Wind Turbine Generator Emulator
with Current Control mode
Satoshi Nagai, Kouki Tokui, Hiroki Watanabe, Jun-ichi Itoh
Dept. of Electrical, Electronics and Information Engineering
Nagaoka University of Technology
Niigata, Japansatoshi_nagai@stn.nagaokaut.ac.jp, k_tokui@stn.nagaokaut.ac.jp, hwatanabe@vos.nagaokaut.ac.jp, itoh@vos.nagaokaut.ac.jp

Abstract—This paper proposes an emulator for a windturbine generator with a current control mode to emulate acurrent source. In this paper, the emulator uses the current
control mode without voltage controller to consider the generator
as a current source. The emulator is used to verify the power
converter connected to the wind turbine. A three-phase inverterwith the current controller is applied in the emulator of the windturbine generator. In addition, the current control command of
the emulator is calculated from the motor equation using theemulator output voltage detection value. Furthermore, the output
filter of the emulator is only used inductor. Therefore, low-passfilter composed by RC filter with low cutoff frequency comparedwith the switching frequency is applied to the emulator voltage
detection. In the experimental results, the emulator operates inaccordance with the motor equation when the current control
response of the emulator and test converter are 4000 rad/s and
100 rad/s. The error voltage between the measurement andtheoretical value is less than 17.5% in the experimental results.
Keywords—Wind turbine emulator; Current control; Motor
equation;

I. INTRODUCTION
In recent years, distributed generation systems such as thewind turbine systems have been actively researched for energy
saving with renewable energy [1-7]. The wind turbine systems
supply the power to the load with the grid-tied converters [811] connecting the power grid. However, the wind turbine withblades are not applied to verify the test converter in the view
point of the scale for the equipment. In addition, the windturbine system applies a generator such as a permanent
magnetic synchronous machine (PMSM) [12-13]. Therefore,
the verification of the grid-tied converter is not easy to achieve,
because the preparation of the generator is also necessary. On
the other hand, there is a case to apply an emulator which
operates as the wind turbine system. The emulator which iscomposed by converters is applied to simulate the powersource or load, when the actual power source or load should
not be used [14-18]. The emulator plays the actual behavior of
the power source.
In [15], a control method is proposed for a synchronous
generator emulator. The ideal transformer method [19] and thefeedback current filtering method [20] are applied in the
proposed control method. The experimental results of thegenerator emulator phase voltage, current, angular velocity,and torque are match to the simulation results in steady state
and transient operation. However, the control delay may occur
due to the feedback current filtering method when the virtual
generator operation is compared with the actual generator. In[16], a control method is proposed for a load emulator of thepower grid. In this paper, the load emulator operates as aconstant-impedance, constant-current, constant-power model,
and a three-phase induction motor model. Moreover, theemulator operates with the current command derived by theemulation model equation [21-22]. However, the detail of thecurrent command calculation for the emulator has not beenshown in this paper.
This paper proposes the emulator of the wind turbine withthe current control mode without a voltage control. In thispaper, the emulator operation is verified to the wind turbine
system composed by PMSM. The original idea of the proposedsystem is that the emulator uses only the current control modein order to operate as the current source. The current commandis derived by the motor equation with the output voltage of theemulator. In addition, the interference of the current control
response between the emulator and the connected converter has
to be considered. The emulator operation is demonstrated byseparating the current control response between the emulator
and the connected converter in the experiments. As the results,
the emulator is demonstrated with the operation based on the
motor equation when the current control responses of theemulator and connected converter are 4000 rad/s and 100 rad/s.
II. SYSTEM COMPONENTS OF WIND TURBINE EMULATOR
Figure 1 shows the circuit configuration of the emulator for
the wind turbine system. Note that Vin is the input voltage ofthe emulator, iudet, ivdet, and iwdet are the output phase current,
vudet, vvdet, vwdet are the output voltage of the emulator, L is theoutput filter inductance, respectively. The emulator iscomposed by the isolated DC/DC converter and the three-phasevoltage source inverter (3 VSI). In this paper, a dual active
bridge converter is applied as the isolated DC/DC converter.
The voltage control is applied in the inverter side of the
DC/DC converter to control the DC-link voltage for 3 VSI in
constant value. Moreover, a three-phase three-level neutralpoint-clamped (NPC) inverter is applied as 3 VSI. The threephase three-level NPC inverter is applied to reduce the current

L
Vin

Isolated
DC/DC
converter

+

3
VSI

L

AC/DC
converter

L

iudet
ivdet
iwdet

LPF
(RC filter)
vudet

vvdet

AC/DC converter(Test converter)

vwdet

Emulator of wind turbine system
Fig. 1. Constitution of emulator for wind turbine system. The emulator is composed by an isolated DC/DC converter and a 3 VSI. In addition, the test
converter is composed by a AC/DC converter.

ripple for switching frequency component and increase theoutput voltage in the future. This paper focuses on the current
control of 3 VSI. In addition, a three-phase AC to DC
converter (test converter) is applied as the grid-tied system.
The three-phase two-level converter is applied as the test
converter. Moreover, the wind turbine system has to detect thefundamental components of the output voltage. However, theoutput voltage is pulse width modulation (PWM) waveform
without the output filter such as the LC filter or LCL filter.
Furthermore, the control model becomes change when the LC
or LCL filter is applied in the output filter. Thus, the RC filter
as the low-pass filter (LPF) is applied in the three-phasevoltage detection. The cutoff frequency of the RC filter is set tothe low frequency in order to detect the fundamental
component of the output voltage without the switchingfrequency components.
Figure 2 shows the control block diagram of the current
controller for the emulator. Note that L is the inductance of 3
VSI, s is the Laplace operator, vddet and vqdet are the voltage
detection value of dq axis, idref and iqref are the current
commands calculated from the motor equation, iLddet and iLqdet
are the detection value for the 3 VSI output current, vddet andvqdet are the detection value for the output voltage, vd and vq are
the output voltage, iLd and iLq are the output current, PI is the PI
controller,  is the rotation speed of the motor,  is the phase
information, LPF is the phase delay due to the LPF, and ’ isphase information for dq transform of the output voltage,
respectively. The PI controller composed by following
equation

PI = K p +

Kp

idref
vddet,
vqdet

Motor eq.
(3), (4)+ iLddet

()

2

PI

Circuit
vd

vddet
+

+
+

vqdet
iqref
+ iLqdet
vudet
vvdet
vwdet

Eq. (5)

1/2p

Eq. (6)

PI

+
+

3/dq
+



1
sL

iLd

1
sL

iLq

vq
+

vddet
vqdet

’

LFP

Fig. 2. Control block diagram of current controller for emulator. The
current commands are derived by the motor equation with the output
voltage.

vd   R
 vq  =  L
   d− Lq  id   0 
 +
R   iq   ()The power flow of the generator is reversed compared with the
motor. Thus, the generator equation is derived by reversing thepower flow in (2). Therefore, the wind turbine generator
applies the motor equation in (2). The current commands of theemulator are expressed from (2) as follows

sTi

K p = 2em L
Ti =Controller

idref =

Rvd det +  Lq vq det −  2 Lq
R 2 +  2 Ld Lq

()

em

where Kp is the proportional gain, Ti is the integral time, s is
Laplace operator,  is the damping coefficient, em is thecurrent control angular frequency, respectively. Moreover, themotor equation is expressed as follows.

iqref =

Rvq det −  Ld vd det − R
R 2 +  2 Ld Lq()

where R is the resistance of the motor,  is back emf
coefficient, and Ld and Lq are the inductance of d and q axis,
respectively. In the case, the transient terms are ignored in the
motor equation. Moreover, LPF with the low-cutoff frequency
compared with the switching frequency is attached in the
output voltage detection circuit. Thus, it is necessary toconsider the compensation of both the gain and phase delay
for the output frequency. The compensation equations of the
LPF gain GLPF and phase delay LPF are expressed as

GLPF =( fout / f LPFcut ) + 1
2

 LPF = tan −1 ( f out f LPFcut )Controller
idout*+ idout_det

Circuitidout
1
sLd + RPId


iqout*+ iqout_det

PIq

()

()

where fout is the output frequency of the emulator, fLPFcut is thecutoff frequency of LPF. (5) is multiplied in the voltage
detection value, and (6) is subtracted from the phase
information to transform the dq axis.

Fig. 3. Control block diagram of current controller for test converter. The
current controller does not apply the disturbance compensation such as
feedfoward. Moreover, the current controller is operated in the low
control response.

TABLE I.

SIMULATION AND EXPERIMENTAL CONDITIONS.
Motor parameter
Resistanse

Figure 3 shows the control block diagram of the test
converter. The test converter is composed by the current
controller applying the PI controller. Note that idout* and iqout*
are the input current commands of the d and q axis, idout andiqout are the input current of the d and q axis, idout_det and iqout_det
are the input current detection values of the d and q axis, PId
and PIq are the PI controller of the d and q axis, respectively.
The PI controllers in the test converter are composed as follows
equations

PI x = K px +Ticonv =Inductance

R

5.7 W

Ld

43.5 mH

Lq

114 mH

Buck emf coefficient

0.54 Vs/rad

Output frequency

fout

10 Hz

Carrier frequency

fc_em

6 kHz

Current control responseem4000 rad/s

Output inductance

Lf

10 mH

Emulator

Cutoff frequency of LPF fLFPcut_1
for voltage detection f

K px
sTiconv

K px = 2conv Lx − R

iqout
1
sLq + R+LFPcut_2(x : d , q)

()

2

conv

where Kpx is the proportional gain of d or q axis, Ticonv is theintegral time, conv is the angular frequency of the current
controller for the test converter, respectively. The current
controller does not use the disturbance compensation.
Moreover, the current control model is composed by the
inductance and winding resistance. In addition, thesynchronous of the phase between the emulator and test
converter is achieved by detecting the phase information fromthe generator emulator.
III. SIMULATION OF WIND TURBINE GENERATOR EMULATOR
From simulation of the emulator, the wind turbine
generator emulator is verified to the operation based on thecurrent command generated by equation (3) and (4).
Table I shows the simulation conditions of the emulator.
Note that two LPF are applied in the voltage detection circuit togreatly reject the high frequency components such as theswitching frequency components in the simulation. The cutoff

800 Hz1300 Hz

Test converter
Carrier frequency

fc_conv

10 kHzCurrent control responseconv

100 rad/s

frequencies for the two LPF are 800 Hz and 1300 Hz,
respectively.
Figures 4 and 5 show the simulation results of the emulator
operation. Note that the emulator output voltage in Fig. 4(a)and Fig. 5(a) are the waveforms which the harmonics such as
the switching frequency components were rejected by LPF.
From each simulation result, the emulator output current
follows each current command value (Fig. 4: idout* = 2 A, iqout*= 0 A, Fig. 5: idout* = 0 A, iqout* = -2 A). Thus, the emulator
current command is generated by following the test converter
current command. Then, the emulator current is controlled by
the generated current command. However, the emulator current
command has error for the test converter current command.
The error is caused by such as the delay time for thediscretization and the non-implementation of the disturbancecompensation.
Table II shows the comparison of the emulator output
voltage between the simulation values and theoretical value
based on the motor equation. The theoretical values in Table

40

V

Emulator output voltage [V]

U

W

20
0

40

-40

W

U

V

W

0

-60
3

U

2

V

W

1
0

2
1
0

-1

-1

-2

-2
50 ms/div-3

50 ms/div-3(a) Emulator output voltage and current.

(a) Emulator output voltage and current.

25

40
q axis30
20
d axis

Output voltage
detection value [V]Output voltage
detection value [V]

V

-40

-60
3

-1

d axis-3

Output current
detection value [A]q axis0-2

q axis20
15

d axis103

101

Output current
detection value [A]

U

20

-20

-20

Emulator output current [A]60

Emulator output current [A]Emulator output voltage [V]60

q axis2
10

d axis-1(b) Emulator output voltage and current detection value (dq axis).

(b) Emulator output voltage and current detection value (dq axis).

Fig. 4. Simulation waveforms for wind turbine emulation. The current
command of test converter; idref = 2 A, iqref = 0 A.

Fig. 5. Simulation waveforms for wind turbine emulation. The current
command of test converter; idref = 0 A, iqref = -2 A.

II(a) are derived from the current command of the test
converter. In Table II(a), error occurs between the theoreticaland simulation value of the emulator output voltage. However,
the error rate between the theoretical and simulation value issmall as less than 5.0 %. Thus, the validity of the generator
emulator operation is shown by the simulation results. In
addition, the theoretical values of the emulator output voltage
in Table II(b) are derived from the current detection value ofthe emulator. In Table II(b), the simulation output voltage ismatch to the theoretical value without error. Therefore, thefactor of the error between the simulation and theoretical valueof the emulator output voltage in Table II(a) occurs due to theerror between the current command of the test converter andthe output current of the emulator.

IV. EXPERIMENTAL RESULTS
In this experiment, the experimental conditions shown in
Table I are used for the emulation system. The current control
response of the emulator is 40 times for the test converter.
Moreover, the output frequency of the emulator is 10 Hz. In
addition, the two LPF for the emulator output voltage detection
are also applied to the experimental circuit. Note that the test
converter current command direction is defined as that thepositive direction is from the test converter to the emulator, theemulator current command direction is defined as that thepositive direction is from the emulator to the test converter,
respectively.
Figure 6 shows the operation waveforms of the emulator
connected to the test converter which the d-axis current of -2 A

U

TABLE II.

ERROR BETWEEN SIMULATION VALUE AND
THEORETIACL VALUE IN SIMULATION RESULTS.

(a) Theoretical value: Derived from output current command of test
converter.

Fig. 4

Fig. 5

vddet

Measure [V]Theory [V]

11.9

11.4

4.4%

Phase current: 2 A/div

W

0

Error

vqdet

38.8

39.4

1.5%

vddet

14.5

14.3

1.4%

vqdet

22.1

22.5

1.8%

Phase voltage detection value: 100 V/div
W

Phase

U

0

V
40 ms/div(b) Theoretical value: Derived from output current detection value.

Fig. 4

Fig. 5

Measure [V]Theory [V]

vddet

11.9

11.9

0%

vqdet

38.8

38.8

0%

vddet

14.5

14.5

0%

vqdet

22.1

22.1

0%

(a) Emulator phase voltage detection value with analog LPF, andemulator output current measured value.

Error

W

0

(c) Emulator current detection value and current commandof test converter.

Fig. 4

Fig. 5Detection [A]Command [A]

id

1.97

2.0

iq

-0.08

0

id

-0.04

0

iq

-2.05

-2.0

Phase current: 2 A/div

U

dq current detection value: 5 A/div

Phase

0A
q axis0d axisis injected from the emulator to the test converter. In Fig. 6 (a),the output current amplitude of the emulator follows to thecurrent command of the test converter. In Fig. 6 (b), the current
command detection value is match to the current command of
dq axis. Thus, the emulator output current is follows to thecurrent command of the test converter. Moreover, the output
current distortion is low. Furthermore, the U phase output
current in Fig. 6 is peak value when the phase is 0 degree. Thus,
the current is output based on the phase information.
Figure 7 shows the operation waveforms of the emulator
connected to the test converter which the q-axis current of 2 A
is injected from the emulator to the test converter. The output
current amplitude of the emulator in Fig. 7 also follows to thecurrent command of the test converter. Moreover, the U phase
output current in Fig. 7 is zero when the phase is 0 degree.
Thus, the current is output based on the phase information.
Table III shows the comparison of the output voltage
between the experimental results and theoretical values. The
theoretical value of the output voltage is derived by the motor
equation with the current command of the test converter. The
error rates of the output voltage detection value vddet, vqdet in Fig.
6 are 17.5% and 2.03%. The error rates of tfhe output voltage
detection value vddet, vqdet in Fig. 7 are 7.69% and 7.56%. Thus,
the error rates of the output voltage detection value in dq axis
are less than 17.5% in the experimental conditions. The causeof the error for 17.5% in Fig. 6 occur due to the voltage
detection error, and the reduction of voltage detection-2.0 A40 ms/div(b) Emulator output current detection value in dq axis, and emulator
output current measured value.
Fig. 6. Experimental waveforms for wind turbine emulation. The
command of test converter; idref = 2 A, iqref = 0 A. Thus, the emulator
output current follows the current command of the test converter.

resolution by operating at low voltage. Therefore, the output
voltage for the output current is output based on the motor
equations. Consequently, the validity of the emulator was
confirmed in the experimental results. Hence, the emulator
operated when the current control response of the emulator isseparated from the current control response of the test
converter.
V. CONCLUSION
This paper proposed the emulation system which simulates
the wind turbine system. The output current and voltage of theemulator were output based on the current command of the test
converter and the motor equations. The error rates of the output
voltage detection values were less than 17.5% compared withthe theoretical values.
Future work is to consider increasing the current control
response for the test converter.
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Phase

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vddet

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