Permanent Magnet Machines for Distributed Generation: A Review

Transcription

1 Permanent Magnet Machines for Distributed Generation: A Review Paper Number: 07GM0593 Authors: Tze-Fun Chan, EE Department, The Hong Kong Polytechnic University, Hong Kong, China Loi Lei Lai, School of Engineering and Mathematical Sciences, City University, UK 1

2 Outline of this presentation Introduction Types of permanent-magnet synchronous generator (PMSG) Radial-flux PMSG for isolated operation Linear PMSG Axial-flux PMSG Variable-speed operation 2

3 Advantages of distributed generation Full exploitation of local energy resources Autonomous operation reduces the need for grid connection and transmission losses Improves reliability /security of supply Generators types: Induction generators Wound-field synchronous generators PM synchronous generators Switched-reluctance generators 3

4 Advantages and disadvantages of PMSG Advantages: Brushless construction Light weight amd small size High reliabilitry High efficiency Less frequent maintenance Disadvantages Excitation is fixed Output voltage varies with load 4

5 PM materials and PM machines Types of PM material: Alnico (since 1940s) Ferrites (since 1950s) SmCo (since 1960s) NdFeB (since 1980s) Types of PM machines: Surface magnet type Interior magnet type Surface inset type 5

6 Radial-flux PMSG for isolated operation Research work: New machine configurations (Binns 1983) Performance analysis using 2-axis model Performance using finite element method (FEM) (Chen, Nayar and Xu,1998; Chan, Yan and Lai, 2004, 2005) Voltage regulation improvement by capacitor compensation (Rahman, 1996) Voltage regulation improvement by exploiting rotor inverse saliency Interior type (Chalmers, 1994) Surface inset type (Chan, Yan and Lai, 2004) 6

7 A surface inset PMSG Soft-iron pole piece L Stator core Permanent magnet T2 T1 Rotor core 7

8 Phasor diagrams of PMSG (a) Unity power factor (b) Lagging power factor 8

9 Analysis (1) From phasor diagram for lagging power factor the following equations may be written V V cosδ = sinδ = I d = I I q = I E X From the equations: tanδ = Z I q I X d q sin( δ φ) cos( δ φ) L X q d cosφ X q I d sinφ I q R R R sinφ R cosφ 9

10 10 Analysis (2) The output voltage is For given load current and power factor angle, the following formula may be using to find V: ) sin( ) cos( cos. φ δ φ δ δ X R Z E Z V d L L = ) ( ) cos sin ( 2 ) ( )sin ( cos q q q d q d X R I R X VI. V X X R I X X VI VI.R V E = φ φ φ φ

11 Condition for zero voltage regulation Zero voltage regulation occurs when δ tan = 2 R X d tan( δ φ) X R tan( δ φ) q For machine with zero resistance δ tan 2 = r 2 r where r = X q /X d = inverse saliency ratio Zero voltage regulation possible when r >2 11

12 Effect of speed on voltage of PMSG with surface inset rotor (Chan, Yan, Lai, 2004) Parameters at nominal speed: E = V, R = Ω, X d = 0.88 Ω, X q = 2.23 Ω. At 4 times nominal speed a level voltage char. is obtained. 12

13 Air gap flux density obtained by finite element method (Chan, Yan, Lai, 2005) Flux density (T) Electrical angle (deg.) Strong flux through interpolar soft iron results in increased output Voltage and hence improved voltage regulation 13

14 Experimental characteristics of PMSG with inset rotor (Chan, Yan and Lai, 2005) 14

15 High-speed PMSGs Suitable for DG driven by micro-turbines Applicable in regions with abundant natural gas resource Generator designed to run at high speeds Main technical issues (Wang, 2002): Electromagnetic design Reduction of iron / stray losses Bearings for high-speed operation Cooling problem 15

16 Rotor of a high-speed PMSG Shaft Retaining ring or tape Rotor yoke Permanent magnet 16

17 Linear PM machines Linear electric machines (LEMs) involve translational motion instead of rotational motion For power generation, short-distance and oscillatory motions are involved Typical application is as a wave generator that captures the energy of perpetual wave motion, e.g., the Archimedes Wave Swing (AWS) (Polinder, 2004) Different machine designs are possible, e.g. transverse flux PMSG (Polinder, 2005), tubular PMSM (Amara, 2005) 17

18 A simple linear PMSG Nonmagnetic stop Permanent magnet Coil Core Linear, oscillatory motion Nonmagnetic collar 18

19 Axial-flux PMSG Electric machine with flat annulus air gap Main flux in axial direction, active conductors in radial directions Suitable for low-speed, direct drive wind energy systems because large number of poles can be accommodated 19

20 Types of axial-flux PMSG Modular PM generator with toroidal stator winding (Muljadi, 1999) Torus generator with double-sided stator (Wu and Chalmers, 1995 and 1999) Axial PMSG with toothed stator core (Hwang, 2004; Parvianen, 2005) Axial PMSG with coreless air gap winding (Chan and Lai, 2007) Outer rotor, single-sided design Zero iron loss, cogging torque Zero magnetic pull Suitable for both vertical or horizontal axis wind turbines 20

21 Test rig for outer-rotor axial-flux PMSM (Chan and Lai, 2007) The outer-rotor axial-flux PMSG is driven by a dc motor through a belt drive in order to emulate the wind turbine 21

22 Test results The experimental machine can deliver 350 W when delivering rated current at a speed of 600 r/min Output voltage is almost sinusoidal Line voltage waveform of axial-flux PMSG on load 22

23 Variable-speed PMSG connected to grid Variable-speed operation needed to optimize energy capture from the wind Frequency converter is required between generator and the power network Maximum power control strategy needs to be devised Using rectifier, boost chopper and inverter (Amei, 2002) Using PWM rectifer, intermediate dc circuit and PWM inverter (Chinchilla, 2006) 23

24 Maximum power control of PMSG with grid connection Variable-speed wind turbine DC link PMSG PWM rectifier PWM inverter Step-up Transformer Grid Pitch control Maximum power control Duty cycle control 24

25 Conclusions (1) PMSG is suitable for distributed generation Advantages are compactness, light weight, and high efficiency Variable machine configurations are possible Radial or axial flux machines Linear machines Outer-rotor design Light-weight design (e.g., spoke-wheel concept) Torus machines Toothless or coreless designs 25

26 Conclusions (2) Future work on PMSG might include: Refined performance analysis Loss or thermal models Design optimization Application of finite element analysis Control for grid integration Niche application areas, such as hybrid EVs, microturbine generators, etc. 26