LVRT Capability Evaluation of Variable-Flux PMSG

LVRT Capability Evaluation of Variable-Flux PMSG based WECS 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— Exponential increase in installed capacity of Wind energy conversion system (WECS) in recent years has led to increased penetration into power grid. Thus, Grid-codes have started demanding better low-voltage-ride-through (LVRT) capability of coming wind generators and behave pretty much like conventional synchronous generator. This paper tests and analyses LVRT capability of a variable-flux PMSG and compares it with conventional blade-pitch-angle (BPA) control technique. Experimental setup to test LVRT capability consists of a Kamantype dual-stator axial-flux permanent-magnet synchronous generator (DSAF PMSG) with provision of angular displacement of one of the stator with respect to other to implement mechanical flux-variation technique as proposed. Keywords—Permanent-magnet synchronous generator, fluxvariation, Low-voltage-ride-through, Wind energy conversion system, Blade-pitch-angle control.

I. INTRODUCTION World in recent years has seen exponential increase in wind energy conversion system (WECS). Thus, penetration of wind energy into the grid has increased by significant portion. This has brought researchers attention to study the effects of higher penetration of intermittent wind energy into the grid. LVRT is defined as the sustainability of WECS under grid-fault. In this situation grid voltage can go as low as zero before fault is cleared. During the fault, the generator needs to supply reactive power to the grid to maintain voltage at pre-fault level. In synchronous generator active control of generator excitation regulates the flow of reactive power into the grid. Thus, any wind generator need to behave very near to a conventional synchronous generator. Grid-codes published by many countries have seen low-voltage-ride-through (LVRT) capability of wind generators as the most stringent requirement [1]. Fig. 1 shows grid codes for different country. In Region above a line WECS should remain connected to the grid and supply reactive power. In region below a line WECS can be disconnected.

Fig. 1 Grid-codes of National Grid of England and Wales (EAW), Eirgrid in Ireland(IRE), Red Electric in Spain (ESP), E.OnNetz in Germany (EON) for LVRT performance[1].

A Permanent-magnet synchronous generator (PMSG) based WECS with full-power converter (FPC) has shown good LVRT capability [2]. FPC acts as a buffer between PMSG and grid. Grid-side inverter in FPC controls the reactive power

supply to grid and thus, the grid voltage. At low grid-voltage, the system suffers from active-power surplus that is reflected at DC-bus. Power stored by DC-bus increases its voltage to dangerous level effecting power converter switches. To control the active-power surplus many techniques are proposed [1,36]. The control techniques are fundamentally based upon two methods. One is by controlling active-power from wind turbine and second is by dissipating surplus energy from DCbus. Former most commonly use turbine BPA control [3], yaw control [4], generator speed control [5] and storing surplus energy in rotor [6] while later is accomplished by using electronic switch controlled resistances across DC-bus [1]. The effectiveness of turbine BPA and yaw control is constrained by maximum rate of pitching angle mechanically possible [3]. Generator speed control is smart way to control the activepower though, the control mechanism is complex and incur higher losses due to use of active generator-side converter. Storing surplus energy in rotor is very effective though active generator-side converter is still required. Inclusion of brakingresistance across DC-bus is economically inefficient due to need of high-capacity resistances and dissipating heat from it. This paper proposes use of a variable-flux PMSG as wind generator with passive generator-side converter. Proposed generator can have active-power regulation at variable wind/rotor speed by regulating generator output voltage. Voltage Regulation (VR) is achieved by varying the field flux of PMSG. Flux variation in PMSG is done in two ways, electrical flux-variation (EFV) and MFV. EFV technique comprises of using external field-coils with permanent magnets. Flux in the machine is varied by controlling fieldcoil current. This technique is further divided based upon orientation of field-coils 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 [7]. Field-coils oriented in parallel to PM are free from risk of magnet demagnetization but have complex construction and thus incur increased manufacturing cost [7]. On the other hand, in MFV technique, flux -weakening and -boosting is achieved mechanically-varying 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. Other techniques of MFV are achieved by rotating rotor/stator (in dual-rotor/dual-stator topology), adding leakage-path or flux-barrier [7]. 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. Subsequently, LVRT capability of the generator has been tested using ASS technique. The paper has been divided among VII sections. Section I introduces the topic. Section II details generator topology and basic principle of voltage regulation detailed in section III. Section IV explains the experimental test bed used. Results and discussions have been presented in section V. Finally, paper is concluded in section VI with references in section VII. 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. 2. 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. 2. 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. 3.

Fig. 2 Constructional details of Dual-stator axial-flux permanent magnet synchronous generator [11]. TABLE I. DESIGN DATA OF THE DSAFPMG. No. of phase

3

Output voltage

58V per phase

Speed

1000 rpm

No. of poles

4

No. of slots

18

Inner radius of stator

48 mm

Outer radius of stator

80 mm

NdFeB magnet

N35

Magnet shape

Arc shape

Magnet dimensions

V 0 = V SS + V RS∠θ

(1)

V 0 = V 2(1 + Cos (θ ) )

(2)

θ is angular shift of RS in electrical angle, V is rms phase voltage in RS and SS winding. Fig. 4 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.

80 mm outer radius 48 mm inner radius

Magnet arc

700mech degree

Magnet width

3 mm

Air gap flux density

1.170 tesla

Air-gap length

5.5 mm

III. VR TECHNIQUE Voltage regulation of the generator for extended speed range is achieved by connecting same phase of each stators in series [12]. 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)

Fig. 3 Fabricated setup of proposed generator [12].

Fig. 5 Emulated turbine Power Vs rotor speed characteristic

Fig. 4 Rectified Output voltage three phase DSAF PMSG as per angular displacement of RS in electrical angle

IV. LVRT TEST BED Experimental setup for studying LVRT capability of proposed generator consist of a wind-turbine emulator (WTE), a DSAF PMSG, passive rectifier, DC-bus-bar capacitor and an electronic-switch controlled resistive load. Wind turbine has been modeled as per eq. (3).

1 Pwind = ρAv3 CP (λ, β) 2 λ=

(3)

ωR v

(4)

Here, Pwind is turbine output power, ρ is air density, A is blade swept area, v is wind velocity, CP is power coefficient, ω is rotor speed in mechanical radians, R is blade radius and λ is tip-speed to wind-velocity ratio. CP is a function of tip-speed ratio, λ and BPA, β, that defines the turbine characteristic. Mathematically CP is given by (5) [13]. C

− 7 C CP (λ, β) = C1 ( 2 − C3β − C4βC5 − C6 )e λi λi

λi =

1 C 1 − 9 λ + C 8 β β3 + 1

(5)

(6)

C1 to C9 are constants whose values are given in table 2. The parameters of turbine i.e. outer-radius of the turbine blade, R, wind velocity, v and rated electrical rotor speed, ωe have been chosen such that to match the generator rating. The turbine parameters have been tabulated in table 2. Based on the parameters Pwind Vs rotor speed, ω, and CP Vs γ for β varying from 0 degree to 15 degree, have been plotted in fig. 6 and fig. 7 respectively. TABLE II. EMULATED TURBINE PARAMETERS R v

ωe C1 C2 C3 C4 C5 C6 C7 C8 C9

Fig. 6 Emulated turbine CPVs λ characteristic

WTE for the setup is drawn from chopper-control of a separately-excited DC motor as given in fig. 8. A PI currentcontroller is used to generate PWM pulses for chopper as to match characteristic of a wind turbine. A DSPACE 1104 board is used to generate PWM signal for chopper. Rotor speed is sensed by a 400 ppr absolute encoder that intern generate reference DC armature-current signal. This signal is compared to DC armature current and fed to PI current-controller. Controller generates PWM pulses for chopper controlling DC armature-current as per turbine characteristic. Sample time has been taken to be 500 µsec. High sample time limits the switching frequency of chopper and consequently limits the controller performance. Kp and Ki of the controller involved is tuned by hit and trial method due to turbine characteristic being highly nonlinear. For testing the Generator under wind conditions, is coupled to WTE. Generator’s Three-phase output is rectified by three-leg H-bridge passive rectifier. Rectified output is then smoothened by DC-bus-bar capacitor. To test the generator under gridfault, it is assumed that DC-bus-bar capacitor is sufficient enough to take care of reactive power need of grid, under fault. Proposed technique is to control the surplus active-power at DC-bus-bar on grid-fault and thus, regulates the same. DCbus-bar is connected to resistive load through an electronic switch. Under grid-fault active-power consumption of the grid reduces by its voltage. For a three-phase symmetrical-fault grid voltage becomes zero thus active-power intake is zero resembling an open-circuit load. Therefore, in experimental setup the electronic switch is opened to test the proposed generator for LVRT capability.

0.75 m 5.4 m/s 50 rad/sec 0.73 151 0.58 0.002 2.14 13.2 18.4 0.02 0.003 Fig. 7 Control strategy of emulating wind turbine from DC motor.

V. RESULT AND DISCUSSION Proposed generator has been tested for three-phase symmetrical grid-fault. To test the generator in extreme condition the fault time has been taken as to be 2 sec. Fig. 9 (a) shows DC-bus-bar voltage, VDC , at no-load, load and fault conditions without any controller in action. It is seen that voltage dips from no-load voltage of 90 volt to 50 volt when machine is loaded and surges back to 90 volt at fault. Fig. 8 (b) presents the voltage dynamics with turbine BPA controller on the same lines as done by Conrey, J.F. et. al. [1]. In the experiment the voltage has been regulated to 40 volts at loaded condition. From fig. 8 (b) it is seen that BPA-control limits the

voltage surge to 60 volts. The oscillations observed are due to the PI controller involved and second-order actuator model considered [1]. Fig. 8 (c) gives the result when MFV is incorporated with BPA-control. It can be seen that the dynamic response of the system has deteriorated though, the voltage surge has been curtailed to 50 volts at fault. Fig. 8 (d) gives the voltage dynamics with only MFV controller. It is observed that the dynamic and steady-state response have been improved with voltage surge curtailed to below 50 Volts. This gives pessimistic conclusion that the deteriorated dynamic response in case of all-controller involved is due to the actuator model and PI controller of BPA-controller that cannot be avoided in the real system.

8 (a)

8 (b)

8 (c)

8 (d)

Fig. 8 Vdc voltage dynamics at fault (a) without controller (b) with BPA controller (c) with BPA controller and flux weakening control (d) with flux weakening control

From the analysis it is concluded that if the actuator to rotate the stator for MFV is fast enough, LVRT capability of the wind generator can be increased comparable to conventional synchronous generator. This allows higher penetration of wind generator into the grid. However, this paper does not consider the reactive power need of the grid at fault condition which is also an important factor to be considered. Though, it is handled by having sufficient large-capacity inverter or other inverter connected to the grid capable enough to supply reactive power need of the grid. VI. CONCLUSION LVRT capability of a DSAF PMSG based WECS at threephase grid-fault has been studied. Mechanical flux-variation and wind BPA-control technique for strengthening LVRT capability of chosen generator have been compared. It is observed that second-order blade-pitch actuator and PI controller involved with BPA-control imparts a sluggish response to the system deteriorating its LVRT capability. With MFV, dynamic response of the system has been improved with better LVRT capability. Also, MFV has been incorporated with blade-pitch control though, it is observed that the dynamic response could not be improved but voltage surge at fault was curtailed. Results conclude that MFV technique for chosen generator has better LVRT capability and

dynamic response given that the actuator used for angularly shifting one of the stators is fast enough. VII. REFERENCES [1]

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