Delta Connection Research Articles

Irreversible Demagnetization Analysis with Respect to Winding Connection and Current Ripple in Brushless DC Motor

In this study, a characteristic analysis of irreversible demagnetization in brushless dc motors (BLDCMs) according to the winding connection type, wye, and delta, is conducted. Depending on the excitation sequence and inverter output current for each winding method, different irreversible demagnetization characteristics are caused under the equal-input-limit condition. However, in contrast with the ideal current waveform, the current ripple inevitably occurs when pulse width modulation (PWM) control is applied. In the case of a large current ripple, the irreversible demagnetization characteristics could deteriorate due to the instantaneous increase in the synthesized magnetomotive force. Therefore, with regard to the switching frequency, we analyze the irreversible demagnetization characteristics of the delta and wye connections for BLDCM with PWM control. The validity of the results is verified through finite element analysis.

Model test and numerical simulation of OC3 spar type floating offshore wind turbine

Nowadays, the study on Floating Offshore Wind Turbines (FOWTs) is being performed globally. Dozens of numerical simulation tools have been developed for designing FOWTs and simulating their performances in combined wave and wind environments. On the other hand, model tests are still required to verify the results obtained from numerical simulation tools. To predict seakeeping performance of the OC3-Hywind platform, a OC3 spar model moored by a 3-leg catenary spread mooring system with a delta connection was built with a 1/128 scale ratio. The model tests were carried out for various sea states, including rotating rotor effect with wind in the Ocean Engineering Wide Tank, University Of Ulsan (UOU). The model test results are compared with the numerical simulations by UOU in-house code and FAST.

Analysis of star and delta connected modular multilevel cascaded converter-based STATCOM for load unbalanced compensation

This paper compares the operating capabilities of STATCOMs based on Modular Multilevel Cascaded Converters (MMCC) using star and delta connections, with special attention to unbalanced load compensation. Zero sequence voltage for star connection, and zero sequence current for delta, need to be applied to overcome the phase cluster DC-voltage unbalance. Expressions are derived for both zero sequence elements as functions of the degree of load unbalance defined as the ratio of negative to positive sequence load current. They show that the zero sequence voltage in star connection reaches a very high level as the degree of load unbalance increases, making the MMCC DC-link voltage too high for correct functioning. However the delta connected MMCC can cope with the high level of load current unbalance. Experimental results are presented to validate this analysis.

Detecting Stress Based on Social Interactions in Social Networks

Psychological stress is threatening people’s health. It is non-trivial to detect stress timely for proactive care. With the popularity of social media, people are used to sharing their daily activities and interacting with friends on social media platforms, making it feasible to leverage online social network data for stress detection. In this paper, we find that users stress state is closely related to that of his/her friends in social media, and we employ a large-scale dataset from real-world social platforms to systematically study the correlation of users’ stress states and social interactions. We first define a set of stress-related textual, visual, and social attributes from various aspects, and then propose a novel hybrid model - a factor graph model combined with Convolutional Neural Network to leverage tweet content and social interaction information for stress detection. Experimental results show that the proposed model can improve the detection performance by 6-9 percent in F1-score. By further analyzing the social interaction data, we also discover several intriguing phenomena, i.e., the number of social structures of sparse connections (i.e., with no delta connections) of stressed users is around 14 percent higher than that of non-stressed users, indicating that the social structure of stressed users’ friends tend to be less connected and less complicated than that of non-stressed users.

Line start permanent magnet motors with double‐barrier configuration for magnet conservation and performance improvement

This study proposes a new configuration for line start permanent magnet (PM) motors with low consumption of PM material, providing improved performance. The configuration is developed by analysing PM flux density distribution in the machine airgap and focusing on the d-axis armature reaction flux path and also different magnetisation types in the PM poles. It features double flux barriers for each rotor pole to facilitate near sinusoidal flux density distribution. Main characteristics of a motor with the proposed configuration are analysed by finite element method (FEM) and compared with the characteristics of a motor with the conventional configuration. Finally, the motor is built and tested. The FEM and experimental results verify the advantages of the proposed configuration. The results show that the required PM material reduces by one-third, while desirable performance is provided. The machine is capable of working with delta connection causing negligible circulating current and low torque ripples.

An Investigation of Motor Topology Impacts on Magnet Defect Fault Signatures

This paper presents a study on the topology-dependent magnet defect fault signatures in permanent-magnet motors. A new analytical approach is introduced to characterize the fault signatures in stator back electromotive force (EMF) and current waveforms using magnetic equivalent circuit. Stator winding configuration, winding connection type and location of damaged rotor magnets are some of the physical properties affecting the fault signature characteristics. Several cases with different number of pole and slot are investigated through the proposed method. In addition, different winding connections (including star and delta connection), different winding configurations (including single and double layer, fractional and full coil pitch), and different magnet defect number and location are scrutinized. It is shown that there are some cases exhibiting different fault patterns than the ones obtained through well-known fault models defined in the literature. It is essential to take these discrepancies into account in order to avoid false alarms. In addition, it is observed that some of the fault signatures show up in the stator back EMF spectrum but not in the current spectrum due to location and severity of magnet defect, and design specs. Comparative 2-D finite-element simulations and experimental results justify the theoretical magnet defect fault analysis and show the efficacy of the proposed approach.

IZDPWM-Based Feedforward Controller for Grid-Connected Inverters Under Unbalanced and Distorted Conditions

In grid-connected inverters, a feedforward controller accurately compensates for a large number of the harmonic components of the injected current. This controller is fast and directly tackles the grid dynamic voltage disturbances without doing any further harmonic analysis. A feedforward controller was introduced to a three-phase grid connection with a wye configuration; however, for the widespread delta connections, such a scheme is less established. Moreover, the nonsinusoidal profile of the grid voltage waveform makes the online conversion of line-to-line values into phase values impossible. In this paper, implicit zero-sequence discontinuous pulse width modulation (IZDPWM) based feedforward control is implemented on the two-level grid-connected inverter. Regardless of the grid topology, the IZDPWM-based controller ensures sinusoidal current injection to the grid under balanced, unbalanced, and distorted conditions.

Novel Auto-Reclosing Blocking Method for Combined Overhead-Cable Lines in Power Networks

This paper presents a novel auto-reclosing blocking method for combined overhead-cable lines in power distribution networks that are solidly or impedance grounded, with distribution transformers in a delta connection in their high-voltage sides. The main contribution of this new technique is that it can detect whether a ground fault has been produced at the overhead line side or at the cable line side, thus improving the performance of the auto-reclosing functionality. This localization technique is based on the measurements and analysis of the argument differences between the load currents in the active conductors of the cable and the currents in the shields at the cable end where the transformers in delta connection are installed, including a wavelet analysis. This technique has been verified through computer simulations and experimental laboratory tests.

Research on a Single Phase-Loss Fault-Tolerant Control Strategy for a New Flux-Modulated Permanent-Magnet Compact In-Wheel Motor

This paper proposed a new fault-tolerant control strategy focusing on phase-loss (PL) fault based on winding's Y/Δ change and phase shift under vector control in electric vehicle driving system with a new flux-modulated permanent magnet compact in-wheel (FMPMCW) motor. The winding is connected in star connection in normal operation and delta connection when fault occurs in which a new phase-shift fault-tolerant scheme is proposed. The proposed strategy has the advantages of the simplicity of fault detecting and fault-tolerant program, symmetrical three-phase sinusoidal inverter current, little size of driving system, and low cost of investment. Simulation and experimental verification of the fault-tolerant control strategy show that the FMPMCW motor driving system can switch from the fault state to the FT operation smoothly.

Three-Phase AC–AC Hexagonal Chopper System With Heterodyne Modulation for Power Flow Control Enhancement

This paper proposes a three-phase ac chopper system for the interconnection of various distributed generation (DG) farms or main utilities to enhance the active and reactive power flow control. The absence of large energy storage component in direct ac–ac converter makes the system footprint small and reliable. As the interface for different ac sources, the presented converter can be configured as star or delta. However, delta connection is preferred as it can trap the potential zero-sequence current and reduce the current rating of the switching devices. In this way, the proposed converter resembles the hexagonal chopper, and it offers an inherent degree of freedom for output voltage phase shifting. Considering the scalability in high-voltage applications, a new version of the hexagonal chopper with half-bridge cell modular multilevel structure is developed. The modular multilevel ac hexagonal chopper (M2AHC) is operated in quasi-two-level mode to suppress the electromagnetic interference (EMI) caused by high-voltage switching. Quasi-two-level operation divides the voltage level transition into multisteps, diminishing the voltage rising and falling rates ( dv / dt ) in high-voltage condition. Then, heterodyne modulation is adopted for the presented chopper system, supplying a new degree of freedom to decouple the phase and amplitude regulation. Based on this idea, system control strategy is developed in synchronous reference frame (SRF). Simulations and experimentations have confirmed the validity of the proposed approaches.

Electric Safety: Practice and Standards [Book Reviews

Northwest Lineman College provides a range of power system training that is designed for entry-, apprentice-, and advanced-level trainees. The importance of effective training and education for line workers is paramount, given the hazards associated with working on and around electric lines and equipment on a daily basis. Reviewing this book was a valuable experience for me since I was able to relate its information to the academic curriculum that we utilize at Northwest Lineman College along with my experience as a lineman. The author, Prof. El-Sharkawi, has an impressive array of credentials and experience related to power systems and associated applications. His experience in authoring various publications was evident as the book is clearly a highlevel publication. The first chapter covers various fundamentals of electricity. Electric and magnetic fields are well explained and easy to comprehend. Alternating C fundamentals include the operation of a three-phase generator and the associated alternating current waveforms. Delta and wye connections are explained with the use of easy-to-understand vectors. I liked the examples that were provided with actual values included in the formulas, making it easier to learn and apply the formulas.

A Universal High-Frequency Three-Phase Electric-Motor Model Suitable for the Delta- and Star-Winding Connections

A universal high-frequency three-phase electric-motor model to be used in the design process of the electric-motor drive systems is proposed. The model can be combined with the models of the filter, cable, and inverter to form a complete drive system. It allows for a common and differential-mode high-frequency analysis in studying the bearing currents, electromagnetic interference, and overvoltages in the motor windings caused by the high-frequency reflections. The model phase is symmetrical meaning that the beginning and end of each phase are equal and the central part is inserted between them. This enables modeling of both the delta and star-winding connections of the three phases. The equations used to calculate the model parameters are presented. They are based on the common and differential-mode impedance measurements. The model can be simplified when considering only electric-discharge machining bearing currents, as they are affected only by the common-mode states. A simplified model to be used in the delta-winding connection is also presented. The model-calculated common and differential-mode impedances are compared with the impedances measured on a 6.5-kW outer-rotor brushless direct-current motor. This paper ends by providing the obtained simulation and measurement results for the common and differential-mode currents and bearing voltages of the studied three-phase delta-winding-connected electric motor.

Oilfield Electric Power Distribution

The uniqueness of power system designs for oilfield distribution is seldom acknowledged or addressed. Utilities provide construction typical of either commercial or rural applications. Oilfields commonly have large concentrated loads separated by thousands of feet. To minimize line losses, medium-voltage power is distributed and stepped down at the point of use. In addition to common load flow analysis and protection coordination, transformers, surge arresters, and grounding connections are important considerations for reliable production operations. Recloser settings typical for residential distribution cause major damage to submersible pumps. In remote areas where long neutral wires are necessary, solidly grounded power contributes to equipment failure but is nevertheless necessary for safety in surface equipment. The corner-grounded delta connection eliminates the neutral wire and is often used by rural utilities, despite its dangers. High-voltage three-wire construction can cause ferroresonance. Grounding influences the effectiveness of lightning and switching surge protection.

Harmonic Mitigation in a Coreless Double-Wound Flywheel Machine: Experimental Verification

Abstract This paper aims at analyzing harmonics and interharmonics present in a flywheel-based driveline developed at Uppsala University. The flywheel machine is a permanent magnet synchronous machine with two sets of windings in the stator, with a Wye–Wye connection as standard. Different sources of harmonic distortion have been evaluated. Flywheel machine simulations are performed in finite element method (FEM) (Comsol) and experimental verifications are presented. Several methods for harmonic mitigation have been evaluated. The driveline suffers almost exclusively from second-order harmonics but the THD (total harmonic distortion) seldom exceeds 5%. Very little interharmonics are present. A Delta–Wye connection of the flywheel eliminates most of the harmonics but at the expense of a high-phase current through the Delta connection. Grounding the neutral conductor in Wye connections increases the distortion.

An Energy-efficient Switching Scheme with Reduced Switching Transients for a Wind-driven Induction Generator

This article presents a scheme for improving the power output of grid-connected induction generator commonly used in wind energy conversion systems. Generally, the stator of the induction generator is connected in a star with a line voltage of √3 times the rated winding voltage to reduce the line current and, hence, conductor size. To extend the generating operation over a wider speed range, delta-star switchable stator windings are also in vogue. In such cases, the stator is star connected in the lower speed range and switched to a delta connection above a threshold speed. In this study, a new switching scheme is proposed wherein the stator coils are always connected in a star, while the stator is connected to different voltages in low- and high-speed conditions. At low wind speeds, nominal winding voltage is applied to the stator, whereas at higher speeds, the stator applied voltage is √3 times higher than the rated winding voltage. The efficacy of the scheme is demonstrated experimentally with a suitable microcontroller-based switching arrangement. Typical results indicate an increase in output with reduced switching transients. A case study on a 3-Φ, 50-kW induction generator is presented to emphasize the performance improvement with the proposed scheme.

Calculation of parameters of balancing devices for traction substation of alternative current with visual simulation

Traction substation with the star - delta connection of transformer windings is considered. Power supply of traction load is carried out from one side of the triangle on the secondary side of the transformer, which creates an unbalance in the power supply system. In turn, current and voltage unbalance reduces the power factor for power sources in each phase and increases losses in the transmission line. The study of electromagnetic processes, conducted using the developed visual model has confirmed the presence of a substantial unbalance in the system. With the help of the visual model and search engine optimization, parameters of balancing device, consisting of reactive elements are found. Herewith, currents in transformer lines and windings are balanced, and reactive power in the power supply system is fully compensated. Due to the perfect balancing and full compensation of reactive power, the losses in the system are minimized.

Comparison of Finite-Element-Based State-Space Models for PM Synchronous Machines

An interior permanent-magnet (PM) motor is modeled by a combined analytical–numerical approach, in which the relationships between the stator currents and flux linkages are identified with static finite-element (FE) analysis. In addition to the previous approaches using the current space vector as the state variable, new models are also developed using the flux-linkage space vector, which leads to more convenient time-integration of the voltage equations. In order to account for the zero-sequence effects in delta connection, the models also include either the zero-sequence flux or current as an additional state variable. Finally, the possibilities of deriving the required quantities as partial derivatives of the magnetic field energy are discussed. The energy-based approaches avoid inaccuracies related to torque computation and thus allow better satisfying the power balance in the state-space model. We show the ability of the developed state-space models to predict the currents and torque equally to a nonlinear time-stepping FE model with much less computational burden. The results are validated by means of measurements for a prototype machine in both star and delta connections. In addition, we also demonstrate the effect of the zero-sequence current on the torque ripple in case of a delta-connected stator winding.

Novel inrush current restraining method for transformers of shipboard power system

Summary In this paper, the process of energizing unloaded transformer in the shipboard power system is analyzed. Because the transformer in shipboard power system is generally delta connection or ungrounded star connection, energization of the first phase will have no influence to the transformer. By mathematical analysis of delta connection transformer and ungrounded star connection transformer, the methods to calculate optimal closing time of the second phase and third phase are presented. And then, a novel time-controlled energization strategy is proposed. The feasibility of this new energization strategy is verified with EMTP. Copyright © 2014 John Wiley & Sons, Ltd.

Star-Delta Switches Evaluation for Use in Grid-Connected Wind Farm Installations

Electrical generators are designed to perform best under permanent rotation velocity and fixed loads conditions. However, such ideal conditions are not practically feasible during the operation of real wind turbines. Generally, the voltage output of electrical generators can be regulated without redesigning the electrical or/and mechanical parts constituting such a system, by simply changing the connection of the generator to the grid from Star to Delta or by using combined windings. The present work attempts to investigate the behavior of grid-connected wind turbines with Star-Delta, Delta, and Star connection switches in a variety of simulation scenarios, by taking into consideration the influence of both internal and external factors such as the inertia factor and the wind speed.

High-frequency motor modelling with high accuracy for electric vehicle drive system

A practical method to construct a high–frequency equivalent circuit model for AC motor of delta connection is present in this paper. The modelling is based on measured data of the motor–port impedance test and vector fitting technique. The procedure of the modelling is described in detail in this study. The proposed model is verified by comparing simulation data with experimental data on a permanent magnet motor in the frequency range from 100 kHz to 100 MHz. The good correlation between the simulated and the measured values has confirmed that the approach is effective. This model can be use to analyse the characteristics of the motor and predict the conducted electromagnetic noise in the motor drive system of electric vehicles.