DFIG Wind Turbine Three Single Phase Grid Side Converters Strategy Control Maryam Bahramipanah Department of Computer and Electrical Engineering University of Tehran Tehran, Iran
[email protected]
Abstract-In
this
paper,
an
effective
_
doubly
fed
induction
generators' grid side converter control strategy is proposed. This
scheme helps in limiting the fault currents as well as in voltage unbalance compensation. In the considered topology three single phase converters is used instead of one three-phase converters. Voltage is injected in series with the transmission line to limit the fault currents as well as to balance the voltages. The advantages of the proposed controller method with previous one are in inverter
size
reduction,
controllability
and
feasibility.
This
scheme allows wind turbines to remain synchronized to the grid during faults or during voltage sags and swings. The effectiveness of the proposed control strategy is confirmed with simulation results in MATLAB/SIMULINK environment.
Keywords-Doubly fed induction generators; fault current limitation; voltage unbalance compensation
I.
Saeed Afsharnia Department of Computer and Electrical Engineering University of Tehran Tehran, Iran
[email protected]
Ghias Farivar Department of Computer and Electrical Engineering University of Tehran Tehran, Iran gh
[email protected]
INTRODUCTION
Nowadays, the use of renewable energies attracts much attention, and electricity generation from them is comparable to the conventional power plants. Wind energy is considered as one of the most promising infinite energy sources to produce electricity. So, a lot of extensive researches are done to use wind energy more effectively. Wind energy is the fastest growing source in the world for electricity generation. It is predicted that the installed capacity of wind farms around the world will reach to 17500 MW by 2012 [1]. Wind turbines are energy converters that can convert kinetic energy of wind into mechanical energy. There are two types of wind turbine used in wind farms: fixed speed and variable speed. Variable speed turbines are divided into two categories based on the generator used in them: Doubly-Fed Induction Generator (DFIG), and Permanent Magnet Synchronous Generator (PMSG). Each of these kinds of wind turbine systems has its own advantages and disadvantages. Fixed speed wind turbines are directly connected to the grid. So, they are simple and cheap. However, the power system using Fixed Speed Induction Generators (FSIG) will face to many problems such as high reactive power absorption from the grid, and large fluctuations in output power. To fix these problems, wind turbine manufacturers tend to employ variable speed wind turbine generators which allow for independent control of active and reactive powers. These turbines produce 8-15% more energy output, as compared to fixed speed wind
turbines. But, they need power electronic converters. So, it imposes economic burden on their cost. Grid connected wind turbines faces many problems. Low voltage ride-through, the grid voltage unbalance, and reactive power compensation are some of these issues. First concern which all kinds of wind turbines suffered from is disconnecting it from the grid during fault occurrence. This occurs since the voltage drops at the point of interconnection of the wind turbine. They will be disconnected from the network if the voltage drops to 10-20 percent of nominal voltage. This situation underlies an increase of the current in the stator and the rotor winding of the generator. Most induction generators are disconnected from the grid when such faults occur to avoid damage to the converter [2]. Several ways are suggested to keep wind turbines connected to the network during voltage sag, such as utilizing SVC or STATCOM or other compensators. Low Voltage Ride through (LVRT) is one the most demanding requirements in network [3]. The voltage requirement code which is suitable for the Nordic is shown in Fig. 1. It is known as Nordel [4].The grid voltage suddenly dropped to 25% due to a line fault, and then it increased with a slow buildup back to 1 pu. During this period, the wind generators must remain connected to the grid. 1.1
1.0
S 8
� � o > u
Z3
0.9
0.8
0,7
0,6
0,5
0.4
0.3 0.25--------- ------ ----0,2 0.1
0,0
0.00
Figure 1.
0.25
0.50
0.75
Time (8)
1.00
1.25
1.50
1.75
Low voltage ride through at Nordic [4]
During fault occurrence, the voltage at the point ofinterconnection of the wind turbine will decrease while the current will increase. So, high-fault currents could damage the generator stator androtor windings. Thus, it is important to
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336
limit the fault currents. Several solutions have been offered to limit the fault current, such as in [5] Thyristor-controlled resistors have been considered as amean to suppress short circuit currents. The required compensation voltage injection is suggested in this paper to limit the fault current. The other problem in wind power is unbalance voltage compensation. Overheating and torque pulsations occur following the grid voltage unbalance [6]. As a result, wind farms will be disconnected from the grid beyond a certain amount of unbalance. In this paper the voltage unbalances is compensated by eliminating negative sequence voltage [7]. The last issue considered in this study is reactive power compensation. Wind farms absorb high reactive power from the network. In FSIGs, this requirement is generally met by shunt capacitors. However, DFIGs can be operated at any power factor, depending on converter ratings. DFIG is considered as wind turbine generator. Rotor side converter use vector control for reactive power and speed control.
dAsq vsq =Rs isq +mA s sd +-dt v rd
=
R r ird - (ms -mr )A rq +
(4) dA
_r_,.·
dt
(6)
Here, Vsd, vsq, Vrd, and Vrq are d and q axes stator and rotor winding voltages. Likewise, isd, isq, ird, and irq are d and q axes stator and rotor winding currents, and Asd, Asq,Ard, and Arq are stator and rotor flux linkages in Wb-turns. (Os is the speed of a reference frame and (Or is the rotor speed in electrical radians per second. R" R" are the induction generator parameters. The stator and rotor flux linkages are defined as: (7)
The rest of the paper is organized as follows: Section II describes DFIG modeling. Section III expresses the converters controllers. Simulation results are shown in Section IV and the proposed scheme is compared with the conventional DFIG scheme. The paper is concluded in Section V. II.
(8) (9) (10)
DOUBLY-FED INDUCTION GENERATORS MODELING
In which:
Primary structure of DFIG wind turbine is shown in Fig. 2 . Wind turbine is connected to DFIG through a mechanical shaft. Both rotor and stator is fed by the grid. The stator is directly connected to the network while the rotor is connected through back to back converters. According to the variables shown in Fig. 2, rotor and stator voltage equations are as follows: vmbc vrabc
=
=
. R .,lmbc
. Rrlrahc
+
dAsabc
�
+
(5)
(1)
(11) (12)
Ls, and Lr are stator and rotor inductance; and Lm, LIs and Llr are the mutual inductance, stator and rotor leakage inductance, respectively. Electromagnetic torque produced by the generator can be expressed as follows: (13)
dArabc
---
(2)
dt
Where, np is the number of pairs of poles. Regardless of the power losses in the stator and rotor resistance, the stator active and reactive power is equal to:
wind turbine
(14) (15)
And the rotor active and reactive power is equal to: Pr Figure 2.
Vsd
=
. Rsl.,·d - m,. Asq +dt
1.5(vrdird +vrqirq)
DFIG
(16) (17)
Using conventional d-q equations, yields:[8] dA.,d
=
(3)
Electromagnetic dynamic equation is obtained by the following equation:
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337
n
P
dmm d
Jt =Tm -Te -Dm(J)m
(18)
(28)
Here, Tm and Teare mechanical torque and electrical torque in newton-meters and ] and Om are moment of inertia and friction constant. III.
All the aforementioned analyses is applied in RSC controller and represented by a block diagram shown in Fig. 3.
CONVERTER CONTROLLERS
Rotor in DFIG is fed through two converters named rotor side converter (RSC), and grid side converter (GSC). The RSC is responsible for reactive power and speed control. Usually, vector control is used in RSC to generate IGBT control gate signals through PWM module. The grid-side converters considered in this paper have three objectives: 1) To maintain the dc link voltage; 2) To compensate for unbalanced voltages; 3) To limit the fault current/low-voltage ride-through. Various methods can be used to achieve these goals. The function of the rotor-side converter used in this study is similar to that in a conventional scheme of DFIG. The stator-flux oriented vector control is used for this purpose. In this approach, the d-axis is always aligned to the stator flux space vector (As). Therefore, the q-axis stator flux (Asq) is zero, and Asd is kept constant. So, DFIG equations are written as follows: Vsd
=
Vsq
=
Wd /Lsq
=
R, isd
+
; /Lsd
(19)
R,is
+
OJdAsd
(20)
q
(v,q -R,i,q ) A,d
=
L,is q
+
d
=
1
__
A sd
[ sq v
In a conventional scheme, GSC generally functions as a STA TCOM, which is connected in parallel to the grid. However, in the considered scheme, the grid-side converter is connected in series instead of in parallel. Also, instead of 1three-phase converter, 3 single-phase converters are used. The scheme is shown in Fig. 4. Unlike the strategy proposed in [9], in the suggested method all the tasks are shared between the three single-phase converters equally. Hence, the ratings of the converters can be decreased to some extent. Moreover, the scheme has many merits compared to conventional scheme. The conventional scheme has not the ability of a good compensation during fault occurrence. However, the proposed scheme offers an acceptable compensation for low-voltage ride-through purposes. Furthermore, the suggested method compensates the voltage unbalances more effectively. In the proposed design, controllability and feasibility is easier than the conventional scheme, as well.
(21)
Lmir q
=
0
(22)
RSC
11
Lm . isq =--l q L, r (J)
(29)
(23) +
� i rq Ts
]
(24)
a;
Where 'ts is the stator time constant L/Rs. Similarly, isd can be written in terms of ird: (25)
. = A", - Lm. lsd L, ---r:1rd
Figure 3.
Vector control scheme for RSC
(26)
(27)
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338
�'7. .� r=t--= =-.-------------1�J--l :;::---+--!--I j--+--+---+----1f----i
Vcgrid
Vbgrid Vagrid
B.
Voltage Balancing
As the International Electrotechnical Commission (lEC) definition, voltage unbalance is given as the ratio of the negative sequence voltage to the positive sequence voltage. Negative sequence voltage in a three-phase system is given by: (33) Where a == lL120°, and van, Vbn, and Ven are phase voltages. If the injection voltage in phase A given by the equation:
ir
1
-
" 1
0.5
0.6
0. 8
0.7
0.9
time(s)
1.1
1.2
1.3
1.4
1.5
Figure 12. DC link Voltage with new controller scheme DC link Voltage with Conventional Controller Scheme
1 -- - --I - - - � - - --I - - - � - - ...j. - - - � - - ...j. - - - 1- - - ...j. ---1 1 1 I I I I I I
0.5
0.6
0.7
0.8
0.9
time (5)
1.1
1.2
1.4
1.5
Figure 13. DC link Voltage with conventional controller scheme
It is seen that the new scheme can maintain the DC link voltage better than the conventional one. V.
CONCLUSION
In this paper, a new method is shown to connect DFIG wind turbines to the Network. Compared with conventional scheme, the proposed scheme not only has better ability of unbalanced voltage compensation during fault occurrence, but also has an ability to limit fault current. Moreover, in the
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341
