Статья поступила в редакцию 10.01.10. Ред. рег. № 692 The article has entered in publishing office 10.01.10. Ed. reg. No. 692
УДК 621.313.2
УПРАВЛЕНИЕ ВЕТРОУСТАНОВКОЙ С ПОМОЩЬЮ АСИНХРОННОГО ГЕНЕРАТОРА
М. Безза, Б-Эль-Муссауи
LEE&TS - Электротехническое отделение Научно-технологический факультет Мохаммедия - Марокко e-mail: m_bezza@yahoo.fr
Заключение совета рецензентов: 20.01.10 Заключение совета экспертов: 25.01.10 Принято к публикации: 30.01.10
В данной статье рассмотрено напряжение управления на выходе асинхронного генератора, подключенного к инвертеру /выпрямителю PWM. Это устройство предназначено для преобразования энергии ветра в случае использования автономной ветроустановки. Испытываются и сравниваются два способа управления: векторное управление магнитным потоком ротора и прямое управление крутящим моментом (DTC). В обоих случаях в конструкции установки учитывается эффект насыщения. Представлены результаты расчетов для случая изменения скорости вращения ротора.
Ключевые слова: асинхронный генератор, ветрогенератор, векторное управление, DTC, насыщение.
INDUCTION MACHINE BASED WIND-TURBINE CONTROL M. Bezza, B-El-Moussaoui
LEE&TS - Electrical Engineering Department Science and Technology Faculty Mohammadia-Morocco e-mail: m_bezza@yahoo.fr
Referred: 20.01.10 Expertise: 25.01.10 Accepted: 30.01.10
In this paper, we study the control voltage at the output of an asynchronous generator connected to an inverter/rectifier PWM. This device is intended for application of wind energy conversion in the case of an isolated site. Two control techniques are tested and compared: vector control in rotor flux oriented and direct torque control (DTC). In both cases, the effect of saturation is taken into account in the design of the machine. Simulation results are presented in the case of variations in rotor speed.
Keywords: induction generator, wind-generator, vector control, DTC, saturation.
Introduction
Asynchronous machines are widely used in the conversion of wind energy [1], in the case of mass production, using more and more structures wound rotor [2-5]. By connecting the rotor winding to the network through a rectifier and PWM inverter, it is possible to shift control and operate the structure in the variable speed. In the case of autonomous operation, the use of cage induction machines is more common [6-12]. These have advantages such as robustness, reduced maintenance and low cost.
The asynchronous machine with no field winding, it is necessary to provide magnetizing energy. In the case of generator operation, this energy can be generated by a capacitor connected in parallel to the stator windings or
using a converter and capacity connected side of DC-link [1, 6]. However, in operating autonomous mode, the speed and load have not fixed, the stator voltage can vary in large proportions. It then becomes necessary to use a system of regulation appropriate to maintain the output voltage amplitude and constant frequency. Works has been done in this meaning [6-9] by controlling the device consists of an asynchronous generator connected to a rectifier and an inverter controlled PWM.
Using a PWM rectifier can, by controlling the frequency reference signals to maintain a negative shift so that the machine operates in generator. Furthermore the phase shift between signals reference and stator currents can control the flow of reactive power and thereby maintain optimum magnetization of the machine.
Международный научный журнал «Альтернативная энергетика и экология» № 1 (81) 2010 © Научно-технический центр «TATA», 2010
Various control strategies have been proposed [6, 7, 13, 14]. In this paper, we present a comparison of two control strategies. The studied system can be reduced to the machine connected to the rectifier feeding a load equivalent; the purpose of the control system is to maintain a constant voltage DC-bus. We then compare the performance of rotor flux oriented vector control and the command based on direct torque control DTC.
After the introduction of the system studied, we present the model of the machine. This from the classical model in the Park axis, taking into account the effects of saturation magnetic by induction variable. The second part is the presentation of the two control strategies tested. Finally, we present simulation results using both strategies during changes in rotor speed.
The system studied
The overall system studied consists of a wind turbine, an asynchronous generator and inverter-rectifier PWM. In the case of autonomous operation with a load balanced, it is possible to reduce the load of continuous side. Therefore, the study of the command can be restricted to that of the voltage at the output of the PWM rectifier. The system studied is then simplified as shown in Fig. 1.
Fig. 1. System studied schematic Рис. 1. Схема исследуемой системы
R is the load resistance reduced side continuous and C filter output rectifier capacity.
The latter is approximated by a polynomial in function of the magnetizing current im. It's lead to the following matrix system [15]:
Xd' -mJs 0 -maLrn isd
Vsq ® als R.. ® aLm 0 lsq
0 - Rr ЮЛ Rr r (lr + Lm ) lmd
0 _ЮЛ -Rr ®r (lr + Lm ) Rr i mq
l. 0 Lm + L
/ md m ' ~m I . I
0 I
i J
r r md mq
m
L + T ' JOL
i2
-l 0 l + T + T' — T'
h U lr -t- bm + L,m I . I bm ..
i i
m
0 -1
i J
md mq
l + l + L'T=t
r m m
with:
<°r — ■ ma- ;
imd — isd + ird ;
i - i + i ;
mq sq rq >
|m| - 1.2 -2 '4imd + imq
T ' — dLm
Lm — .
im
di.d dt
di.q dt
dimd dt dimq
mq dt
(1)
(2)
(3)
(4)
(5)
(6)
This model of the induction machine can be operated in a marker linked to the rotating field (ma = ros) for the implementation of vector control and a marker linked to the stator (ma = 0) for the direct control of torque.
The evolution of the magnetization inductance of the machine designed according to the current im is shown in the following Fig. 2 [15].
Asynchronous generator model
The model used for simulating the operation of the asynchronous machine takes into account the effect of saturation of materials. Indeed, the gap of asynchronous machines is generally low; the nonlinearity of magnetic materials has a significant effect [1, 15]. This effect is difficult to understand in the case of conventional three-phase models. Therefore, it usually takes two phase models to reflect a comprehensive manner. This obviously implies that the induction is homogeneous in the whole structure. In our approach, we adopt the model of Park dq induction machine. The effect of saturation is taken into account through a magnetizing inductance Lm.
Fig. 2. Magnetization curve Рис. 2. Кривая намагничивания
International Scientific Journal for Alternative Energy and Ecology № 1 (81) 2010
© Scientific Technical Centre «TATA», 2010
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Asynchronous generator control
The objective is to control the voltage input of the inverter VDC. From the desired value of the voltage, it is possible to express the power of reference by
Vdc-ref^dc Pref '
(7)
where idc is the output of the rectifier.
Neglecting the different losses, we obtain directly the expression of electromagnetic torque:
T — Pref
em — Q
(8)
T — pФ i .
em Г l r sq
(9)
L i d
ф _ m sd
1 + Ts
(10)
maintained at this value, determining other parameters of the model order. The values obtained are not accurate transient but it simplifies the control and not to resort to adaptive correction.
Thus, considering the steady controlled, rotor flux is only carried by the axis d and maintained at the value &rd f By introducing the expression module of the magnetizing current, we can write:
L
ф
md
rd-ref _ I 2 Фг-ref
3 ' '
M„
(12)
The control voltage Vdc can be done through the setting of the electromagnetic torque, which amounts to the same approach as that used in the case of a classical motor control operating.
Driving vector The model of induction machine made for the development of the command is the model linearized in a landmark associated with rotating field and the command is classical in rotor flux oriented vector: orientation of reference is chosen so that = ®r and we order to keep constant.
The implementation of the command needs to estimate the electromagnetic torque, the rotor flux and the stator pulsation The electromagnetic torque is expressed from the current isq by:
The intersection between the characteristic Lm (im) and the curve defined by (12) determines the value of the magnetizing inductance Lm to be considered in the command. Fig. 3 summarizes the algorithm of vector control adopted.
pa ■
isd ■
is,
I/ d t Tem-ref Isq-ref
pi -
Ib-ref
The rotor flux is, in turn, according to the current isd and the rotor time constant Tr = Lr/Rr:
Pf-
Fig. 3. Algorithm of the vector drive Рис. 3. Алгоритм векторного управления
Knowing ensures the validity of equations because the repository "dq" must always follow the rotating field. For this it uses the internal angular relationship = ror + ro with ro = pQ. The speed of the machine is measured and that the rotor field is estimated. Then obtained for ros:
ю s — -
L i
m sq
T„ Ф,
- + pQ.
(11)
The flow is controlled through the current isd and torque by isq.
The implementation of the command requires knowledge of various parameters of the asynchronous machine (Lm, Ls and Tr) these are not constant from the non-linear materials. However, if the flow is controlled at a constant value these parameters can be considered constant for the magnetic state. We can then determine the value taken by the magnetizing inductance Lm for the reference flux and, assuming that the magnetic state is
Note that the reference power is calculated in agreement on the load receptor, we reverse the sign of the current isq-ref to ensure a functioning generator.
Direct torque control (DTC)
Classically DTC is developed from the expressions of the model of induction machine expressed in the stator reference [16]. Thus, the amplitude of st
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