25kw dfig wind energy conversion system prototype for ... · doubly-fed induction generator ......

Anais do V Simpósio Brasileiro de Sistemas Elétricos, Foz do Iguaçu – PR, Brasil. 22-25/04/2014 ISSN 2177-6164

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Abstract – In the last few years the number of grid

connected wind conversion systems (WECS) has increased

rapidly. The most commercialized WECS technology is the

doubly-fed induction generator (DFIG) that offers

operational and economical advantages over other ones.

Nevertheless, this system has as the main drawback the

high susceptibility to grid disturbances, for example,

voltage sags. In this work it is presented a 25kW test bench

which emulates a WECS using the DFIG technology for

tests under symmetrical and asymmetrical voltages sags.

The design of the main components is shown: the power

filter, signal conditioning board, control structure and the

voltage sag generator. The first experimental results are

also shown and briefly analyzed.

Keywords - Doubly-fed Induction Generator (DFIG),

Wind Conversion Systems (WECS), Voltage Sags, Turbine

Simulator and LCL Filter.

I. INTRODUCTION

The massive investments in renewable energy have

reduced the prices of the electrical energy generated from

these sources, becoming competitive in comparison with the

traditional sources. One of the most competitive renewable

technologies is the wind energy conversion system (WECS).

The number of wind power plants worldwide has increased so

fast in the last few years. Brazil has more than 2GW of

installed power, a small number when compared with the top

five countries (China 75GW, USA 60GW, Germany 32GW,

Spain 23GW and India 18GW) [1], but the number of wind

power plants are growing fast.

There are different WECS technologies using different

types of generators. The most common technologies are the

synchronous generator using full scale converters (FC-SG) and

the doubly-fed induction generator (DFIG), both depicted in

Fig. 1. The main advantage of the DFIG is the use of a

converter connected to the rotor dimensioned for just 30% of

the machine rated power, so reducing the equipment costs

This work has a financial support provided by CAPES, CNPQ and

FAPEMIG.

Victor F. Mendes ([email protected]) and Selênio Rocha Silva

([email protected]) are with Department of Electrical Engineering of

Federal University of Minas Gerais, Belo Horizonte, Brazil.

Guilherme M. Rezende ([email protected]) and Clodualdo V.

de Sousa ([email protected]) are with Federal University of

Itajubá, Itabira, Brazil.

(a)

(b)

Fig. 1. Wind conversion systems: (a) FC-SG, (b) DFIG.

when compared with the FC-SG which employs a full

converter. Nevertheless, the DFIG topology has two main

disadvantages: the use of a gearbox, which is a fragile part in

the whole system, and the direct connection of the stator to the

grid, increasing the system susceptibility to network

disturbances, such as voltages sags. The synchronous

generator can be constructed with high pole number,

eliminating the gearbox, and the full scale converter decouples

the generator and the grid [2].

Within this context, it is important to study the WECS

behavior during grid disturbances. In the last few years several

papers on this topic have been published, studying the voltage

sags impact on the WECS and developing some strategies to

improve the low voltage ride-through capability (LVRT) as

required in the modern grid codes [3], [4].

Papers [5]-[9] analyze the DFIG behavior during voltage

sags using mathematical modeling and show simulation and

experimental results. Based on the system behavior, other

papers focus the studies on the proposal of different solutions

for the LVRT improvement of the DFIG technology [8], [10]-

[15]. The impact of the voltage sags on the FC-SG and ride-

through solutions are addressed in [2], [16]-[21].

Most of the previous papers prove their developments using

experimental results obtained in small power test benches

(<5kW). In this context, this work presents the design of a test

rig using a 25kW DFIG for voltage sags studies and the future

development of new control strategies to improve its LVRT

capability.

This work is focused in the design of the test bench since

the details of the prototypes used in the literature are not

25kW DFIG Wind Energy Conversion System

Prototype for Voltage Sag Tests

Victor F. Mendes, Guilherme M. Rezende, Clodualdo V. de Sousa and Selênio Rocha Silva


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generally described. Furthermore this test bench has higher

power than the ones generally used in the literature,

representing advantages, as: better signal-to-noise ratio, higher

inertia, smaller stator and rotor resistances, etc.

The text is organized as following: Section II is dedicated to

describe the general structure of the test bench and its

components, in Section III the DFIG classical modeling and

the control strategy are described, Section IV show some

experimental results and finally in Section V the conclusions

and the proposal of future works are presented.

II. THE TEST BENCH

The test bench presented in this work represents a WECS

with the DFIG technology. Fig. 2 depicts the prototype

diagram. In the following subsections each component will be

described.

A. Generator and Converter

The generator is a wound rotor induction machine

manufactured by VEM, model SPER225M4HW. The

parameters are listed in Table I. This machine has no especial

construction characteristics and the power was chosen based

on the available converter rated power. It is supposed to

operate with speeds varying between +30% slip (1260RPM)

and -30% slip (2340RPM).

TABLE I – GENERATOR PARAMETERS

Variable Value Variable Value

Rated Power 30 kW Rotor Resist. (Rr) 0.3 Ω

Frequency (fs) 60 Hz Stator Induct (Ls) 102.4 mH

Stator/Rotor Volt. (Ksr) 400/300V Stator Rest. (Rs) 0.24 Ω

Magnet. Inductance (Lm) 100 mH Inertia (J) 0.476Kg.m2

Rotor Self Induct. (Lr) 102.4 mH Friction (B) 0.010 N.m.s

The power converter is constituted of two three-phase

converters from Semikron connected in the so called back-to-

back configuration, as depicted in Figure 1(b) and 2. The rotor

side converter (RSC) has three IGBT's modules (arms) of

1200V, 63A (SK60GB128). In the grid side converter (GSC)

the modules are 1200V, 35A (SK30GB128). The converter

also includes a chopper (SK30GAL123) for the DC-link

overvoltage protection. The IGBT's are commanded through

dual IGBT drivers (SKHI20opA).

Since the rotor voltage is much smaller than the grid

voltage, the RSC has the higher current capability. Due to the

high difference between grid and rotor voltage, in order to

reduce the DC-link voltage (used=450V), improving the

modulation index on the rotor side, it is used a 380/220V

transformer for the connection of the GSC to the grid.

On the other hand, the grid voltage reduction diminishes the

maximum converter power. In fact the maximum active power

(unity power factor) flowing through the converter is

approximately 8kW ( 3.220 .25V A ), since 25A is the IGBT

rated RMS current with 80oC. Therefore, in this configuration

operating the generator with the maximum slip of -30% the test

bench can handle only 26.5kW (8kW/0.3). For safety reasons

the rated power of the prototype was set to 25kW for now.

B. Turbine Simulator

The wind turbine is mechanically coupled to the generator

through a gearbox in real WECS. Since the objective of the

test bench is the studies related to the electrical part, the wind

turbine is replaced by a simulator. The turbine simulator has

the advantage of testing the generator under different operation

(power) conditions independent of meteorological conditions

(wind speed).

The turbine is emulated using a 37kW squirrel cage

induction motor manufactured by Voges which is controlled

by a commercial back-to-back converter ABB 380V/42A. This

converter receives an external reference signal to control the

machine torque.

The wind turbine torque is calculated through [22]:

2

( , )1

2

pCT ARV

λ βρ

λ= , (1)

where ρ is the air density (typical value 1.225Kg/m³), A is the

swept area of blades (m²), R is the rotor blades radius (m), V is

the wind speed (m/s), Cp is the power coefficient, β is the blade

angle (degrees) and λ is the tip-speed ratio given by:

R

V

ωλ = , (2)

with ω being the turbine angular speed (rad/s). In the literature

several models are proposed to calculate the power coefficient

Cp. One of the most used models is given by [22]:

Fig. 2. Diagram of the test bench representing a wind energy conversion system with the DFIG technology


3

i

12.5

i

116( , ) 0.22 0.4 5

pC e

λλ β βλ

− = − −

, (3)

where:

1

3

1 0.035

0.08 1i

λλ β β

−

= − + +

. (4)

With the restriction of power being 25kW and choosing a

typical wind speed range for a turbine with such power (4-

10m/s), the blades radius is calculated based on Eq. (1).

Furthermore, it is necessary to emulate a gearbox and chose

the gear ratio based on the maximum generator speed

(2340RPM). Table II shows the turbine parameters and Fig.3

depicts the power characteristics for different wind speeds with

fixed blade angle β=00.

TABLE II – TURBINE PARAMETERS

Variable Value

Radius (R) 5.45 m

Rated Wind Speed (Vnom) 10 m/s

Cut-in Wind Speed (Vcutin) 4m/s

Gear Ratio 21.12

Rated Power 25 kW

Rated Speed (high speed side) 2340 RPM

Fig. 3. Emulated turbine power for different wind speeds and β=0

The pitch control (β) is not represented in the turbine

simulator, because, as mentioned, the electrical part is of main

interest. Other effect not represented is the shaft torsional

oscillations.

Eq. (1)-(4) are implemented in a dSpace 1103 platform,

described in Section II.E, where the torque reference for the

turbine simulator is calculated using a wind speed (V) profile

set by the user.

C. LCL Filter

The LCL filter function is to reduce the harmonics produced

by the switching of the converters, being of great importance

for the overall system performance. This filter is designed to

ensure the harmonic level within the range recommended by

the standard [23].

The procedure for the choice of the parameters uses the

converter power ( ), the rated peak secondary transformer

voltage ( ), the grid frequency ( ) and the switching

frequency ( ).

The filter inductor is calculated as a function of the

maximum current ripple and can be obtained as [24]:

(5)

The inductor value at the converter side ( ) is related to the

transformer inductance ( ) by the parameter as seen in (6).

The value of the capacitor is limited by the allowed reduction

of the power factor through the parameter ( ) as

shown in (7) [24].

(6)

(7)

The value is calculated by (8), where . This

equation represents the current attenuation as a function of the

calculated parameters:

(8)

Usually, the attenuation value is chosen to be , in order

to achieve good practical results [24]. The resonant frequency

of the filter, , must be in an interval which is not affected

by the grid frequency nor the switching frequency. Normally,

it is between ten times the grid frequency and 50% of the

switching frequency . The resonant

frequency is calculated by [24]:

(9)

The value of the damping resistor is initially defined as

twice the value of the capacitor’s impedance at the resonant

frequency [24]. The higher , the higher the filter damping

and the gain at the resonant frequency is reduced. The best

value for the resistor is chosen by considering the filter

dynamics, the resonance and the generated losses [25].

Considering the equations above and performing some

simulations, the values of the designed LCL filter are

presented in Table III.

TABLE III – LCL FILTER COMPONENTS VALUES

Parameter Value

Inductor ( ) 2mH

Capacitor ( ) 20µF

Resistor ( ) 3Ω

For the inductors construction, iron powder cores with

distributed air gap were used. It was chosen the E-type core

from Micrometals, model E610-29. Details about the

calculation of the necessary turns can be found in [26].

D. Voltage Sag Generator

For the experimental tests it is necessary the emulation of

voltage sags in the test bench. Two equipments were used for


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this purpose: the impedance sag generator (ISG) and the

industrial power corruptor (IPC).

For the symmetrical voltage sag tests, the ISG recommended

in the standard IEC61400-21 [27] is used, which is depicted in

Fig. 4. When the switch "S" is closed the currents flowing

through Z2 cause a voltage drop in Z1 which represents a

voltage sag in the terminals of the system "WT", calculated as:

2

1 2

Zsag(%)= .100%

Z Z+. (10)

Several combinations of Z1 and Z2 are possible. Due to

restrictions in the size of the inductors two 1.2mH and one

0.3mH inductors were constructed, permitting the test of

different levels of voltage sags. These inductors also present a

medium tap which divides by two the total impedance. Since

the impedance is low, the reactors are supposed to operate

with currents around 1000A without saturation.

Fig. 4. Impedance sag generator [27]

The IPC is a commercial equipment, used of asymmetrical

voltages sags test, following the IEC6100 standard [28]. It can

control the sag level, duration and start time, allowing a wide

variety of tests.

E. Measurement System and Control Platform

The control strategies, described in the next section, the

signal command logics, the turbine simulator algorithm and the

software protections are implemented in a dSpace 1103

controller board. This platform is designed to meet the

requirements of modern rapid control prototyping and it is

used worldwide in the main university and industry research

centers. Its main features includes an 1GHz processor, 50 bit-

IO channels, 36 A/D channels, 8 D/A channels, a slave DSP,

10 PWM outputs, etc.

The dSpace platform was employed in the test bench for two

main reasons: the processing and memory capacity which

permit the implementation of complex control strategies (next

step in the development) and its Real-Time Interface in

conjunction with the software ControlDesk permits the

recording of several variables with a high sampling rate.

These features meet the requirements for the voltage sags

studies intended with the test rig. Furthermore, the

programming is simple and can be performed using the

Matlab/Simulink or the C language. Latter programming

technique was used.

For the control of the generator is necessary to measure the

rotor (RSC) currents, the stator currents and the GSC currents,

as illustrated in Fig.2. Furthermore, the DC-link voltage, rotor

voltage and the low voltage side of the transformer (GSC

voltage) are also measured. For this purpose it is used the

current LEM LA-55P/SP1 and voltage LEM LV-25P

transducers. These are Hall Effect sensors and are employed

due to its good linearity, high bandwidth and great accuracy.

The output of the transducers (measurement signal) is a

0/25mA current signal. In order to minimize electromagnetic

interferences (EMI) the transducers are localized close to the

measured signals and the measurement signals are transmitted

in current until the conditioning board (Fig.2).

The signal conditioning board is responsible for the

conversion of the measurement for an adequate voltage level

(±10V). This voltage was chosen in accordance with the input

level of the A/D converter of the dSpace platform. Before the

A/D conversion, there is a filtering stage to prevent aliasing.

This anti-aliasing filter has 3 kHz of cut-off frequency, since

the sampling rate was set to 6 kHz, and it is implemented via

an Universal Active Filter (UAF42 chip). This chip is largely

used in active filter implementation, due to the good precision

of its internals capacitances, and so its smalls deviations of the

designed filter’s cut-off frequency. Furthermore, the Besel

topology was chosen because of its linear phase response and

so its excellent pulse response with minimal overshoot. For

circuit safety, the filters outputs are limited to -10/+10V using

Zener diodes and the signals supply the A/D converter of the

dSpace. Fig. 5 depicts the measurement signals transmission.

The control algorithm runs with the same rate of the signal

sampling (6kHz), generating the voltage reference signals for

the converters. The dSpace has a PWM dedicated module

which calculates the outputs based on the voltage reference

signals. These outputs are 0/5V digital signals that must be

converted to 0/15V digital signal in accordance to the gate-

drive circuit input. This conversion is made by the signal

conditioning board using n-channel MOSFET (2N7002), due

to its very fast switching, and so minimal delay of reference

signal.

Fig. 6 shows some pictures of the test bench emphasizing

the main components.

Fig. 5. Signal transmission of the measurement system

III. DFIG CLASSICAL CONTROL STRATEGY

For the first voltage sag tests it was implemented the

classical DFIG control, depicted in Fig. 7 which shows the

block diagram of the control structure for the two converters:

• Grid side converter (GSC): internal loops controlling the

currents flowing through the filter using the grid voltage


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angle orientation and external loops controlling the dc-

link voltage and the reactive power;

• Rotor side converter (RSC): internal loops controlling the

rotor currents using the stator voltage angle orientation

and external loops controlling the active and the reactive

stator power.

In order to estimate the grid voltage angle for the control

orientation, it used the PLL proposed by [29]. This PLL is

based on a "Dual Second Order Generalized Integrator”

(PSOGI) which decomposes the positive and negative

sequence components of the voltage. Therefore, this PLL is

robust for unbalanced voltage conditions as expected.

For the control adjustment it was used the techniques of

"Modulus Optimum" and "Symmetrical Optimum" [30]. More

details about the system modeling and the tuning process can

be found in [31].

It is important to mention that the classical control strategy

was first tested, because it is more employed in commercial

equipments. Just this strategy, as highlighted hereafter, cannot

guarantee the LVRT capability. For this purpose, commercial

equipments also use the so called crowbar device.

Improvements of the classical strategy will be the future works

with the test bench presented in this paper.

IV. EXPERIMENTAL RESULTS

This section presents some experimental results of the test

bench. The objective here is to show the operation and the

voltage sag tests. The complete explanation of the results is not

addressed in this work.

For the first test it was considered a 50% three-phase

voltage performed with the ISG, as depicted in Fig. 8. The

generator was operating at synchronous speed (1800RPM)

generating 10kW.

Fig. 9 shows the rotor current phase A. This is the main

variable to be analyzed during the voltage sag, because it

reaches high values that can trip or even damage the RSC. In

the Fig. 9 one can notice that the first peak current is almost

the converter rated current. The rotor currents oscillate with

the grid frequency (60Hz) due to the flux natural response, fact

explained in [5] and [8].

Fig. 7. Block diagram of the DFIG control structure

Another important variable to be analyzed is the DC-link

voltage. One can see in Fig. 10 that during the sag this voltage

increases, so it is necessary the use of the discharge chopper

for safety operation of the test rig during the tests.

For the unbalanced test it was considered a 65% phase-to-

phase voltage sag (Fig. 11) with the generator operating at

2070RPM. Fig. 12 depicts the rotor currents, showing the high

peak values compared with the peaks before the sag. These

high values can be explained by the flux natural response and

also by negative sequence component [6]. Fig. 13 shows the

dq rotor currents used by the control. One can see that just the

mean values of the currents are controlled.

Through these results it is possible to see that dSpace

platform permits the record of several variables, fact extremely

important for the posterior analysis of the data.

V. CONCLUSIONS

In this work the design and selection of the main

components of as 25kW WECS test bench were described.

This prototype was built for voltage sag tests presenting the

Fig. 6. Test bench pictures


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DFIG technology, since this type of generator is very

susceptible to this kind of disturbances.

The first experimental results were shown in order to show

the system operation and point out the main variables of

interest.

The next step in the studies will be the analysis of the results

and the development of new strategies to improve the system

ride-though fault capability. Therefore, the future works intend

to contribute for the integration of wind turbines to the grid,

attending the modern grid codes.

ACKNOWLEDGEMENT

The authors would like to thank the Capes and DAAD for

the support of the Probral cooperation program between the

UFMG and the TU Dresden as well as the CNPQ and

FAPEMIG for the financial support.

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Fig. 8. 50% three-phase (3P) voltage sag

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Fig. 11. 65% phase-phase (2P) voltage sag

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Fig. 9. Rotor currents - 50% 3P voltage sag,

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Fig 12. Rotor currents - 65% 2P voltage sag,

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Fig. 10. DC-link voltage - 50% 3P voltage

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Fig 13. dq rotor currents - 65% 2P voltage

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25kw dfig wind energy conversion system prototype for ... · doubly-fed induction generator ......

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