Content uploaded by Ehsan Abdi

Author content

All content in this area was uploaded by Ehsan Abdi on Mar 24, 2021

Content may be subject to copyright.

IEEE TRANSACTIONS ON INDUSTRIAL ELECTRONICS, VOL. 60, NO. 7, JULY 2013 2833

Crowbarless Fault Ride-Through of the Brushless

Doubly Fed Induction Generator in a Wind Turbine

Under Symmetrical Voltage Dips

Teng Long, Student Member, IEEE, Shiyi Shao, Member, IEEE, Paul Malliband,

Ehsan Abdi, Member, IEEE, and Richard A. McMahon

Abstract—The brushless doubly fed induction generator

(BDFIG) shows commercial promise for wind power generation

due to its lower cost and higher reliability when compared with the

conventional DFIG. In the most recent grid codes, wind generators

are required to be able to ride through a low-voltage fault and

meet the reactive current demand from the grid. A low-voltage

ride-through (LVRT) capability is therefore important for wind

generators which are integrated into the grid. In this paper, the

authors propose a control strategy enabling the BDFIG to success-

fully ride through a symmetrical voltage dip. The control strategy

has been implemented on a 250-kW BDFIG, and the experimental

results indicate that the LVRT is possible without a crowbar.

Index Terms—AC generators, doubly fed induction generators

(DFIG), fault ride-through, wind energy.

NOMENCLATURE

PW, CW Power winding and control winding.

V,I,ΨVoltage, current, and ﬂux vectors.

v, i, ψ Voltage, current, and ﬂux scalars.

P, Q Active and reactive powers.

ωAngular velocity of the arbitrary rotating refer-

ence frame.

ω1,ω

2,ω

rAngular velocities of PW, CW, and rotor.

θ1,θ

rAngular positions of PW ﬂux frame and rotor.

p1,p

2Pole-pair numbers of PW and CW.

R1,R

2,R

rResistances of PW, CW, and rotor.

L1,L

2,L

rSelf-inductances of PW, CW, and rotor.

L1r,L

2rCoupling inductances between stator windings

and rotor.

snSlip.

sDifferential operator, i.e., d/dt.

¯

FComplex conjugate of vector F.

subscripts

1, 2, rPW, CW, and rotor.

Manuscript received February 2, 2012; revised May 1, 2012; accepted

June 9, 2012. Date of publication July 16, 2012; date of current version

February 28, 2013.

T. Long and R. A. McMahon are with the Department of Engineering, Uni-

versity of Cambridge, CB3 0FA Cambridge, U.K. (e-mail: tl322@cam.ac.uk;

ram1@cam.ac.uk).

S. Shao, P. Malliband, and E. Abdi are with Wind Technologies Ltd., CB4

0EY Cambridge, U.K. (e-mail: shaoshiyi@gmail.com; ea257@cam.ac.uk).

Color versions of one or more of the ﬁgures in this paper are available online

at http://ieeexplore.ieee.org.

Digital Object Identiﬁer 10.1109/TIE.2012.2208437

d, q Rotating dq frame axis.

α, β Stationary reference frame of PW.

pForward sequence component.

¯

FComplex conjugate of vector F.

superscripts

(2) Stationary reference frame of converter.

+PW synchronous reference frame.

I. INTRODUCTION

THE BRUSHLESS doubly fed induction generator

(BDFIG), also known as the brushless doubly fed ma-

chine, promises signiﬁcant advantages for wind power gen-

eration [1], since only a fractionally rated converter is

required and slip rings and brush gear are removed, enhancing

reliability [2].

With increasing penetration of wind power, wind generators

are expected to remain connected and supply reactive current

to the grid during grid voltage dips [3]. In the DFIG, the

stator ﬂux is exposed directly to the grid, and any voltage

dip will result in a sudden loss of the machine magnetization,

producing a current surge in the machine-side converter [4],

[5]. This current is typically large and, without appropriate

control strategies, may cause damage to the converter unless it

signiﬁcantly overrates the converter [6]. Hence, an appropriate

control strategy for riding through low-voltage faults is required

to integrate a wind generator into the grid.

According to the grid code from E.ON, when the grid expe-

riences a symmetrical low-voltage fault, wind turbines need to

perform the following: 1) ride through a period of zero voltage

up to 0.15 s and a further period of grid voltage recovery up to

1.35 s and 2) inject up to rated current of the reactive current to

the grid during the entire 1.5-s low-voltage fault time, shown in

Fig. 1. It is worth distinguishing the symmetrical low-voltage

faults from the asymmetrical low-voltage faults. Symmetrical

faults are more severe regarding the current surge but less likely

to happen. Asymmetrical faults will introduce a backward

sequence to the generator, although the current surge is less

severe and asymmetrical faults are more likely to happen [7],

[8]. Since the mathematical derivations are different between

these two situations, this paper only focuses on the symmetrical

low-voltage ride-through (LVRT). In addition, according to the

grid code, the reactive current injection requirements are also

different in these two situations; thus, the control strategies for

riding through different faults are distinguished too.

0278-0046/$31.00 © 2012 IEEE

Authorized licensed use limited to: UNIVERSITY OF SOUTHAMPTON. Downloaded on March 24,2021 at 14:11:22 UTC from IEEE Xplore. Restrictions apply.

2834 IEEE TRANSACTIONS ON INDUSTRIAL ELECTRONICS, VOL. 60, NO. 7, JULY 2013

Fig. 1. Grid code from E.ON.

LVRT control strategies for the DFIG have been widely

investigated. A widely used solution to protect the machine-

side converter from overcurrents is implementing a crowbar

circuit to short circuit the rotor connections of the DFIG

during the fault, such that the overcurrent ﬂows through the

crowbar shorting resistors [9]–[11]. The use of crowbar re-

quires extra hardware, thereby increasing the cost of the whole

system. In addition, the implementation of a crowbar limits

the reactive current injection, although many papers propose

complex control algorithm to increase reactive current injection

[12], [13]. Other than using a crowbar, the authors in [14]–

[16] proposed zero-sequence-compensation-based solutions to

reduce the transient overcurrent using different control ref-

erences, and the authors in [17] discussed the uncertainties

of the compensation-based LVRT solution with an improved

controller. In [18], the authors raised a novel control feedback

loop which created a virtual rotor resistor which was able to

increase the rotor resistance to damp the overcurrent without

extra crowbar or zero-sequence compensation. However, all of

those methods require complex control schemes and are, in any

case, inadequate during large voltage dips.

In contrast, the BDFIG typically has a larger series leakage

reactance and thereby experiences a reduced transient current

when compared to an equivalent DFIG [19] from the view

of the machine-side converter. As a result, it may be possible

for the BDFIG to ride through a low-voltage fault without the

need of a crowbar circuit or additional zero-sequence current

compensation or creating any virtual impedance by using extra

control feedback loop. Hence, the system cost will be reduced,

and the machine-side converter can also be utilized to supply

reactive current during the fault to satisfy grid regulations.

In this paper, a novel control scheme is applied to the

machine-side converter of the BDFIG for the LVRT. The dc-

link voltage is assumed being stable by using a commercial

grid-side converter in the experiment. The coordinated control

scheme combining both grid- and machine-side converters is

beyond the scope of this paper and will be investigated in the

future.

II. BDFIG OPERATION AND GENERAL VECTOR MODEL

The stator of the BDFIG has two separate windings, namely,

PW and CW, with different pole-pair numbers to avoid direct

coupling between the windings. A specially designed rotor

is able to couple both the PW and CW. The PW is directly

connected to the grid, whereas the CW is connected to the

grid through a bidirectional variable-voltage variable-frequency

converter, which handles a fraction of rated power and regulates

the dynamic performance of the whole BDFIG.

The BDFIG is normally operated at the synchronous (dou-

bly fed) mode, where the shaft angular velocity ωris deter-

mined by the excitation frequencies of two stator windings, as

expressed by

ωr=ω1+ω2

p1+p2

(1)

when the CW angular velocity ω2is equal to zero, the angular

velocity of the shaft is deﬁned as the natural angular velocity

ωn, as expressed by

ωn=ω1

p1+p2

.(2)

In this paper, the pole-pair numbers of the PW and CW are

two and four, respectively, and the grid frequency is 50 Hz.

Thus, the natural speed is equal to 500 r/min.

The vector model of the BDFIG, aligned in an arbitrary

rotating reference frame with the angular velocity of ω,is

expressed as [20]

V1=R1I1+dΨ1

dt +jωΨ1(3)

Ψ1=L1I1+L1rIr(4)

V2=R2I2+dΨ2

dt +j(ω−(p1+p2)ωr)Ψ2(5)

Ψ2=L2I2+L2rIr(6)

Vr=RrIr+dΨr

dt +j(ω−p1ωr)Ψr(7)

Ψr=LrIr+L1rI1+L2rI2.(8)

The active and reactive powers of the PW are expressed as

P1=3

2Re[V1¯

I1](9)

Q1=3

2Im[V1¯

I1].(10)

III. EQUIVALENT CIRCUIT MODEL OF THE BDFIG FOR

LVRT A NALYSIS

For simplicity, the PW stationary reference frame is used for

deriving the voltage and currents of the CW in this paper. Thus,

the arbitrary angular velocity ωin (3)–(8) equals zero.

By substituting (8) into (7), the rotor voltage in the Laplace

domain is expressed as

Vr=(sL1r−jp1L1rωr)I1+(sL2r−jp1L2rωr)I2

+(Rr+sLr−jp1Lrωr)Ir.(11)

As the rotor winding is shorted in the BDFIG, the rotor

voltage Vris zero.

Authorized licensed use limited to: UNIVERSITY OF SOUTHAMPTON. Downloaded on March 24,2021 at 14:11:22 UTC from IEEE Xplore. Restrictions apply.

LONG et al.: FAULT RIDE-THROUGH OF THE BDFIG IN A WIND TURBINE UNDER SYMMETRICAL VOLTAGE DIPS 2835

By substituting (4) into (11), the rotor current can be ex-

pressed as

Ir=L1rΨ1+L2rL1I2

L2

1r−L1Lr−RrL1

s−jp1ωr

.(12)

Substituting (12) into (6), the CW ﬂux is expressed as

Ψ2=L1rL2r

L2

1r−L1Lr+L1Rr

s−jp1ωr

Ψ1

+L1L2Lr−L1L2

2r−L2L2

1r+L1L2Rr

s−jp1ωr

L1Lr−L2

1r+L1Rr

s−jp1ωr

I2.(13)

It is worth noting that s−jp1ωris very large compared with

other terms in (13), and its reciprocal can be neglected. By

substituting (13) into (5), the CW voltage is given as

V2=Vx2 +EΨ1 (14)

where

Vx2 =(R2+Ll(s−j(p1+p2)ωr)) I2(15)

Eψ1=L1rL2r

L2

1r−L1Lr

(s−j(p1+p2)ωr)Ψ1(16)

Ll=L1L2Lr−L2

1rL2−L1L2

2r

L1Lr−L2

1r

.(17)

From (14), the CW voltage vector is split into two terms. The

electromotive force (EMF) induced by the PW ﬂux is referred

to as EΨ1, and Vx2 is the voltage drop across both the CW

resistance and an equivalent leakage inductance between the

CW and the PW caused by the CW current, ignoring the PW

resistance.

Neglecting the resistance of the PW R1=0,from(3)

dΨ1

dt =V1(18)

Ψ1=V1

jω1

=|V1|

jω1

ejω1t.(19)

Substituting (18) and (19) into (16) in the time domain, the

term EMF due to the PW ﬂux is given as

Eψ1=L1rL2r

L2

1r−L1Lr

(V1−j(p1+p2)ωrΨ1)(20)

=L1rL2r

L2

1r−L1LrV1−j(p1+p2)ωr

jω1

V1(21)

=L1rL2r

L2

1r−L1Lr

sn|V1|ejω1t(22)

where snis deﬁned as the slip of BDFIG

sn=ωn−ωr

ωn

=ω1−(p1+p2)ωr

ω1

.(23)

Transferring the reference frame of (20) from the PW to the

CW stationary reference frame where the superscript (2) indi-

cates that the vector is seen from the CW stationary reference

frame

E(2)

Ψ1=L1rL2r

L2

1r−L1Lr

sn|V1|ej(ω1−(p1+p2)ωr)t.(24)

Fig. 2. Equivalent circuit analysis for LVRT.

Similarly, the voltage drop due to the CW current is given in

the CW stationary reference frame in the time domain

V(2)

x2 =(R2+j(ω1−(p1+p2)ωr)Ll)I(2)

2.(25)

Hence, from the viewpoint of the converter (CW stationary

reference frame), the CW voltage related to the PW ﬂux and

voltage drop caused by the CW current becomes

V(2)

2=E(2)

Ψ1 +V(2)

x2 .(26)

This yields the equivalent circuit in Fig. 2.

IV. ANALYSIS OF THE BEHAVIOR OF THE BDFIG UNDER

ASYMMETRICAL FULL VOLTAGE DIP

According to the grid code, the voltage dip is assumed to

occur instantly, and grid faults are rapid in nature [21]. In this

paper, symmetrical short circuit of the grid is considered during

which the voltage collapses to zero.

Assuming that the short circuit fault happens at t=t0, then

V1=V1ejω1t,if t<t

0

0,if t≥t0.(27)

A. CW Open Circuit

At the moment when the fault happens, the PW ﬂux linkage

is trapped in the machine, and the rotor windings continue

to cut the trapped ﬂux at the prefault speed, i.e., 625 r/min,

inducing a signiﬁcantly larger EMF EΨ1. The presence of EΨ1

is observed at the CW through the coupling between the rotor

and the CW. In this section, the CW of the BDFIG is assumed

open circuited, and no current ﬂows through the winding.

Since the ﬂux is a state variable and cannot be discontinuous,

the PW ﬂux evolves from the initial value to zero exponentially;

then

Ψ1=|V1|

jω1ejω1t,if t<t

0

|V1|

jω1ejω1t0e

−(t−t0)

τ1,if t≥t0.(28)

Combining (5), (3), and (12), the open-circuit time constant

of the ﬂux decay is derived as

τ1=L1Lr−L2

1r

R1Lr

.(29)

According to (28), before the fault (t<t

0), the steady-

state ﬂux vector rotates at the synchronous angular velocity ω1

with a constant magnitude. When the fault happens, the ﬂux

Authorized licensed use limited to: UNIVERSITY OF SOUTHAMPTON. Downloaded on March 24,2021 at 14:11:22 UTC from IEEE Xplore. Restrictions apply.

2836 IEEE TRANSACTIONS ON INDUSTRIAL ELECTRONICS, VOL. 60, NO. 7, JULY 2013

vector stops at the t0position (t=t0)and decays exponentially

afterward (t>t

0).

Substitute (28) and (27) into (20) for t≥t0, with respect to

the PW stationary frame, given the induced EMF at the CW as

EΨ1 =L1rL2r

L2

1r−L1Lr

(−j(p1+p2)ωrΨ1)(30)

=L1rL2r

L2

1r−L1Lr

(1 −sn)|V1|ejω1t0e

−(t−t0)

τ1.(31)

Since the CW current is assumed to be zero, the CW voltage

equals the induced EMF. Due to the frozen ﬂux, this CW

voltage is a vector ﬁxed to the PW stationary frame where

magnitude decays exponentially. Hence, from the view of the

converter, i.e., the CW stationary frame, the CW voltage vector

reverses its directional rotation and has an angular velocity of

(p1+p2)ωr[5]

V(2)

2=E(2)

Ψ1

=L1rL2r

L2

1r−L1Lr

(1−sn)|V

1|ejω1t0e

−(t−t0)

τ1ej(p1+p2)ωrt.

(32)

From (32), the maximum value of the CW voltage is achieved

at the moment of the grid voltage dips, given by

|V2|=L1rL2r

L2

1r−L1Lr

|1−sn||V1|.(33)

This maximum voltage is proportional to |1−sn|. Since

the slip of a BDFIG for a wind turbine is usually designed

from −0.3 to 0.3, corresponding to a ±30% speed range, the

highest value of the CW voltage during the fault is obtained

when the machine is running at the highest speed, i.e., 130% of

the natural speed. In addition, when the machine is operated

normally, according to (20), the magnitude of the voltage is

proportional to |sn|. Therefore, the maximum voltage at the

fault time is signiﬁcantly larger, 4.3 times than the normal CW

voltage.

B. CW Connected to a Converter

In normal operation, the BDFIG is operated by controlling

the CW voltage through the voltage source converter which

is connected to the CW. According to (26) and Fig. 2, the

CW current can be controlled if the voltage fed from the

converter is capable of balancing the induced EMF from the

PW. For a BDFIG with a −0.3 to 0.3 slip range, a fractionally

rated converter is able to supply appropriate voltage for normal

operation.

According to (25), (26), and (33), during the fault t≥t0with

the respect to the converter, the CW current is expressed as

I(2)

2=1

Ll

t

t0V(2)

2−E(2)

Ψ1dν (34)

where the CW resistance R2is ignored

Equation (34) shows that the converter current is regulated

only if (V(2)

2−E(2)

Ψ1 )is under control. When the machine is

initially running at 30% above the natural speed, i.e., 650 r/min,

the slip snbecomes −1.3. According to (33), the maximum CW

open-circuited terminal voltage at the fault is 4.3 times greater

than the normal value. To balance this voltage and regulate

the current, the converter is of need to apply a signiﬁcantly

increased voltage

V2(t≥t0)≥L1rL2r

L2

1r−L1Lr

|1−sn||V1|=4.3|V2rated |.

(35)

It is worth noting that the overcurrent is introduced by the

induced EMF when the machine-side converter is not able

to balance the enlarged EMF. Similar to the DFIG where

the rotor rated voltage is designed to be close to the sta-

tor rated voltage, the BDFIG CW rated voltage is 90% of

the PW rated voltage due to the maximum voltage modula-

tion. As the machine-side inverter works as a voltage source,

when the low-voltage faults happen, the induced overvoltage

estimated from (35) cannot be observed from the machine-

side converter, but an overcurrent is generated simultaneously

[4]. In other words, the normal fractionally rated converter

is unable to balance this induced EMF, so the control of

the current is lost. This transient overcurrent can damage the

converter [11].

In order to prevent the converter from this transient over-

current, crowbar circuits are always applied for absorbing the

current in conventional DFIGs. However, an effective crowbar

circuit needs an active chopper with a fast dynamic response,

which is complicated and expensive [10]. In addition, crowbar

circuits will reduce the ability of reactive current injection

required during the fault by the grid code [22].

The maximum value of the overcurrent depends on the

leakage inductance of the stator and rotor [4], [21]. From

(17), the BDFIG, in contrast to the conventional DFIG, has

the characteristic of a larger leakage inductance Llfrom the

special rotor design [23]. Hence, according to (34), the transient

overcurrent has reduced amplitude.

V. C ONTROL DESIGN FOR RIDE THROUGH SYMMETRICAL

LOW-VOLTAGE FAULTS

If a reference frame is synchronized with the PW ﬂux, the

stationary variables of both the PW and CW can be transferred

as the vector variables at the synchronous reference frame by

F+

1dq =F1αβe−jω1t(36)

F+

2dq =¯

F2αβe−j(ω1−(p1+p2)ωr)t.(37)

From (3) and (4), based on this reference frame, the arbitrary

angular velocity ωin (3)–(8) is replaced by ω1. Assuming that

the direct axis of reference frame is aligned to the PW ﬂux

LONG et al.: FAULT RIDE-THROUGH OF THE BDFIG IN A WIND TURBINE UNDER SYMMETRICAL VOLTAGE DIPS 2837

Fig. 3. Schematic of the control system for riding through symmetrical voltage faults.

Fig. 4. PLL block diagram.

vector and the resistance of the PW can be neglected, (3) can

be split in terms of dq as

Ψ+

1=ψ+

1d+jψ+

1q=ψ+

1d(38)

V+

1≈jω1Ψ+

1(39)

v+

1d≈ψ+

1q=0 (40)

v+

1q≈|V1|=ψ+

1d.(41)

Similarly, the PW real and reactive powers can be expressed

in terms of dq by splitting (3)–(8) and (9)–(10)

P1=3

2v+

1di+

1d+v+

1qi+

1q≈3

2v+

1qi+

1q(42)

Q1=3

2−v+

1di+

1q+v+

1qi+

1d≈3

2v+

1qi+

1d.(43)

Therefore, the real and reactive powers of the PW can be

regulated by i+

1qand i+

1d, respectively. From [19] and [24],

mathematical analysis and simulation results show that i+

1qand

i+

1dcan be controlled by i+

2qand i+

2dwith a linear gain K1pand

perturbation terms D1dp and D1qp arising from cross-coupling

from the BDFIG [25]

i+

1d=K1pi+

2d+D1dp (44)

i+

1q=K1pi+

2q+D1qp.(45)

TAB L E I

PROTOTYP E MACHINE SPECIFICATIONS

Detailed expressions for K1p,D1dp, and D1qp can be found

in [26].

Since the CW is connected to a voltage source converter,

it is necessary to relate the CW current to the CW voltage.

According to the mathematical derivation from [26] and [27],

the relationship between the currents and voltages in the CW in

terms of dq can be written as

i+

2d=K2pv+

2d+D2dp (46)

i+

2q=K2pv+

2q+D2qp.(47)

Detailed expressions for K2p,D2dp, and D2qp can be found

in [26].

The outer loop is called the power loop, in which i+

2dand i+

2q

are generated by two proportional–integral (PI) controllers from

the active and reactive power errors. The inner loop generates

v+

2dand v+

2q, and it is called as the current loop. Generally, the

current loop requires much faster dynamics. The control chain

of the normal operation can be summarized as

P1⇒i+

1q⇒i+

2q⇒v+

2q

Q1⇒i+

1d⇒i+

2d⇒v+

2d.

2838 IEEE TRANSACTIONS ON INDUSTRIAL ELECTRONICS, VOL. 60, NO. 7, JULY 2013

Fig. 5. Schematic diagram and photograph of the 250-kW BDFIG LVRT test rig. (a) Schematic of the test rig. (b) Photograph of the test rig.

A hysteresis-comparator-based low-voltage detector is used,

and the controller is triggered into the LVRT mode, when the

fault detector observes a symmetrical low-voltage fault.

According to the grid code, the required reactive current

injection is the rated current of the converter. For maximizing

the capability of the reactive current injection, the PW real

current i+

1qis controlled to zero, and the PW reactive current

i+

1dis controlled to the PW rated current. Another pair of PI

controllers is applied for the control loop during the fault. The

control chain of the LVRT mode is shown as follows:

0⇒i+

1q⇒i+

2q⇒v+

2q

|I1rated|⇒i+

1d⇒i+

2d⇒v+

2d.

When the low-voltage fault is cleared, the voltage detector

will trigger the controller back to the normal operation control

mode.

The complete control scheme is shown in Fig. 3. PW currents

and voltages are used to estimate the position of the PW ﬂux

through a phase locked loop (PLL) shown in Fig. 4. Since

only forward sequence is controlled in the fault control mode,

zero sequence and backward sequence, caused by transient

response and the possible imbalance, respectively [28], need

to be ﬁltered. The PLL contains two band-stop ﬁlters, and

components at 50 Hz for the zero sequence and 100 Hz for

the backward sequence are trapped [29]. Hence, according to

(41), the forward sequence at the synchronous reference frame

is locked when ψ+

1q=0, implemented by a PI controller.

VI. EXPERIMENTAL RESULTS

A. Experimental Setup

An experimental setup was used to evaluate the performance

of the proposed control scheme using a 250-kW BDFIG, the

speciﬁcation of which was given in Table I.

The prototype BDFIG is coupled to an induction machine

equipped with a commercial ac drive (ABB-ACS800). The in-

duction machine operates at a constant speed, and the BDFIG is

in the power loop control mode, as explained in the last section.

An incremental encoder with 10 000 pulses per resolution is

used to measure the shaft rotational speed. The voltages and

currents of each stator phase are measured by LEM LV25-p and

LEM LTA 100-p transducers, respectively.

The LVRT test rig schematic diagram and photograph are

shown in Fig. 5. The PW of the BDFIG is connected to the

grid through a symmetrical low-voltage fault emulator compris-

ing an autotransformer and six programmable-logic-controller-

controlled contactors. The BDFIG is normally operated with

Su1,S

v1,and Sw1closed and others open. The PW voltage is

symmetrically pulled down to zero only if all six switches are

closed at the same time.

The grid-side converter is directly connected to the grid

shown in Fig. 5, and the grid-side converter is a commercial

inverter supplied from Control Techniques (Unidrive SP6601).

Therefore, this paper only focuses on the control algorithm of

the machine-side converter, assuming that the grid-side con-

verter stabilizes the dc-link voltage properly.

B. Experimental Result Analysis

To be grid code compliant, the converter of the BDFIG

should not trip or be damaged by the transient overcurrent

arising from the sudden voltage dip. It is widely accepted that

the insulated-gate bipolar transistors (IGBTs) in a converter

can stand 2 per unit (p.u.) peak current for 1 ms [14]. This

means that, if the surge current is less than 2 p.u. of the IGBT

current rating in the converter, no extra capacity of the converter

or additional hardware such as crowbar circuit is required. In

addition, recalling the grid code shown in Fig. 1, fast reactive

current injection in the PW is expected by using the proposed

control scheme. For the most severe case, according to (26)

and (34), the machine is operated at a speed of 625 r/min. The

commercial converters for DFIGs are slightly larger than the

theoretical 33% of the total rating for a margin [30]. Similarly,

the converter rating for this BDFIG is also slightly larger, which

is 40% of the machine rating in this paper.

From Fig. 6, it is seen that the surge current of the BDFIG is

lower than 2 p.u. of the transient IGBTs’ current rating because

of the larger leakage inductance of the BDFIG. Hence, no active

compensation voltage needs to be applied to reduce the surge

LONG et al.: FAULT RIDE-THROUGH OF THE BDFIG IN A WIND TURBINE UNDER SYMMETRICAL VOLTAGE DIPS 2839

Fig. 6. Experimental results at voltage dip moment. The converter current and

voltage are the line–line values; the PW dq currents are the peak values.

current. Without the obligation of reducing the surge current,

the converter is fully committed to reactive current injection.

From the PW real and reactive currents shown in Fig. 6, before

the fault, the real current in the PW is controlled to be the

Fig. 7. Experimental results during the entire voltage dip fault. The converter

current and voltage are the line–line values; the PW dq currents are the peak

values.

full rating, and the reactive power current is controlled to be

zero full-load operation with unity power factor. When the low-

voltage fault happens at t=1s, the fault detector triggers the

controller, which switches to the control loop for LVRT (see

2840 IEEE TRANSACTIONS ON INDUSTRIAL ELECTRONICS, VOL. 60, NO. 7, JULY 2013

Fig. 3), and a pair of new targets, which are rated reactive

current and zero real current, is set. The proposed control loop

is capable of achieving these targets with a fast response, which

is less than 40 ms. The current of the converter has two compo-

nents: a transient overcurrent from the induced EMF decaying

exponentially with the angular frequency (p1+p2)ωrdecaying

exponentially and the control current for injecting reactive

current with a normal slip angular frequency ω1−(p1+p2)ωr.

It is worth noting that, according to the converter voltage and

current waveforms shown in Fig. 6, from t=1.5s until the

fault is cleared t=3 s, the converter still has a potential to

inject more reactive current beyond the grid code required.

Fig. 7 shows the entire proﬁle of the LVRT test. The fault

clears at t=3 s, and the grid voltage recovers instantly. A

transition shown from 3 to 12 s is resulted from the transient

response of the ABB induction machine drive system. Never-

theless, it does not affect the analysis of the LVRT and veriﬁ-

cation of the proposed controller. Although the drive machine

is set to keep a constant speed, a small speed increase appears

due to the energy conservation; a sudden torque drop causes

the speed to increase. However, this variation does not affect

the severity of the surge current as the exponential decay of

the voltage is much faster than the speed deviation, and even

though the speed increases by 25 r/min during the fault, the CW

current still keeps decreasing until the fault is cleared. In reality,

because the response time of the speed variation is about 0.5 s,

which is slower than the electromagnetical response, the pitch

control of the wind turbine can be applied to limit the speed

deviation.

VII. CONCLUSION

This paper has proposed a practical control scheme for

BDFIG to ride through symmetrical low-voltage faults (LVRT).

The experimental results show that the BDFIG with the pro-

posed controller is able to meet the stringent grid regulations

without additional protective hardware such as crowbar circuits.

The BDFIG-based wind turbine with the proposed control

scheme shows high stability and low-cost grid integration.

REFERENCES

[1] R. McMahon, P. Roberts, X. Wang, and P. Tavner, “Performance of BDFM

as generator and motor,” Proc. Inst. Elect. Eng.—Elect. Power Appl.,

vol. 153, no. 2, pp. 289–299, Mar. 2006.

[2] P. J. Tavner, A. Higgins, H. Arabian, H. Long, and Y. Feng, “Using an

FMEA method to compare prospective wind turbine design reliabilities,”

in Proc. EWEC, 2010, pp. 1–10.

[3] R. Piwko, N. Miller, R. Girad, J. MacDowell, and K. Clark, “Generator

fault tolerance and grid codes,” IEEE Power Energy Mag., vol. 8, no. 2,

pp. 18–26, Mar./Apr. 2010.

[4] J. Morren and S. W. H. de Haan, “Short-circuit current of wind turbines

with doubly fed induction generator,” IEEE Trans. Energy Convers.,

vol. 22, no. 1, pp. 174–180, Mar. 2007.

[5] J. López, P. Sanchis, X. Roboam, and L. Marroyo, “Dynamic behavior of

the doubly fed induction generator during three-phase voltage dips,” IEEE

Trans. Energy Convers., vol. 22, no. 3, pp. 709–717, Sep. 2007.

[6] S. Seman, J. Niiranen, and S. Kanerva, “Performance study of a doubly

fed wind-power induction generator under network disturbances,” IEEE

Trans. Energy Convers., vol. 21, no. 4, pp. 883–890, Dec. 2006.

[7] O. Gomis-Bellmunt, A. Junyent-Ferré, A. Sumper, and J. Bergas-Jané,

“Ride-through control of a doubly fed induction generator under un-

balanced voltage sags,” IEEE Trans. Energy Convers., vol. 23, no. 4,

pp. 1036–1045, Dec. 2008.

[8] J. Lopez, E. Gubia, P. Sanchis, X. Roboam, and L. Marroyo, “Wind

turbines based on doubly fed induction generator under asymmetrical

voltage dips,” IEEE Trans. Energy Convers., vol. 23, no. 1, pp. 321–330,

Mar. 2008.

[9] J. Morren and S. DeHaan, “Ridethrough of wind turbines with doubly-fed

induction generator during a voltage dip,” IEEE Trans. Energy Convers.,

vol. 20, no. 2, pp. 435–441, Jun. 2005.

[10] G. Pannell, D. Atkinson, and B. Zahawi, “Minimum-threshold crowbar

for a fault-ride-through grid-code-compliant DFIG wind turbine,” IEEE

Trans. Energy Convers., vol. 25, no. 3, pp. 750–759, Sep. 2010.

[11] J. López, E. Gubia, E. Olea, J. Ruiz, and L. Marroyo, “Ride through

of wind turbines with doubly fed induction generator under symmetrical

voltage dips,” IEEE Trans. Ind. Electron., vol. 56, no. 10, pp. 4246–4254,

Oct. 2009.

[12] L. Meegahapola, T. Littler, and D. Flynn, “Decoupled-DFIG fault

ride-through strategy for enhanced stability performance during grid

faults,” IEEE Trans. Sustain. Energy, vol. 1, no. 3, pp. 152–162,

Oct. 2010.

[13] J. Yang, J. Fletcher, and J. O’Reilly, “A series dynamic resistor based

converter protection scheme for doubly-fed induction generator during

various fault conditions,” IEEE Trans. Energy Convers., vol. 25, no. 2,

pp. 422–432, Jun. 2010.

[14] D. Xiang, L. Ran, P. Tavner, and S. Yang, “Control of a doubly fed

induction generator in a wind turbine during grid fault ride-through,”

IEEE Trans. Energy Convers., vol. 21, no. 3, pp. 652–662, Sep. 2006.

[15] J. Liang, W. Qiao, and R. Harley, “ Feedforward transient current control

for low-voltage ride-through enhancement of DFIG wind turbines,” IEEE

Trans. Energy Convers., vol. 25, no. 3, pp. 836–843, Sep. 2010.

[16] F. Lima, A. Luna, P. Rodriguez, E. Watanabe, and F. Blaabjerg, “Ro-

tor voltage dynamics in the doubly fed induction generator during grid

faults,” IEEE Trans. Power Electron., vol. 25, no. 1, pp. 118–130,

Jan. 2010.

[17] J. Da Costa, H. Pinheiro, T. Degner, and G. Arnold, “Robust controller

for DFIG of grid connected wind turbines,” IEEE Trans. Ind. Electron.,

vol. 58, no. 9, pp. 4023–4038, Sep. 2010.

[18] S. Hu, X. Lin, Y. Kang, and X. Zou, “An improved low-voltage ride-

through control strategy of doubly fed induction generator during grid

faults,” IEEE Trans. Power Electron., vol. 26, no. 12, pp. 3653–3665,

Dec. 2011.

[19] S. Shao, E. Abdi, and R. McMahon, “Dynamic analysis of the brushless

doubly-fed induction generator during symmetrical three-phase voltage

dips,” in Proc. PEDS, Nov. 2009, no. 1, pp. 464–469.

[20] J. Poza, E. Oyarbide, D. Roye, and M. Rodriguez, “Uniﬁed reference

frame dq model of the brushless doubly fed machine,” Proc. Inst. Elect.

Eng.—Elect. Power Appl., vol. 153, no. 5, pp. 726–734, Sep. 2006.

[21] G. Pannell, D. Atkinson, and B. Zahawi, “Analytical study of grid-fault

response of wind turbine doubly fed induction generator,” IEEE Trans.

Energy Convers., vol. 25, no. 4, pp. 1081–1091, Dec. 2010.

[22] C. Wessels, F. Gebhardt, and F. Fuchs, “Fault ride-through of a DFIG

wind turbine using a dynamic voltage restorer during symmetrical and

asymmetrical grid faults,” IEEE Trans. Energy Convers., vol. 26, no. 3,

pp. 807–815, Mar. 2011.

[23] X. Wang, R. McMahon, and P. Tavner, “Design of the brushless

doubly fed (induction) machine,” in Proc. IEEE IEMDC, 2007, vol. 2,

pp. 1508–1513.

[24] S. Shao, T. Long, E. Abdi, R. Mcmahon, and Y. Wu, “Symmetrical low

voltage ride-through of the brushless doubly-fed induction generator,” in

Proc. IEEE IECON, Nov. 7–10, 2011, pp. 3209–3214.

[25] B. Hopfensperger, D. Atkinson, and R. Lakin, “Combined magnetizing

ﬂux oriented control of the cascaded doubly-fed induction machine,”

Proc. Inst. Elect. Eng.—Elect. Power Appl., vol. 148, no. 4, pp. 354–362,

Jul. 2001.

[26] S. Shao, E. Abdi, F. Barati, and R. McMahon, “Stator-ﬂux-oriented vector

control for brushless doubly fed induction generator,” IEEE Trans. Ind.

Electron., vol. 56, no. 10, pp. 4220–4228, Oct. 2009.

[27] K. Protsenko and D. Xu, “Modeling and control of brushless doubly-fed

induction generators in wind energy applications,” IEEE Trans. Power

Electron., vol. 23, no. 3, pp. 1191–1197, May 2008.

[28] K. Lee, T. Jahns, and T. Lipo, “New control method including state

observer of voltage unbalance for grid voltage-source converters,” IEEE

Trans. Ind. Electron., vol. 57, no. 6, pp. 2054–2065, Jun. 2010.

[29] L. Xu and Y. Wang, “Dynamic modeling and control of DFIG-based wind

turbines under unbalanced network conditions,” IEEE Trans. Power Syst.,

vol. 22, no. 1, pp. 314–323, Feb. 2007.

[30] M. Liserre and M. Molinas, “Overview of multi-MW wind turbines and

wind parks,” IEEE Trans. Ind. Electron., vol. 58, no. 4, pp. 1081–1095,

Apr. 2011.

LONG et al.: FAULT RIDE-THROUGH OF THE BDFIG IN A WIND TURBINE UNDER SYMMETRICAL VOLTAGE DIPS 2841

Teng Long (S’10) received the B.Eng. degree from

Huazhong University of Science and Technology,

Wuhan, China, and the ﬁrst class B.Eng.(Hons.)

degree from the University of Birmingham,

Birmingham, U.K., in 2009. He is currently working

toward the Ph.D. degree at the University of

Cambridge, Cambridge, U.K.

He was an exchange student in The Hong Kong

Polytechnic University, Kowloon, Hong Kong, in

2008. His research interests include electric ma-

chines, machine drive and control, power electronics,

and renewable energy.

Shiyi Shao (M’08) received the B.Eng. and

M.Phil. degrees from Shanghai Jiao Tong University,

Shanghai, China, in 2003 and 2006, respectively, and

the M.Phil. and Ph.D. degrees in electrical engineer-

ing from the University of Cambridge, Cambridge,

U.K., in 2008 and 2010, respectively.

He is currently with Wind Technologies Ltd.,

Cambridge, as a Chief Engineer involved in electri-

cal system design and machine drive and control.

Paul Malliband received the M.S. and Ph.D. de-

grees in electrical engineering from the University of

Cambridge, Cambridge, U.K.

He has over 15 years of design and project man-

agement experience covering the aerospace, auto-

motive, construction, and manufacturing sectors. He

is currently the Vice-President of Engineering with

Wind Technologies Ltd., Cambridge, where he is

responsible for the engineering activities and has

overseen the development and integration of the

brushless-DFIG technology into a 20-kW wind tur-

bine and, most recently, the design, manufacturing, and testing of a medium-

scale 250-kW brushless DFIG and associated converter system.

Ehsan Abdi (M’06) received the B.Sc. degree in

electrical engineering from Sharif University of

Technology, Tehran, Iran, in 2002 and the M.Phil.

and Ph.D. degrees in electrical engineering from the

University of Cambridge, Cambridge, U.K., in 2003

and 2006, respectively.

He is currently with Wind Technologies Ltd.,

Cambridge, aiming at exploiting the brushless dou-

bly fed machine for commercial applications. He is

also an Embedded Researcher with the Electrical

Engineering Division, University of Cambridge. His

main research interests include electrical machines and drives, wind power

generation, and electrical measurements and instrumentation.

Richard A. McMahon received the B.A. degree in

electrical sciences and the Ph.D. degree from the

University of Cambridge, Cambridge, U.K., in 1976

and 1980, respectively.

He has been with the Department of Engineering,

University of Cambridge, where he was appointed a

University Lecturer in electrical engineering in 1989

following his postdoctoral work on semiconductor

device processing and became a Senior Lecturer in

2000. His research interests include electrical drives,

power electronics, and semiconductor materials.