Comparison of Variable-Flux PMSG for Extended Speed-Range based on Magnet Arc-Length to Pole-Pitch Ratio Shailendra Kumar Gupta
R K Srivastava
Department of Electrical Engineering IIT BHU Varanasi, India
[email protected]
Department of Electrical Engineering IIT BHU Varanasi, India
[email protected]
Abstract— This paper discusses voltage regulation (VR) of PMSG for a wind energy conversion system (WECS) under varying wind conditions. Mechanical flux-variation technique (MFV) has been used for VR of a kaman-type dual-stator axialflux PMSG at above rated rotor/wind speed extending the speedrange of generator. An experimental test-bed has been set up to implement MFV in the generator and subsequently, the implications of proposed technique on the performance of generator has been analyzed. Total harmonic injected in generator output due to proposed technique has been calculated by FFT analysis and compared for two types namely, sinusoidal and trapezoidal back-emf characteristics. Keywords—Permanent-magnet synchronous generator, fluxvariation, Voltage regulation, Wind energy conversion system.
I.
INTRODUCTION
Increasing global pollution and temperature have shifted onus from conservative non-renewable energy resources to renewable energy resources. Our mother earth’s diverse geography has made some region rich in wind energy. To capture maximum wind energy through WECS, wind generators operate in two modes, constant-speed constantfrequency (CSCF) and variable-speed constant-frequency (VSCF). Induction generator (IG) and doubly-fed induction generator (DFIG) based system work in CSCF mode of operation. Though the system are very popular works at low power factor, low power density and operating speed-range is small [1]. Contrary to DFIG, Permanent-magnet synchronous generator (PMSG) works in VSCF mode of operation. In VSCF mode power produced by variable wind speed is first rectified and then inverted to required frequency by an inverter. Rectifier and inverter is coupled by a DC-link to stabilize rectifier output containing ripples. Being operated at variable speed in VSCF mode and due to unavailability of field flux control in PMSG, system faces problem of voltage fluctuations. The output voltage is regulated either by generator-side control [2] or grid-side control [3] or both [45]. Though, these control strategies are being used in stronggrid as well as weak-grid, the system becomes less reliable, complex and less economical owing to increased no. of switches and using complex and aggressive control techniques. Voltage regulation of PMSG based system in VSCF mode is also done by varying the field flux of PMSG. Flux
variation in PMSG is done in two ways, electrical fluxvariation (EFV) and MFV. EFV technique comprises of using external field-coils with permanent magnets. Flux in the machine is varied by controlling field-coil current. This technique is further divided based upon orientation of fieldcoils with respect to PM. Field-coils oriented in series to PM has boosting as well as weakening capabilities but there is risk of magnet getting demagnetized [6]. Field-coils oriented in parallel to PM are free from risk of magnet demagnetization but have complex construction and thus incur increased manufacturing cost [6]. On the other hand, in MFV technique, flux -weakening and -boosting is achieved mechanicallyvarying the inner construction of machine. Basically, only flux-weakening is achieved through mechanical manners with exception of having flux-boost when air-gap in the machine is varied. Varying the air-gap of the machine requires big actuators due to the fact that to vary the air-gap we need to supply power equivalent to energy-stored in the air-gap [7]. Other techniques of MFV are achieved by rotating rotor/stator (in dual-rotor/dual-stator topology), adding leakage-path or flux-barrier [6]. In this paper a kaman-type DSAF PMSG with provision of angularly-shifting one of the stator (ASS), rotatable stator (RS), with respect to stationary stator (SS) has been chosen to be experimentally tested as a wind generator. Here output voltage regulation (VR) of the generator is achieved by angularly-shifting RS with respect to SS whose respective phases are connected in series. The technique has been explained in detail in section III. Capponi F. G. et. al. [8] implemented same technique for VR on a torus-type AFM by bifurcating the stator into two and shifting the same with respect to each other. The paper has been divided among six sections. Section II gives construction details of the generator and mechanical arrangement of misaligning two stators. Section II describe basic principle of VR in proposed generator. Section IV presents results and discussions. Section V finally concludes the topic with references in section VI. II.
GENERATOR CONFIGURATION
A Kaman-type dual-stator sandwiched-rotor axial-flux permanent magnet synchronous generator (DSAF PMSG) has been chosen for experimental hardware development owing to
its easy construction, higher power density, higher torque to inertia ratio and efficiency [9-10]. Further, it was easy to execute MFV technique in chosen generator as outer-stator topology provide easy excess to stator for angular shift of the same. The construction details have been presented in fig. 1. A disc-rotor with PM pasted on both surfaces is sandwiched between two pancake-shaped stators. Both Stators are supported by potting arrangement that rest on the shaft through bearings. Shaft consists of a flange at the centre on
which rotor is pinned. On both sides of rotor, shaft is stepped where potting-bearing sits to maintain constant air-gaps between each side of rotor and stators as given in fig. 1. As per the output voltage regulation requirement, mechanical provision of angular shift of RS with respect to SS is provided by fixing SS to generator platform and fixing a worm wheel to the periphery of RS potting. Gear to rotate the worm wheel is coupled to a stepper motor fixed to the platform as given in fig. 2.
Fig. 1 Constructional details of Dual-stator axial-flux permanent magnet synchronous generator [11].
In the paper two topologies of DSAF PMSG have been compared. Topology 1 (T1) and Topology 2 (T2) have characteristic difference in its back-emf waveform due to T1 and T2 having 1800 and 1400 electrical degree magnet arclength. Due to absence of interpolar region in T1 the back-emf waveform has high-peak sinusoidal (HPS) while in T2 the waveform is regular sinusoidal as shown in fig. 5 (a) and 6 (a) respectively. Further, T1 has non-overlapped concentrated coil with coil span of 900 and T2 has overlapped coil with coil span of 1200. Though the winding is different in both topology, winding distribution factor has been maintained to be one therefore, there is no waveform change introduced by windings. The air-gap, magnet axial-length and magnet material in both topologies have been maintained same to produce same air-gap flux density. Other design parameters of both topologies have been tabulated in table 1 and table 2. III.
θ is angular shift of RS in electrical angle, V is rms phase voltage in RS and SS winding. Fig. 3 shows variation in noload rectified output voltages without filter-capacitors as per θ for rotor rotated at 1000 rpm, 750 rpm and 500 rpm. For smoothening of the rectified output voltage filter-capacitor is required.
VR TECHNIQUE
Voltage regulation of the generator for extended speed-range is achieved by connecting same phase of each stators in series [13]. For rotor/wind speed above rated value, RS is angularly shifted to introduce a phase difference between same phase of SS and RS as to be VRS and VSS respectively. As the phases are connected in series, output voltage, V0 , is equal to phasor sum of VRS and VSS . Mathematically V0 is given as (1) and amplitude as in (2)
V 0 = V SS + V RS∠θ
V 0 = V 2(1 + cos(θ ) )
(1) (2)
Fig. 2 Experimental Set-up of MFW enabled DSAF PMSG.
TABLE I. DESIGN DATA OF TOPOLOGY T1 [12]. No. of phase 2 Output voltage 26.5 V per phase per stator
TABLE II. DESIGN DATA OF THE TOPOLOGY T2. No. of phase 3 Output voltage 29 per phase per stator
Speed No. of poles No. of slots
1000 rpm
Speed No. of poles
1000 rpm 4
No. of slots
18
Inner radius of stator
35 mm
Inner radius of stator
48 mm
Outer radius of stator
80 mm
Outer radius of stator
80 mm
No. of turn per coil Winding type
45 Double layer Concentrated non-overlapping N35
No. of turns per coil
45
Winding type NdFeB magnet
Double layer overlapped N35
Magnet shape Magnet dimensions
Arc shape
6 12
NdFeB magnet Magnet shape Magnet dimensions Magnet arc-length Magnet width
Arc shape 76.15 mm outer dia 36.75 mm inner dia 600 mech degree 3 mm
Air-gap flux density
1.170 tesla
Air-gap length
6 mm
IV.
Magnet arc-length Magnet width
80 mm outer radius 48 mm inner radius 700 mech degree 3 mm
Air-gap flux density
1.170 tesla
Air-gap length
5.5 mm
RESULT AND DISCUSSION
MFV technique of regulating generator output voltage has been examined to see its implications on generators having different back-emf characteristic. The technique’s crux is to add two voltages with phase difference. Thus, the output voltage waveform depends upon the waveform of the voltage itself and the phase difference. Phase difference is a controller variable but voltage waveform characteristic depends upon magnet shape, α, air-gap length and winding configuration. Effects owing to stator slots have been neglected to focus upon effects due to angular shift of RS. Topologies T1 and T2 being compared has arc-shape magnets, high air-gap length, unity winding distribution factor and leakage flux between magnets that develops a sinusoidal airgap flux density and thus, voltage induced in both stators are also sinusoidal. Adding two sinusoidal quantities with or without phase difference generates another sinusoidal quantity therefore, output voltage is also a sinusoidal. Fig. 5 and 6 shows variations in output line voltage waveform for T1 and T2 topologies as per RS angular shift. The dips observed in Fig. 5(b) and 5(c) is due to use of full-pitched magnets producing a characteristic HPS back-emf that is added with a phase difference. Higher the phase difference higher the dip observed. On the other hand, no distortion has been observed in T2 that uses a short-pitched magnet producing a near to sinusoidal back-emf. To study the total distortion in output voltages produced by angular shift of RS in both topologies FFT analysis of the same have been done. Also, effect of MFV on a simulated trapezoidal characteristic back-emf has also been examined to include trapezoidal-shaped air-gap flux density as observed in BLDC machine. The variations of THD in output voltages as per angular shift of RS for sinusoidal and trapezoidal backemf has been presented in Fig. 7(a). Result suggests high THD
Fig. 3 Rectified Output voltage three phase DSAF PMSG as per angular displacement of RS in electrical angle
content in trapezoidal back-emf characteristic generator. FFT analysis of the waveforms shows HPS back-emf to have high content of 2nd harmonics while in case of trapezoidal backemf, 3rd and 5th harmonics have been found to be comparably high. The comparison has been plotted in Fig. 7 (b). Though the output voltages for HPS and Trapezoidal characteristic back-emf generators has seen high content of THD, effective voltage-regulation capabilities is same as that of normalsinusoidal back-emf generators. Fig. 8 shows effective voltage regulation of HPS, trapezoidal and normal-sinusoidal back-emf characteristics as per RS angular shift. It is inferred from experimentation that WECS with DSAF PMSG using MFV for output voltage regulation should be filtered for 2nd and 3rd harmonics and PMSG is to be used without any filters before supplying to grid/load.
Fig. 5 (a) Fig. 5 (b) Fig. 5 Output voltage waveform of T1 for RS angular shift in electrical degree of (a) 00 (b) 750 (c) 1150.
(a)
Fig. 5 (c)
(b)
(c)
Fig. 6 Output voltage waveform of T2 for RS angular shift in electrical degree of (a) 00 (b) 750 (c) 1150
(a)
(b)
Fig. 7 (a) THD content in Output voltage as per angular shift of RS for Trepezoidal and HPS back emf waveform 8 (b) content of 2nd and 3rd harmonics in HPS and Trepezoidal back emf waveform as per angular shift of RS
Fig. 8 Amplitude of Output voltage as per angular shift of RS for trapezoidal, HPS and regular sinusoidal back emf waveform
V.
CONCLUSION
The paper has discussed the implications of MFV technique for voltage regulation of PMSG based WECS under varying wind conditions. Experimental setup with a DSAF PMSG has been fabricated and MFV is achieved by mechanically rotating RS with respect to SS. The effects of angular shift of RS on generator performance have been compared for different backemf characteristic. It is observed through experimentation that for fractional α, generating a regular-sinusoidal back-emf, is most appropriate for proposed technique. It is observed that fractional α generators incur no distortion in output voltage upon MFV. On the other hand, trapezoidal back-emf generators have seen appreciable amount of 2nd, 3rd and 5th harmonics during angular shift of RS. It is suggested to use filters for 2nd, 3rd and 5th harmonics for MFV controlled PM generators with trapezoidal back-emf characteristic as found in BLDC and PMSG is to be used without any filters. VI. [1]
[2]
[3]
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Transportation Electrification Conference (ITEC), 2015 IEEE International, Chennai, Aug 2015, pp. 1-6.
