Six-phase System Research Articles

Six-phase motor drive supplied by four voltage source inverters with synchronized space-vector PWM

Six-phase motor drive supplied by four voltage source inverters with synchronized space-vector PWMNovel method of space-vector-based pulsewidth modulation (PWM) has been disseminated for synchronous control of four inverters feeding six-phase drive based on asymmetrical induction motor which has two sets of windings spatially shifted by 30 electrical degrees. Basic schemes of synchronized PWM, applied for control of four separate voltage source inverters, allow both continuous phase voltages synchronization in the system and required power sharing between DC-sources. Simulations show a behavior of six-phase system with continuous and discontinuous versions of synchronized PWM.

A Two-Frequency Quasi Six-Phase Voltage Source Inverter Based on Space Vector PWM

With development of series multi-phase motor drives and harmonic injection technique to enhance torque for multi-phase motor drives, there is an immense need for multifrequency PWM voltage source inverters (VSI).The PWM techniques used in the previous works are either of ramp-comparison or of hysteresis type. The first suffer from implementation difficulties and the second has the high variable switching frequency drawback (difficulties of LC input EMI filter design). SVM possesses many advantages over the mentioned PWM techniques and it can be used in multiphase drive applications.One of the most popular multi-phase machines is quasi six-phase (QS) machine. The phasor diagram of a QS system and a two level six-phase VSI are depicted in Fig. 1. Despite the three-phase case the numbers of switching vectors are very high (refer to Fig. 2) and the removal of low-order harmonics in SVM six-phase systems is so complicated. In this paper, a two frequency QS SVM VSI is designed and implemented. The approach improves the performance of the system in terms of total harmonic distortion (THD) and DC-bus utilization. The resulted SVM VSI is used to generate two reference vectors for two series QS machines.Some analysis and discussions are given through the development of the SVM technique, computer simulation and practical results.

Generalized 12n-phase converter

Topological features and calculation relationships are considered for a 12n-phase converter with a multistage configuration of six-phase asymmetric voltage systems of secondary transformer windings. An example of a synthesized circuit of the phase number and shift converter is given for a 24-pulse rectifier recommended for use in networks with distorting factors.

The Geographical Spread of Avian Influenza A (H5N1): Panzootic Transmission (December 2003–May 2006), Pandemic Potential, and Implications

The World Health Organization (WHO) believes that the advent of highly pathogenic avian influenza A (H5N1) has moved the world closer to a further global pandemic of human influenza than at any time since the Hong Kong (H3N2) pandemic of 1968–1969. The immediacy of the perceived threat is underscored by the current classification of the world at Phase 3 of WHO's operative six-phase system of pandemic alert, with H5N1 having met all the prerequisites for the onset of a human pandemic but one: the efficient and sustained person-to-person transmission of the virus. With preparations for an anticipated pandemic now recognized as a global health priority, the purpose of this article is to provide a foundation for geographical research on avian influenza A (H5N1). The article introduces geographers to the complex nature and ecology of H5N1, the principal data sources available to analyze the global occurrence of the virus in birds and humans, and evidence regarding its geographical origins and international dispersal during the first thirty months of the ongoing panzootic in wild birds and poultry, from December 2003 through May 2006. Key epidemiological facets of the disease in humans are examined. We conclude with a review of the incurred and projected economic costs of H5N1, global plans for pandemic aversion and mitigation, and prospects for the future geographical expansion of the virus. Areas in which geographers can make an effective contribution to knowledge about the virus and the disease are considered.

Inter‐Observer Variation in Methodologies Involving the Pubic Symphysis, Sternal Ribs, and Teeth*

For the skeletal age of a victim to be useful in victim identification, the methods on which it is based must be reliable, accurate, and the results easily duplicated. The ability of multiple investigators to duplicate results is an interesting and complex issue. The purpose of this study is to investigate how consistently multiple investigators assign skeletal traits to rib, pubic symphyseal, or tooth "phases" and measure teeth. The skeletal data from identified individuals in Kosovo are used to test inter-observer variation for a variety of skeletal and dental aging techniques. Two hundred and ninety-six (n = 296) pubic symphyses were scored in the manners of the Todd's ten-phase system and the Suchey-Brooks six-phase system. Six hundred and twenty-two (n = 622) sternal rib ends were scored in the manner of Işcan and co-author's nine-phase system. Four hundred and twelve (n = 412) single-rooted teeth were measured in the manner of Lamendin and colleagues and scored for the amount of tooth wear using Smith's nine-phase system. Repeat measures were taken by multiple observers. There appears to be a wide range of variation, even among experienced investigators in the assignment of phase or metric data. Inter-observer variation, investigated through Pearson's r correlation coefficients, the Wilcoxon signed ranks test, and paired samples t-tests demonstrate significant differences using all methods. How this variation affects the accuracy of age estimation is subject to further investigation, but what is clear is that even with collaboration among investigators to calibrate with one another, the repeatability of numerous aging methodologies is difficult to achieve. Through this investigation it appears the problem lies in the qualitative nature of broad descriptive phase categories, which contain multiple skeletal features and traits that are open to interpretation.

Reconsidering the Auricular Surface as an Indicator of Age at Death

Using standards established by Lovejoy et al. to estimate age at death from auricular surface morphology, 266 individuals of documented age, sex, and ancestry from the Terry and Bass Donated Collections were scored. Analysis of covariance (ANCOVA) indicates that for the factors that could be controlled, age is the sole influence on auricular surface morphology. Ancestry and sex had no significant effect on auricular phase expression. No evidence of secular changes was detected when comparing the Terry Collection (early 20th century) to the Bass Collection (later 20th century). Pearson correlations reveal that several of the subcomponents of the auricular surface (superior and inferior demifaces, left and right sides, transverse organization, texture) correspond with age equally well, although a combined scoring of all features performs slightly better than any one indicator taken alone. Not surprisingly, only 33% of the sample was correctly aged when using the 5-year age ranges provided by Lovejoy et al., suggesting that the published ranges are much too narrow to be used in forensic contexts. To assess the variation in age per phase, standard descriptive statistics and error ranges were calculated and can be employed by forensic anthropologists when estimating the age of an unidentified decedent. Because the mean ages of some of the eight phases did not differ significantly from one another, a modified six-phase system is presented. The auricular surface performs as well as any other single skeletal indicator of adult age. This research suggests that a statistically-informed approach should be taken in order to fully understand the drawbacks and limitations of any aging method.

Six-phase transmission line-propagation characteristics and new three-phase representation

A new component transformation is proposed for six-phase systems, applicable for six-phase transmission lines studies, allowing its representation by two uncoupled three-phase lines. Cyclic transposition allows additional properties of the two three-phase groups that are presented. Transposed and nontransposed six-phase lines are analyzed, and the main characteristics of their natural modes are discussed in the frequency domain. An eigenvector normalization method for nontransposed lines that assures null modal conductance and, as a consequence, positive modal resistance, is proposed. Numeric results are presented in relation to an example line of 500 kV/sub rms/, phase to ground, with characteristic power of 5176 MW. >

Multiphase railgun systems: a new concept

The authors investigate multiphase railguns powered by multiphase compulsators. The polyphase system offers several advantages over the single-phase system. The multiphase compulsator relaxes the strong dependence between the current pulse width necessary for the railgun and the design parameters of the generator, thus allowing the compulsator to be designed for optimum power density and electromechanical energy conversion. The authors examine the two-, three-, and six-phase systems. They also explore different methods of achieving high acceleration ratios in multiphase railgun systems. Some of the methods analyzed are ramping up the field current of the compulsator to counter the back electromotive force of the gun, utilizing a railgun with varying inductance per unit length, and using an external variable inductor in series with the compulsator. The different features of each method are highlighted using simulation results. >

A method of analyzing unsymmetrical faults on six-phase power systems

The use of six-phase (and other higher order phase) systems for power transmission was proposed as an alternative for using electrical rights-of-way more effectively and efficiently. One of the important items in applying the six-phase alternative in transmission planning is the design of an adequate protective scheme. This requires a detailed and realistic fault analysis. The 23 significant fault types (symmetrical or asymmetrical) that can occur in six-phase power systems are analyzed. The theory of three-phase symmetrical components is used to develop a method for such analysis. The validity of the proposed method is checked by application to a simple system under a single line-to-ground fault, and three line-to-ground, five line-to-ground, and six line-to-ground faults. The advantage of the method is its applicability to all significant faults that occur on six-phase power systems.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Excessive neutral currents in three-phase fluorescent lighting circuits

Causes for excessive neutral currents amounting to 75% and 157% of phase current values in two installations supplying predominantly fluorescent lighting circuits have been identified. These high neutral currents are accounted for by three main components, namely fundamental phase current unbalance, third-harmonic currents caused by saturation of ballasts, and more third-harmonic current caused by third-harmonic voltage waveform distortion in the supply. Solutions are presented to overcome the problem, including a scheme that cancels third-harmonic currents along a common neutral conductor by means of a six-phase system supplied by two transformers connected in antiphase.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

A general treatment of phase faults on a six-phase generator

One of the problems that must be thoroughly investigated from the operational point of view is the fault analysis of a multiphase power system. The variety of faults in a six-phase system is far more complicated than in a three-phase system. This paper presents three simple algebraic equations for sequence currents, actual fault currents and post-fault terminal voltages for the most general phase(s) fault on a six-phase generator. The sequence currents, when written in the format suggested, immediately indicate the method of connecting sequence networks for the simulation of faults on a network analyser. The connections of the sequence networks for some faults not discussed in the literature so far have also been given.

Series faults in six-phase electric poer systems

The planning and design of transmission systems require the analysis of systems under steady-state as well as under transient conditions. This paper addresses a group of medium-fast transients in six-phase systems created by faults which do not involve any connection between conductors or between conductors and ground. This type of imbalance is referred to as a series fault; it arises, in most instances, when poles of a circuit breaker fail to open, thus creating an unbalanced series impedance condition. The various cases of series faults or open-line faults for six-phase transmission systems are analyzed in this work using the method of symmetrical components and Thévenin's theorem for two-port networks. A numerical algorithm for each of the various series fault cases is also developed here. This will allow us to determine the unbalanced currents resulting from an unbalanced series impedance condition.

Power transmission through ring transmission systems

Abstract Now that large blocks of power need to be transmitted to load centers, modifications to traditional transmission systems may be necessary. Although multi-phase (six-phase) systems have been suggested, new connections between the phases of double-circuit transmission lines might improve the transmission performance. This paper presents a mathematical analysis of the main parameters of multi-phase transmission lines. The mutual effect between the phase voltages is included. A ring-type transmission system is proposed for power transmission over lines of a network. Different configurations for such ring systems are investigated. The best connection diagram for this type of transmission is determined according to the condition of maximum transmitted power. The voltage distribution in ring transmission systems is compared with that of ordinary double-circuit transmission lines. The currents through both ring and ordinary lines are estimated. The circulating power during the no-load operation of ring transmission systems is evaluated. The transmitted power over both ordinary and ring double-circuit transmission lines is formulated and computed. Also, the transmission efficiency is calculated with respect to the transmission distance. The performance of ring transmission systems varies according to the angle between the voltages of both phases of the two circuits of double-circuit lines. Abnormal conditions are compared for three types of power transmission, namely, single, double and ring transmission lines. The proposed ring transmission system is recommended for networks with bad stability characteristics.

Six-phase (multiphase) power systems: Transient stability analysis

This paper addresses itself to the investigation of six-phase (multiphase) power systems which are currently being developed as potential alternatives to the conventional three-phase systems. Various aspects of six-phase systems, viz. feasibility, modelling, load flow and fault analysis, have already been examined. In this paper, the three-phase techniques of transient stability analysis are extended to six-phase systems. The equivalent single-phase as well as phase coordinate representations are employed to simulate the transient stability behaviour of six-phase systems. A sample system is worked out to illustrate the procedure and to focus on certain salient features of the performance of six-phase systems compared with that of conventional three-phase systems.

Six-Phase Transmission Systems: Capacitance Switching

This paper is aimed at studying the capacitance switching phenomenon in a six-phase power transmission system and comparing it with that of a three-phase system. The study is conducted by first deriving the analytical expressions for the transient recovery voltages associated with capacitance switching for three representative situations, and then verifying these analytical results by simulating the six-phase system on the digital computer using an electromagnetic transients program. The results indicate that the capacitance switching imposes less onerous transient recovery voltages on a six-phase system when compared to a three-phase system.

Six-Phase Voltage Source Inverter Driven Induction Motor

A six-phase six-step voltage-fed induction motor is presented. The inverter is a transistorized six-step voltage source inverter, while the motor is a modified standard three-phase squirrel-cage motor. The stator is rewound with two three-phase winding sets displaced from each other by 30 electrical degrees. A model for the system is developed to simulate the drive and predict its performance. The simulation results for steady-state conditions and experimental measurements show very good correlation. It is shown that this winding configuration results in the elimination of all air-gap flux time harmonics of the order (6v ±1, v = 1,3,5,...). Consequently, all rotor copper losses produced by these harmonics as well as all torque harmonics of the order (6v, v = 1,3,5,...) are eliminated. A comparison between-the measured instantaneous torque of both three-phase and six-phase six-step voltage-fed induction machines shows the advantage of the six-phase system over the three-phase system in eliminating the sixth harmonic dominant torque ripple.

Six-phase (multiphase) power systems: Some aspects of modelling and fault analysis

This paper presents a further step in the mathematical modelling and analysis of six-phase (multiphase) systems. In earlier publications, a six-phase line, several three-phase/six-phase transformers, loads and a six-phase generator were modelled. The three-phase equivalent representations of various elements were obtained for analysis of the overall system on a three-phase basis. In this paper the equivalent six-phase representations of various elements of a composite three-phase/six-phase system are derived for investigating the phenomena on the six-phase side of the network. Utilizing the element models discussed in the paper, several alternative schemes for fault analysis of a six-phase system employing transformation as well as phase coordinate methods are developed. The validity and effectiveness of the proposed schemes are demonstrated with the help of a sample system.

A Method of Supressing Fault Currents and Improving the Ground level Electric Field in a Novel Six-Phase Power Transmission system

In this paper, it is shown that the maxnuma electric field under the six-phase line with a modified phase arrangement becomes as low as that under the three-phase double-circuit ( conventional ) line. Then, a method is proposed for limiting line-to-ground fault currents and short-circuit currents without causing the fall in unfaulted phase voltages in the six- phase system with its neutrals solidly grounded. The fault analysis and the experimental result demanstrate that for a single line-to-ground fault at the sending end, the fault current in the proposed system is half that in the conventional system. As for the three-phase short-circuit fault at the sending end, the fault current in the proposed system is about 60% of that in the conventional system. It is concluded that the proposed six-phase system is superior in the system transient stability to the conventional system.

Fault analysis of six-phase systems

This paper presents an overview of the feasibility of six-phase transmission, symmetrical and Clarke's component transformations for six-phase systems derived from firm group theoretic considerations, and an analysis of the various faults likely to occur on six-phase systems. A numerical example is given to compare the magnitudes of fault currents involved for a six-phase and a double-circuit three-phase system for the most severe and most common cases. Finally, the future scope of the present investigations is indicated.

Six-phase (multi-phase) power transmission systems: Fault analysis

The motivation for writing this paper stems from the question, If the concept of six- (multi-) phase systems become feasible, how to plan such systems? One of the important aspects of transmission planning is the design of adequate protective scheme which requires a detailed fault analysis. It is well-known that the theory of symmetrical components is an important tool for such analysis. The first part of the paper is devoted to the development of the symmetrical component theory for six-phase systems. The remaining part of the paper deals with both symmetrical and unsymmetrical fault analysis of six-phase transmission systems. The results of the paper could readily be used by power system planners.