A NEW INTERCONNECTING METHOD FOR WIND TURBINE/GENERATOR IN A WIND FARM
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#1
18-12-2010, 08:45 PM


A NEW INTERCONNECTING METHOD FOR WIND TURBINE/GENERATOR IN A WIND FARM
Presented by:
Aswin Das
EEE
College Of Engineering, Trivandrum


.pptx   A NEW INTERCONNECTING METHOD FOR WIND TURBINEGENERATOR IN A WIND FARM.pptx (Size: 1.55 MB / Downloads: 109)

CONTENTS
Objectives
Introduction
Selection of site
New method
How system works
Basic equations
Measures against halt
Operating methods
Control strategies
Basic characteristics


OBJECTIVES
A new interconnecting method for wind turbine/generator is proposed.
Through this method electrical energy of high quality with high reliability can be produced.

INTRODUCTION
Major components.
Advantages
Disadvantages of existing methods.

MAJOR COMPONENTS
ADVATAGES
Wind power systems are non polluting , so no adverse effect on environment.
Avoid fuel provision and transport.
On a small scale up to a few kilowatt system is less costly.


DISADVANTAGES OF EXISTING METHODS
Doubly fed induction generator system(DFIG) is more complicated.

Low voltage ride through characteristics are poor.

SELECTION OF SITE
High annual average wind speed.

Wind structure of the proposed site.

Altitude of the proposed site.

Nearness of site to local centers/users.

NEW METHOD
HOW SYSTEM WORKS

Output of AC generator are rectified.
Rectified outputs are integrated in DC link.
The resultant output is converted again with thyristor inverter.
Optimum site can be selected with a long distance DC transmission line.


BASIC EQUATIONS OF THE SYSTEM


MEASURES AGAINST SYSTEM HALT
Short circuit the corresponding converter by means of giving signals to all the gates of the thyristors.
Connect a diode in parallel with each thyristor converter.
OPERATING METHODS
CONTROL STRATEGIES
CONTROL STRATEGIES
Basic Characteristics for Case of Two Wind Turbine Generators

is set to zero.
constant
changes in the range of 0-11m/s.







CONCLUSION
Only one externally commutated thyristor inverter is used.
High quality output power with high quality.
Optimum site for wind turbines can be readily selected.
Dc transmission system is entirely appropriate for the proposed system.
REEFERENCES
[1] Shoji Nishikata, Fujio Tatstuta, “A New Interconnecting Method for Wind Turbine/Generators in a Wind Farm and Basic Performances of the Integrated System,” IEEE Transactions on Industrial Electronics,vol.57,no:2,February 2010.
[2] Shoji Nishikata, Fujio Tatstuta, “A New Interconnecting Method for Wind Turbine/Generators in a Wind Farm and Basic Performances of the Integrated System,” in Proc.13th international Power Electronics and Motion Control Conference, Poznan Poland, Sep 1-3,2008,pp 2342-2348.
[3]M.P Ramesh, “Grid Interconnection of Wind Turbines”, Presentation to GERC Ahmedabad,7 February 2009.
[4]Thomas.A.Wind, ”Distributed wind generation and interconnection”, Transmission interconnection and integrating issues, May 19,2010.
[5] C. Ghita, A.– I. Chirila, I. – D. Deaconu, and D. I.
Ilina, “The magnetizing field of a linear generator
used to obtain electrical energy from waves
Energy”, in Proc. ICREPQ’07, pp. 207-208.
[6] H. James Green, Thomas W, Wind Utility Consulting,” The IEEE Grid Interconnections Standard: How Will it Affect
Wind Power?”, Presented at AWEA’s Wind Power 2000 Conference Palm Springs, California April 30–May 4, 2000


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science projects buddy
Active In SP
**

Posts: 278
Joined: Dec 2010
#2
18-12-2010, 09:15 PM

A NEW INTERCONNECTING METHOD FOR WIND TURBINE/GENERATOR IN A WIND FARM
Presented by
Aswin Das
EEE
College Of Engineering, Trivandrum
2007-11 batch



.docx   A NEW INTERCONNECTING METHOD FOR WIND TURBINEGENERATOR IN A WIND FARM.docx (Size: 690.09 KB / Downloads: 84)

1. INTRODUCTION
There is a need to construct a large-scale wind farm for generating a large amount of electric power by kinetic energy of wind, and such a site should be far distant from urban areas or off shore. In addition, the interconnecting method for the wind turbine/generators is essential in forming the wind farms.

But the present system uses complicated doubly fed induction generator system (DFIG) and has certain drawbacks such as poor low voltage ride through characteristics and immense noise.

Through this seminar and presentation a new interconnecting method for a group of wind turbine/generators is proposed. In this method, the outputs of each ac generator coupled with the wind turbine are rectified, and these rectified outputs are connected in series and integrated in the dc link. The resultant output dc power is converted again to ac power with the thyristor inverter. Thus, only one inverter is enough, making the system very simple, and the inverter can be placed everywhere without restraint. Thereby, we can select the optimum site for wind turbine/generators with a long-distance dc transmission line.

In addition to the aforementioned advantages, the proposed system not only has an ability of standalone operation without the existence of utility but also it cooperates with the utility. Since this system is provided with a synchronous compensator on the output side as an essential component, the low-voltage ride-through characteristic of the system is improved greatly when compared with the conventional system.





2. NEW METHOD
2.1. BASIC EQUATIONS OF THE SYSTEM


Fig.2.1.1 Wind turbine generator and thyristor converter

As a wind turbine generator, a permanent magnet synchronous generator (PMSG) is used here. In Fig. 1, mechanical energy is acquired from kinetic energy possessed by wind through a wind turbine, and the PMSG converts it to electrical energy. The output of PMSG is converted to dc power through a thyristor converter. The output power of the wind turbine Pt, which is equal to converted dc power if the losses in the generator and converter are neglected, is given by

〖 P〗_t=1/2 C_p (λ,β)ρA_W V_wind^3=V_d I_d (1)

where ρ, A_W, V_wind , V_d , I_d and are the air density, rotor swept area, wind velocity flowing into the wind turbine, output voltage, and current of the converter, respectively. In addition, C_p is the performance coefficient which is the conversion ratio of turbine output to wind power .

Here, it should be noted that C_p is expressed as a function of tip speed ratio λ and blade pitch angle β. The tip speed ratio λ
is defined as
λ=ω_t R_rotor/V_wind (2)

where ω_t is the angular velocity of the wind turbine and R_(rotor )is the blade radius.
In general, the wind turbine should be driven to keep λ a constant value to obtain〖 C〗_p as large as possible. Thus C_p , λ and are assumed to be constant for a given pitch angle β in this discussion for the sake of simplicity. In this case, ω_t the angular velocity is proportional to the wind velocity〖 V〗_wind.

On the other hand, it should be recognized that the noncontrol output voltage of the converter V_(d0 )is almost proportional to ω_t since PMSG is used as the wind turbine generator.
When the coefficient of proportion between〖 V〗_(d0 ) and ω_(t )is assumed to be K_d, we have (3) for controlled output voltage of the converter.

V_d=〖 V〗_(d0 ) cos⁡〖α=K_d 〗 ω_t cos⁡〖α=〗 (K_d λ)/R_rotor V_wind cos⁡〖α (3)〗

where α is the control angle of converter and 〖 V〗_(d0 )=K_d ω_t
As for the torque of the wind turbine ,T_t we obtain the following equation from (1)–(3):

T_t=P_t/ω_t = (〖 C〗_p ρA_W V_wind^2)/2λ = I_d K_d cos⁡α (4)

From (4), it is recognized that the wind turbine torque T_t and hence, the operating point of the system can be controlled with the control angle α. The increase in α, however, results in an increase in reactive power of the system, and hence, α should be kept small to use the PMSG as effectively as possible.











2.2. SYSTEM CONFIGURATION



Fig. 2.2.1 Proposed interconnecting method for wind turbine/generators and system configuration
Fig. 2.2.1 shows the configuration of the proposed wind turbine generating system composed of two or more sets of wind turbine and PMSG. As in the figure, the output of each thyristor is connected in series, and the unified dc output is fed to the current source thyristor inverter through a dc transmission line. It should be noticed that the whole system seems likely to come to a halt when one of the wind turbines is lost for some reason, including the lack of wind. However, we can prevent such a system shutdown by short-circuiting the corresponding converter by means of giving signals to all the gates for the thyristors in the converter. Another means for protecting the system against the shutdown is to connect a diode in parallel with each thyristor converter in Fig. 2.2.1 That is, once one of the output voltages of the converters is lost for some reason, the corresponding parallel-connected diode turns into ON-state immediately, continuing dc-link current I_(d )to flow and enabling the whole system to operate without interruption.
The synchronous compensator connected to the inverter through a duplex reactor provides reactive power needed for commutation of the inverter thyristors and that required in the ac output as well. Since the presence of this synchronous machine (compensator) allows the system output voltage to control, the proposed system can be operated as either an isolated generating system or an incorporated generating system into the utility. Hence, a good low-voltage ride-through characteristic can be obtained with this system Moreover, when we drive the synchronous machine by a prime mover to generate active power as well, a new hybrid-type wind turbine generating system can be realized .

It should also be reminded that the output voltage distortion in the inverter caused by the commutation of thyristors is completely compensated with the well-designed duplex reactor so as to cancel the subtransient inductance of the synchronous compensator .

The system controller shown in the figure collects observed data on the wind velocities V_(wind i )(i = 1, 2, . . . , n) at each wind turbine. In this controller, the dc-link current Id and then the leading angle for commutation of the inverter γ are calculated based on V_(wind i ) (i.e., total output power gained by the wind farm ΣP_ti), and the control angles of converter α_i(i = 1, 2, . . . , n) for the individual converter [output voltage V_di(i = 1, 2, . . . , n)] are also determined.












3. OPERATING METHODS

3.1. Load Connected Through Thyristor Invertor


Fig 3.1.1 Two wind turbine/generators connecting an inverter load

We discuss here the way of controlling α_i for individual thyristor converter when the system loads are connected in the dc link through the thyristor inverter as in Fig. 2.2.2

For this case, let V_wind1∼〖 V〗_(wind n) be the wind velocities flowing into #1 ∼ #n wind turbines, respectively, and let V_(w max) be the maximum value among these wind velocities.
That is



V_(w max)=max⁡(V_wind1,V_wind2,…〖 V〗_(wind n)) (5)


As previously mentioned, the control angle for the converters α should be controlled as small as possible in order to reduce the reactive power of the system. Consequently, we set the


control angle for the converter at which wind velocity is V_(w max) as zero. Then, the output power and output dc voltage for this wind turbine become maximum as

〖 P〗_(t max)=1/2 C_p ρA_W V_(w max)^3 (6)

〖 V〗_(d max)=(K_d λ)/R_rotor V_wmax (7)

As a result, the dc-link current turns into

I_d=P_(t max)/〖 V〗_(d max) =(C_p ρA_W V_(w max)^3 R_rotor)/(2K_d λ) (8)

Aside from the wind turbine with the maximum wind velocity, the output power P_(ti )and output dc voltage V_di for #i wind turbine are given as

〖 P〗_(t i)=1/2 C_p ρA_W V_(w i)^3 = V_di I_(d ) (9)

〖 V〗_(d i)=(K_d λ)/R_rotor V_(wind i) cos⁡〖α_i 〗 (10)


Since the dc-link current〖 I〗_d is the same for all the converters because of dc link, the following relationships are obtained:


〖 P〗_(t i)/P_(t max) =〖 V〗_(d i)/〖 V〗_(d max) =(V_(w i)^3)/(V_(w max)^3 )=(V_(wind i) cos⁡〖α_i 〗)/V_(w max ) (11)


Hence, control angle α_(i )for converter #i should be controlled as

α_i=cos^(-1)⁡〖(V_(wind i)^2)/(V_(w max)^2 )〗 (12)


Based on the control strategy introduced here, we can acquire the maximum wind power from the whole system, in which the individual wind turbine can be operated most effectively.


The total dc-link voltage V_(d )and the total output power, P_(t total) which are the input voltage and input power, respectively, to the inverter, become

V_d=(K_d λ)/R_rotor ∑_(j=1)^n▒〖〖(V〗_(wind j ) cos⁡〖αj)〗 〗 (13)

P_ttotal=1/2 C_p ρA_w ∑_(j=i)^n▒〖V_(wind j)^3 (14)〗


It is recognized here that the dc-link current I_d given in (8) should be governed by controlling the leading angle of commutation for inverter thyristors γ.
If the angle of overlap in the inverter is neglected, the equation for inverter dc side voltage E_d is given as follows:
E_d=V_d-I_d R_d=(3√2)/π V_(l-l) cos⁡γ (15)

where〖 R〗_d is the total resistance in the dc link and V_(l-l) is the line-to-line rms voltage of the inverter output.
Hence, we have the equation for γ as

γ=cos^(-1)⁡〖{π(V_d-I_d R_d )/(3√2 V_(l-l) )} (16)〗

It is also noted that γ should be controlled so that the margin angle for commutation γ −u > 0 (u: overlapping angle of commutation) to secure a stable operation of the inverter.









3.2. Resistive Load Connected in DC Link


.

Fig. 3.2.1 Operation with the resistor load.


As another primitive investigation, we discuss the method of controlling the output voltage for each converter for the case of a load of constant resistance connected in the dc link instead of the thyristor inverter shown in Fig. 2.2.2.

The total power derived from all the wind turbines P_(t total )is given with (14), and the dc-link voltage V_(d )applied to the load resistance turns into

V_(d )=√(〖(R〗_L ) 〖 P〗_ttotal)=√(1/2 R_L C_p ρA_w ∑_(j=1)^n▒〖(V_(wind j)^3)〗) (17)

Since the same dc current I_(d )flows in all of the converters because of dc link, the output dc voltage for the individual converters has to be controlled, depending on the wind conditions for the turbines in order to provide a stable operation of the system. That is, the control angles should be determined based on the relationships between the power obtained by the wind and the consumption power.




Hence, the contribution of each wind turbine to the total power is assigned as

P_ti/P_(t total) =(V_windi^3)/(∑_(j=1)^n▒V_windj^3 ) (18)



V_di=V_d (V_windi^3)/(∑_(j=1)^n▒V_windj^3 )=√((R_L C_p ρA_w)/(2∑_(j=1)^n▒V_windj^3 )) V_windi^3=(K_d λ)/R_rotor V_(wind i) cos⁡〖αi (19)〗


As a result, the control angle for #i converter can be calculated as


α_i=cos^(-1)⁡〖((R_rotor V_windi^2)/(K_d λ))〗 √((R_L C_p ρA_w)/(2∑_(j=1)^n▒V_windj^3 )) (20)




















4. BASIC CHARACTERISTICS FOR THE CASE OF TWO WIND TURBINE GENERATORS

On the basis of the system equations derived earlier, we explore here the system characteristics for the case of two sets of wind turbine/generator as a basic investigation. In this case, n = 2, and the system is given by Fig. 3 for the inverter load. The whole load is shared between two generators, and the control angles of each converter should be properly controlled depending on the wind velocity. It should be remembered here that the output voltage and frequency of the inverter can be kept constant by controlling the field current of the synchronous compensator shown in Fig. 2.2.2 as well as the leading angle of commutation of inverter for the case when the system is used in the standalone operation, while the voltage and frequency of the system when connected with the utility depend solely on those of the grid.

When V_wind1>V_wind2, then α_(1 )is set to be zero according to foregoing discussion and α_(2 ) is calculated with (12). Figs. 4.1 and 4.2 show examples of calculated results for the cases when〖 α〗_(1 ) is fixed to be zero and V_wind1= 11 m/s = constant; meanwhile V_wind2 changes in the range of 0-11 m/s. In Fig. 4.1, the characteristics of angular velocities of the wind turbines, ω_t1, ω_t2, and〖 α〗_(2 ) versus V_wind2 are shown. It is clarified that, although ω_t2increases with〖 V〗_wind2, ω_t1is kept constant because of a constant tip speed ratio λ and that control angle α_(2 )decreases with an increase in V_wind2 according to (12).











Table 4.1

Fig 4.1 Angular velocities of wind turbine ω_(t1 ,) ω_(t2 ) and control angle of converter α_(2 ) versus V_wind2








Fig 4.2.1 〖 I〗_(d,) V_(d1,) V_(d2 ) versus V_(wind2.)



Fig 4.2.2 P_(t1,) P_(t2,) versus V_wind2

Fig 4.2 Characteristics of dc link voltages and current , and output powers versus wind velocity at turbine #2 V_(wind2.)





Fig. 4.2 shows the characteristics of the dc-link current〖 I〗_d, dc
voltages of the converters V_d1 and V_d2 [in Fig.4.2.1], and system output P_(t total ), which is equal to P_t1+P_t2[in Fig. 4.2.2], when V_wind2, changes. It can be seen from this figure that the dc voltage V_(d2 )and output power P_(t2 )increase with an increase in〖 V〗_wind2, while V_d1 and P_t1, as well as I_d, are kept to be constant independently of V_wind2 , since V_(wind1 )and α_(1 )are constant in this case.



























5. CONCLUSION


Through this paper a new interconnecting method of two or more sets of wind turbine/generators used in a wind farm has been proposed, and basic characteristics of the integrated wind turbine generating system have been discussed.

In the proposed system, only one externally commutated thyristor inverter is required for a cluster of wind turbines, and output voltage without distortion can be achieved with ease, realizing a very simple configuration of wind farm with high quality of output power as well as high reliability.

In addition to these advantages, only one dc link is used,and the optimum site for wind turbines, such as off shore, can be readily selected in order to obtain more power from wind because dc transmission system is entirely appropriate for the proposed system. It should be recognized that, in general, the dc-link voltage of the proposed system is changed fairly with the change in the wind velocity, so careful attention has to be made in designing the dc transmission system as for the insulation deterioration and losses. In addition, the voltage levels to the ground for the system components such as PMSGs and thyristor converters increase considerably when compared with the case of single turbine/generator, and careful considerations should be made for the insulation class of the individual components.









6 .REFERENCES

[1] Shoji Nishikata, Fujio Tatstuta, “A New Interconnecting Method for Wind Turbine/Generators in a Wind Farm and Basic Performances of the Integrated System,” IEEE Transactions on Industrial Electronics,vol.57,no:2,February 2010.
[2] Shoji Nishikata, Fujio Tatstuta, “A New Interconnecting Method for Wind Turbine/Generators in a Wind Farm and Basic Performances of the Integrated System,” in Proc.13th international Power Electronics and Motion Control Conference,Poznan Poland,Sep 1-3,2008,pp 2342-2348.
[3]M.P Ramesh, “Grid Interconnection of Wind Turbines”, Presentation to GERC Ahmedabad,7 February 2009.
[4]Thomas.A.Wind,”Distributed wind generation and interconnection”,Transmission interconnection and integrating issues,May 19,2010.
[5] C. Ghita, A.– I. Chirila, I. – D. Deaconu, and D. I.Ilina, “The magnetizing field of a linear generator used to obtain electrical energy from waves Energy”, in Proc. ICREPQ’07, pp. 207-208.
[6] H. James Green, Thomas W, Wind Utility Consulting,” The IEEE Grid InterconnectionsStandard: How Will it Affect Wind Power?”,Presented atAWEAs WindPower 2000Conference Palm Springs, California April 30–May 4, 2000.


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