoltages of the PV power plants.•High-frequency models for all PV plant components (air-termination, grounding system, surge protective devices, ...PV string, inverters, underground cables, and power transformers) using the ATP/EMTP simulation software.•Design the PV grounding system using COMSOL Multiphysics.•Evaluate the PV grounding system toward decreasing the backflow lightning overvoltage magnitudes.
Photovoltaic (PV) systems are subjected to lightning strikes that contribute to losing their sustainable electrification service. Furthermore, they are subjected to backflow lightning overvoltages due to the installation in high soil resistivity areas, however, such the study is not aforementioned well in the literature. Therefore, the analyses and reductions of backflow lightning overvoltages are investigated in this paper, considering a PV power plant as an example installed in Taif city, KSA. All PV plant components are modeled using high-frequency models, in which they are such as air-termination, grounding system, surge protective devices, PV string, inverters, underground cables, and power transformers. To decrease the lightning overvoltages in the PV power plant, a modified PV grounding system design is introduced and evaluated. The evaluation is performed considering step voltage profiles using COMSOL Multiphysics and backflow lightning overvoltage magnitudes at different points in PV plant using ATP/EMTP. The results provide the motivation of studying backflow lightning overvoltages in PV plants and evidence of the efficacy of the proposed grounding system design in reducing the overvoltage magnitudes.
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