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Symmetrical Low Voltage Ride-Through of a 250 kW Brushless

DFIG

T. Long*, S. Shao *, E. Abdi*, P. Malliband*, M.E. Mathekga*, R.A. McMahon*, P.J. Tavner§

*Electrical Engineering Divison, Cambridge University, 9 JJ Thomson Avenue, Cambridge CB3 0FA,UK

:LQG7HFKQRORJLHV/WG&DPEULGJH6FLHQFH3DUN&DPEULGJH&%(<8.

§School of Engineering, Durham University, South Road, Durham DH1 3LE, UK

Keywords: Brushless DFIG, DFIG, LVRT, Wind Turbines,

Converters.

Abstract

The Brushless Doubly-Fed Induction Generator (Brushless

DFIG) shows commercial promise for wind power generation

due to its lower cost and higher reliability when compared

with the conventional Doubly-Fed Induction Generator

(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. Hence, a

Low-Voltage Ride-Through (LVRT) capability is important

for wind generators which are integrated into the grid. In this

paper the authors propose a control strategy enabling the

Brushless DFIG to successfully ride through a symmetrical

voltage dip. The control strategy has been implemented on a

250 kW Brushless DFIG and the experimental results indicate

that LVRT is possible without a crowbar.

1 Introduction

The Brushless DFIG promises significant advantages for

wind power generation [2], since only a fractionally-rated

converter is required and slip-rings and brush gear are

removed, enhancing reliability. The Brushless DFIG has two

stator windings, the power winding (PW) which is connected

directly to the grid, and the control winding (CW) which is

fed by a frequency converter.

The Brushless DFIG is normally operated at the synchronous

mode, where the angular shaft speed Ȧr is determined by the

excitation frequencies of the PW, f1, and the CW, f2.

12

12

2

r

ff

pp

ZS

(1)

When the CW is shorted, i.e.

2

Z

equals to zero, the shaft

speed is defined as the natural angular speed,

n

Z

.

With increasing penetration of wind power, wind generators

are expected to remain connected during grid voltage dips.

Furthermore, according to the most recent grid codes, wind

generators are required to supply reactive current to the grid

during faults. In doubly-fed machines, the stator flux (PW

flux in a Brushless DFIG) is exposed directly to the grid and

any voltage dips will result in a sudden loss of machine

magnetisation, producing a current surge in the machine side

converter. This current is typically large and without

appropriate control strategies may cause damage to the

converter. Hence, an appropriate control strategy for riding

through low voltage faults is required to integrate the wind

generator into the grid.

According to the grid code from E.ON, when the grid has a

symmetric low voltage fault, the wind turbines need to 1) ride

through interval of zero voltage up to 0.15s and interval of

grid voltage recovery up to 1.35s; 2) inject up to rated current

of the reactive current to the grid during the entire 1.5s low

voltage fault time.

LVRT control strategies for the DFIG have been widely

investigated. One proposed solution is zero sequence current

control which can be used to reduce the over-current, but this

requires a complex control scheme and is not sufficient

during large voltage dips [8]. Thus a widely-used solution to

protect the converter from over-currents is implementing a

crowbar circuit to short circuit the rotor connections of the

DFIG during the fault, such that the over-current flows

through the crowbar shorting resistors instead of the power

switches [3]. In contrast, the Brushless DFIG typically has a

larger series leakage reactance, and thereby experiences a

reduced transient current when compared to an equivalent

DFIG [6]. As a result, it may be possible for the Brushless

DFIG to ride through a low voltage fault without the need for

a crowbar circuit or additional zero sequence current control.

Hence, the system cost will be reduced and the machine side

converter can also be utilised to help to supply reactive

current during the fault to satisfy grid regulations.

Time/ s

voltage

100%

90%

1.5

0.150

0

2

>@

2

0,

drop

drated

rated

drated

U

II

U

II

'

'

E.ON

Test in this paper

Rising time <20 ms

Figure.1: Grid Code

Authorized licensed use limited to: UNIVERSITY OF SOUTHAMPTON. Downloaded on March 24,2021 at 11:36:48 UTC from IEEE Xplore. Restrictions apply.

In this paper, the authors analyse the nature of the transient

current in the Brushless DFIG when experiencing a

symmetrical voltage dip and will demonstrate that this current

is much smaller than that of an equivalent DFIG. The

emulated low voltage fault is similar to the grid code of E.ON

but the total low voltage fault lasts 2s and zero voltage during

whole fault interval. This test is even more severe than the

grid code of E.ON. A control scheme based on PW flux

oriented Vector Control (VC) for the Brushless DFIG is

demonstrated and verified for the capability of riding through

the symmetric low voltage fault.

2 Equivalent Circuit Model of Brushless DFIG

for LVRT Analysis

Based on the vector model of Brushless DFIG [5], the CW

voltage equation can be expressed as:

212x

VE V

\

(2)

where

1

E

\

WKH LQGXFHG (0) IURPWKH 3:¶V IOX[ DQG

2x

V

is

the voltage drop across the equivalent leakage impedance

between the CW and the PW [6].

Identifing the slip of the Brushless DFIG as:

112

1

()

nr r

n

n

pp

s

ZZ ZZ

ZZ

(3)

The Equation (2) can be expanded as:

112

(( ))

12

11

2

112

jppt

rr

n

r

LL

EsVe

LLL

ZZ

\

(4)

112

(( ))

22 2

()

jppt

xln

VRLsIe

ZZ

(5)

where:

22

12 1 2 12

2

11

rr r

l

rr

LLL L L LL

LLL L

(6)

All voltages and currents here are vectors of the respect to the

CW stationary reference frame.

1

L

,

2

L

,

r

L

,

1r

L

,

2r

L

present

the PW self inductance, the CW self inductance, the rotor self

inductance, the mutual inductance between the PW and the

rotor and the mutual inductance between the CW and the

rotor respectively.

The equivalent circuit is shown in Figure. 1.

3 Analysis of the Behaviour of Brushless DFIG

under a Symmetrical Full Voltage Dip

The voltage dip is assumed to occur instantly according to the

grid code shown in the Figure 1. The electromagnetic

response of the generator would be far less dramatic, if the

voltage dipped gently; grid faults are rapid in nature [4]. In

this paper, symmetric short circuit of the grid is considered;

the voltage collapses to zero.

Assuming a short circuit fault happens at t = t0, then:

1

10

1

0

,

0,

jt

Ve t t

Vtt

Z

°

®t

°

¯

(7)

3.1 Control Winding is Open-circuit

When the Brushless DFIG is operated its synchronous mode,

the induced electromotive force (EMF) in the CW is

proportional to the product of PW flux Ȍ1 and its rotational

speed, Ȧ1-(p1+p2)Ȧr. Since the PW flux vector is the

integration of the grid voltage, when a full symmetrical

voltage dip occurs, the PW flux vector is frozen at the time

that the dip starts. The flux is a state variable and cannot be

discontinuous, the PW flux decays from the initial value to

zero exponentially:

1

1

0

10 1

1

1

0

1()

1

0

,

,

jt

tt

jt

Vett

j

Vee tt

j

Z

ZW

Z

Z

°

°

<

®

°t

°

¯

(8)

Where

1

W

is the time constant associated with the machine

parameters as:

2

11

1

1

rr

r

LL L

RL

W

(9)

Therefore, resolving Equation (4) and Equation (8), from the

perspective of the CW, the induced EMF from the PW flux

can be derived similarly when the grid voltage is zero.

Because the CW is assumed open-circuit, the voltage of the

CW equals to the induced EMF. Hence, from the view of the

converter, during the short circuit fault, the voltage is derived

as:

0

10 12 1

0

()

()

12

2( ) 1

2

112

(1 )

r

tt

jt jp p t

rr

tt n

r

LL

VsVeee

LLL

ZZW

t

(10)

From Equation (4) and Equation (10), before the fault (t<t0),

the steady state flux vector rotates at the slip speed,

1

n

s

Z

,

from the view of the converter (the CW stationary frame).

When the fault happens, due to the frozen PW flux, the CW

open-circuit voltage is fixed to the PW stationary winding and

decays exponentially depending on the time constant. In the

other word, this decaying voltage vector reversely rotates

with an angular frequency of (p1+p2)Ȧr. instead of the slip

speed with the respect to the converter. The maximum value

of this voltage is achieved at the moment at the grid voltage

falls:

1

ȥ

E

2

V

+-

2

I

l

L

2

R

2x

V

Figure 1: The equivalent circuit for LVRT analysis

Authorized licensed use limited to: UNIVERSITY OF SOUTHAMPTON. Downloaded on March 24,2021 at 11:36:48 UTC from IEEE Xplore. Restrictions apply.

0

12

2( ) 1

2

112

1

rr

tt n

r

LL

VsV

LLL

(11)

This maximum voltage is proportional to

1n

s

. Since the

slip of a Brushless DFIG 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 fastest

speed, i.e. 125% of the natural speed.

From Equation (4), when the machine is normally operated,

the magnitude of the voltage is proportional to

n

s

. Hence,

it is clearly shown that the maximum voltage during the

fault is significantly larger than the normal CW voltage.

3.2 Control Winding is connected to a Converter

In normal operation, the Brushless DFIG is operated by

controlling the CW current through the voltage source

converter which is connected to the CW. According to

Equation (2) and Figure.1, 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

Brushless DFIG with a -0.3 to 0.3 slip range, a fractionally

rated converter is able to supply appropriate voltage for

normal operation.

However, when a low voltage fault is experienced, it is

shown in the last section, a large induced voltage by the

frozen PW flux is introduced to the terminal of the CW. For

example, when the machine is initially running at 30%

above the natural speed, i.e.650rpm, according to Equation

(11), the maximum CW terminal voltage during the fault is

4.3 times larger than the normal value. The fractionally

rated converter is usually unable to control the balance the

voltage induced from the PW so the control of the current

is lost. Subsequently, this outrushing current can eventually

damage the converter.

In order to prevent the converter from this outrushing

current, for conventional DFIGs, crowbar circuits are

always applied for damping the current. However, an

effective crowbar circuit needs an active crowbar chopper

with a fast dynamic response which is complicated and

expensive. In addition, crowbar circuits will reduce the

ability of reactive current injection which is required during

the fault by the grid code.

Compensation based crowbarless control strategies are

investigated [8] but the injected compensation current for

reducing the rushing current is limited by the zero sequence

voltage resulting from the remaining PW flux.

(a)

(b)

(c)

Figure.3: The transient response of the low voltage fault.

(a): The grid line voltage; (b) The converter current; (c) The

shaft speed.

0.5 1 1.5 2 2.5 3 3.

5

-1500

-1000

-500

0

500

1000

1500

PW voltage (V)

Time (s)

0.5 11.5 22.5 33.5

-400

-300

-200

-100

0

100

200

300

400

CW line current (A)

Time (s)

2 p.u. IGBT rated current

1 p.u. IGBT rated current

1 p.u. IGBT rated current

2 p.u. IGBT rated current

0.5 1 1.5 2 2.5 3 3.5

575

600

625

650

675

700

shaft speed (rpm)

Time (s)

Authorized licensed use limited to: UNIVERSITY OF SOUTHAMPTON. Downloaded on March 24,2021 at 11:36:48 UTC from IEEE Xplore. Restrictions apply.

The Brushless DFIG, by contrast to the conventional DFIG,

has the characteristic of a larger leakage inductance Ll from

rotor harmonic inductance due to the special rotor design [2].

From Equation (2), the outrushing current can be damped

through the equivalent leakage inductance and the CW

resistance. This advantage enable the Brushless DFIG to be

naturally capable of constraining the over rated current.

It is widely accepted that the IGBT can stand 2 p.u. peak

current for 1ms [8], this means if the surge current is under 2

p.u. of the IGBT in the converter, no extra capacity of the

converter or hardware such as crowbar circuit is required. The

commercial converter for the DFIG is slightly larger than the

theoretical 33% of the total rating. Similarly, the converter

rating for this prototype Brushless DFIG is also slightly

larger, which is 40% of the machine rating. The analysis in

the last section demonstrates the most severity happens at the

time of the dip when machine is initially running at high

speed. Figure.3 is the test result of the 250 kW Brushless

'),*¶V UXVKLQJ FXUUHQW DW WKH IDXOW PRPHQW ZKHQ WKH

machine is initially operated at 625rpm, 25% supernatural

speed at the full load with a unity power factor. It can be seen

that WKH PRVW VHYHU FXUUHQW LV EHORZ SX RI WKH ,*%7¶V

rated current.

It is worth noting that, although the drive machine is set to

keep a constant speed, a small speed deviation appears due to

WKH GULYH PDFKLQH¶V WUDQVLHQW UHVSRQse. 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 the speed increased 25rpm during

the fault, the CW current decreased continuously.

4 Control Scheme for Riding through the

Symmetric Low Voltage Fault

The PW flux oriented vector control method described in [7]

was utilised in this paper [7]. Two power loops, real and

reactive power, are established based on the current link

between the PW and the CW through a pair of PI controllers.

The control chains are shown below:

11 2 2

11 2 2

qq q

dd d

Pi i v

Qi i v

(12)

When the fault detector observes a symmetrical low voltage

fault, the controller is triggered to the fault control mode.

According to the grid code, the required reactive current

injection is the rated current of the converter. For maximising

the capability of the reactive current injection, the PW real

current,

1q

i

, is controlled to zero and the PW reactive current,

1d

i

, is controlled to the PW rated current. Another pair of PI

controllers is applied for control loop during the fault. The

control chain during the fault is showen below:

12 2

1122

0

qq q

rated d d d

ii v

Iiiv

(13)

The entire schematic control loop is shown in Figure.4.

Figure 4: Schematic of the control system

5 Experimental Results

5.1 Experiment setup

An experimental setup was used to evaluate the performance

of the proposed control scheme using a 250 kW Brushless

DFIG, the specification of which is given in Table 1.

Parameter Value Parameter Value

PW/CW pole-pairs 2/4 PW/CW connection <ǻ

PW rated voltage 690 V CW rated voltage 690V

Operating speed 350~650 rpm Rated torque 3750Nm

Table.1 Prototype Machine Specification

The LVRT test rig schematic is shown in Figrue 5. The PW

of the Brushless DFIG is connected to the grid through the

fault hardware. The fault hardware employs PLC controlled

contactors for creating the short circuit fault, as shown by

Figure 6.

The supply of the grid side converter is always connected the

grid bypassing the grid fault hardware. Therefore, this paper

only focuses on the control algorithm of the machine side

converter, assuming that the grid side converter stabilises the

DC link voltage appropriately [8].

The prototype Brushless DFIG is coupled to an induction

machine equipped with a commercial AC drive (ABB-

ACS800). The induction machine operates at constant speed

and the Brushless DFIG is in the power control mode. An

incremental encoder with 10,000 pulses per revolution 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 control algorithm was implemented in MATLAB/

Simulink in an xPC Target computer which receives all the

measurements and generates PWM signals for the machine

side converter. The sampling time of the control loop is 0.4

ms.

5.1 Experimental results analysis

Figure.7 shows the symmetric low-voltage ride-through

performance of the 250kW Brushless DGIG. Figure.7 (a)

shows that the grid witnesses a three phase short circuit fault

from 1s to 2s. (b) shows the most severe outrushing current is

below 2.p.u of the converter rated current. Figure.7 (c) shows

that before the fault, the machine was running at the full load

and unity power factor; real current I1q achieves the rated

current, 144A (peak value) and the reactive power current I1d

is controlled to zero.

At 1s, the control system detects the fault and triggers the

controller into the fault mode; immediately resets the real

current to zero for maximising the reactive current injection

from the converter. Since a new pair of PI controllers is

applied in this mode, the dynamic response of the reactive

FXUUHQWLQMHFWLRQLVIDVWDQGDFKLHYHVWKH(21¶VUHTXLUHPHQW

shorter than 40ms.

transformer

Grid-side

filter

Grid-side

inverter

CW-side

inverter

CW-side

filter

Brushless DFIG

CW

415 V 690 V

Induction

machine

Grid fault

hardware

PW

V

in_u

V

in_v

V

in_w

V

out_u

V

out_v

V

out_w

Figure.5: Schematic and photo of the 250 kW Brushless DFIG LVRT test rig

Auto transformer PLC controlled

contactor

U

VW

S

U1

S

U2

S

w2

S

w1

S

V2

S

V1

Nomal operation: S

X1

=1,S

X2

=0

LV operation: S

X1

=1,S

X2

=1

X=U,V,W

V

in_u

V

in_v

V

in_w

V

out_u

V

out_v

V

out_w

Inductor

Figure.6: Symmetric Low Voltage Fault hardware

At 3s, the fault cleared and the controller is switched back to

the normal mode and the machine is required to be controlled

to the initial statue finishing at 12s.

5 Conclusions

This paper investigates the LVRT behaviour of the Brushless

DFIG and proposes a practical control strategy to ride through

faults and meet the ever more stringent grid regulations. The

experimental results show that the Brushless DFIG has a

significantly improved LVRT performance when compared

with the DFIG. The requirement for a crowbar or other

protective equipment to protect the converter during the fault

can thereby be reduced.

6 References

[1] -/RSH] 3 6DQFKLV ; 5RERDP / 0DUUR\R ³'\QDPLF

Behaviour of the Doubly Fed Induction Generator During Three-

3KDVH 9ROWDJH 'LSV´ Energy Conversion, IEEE Transactions,

22(3):709-717, 2007

[2@50F0DKRQ35REHUWV;:DQJDQG37DYQHU³3HUIRUPDQFH

RI %')0 DV JHQHUDWRU DQG PRWRU´ Electrical Power Applications,

IEE Proceedings, 153(2):289-299, 2006.

[3] J. Morren, and W. de Haan ³6KRUW-circuit current of wind

WXUELQHV ZLWK GRXEO\ IHG LQGXFWLRQ JHQHUDWRU´ Energy Conversion,

IEEE Transactions, 22(1):174-180, 2007

[4] *3DQQHOO'-$WNLQVRQ%=DKDZL³$QDO\WLFDO6WXG\RI*ULG -

Fault Response of Wind Turbine Doubly Fed InductiRQ*HQHUDWRU´

Energy Conversion, IEEE Transactions, 25(4):1081-1091, 2010.

[5] -3R]D(2\DUELGH '5R\H05RGULJXH]³8QLILHG UHIHUHQFH

IUDPH GT PRGHO RI WKH EUXVKOHVV GRXEO\ IHG PDFKLHQ´ Electric

Power Application, IEE Proceeding, 153(5):726-734, 2006

[6@ 6 6KDR ($EGL DQG 5 0F0DKRQ ³'\QDPLFDQDO\VLV RI WKH

brushless doubly-fed induction generator during symmetrical three-

SKDVH YROWDJH GLSV´ Power Electronics and Drive Systems, PEDS,

464-469, 2009.

[7] S. Shao, E. AbdiDQG5 0F0DKRQ³6WDWRU-flux-oriented vector

FRQWURO IRU EUXVKOHVV GRXEO\ IHG LQGXFWLRQ JHQHUDWRU´ Industrial

Electronics, IEEE Transactions, 56(10):4220-4228, 2009.

[8@';LDQJ/5DQ37DYQHUDQG6<DQJ³&RQWURORIDGRXEO\

fed induction generator in a wind turbine during grid fault ride-

WKURXJK´ Energy Conversion, IEEE Transactions, 21(3):652-662,

2006.

(a)

(b)

(c)

(d)

Figure.7: Test result of LVRT (a): grid voltage; (b):

converter current; (c): PW current injection; (d): total

p

ower

0 2 4 6 8 10 12

-1500

-1000

-500

0

500

1000

1500

PW voltage (V)

Time (s)

0 2 4 6 8 10 12

-400

-300

-200

-100

0

100

200

300

400

CW line current (A)

Time (s)

1 p.u. IGBT rated current

1 p.u. IGBT rated current

2 p.u. IGBT rated current

2 p.u. IGBT rated current

0 2 4 6 8 10 12

-300

-250

-200

-150

-100

-50

0

50

100

PW real and reactive current (A)

PW reactive current, I1d

PW real current, I1q

PW rated current (peakvalue)

0 2 4 6 8 10 12

-25

-20

-15

-10

-5

0

5

x 10

4

Total power (W)

Time (s)