- Email: [email protected]
EMTDC-based simulation of wind power generation system
ARTICLE IN PRESS
Renewable Energy 32 (2007) 105–117 www.elsevier.com/locate/renene
PSCAD/EMTDC-based simulation of wind power generation system S.G. Hana, I.K. Yua,, M. Parkb a
Department of Electrical Engineering, Changwon National University, 9 Sarim-dong, Changwon 641-773, Korea b Center for Applied Superconductivity Technology, 28-1 Seongju-dong, Changwon 641-120, Korea Received 20 January 2004; accepted 14 December 2005 Available online 7 February 2006
Abstract This paper proposes a novel simulation method of wind power generation system (WPGS) using PSCAD/EMTDC. The pitch control-based rotation speed control scheme of turbine under variable wind speed is implemented. For the purpose of achieving effective and user-friendly simulation method for utility interactive (grid connected) WPGS, real weather condition-based WPGS simulation (RW–WPGS) is performed using PSCAD/EMTDC. It is not easy, in general, to consider the RW conditions in the WPGS simulation using the EMTP or PSPICE type of simulators. External parameters of the RW conditions, however, are necessary to improve the simulation accuracy. The components modeling of wind turbine system is also studied and the real weather conditions are introduced by the interface method of a non-linear external parameter of the PSCAD/EMTDC. The outcomes of the simulation demonstrate the effectiveness of the proposed simulation scheme. r 2006 Elsevier Ltd. All rights reserved. Keywords: Pitch control; PSCAD/EMDC; Real weather condition; Wind power generation
1. Introduction In recent years, there has been growing interest in renewable energy systems due to the environmental problem and the economic benefits from fossil fuel savings. Such systems are usually connected to the existing power grid for fuel displacement purposes as well as of earning some capacity credit. Wind Power Generation System (WPGS) is one of the Corresponding author. Fax: +82 55 263 9956.
E-mail address: [email protected] (I.K. Yu). 0960-1481/$ - see front matter r 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.renene.2005.12.008
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most useful energy resources using natural environment. The WPGS production is undoubtedly accompanied with minimization of environmental pollution, reduction of losses in power system transmission and distribution equipments, and supports the utility in demand side management [1,2]. The simulation schemes that can be applied to the utility interactive WPGS readily under various conditions considering the sort of wind turbine generators, the system capacity and the converter system as well are strongly expected and emphasized among researchers. The generation of electric power by wind turbine generators depends on the wind power source which changes with time and position, it is, therefore, necessary to have a proper control strategy to maintain a rated value of the WPGS and the angular velocity regardless of the wind speed changes [3]. This work is to study the behavior of the wind turbine generator operated under variable speed with pitch-control capability under turbulent winds. The control algorithm is chosen based on the constant output torque of wind turbine. Pitch control is relatively fast and can be used to limit the rotor speed by regulating input aerodynamic power flow, especially, near the high-speed limit. The utility interactive WPGS is simulated using the real weather condition-based WPGS simulation (RW–WPGS) in this paper. Fig. 1 shows the conceptual diagram of the RW–WPGS. Three important topics, which have been emphasized by the researchers related with simulation study of the WPGS, are user-friendly method, applicability to gridconnected systems and the RW conditions included [4,5]. As shown in Fig. 1, the RW–WPGS could be a peculiar simulation method to realize the requirements of researchers mentioned above. This paper presents a computer simulation model of a unified wind turbine–synchronous generator interface scheme using PSCAD/EMTDC [6]. Details of the dynamic simulation models and the transient results are presented. It is difficult to imitate an actual weather
User-friendly (Graphic User Interface)
EMTP/ATP PSCAD/EMTDC
Applicability for Grid-connection
User (Researchers for WPGS)
Applicability for Grid-connection
Closer to reality
Using the empirical formula of a wind turbine
Using the real weather conditions
Fig. 1. Conceptual diagram of the RW–WPGS.
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condition in the simulation of the WPGS with a transient phenomenon digital simulation tool for the electric power systems such as EMTP type of simulators and PSPICE, etc. The reliability is not guaranteed in the simulation for which an external parameter of the weather condition is necessary [7]. Thus, the simulation of the WPGS using the RW condition was achieved by introducing the interface method of non-linear external parameters and FORTRAN by using PSCAD/EMTDC in this paper. The components of wind turbine system have been modeled by the proposed scheme. The output torque of wind turbine and the output power of generator are calculated by using the real weather conditions. Speed control of turbine under variable wind speed is performed using pitch control. Moreover, when wind speed exceeds cut-out wind speed, the turbine will be stopped by controlling pitch angle to 901, and when wind speed is under cut-in wind speed, it will be controlled to steady-state operation. The simulation of long-term, short-term, over cut-out wind speed and under cut-out wind speed can be performed by the simulation method effectively. The efficiency of wind power generator, power converter and energy flow are analyzed by the long-term and short-term simulations, and the total harmonic distortion (THD) can be calculated by analysing the results as well. Finally, transient-state of the WPGS can be analyzed by the simulation results of the over cut-out wind speed and under cut-out wind speed, respectively. 2. Wind turbine simulation using real weather conditions 2.1. Wind turbine characteristics The blades convert the wind energy into rotational forces that drive the generator. Although these blades are rigidly attached to the generator, the pitch angle of the blades is variable. The blades change pitch during operation by passively twisting. The blades start at pitch-up position and flatten-out as the turbine speed increased. This makes the pitch angle is being adjusted in such a way to operate the wind turbine at its optimal performance. Hence, the power extracted from the wind turbine is being maximized in a wide variety of wind speed. The wind turbine is characterized by a non-linear curve of performance coefficient Cp as a function of tip speed ratio l. The l is the ratio of linear speed of the tip of blades to the rotational speed of wind turbine. The l can be expressed as in (1). l¼
rom , nm
(1)
where r is the radius of the rotor (m), om the mechanical angular velocity of the rotor and vm the wind speed (m/s). The Cp is also known as power coefficient. The Cp versus l is given in Fig. 2. For the wind turbine used in the study, the Cp is expressed as a function of l. That is, pðl 3Þ C p ¼ ð0:044 0:0167BÞ sin 0:00184ðl 3ÞB, (2) 15 0:3B where B is the pitch angle.
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Cp (power Coefficient)
0.4
0.2
0.0 0
20 λ (Tip Speed Ratio) Fig. 2. Power coefficient versus tip-speed ratio.
The output power and torque of the wind turbine Pwind and Twind can be calculated by the following equations: 1 Arv3m C p ðlÞ 2 1 C p ðlÞ 2 1 vm ¼ rArC T ðlÞv2m , ¼ rAr 2 l 2
Pwind ¼
Pwind 1 v3 ¼ ArC p ðlÞ m , 2 om om
(4)
P ¼ r0 1:194 104 H m , RT
(5)
T wind ¼ r¼
ð3Þ
C p ðlÞ : torque coefficient; l
(6)
r0 ¼ 1:225 kg=m2 ð14:7 psi; 15 CÞ,
(7)
C T ðlÞ ¼
where A is the area swept by the rotor blades (m2), r the air density, Hm the site elevation (m), P the air pressure, T the temperature on the absolute scale and R the gas constant. 2.2. Interface of the real weather conditions It is difficult to imitate a RW condition in the simulation of the WPGS with a transient phenomenon digital simulation tool for the electric power systems such as EMTP/ATP
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109
Hardware : in case of EMTDC Personal computer Fortran Interface
PSCAD EMTDC
A/D converterr +
Power Supply
Software : in case of EMTDC Draft file
Graphic file
Compile
Wind speed
Fortran file
Plot of result
Data file
Run
Interface
Data file
Fig. 3. Simulation concept using real weather conditions.
Vm
Air density Air
Wind speed To Output torque m
Prototype second-order system
Tm
Mechanical angular velocity of the rotor
B Pitch angle
Pitch angle data
Tr
+
Regular torque
-
PRES_WIND3
PI controller
Terr
Fig. 4. Conceptual diagram of a wind turbine simulation.
and PSCAD/EMTDC. Therefore, the reliability is not guaranteed in the simulation for which an external parameter of the weather condition is necessary. RW condition-based simulation of the WPGS is achieved by introducing the interface method of a non-linear external parameter and FORTRAN using the PSCAD/EMTDC in this study. Fig. 3 shows the flow chart of the proposed simulation method considering the non-linear external parameter using the PSCAD/EMTDC. The measured data of wind interfaced to the simulation by the interface method with FORTRAN.
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2.3. Modeling of wind turbine Fig. 4 shows the conceptual diagram of simulation component for a wind turbine in the PSCAD/EMTDC. The real data of wind speed are inserted into the wind turbine characteristic equation, and the obtained output torque Tm is treated as an input torque of generator. Moreover, the inertia of turbine is also considered in the component, and the pitch angle control is conducted by using the prototype second-order system. Using the turbine component proposed, the turbine capacity and the length of blade can also be considered during the simulation, thus research period and cost for the study of WPGS may possibly be reduced. Fig. 5 shows the control algorithm of the pitch angle. By inputting the wind speed and air density, output torque is computed by the characteristic equation of wind turbine. When wind speed exceeds the cut-out wind speed, the turbine will be stopped by Wind speed Air density
Vci< wind speed
No
Yes
T b =0
Wind turbine Characteristics formula
Inertia of wind turbine considered by prototype second order system
Twind > Trated Yes Err = Twind -T rated PI control Pitch control
Vci Vco Tb Twind Trated
: : : : :
Cut in wind speed Cut out wind speed Input torque of generator Output torque of wind turbine Rated output torque Fig. 5. Control algorithm of pitch angle.
No Twind = T b
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111
30
Windspeed
Windspeed [m/s]
27
24
21
18
15
0
20
40
60
80
100
Time [sec]
Fig. 6. The real data of wind speed for 100 s.
Table 1 The characteristic parameters of wind turbine Radius of turbine Rated power Cut-in wind speed Cut-out wind speed Rated wind speed
24 m 750 kW 4 m/s 25 m/s 16 m/s
35
Case 2 Case 3 Case 1 Cut-out wind Turn-on Short term speed section section simulation section
Torque [pu]
1.2
Tout Pitchangle
30
1.0
25
0.8
20
0.6
15
0.4
10
0.2
5
0.0 0
10
20
30
40
50
60
70
80
90
Time [sec] Fig. 7. Pitch angle of blade and output torque of wind turbine.
0 100
Pitch angle [deg]
1.4
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Wind
G SG
AC EG
AC
DC
REC
Inv
ER
Iout
EI
Wind turbine
Fig. 8. Conceptual diagram of WPGS connected to utility network. 900
Pgen 750
Pgen [kW]
600
450
300
150
0 40
60
80
100
Time [sec]
Fig. 9. Output power of generator at long-term simulation.
800
Pout
Pout [kW]
600
400
200
0
0
20
40
60
80
100
Time [sec]
Fig. 10. Output power of inverter at long-term simulation.
controlling pitch angle to 901, otherwise it will be controlled to steady-state operation. Because the inertia of wind turbine cannot be ignored under the active region of wind speed, it should be considered during solution process. Therefore the inertia of wind
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113
100
Igen_a
Igen [A]
50
0
-50
-100 1.90
1.92
1.94
1.96
1.98
2.00
Time [sec]
Fig. 11. Output current of generator (case 1).
Iout_a
100
Iout [A]
50
0
-50
-100 1.90
1.92
1.94
1.96
1.98
2.00
Time [sec]
Fig. 12. Output current of inverter (case 1).
turbine is considered using prototype second-order system. Pitch control governs the output torque with the pitch angle determined by the feedback of error between reference torque and output torque. After all, rotation speed of wind turbine will be within a permitted limit of synchronous generator. 2.4. Simulation of wind turbine Fig. 6 represents the measured data of the real wind speed for 100 s. The components enable the actual data of wind speed and air density to be applied to the parameter of wind turbine. Thus it is possible to use the RW condition in the wind turbine components. The characteristic parameters of a wind turbine are given in Table 1. The output torque of
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8
Eouta 6 4
Eout [kV]
2 0 -2 -4 -6 -8 1.90
1.92
1.94
1.96
1.98
2.00
Time [sec]
Fig. 13. Utility voltage (case 1).
Igen_a Igen_b Igen_c
100
Igen [A]
50
0
-50
-100
1.6
1.8
2.0
2.2
2.4
Time [sec]
Fig. 14. Output current of generator (case 2).
750 kW wind turbine with pitch angle control is shown in Fig. 7. The blade is operated between cut-in and cut-out wind speeds. Fig. 7 depicts that no output torque of the turbine is obtained during over cut-out wind speed. The controlled pitch angle (81/s of ramp rate) is also given in Fig. 7. The pitch angle shows a very active movement to control the output torque constant in the high-wind-speed region. The pitch angle follows the trend of wind speed. As the wind speed increases, the average pitch angle also increases. Similarly, when the wind speed decreases, the average pitch angle also decreases. Cases for long-term, short-term, over cut-out and under cut-out wind speeds are simulated, respectively. The long-term case is simulated for 100 s, for the short-term, over cut-out and under cut-out wind speeds are simulated and termed as cases 1, 2 and 3 (turnon section) in Fig. 7, respectively.
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6.7
Eout_a
Eout_b
Eout_c
6.6
Eout [kV]
6.5
V 6.4
6.3
6.2 2.05
2.10
2.15
2.20
Time [sec]
Fig. 15. Variation of system voltage for short-term simulation (case 2).
Igen_a
100
Igen_b
Igen_c
Igen [A]
50
0
-50
-100 5.8
6.0
6.2
6.4
Time [sec]
Fig. 16. Output current of generator (case 3).
3. Simulation results Fig. 8 presents the conceptual diagram of the WPGS connected to utility network. Utility interactive WPGS is simulated and analyzed using the proposed RW–WPGS simulation method in this work. Long-term, short-term and the case of over cut-out, under cut-out wind speeds of the utility interactive WPGS are simulated. Figs. 9 and 10 represent the output power of generator (Pgen) and inverter (Pout) for the long-term simulation. The data given in Fig. 6 are used as input wind speed for the simulation. Fig. 11 shows the output current of generator (Igen), which is distorted by rectifier and reactor. Figs. 12 and 13 depict the output current of inverter (Iout) and the utility voltage (Eout), respectively. The output current of generator for the case 2 is given in Fig. 14. When wind speed exceeds the cut-out wind speed, wind turbine will be stopped by the rapid increased pitch angle of blade to protect the wind turbine and generator. As a result, the inverter output
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116 7.0
Eout_a
Eout_b
Eout_c
6.9
6.8
Eout [kV]
V 6.7
6.6
6.5
6.4
6.3 6.10
6.12
6.14
6.16
6.18
6.20
Time [sec]
Fig. 17. Variation of system voltage for short-term simulation (case 3).
will be decreased, and the system voltage is slightly dropped affected by the phenomenon as shown in Fig. 15. Fig. 16 represents the generator output current of the case 3. Pitch angle of blade will be changed until the output of wind turbine reaches a rated value by the pitch control. Inverter output will be increased, and the system voltage will also be risen as in Fig. 17. In case 1, the output of turbine, generator and power converter system are 750, 680 and 630 kW, respectively. Thus, the efficiency of wind power generator and power converter are 90% and 93%. The THD is very low (about 0.9%) because a current control voltage source type of inverter has been used in this study case, which means that the inverter causes little THD due to the rapid following of output current to the reference current. 4. Conclusion This paper proposes a novel simulation method of WPGS using PSCAD/EMTDC. Details of the dynamic simulation models and the transient results are presented, and the simulation of the WPGS using the real weather conditions was achieved by introducing the interface method of non-linear external parameters and FORTRAN using the PSCAD/ EMTDC. The components of wind turbine system have been modeled by the proposed scheme. The real weather condition-based output torque of wind turbine and the output power of generator are calculated. The efficiency of wind power generator, power converter and flow of energy are analyzed by wind speed of the long-term simulation. The generator output power and current supplied into utility are obtained through the short-term simulation, and THD could be calculated by analyzing the results as well. The transient-state of the WPGS can also be analyzed by the simulation results of the over cut-out wind speed and under cut-out wind speed, respectively. Those phenomenon possibly occurred in the WPGS may readily be analyzed by the simulation of long-term, cases 1, 2 and 3, respectively. Especially, it is possible to analyze the transient phenomenon considering various conditions of utility systems. The outcomes of the simulation demonstrate the effectiveness of the proposed simulation scheme.
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Acknowledgements This work was supported in part by EESRI (02340-17), which is funded by Ministry of commerce, industry and energy (MOCIE), and in part by Korea Energy Management Cooperation (KEMCO). References [1] Patel MR. Wind and solar power systems. New York: CRC Press; 1999. p. 35–69. [2] Belhomme R. Wind power developments in France. IEEE Power Eng Rev 2002:21–4. [3] Borowy BS, Salamech ZM. Dynamic response of stand-alone wind energy conversion system with battery energy storage to a wind gust. IEEE Trans Energy Convers 1997;12(1):73–8. [4] Park M. A novel simulation method for PV power generation system using real field weather condition and its application. Trans IEE Jpn 2001;121-B(1):1499–505. [5] Murdoch A, Winkelman JR, Javid SH. Control design and performance analysis of a 6 MW wind turbinegenerator. IEEE Trans PAS 1983;102(5):1340–7. [6] [PSCAD/EMTDC] Power system simulation software manual. Monitoba HVDC Research Center; 1995. [7] Benjamin CK. Automatic control systems. Englewood Cliffs, NJ: Prentice-Hall; 1993. p. 340–51.
Renewable Energy 32 (2007) 105–117 www.elsevier.com/locate/renene
PSCAD/EMTDC-based simulation of wind power generation system S.G. Hana, I.K. Yua,, M. Parkb a
Department of Electrical Engineering, Changwon National University, 9 Sarim-dong, Changwon 641-773, Korea b Center for Applied Superconductivity Technology, 28-1 Seongju-dong, Changwon 641-120, Korea Received 20 January 2004; accepted 14 December 2005 Available online 7 February 2006
Abstract This paper proposes a novel simulation method of wind power generation system (WPGS) using PSCAD/EMTDC. The pitch control-based rotation speed control scheme of turbine under variable wind speed is implemented. For the purpose of achieving effective and user-friendly simulation method for utility interactive (grid connected) WPGS, real weather condition-based WPGS simulation (RW–WPGS) is performed using PSCAD/EMTDC. It is not easy, in general, to consider the RW conditions in the WPGS simulation using the EMTP or PSPICE type of simulators. External parameters of the RW conditions, however, are necessary to improve the simulation accuracy. The components modeling of wind turbine system is also studied and the real weather conditions are introduced by the interface method of a non-linear external parameter of the PSCAD/EMTDC. The outcomes of the simulation demonstrate the effectiveness of the proposed simulation scheme. r 2006 Elsevier Ltd. All rights reserved. Keywords: Pitch control; PSCAD/EMDC; Real weather condition; Wind power generation
1. Introduction In recent years, there has been growing interest in renewable energy systems due to the environmental problem and the economic benefits from fossil fuel savings. Such systems are usually connected to the existing power grid for fuel displacement purposes as well as of earning some capacity credit. Wind Power Generation System (WPGS) is one of the Corresponding author. Fax: +82 55 263 9956.
E-mail address: [email protected] (I.K. Yu). 0960-1481/$ - see front matter r 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.renene.2005.12.008
ARTICLE IN PRESS 106
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most useful energy resources using natural environment. The WPGS production is undoubtedly accompanied with minimization of environmental pollution, reduction of losses in power system transmission and distribution equipments, and supports the utility in demand side management [1,2]. The simulation schemes that can be applied to the utility interactive WPGS readily under various conditions considering the sort of wind turbine generators, the system capacity and the converter system as well are strongly expected and emphasized among researchers. The generation of electric power by wind turbine generators depends on the wind power source which changes with time and position, it is, therefore, necessary to have a proper control strategy to maintain a rated value of the WPGS and the angular velocity regardless of the wind speed changes [3]. This work is to study the behavior of the wind turbine generator operated under variable speed with pitch-control capability under turbulent winds. The control algorithm is chosen based on the constant output torque of wind turbine. Pitch control is relatively fast and can be used to limit the rotor speed by regulating input aerodynamic power flow, especially, near the high-speed limit. The utility interactive WPGS is simulated using the real weather condition-based WPGS simulation (RW–WPGS) in this paper. Fig. 1 shows the conceptual diagram of the RW–WPGS. Three important topics, which have been emphasized by the researchers related with simulation study of the WPGS, are user-friendly method, applicability to gridconnected systems and the RW conditions included [4,5]. As shown in Fig. 1, the RW–WPGS could be a peculiar simulation method to realize the requirements of researchers mentioned above. This paper presents a computer simulation model of a unified wind turbine–synchronous generator interface scheme using PSCAD/EMTDC [6]. Details of the dynamic simulation models and the transient results are presented. It is difficult to imitate an actual weather
User-friendly (Graphic User Interface)
EMTP/ATP PSCAD/EMTDC
Applicability for Grid-connection
User (Researchers for WPGS)
Applicability for Grid-connection
Closer to reality
Using the empirical formula of a wind turbine
Using the real weather conditions
Fig. 1. Conceptual diagram of the RW–WPGS.
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107
condition in the simulation of the WPGS with a transient phenomenon digital simulation tool for the electric power systems such as EMTP type of simulators and PSPICE, etc. The reliability is not guaranteed in the simulation for which an external parameter of the weather condition is necessary [7]. Thus, the simulation of the WPGS using the RW condition was achieved by introducing the interface method of non-linear external parameters and FORTRAN by using PSCAD/EMTDC in this paper. The components of wind turbine system have been modeled by the proposed scheme. The output torque of wind turbine and the output power of generator are calculated by using the real weather conditions. Speed control of turbine under variable wind speed is performed using pitch control. Moreover, when wind speed exceeds cut-out wind speed, the turbine will be stopped by controlling pitch angle to 901, and when wind speed is under cut-in wind speed, it will be controlled to steady-state operation. The simulation of long-term, short-term, over cut-out wind speed and under cut-out wind speed can be performed by the simulation method effectively. The efficiency of wind power generator, power converter and energy flow are analyzed by the long-term and short-term simulations, and the total harmonic distortion (THD) can be calculated by analysing the results as well. Finally, transient-state of the WPGS can be analyzed by the simulation results of the over cut-out wind speed and under cut-out wind speed, respectively. 2. Wind turbine simulation using real weather conditions 2.1. Wind turbine characteristics The blades convert the wind energy into rotational forces that drive the generator. Although these blades are rigidly attached to the generator, the pitch angle of the blades is variable. The blades change pitch during operation by passively twisting. The blades start at pitch-up position and flatten-out as the turbine speed increased. This makes the pitch angle is being adjusted in such a way to operate the wind turbine at its optimal performance. Hence, the power extracted from the wind turbine is being maximized in a wide variety of wind speed. The wind turbine is characterized by a non-linear curve of performance coefficient Cp as a function of tip speed ratio l. The l is the ratio of linear speed of the tip of blades to the rotational speed of wind turbine. The l can be expressed as in (1). l¼
rom , nm
(1)
where r is the radius of the rotor (m), om the mechanical angular velocity of the rotor and vm the wind speed (m/s). The Cp is also known as power coefficient. The Cp versus l is given in Fig. 2. For the wind turbine used in the study, the Cp is expressed as a function of l. That is, pðl 3Þ C p ¼ ð0:044 0:0167BÞ sin 0:00184ðl 3ÞB, (2) 15 0:3B where B is the pitch angle.
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108
Cp (power Coefficient)
0.4
0.2
0.0 0
20 λ (Tip Speed Ratio) Fig. 2. Power coefficient versus tip-speed ratio.
The output power and torque of the wind turbine Pwind and Twind can be calculated by the following equations: 1 Arv3m C p ðlÞ 2 1 C p ðlÞ 2 1 vm ¼ rArC T ðlÞv2m , ¼ rAr 2 l 2
Pwind ¼
Pwind 1 v3 ¼ ArC p ðlÞ m , 2 om om
(4)
P ¼ r0 1:194 104 H m , RT
(5)
T wind ¼ r¼
ð3Þ
C p ðlÞ : torque coefficient; l
(6)
r0 ¼ 1:225 kg=m2 ð14:7 psi; 15 CÞ,
(7)
C T ðlÞ ¼
where A is the area swept by the rotor blades (m2), r the air density, Hm the site elevation (m), P the air pressure, T the temperature on the absolute scale and R the gas constant. 2.2. Interface of the real weather conditions It is difficult to imitate a RW condition in the simulation of the WPGS with a transient phenomenon digital simulation tool for the electric power systems such as EMTP/ATP
ARTICLE IN PRESS S.G. Han et al. / Renewable Energy 32 (2007) 105–117
109
Hardware : in case of EMTDC Personal computer Fortran Interface
PSCAD EMTDC
A/D converterr +
Power Supply
Software : in case of EMTDC Draft file
Graphic file
Compile
Wind speed
Fortran file
Plot of result
Data file
Run
Interface
Data file
Fig. 3. Simulation concept using real weather conditions.
Vm
Air density Air
Wind speed To Output torque m
Prototype second-order system
Tm
Mechanical angular velocity of the rotor
B Pitch angle
Pitch angle data
Tr
+
Regular torque
-
PRES_WIND3
PI controller
Terr
Fig. 4. Conceptual diagram of a wind turbine simulation.
and PSCAD/EMTDC. Therefore, the reliability is not guaranteed in the simulation for which an external parameter of the weather condition is necessary. RW condition-based simulation of the WPGS is achieved by introducing the interface method of a non-linear external parameter and FORTRAN using the PSCAD/EMTDC in this study. Fig. 3 shows the flow chart of the proposed simulation method considering the non-linear external parameter using the PSCAD/EMTDC. The measured data of wind interfaced to the simulation by the interface method with FORTRAN.
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2.3. Modeling of wind turbine Fig. 4 shows the conceptual diagram of simulation component for a wind turbine in the PSCAD/EMTDC. The real data of wind speed are inserted into the wind turbine characteristic equation, and the obtained output torque Tm is treated as an input torque of generator. Moreover, the inertia of turbine is also considered in the component, and the pitch angle control is conducted by using the prototype second-order system. Using the turbine component proposed, the turbine capacity and the length of blade can also be considered during the simulation, thus research period and cost for the study of WPGS may possibly be reduced. Fig. 5 shows the control algorithm of the pitch angle. By inputting the wind speed and air density, output torque is computed by the characteristic equation of wind turbine. When wind speed exceeds the cut-out wind speed, the turbine will be stopped by Wind speed Air density
Vci< wind speed
No
Yes
T b =0
Wind turbine Characteristics formula
Inertia of wind turbine considered by prototype second order system
Twind > Trated Yes Err = Twind -T rated PI control Pitch control
Vci Vco Tb Twind Trated
: : : : :
Cut in wind speed Cut out wind speed Input torque of generator Output torque of wind turbine Rated output torque Fig. 5. Control algorithm of pitch angle.
No Twind = T b
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111
30
Windspeed
Windspeed [m/s]
27
24
21
18
15
0
20
40
60
80
100
Time [sec]
Fig. 6. The real data of wind speed for 100 s.
Table 1 The characteristic parameters of wind turbine Radius of turbine Rated power Cut-in wind speed Cut-out wind speed Rated wind speed
24 m 750 kW 4 m/s 25 m/s 16 m/s
35
Case 2 Case 3 Case 1 Cut-out wind Turn-on Short term speed section section simulation section
Torque [pu]
1.2
Tout Pitchangle
30
1.0
25
0.8
20
0.6
15
0.4
10
0.2
5
0.0 0
10
20
30
40
50
60
70
80
90
Time [sec] Fig. 7. Pitch angle of blade and output torque of wind turbine.
0 100
Pitch angle [deg]
1.4
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Wind
G SG
AC EG
AC
DC
REC
Inv
ER
Iout
EI
Wind turbine
Fig. 8. Conceptual diagram of WPGS connected to utility network. 900
Pgen 750
Pgen [kW]
600
450
300
150
0 40
60
80
100
Time [sec]
Fig. 9. Output power of generator at long-term simulation.
800
Pout
Pout [kW]
600
400
200
0
0
20
40
60
80
100
Time [sec]
Fig. 10. Output power of inverter at long-term simulation.
controlling pitch angle to 901, otherwise it will be controlled to steady-state operation. Because the inertia of wind turbine cannot be ignored under the active region of wind speed, it should be considered during solution process. Therefore the inertia of wind
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100
Igen_a
Igen [A]
50
0
-50
-100 1.90
1.92
1.94
1.96
1.98
2.00
Time [sec]
Fig. 11. Output current of generator (case 1).
Iout_a
100
Iout [A]
50
0
-50
-100 1.90
1.92
1.94
1.96
1.98
2.00
Time [sec]
Fig. 12. Output current of inverter (case 1).
turbine is considered using prototype second-order system. Pitch control governs the output torque with the pitch angle determined by the feedback of error between reference torque and output torque. After all, rotation speed of wind turbine will be within a permitted limit of synchronous generator. 2.4. Simulation of wind turbine Fig. 6 represents the measured data of the real wind speed for 100 s. The components enable the actual data of wind speed and air density to be applied to the parameter of wind turbine. Thus it is possible to use the RW condition in the wind turbine components. The characteristic parameters of a wind turbine are given in Table 1. The output torque of
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8
Eouta 6 4
Eout [kV]
2 0 -2 -4 -6 -8 1.90
1.92
1.94
1.96
1.98
2.00
Time [sec]
Fig. 13. Utility voltage (case 1).
Igen_a Igen_b Igen_c
100
Igen [A]
50
0
-50
-100
1.6
1.8
2.0
2.2
2.4
Time [sec]
Fig. 14. Output current of generator (case 2).
750 kW wind turbine with pitch angle control is shown in Fig. 7. The blade is operated between cut-in and cut-out wind speeds. Fig. 7 depicts that no output torque of the turbine is obtained during over cut-out wind speed. The controlled pitch angle (81/s of ramp rate) is also given in Fig. 7. The pitch angle shows a very active movement to control the output torque constant in the high-wind-speed region. The pitch angle follows the trend of wind speed. As the wind speed increases, the average pitch angle also increases. Similarly, when the wind speed decreases, the average pitch angle also decreases. Cases for long-term, short-term, over cut-out and under cut-out wind speeds are simulated, respectively. The long-term case is simulated for 100 s, for the short-term, over cut-out and under cut-out wind speeds are simulated and termed as cases 1, 2 and 3 (turnon section) in Fig. 7, respectively.
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6.7
Eout_a
Eout_b
Eout_c
6.6
Eout [kV]
6.5
V 6.4
6.3
6.2 2.05
2.10
2.15
2.20
Time [sec]
Fig. 15. Variation of system voltage for short-term simulation (case 2).
Igen_a
100
Igen_b
Igen_c
Igen [A]
50
0
-50
-100 5.8
6.0
6.2
6.4
Time [sec]
Fig. 16. Output current of generator (case 3).
3. Simulation results Fig. 8 presents the conceptual diagram of the WPGS connected to utility network. Utility interactive WPGS is simulated and analyzed using the proposed RW–WPGS simulation method in this work. Long-term, short-term and the case of over cut-out, under cut-out wind speeds of the utility interactive WPGS are simulated. Figs. 9 and 10 represent the output power of generator (Pgen) and inverter (Pout) for the long-term simulation. The data given in Fig. 6 are used as input wind speed for the simulation. Fig. 11 shows the output current of generator (Igen), which is distorted by rectifier and reactor. Figs. 12 and 13 depict the output current of inverter (Iout) and the utility voltage (Eout), respectively. The output current of generator for the case 2 is given in Fig. 14. When wind speed exceeds the cut-out wind speed, wind turbine will be stopped by the rapid increased pitch angle of blade to protect the wind turbine and generator. As a result, the inverter output
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Eout_a
Eout_b
Eout_c
6.9
6.8
Eout [kV]
V 6.7
6.6
6.5
6.4
6.3 6.10
6.12
6.14
6.16
6.18
6.20
Time [sec]
Fig. 17. Variation of system voltage for short-term simulation (case 3).
will be decreased, and the system voltage is slightly dropped affected by the phenomenon as shown in Fig. 15. Fig. 16 represents the generator output current of the case 3. Pitch angle of blade will be changed until the output of wind turbine reaches a rated value by the pitch control. Inverter output will be increased, and the system voltage will also be risen as in Fig. 17. In case 1, the output of turbine, generator and power converter system are 750, 680 and 630 kW, respectively. Thus, the efficiency of wind power generator and power converter are 90% and 93%. The THD is very low (about 0.9%) because a current control voltage source type of inverter has been used in this study case, which means that the inverter causes little THD due to the rapid following of output current to the reference current. 4. Conclusion This paper proposes a novel simulation method of WPGS using PSCAD/EMTDC. Details of the dynamic simulation models and the transient results are presented, and the simulation of the WPGS using the real weather conditions was achieved by introducing the interface method of non-linear external parameters and FORTRAN using the PSCAD/ EMTDC. The components of wind turbine system have been modeled by the proposed scheme. The real weather condition-based output torque of wind turbine and the output power of generator are calculated. The efficiency of wind power generator, power converter and flow of energy are analyzed by wind speed of the long-term simulation. The generator output power and current supplied into utility are obtained through the short-term simulation, and THD could be calculated by analyzing the results as well. The transient-state of the WPGS can also be analyzed by the simulation results of the over cut-out wind speed and under cut-out wind speed, respectively. Those phenomenon possibly occurred in the WPGS may readily be analyzed by the simulation of long-term, cases 1, 2 and 3, respectively. Especially, it is possible to analyze the transient phenomenon considering various conditions of utility systems. The outcomes of the simulation demonstrate the effectiveness of the proposed simulation scheme.
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Acknowledgements This work was supported in part by EESRI (02340-17), which is funded by Ministry of commerce, industry and energy (MOCIE), and in part by Korea Energy Management Cooperation (KEMCO). References [1] Patel MR. Wind and solar power systems. New York: CRC Press; 1999. p. 35–69. [2] Belhomme R. Wind power developments in France. IEEE Power Eng Rev 2002:21–4. [3] Borowy BS, Salamech ZM. Dynamic response of stand-alone wind energy conversion system with battery energy storage to a wind gust. IEEE Trans Energy Convers 1997;12(1):73–8. [4] Park M. A novel simulation method for PV power generation system using real field weather condition and its application. Trans IEE Jpn 2001;121-B(1):1499–505. [5] Murdoch A, Winkelman JR, Javid SH. Control design and performance analysis of a 6 MW wind turbinegenerator. IEEE Trans PAS 1983;102(5):1340–7. [6] [PSCAD/EMTDC] Power system simulation software manual. Monitoba HVDC Research Center; 1995. [7] Benjamin CK. Automatic control systems. Englewood Cliffs, NJ: Prentice-Hall; 1993. p. 340–51.