2004 IEEWPES Transmission & Distribution Conference & Exposition: Latin America 1

Wind Generation Dynamic Simulation Connected to an Electric Power System

W. S. Mota, Senior Member IEEE, L. S. Barros, F. M. P. Pamplona, Student Member IEEE, A. N. Epaminondas, E. R. B. Filho, A. M. Formiga, and A. A, F. Santos

Abstract- This work presents a methodology developed for dynamic wind generation simulation connected to an electric power system. It presents wind generators models and rotor excitation control strategies. The turbine mechanical power i s obtained from the wind speed x time characteristic in combination with turbine power curve. The methodology has been applied to the CELPE sub-transmission System connected to the CHESF System using a transient stability program StabTnface* (UFCG-CELPE, 2002), implemented on this work. Two types of induction machines have been considered to model wind generators: squirrel cage and double fed (DFIG). It emphasizes voltagelreactive power control a t the DFIG machines and electric power and generator terminal voltage behavior when fast wind speed variation occurs.

Index Terms- Dynamic simulation, power system transient stability, wind energy.

1. INTRODUCTION

oday, electric energy generation by alternative sources is T an unquestionable tendency. The generation and grid connection technologies of alternative sources seem to be too different from those for conventional hydraulics or thermal systems. On this scenery, wind energy becomes an alternative generation with strong potential of utilization. In wind generation, the wind turbine’s mechanical energy in electric energy conversion is more economically performed through induction machines. Therefore, for dynamic studies of wind systems, induction machines and wind turbines modeling are necessary. Here, a turbine model taking into account the stochastic behavior of the wind speed is considered. [l]. This work proposes a methodology for dynamic simulation of power systems containing wind generation. The proposed methodology has modeled two induction machine types: squirrel cage and double fed (DFIG), including the machine excitation control, (21. The proposed methodology will be applied to a real case; consisting the CELPE sub-transmission system connected to the CHESF system.

This work i s part of the Research and Development Project “Development and Validation of a Methodology for Distributed Generation Analysis”, executed by Universidade Federal de Campina Grande (UFCG) in connection with Companhia de Eletricidadc de Pemambuco (CELPE), which provided financial support.

11. ARMATURE EQUIVALENT CIRCUIT FOR SIMULATION STUDIES

A . Squirrel Cage Induction Generator Connected to Transmission Grid Simulation

The squirrel cage induction machine can be represented by the equivalent circuit shown in Fig. 1, referred to stator, for grid connection through terminal voltage, where the used model is an internal voltage behind the transient impedance.

Y Fig. 1. Squirrel cage induction generator equivalent circuit.

V, = vdr + jvqs (1)

I , = ids + j i , (2)

V‘ = vb + jvh

where,

(3)

(4)

is the induction generator transient reactance.

variation is calculated using (5) and (6), [3]. The squirrel cage induction generator internal voltage

where,

T’ =& R,.

0-7803-8775-9/04/$20.00 02004 IEEE 179

2

is the open circuit transient time constant, and where, E , and E , are the AC voltage components from

xs =%L is the stator leakage reactance.

(8) the conversor at the rotor side, applied to the rotor, referred to stator, Fig. 3. ThroughEFD, the generator reactive power or the terminal voltage can be controlled. Simultaneously, the generator real power can be controlled through E, . The

For this type of generator, the machine can be represented by an equivalent circuit as shown in Fig. 2, referred to stator, for grid connection through terminal voltage. The current source I , represents the current through the conversor at the stator side.

X V W

Fig. 4. Conversor control at the rotor side.

111. ELECTROMECHANICAL OSCILATIONS

Fig. 2. Doubte fed induction generator equivalent circuit. The mechanical coupling between wind turbine and generator can be expressed by (1 l), [73.

I The current 1, is calculated after the definition of the real (PC2) and reactive (QC2) powers, which will be provided to grid through the conversor at the grid side, Fig. 3, [4], [SI. In this case, the time constants associated to the DC circuit

dwr --(T, - T,) -- dt 2H

where between the conversors, was negligenced, [6] .

Conversor CI Conversor C2 Rotor side Grid side

The available mechanical power p,, at the wind turbine axis, is obtained from the wind speed x time characteristic and fkom the turbine power curve.

To obtain the mechanical power x time behavior, information about the wind speed for a considered interval and turbine nominal data are provided by the manufactory.

Then, it adopted the following procedure: From the Power x Wind speed curve, provided by the

manufactory, Fig, 5, the wind speed for the turbine starting operation v I , the wind speed which the turbine achieve its

Fig. 3. DFlG type generation system. v nominal power v2, and the maximum turbine operation speed

v 3 , should be identified. The double fed induction generator internal voltage 0 With the identified speeds, Power as a function of the

Wind Speed can be obtained by using a cosine approximation for the ascendant part of the curve:

variation is calculated using (9) and (10).

which reproduce a curve as in Fig. 5.

power can be obtained by the following steps below.

1 [ v b + ( x ~ - x ~ ~ i d s l - s ~ s u & + O s E , ( lo ) Finally, from the wind speed x time, the mechanical I;' =-- Td q

180

3

Nominal Power Power Generation

R ,

1. For the instant t , identify the wind speed Y,, at the V. SIMULATION

_.._

38,4 MW 38,4 MW 0,34%

wind speed x time curve; 2. If vw 2 v l , then P, = 0,O; 3. If vl < v, 5 v2, then the mechanical power is

calculated through (1 3); 4. If v2 < v, I v 3 , then P,,, = 1 ,O PA.; 5. If v, > v g , then P, =O,O.

R , X .

1 .o h

0,41%

4,67%

=! 4 0.8

xr X ,

3 0.4

0.2 .- B i )

5 0.0

4,10%

I 1 S8,40?'0

0 5 10 15 20 25 Wind Speed ( 4 s )

Fig. 5 . Power x Wind Speed curve.

Control Type

TV. GRID SOLUTION

PI

The grid solution is obtained through the computation of terminal currents of the machines I , , and bus voltages V , where the machine equivalent circuit is included in the admittance matrix shown below:

To represent the unknown variables on the right side, this equation can be handled by matricial algebra, [3]. Resulting:

Matricial techniques are used to avoid the reinvertion of the matrix during the solution process. This allows the grid modification to simulate disturbances, where, only the rows and columns corresponding to the buses or transmission lines in fault are modified.

For this grid modeling, the loads are represented by constant impedance andor current. For constant current load, it is a function of the applied voltage. The most realistic load representation it would be a dynamic model, dependent of the voltage and frequency variations, where the model parameters should be acquired from real data.

The simulation consists of a real case of CELPE, where the Poq50 Wind F m is connected to the Tacaimbb Regional 69 kV System.

To the steady-state simulations (Load Flow), the CEPEL ANAREDE Program* was used. The dynamic simulations were carried out using the STABEOLICA@ routine, which is part of the Stabhface@ package, [3]. Initially, the brazilian electrical system data, provided by ONS (Operador Nacional do Sistema) is considered. Then, a simplified equivaIent regional system was elaborated, regarding the east area of the CHESF System, Fig. 6 .

For this simulation, the generations of CHESF System were represented by classical models. Only the local generation, Temopernambuco Thermal-Electrical Plant and Po@o Wind Farm, was represented by realistic models. The wind generation estimated data are in Table I.

TABLE I PoCKO WWD FARhi ESTIMATED DATA.

Double Fed Induction Generator - DFlG

Type

I,. L

H I 25s (Generator + Turbine)

For the Double Fed Induction Generator (DFIG) rotor control, two PI controllers are used as shown in Fig. 4. The excitation system parameters are in Table 11.

TABLE I1 EXCITATION COmROL PARAMETERS

2,oos

0,oo I

0,005s

10,oo

0,ws

0,001 TSE

181

m

Pirapama Termopernambuco :+>*

Fig. 6. East CHESF simplified regional system.

A . Dynamic Simulations Carried Out A simulation of a 150ms three phase to ground fault at the

Tacaimbo 69kV bus is performed. The electrical power variation for squirrel cage induction generator is shows in Fig. 7. From this curve, the oscillation frequency and damping can be evaluated, For this case, the oscillation frequency is 1.6 Hz, and the damping, according to logarithmic decrement, is 1.74.

0.6 I , . I , I , I

I I I I I I I I I 0 8 - -1- --I- - J ~ - I - - L - -c - - 1 - - -1 - - 1 -

045

Time In SEZondr

Fig. 7. Electric power of the P q b squirrel cage induction generator.

It emphasizes two important aspects at the squirrel cage induction generator: slip stability and voItage decrease for some types of disturbance. For this simulation the machine is stable, because the slip stabilizes itself, Fig. S.

Fig. 8. Slip in % for the squirrel cage induction generator.

Another disturbance simulated was a 150ms three phase to ground fault at the Tacaimb6 69kV bus followed by a load rejection at the same bus. The terminal voltage is shown in Fig. 9. A voltage drop at the squirrel cage generator can be seen after the disturbance. Also, the frequency variation is shown in Fig. 10, may be used for protection purposes in order to avoid machine turning off For this case, the maximum frequency achieved was 61 2 H z .

If the generator is Double Fed type, it can provide reactive power, until its maximum capacity, to maintain its nominal terminal voltage. In Fig. I 1 is shown the terminal voltage for the double fed induction generator when the same disturbance is applied.

182

5

l l r , , , , , , , , , ,

I l l I 1 I I I I l l

B. Fust Wind Speed Variafion Simulation The objective of this simulation is to observe the wind

generator dynamic behavior, through the level flick of its terminal voltage, when fast wind speed variation occurs.

For this simulation the wind speed variation, as shown in Fig. 12, generated by (16), has been used. It provides a random wind speed with a defined average value.

6UOt 600t 12 15

v , = v, -i+zv, +sen(-,l-2cos(-)

Y, is the average speed of interest; V, is a random variable between 0 and I , and t is the time in seconds.

I I I I I I I I 1 1 I I 40 0 2 0 4 0 6 0 8 1 1 2 1 4 16 1 8 2

=me in -a

Fig. 12. Wind speed fast variation.

The mechanical power provided by wind turbine was calculated as a speed wind function, according to step 3 of the wind speed x time curve construction. It can to observe in Fig. 13.

0 0 2 04 0 6 0 8 1 t 2 1 4 1 6 1 8 2 l m e in recan&

Fig. 13. Mechanical power provided by the wind turbine when fast wind speed variation occurs.

The terminal voltage for the double fed induction generator is shown in Fig. 14.

Tempo ~m SWundoB

Fig. 14. Double fed induction generator terminal voltage when fast wind speed variation DCCUTS.

where,

183

6

In Table 111, the variables along the text are defined.

TABLE 111 VARIABLES IN THE TEXT

I V I Machine intemal voltage 1 I v, I Machine terminal voltage I

1, I Armature current 0. Stator angular speed

I 0. I Rotor angular speed 1 I L, I Mutual inductance between stator and rotor I I L, I Stator inductance added to L. I I I.” I Rotor inductance added to L. I 1 R. I Stator resistance I I R, I Rotor resistance I I X, I Mutual reactance between stator and rotor I 1 x 1 Stator reactance I 1 x 1 Rotor reactance I ! H I Machine inertial constant I I r. I Electrical toraue I I ~ , - r Mechanical torque I I Yn 1 EIement of the Y-bus mahix I

VI. CONCLUSION This work presented results of dynamic simulations for a

real power system containing wind generation. According to obtained cuwes, one can to observe the system behavior and verify if the established criterion are satisfied at the grid connection of wind generators.

For the used modeling, the wind turbine’s mechanical power estimation from the Power x Wind Speed curve, is emphasizes. In this methodology, a cosine approximation for the ascendant part of the curve has been used. It becomes a very simple way to estimate the mechanical power from the wind speed.

VII. REFERENCES

Rosas, P. A. C e Estanqueiro, A. I. “Guia de Projeto EI6trico de Centrais Edicas”. Vol. I Centro Brasileiro de Energia Eblica, Recife, 2003; Mdler, S., Deicke, M. and Rik W. De Doncker, “Double Fed Induction Generator System for Wind Turbines”, IEEE Industry Applications Magazine, MayiJune 2002; Mota, W. S. Program de estabilidade transitbda “Stabhface” produto de um P&D entre a UFCG e a CELPE, “Programa Computacional para simulago e d l i s e de Geraqlo E6licdDiesel em Femando de Noronha” Campina Gmnde, 2003; Akhmatov, V., “Variable-speed Wind Turbines with Doubly-fed Induction Generators - Part I: Modeling in Dynamic Simulation Tools”, Wind Engineering Vol. 26, No 2, pp85-108,2002; Akhmatov, V., “Variable-speed Wind Turbines with Doubly-fed Induction Generators - Part 11: Power System Stability”, Wind Engineering Vol. 26, No 3, ppI71-188,2002; Poller, Markus A. “Doubly-Fed Irtduction Machine Models for Stability Assessment of Wind Farms”. 2 0 3 Bologna Power Tech Conference, June 23’” - 26*, 2003 Bologna, Italy; Kundur, P. “Power System Stability and Control”. Book, Mc.Graw Hill, 1994.

VIII. BIOGRAPHIES

i Wellington S. Mota was born in Jolo Pessoa, Brazil, 1946 He received the B Sc and MSc in Electncal Engineering from UFPB (Federal University of Paraiba) Brazil in 1970 and 1972 respectrvely He got the Electr~cal Engineenng Ph.D from Waterloo, University of Waterloo, Canada in 1981 Since 1971 he has been with the Department of Electncal Engrneenng, UFPB where currently is a Senior Professor. From 1973 to 1977 he worked at the Sao Francisco hver Hydro (CHESF) III power system planning His research interests include

Power System Control and Stability including recently wind farms

Lucian0 S. B a r m was born in Campina Grande, Brazil, 1975 He received the B Sc degree in Electrical Engineering from the Federal University of Paraiba (UFPB) in 2000, and the M.Sc. from the Federal University of Campina Gmnde (UFCG) in 2002. Currently, he is working for his D Sc. degree at the UFCG. His research areas include Power System Dynamic and Control, Stability and Wind Generation.

Franklin M, P. Pamplona was bom in Brasilia, Brazil, 1970. We received the B.Sc. and MSc. degrees in Electrical Engineering from the Federal University of Paraiba (UFPB) and is currently working for his D.Sc. degree at the Federal University of Campina Grande (UFCG). He is on the faculty of Electrical Technology at the CEFET-AL, Brazil. His a m of interest are Power Quality, and Distribution Systems Analysis and Planning.

Antanio do N. Epaminondas was born in Currais Novos, Brazil, 1956 He received the B.Sc. and MSc degrees in Electrical Engineenng from the Federal University of Paraiba (UFPB) in 1980 and 1986 respectivety. Currently, he is professor o f the Department of Electrical Engineering, UFPB. His area of interest is the application of computational methods to electrical power systems.

Aldo M. Fordga was bom in Recife, Brazil, 1966. He received the B.Sc. degree in Electrical Engineering and the M.Sc. degree in Quality and Productivity Administration from the University of Pemambuco (UFE), in 1995 and 1996 respectively. Currently, he is woking for Electrical Company of Pemambuco (CELPE) in the areas of Power Systems Control, Planning and Management.

Antdnia A. F. dos Santos was born in Recife, Brazil, 1966. She received the B.Sc. degree in Electrical Engineering from the University of Pemambuco (UFE) in 1991, and the M.B.A. in Managerial Administration. Currently, she is working for Electrical Company of Pemambuco (CELPE) in the areas of Power Systems Protection and Planning.

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