In order to select a reliable and economical type of transformer on-load tap-changer (OLTC), it is necessary to accurately determine the maximum step voltage in the process of engineering design. ...Because both configuration and operating conditions of a phase-shifting transformer (PST) are more complicated than those of conventional transformers, it is far harder to accurately calculate the maximum step voltage. The existing calculation methods have the shortcomings of inadequate accuracy or weak applicability. To solve these problems, based on the equivalent circuit of a PST, this paper at first derives the quantitative relations between operating conditions and the step voltage. Subsequently, the effect of different load conditions, including the load current and the phase angle difference between the load voltage and current, on the step voltage is fully investigated. This investigation reveals a law about the changes in step voltage under different load conditions. Next, a new calculation formula for the maximum step voltage is proposed, and its accuracy is verified with two practical examples. Finally, an error analysis is carried out to demonstrate the generality of this approach. The calculation method offers appealing accuracy and is very suitable for the economical and reliable selection of OLTC for symmetrical two-core PSTs in the process of engineering design.
This paper proposes a double-time-scale coordinated voltage control scheme for distribution networks with distributed generators (DGs) based on model predictive control (MPC) to regulate the voltage ...profile across a network. The slow-time-scale control scheme is designed to correct long-term voltage deviations while reducing the number of actions of the on-load tap changer, step voltage regulators, and capacitor banks. The MPC problem is formulated as a mixed-integer quadratic programming (MIQP). A tailored exaction solution method based on the branch-and-bound algorithm embedded with an alternating direction method of multiplier-based QP solver is developed to efficiently solve the MIQP problem. In the fast-time-scale control, the active and reactive power outputs of DGs are optimally coordinated to handle the fast voltage fluctuations as well as capture more renewable energy. An efficient analytical sensitivity calculation method is used to update the voltage sensitivities online. The effectiveness of the proposed control scheme along with the exact solution method is verified on a modified real 20 kV distribution system.
TNAs have been synthesized using a two-step anodization method by increasing the second-step voltages. The titanium (Ti) foil was used as the anode, and the first step of anodization layer was ...removed by sonication in the deionized water. The second anodization step was conducted using the same electrolyte but at a higher potential. The modification of the second-step voltage was discussed with correlation for the morphology and photoelectrochemical performance of TNAs. Increasing the second voltage of two-step anodization leads to a decrease in the bandgap of the anatase phase of TNAs at 3.09 eV. The highly ordered TNAs were formed by increasing the second-step voltage based on the Fast Fourier Transform (FFT) images with a higher regularity ratio (RR) with 30 V for 30 min in the first step and 40 V for 30 min in the second step. The tube length of TNAs was produced in a short time of anodization. The photocurrent density of TNAs prepared by increasing voltage in the second step is higher than that of the one-step anodization.
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Insulated-gate bipolar transistor (IGBT) modules and dc-link capacitors are important parts in the majority of power electronic converters which contribute to cost, size, and failure rate on a ...considerable scale. This paper presents an online condition monitoring method for both IGBT modules and dc-link capacitors of power converters based on short-circuit current of an IGBT module, which is a good condition indicator according to the theory analysis. The failure prediction of dc-Link capacitor is realized by equivalent series resistance, which can be calculated by short-circuit current and a step voltage, and the bond wires fatigue can also be identified by the short-circuit current. The proposed method is capable for detecting small changes in the failure indicators of IGBT modules and electrolytic capacitors, and its effectiveness is validated by a confirmatory experiment. The novelty of the proposed method is that the degradations of IGBT modules and capacitors can be identified simultaneously, and has the merits of low cost and circuit simplicity.
Automotive applications demand high efficiency, good reliability, and a wide operating speed range within limited dc bus voltage constraints. In this article, a zero degree displacement dual ...three-phase permanent magnet synchronous machines (PMSM) is derived from an original three-phase PMSM to enhance the power rating as well as the operating speed range. The proposed topology can be used as an alternative to the conventional configuration of three-phase PMSM with a bidirectional dc-dc converter for electric vehicles. Even though the commonly used dual three-phase PMSM with split-phase winding eliminates the sixth harmonic torque pulsation, a large circulating current is generated during six-step operation, which results in additional stator copper loss. In this article, six-step operation of the symmetric dual three-phase PMSM with zero degree winding displacement is proposed to achieve maximum inverter utilization. Both harmonic circulating current and torque pulsation were observed to be within satisfactory levels using this configuration. A changeover algorithm is also proposed to enable a smooth transition between closed-loop current vector control during low and medium speed operation to open-loop six-step voltage angle control for high speed operation. The performance of the proposed method is experimentally validated on a 3-kW dual three-phase PMSM drive.
Liquid crystals have great potential for developing photonic devices that control the optical behaviors of liquid crystals in smart devices with external stimulation. In this study, we have ...demonstrated a series of bistable cholesteric liquid crystal devices using the dual-frequency nematic liquid crystal HEF951 assisted by a predesigned chiral ferroelectric liquid crystal. The synthesized chiral ferroelectric liquid crystal was used to induce the formation of the cholesteric liquid crystal phase and decrease the driving voltages and response times of the fabricated bistable liquid crystal devices. The fabricated ferroelectric liquid crystal-assisted bistable cholesteric liquid crystal devices showed a stable opaque focal conic state and stable transparent planar state without any energy consumption once they were turned "from focal-conic to planar" and "from planar to focal-conic". To enhance the reliability, the fabricated bistable cholesteric liquid crystal devices were further stabilized by polymer matrixes. Switching of the bistable liquid crystal cells from one state to another was achieved by a one-step voltage bias with various frequencies. Furthermore, the response time of the sample cell was calculated as 1.7 ms. These results suggest that fabricated ferroelectric liquid crystal-assisted bistable cholesteric liquid crystal devices can be applied to produce energy-saving green liquid crystal displays and other related smart devices.
Fabricated polymer-stabilized bistable cholesteric liquid crystal devices show a stable opaque focal conic state and a stable transparent planar state without any energy consumption once they are turned.
With more distributed power generation and renewable energy penetration, voltage fluctuation and power generation variation can be observed more frequently in the modern power distribution network. ...More tap change operations are required for the step voltage regulator (SVR) to regulate the line voltage. However, the tap changing mechanism of the conventional SVR always generates an electric arc when tap changes, which imposes constraints on conventional SVRs, such as lifetime and maintenance period. This article proposed a new topology of arcless step voltage regulator based on a series-connected converter for current suppression. The system efficiency is the same as the conventional SVR in the normal operation, while the converter power rating is only 0.3% of the total system power. A scaled-down prototype of the arcless SVR is developed to verify the proposed arcless tap changing method. The hardware test results verified that the proposed arcless step voltage regulator can eliminate arcing during the tap change and reduce the contact erosion rate by over 10000 times the conventional arcing SVR. Other advantages of the novel method over the conventional SVR, such as advanced load voltage regulation and volt/var control, are also verified.
This paper studies and proposes the vector space decomposition-based direct torque control (DTC) scheme for the T-type neutral-point-clamping (T-NPC) three-level inverters-fed double-stator-winding ...permanent magnet synchronous machine (PMSM) drive, which provides an effective solution for high-power high-reliability applications. The key is to propose a simple but effective space vector modulation (SVM) for DTC of T-NPC double-stator-winding drives-based two-step voltage vector synthesis, in such a way that good dynamic response and harmonic performance are obtained. The closed-loop controllers on harmonic subspace are incorporated to suppress the possible harmonics induced from back electromotive force and unbalanced parameters in phase windings of electrical machine. Furthermore, a hybrid current control is proposed for fault-tolerant operation of the T-NPC double-stator-winding PMSM drives under one-phase open-circuit conditions. In the hybrid current controller, the healthy winding still uses the SVM-DTC control while the faulty winding uses the closed-loop current controller to track the optimized current references. Both simulation and experimental results are presented to verify the performance of the proposed switching strategies and control schemes.
•Theoretical and FEM simulation analysis together with a scaled laboratory test have been applied to verify the effectiveness of the proposed tower footing resistance reduction method.•To get the ...most effective way of grounding resistance reduction, two types of tower footing model, including single-pile tower footing model and multi-pile tower footing model, have been considered in detail under different conditions.•Comparison has been done among five practical grounding grids, and recommended grounding type is given.
The excellent performance of earth electrodes is the key to stable and safe operation of transmission lines. However, corroded earth electrodes in practical cases can increase the tower earth resistance. In this paper new Flexible Graphite Earth Electrodes (FGEE) are introduced to address the corrosion problem and its new wrapping method is proposed to reduce the tower earth resistance. The proposed earth resistance reduction method is effective by theoretical analyses. Lots of simulation work is conducted to analyze the effect of factors on both single-pile and multi-pile tower earth resistance. Comparisons are conducted on five commonly used types of tower earth-electrode network, including sole tower foundation, tower foundation with ring earth conductor or FGEE, tower foundation with both ring earth conductor and FGEE, and tower foundation with ring earth conductor and horizontal radial electrode of 20 m on four directions.. Besides, grounding tests on scaled foundation earth electrodes are carried out to verify the validity of the proposed method. Simulation results show that the tower earth resistance is reduced by 15~60% in 500~3000 Ω•m soil. For single-pile tower foundation, the lower the wrapping position, the smaller the earth resistance, and the earth resistance is negatively related to the wrapping radius, while down lead wire connection types have little influence on earth resistance; besides, all the three factors above have impacts on current density. For multi-pile tower foundation, earth resistance is negatively related to the distance between two piles and separate wrapping is a preferred type of wrapping. Moreover, among the proposed five commonly used grounding types, Type V is preferred. Finally, scaled laboratory test results indicate that the cylinder FGEE can effectively reduce the tower earth resistance.
With the rapid development of photovoltaic (PV) technology, more utility-scale PV systems are being integrated into rural areas where abundant and low-cost land is available. In these areas, ...step-voltage regulators (SVRs) with an open-delta configuration are widely used for voltage regulation. However, since SVRs were initially designed for tackling slow and smooth load variations, frequent PV output fluctuations can cause excessive tap changes. Moreover, rural communication facilities are normally expensive and underdeveloped, so only limited parameters are recorded with low resolution and reliability. Thus, much needed tap information cannot be properly monitored in a cost-effective way. In contrast, a downstream load center or utility-scale PV site is commonly well equipped with modern sensors and data acquisition systems, which provide the possibility for tap evaluation. Therefore, this paper proposes a new remote tap position estimation approach to accurately assess the upstream tap positions of open-delta SVRs from downstream measurements. Furthermore, the proposed approach is firmly validated through field testing under the support of the local utility and PV plant owner. This novel method not only enables reliable tap position monitoring with high precision for utilities to research PV-SVR interaction, but also provides valuable information to PV owners for correctly analyzing downstream voltage performance.
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