Recent Research Trend on Multi-phase Induction Machines

Transcription

1 Proc. of Int. Conf. on Control, Communication and Power Engineering, CCPE Recent Research Trend on Multi-phase Induction Machines K. B. Yadav 1, Alok Kumar Mohanty 2, Prabhat Kumar 3 1 Department of Electrical Engineering, National Institute of Technology, Jamshedpur, India yadavbkrishna@rediffmail.com 2 Department of Electrical Engineering, National Institute of Technology, Jamshedpur, India alokmoh@gmail.com 3 Department of Electrical Engineering, National Institute of Technology, Jamshedpur, India prabhat_kr135@sify.com Abstract This paper presents a detailed recent research trend on multi-phase induction machines, which include multiphase induction motor as well as the multiphase induction generator. Electrical power system developed so far was of three phases since the beginning of the last century but research in the field of multi-phase machines have attracted increased interest in recent years due to the several advantages that they offer as compared to the conventional three-phase ones, such as high power handling capability by dividing the required power between multiple phases, reduced torque pulsations and higher reliability. In particular, unlike in a three phase drive, the loss of stator phase does not prevent the machine from starting and running. Increased torque per ampere for the same volume machine due to reduced stator copper losses and reduced rotor harmonic currents have paved its wider acceptability of utilization. Index Terms Multi-phase machines, Induction motor, Induction generator. I. INTRODUCTION Multi-phase machines are AC machines characterized by a stator winding composed of generic number of phases. In today s electric drive & generation technology multiphase machine has several advantages over the traditional three phase machine such as reducing the amplitude and increasing the frequency of torque pulsation, reducing the rotor harmonic current per phase without increasing the voltage per phase, lowering the dc-link current harmonics and higher reliability, high fault tolerance [1,2]. Earlier, multiphase motor were not in wide use because of the lack of multiphase supply for the multi phase motor. With the advancement in power electronics, interest in multi-phase machine has been increased tremendously as high power electronic devices are used as a switch in Voltage Source Inverter (VSI), the output of the VSI is given to the multiphase machine. In a multi-phase machine drive system, more than three-phase windings are housed in the same stator of the electric machine, and the current per phase in the machine is, thereby reduced. In the most common of such structures, two sets of three-phase windings are spatially phase shifted by 30 electrical. In such systems, each set of the three-phase stator winding is excited by a three-phase inverter; therefore, total power rating of the system is theoretically doubled. In addition to enhancing power Elsevier, 2014

2 multiphase machine has many advantages[1-20] over the traditional three-phase motor drives such as reducing the amplitude and increasing the frequency of torque pulsation, reducing the stator current per phase without increasing the voltage per phase, lowering the dc link current harmonics and higher reliability. For that multiphase machine with capabilities to start and run even with one or more of its stator phases opencircuited when the winding structure is multiphase. II. VARIOUS REASONS FOR CONSIDERATION OF MORE PHASES The various reasons for which multiphase induction machines are considered advantageous as compared to their three phase counterpart are as follows: A. To improve reliability since loss of one of many phases does not prevent the machine from starting and running. B. To reduce the current without increasing the voltage per phase. C. Due to a larger number of phases, multiphase machines are characterized with much better fault tolerance than the three-phase machines. D. For a given machine s output power, utilization of more than three-phases enables splitting of the power across a larger number of inverter legs, thus enabling use of semiconductor switches lower rating. III. GENERAL DESCRIPTION A machine can have as many phases as coils per pole pair. The number of phases for a machine is assumed to be the same as number of stator terminals or leads, excluding neutral. However, giving the number of phases is not always an adequate description because two machine versions are possible based on two possible values of the phase belt angle for a given number of phases [1, 2, 3, 4]. Almost all the three-phase machines have 60 phase belts but sometimes these machines are wound with 120 phase belts, and have some different characteristics from 60 version. A six-phase machine can easily be constructed by splitting the 60 phase belt into two portions each spanning 30.The Table-1 given below [1,2] shows the names of different multiple phases and relate them to the phase belt angle and the minimum number of stator leads required for them. Based on the equivalent circuit of a three winding transformer, the equivalent circuit of the multi-phase induction machine having a double stator winding can be realized [2] as shown in Fig. 1.The common mutual leakage reactance, X Lm, in the figure represents the fact that the two sets of three-phase stator windings occupy the same slots and are, therefore, mutually coupled by a component of leakage flux. Although the circuit is termed per phase ; in reality the circuit is drawn with two stator circuits, one per three-phase group. The equivalent circuit based on the generalized mathematical model developed by Singh and Pant [14, 15, 16] shown in Fig. 2. The common mutual leakage inductance L lm in Fig. 2 represents the fact that the two sets of stator windings occupy the same slots and are, therefore, mutually coupled by a component of leakage flux [14]. TABLE I. MULTIPHASE MACHINES HAVING MULTIPLES OF THREE PHASES belt angle(β) No. of Phase belt per pole No. of stator terminals (Minimum) Connection Name Phase Semi 6- Phase 6-Phase 9-Phase Semi 12- Phase 12 Phase Semi 18- phase 18- Phase Schematic diagram of star connection & voltage phasor diagram Alternate diagram or Common name Three Phase 581

3 Fig.1.Equivalent circuit of induction machine double stator winding Fig.2. Equivalent circuit based on the generalized mathematical model This mutual leakage inductance, L lm has an important effect on the harmonic coupling between the two stator winding sets and depends on the winding pitch and the displacement angle between the two stator winding sets.the coil pitch affects the leakage reactance of the stator winding. Lipo [11, 12, 13] has explained this in detail and has given the technique for finding slot reactance. IV. MULTI-PHASE INDUCTION MOTOR A multi-phase system comprises more than the conventional three phases, the machine output power can be divided into two or more solid state inverters that could each be kept within prescribed power limits. Also, having additional phases to control means additional degrees of freedom available for further improvements in the drive system. Dual-stator machines are similar to split-phase machines with the difference that the stator groups are not necessarily equal. Usually these three-phase groups are displaced by thirty electrical degrees from each other.this arrangement composes an asymmetrical six-phase machine since the angular distance between phases is not all the same. The analysis of an induction machine for multiple phases and arbitrary displacement between them is presented. The dq0 model for a six-phase machine was developed. Reliability is one of the advantages in using six-phase systems. In the case of losing one phase, the six-phase machine can continue to be operated as a five-phase machine. Among the different multi-phase induction drives solutions, the dual-3-phase induction machine having two stator winding sets spatially shifted by 30 electrical degrees with separated neutral points has important advantages. These electrical machines are convenient in high power and high current applications offering an interesting alternative to the conventional 3-phase counterpart. The current stress of each semiconductor power device is reduced by one half compared with the 3-phase machine counterpart, while the dual-3-phase solution can benefit of the wide availability of components dedicated to 3- phase system. There are several reasons for opting for a multiphase machine. Some of the principal are stated below. 1. The stator excitation in a multiphase machine produces a field with a lower space-harmonic content, so that the efficiency is higher than in a three-phase machine. 2. Multiphase machines have a greater fault tolerance than their three-phase counterparts. If one phase of a three-phase machine becomes open-circuited the machine becomes single-phase. It may continue to run but requires some external means for starting, and must be massively de-rated. 582

4 3. Multiphase machines are less susceptible than their three-phase counterparts to time-harmonic components in the excitation waveform. Such excitation components produce pulsating torques at even multiples of the fundamental excitation frequency. The preliminary investigation of an inverter fed five-phase induction motor had been presented by Ward and Harer [8] and suggested that the amplitude of torque pulsation can be reduced by increasing the number of stator phases. The computer simulation on three types of six-phase motors using an inverter source has been carried out by Nelson and Krause [9]. They found that by using a motor with 30 phase belts, the sixth harmonic torque pulsation usually encountered in inverter driven three-phase motors was eliminated, though the peak stator currents were increased. Danzer has reported the test results on five-phase motors. The reason given for using five phases was to reduce the current such that it would match the rating of the available thyristors, for inverter source. However, the third harmonic current was found to be excessive when it was supplied by the inverter. Motors with many phases have been proposed for high degree of reliability. The derivation of the voltage equations in phase variables and the transformation to the d q o reference frame of a multi-phase machine with unsymmetrical phase displacement has been reported by Nelson and Krause [9]. Analysis of six-phase machine with 0 phase displacement between two winding sets has been given by Singh et al [2]. A model for inverter fed dual three phase (spatially phase shifted by 30 electrical) induction machine drive system have been reported by Abbas and Lipo [10,11]. Two separate models have been used by Zhao et al [12, 13] to analyze the dynamic behaviour of machines for balanced and unbalanced excitation due to open circuit. These models are silent about the analysis of unbalanced condition caused by the short circuit at stator terminals. The two-axis (d q) model of the multi-phase machine in an arbitrary reference frame was developed by Singh et al[6,7] and a detailed analysis of the machine under balanced, and unbalanced (open circuit and short circuit both) operating condition has been carried out. The characteristics of several high phase order induction motors were examined by Klingshirn [1]. A detailed performance analysis (no-load and load test) of the six-phase induction machine have been presented by Singh et al [5, 6, 7]. V. MULTIPHASE INDUCTION GENERATOR Multiphase system expands the universe for drive and control purpose. The use of renewable energy sources becomes essential and therefore, the study of self excited induction generator has regained importance as it is particularly suitable for wind and hydro power plants.eventhough three phase induction generator are used for this purpose but now-a-days multi-phase are being considered due to the advantages of multiphase machines as compared to the three phase counterpart machines. Having more phases means added degrees of freedom that can be explored in these systems. In a dual stator winding induction machine with two stators, one of stator winding used for electromechanical power conversion and the other one for excitation [14, 15, 16]. In an induction generator consisting of double stator winding configuration with extended rotor common to both stators.. Split-phase electrical machines consist of two similar stator windings sharing the same magnetic circuit. Such a construction made it possible to extend the power range of solid-state based drives by sharing the total power between two drives. Usually a split-phase machine is built by splitting the phase belt of a conventional three phase machine into two equal parts with spatial phase separation of 30 electrical degrees. Singh G. K., Yadav K. B., and Saini R. P et al have modelled and analyzed of multi-phase (six-phase) self-excited induction generator [14]. Singh G. K., Yadav K. B., and Saini R. P. have also analyzed the characteristic of a saturated multi-phase (six-phase) self-excited induction generator [15]. Singh G. K., Yadav K. B., and Saini R. P [16] had also analyzed on how a self exited six phase induction generator can be useful in renewable energy generation purpose. Singh et al have reported the steady-state performance of a six-phase self-excited induction generator for renewable energy generation. Singh and Senthil Kumar [17, 18] presented a simple method to find out the minimum value of capacitance crucial to initiate self excitation process in six-phase induction generator. The problem was formulated as multivariable unconstrained nonlinear optimization problem. Singh et al also reported paper discusses the steady-state analysis and performance of six-phase self-excited induction generator for stand-alone renewable energy generation. The basis of the analysis is the nodal admittance method based on graph theory as applied to the equivalent circuit model. Feifei Bu, Wenxin Huang, Yuwen Hu, Chendan Li, Kai Shi, Qianshuang Wang presented the static and dynamic characteristics of 6/3-phase DWIG system with a rectifier load, harmonic MMF in generator are analyzed in detail. The corresponding relationship between the harmonic current and harmonic MMF is achieved. Singh & Senthil Kumar reported a simple method to determine the performance of a Self-regulated, Self-Excited Six-Phase Induction Generator. The problem is formulated as multivariable 583

5 unconstrained non linear optimization problem. For Selecting the value of exciting capacitance required for multiphase induction generator, an analysis of the six phase self exited induction generator indicates that for different operating conditions such as change in speed, power factor of load, different values of shunt and series capacitance etc., results are sometimes satisfactory from the view point of safe operation of the machine but may violate the various constraints [17].Therefore, it becomes necessary to optimize the values of shunt and series capacitances so as to achieve favourable results on loading the machine to its rated full load. Singh et al have proposed simple method for computing the minimum value of capacitance to initiate self excitation in six-phase self-excited induction generator. The methodology presented by Singh is equally applicable for minimum capacitance value analytically under both the conditions i.e. when excitation capacitance is connected across single three-phase winding set, and when both the winding sets for operation at no load and pure resistive load. The value of series capacitors can be selected for the two winding sets depending on the loading conditions [18]. The model is equally applicable to the machine with any arbitrary phase displacement between the two winding sets [14, 15, 16] as shown in Fig.3. Fig.3. Per phase equivalent circuit of a six-phase self-excited induction generator Since the conventional supply and uses are three phase and single phase, it seems necessary to mention here that the combination with two three-phase windings displaced 30 in phase is the configuration of greatest practical interest for very large generators as it permits recombination of two three-phase power in the stepup transformer bank without the need for increased transformer KVA rating for phase shifting. This indicates the need for a coordinated design of the 3-winding step-up transformer or of two separate transformers. VI. DUAL-STATOR WINDING INDUCTION GENERATOR The dual-stator winding induction generator(dwig) at the beginning of this century overcome the shortcomings of traditional cage-type induction motor, arouses widespread concern in academics by its unique structure, and the optimized design of this generator, voltage control strategy, the system stability and variable-speed operation have been studied deeply. According to the current study, the generation system consisting of DWIG has better performance. Because its output is stable DC power, it can be transmitted by the way of HVDC, which can be applied for offshore wind power generation. In the usual 3/3-phase DWIG system, the power windings have only three phases, and it is connected to a rectifier load, which causes larger harmonic current and harmonic MMF. When it is applied to wind power, there exist lots of vibrations and noise in the system. To overcome these problems, the power windings of 3/3 phase DWIG is increased from three phases to six phases, called as 6/3 phase DWIG as shown in Fig.4. The 6-phase windings are divided into two Y windings shifted by 30 electrical angles so as to reduce the harmonic MMF and torque ripple, and improve the performance of the generator. Y. Li, Y. Hu, W. Huang, Y. Zhang, Z. Hao, and F. Teng, have presented a Dual stator winding induction generator based automotive power generation system using direct power control, its behaviour for wide speed-range operation [19]. In the usual 3/3-phase DWIG system, the power windings have only three phases, and it is connected to a rectifier load, which cause larger harmonic current and harmonic MMF. When it is applied to wind power, there exist lots of vibration and noise in the system. To overcome these problems, the power windings of 3/3 phase DWIG is increased from three phases to six phases, called as 6/3 phase DWIG. The 6-phase windings are divided into two Y windings shifted by 30 electrical angles so as to reduce the harmonic MMF and torque ripple, and improve the performance of the generator. Each three phase power windings of the 6/3 phase DWIG system is connected to a rectifier load in parallel though balanced reactor, so the current in the stator windings is not sinusoidal. Except for the elementary current, there is a series of high harmonic current, which results in elementary MMF, as well as high 584

6 harmonic MMF. The relationship between harmonic current and harmonic MMF is very import to the optimization design of the system and improvement of performance. Bu Feifei, Huang Wenxin, Hu Yuwen, Fig.4.The 6/3 phase dual stator winding Induction Generator Chen Xiaobo, Shi Kai, Wang Qianshuang [20] proposed a 6/3-phase DWIG and also reported on harmonic magneto motive force and Static-Dynamic Characteristics of the 6/3-Phase Dual Stator-winding Induction Generator. It has three-phase control winding, the same as the 3/3-phaseDWIG, and two sets of Y power winding shifted by 30 electrical degrees. Each Y power winding is rectified respectively and paralleled to power the load via the balancing reactor. Therefore, the current of the power winding is more continuous, which can reduce the harmonic MMF in the generator. According to the analysis, when the elementary current is flowing though the six-phase power windings, there are only 1, 11, 13, 23, 25 th harmonic MMF without 5,7, 17, 19 th ones. When the elementary current is flowing though the control windings, it creates 1, 5, 7, 11, 17, 19 th harmonic MMF with 5, 7, 17, 19 th ones. This is the difference between the control and power windings. Moreover, in the six-phase power windings, compared with three-phase power windings, the number of harmonic MMF is reduced. Therefore, compared to 3/3-phase DWIG, the 6/3-phase DWIG can reduce the content of the harmonic MMF, thus improve the performance of the generator and weaken the vibration of the system. VII. CONCLUSION In this paper, a survey has been made on the recent developments in the area of multiphase induction machines. Multiphase machines are currently being used where both the machine and its control electronics are designed as a system, rather than as individual components. Multiphase machines has many advantages over the conventional one such as improved reliability as the machine continues running with one of its many phases open- or short-circuited, reduced iron loss, lower current per phase without increase in per phase voltage, reduction in per phase power handling requirement, increased torque per rms ampere for the same volume machine. REFERENCES [1] E. A. Klingshirn, High phase order induction motors Part I: experimental results, IEEE Trans. Power Applications [2] Singh G.K., Multi - phase induction machine drive research a survey, Electr. Power Syst. Res., 2002, 61, pp [3] E. Levi, R. Bojoi, F. Profumo, H.A. Toliyat and S. Williamson, Multiphase induction motor drives a technology status review, IET Electr. Power Appl., 2007, 1, (4), pp [4] Emil Levi, Multiphase Electric Machines for Variable-Speed Applications, IEEE transactions industrial electronics, vol. 55, no. 5, may [5] G.K. Singh, M. Bhattacharyya, V. Pant, Improved reliability and high power rating in ac drives by means of phaseredundancy some analytical and experimental results, Journal of Institution of Engineers (India) 82 (2001)pp [6] V. Pant, G.K. Singh, S.N. Singh, Modeling of a multiphase induction machine under fault condition, in: Proceedings IEEE The Third International Conference on Power Electronics and Drive Systems, PEDS 99, Hong Kong, July 26 29, 1999, Vol. 1, pp [7] G.K. Singh, V. Pant, Analysis of multiphase induction machine under fault condition in a phase-redundant ac drive system, Int. J. Electric Machines Power Syst. 28 (6) (2000) pp

7 [8] E.E. Ward, H. Harer Preliminary investigation of an inverterfed 5-phase induction motor, Proc. IEE 116 (6) (1969) pp [9] R.H. Nelson, P.C. Krause, Induction machine analysis for arbitrary displacement between multiple winding sets, IEEE Trans. 93 (1974) pp [10] M.A. Abbas, R. Christen, T.M. Jahns, Six-phase voltage source inverter driven induction motor, IEEE Trans. Ind. Appl. IA-20 (5) (1984) pp [11] T.A. Lipo, A d q model for six-phase induction machine, in: Proceedings on International conference, Electric machines, Athens, Greece, 1980, pp [12] Y Zhao, T.A. Lipo, Space vector PWM control of dual three phase induction machine using vector space decomposition, IEEE Trans. IA-31 (5) (1995)pp [13] Y. Zhao, T.A. Lipo, Modeling and control of multi-phase induction machine with structural unbalance, part I- Machine modeling and multi dimensional current regulation, IEEE Trans. Energy Conversion EC-11 (3) (1996) pp [14] Singh, G. K., Yadav, K. B., and Saini, R. P., Modeling and analysis of multi-phase (six-phase) self-excited induction generator, Proc. IEEE Conf. The Eighth International Conference on Electrical Machines and Systems, ICEMS 05, Vol. 3, pp , September 27-29, [15] Singh, G. K., Yadav, K. B., and Saini, R. P., Analysis of a saturated multi-phase (six-phase) self-excited induction generator, International Journal of Emerging Electric Power Systems, Vol. 7, pp.1-23, [16] Singh, G. K., Yadav, K. B., and Saini, R. P., A Self-excited six-phase induction generator for stand-alone renewable energy generation, Proc. IEEE International Agean Conference on Electric Machines, Power Electronics and Electromotions, ACEMP 07, pp , Bordrum, Turkey, September, [17] Singh GK and Senthil Kumar A Saini RP, Selection of Capacitor for the Self-Excited Six-Phase Induction Generator,2009 Third International Conference on Power Systems, Kharagpur, INDIA December [18] A. Senthil Kumar, G.K. Singh, Performance Analysis of Self-Regulated and Self-Excited Six-Phase Induction Generator, Chennai and Dr.MGR University, Second International Conference on Sustainable Energy and Intelligent System (SEISCON 2011) [19] Y. Li, Y. Hu, W. Huang, Y. Zhang, Z. Hao, and L. Liu, Decoupling control of the dual stator-winding induction generator using SVM, IEEE Power Electronics Specialists Conference, PESC, pp ,June, [20] Bu Feifei, Huang Wenxin, Hu Yuwen, Chen Xiaobo,Shi Kai, Wang Qianshuang, A Novel 6/3-Phase Dual Stator- Winding Induction Generator System Applied in Wind Power Generation, PEDS