converter rating optimization for a brushless doubly fed induction generator

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Published in IET Renewable Power GenerationReceived on 16th July 2014Revised on 29th September 2014Accepted on 26th October 2014doi: 10.1049/iet-rpg.2014.0249

T Renew. Power Gener., pp. 1–8oi: 10.1049/iet-rpg.2014.0249

ISSN 1752-1416

Converter rating optimisation for a brushless doublyfed induction generatorAshknaz Oraee1, Ehsan Abdi2, R.A. McMahon1

1Electrical Engineering Division, Cambridge University, 9 JJ Thomson Avenue, Cambridge CB3 0FA, UK2Wind Technologies Ltd, St John’s Innovation Park, Cambridge CB4 0WS, UK

E-mail: [email protected]

Abstract: This article studies the converter rating requirement of a Brushless doubly-fed induction generator (DFIG) for windturbine applications. Practical constraints such as the generator torque-speed requirement, reactive power management andgrid low-voltage ride-through (LVRT) are considered. Experimental data have been used to obtain a realistic system model ofa Brushless DFIG in a wind turbine. The study shows that there is a minimum converter rating, dependant on operating point,but this may not be achievable without machine derating. The use of a capacitor bank enables the converter ratings to bereduced without compromising the machine’s performance. The effect of the machine’s leakage inductance on the interplayof LVRT, achievable system power factor, converter rating and losses is investigated.

1 Introduction

The Brushless doubly-fed induction generator (DFIG) is apromising replacement for the widely used conventionalDFIG offering improved reliability, reduced capital andmaintenance costs and significantly greater low-voltageride-through (LVRT) capability [1, 2]. It is intrinsically amedium-speed machine, enabling the use of a simplifiedone or two stage gearbox, hence potentially reducing theweight of the overall drivetrain and further improvingreliability [3]. The low-cost advantage of the DFIG systemis retained as only a fractionally rated converter is neededand in addition the machine does not use permanent magnetmaterials. A schematic of a wind turbine with a BrushlessDFIG is shown in Fig. 1.The rating of the converter, comprising an AC to DC grid

side inverter (GSI) and a DC to AC machine side inverter(MSI) is determined not only by the generator speed rangeand operating conditions, but also by grid requirements forreactive power and LVRT. The real and reactive powers ofthe power winding (PW) are controlled by the MSI [2]. TheGSI stabilises the DC-link voltage and is also able toprovide reactive power directly to the grid. Little attentionhas been devoted to the optimisation of converter ratings forthe Brushless DFIG. In [4], Wang et al. proposed a methodbased on minimising the sum of MSI and GSI ratings forgiven steady-state conditions. This study assumed a constanttorque over the entire speed range and did not considertransient currents during grid faults. A reactive powermanagement study was also presented by Carlson et al. in[5], considering the converter requirement for a 75 kWBrushless DFIG. They proposed the use of equally ratedMSI and GSI and show that by utilising a capacitor bank,improved efficiency and overall cost could be achieved.

However, the effect of transients on converter rating wasnot discussed. The converter rating is essentially determinedby its maximum voltage and current to be handled. Hence,the optimisation process must consider the requirements onboth voltage and current individually over the operatingrange of the generator as opposed to simply minimising theproduct of voltage and current, as proposed by [4, 5].This paper investigates the effects of grid requirements

under steady-state and transient conditions, includingreactive power management and LVRT requirements, on theconverter rating of the MSI and GSI for the Brushless DFIGwind turbine. A practical operating speed range, load torqueand power factor are considered for the generator, based onthose found in commercial wind turbines. The optimisationprocedure determines minimum requirements on voltage andcurrent individually, thereby obtaining a minimum converterrating. The effect of utilising a capacitor bank in reducingthe converter rating is also discussed.The design of the Brushless DFIG determines its

magnetising and leakage inductances, both of which affectreactive power management, hence the inverter ratings. Theexcitation level of the CW determines the generation ofreactive power which in turn affects the MSI rating. Theleakage inductances, in particular the rotor leakageinductance, limit LVRT transient currents, hence reducingthe size of MSI. Therefore the assessment of rotor leakageinductance is important in system optimisation and thispaper investigates its effects on converter rating andmachine copper losses.

2 Operation of the Brushless DFIG

The Brushless DFIG comprises two electrically separatedstator windings, the PW being connected directly to the grid

1& The Institution of Engineering and Technology 2014

Fig. 1 Wind turbine with a Brushless DFIG

Fig. 2 Brushless DFIG system and its test rig

a 250 kW brushless DFIG (right front) on test bedb Converter, comprising GSI (left) and MSI (right)

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and the CW supplied by a variable voltage and frequencyconverter. The pole numbers and winding configurations ofthe two stator windings are chosen to avoid direct couplingbetween the windings and a specially designed rotorcouples to both stator windings. The synchronous mode ofthe Brushless DFIG is the most desirable as the shaft speed,N, is then independent of the torque exerted on the machineand is determined by the pole numbers and the frequencyof mains and converter

N = 60f1 + f2p1 + p2

(1)

where f1 and f2 are PW and CW supply frequencies and p1 andp2 are their pole pair numbers, respectively.Wind turbine generators operate at variable power and

torque over a speed range which is approximately ±30% ofthe generator synchronous speed [1] or in the case of theBrushless DFIG, its natural speed. The torque-speed profile

Table 1 Specifications of the prototype 250 kW D400Brushless DFIG

frame size 400 PW polenumber

4

speedrange

500 rpm± 36% PW ratedvoltage

690 V (50 Hz,delta)

ratedtorque

3670 Nm PW ratedcurrent

178 A (line)

ratedpower

250 kW at 680rpm

CW polenumber

8

powerfactor

0.95 lag tounity

CW ratedvoltage

620 V (18 Hz,delta)

efficiency >95% CW ratedcurrent

73 A (line)


adapted for this study is based on the 5 MW wind turbinedescribed in [6] and has been scaled to give a maximumoutput power of 250 kW, to match that of the BrushlessDFIG prototype shown in Fig. 2. The converter is alsoshown in Fig. 2 and its specifications are listed in Table 2.The natural speed for the 4/8 pole Brushless DFIGprototype is 500 rpm at 50 Hz. The prototype machine wasdesigned to produce 250 kW output at 680 rpm with apower factor between 0.95 lagging and unity; itsspecifications are listed in Tables 1 and 2 and the scaledtorque-speed curves are shown in Fig. 3. As can be seen,the speed range is symmetrical around the natural speed andthe generator operates at its maximum speed of 680 rpmwhen the output power has reached about half of its ratedvalue.The speed deviation from natural speed determines the

theoretical minimum value for the converter ratings [1]. Asa minimum, the converter rating must be high enough toaccommodate real power transfer from the CW to the grid.

Table 2 Converter specifications [7]

Specifications for the GSI Specifications for the MSI

Type Controltechniques

Unidrive SP6601

Type SemikronSemistack SKS

84F

ratedvoltage(rms)

690 V ratedvoltage(rms)

690 V

ratedcurrent(rms)

100 A ratedcurrent(rms)

84 A

frequency 50 Hz frequency 0–18 Hz

IET Renew. Power Gener., pp. 1–8doi: 10.1049/iet-rpg.2014.0249

Fig. 3 Brushless DFIG characteristics for a wind turbine of [6]

a Power against generator speed for different wind speedsb Torque against generator speed

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However, grid code requirements including reactive powermanagement and LVRT, lead to an increase in the convertersize. The required power factor from wind turbines istypically within the range of 0.95 lag to 0.95 lead, asspecified by E.ON [8], posing a constraint on the size of theconverter, specifically at 0.95 lead. In addition, in recentgrid codes, wind generators are required to stay connectedand ride through low voltage faults and meet the reactivecurrent demanded by the grid [9], as seen in Fig. 4. LVRTcapability is therefore important for wind turbines which areintegrated into the grid.Crowbars are commonly used in DFIGs to protect

converters during voltage dips, but reduces the dynamics ofreactive current injection. In the Brushless DFIG, transientcurrents during faults are significantly lower than anequivalent DFIG [2], hence a crowbar-less converter maybe utilised. Consequently, the MSI must be able to handlethe transient currents during LVRT events. The grid coderequirements on LVRT affect the size of the MSI becauseunder voltage control large currents will flow in the CW,producing a current surge in the MSI because of suddenloss of magnetisation on the PW.The majority of converters used in wind turbine

applications have a voltage rating between 690–900 V,hence the generator is designed to operate within a similarrange considering an allowance for transients resulting fromthe action of the controller. In the following sections the

Fig. 4 LVRT requirement by E.ON grid code [8]


VA converter rating is established and the presumption isthat the CW voltage will be set to match the converter’soutput range. A procedure is presented to obtain theminimum rating for the MSI and GSI consideringsteady-state and transient operation of the Brushless DFIGin a practical wind turbine.

3 Converter rating for steady-state operation

In steady-state operation, the generator is expected to operateaccording to the torque-speed curve shown in Fig. 3 and atthe same time provide the required reactive power to thegrid. The minimisation method aims to find the minimumsum of the MSI and GSI ratings. During this procedure, theCW excitation is varied using the MSI for a given operatingpoint of the generator. The GSI is then rated to provide thebalance of the required reactive power. The procedure isapplied over the speed range of the generator and theminimum sum for each point shown in Fig. 3 is derived. Itis evident that the most demanding operating point, fromthe converter point of view, is when the generator isrunning at rated power and speed and injecting ratedleading reactive power. For the Brushless DFIG understudy, this is equivalent to 250 kW at 680 rpm at a powerfactor of 0.95 lead. Therefore the analysis presented in thispaper is for this operating point.The equivalent circuit of the Brushless DFIG is utilised for

the steady-state analysis. The simplified per-phase equivalentcircuit is shown in Fig. 5 [1], where R1, R2 and Rr are the PW,CW and rotor resistances, Lm1 and Lm2 are the PW and CWmagnetising inductances and Lr represents the combinedstator and rotor leakage inductance. The parameter valuesfor the 250 kW D400 Brushless DFIG are given in Table 3.

Fig. 5 Referred per phase equivalent circuit of Brushless DFIG


Table 3 Equivalent circuit parameters of D400 Brushless DFIGreferred to the PW [6]

(a) Parameter definitionsparameter PW Rotor CWresistance R1 Rr R2magnetisinginductance

Lm1 – Lm2

leakage inductance – Lr –

(b) Parameter valuesR1, Ω Lm1, mH R′r, Ω L′r, mH R2″, Ω Lm2″ , mH0.097 104 0.114 12.5 0.102 53

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The slips are defined as follows

s1 =v1 − p1vr

v1(2)

s2 =v2 − p2vr

v2(3)

where ω1 and ω2 are the angular frequencies of the PW andCW, and ωr is the shaft angular speed. The MSI rating isdetermined by the rating of the CW, calculated as

SMSI = S2 = 3 I2V2

∣∣ ∣∣ (4)

Under normal operation, the Brushless DFIG needs acontroller to ensure stable operation over the desired speedrange and achieve effective control and rapid updating ofthe frequency and voltage. The PW real and reactivepowers, P1 and Q1, can be controlled by a vector controlalgorithm, implemented using a d–q model for theBrushless DFIG. A block diagram of the control system isshown in Fig. 6 and the principles of its design arepresented in [10]. The d-q components of the CW voltage,v2d and v2q are regulated by the CW current i2d and i2q,which are determined by the PW current i1d and i1q,respectively. Since there are linear relationships between P1

and i1q, and Q1 and i1d, P1 and Q1 can be controlled by theCW voltage, supplied by the MSI. The control chain istherefore summarised in Fig. 6.In the above control scheme, the MSI is used to control

both the PW real and reactive power and therefore the MSImust be sized to reflect this.At a grid power factor of cos f, the total reactive power

output, Qtot, is

Qtot = Ptot tanf = (P1 + P2) tanf (5)

Qtot comprises both the reactive power contribution of thePW, Q1 and the reactive power provided by the GSI, QGSI.Therefore QGSI can be written as

QGSI = Qtot − Q1 (6)

Fig. 6 Control scheme for the PW real and reactive power


and its rating, SGSI, is given by

SGSI =��P22 + Q2

GSI

√(7)

For unity power factor, (7) becomes

SGSI =��P22 + Q2

1

√(8)

The total inverter rating, Stot, is the sum of the GSI and MSIratings, given by

Stot = SGSI + SMSI (9)

The sum of the required ratings for the MSI and GSI isminimised, subject to 0.95 leading power factor, for the250 kW Brushless DFIG wind turbine with a power curveof Fig. 3. The total converter rating, that is, the sum of theMSI and GSI ratings, to achieve 0.95 leading grid powerfactor over the entire speed range is shown in Fig. 7a. Thecorresponding individual MSI and GSI ratings are alsoshown. As can be seen from the figure, a higher totalconverter rating is required at the upper end of the speedrange and the required GSI and MSI ratings are notnecessarily equal. The required rating for the MSI issignificantly higher than the GSI, because the former mustcontribute to the machine’s magnetising currents. Fig. 7bshows the required MSI and GSI ratings against variation ofthe excitation voltage, that is, the MSI voltage, when thegenerator operates at its most demanding operating point atfull load, maximum speed and 0.95 leading power factor.These curves are used to find the minimum sum at anyspecific operating point shown in Fig. 7a. The GSI ratingvaries substantially with change in the excitation voltage asthe reactive power contributed by the PW is determined bythe CW excitation voltage and the GSI must provide thebalance of the total required reactive power. The CWvoltages at which the minimum sum of the MSI and GSIratings are obtained, taken from Fig. 7a are shown inFig. 7c as a function of speed. However, as can be seenfrom Figs. 7b and c, the minimum sum at speeds above680 rpm is achieved by exceeding the rated CW voltage(620 V); at maximum speed a converter voltage of 870 V isneeded. Operating at voltages significantly above rated isnot practical.Another consideration is the total copper loss in the stator

and rotor windings and the required converter rating isplotted in Fig. 7d against the excitation voltage for thesame operation shown in Fig. 7b. It shows that, asexpected, under and over excitation of the CW increasescopper losses by increasing the currents in machinewindings required to transfer reactive power from one statorwinding to another. Therefore significant over and underexcitation of the CW should be avoided to maintain goodefficiency. It should be noted that this operating point is notachievable since the losses when overexciting the machineare likely to be higher than shown in Fig. 7d because of thepresence of saturation effects. Iron losses are estimatedaround 5 kW at the rated CW voltage, less than half of thetotal copper losses at full load. Accurate modelling of theiron losses in the Brushless DFIG is complex and hencethese have not been included in Fig. 7d.


Fig. 7 Converter rating minimisation for 250 kW Brushless DFIG

a Converter rating for 250 kW Brushless DFIG wind turbineb Converter ratings against CW voltage when the generator operates at full load, 680 rpm and 0.95 leading power factorc CW voltage for minimum sum of the MSI and GSI obtained in (a)d Total copper loss and converter rating against converter voltage when operating at full load, 680 rpm and pf = 0.95 lead

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3.1 Use of a capacitor bank

Capacitor banks may be used with the machine to reduce thereactive power requirement of the converter, therebyminimising the size of the GSI. This brings the operatingwithin the generator design rating and reduces the machinelosses. Fig. 8 shows the required GSI rating against the sizeof the capacitor bank when the CW is excited at its rateddesign voltage, that is, 620 V, for the same operatingconditions of Fig. 7a. Fig. 9 shows the reduction inconverter rating when a capacitor bank is used to contributeto the generation of reactive power. For example, eightcommercially available capacitor units, each rated at23.1 kVArs, would provide 184.4 kVArs in total. Anappropriate number of units can be switched out using apower factor controller to adjust reactive power generationat lower loads. Considering that the Brushless DFIGconsumes 72 kVAr of magnetising VArs and the exported

Fig. 8 GSI rating against capacitor bank VArs, full load, 680 rpmand pf = 0.95 lead


reactive power at full load and 0.95 leading power factor is78 kVArs, the majority of the VArs generated by thecapacitor bank effectively provides the magnetising andexported VArs, easing the VAr generation burden on theGSI and hence reducing its size. An alternative approach isto choose a capacitor bank with a rating of 72 kVArs tosupply only the magnetising VArs, therefore preventingunnecessary export of the VArs to the grid at part load.Fig. 9a shows the minimum required sum of the MSI and

GSI and the individual ratings over the speed range. Theratings are very similar to the ones shown in Fig. 7a.However, as can be seen from Fig. 9c, the minimum ratingsare achieved at significantly lower CW voltages, lower thanthe design rated values. Fig. 9b shows the inverter ratingsfor the same operating conditions as Fig. 7a, when thecapacitor bank is used. As can be seen, the minimum sumof the MSI and GSI rating is obtained when the CW isexcited at 630 V with MSI and GSI ratings of 145 and77 kVA, respectively. The average rating is therefore,111 kVA, equivalent to a converter rated at 44% of the totalmachine output power for a speed deviation of 36%.

4 Converter rating for transient operation

In this section, the effects of grid code requirements for windturbines under LVRT on the converter rating, specifically therating of the MSI are discussed. During LVRT, if the CWvoltage is controlled, the CW current rises substantially, tomultiples of its rating for a short time, hence a crowbar isoften required to manage the large currents. The aim is todetermine the required rating for the MSI so that the needfor a crowbar is eliminated.The simulation results performed on the 250 kW Brushless

DFIG for a symmetrical three-phase short-circuit are shown inFig. 10. The machine was initially running at 625 rpm with a


Fig. 9 Converter rating minimisation when a capacitor bank is used

a Converter rating for 250 kW Brushless DFIG wind turbineb Converter ratings against CW voltage when the generator operates at full load, 680 rpm and 0.95 leading power factorc CW voltage for minimum sum of the MSI and GSI obtained in (a)

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nominal torque of 3670 Nm. As shown in Fig. 10b, the peakMSI current is less than the transient rating of the MSI givenby the manufacturer’s datasheet, hence a crowbar is notrequired. The transient rating of commercial inverters forfault currents of this duration (<0.2 s) is typically up to2.5 p.u. of its steady state rating. Hence, the minimumconverter rating obtained in the previous section for thesteady-state requirements is sufficient to achieve acrowbar-less LVRT operation.

Fig. 10 Grid voltage and MSI line currents during LVRT fault, full loa

a Grid voltage during LVRT faultb MSI line currents during LVRT fault


5 Effect of leakage inductance on inverterrating

The series leakage inductance, Lr, in the Brushless DFIG’sequivalent circuit, shown in Fig. 5, represents the stator androtor windings flux leakage and has an important effect onthe rating of the converter. A higher leakage inductancemakes it more difficult to control the PW reactive power bythe MSI, but limits the MSI transient currents during LVRT

d, 625 rpm and pf = 0.95 lead


Fig. 11 MSI rating and total copper loss with variation in rotor leakage inductance, full load, 680 rpm and pf = 0.95 lead

a MSI rating requirement against variation of L′rb Total copper loss variation with L′r

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fault conditions. It is therefore important to assess thetrade-off during the generator design.Fig. 11a shows the variation of MSI rating against the

series leakage inductance to satisfy the required powerfactor of 0.95 leading, and allows a crowbar-less LVRToperation. As expected, an increase in rotor leakageinductance reduces the MSI rating needed to ride throughgrid faults, but comes at the price of reduced capability forreactive power management. The crossover in Fig. 11a, thatis, Lr = 11.2 mH, is the series inductance at which MSIrating is minimum while satisfying both steady-state andtransient requirements. The actual Lr of the 250 kWBrushless DFIG is 12.5 mH which is larger than theoptimal value for a minimum converter rating without usingcapacitor banks.The leakage inductance may also affect other performance

measures of the generator; an important one is the machinelosses. Fig. 11b shows the effect of rotor leakageinductance on total copper loss. As shown, a limited changein rotor leakage inductance within the range of interest doesnot have a significant effect on the machine’s copper loss.Hence, the optimum value of the leakage inductanceobtained from Fig. 11b may be aimed for during the designstage without greatly affecting the efficiency.A desired value of Lr, which represents stator and rotor

windings leakage effects, can be achieved by appropriatedesign of stator and rotor. For example, the effects ofnested-loop and series-wound rotor windings on the rotorleakage inductance are discussed in [11]. An alternativeway is to manage the stator slot leakage by suitable designof stator slot magnetic wedges [12, 13]. Clearly, the changein the design parameters would also affect otherperformance measures of the machine, hence an overalloptimisation must be carried out.

6 Conclusions

The converter rating for the Brushless DFIG wind turbine hasbeen studied with respect to reactive power management andgrid LVRT requirements. It has been shown that at a givenspeed, taking into account both steady-state and transientoperating constraints, there is a minimum sum of the MSIand GSI ratings but this condition may require anundesirably high CW voltage hence the minimum ratingmay not be practically achievable. Furthermore, it is shownthat the MSI rating is typically significantly larger than theGSI rating since the former contributes to the supply of


machine’s magnetising currents and must handle largetransient currents for crowbar-less LVRT operation.The effects of a capacitor bank in reducing the rating of the

GSI is also discussed. It is shown that with the use of asuitably rated capacitor bank, the minimum sum of the MSIand GSI ratings can be achieved at rated CW excitationvoltage. For example, at maximum speed, using a capacitorbank producing 72 kVArs leads to a reduction of the CWexcitation voltage from 870 V to the nominal 620 V. At thismaximum speed deviation of 36% around the natural speed,a converter rated at 44% of the machine rating is required.The effects of the machine’s leakage inductance on MSI

rating and efficiency have also been studied. A higherleakage inductance limits the currents during LVRT butcomes at the price of a restricted power factor range withoutde-rating the system. The effect of leakage inductance onmachine losses is investigated and these can risesignificantly when operating at leading power factors.However, the problem becomes less critical for largerBDFMs, that is, MW scale machines, since the per unitvalue of the rotor reactance will drop with size [14]. Theuse of capacitor banks is therefore more valuable withsmaller machines unless the operating speed range abovethe natural speed can be limited to around 20%.

7 Acknowledgments

The research leading to these results has received fundingfrom the European Union’s Seventh Framework Programmemanaged by REA – Research Executive Agency (FP7/2007_2013) under grant agreement N.315485.

8 References

1 McMahon, R., Wang, X., Abdi, E., Tavner, P., Roberts, P., Jagiela, M.:‘The BDFM as a Generator in Wind Turbines’. Power Electronics andMotion Control Conf., Portoroz, Slovenia, 2006, pp. 1859–1865

2 Long, T., Shao, S., Abdi, E., et al.: ‘Symmetrical low voltage ride-through of a 250kW Brushless DFIG’. Sixth IET Int. Conf. on PowerElectronics, Machines and Drives (PEMD), Bristol, UK, March 2012,pp. 1–6

3 Polinder, H., van der Pijl, F.F.A., de Vilder, G.-J., Tavner, P.J.:‘Comparison of direct-drive and geared generator concepts for windturbines’, IEEE Trans. Energy Convers., 2006, 21, (3), pp. 725–733

4 Wang, X., Roberts, P., McMahon, R.A.: ‘Studies of inverter ratings ofBDFM adjustable speed drive or generator systems’. Int. Conf. onPower Electronics and Drives Systems, 2005, vol. 1, pp. 337–342

5 Carlson, R., Voltolini, H., Runcos, F., Kuo-Peng, P., Batistela, N.J.:‘Performance analysis with power factor compensation of a 75 kwbrushless doubly fed induction generator prototype’. IEEE Int. ElectricMachines and Drives Conf., Antalya, Turkey, May 2007, pp. 1502–1507


www.ietdl.org
6 Byars, R.: ‘Power system architecture: finding the best solution for a
5MW wind turbine’. EWEA Offshore Wind Conf., Amsterdam,Netherlands, November 2011

7 Abdi, E., McMahon, R., Malliband, P., et al.: ‘Performance analysis andtesting of a 250kW medium-speed brushless doubly-fed inductiongenerator’, IET Renew. Power Gener., 2013, 7, pp. 631–638

8 ‘Requirements for offshore Grid Connections in the E.ON NetzNetwork’, http://www.eon-netz.com/pages/ehn_de/Veroeffentlichungen/Netzanschluss/Netzanschlussregeln/080702ENENAROS2008eng.pdf,updated 1 April 2008

9 Piwko, R., Miller, N., Girad, R., MacDowell, J., Clark, K.: ‘Generatorfault tolerance and grid codes’, IEEE Power Energy Mag., 2010, 8,pp. 18–26

10 Shao, S., Abdi, E., McMahon, R.: ‘Dynamic analysis of the brushlessdoubly-fed induction generator during symmetrical three-phase


voltage dips’. Int. Conf. on Power Electronics and Drive Systems,Taipei, Taiwan, November 2009, pp. 464–469

11 McMahon, R.A., Tavner, P., Abdi, E., Malliband, P., Barker, D.:‘Characterising brushless doubly fed machine rotors’, IET Electr.Power Appl., 2010, 7, pp. 1–6

12 Abdi, S., Abdi, E., Oraee, A., McMahon, R.: ‘Investigation of magneticwedge effects in large-scale BDFMs’. IET Renewable Power GenerationConf., Beijing, China, September 2013, pp. 1–4

13 Abdi, E., Oraee, A., Abdi, S., McMahon, R.A.: ‘Design of the BrushlessDFIG for optimal inverter rating’. Seventh IET Int. Conf. on PowerElectronics, Machines and Drives (PEMD), Manchester, UK, April2014, pp. 1–6

14 Tohidi, S., Tavner, P., McMahon, R.A., et al.: ‘Low voltageride-through of DFIG and brushless DFIG: similarities anddifferences’, Electr. Power Syst. Res., 2014, 110, pp. 64–72


converter rating optimization for a brushless doubly fed induction generator

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