Multi-structure model to optimize a Hybrid Excitation Synchronous Generator M. Ployard, A. Ammar, J. Iamamura, F. Gillon, L. Vido, D. Laloy
Φ
Abstract -- This paper proposes a multi-structure model combining eight rotating machines. The interest of double excitation is considered through magneto-motive sources combinations. An 1MV prototype of Hybrid Synchronous machine with permanent magnets mounted on pole surfaces is presented and used to validate the studied model at no-load operating mode for two topologies. An optimization problem provides data for discussion on the industrial choices of the prototype structure. Index Terms -- Hybrid Excitation, No-load operating mode, Reluctance network, Synchronous generator prototype, Multiobjective optimization
I.
INTRODUCTION
T
he requirements of a higher efficiency in electrical systems such as rotating electrical machines is constantly improving. The challenge is to design an electric generator which operates efficiently at various speeds and loads. Wound Rotor Synchronous Machines (WRSM) are formed on the rotor by a succession of poles which are themselves surrounded by excitation coils. WRSM have a known and robust design but needs a power supply system at the rotor. This kind of machines offers an easy flux control by acting on the excitation current [1]. The presence of rotor windings induces an increase of copper losses and therefore an efficiency reduction. Permanent Magnet Synchronous Machines (PMSM) are widely used because of the high power and torque to weight ratio, in fact they are more efficient than WRSM. Many industrial applications use PMSM in a large power range from a few kW to many MW, such as power generators [2]. The use of permanent magnets involves a constant magnetic flux [1] and also no copper losses. Nevertheless, the control of PMSM is difficult and costly, especially for high power applications [3]. Moreover, the high cost of magnets must be taken into account according to the chosen This work was supported by MEDEE. MEDEE is co-financed by European Union. Europe is moving in Nord Pas-de-Calais with the European Regional Development Found. M. Ployard is with the University of Lille Nord de France, F-5900 Lille, France, EC Lille, L2EP, F-59650 Villeneuve d’Ascq, France (e-mail:
[email protected]) A. Ammar is with JEUMONT Electric, F-59460 Jeumont, France (email:
[email protected]) J. Iamamura is with the University of Lille Nord de France, F-5900 Lille, France, EC Lille, L2EP, F-59650 Villeneuve d’Ascq, France (e-mail:
[email protected]) F. Gillon is with the University of Lille Nord de France, F-5900 Lille, France, EC Lille, L2EP, F-59650 Villeneuve d’Ascq, France (e-mail:
[email protected]) L. Vido is with SATIE laboratory, Univerty of Cergy Pontoise, CNRS, rue d’Eragny, Neuville sur Oise, F-95031 Cergy Pontoise, France (e-mail:
[email protected]) D. Laloy is with JEUMONT Electric, F-59460 Jeumont, France (email:
[email protected])
technology. These technologies led to the Hybrid Excitation Synchronous Machine (HESM) [1], [3]–[5], also called Double Excitation Synchronous Machine (DESM) [6], [7]. This machine combines the advantages of WRSM and PMSM, since the presence of permanent magnets allows reducing copper losses, and the excitation winding permits easy flux control. Due to the fact that there are two kinds of flux sources, the design of HESM involves a certain degree of freedom. The benefits obtained by these machines make that HESM catches researchers and industrials attention. Several applications have been designed in different fields, such as electrical vehicle traction [1], vehicle alternator [8], as a generator for island operation [2] or wind power generation [3]. In this paper, the HESM principle is presented and two topologies are discussed. At no-load, multi-structure magnetic model based on reluctance network is established. The model takes into account the magnetic saturation. An HESM prototype of 1MVA is presented. Then experimental results are confronted to those obtained with the multistructure model. Finally, an optimization problem is formulated from this model by taking into account total cost and copper losses of the machine while maintaining the general geometries of the prototype. A set of compromises is deduced and can be confronted to the industrial choice. The originality of this study is to have a multi-structure model of eight rotating machines. An optimization problem at no-load operating mode is used to propose a set of optimal machines with different structures. II.
CLASSIFICATION OF HYBRID EXCITATION SYNCHRONOUS MACHINES
Different topologies of HESM exist in the literature, they may be classified by different criteria [4]–[6]. For instance, they can depend on magneto-motive source locations but also on 2D or 3D structures. In this paper, HESM are defined by their flux source combinations. Thus, two groups of HESM are defined: series and parallel HESM. Series HESM topology is composed of two flux sources magnetically in series. Fluxes generated by windings and permanent magnets follow the same path in the machine [3], [5]–[7]. In parallel HESM topologies, flux sources are placed in parallel, thus the flux created by the windings does not pass through the permanent magnets, taking a different path [3], [5]–[7]. These topology principles are described, based on a classical salient pole machine, and their advantages and drawbacks are identified. Different examples of both main topologies are briefly discussed.
A.
Series HESM topologies
In a series HESM, the magnetic fluxes created by both sources take the same path. (Fig. 1) In this topology, the natural magnetic flux of the permanent magnets can be added or subtracted to the winding flux. In Fig. 1, the flux of the magnets is added to the winding flux, increasing the total magnetic flux in the machine.
Fig. 3. Parallel Hybrid Synchronous Machine principle
Fig. 1. Series Hybrid Synchronous Machine principle: conventional salient pole machine with permanent magnets mounted on pole surfaces
The increase or decrease of total magnetic flux is achieved by injecting a positive or negative current in the excitation winding (1). ƒ
(1)
where φu represents the useful flux, φw is the winding flux, φPM is the permanent magnet flux and ƒ can be expressed as a function. The electrical power supply is sized just for the excitation power. However, a bidirectional power supply leads to an increase of total cost. This structure of salient pole machine with magnet mounted on the rotor surface involves an important magnetic air gap, due to the fact that magnet permeability value is close to that of air [1], [5], [6]. Consequently, the efficiency of the winding is reduced, leading to more copper losses. Furthermore, permanent magnet and winding fluxes follow the same trajectory, so the risk of demagnetization remains important. This is why flux weakening operating mode is limited on series topologies. Nevertheless, different machines were developed for traction applications [7], or island operation [2]. B.
Parallel HESM topologies
Parallel HESM topologies are composed of two magnetic sources, each one creating a magnetic flux which follows a different path in the machine. Compared to series HESM, there is no risk of demagnetization [4]. Many parallel double excitation topologies are possible and exist [4], [5]. On Fig. 3, the principle of salient pole machine with inter-polar magnets is represented.
The operating mode of this machine can be divided in two parts. When there is no current supply in excitation windings, natural flux created by permanent magnets is short-circuited on the wound rotor [4], [7]. No flux traverses the stator when the excitation current is null. In short circuit case, the EMF can be adjusted to zero. When windings are powered, they saturate the rotor yoke of the HESM. The permanent magnets and winding fluxes are added and improve the useful magnetic flux (2). ƒ
,
(2)
The contribution of the magnets in the useful flux depends on the saturation and therefore, on the flux created by the windings. The main drawbacks of this topology are the need to bring power to feed the rotor windings and also the structure complexity [3]. A hybrid claw pole alternator with inter-polar magnets has been investigated [8], where the polarization of magnets is opposite to the leakage flux between claws. Its operating mode is based on parallel HESM topology. All these structures employing a combination of permanent magnets and windings require to be properly sized. Contribution of magnets must be estimated in relation to performance and cost to find the best compromise [7]. III. MULTI-STRUCTURE MODEL OF HESM A.
Magnetic model
The multi-structure model principle is based on a salient pole synchronous machine composed of 3 sources: windings, parallel and series magnets. The model of the Fig. 5 is based on a reluctance network method which consists in decomposing a magnetic device in a set of flux sources and passive elements. It is a static model at first harmonic, taking into account saturation in ferromagnetic parts at no-load operating mode. Fig. 5 represents a 3branch-model used in preliminary design study of a machine.
is reduced by half, according to the symmetries on one pole pitch. The general reluctance expression is given in (3) [7], with h the height, μ the magnetic permeability of the material, w and L respectively the width and length. (3)
. .
The MMF source of parallel permanent magnets is expressed by .
(4)
with BrPPM the remanent induction of the magnet, lPPM the length of the magnet and μPPM the magnetic permeability. The air gap reluctance is calculated by taking into account the Carter coefficient: (5) .
where τ is the tooth pitch, ws represents the slot width and hageq is the equivalent air gap height. The equivalent leakage reluctance is decomposed as shown in Fig. 6. RPPM, Rl1 and Rl2 are calculated according to (6), and then placed in parallel (Fig. 7 [9]). Fig. 5. Reluctance network at no-load operating mode. Rry and Rsy, are respectively the reluctances of rotor and stator yokes. Rt is the reluctance of the stator tooth. Rag, RSPM and RPPM are, respectively, the reluctances of the air gap, series and parallel permanent magnets. Rp0, Rp, Rp1, Rp2 and Rp3 are the pole reluctances. MMFw, MMFSPM, MMFPPM represent the MMF of windings and series and parallel permanent magnets. φr, φu, φip are respectively rotor, useful and inter-polar flux.
. . . ln
(6)
Hypotheses are made on the leakage reluctance, directly placed at the polar pieces (Fig. 6). The effects of rotor winding distributions on the MMF are not considered, in order to keep a simple multi-structure reluctance network.
The use of 3 magnetic flux sources allows this model to provide a combination of 8 potential machines, presented in Table I. TABLE I DIFFERENT ROTATING MACHINES FROM RELUCTANCES NETWORK AT NO-LOAD OPERATING MODE Configuration number 1 2 3
Parallel magnets 0 0 0
Series magnets 0 0 1
4
0
1
1
5
1
0
0
6
1
0
1
7 8
1 1
1 1
0 1
Windings
Machine
0 1 0
SRM WRSM SMSM Series HESM IMSM Parallel HESM SIMSM TESM
Fig. 6. Decomposition of leakage reluctances.
SRM: Switched Reluctance Motor; WRSM: Wound Rotor Synchronous Machine; SMSM: Surface Magnets Synchronous Machine; Series and Parallel HESM: Series and Parallel Hybrid Excitation Synchronous Machine; IMSM: Interpolar Magnets Synchronous Machine; SIMSM: Surface and Interpolar Magnets Synchronous Machine; TESM: Triple Hybrid Excitation Synchronous Machine
However, this model implies several simplification hypotheses, for instance, the section of flux tubes modeled by reluctances are crossed by a uniform magnetic flux. The induction in stator teeth is considered sinusoidal. Factor 2 in front of reluctances is due to the fact that the flux surface
Fig. 7. Trapezoidal element
The non-linear system corresponding to this multistructure model can be solved by means of the equivalent magnetic circuit (7).
1
.
(7)
Assuming a perfectly sinusoidal spatial distribution of flux, the EMF value is determined with (8) 2.
. .
√2
.
.
(8)
where EMF represents the electromotive force, f is the frequency, Nsp is the number of turns in series per phase, Kw is the winding factor and φu is the useful flux. B.
Fig. 9. Rotor of the series HESM with segmented magnets mounted on one pole
Losses calculation
Iron losses at no-load operating mode are calculated with the analytical expression proposed by Jordan [10], which separates iron losses in two components, hysteresis losses and eddy current losses (9). . .
.
.
(9)
where α, Kh, Kec are coefficients determined experimentally or after constructor data, Bm is the maximal magnetic induction and Piron is the loss density, expressed in [W/kg]. During no-load operating mode, copper losses are only present in the rotor (10). . ²
(10)
where Re represents the excitation resistance and J is the excitation current. C.
Fig. 10. Stator of the prototype
Economic model
The economic model of this structure depends on the weight of each machine element (11). Copper and electrical steel coefficients, respectively Kc and Ks, are expressed in [€/kg]. The cost of magnets is considered as a linear expression which depends on their masses MPM and their remanent induction Br (11). .
.
.
The main design parameters are listed in Table II. Permanent magnets employed on Fig. 9 are NdFeB with a remanent induction equal to 1.2T. Magnet pieces are fixed on the head parts of each rotor pole. This topology requires a retaining sleeve to maintain permanent magnets. TABLE II MAIN DESIGN PARAMETERS OF SERIES HESM
(11)
IV. PROTOTYPE The studied prototype, designed in [3], is a 1MVA salient pole synchronous machine with permanent magnets mounted on pole surfaces. This prototype allows studying successively two structures. The first one is a classical WRSM without permanent magnets (Fig. 8). After first experimentations, the rotor was extracted, and the permanent magnets were mounted on the rotor poles, providing a series HESM (Fig. 9). For both structures, the stator stays the same (Fig. 10).
Design parameters
Values
Speed
750 rpm
Pairs of poles Exterior diameter Number of stator slots Core length
96 750 mm
By testing the two different machines with the same geometry, a comparison is made between WRSM and series HESM. Furthermore, the multi-structure model can be validated for these two cases. V.
Fig. 8. Rotor of the WRSM prototype during balancing phase
4 860 mm
EXPERIMENTAL RESULTS AND VALIDATION
In order to investigate characheristics and performances of WRSM and HESM prototypes at no-load operating mode, the EMF of each machine is confronted to the one of the corresponding model. The reluctance network defined on Fig. 5 is previously set to consider these cases. The multi-structure model offers a good compromise between accuracy and simplicity. Fig. 11 shows a good agreement between theoretical model and experimental results. By adding surface series magnets, their constant flux improves the total magnetic flux. Thanks to the contribution of the
permanent magnets, the excitation current decreases by half for a rated EMF of 3000V. Therefore, rotor copper losses are considerably reduced in this no-load operating mode. 4000
3500
3000
EMF (V)
2500
2000
SRM and IMSM are not represented on Fig. 13 because at no-load they have no EMF. SMSM provides a constant magnetic flux. Without a high level of saturation, the flux generated by parallel magnets is short-circuited on the rotor. For SIMSM, there is no additional flux comparing to the SMSM. For the prototype dimensions with four pole pairs, parallel magnets do not involve a lot of advantages besides the leakage flux compensation. By adding series magnets, excitation current decreases of 50% at the rated EMF of 3kV for series HESM, comparing to WRSM.
1500
VI. OPTIMIZATION
1000
WRSM model WSRM mesures HESM model HESM mesures
500
0 -20
0
20
40
60
80
100
Excitation current (A)
Fig. 11. Analysis of WRSM and series HESM, model and experimental test at no-load operating mode
In this study, iron losses are supposed to be present only in the stator. On Fig. 12, measured iron losses of WRSM are compared to the iron losses model (9). The iron losses model provides a good approximation compared to the experimental results.
In order to discuss industrial choices of prototype, a multi-objective optimization problem is defined in (12) by using multi-structure model at no-load operating mode. The aim of this optimization is to observe the structure evolution in function of the objectives: cost (11) and total losses (9), (10). The multi-objective optimization problem is solved by an epsilon-constraint method with Sophemis, a tool developed at L2EP laboratory. The optimizer looks for a set of optimal Pareto solutions which respect the inequality and equality constraints given in (12).
12000
10000
0
8000
Iron Losses (W)
100 3 1,2 1,2
0 0 0
6000
12
4000
2 2000
Model Measures 0
0
500
1000
1500
2000
2500
3000
3500
EMF (V)
Fig. 12. Analysis of WRSM iron losses at no-load operating mode, theoretical model and experimental test
The model has been validated for both machines. Now, structures defined on Table I can be analyzed taking prototype dimensions into account. Fig. 13 shows the EMF values for 6 structures.
where HSPM is the height of series magnets (mm), BrSPM and BrPPM represent the remanent induction of surface and interior magnets (T), Hag represents the minimal air gap height (mm) ensuring technical feasibility. The length of parallel magnets is imposed by the distance between poles. On Table III, four optimizations are successively run according to each source. Then, the global optimization problem with the 4 variables (12) is also run. The corresponding results are on Fig. 14. TABLE III DESCRIPTION OF FOUR OPTIMIZATION PROBLEM
4000
3500
Variables
EMF (V)
3000
3000
1st problem
2nd problem
W
3rd problem
4th problem
×
×
S
T
2500
×
×
2000
-
×
-
3
1500
-
×
-
1,2
-
-
×
WRSM SMSM Series HESM Parallel HESM SIMSM THESM
1000
500
0 -20
0
20
40
60
80
100
120
140
Excitation current (A)
Fig. 13. Analysis of machines EMF from multi-structure model
160
×
For the first optimization, which analyses the WRSM, the optimal machine corresponds to only one point on Fig. 14. This Pareto optimal solution represents the best WRSM with lowest excitation current and therefore lowest copper losses. This point corresponds to the WRSM prototype that has been built.
4
1.55
x 10
WRSM Parallel HESM Series HESM TESM Global Optimization Prototype
1.5 1.45 1.4
Losses (W)
1.35 1.3 1.25 1.2 1.15 1.1 1.05 1 0.4
0.5
0.6
0.7
0.8
0.9
1
Cost (p.u.)
Fig. 14. Pareto fronts comparing different machine structures
The second optimization acts on the height and remanent induction of permanent magnets of the series HESM, which increase progressively with the cost, as losses decrease. The third optimization corresponds to the parallel HESM. From the analysis of these two Pareto fronts on Fig. 14, obviously series magnets are more effective in terms of copper loss reductions than parallel magnets. Fluxes generated by series magnets are directly considered as useful fluxes, and allow the decrease of the excitation current. For parallel magnets, when the poles are saturated, their fluxes are added to the useful flux. The contribution of magnets is most effective when they are mounted on pole surface, despite a higher cost. The fourth problem represents a set of optimal solutions with a combination of three magnetic sources. The height and remanent induction of series magnets are fixed according to Table III, only BrPPM varies. Economically, the TESM is the least interesting configuration. When combining the two types of magnets, price rises significantly. At last, Fig. 14 shows the global optimization problem and the location of the series HESM prototype. The technical choices of the prototype are justified, since the no-load operating mode. Surface magnets allow easily decreasing the excitation current and, accordingly, total losses. Even if this structure is suitable for prototype dimensions, risks of demagnetization are strong, due to the fact that the magnetic flux generated by both sources share the same path. In what concerns parallel magnets, they will be more efficient for structures with a large number of poles. VII.
CONCLUSION
A multi-structure model for eight potential machines was developed for no-load operating mode. This model can be employed in preliminary design study, combining a reluctance network for magnetic parts, analytical formulations for losses and economic aspects. A series HESM prototype allowed the validation of this model for two types of machines. An optimization problem was defined to confirm the industrial choices of the prototype structure. Despite the risk of demagnetization and high magnetic air gap, using series magnets is a good solution for the prototype
dimensions. It allows decreasing by half the excitation current and, consequently, copper losses. VIII.
REFERENCES
[1]
Y. Amara, L. Vido, M. Gabsi, E. Hoang, A. Hamid Ben Ahmed, and M. Lecrivain, “Hybrid excitation synchronous machines: Energyefficient solution for vehicles propulsion,” IEEE Trans. Veh. Technol., vol. 58, no. 5, pp. 2137–2149, 2009. [2] K. Kamiev, J. Nerg, J. Pyrhonen, V. Zaboin, and J. Tapia, “Feasibility of an armature-reaction-compensated permanent magnet synchronous generator in island operation,” IEEE Trans. Industrial Electronics, to be published . [3] A. Ammar, A. Berbecea, F. Gillon, and P. Brochet, “Influence of the ratio of hybridization on the performances of synchronous generator with Hybrid Excitation,” in International Conference on Electrical Machines (ICEM), 2012 XXth, 2012, pp. 2921–2926. [4] C. Zhao and Y. Yan, “A review of development of hybrid excitation synchronous machine,” in Proceedings of the IEEE International Symposium on Industrial Electronics, 2005. ISIE 2005., 2005, vol. 2, pp. 857–862. [5] W. Shanming, X. Yonghong, W. Xiangheng, S. Pengsheng, H. Shaogang, and Z. Anming, “State of the art of hybrid excitation permanent magnet synchronous machines,” in International Conference on Electrical Machines and Systems (ICEMS), 2010, pp. 1004–1009. [6] Y. Amara, G. Barakat, and M. Gabsi, “Comparison of flux control capability of a series double excitation machine and a parallel double excitation machine,” in Vehicle Power and Propulsion Conference (VPPC), 2010 IEEE, 2010, pp. 1–6. [7] D. Fodorean, A. Djerdir, I.-A. Viorel, and A. Miraoui, “A double excited synchronous machine for direct drive application—Design and prototype tests,” Energy Convers. IEEE Trans., vol. 22, no. 3, pp. 656–665, 2007. [8] C. D. Syverson and W. P. Curtiss, “Hybrid alternator with voltage regulator,” US Patents, 1997. [9] C.-S. Jin, Y.-H. Kim, J. Lee, T.-B. Im, and Y.-H. Cho, “Nonlinear analysis of 2 phase hybrid stepping motor using 3D equivalent magnetic circuit network method,” in Digest of Technical Papers Magnetics Conference INTERMAG Europe, IEEE International, 2002, p. FT10–. [10] A. Krings and J. Soulard, “Overview and Comparison of Iron Loss Models for Electrical Machines,” J. Electr. Eng., vol. 10, no. 3, pp. 162–169, 2010.
IX. BIOGRAPHIES Maxime Ployard received a M.S. degree in electrical engineering from Université de Lille 1. He is currently working towards the Ph.D. degree at Ecole Centrale de Lille. His research focusses on efficiency of rotating electrical machines. Aymen Ammar obtained an engineering degree from National Engineering School of Tunis. He got a M.S and a Ph.D degrees in 2009 and 2013 at Ecole Centrale de Lille. He is currently an electric rotating machines designer at JEUMONT Electric. Juliana Luísa Müller Iamamura obtained her doctor degree in the Lille 1 University in France, in cotutelle with the Federal University of Santa Catarina, in 2012. Nowadays she works as a postdoc researcher at Ecole Centrale de Lille, France. Frederic Gillon obtained an Engineer Diploma in 1992 and a Ph.D. in 1997 at Université des Sciences et Technologies de Lille. He is currently Assistant Professor at Ecole Centrale de Lille since 1999. His main research subjects are the design by optimization of electric systems and the study of electrical machines for applications such as linear, axial and radial synchronous motors and railway propulsion systems. Lionel Vido got a M.S. and Ph.D. degrees from the Ecole Normale Supérieure de Cachan, respectively in 2001 and 2004.Since 2005, he has been an Associate Professor wih the University of Cergy-Pontoise. He is currently with the Systèmes et Applcations des Technologies de l’Information et de l’Energie, ENS Cachan with the Centre National de la Recherche Scientifique and also with UniverSud Paris. His main research interest is the design of special electrical machines. Daniel Laloy obtained an engineering degree from Ecole Supérieur d’Electricité Supélec in 1974. He is currently .the director of research and development of JEUMONT Electric.
