Search

An integral domain D is called a locally GCD domain if DM is a GCD domain for every maximal ideal M of D. We study some ringtheoretic properties of locally GCD domains. For example, we show that D is a locally GCD domain if and only if aD ∩ bD is locally principal for all 0 ≠ a; b ∈ D, and at overrings of a locally GCD domain are locally GCD.We also show that the t-class group of a locally GCD domain is just its Picard group. We study when a locally GCD domain is Prüfer or a generalized GCD domain. We also characterize locally factorial domains as domains D whose minimal prime ideals of a nonzero principal ideal are locally principal and discuss conditions that make them Krull domains. We use the D + XDS[X] construction to give some interesting ex- amples of locally GCD domains that are not GCD domains. [ABSTRACT FROM AUTHOR]

SUMMARY In this contribution, a control strategy in phase coordinates abc is proposed as the reference current generator to be used by the shunt active power filter (APF) compensation device. The development of the proposed reference current generator is described in detail. Its control design is kept simple, and transformation between different frameworks is not needed, thus allowing the control strategy to be applicable for both three-phase and single-phase electric systems maintaining theoretical and practical advantages. The proposed control strategy for the reference current generator is tested using Matlab/Simulink® with the incorporation of the Real-Time Workshop® for the simulation of the shunt APF compensation in real time. Based on the reported real-time application, it is possible to obtain simulation conditions that are close to the true real-time environment under study using only a single computer, giving accurate results with relatively low-cost hardware and providing a more complete evaluation of the applied control strategy. The dynamic behavior and performance of the shunt APF, using the proposed reference current calculator, have shown remarkable results. Copyright © 2012 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

This paper presents a review with a concise description and analysis of the fundamentals, characteristics, analytical details, merits, and drawbacks associated with existing methods in frequency and time domain for harmonic analysis in practical power networks. The description and analysis are centered on methods developed in the harmonic domain, hybrid frequency-time domain, and time domain, respectively. Validation of the reviewed methods for harmonic analysis, against one of the widely accepted digital simulators, such as EMTP, EMTDC, or MATLAB/SIMULINK, is reported in the cited individual contributions. [ABSTRACT FROM PUBLISHER]

This task force paper presents multi-domain tools and their interfacing issues to perform simulations that require expertise in multiple areas of physics, such as electrothermal, electromechanical, electrochemical, and electromagnetic simulations, etc. Features of single multi-domain simulation tools, which are capable of simulating more than one system such as electrical chemical, mechanical, etc., are discussed. In addition, programs that are not capable of performing multi-domain simulation independently but can be interfaced with other programs to perform multi-domain simulation are also covered. Two case studies are reported where multi-domain simulations are performed in offline and real-time environments through the interfacing of more than one simulation tool. [ABSTRACT FROM PUBLISHER]

A microgrid is a hybrid power system consists of several distributed generation resources and local loads, which provide the solution to supply premium power to remote or specific areas. However, some undesired effects are accompanied with their installations and operations, such as imbalance, voltage fluctuation, and harmonics, as well as inter-harmonics. To the aspect of voltage quality, the switching on and off of the distributed generation resources may cause power fluctuation, hence the associated power quality disturbances are produced and affect the connected power system. To the design of a hybrid power system such as an ac or dc microgrid, it is necessary to perform computer simulation for the planning and study. Before the actual system is built, all potential responses of the system should be known and then the design can be modified to avoid negative impacts to the system. This paper presents an application of the real-time simulation tool for power quality studies of the dc micro grid planning. Experiences of modeling a dc microgrid components are presented and simulation results are reported. [ABSTRACT FROM AUTHOR]

The electromagnetic transient programs (EMTP-like tools) are based on the nodal (or modified nodal) equations that enable an efficient numerical solution and, subsequently, fast time-domain simulations. The state-variable-based simulation programs, such as Simulink, are also used for studying the dynamics of electrical systems. Both the offline and real-time versions of these two types of simulation tools are widely used by the researchers and engineers in industry and academia to study the transient phenomena and dynamics in power systems with rotating electrical machines. This paper provides a summary of the interfacing techniques that are utilized to integrate the general-purpose models of electrical machines with the rest of the power system network for these studies. The interfacing methods are broadly classified as indirect and direct approaches. The paper also describes the numerical properties as well as limitations imposed by the interfacing of the commonly used machine models that should be considered when selecting the simulation parameters and assessing the final results. [ABSTRACT FROM AUTHOR]

The effective harmonics estimation for measuring power signals has become an important issue in the power quality assessment. By reviewing those commonly used Kahnan filter-based models, some limitations for harmonics estimation can be observed. In this paper an extended real model of Kalman filter combined with a resetting mechanism for accurately tracking time-varying harmonic components of power signals is presented. The usefulness of the proposed algorithm is demonstrated by a simple laboratory setup with LabVIEW program and the dedicated hardware for harmonics monitoring. Results show that the proposed method can achieve more accurate and robust measurement of harmonic amplitudes and phase angles for the time-varying power signals among compared methods while the uncertainty testing performances required by LEC Standard 61000-4-30 are satisfied. [ABSTRACT FROM AUTHOR]

A useful time-domain algorithm to characterise harmonic currents produced by a cluster of three-phase AC/DC converters operated under either balanced or imbalanced supply voltages and asymmetrical source impedances with the consideration of load variations is proposed. The proposed method is based on the Poincaré map concept for increasing the solution speed. An actual industrial rolling mill system with several DC motor drives operating in parallel is under test to show the usefulness of the proposed method. Simulations results are compared with those obtained by actual measurements. It shows that the harmonic currents determined by the proposed method agree well with those obtained by actual measurements. The proposed algorithm is relatively accurate, computationally efficient and robust. [ABSTRACT FROM AUTHOR]

The fast Fourier transform (FFT) has been widely used for the signal processing because of its computational efficiency. Because of the spectral leakage and picket-fence effects associated with the system fundamental frequency variation and improperly selected sampling time window, a direct application of the FFT algorithm with a constant sampling rate may lead to inaccurate results for continuously measuring power system harmonics and interharmonics. An improved FFT-based algorithm to measure harmonics and interharmonics accurately is proposed. In the proposed algorithm, a frequency-domain interpolation approach is adopted to determine the system fundamental frequency, and the interpolatory polynomial method is applied to reconstruct the sampled time-domain signal; it is followed by using the FFT to calculate the actual harmonic components. Then, the frequency-domain interpolation is again applied to find the interharmonic components. The performance of the proposed algorithm is validated by testing the actual measured waveforms. Results are compared with those obtained by directly applying a typical FFT algorithm and by the IEC grouping method. It shows that the solutions determined by the proposed algorithm are more accurate, and a reasonable computational efficiency is maintained. [ABSTRACT FROM AUTHOR]

An efficient approach to characterising harmonic currents generated by a cluster of parallel-operated AC/DC converters in the steady state is presented. The approach is carried out in the time domain and the interactions between converters are considered. In calculating harmonic currents generated by a single converter, the Poincaré map-based approach is applied to increase the computational efficiency. A solution procedure that is sequentially solving a single-converter problem with detailed model in the time domain, while the others are fixed as equivalent AC current sources, is proposed to find the overall source-side harmonic currents. The computed AC-side harmonic components of each converter current are then extracted via FFT. Solutions obtained by the proposed method are compared with those obtained by using a brute-force time-domain simulation tool, Simulink. It is shown that the AC-side harmonic currents determined by the proposed approach agree well with those obtained by the time-domain simulation tool, but the solution time is significantly reduced. Therefore, the proposed approach is useful for quantifying harmonic currents generated by a cluster of AC/DC converters and to facilitate the harmonic mitigation implementations, such as passive and active harmonic filter designs, for complying with harmonic standards such as IEC 61000-3-4 and IEEE-519. In addition, the proposed approach is suitable for characterising harmonic currents generated by other types of power-electronic devices, such as AC and/or DC drives, operated in parallel simultaneously. The solution algorithm can also be integrated into a harmonic power flow program for penetration studies to provide a fast quantification of the impact of multiple harmonic sources on the power system. [ABSTRACT FROM AUTHOR]

A new theory and a conceptual design for the three-phase shunt active power filter (AFF) control strategy under imbalanced and distorted supply voltages are presented. Conventionally, the compensation currents injected by the APF are determined according to reference frame transformation-based approaches, where the load instantaneous real and reactive powers are calculated in the new reference frame. The APF injection currents are then computed to maintain the balanced and sinusoidal source currents, as well as the least active power consumption by the APF. However, if current harmonics set by harmonic standards or other constraints are considered, the load harmonic currents after APF compensation can be controlled to meet the requirements. Therefore, the APF may have an adjustable performance. An optimal algorithm for the APF control in steady state is proposed, in which the individual and total harmonic current distortion limits, the minimum load power factor, levels of current imbalance and the size of the APF can be taken into account. The planned APF does not consume or produce active power and only the passive energy-storage elements are required in the new APF design. Results obtained by simulations with Matlab and Simulink show that the proposed approach is more flexible than conventional approaches for compensating reactive power and harmonic/neutral currents of the load, even if the source voltages are severely distorted and imbalanced. The new control strategy is very suitable for planning the APF when several alternative objective functions and constraints are under considerations. [ABSTRACT FROM AUTHOR]

A series of cells representing normal, non-tumorigenic cell lines, as well as differentiating neoplastic and undifferentiated neoplastic rat tracheal epithelial cell populations were evaluated for their ability to establish homologous and/or heterologous cell-cell gap junction communication in culture. Gap junction communication was evaluated by flow cytometric quantitation of the transfer of the fluorescent dye calcein from a donor to a recipient cell population via gap junctions. The data indicate that normal primary cultures of rat tracheal epithelial cells, as well as non-tumorigenic cell lines and squamous cell carcinomas cell populations, retain the ability to establish both homologous and heterologous gap junction communication. In all cases an average of >48% of recipient cells had acquired calcein label during a 5-h interval of co-culture of donor and recipient cells at confluent densities. Cells harvested directly from squamous cell carcinoma tumors exhibited similar levels of cell-cell communication. In contrast, cells giving rise to undifferentiated carcinomas, as well as cells harvested from undifferentiated carcinomas, exhibited very low levels or no homologous or heterologous cell-cell communication. Cell populations exhibiting distinctly different communication phenotypes were evaluated by Northern blot analysis for expression of connexins (Cx 26, 32 and 43) and E-cadherin. Neither communicating nor non-communicating cells expressed connexin 32. Those cell populations, which established functional gap junctions, expressed E-cadherin as well as connexin 26 and/or 43. In contrast, those cell populations that lacked the ability to communicate universally lacked expression of E-cadherin, and a quarter also lacked expression of detectable levels of connexin. [ABSTRACT FROM PUBLISHER]

This letter proposes a curve-fitting approach for modeling fluorescent lamp voltage--current characteristics based on measured lamp voltage and current. The fluorescent lamp conductance is considered as a series of piecewise cubic polynomials that describes the nonlinear voltage--current characteristic of the lamp. The proposed model has been tested with a 10-W conventional 60-Hz fluorescent lamp and a 20-W high-frequency energy-saving lamp. By comparing the measured and simulated results, it shows that the proposed model can accurately model the lamps for harmonic studies. [ABSTRACT FROM AUTHOR]

This letter presents an improved backward/forward sweep algorithm for three-phase load-flow analysis of radial distribution systems. In the backward sweep, Kirchhoff's Current Law and Kirchhoff's Voltage Law are used to calculate the upstream bus voltage of each line or a transformer branch. Then, the linear proportional principle is adopted to find the ratios of the real and imaginary components of the specified voltage to those of the calculated voltage at the substation bus. In the forward sweep, the voltage at each downstream bus is then updated by the real and imaginary components of the calculated bus voltage multiplying with the corresponding ratio. The procedure stops after the mismatch of the calculated and the specified voltages at the substation is less than a convergence tolerance. The proposed algorithm is tested with three IEEE benchmark distribution systems. Results show that the algorithm is accurate and computationally efficient in comparing with two other commonly used methods. [ABSTRACT FROM AUTHOR]

This letter presents a new control strategy to determine the reference compensation voltages of a series active power filter (APF) operated under nonideal supply voltages in the steady-state. By employing the proposed control strategy, the load-side voltages can be maintained as being sinusoidal and balanced. The effectiveness of the proposed control strategy is demonstrated through simulations by the use of Pspice. Results show that the expected series APF performances are achieved. [ABSTRACT FROM AUTHOR]

This paper presents an efficient approach for tracking the location of the electric utility shunt capacitor bank that generates switching transients by a limited number of power quality (PQ) meters. The approach is based on measuring the voltage-disturbance energy variations at the meter buses and phase angle changes at the connected branch current due to the reactive power supplied by the switched-on utility capacitor. Both measured energy and phase angle variations are used to identify the location of the switched capacitor through a rule-based tracking algorithm. The proposed method is then tested with the IEEE 30-bus transmission system to show its usefulness. [ABSTRACT FROM AUTHOR]