Analysis and Simulation of Wind Farm Connected to Grid

Analysis and Simulation of Wind Farm Connected to Grid
International Journal of Applied Science Engineering and Management VOL 1, ISSUE 3
ISSN 2454-9940
Analysis and Simulation of Wind Farm Connected to Grid
Gaurav Sharma
Department of Electrical Engineering, Bhilai Institute of Technology, Durg, Chhattisgarh
A bstract: The statistical data conveys that Doubly-fed Induction Generator (DFIG) based wind turbine with variable
speed is the most common wind turbine in the growing wind farm market. [1] This machine is usually used on the
grid connected wind energy conversion system. For wind farm reactive power compensation is essential because
DFIG requires reactive power for creating its own magnetic flux, which weakens the power factor and
reduced the
voltage regulation of wind farm. [2] This paper describes the study undertaken to assess the steady state current of a
doubly fed induction generator (DFIG) driven by wind turbine of a wind farm which is directly connected to the grid.
The rotor side converter (RSC) and the grid side converter (GSC) provide the power flow between the DC bus and
the AC side. [3] The induction machine runs at above synchronous speed and rotor is attached to wind blade. Various reactive power compensation devices compensate the reactive power and with the help of reactive power compensation we reduce the different current which flow in the different line of wind farm. Behaviour under slip is typically observed in wind turbines. A MATLAB computer simulation study was undertaken and results on 4 kW wind
turbine are presented indicating current before and after compensation and sending end voltage before and after
compensation keeping receiving end voltage constant.
Keywords : Wind farm, doubly fed induction generator, rotor side convertor, grid side convertor, slip, and grid
1 INTRODUCTION
D ue to depletion of fossil fuels and increase of polluted emissions, renewable energy production is rapidly growing. Wind generators play an important role among all possible renewable energy resources, they considered as the most promising in terms of competitiveness in electrical power production. Energy of the wind is user for more than thousand years for water pumping, grinding
grain, and other low-power applications. There were several early attempts to build large-scale wind powered systems to generate the
electrical energy. Today, it is one of the rapidly growing technologies in markets. By the end of 2011 [(EWEA) The European wind
energy association] the total installed capacity of wind energy is estimated to be more than 150 GW all around the world [4].
In a grid connected wind energy conversion system with squirrel cage induction machine, the grid can be connected in three ways,
such as direct grid connection method, grid connection via direct-current intermediate circuit method (with thyristor converter, with
pulse inverter) and grid connection via direct AC converter method. If the machine is a wound rotor induction machine, doubly fed
induction machine method are used. Double fed configuration is best suited for variable speed generation, because nature of the wind
in variable. The doubly-fed machine can be operated in generating mode in super-synchronous modes. In this mode the rotor speed is
much higher than the stator magnetic flux speed. A gear system is attached in between wind blade and rotor which maintain the rotor
speed is above synchronous speed. The slip power requires for doubly fed induction generator is fraction of 30 % (Slip Power) of the
total output power. [6] All these advantages make the DFIG a favourable candidate for variable speed operation. In recent years, DFIG
have received increased attention and they have been widely employed as suitable isolated power sources and grid-connected in wind
energy applications. For stand alone or autonomous operation, mostly single induction generator or parallel operated induction generators are focused according to available analyzed references. These induction generator driven by the individual prime movers employed excitation capacitor bank to build-up desired voltage via self-excited phenomena. Hence the value of the excitation capacitor
bank and the rotor speed determine the magnitude of the generated voltage and its frequency. Both voltage and frequency need to be
controlled to feed the power to the load. But for grid connected operation, there are two types of generators are used (i.e., single output
and double outputs).In order to feed the active power to the grid, the machine should run at a speed greater than the synchronous speed
of the revolving magnetic field. (i.e. slip should be negative).The single output generator feeds active power to the grid via only stator
side and double output generator feeds electrical power to the grid via both stator as well as rotor side. This is only the generator
which generates the power more than rated power without overheating. Wind turbines often do not take part in voltage and frequency
control and if a disturbance occurs, the wind turbines are disconnected and reconnected when normal operation has been resumed. As
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International Journal of Applied Science Engineering and Management , VOL 1, ISSUE 3
ISSN 2454-9940
the wind power penetration continually increases, power utilities concerns are shifting focus from the power quality issue to the stability problem caused by the wind power connection. [5] In such cases, it becomes important to consider the wind power impact properly
in the power system planning and operation. This paper will focus on the directly grid-connected induction generator feeding power
with DFIG during steady state conditions.
This paper presents steady state condition of a double-output induction generator (DFIG) based on mathematical modelling which
is used to show the power flow of an induction generator feeding power to the utility grid. This paper is organized as follows- Section
II introduces the nomenclature of the studied system. Section III explain the basic study of the system Section IV describe the steadystate analyses of DFIG with the help of torque-slip characteristics. Section V shows the steady state response of current before and
after compensation and voltage regulation of the system after reactive power compensation. Section VI addresses the conclusion part
of this paper.
2. Nomenclature
is prime mover electrical power per phase.
is air gap power.
is reactive power supplied by capacitor bank.
γ is propagation constant.
δ is load angle of inverter voltage.
3. Basic Study of System
Wind turbine converts the kinetic energy present in the wind into mechanical energy by means of producing torque. Since the energy contained by the wind is in the form of kinetic energy, its magnitude depends on the air density and the wind velocity. The wind
power developed by the turbine is given by the following equation.
(1)
Where
is the power co-efficient, ρ is the air density in kg/m3, A is the area of the turbine blades in m2 and V is the wind velocity in m/sec.
gives the fraction of the kinetic energy that is converted into mechanical energy by the wind turbine. It is
The power coefficient
a function of the tip speed ratio λ and depends on the blade pitch angle for pitch-controlled turbines. The tip speed ratio may be defined as the ratio of turbine blade linear speed and the wind speed. Variable speed turbines can be made to capture this maximum energy in the wind by operating them at a blade speed that gives the optimum tip speed ratio. This may be done by changing the speed of
the turbine in proportion to the change in wind speed.
Fig-1 Power Coefficient (Cp) & Tip Speed Ratio (λ) Curve
Fig. (1) Shows how variable speed operation will allow a wind turbine to capture more energy from the wind. As one can see, the
maximum power follows a cubic relationship (by eq (1)). A commonly used model for induction generator converting power from the
wind to serve the electric grid is shown in Fig.2. and the rotor side is fed via the back-to-back converter. In the given fig 2 the wind
blade is directly connected to the rotor and stator is connected to the grid. The gear box is required to rotate the rotor above synchronous speed. For rotating the rotor above synchronous speed low speed shaft and high speed shaft are placed in the gear box system.
This system is rotating the rotor above synchronous speed, when the wind velocity is below the rated speed. With the help of high
speed shaft the rotor is rotates higher speed in comparison of stator magnetic field. Thus induction motor is operated in generating
mode.
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International Journal of Applied Science Engineering and Management , VOL 1, ISSUE 3
ISSN 2454-9940
Fig. 2 Induction generator connected to grid
The stator of the wound rotor induction machine is connected to the balanced three-phase grid. The grid side converter controls
the power flow between the DC bus and rotor and allows the system to be operated in super synchronous speed.
4. Steady State Analysis
How the rotor speed of induction generator involves on the power flow of the studied system is discussed below. Fig. 3 shows the
torque slip characteristic of induction motor.
Fig. 3 Torque slip characteristics
It is observed that the induction machine operated as generator mode when the rotor is driven above the synchronous speed. The
negative sign of active power means that the power absorbed by the induction machine, while the rotor runs at a speed more than synchronous speed of the revolving magnetic field and the active power is supplied from induction generator to the grid. The reactive
power always absorbed by the induction machine, despite its Motoring mode. From the characteristics it is clear that when machine is
in motoring mode the speed of rotor is less the stator magnetic field speed. In motoring mode the machine has absorbing active as well
as reactive power while in generating mode machine has absorbs reactive power and supplies active power.
5. Steady State Response
In this section the study of current are observed, with and without reactive power compensation.
For compensating the reactive power various reactive power compensators devises are employed. Fig-4 shows the plot between current of line-1(line between grid and stator) before and after compensation of reactive power. It is clearly shows that after compensation
of reactive power the current level is reduced.
Fig. (4) Shows the rotor circuit current before and after compensation of reactive power. It is observed that the reactive power absorbed by the induction is also decreases rotor current rapidly after compensation.
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International Journal of Applied Science Engineering and Management , VOL 1, ISSUE 3
ISSN 2454-9940
before compensation
after compensation
Rotor circuit current in Amp.
30
20
10
0
-10
-20
-30
0
0.01
0.02
0.03
Time in sec.
0.04
0.05
0.06
Fig. 4 Response of rotor current before and after compensation
Fig.(5) Shows the comparison between receiving end voltage and sending end voltage after compensation of the reactive power. It
is clear that the amount of voltage will reduced after compensation of reactive power.
4
Receiving & Sending end voltage in volts
1
x 10
after compensation
before compensation
0.5
0
-0.5
-1
0
0.01
0.02
0.03
Time in sec.
0.04
0.05
0.06
Fig. 5 Comparison of receiving end voltage and sending end voltage
6. Conclusion
In this paper, steady state characteristics of double-out induction generator have been studied during normal conditions. This paper
shows the mathematical modelling of wind turbine driven doubly-fed induction generator which feeds active power to the utility grid
and draw reactive power from the grid. During steady state conditions, the DFIG feeds active power to the grid and reactive power is
supplied to the machine and the rotor speed of the machine highly influence on the active power production on the machine. In fact
active power produced by the machine is higher at higher speeds and VA rating decides the converter rating on rotor and grid side respectively (normally rating of converter is 30% of machine rating). In this paper the power factor of wind farm as well as voltage
regulation of load side is analyse and overall behaviour of power and power flow in wind farm is observed.
7. References
[1] [(EWEA) The European wind energy association] the European offshore wind industry 2011 trends and statistics, Jan 2012 Edition
[2] M. Zellagui., “Variable speed of the wind turbine generator with DFIG connected to electric grid” Revue des Energies Renouvelables Vol. 11
N°3 (2008)
[3] B.Chitti Babu , K.B.Mohanty “Doubly-Fed Induction Generator for Variable Speed Wind Energy Conversion Systems- Modeling & Simulation”,
International Journal of Computer and Electrical Engineering, Vol. 2, No. 1, February, 2010 1793-8163
[4]IRENA renewable energy technology, volume 1, power sector
[5] Osama S. Ebrahim, Praveen K. Jai and Goel Nishith, “New Control Scheme for the Wind-Driven Doubly Fed Induction Generator under Normal
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International Journal of Applied Science Engineering and Management , VOL 1, ISSUE 3
ISSN 2454-9940
and Abnormal Grid Voltage Conditions”, Journal of Power Electronics, Vol. 8, No. 1, January 2008.
[6] Dr John Fletcher and Jin Yang,” Introduction to Doubly-Fed Induction Generator for Wind Power Applications”
[7] Lucian Mihet-Popa, Frede Blaabjerg, “Wind Turbine Generator Modeling and Simulation Where Rotational Speed is the Controlled Variable”,
IEEE transaction on industry application, VOL. 40, NO. 1, JANUARY/FEBRUARY 2004
[8] Satish Choudhury, Kanungo Barada Mohanty, B.Chitti Babu, “Performance analysis of doubly fed induction generator for wind energy conversion system”, the 5th PSU-UNS International Conference on Engineering and Technology (ICET-2011), Phuket, May 2-3, 2011
[9] M.B.Mohamed, M.Jemli, M-Gossa, K. Jemli,” Doubly fed induction generator in wind turbine Modelling and power flow control” 2004 IEEE
International Conference on Industrial Technology (ICIT)
[10] C. R. Fuerte-Esquivel, J. H. Tovar-Hernández, G. Gutierrez-Alcaraz, and F. Cisneros-Torres, “Discussion of Modelling of Wind Farms in the
Load Flow Analysis” IEEE transaction on power system, VOL. 16, NO. 4, NOVEMBER 2001
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