- Email: [email protected]
Current Control of the Isolated Self-Excited Induction Generator using Shunt Active Filter
Available online at www.sciencedirect.com
Energy Procedia 18 (2012) 349 – 358
Current Control of the Isolated Self-Excited Induction Generator using Shunt Active Filter A.M. BOUZIDa, M. BENGHANEMb, B.HAMANEc, A.BELABBESd, M.BOUHAMIDAe,A.DRAOUf,a* a,b,c,d,e
LDDE laboratory members, Université Mohamed Boudiaf USTO 1505Bp El Mnaouer,Oran 31000, Algeria f,
Senior MIEEE, Department of Electrical Engineering,Hail University,Hail,Saudi Arabia
Abstract The Self Excited Induction Generator (SEIG) is an isolated power source whose terminal voltage and frequency are controlled by the excitation of the capacitance or the load impedance. This paper presents a method for calculating the minimum excitation capacitance using the equivalent circuit approach for analyzing the steady state operation of SEIG. A new strategy based on an active power filter (APF) for controlling the current and power quality of the self excited induction generator (SEIG) is also presented in this paper. The shunt active power filter was implemented using a three phase PWM current controlled voltage source inverter (VSI) and connected to the wind generator and loads in order to compensate the current harmonics and reactive power. The PWM-VSI gate control signals are derived from hysteresis band current controller. The proposed active filter proved to play an important role and give good dynamic response and robust behavior upon changes in load parameters. This investigation demonstrated that power average control strategy can facilitate the improvement of the power quality. The proposed control method extracts fundamental (reference) components of the source current for the shunt active power line conditioners for nonlinear and unbalanced loads. The Power average approach additionally maintains the voltage of the capacitor (of the PWM inverter) nearly constant without any external control circuit. The shunt APF in conjunction with the proposed controller performs perfectly under different steady state and transient conditions. The simulation results with nonlinear loads and unbalanced loads have showed the effectiveness of the proposed scheme for harmonic reduction in Wind based Power Generation.
© 2012 Ltd. Selection and/or peerpeer-review review underunder responsibility of TheofTerraGreen Society. 2010Published PublishedbybyElsevier Elsevier Ltd. Selection and/or responsibility [name organizer] Keywords: Shunt Active Filter, Self Excited Induction Generator (SEIG), Wind Energy, PWM Inverter, Renewable Energy.
* Corresponding author. Tel.:+213-550-023-222; fax:+213-41-560-328. E-mail address: [email protected] (A.M BOUZID) / [email protected] (M.BENGHANEM) [email protected] (A.DRAOU).
1876-6102 © 2012 Published by Elsevier Ltd. Selection and/or peer review under responsibility of The TerraGreen Society. doi:10.1016/j.egypro.2012.05.046
350
A.M. Bouzid et al. / Energy Procedia 18 (2012) 349 – 358
1. Introduction
I
n recent years, the Self-Excited Induction Generator (SEIG) has emerged as the best electromechanical energy converter to replace the conventional synchronous generator in isolated power generators driven by renewable energy resources: biogas, micro-hydroelectric, wind etc. The main advantages of the SEIG are: low cost, ruggedness, absence of a separate DC source for excitation brushless rotor construction (squirrel cage construction) and ease of maintenance. The fundamental problem with using the SEIG was its inability to control the terminal voltage and frequency under varying load conditions. The analysis of the SEIG under steady-state conditions and imposed speed is already known [1]-[2]; however there are few papers about transient state operation. Active Power Filters (APF) are often used in applications where low current harmonics are desirable and/or improvement of quality of energy taken from the power grid are needed. With the use of APF, it is possible to draw near perfect sinusoidal currents and voltages from the grid or renewable distributed power sources. Moreover, it will be possible to balance load currents in different phases which itself is important in stand-alone power generation like wind turbines as for the case of unsymmetrical load currents, it could lead to torque pulsation in generator’s shaft and a decrease of reliability. With the use of APF it is also possible to control reactive power and keep unity power factor that is why they are mainly used in industry where DC current is needed e.g. aluminum plants, train power substations, arc welders. The currents taken by household and office consumers have usually high harmonic contents which is related to an increased number of non-linear loads such as rectifiers and capacitors, where the current is drawn at the peak of sinusoidal voltage. At last, it can be said the APF could be used to prevent any kind of harmonic generation (computer’s power supply, energy savings lamp, etc.), to reduce: harmonic contents in the grid, peak value of the current drawn from the grid, the inrush current taken from the grid, and to compensate the neutral line current, and correct the active power factor correction, and thus transformers will not be necessary [3]. 2. Description of the proposed system A schematic of the proposed system is shown in Fig. 1. It consists of a three phase star-connected induction generator driven by an uncontrolled micro hydroelectric turbine. The generator is operated as an load [4]. When SEIG supplies a non-linear load, the load draws a fundamental component of current and harmonic current from the generation systems, which are to be properly controlled. The shunt APF can compensate the harmonic current by continuously tracking the changes in harmonic content. APF’s consists of a voltage fed converter with a PWM current controller and an active filter controller that realizes an almost instantaneous control algorithm shown in Fig.1. As the input power is nearly constant, the output power of the SEIG must be held constant at all consumer loads. Any decrease in load may accelerate the machine and raise the voltage and frequency levels to prohibitively high values, resulting in large stresses on other connected loads.
SEIG Variable load V PWM Fig. 1. Block diagram of the APF with SEIG
APF Controller
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A.M. Bouzid et al. / Energy Procedia 18 (2012) 349 – 358
3. Modeling of the SEIG The dynamic model of the three-phase squirrel cage induction generator is developed by using a stationary d-q axes references frame [5] and the relevant volt ampere equations are as follows: [ ] = [ ][ ] + [ ] [ ] + [ ][ ] (1) Thus, the current derivative can be expressed as: [ ] = [ ] {[ ] [ ][ ] [ ][ ]}
(2)
Where,[ ], [ ] , [ ] , [ ] and [ ] are defined in the Appendix (Section 9). The SEIG operates in the saturation region and its magnetization characteristics are non-linear in nature. Thus, the magnetizing current should be calculated at every step of integration in terms of stator and rotor currents as in [6]: =
(
+
) +
+
(3)
Magnetizing inductance Lm (H)
against , Magnetizing inductance is calculated from the magnetization characteristics plotted as and is obtained by as shown in Fig. 2 for the machine under test. The relation between synchronous speed test. 0.1 0.08 0.06 0.04 0.02
0
5
10 15 20 Magnetizing current Im (A)
25
Fig. 2. Variation of magnetizing inductance as a function of magnetizing current
The developed electromagnetic torque of the SEIG is: = (3 /4)
(4)
The torque balance equation is: = + (2/ )
(5)
The derivative of the rotor speed from (4) is: = (2/ ) /
(6)
4. Process of self-excitation The process of self-excitation can be compared with the resonance phenomenon in an RLC circuit (Elder et al., 1984), (Grantham et al., 1989). In whose transient solution is of the exponential form the solution, K is a constant, and root 1 is a complex quantity, whose real part represents the rate at which the transient decays, and the imaginary part is proportional to the frequency of oscillation. In real circuits, the real part of 1 is negative, meaning that the transient vanishes with time. With the real part
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A.M. Bouzid et al. / Energy Procedia 18 (2012) 349 – 358
of p1 positive, the transient (voltage) build-up continues until it reaches a stable value with saturation of iron circuit. In other terms, the effect of this saturation is to modify the magnetization reactance , such that the real part of the root 1 becomes zero in which case the response is sinusoidal steady-state corresponding to continuous self-excitation of SEIG. Any current (resulting from the voltage) flowing in a circuit dissipates power in the circuit resistance, and an increasing current dissipates increasing power, which implies some energy source is available to supply the power. The energy source, referred to above is provided by the kinetic energy of the rotor (Grantham et al., 1989). With time varying loads, new steady-state value of the voltage is determined by the self-excitation capacitance value, rotor speed and load [7] [8] [9]. The formula for calculation of minimum capacitance is: =1 (7) 5. Reference current generation using average power method The average power method gives accurate results even if the current is distorted. A PLL based unit vector template is used to obtain fundamental component of mains voltage. To get unit vector templates of voltage, the input voltage is sensed and multiplied by a gain equal to 1/vpk where vpk is the peak amplitude of fundamental supply voltage. These unit vectors are then passed through a PLL for synchronization of signals. Three phase fundamental components are multiplied by vpk to get fundamental mains voltage. The Power average method needs reduced calculation, since it works directly with abcphase voltage and line currents. The elimination of the Clark transformation makes this control strategy simple [10] [12]. The Power average method presents a minimum rms value to draw the same three phase average active power from the source as the original load current. The control strategy principle for the shunt active power filter based on three-level inverter is illustrated in Fig.3.
Compute Average Power
Compute source reference currents
Compute command shunt APF currents
Hysteresis based Current Controller
Gating signals Fig. 3. Block diagram of the proposed shunt active power filter control scheme
6. Analysis and modeling The three phase instantaneous source current can be written as ( )= ( ) ( )
(8)
The instantaneous source voltage is given by ( )= sin
(9)
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A.M. Bouzid et al. / Energy Procedia 18 (2012) 349 – 358
If a nonlinear load is applied, then the load current will have a fundamental component and harmonic components, which can be written as ( )=
(
)=
+
(
+
)+
(
+
)
(10)
The reduction of current harmonics in the load current is achieved by injecting equal but opposite current harmonic components at the point of common coupling, thus cancelling the original distortion and improving the power quality. The system comprises an ac source, non-linear load, unbalanced load, the APF and the new control scheme. The components of the system are analyzed separately and integrated to develop the complete model for the simulation [13]. 6.1. Computation of the average power The sensed load currents ( , , ) and bus voltages( the instantaneous power as given by ( )=
( )
( )+
( )
( )+
( )
,
,
) through PLL are used to derive
( )
The three phase instantaneous reactive power in each phase becomes [12]: = = =
(11)
(12)
The instantaneous active and reactive power delivered to a nonlinear load must satisfy (12) and (13). = + = + (13) = , = , , (14) Where - Instantaneous active power supplied by the source - Instantaneous active power supplied by the APF - Instantaneous active fundamental power of the load - Instantaneous harmonic power of the load - Instantaneous reactive power generated by the APF at phase k. In order to ensure that the fundamental active power is supplied to the load from the source, the instantaneous reactive power and harmonic power must be compensated by the APF. When considering is expressed as: the compensation of both harmonic and reactive power, ( )=
( )
( )+
( )
( )+
( )
6.2. Computation of source reference currents
( ) ,
(15)
,
From (14) and (15), the reference compensating currents are determined as:
= = =
(16)
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A.M. Bouzid et al. / Energy Procedia 18 (2012) 349 – 358
6.3. Computation of the command shunt APF currents
,
,
Finally the desired 3-phase references of the APF currents( , , ) are computed by taking the difference between the three phase instantaneous reference source currents ( , , ) and the actual source currents( , , ) as below: = = (17) = 6.4. Hysteresis current controller The actual source currents are monitored instantaneously, and then compared to the reference currents generated by the proposed algorithm. In order to get accurate and instantaneous control, switching of the IGBT devices should be such that the error signal approaches zero, thus providing quick response. For this reason, hysteresis current controller with fixed band is used to derive the switching signals of the three phase IGBT based VSI bridge. The upper device and the lower device in one phase leg of VSI are switched in complementary manner otherwise a dead short circuit will be take place [17]. The APF reference currents ( , , ) , compared with, the sensed source currents ( , , ) , and the error signals are operated by the hysteresis current controller to generate the firing pulses which activate the inverter power switches in a manner that reduces the current error. The switching logic for ‘phase-a’ is ), then upper switch is OFF and lower switch is ON in the phase formulated as follows: If <( ), then upper switch is OFF and lower switch is ON in the phase ‘a’ leg then (SA=1). If >( ‘a’ leg then (SA=0). In the same fashion, the switching of phase-b and c devices can be derived using as the width of hysteresis band. The switching functions SB and SC for phases b and c are determined in a similar manner. 7. Simulation results and analysis The performance of the proposed control strategy is evaluated through simulation using SIMULINK toolbox in the MATLAB. The system parameters values are: Line to line source voltage is 380 V; System frequency (f) is 50 Hz; Source impedance of RS, LS Rc, Lc L, LL respectively; DC voltage (VDC) is 500V;
= 1100
; Power devices used are IGBT/Diode.
200
20
Vds
C ur rent Ids ( A)
Voltage Vds (V)
7.1. Performance of self excited induction generator
0 -200
Ids
0
-20 0
0.2
0.4
0.6
0.8
1
0
0.2
Time (s)
Fig. 4. Simulation results of stator voltage and current of the SEIG with saturation
0.4
0.6 Time (s)
0.8
1
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A.M. Bouzid et al. / Energy Procedia 18 (2012) 349 – 358
=1500 rpm, the generated voltage and current attain their steady state values of 380 Volts and 19 A in 0.8 sec as shown in Fig. 4. 7.2. Shunt active power filter system performance
Ia-rectifier
Iabc-load(A)
The computer simulation results are provided to verify the effectiveness of the proposed control scheme. The unbalanced load RL current before compensation is shown in Fig 5(a) and the six-pulse diode rectifier RL load current or source current before compensation is shown in Fig 5(b). 2 5
0
-5
0
0.05 Time (s)
0
-2
0.1
0
(a)Unbalanced load currents;
0.05 Time (s)
0.1
(b) Non linear load current
Fig. 5. (a) Simulation results of the load currents
5 IFabc (A)
Isabc (A)
5 0 -5 0
0.05 Time (s)
0
-5
0.1
0
(a)Load currents or source current before compensation ;
0.02
0.04 0.06 Time (s)
0.08
(b) Reference currents before APF
Fig. 6. (a) Simulation results of source and reference currents
2 Isabc(A)
IFabc*(A)
5
0
-5
0
0.05 Time (s)
0.1
(a) Reference currents by the power average control algorithm Fig. 7. Simulation results of source and reference currents
0 -2 0
0.05 Time (s)
0.1
(b) Currents source after compensation
0.1
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A.M. Bouzid et al. / Energy Procedia 18 (2012) 349 – 358
Fig. 6(a) shows the simulated results of the load currents. The harmonic currents of a nonlinear load and unbalanced load are compensated by the shunt active power filter. The actual reference currents for the three phases are shown in Fig. 6(b). This waveform is obtained from the proposed average power controller. The source current after compensation is illustrated in Fig. 7(b) which indicates that the current becomes sinusoidal. After active filter operation the AC-source current only supplies the active fundamental current to the load. The shunt APLC supplies the compensating current that is shown in Fig. 7(a). The current after compensation shown in Fig. 7(b) would have taken a shape as shown in Fig. 6(b) without APF. It is clearly visible that this waveform is sinusoidal with some high frequency ripples. 7.3. Measurement of the total harmonic distortion measured The total harmonic distortion is measured using the source current waveform and presented in Table 1 both with and without APLC. Table 1. Total harmonic distortion (THD %) of source current
Condition THD Steady state
Source current (Is) Without APLC 23.08%
Source current (Is)With APLC 2.01%
The FFT analysis that was carried out confirms that the active filter brings the THD of the source current down to less than 5% which is in compliance with IEEE-519 standards for harmonics. In general, the THD values for both current and voltage in advanced aircraft electric power system in presence of APF are lower than those for conventional aircraft system [18]. 8. Conclusion This paper has presented the implementation of a cage-rotor IG system completely isolated from the utility grid, in order to supply rural sites or isolated areas. It has been demonstrated that the system is able to feed resistive and inductive loads with regulated current and satisfactory energy quality. In this paper we also discussed the problem of terminal current stabilization of the self excited induction generator (SEIG) in standalone mode from which a new method of stabilization of the current is used to improve the performance characteristics of the SEIG. This investigation demonstrated also that the generalized Power average control strategy can facilitate the improvement of the power quality. Simulation results are included in order to validate the proposed control technique. The shunt APF has been implemented with a three phase PWM current controlled voltage source inverter and is connected to the AC mains in order to compensate the current harmonics and reactive power. It has been shown that the Power average approach additionally maintains the voltage of the capacitor (of the PWM inverter) nearly constant without any external control circuit. Different types of linear and non linear loads for reactive power and current harmonics compensation have been connected to the APF to analyse the steady-state and transient performance of the system. The APF has been proved to remarkably eliminate the harmonic and reactive components of load current resulting in sinusoidal and unity power-factor source currents.
A.M. Bouzid et al. / Energy Procedia 18 (2012) 349 – 358
References [1] D. Joshi, K. S. Sandhu, and M. K. Soni’, Performance Analysis of Self-Excited Induction Generator Using Artificial Neural Network’, IRANIAN JOURNAL OF ELECTRICAL AND COMPUTER ENGINEERING, VOL. 5, NO. 1,pp 57-62, WINTERSPRING 2006 [2] Avinash Kishore, G. Satish Kumar, ‘ Dynamic modeling and analysis of three phase self-excited induction generator using generalized state-space approach’,IEEE International Symposium on Power Electronics, Electrical Drives, Automation and Motion, SPEEDAM, pp 52-59, 2006. [3] Arkadiusz Kulka,’ Digital Control of Power Electronics for Reliable Distributed Power Generation’, PhD Projects 2006 at Dep. of Electrical Power. Eng. University of Science and Technology. Norwegian, Jan 2006. [4] Li Wang, Member, ‘Transient Performance of an isolated induction generator under unbalanced excitation capacitor’, IEEE Transaction on Energy conversion, Vol 14,no 4, pp 887-893, Dec. 1999. [5] Murthy, S.S., Malik, O.P., and Tandon, A.K.: ‘Analysis of self-excited induction generator’, IEE Proc. C, Gener. Transm. Distrib. 1982, 129, (6), pp. 260–265. [6] B. Singh, S.S. Murthy and S. Gupta: ‘analysis and implementation of an electronic load controller for a self-excited induction generator’, IEE Proc. C, Gener. Transm. Distrib, vol. 151, pp. 51-60, Jan. 2004. [7] Al Jabri A. K. and Alolah A. I, (1990) “Capacitance requirements for isolated self-excited induction generator,” Proceedings, IEE, pt. B, vol. 137, no. 3, pp. 154-159 [8] Ofualagba, G and Ubeku, E.U,’ The Analysis and Modelling of a Self-excited Induction Generator Driven by a Variable Speed Wind Turbine’, Federal University of Petroleum Resources, Effurun, Nigeria online www.intechopen.com/download/pdf/pdfs_id/1625. [9] Dawit Seyoum,’ the dynamic analysis and control of a Self-excited induction generator driven by a wind turbine’, Ph.D. dissertation, Dept. School of Electrical Engineering and Telecommunications, Univ. of New South Wales, 2003. [10] Madhukar Waware, Pramod Agarwal, ‘Comparison of Control Strategies for Multilevel Inverter based Active Power Filter used in High Voltage Systems’, IEEE/PEDES Power Electronics, Drives and Energy Systems, Dec. 2010. [11] Chennai Salim, Benchouia M-T, ‘Shunt Active Filter based on three–level (NPC) Inverter using Current and DC Voltage Artificial Neural Network Controllers’, International Electrical Engineering Journal (IEEJ), Vol. 1 (2011) No. 1, pp. 523-528. [12] R.SHANMUGHA SUNDARAM K.J.POORNASELVAN N.DEVARAJAN, ‘Comparison of Reference Compensating Current Estimation Techniques for Shunt Active Filter’, Conferences-2005-prague papers pp 493-149,www.wseas.us/elibrary/conferences/2005prague/papers/493-149.pdf. [13] Bhim Singh, Kamal Al-Haddad, and Ambrish Chandra: ‘A New Control Approach to Three-phase Active Filter for Harmonics and Reactive Power Compensation’ ,IEEE Transactions on Power Systems, Vol. 13, No. 1, pp. 133-138, Feb. 1998. [14] K. Vinoth Kumar, G. Surendar, M. P. Selvan, ‘Performance Comparison of Shunt Active Filter and Hybrid Active Filter’, XXXII National Systems Conference, Nsc 2008, pp. 71-76, December 17-19, 2008. [15] H. Dalvand J. S. Moghani and N. Talebi, ’An adaptive hysteresis band current controller for hybrid power filter’, IEE Trans, First International Conference on Industrial and Information Systems, ICIIS 2006, pp 613-618, August 2006. [16] Wenjin Dai, Yongtao Dai and Tingjian Zhong, ‘A New Method for Harmonic and Reactive Power Compensation’, IEEE ICIT International Conference on Industrial Technology, Apr. 2008. [17] Gupta, N. ; Singh, S.P. ; Dubey, S.P. , ’PLL Less Shunt Active Filter with Direct Current Control for Power Quality Conditioning’, IEEE 5th Conference on Industrial Electronics and Applications, pp 936-941 (ICIEA), Jun.2010. conventional and advanced aircraft electric power systems’, Elsevier, Electric Power Systems Research 79, pp80–88,2009. [19] Bhim Singh, S. S. Murthy, and Sushma Gupta, “Analysis and Design of Electronic Load Controller for Self-Excited Induction Generators”, IEEE transactions on energy conversion, vol. 21, no. 1, march 2006. [20] Vargil Kumar E, Narasimham PVRL, Sarma AVRS, “Steady State Investigation of Self Excited 3 Phase Induction Generator with Novel Leading VAR Controller and Mitigation of Harmonics Using Active Power Filter’, IEEE International Conference on Power and Energy (PECon2010), pp 495-500,Nov 29 - Dec 1, 2010.
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[21] Dheeraj Joshi, Kanwarjit Singh Sandhu, and Mahender Kumar Soni,’ Constant Voltage Constant Frequency Operation for a Self Excited Induction Generator’, IEEE TRANSACTIONS ON ENERGY CONVERSION, VOL. 21, NO. 1, pp 228-234, MARCH 2006. [22] A.M.Sharaf, Subramanian Kanthi Murugan,’ Dynamic Power Filter & Capacitor Compensator for Isolated Self-excited Induction Generator driven by a Wind Turbine’, IEEE 11th International Conference on Harmonics and Quality of Power, pp 46-49,sept 2004.
Appendix A. Modeling of SEIG The matrices of (1) are defined as follows: ; [ ]=
[ ]=
[ ]=
+ 0
0 + 0
0
; [ ]=
0 0 + 0
0 +
[ ]=
0 0 0
[
0 0 0
Here, suffixes and refer to and axis (in stator reference frame), rotor; refers to magnetizing component. Appendix B. Parameters of SEIG Table. Generator Rating and Parameters
Rated Power Rated Line to Line Voltage Rated line to line Current Rated Frequency Number of poles, P Rated Rotor speed Nn Stator Resistance, Rs Stator Leakage inductance Lls Rotor Resistance, Rr Rotor leakage inductance, Llr Capacitance for excitation C
3.5KW 380 V 14 A 50 Hz 4 1410 rpm 0.003mH 0.003mH 270 μF
]
0 0 0 +
and
0 0 + 0 refer to stator and
Energy Procedia 18 (2012) 349 – 358
Current Control of the Isolated Self-Excited Induction Generator using Shunt Active Filter A.M. BOUZIDa, M. BENGHANEMb, B.HAMANEc, A.BELABBESd, M.BOUHAMIDAe,A.DRAOUf,a* a,b,c,d,e
LDDE laboratory members, Université Mohamed Boudiaf USTO 1505Bp El Mnaouer,Oran 31000, Algeria f,
Senior MIEEE, Department of Electrical Engineering,Hail University,Hail,Saudi Arabia
Abstract The Self Excited Induction Generator (SEIG) is an isolated power source whose terminal voltage and frequency are controlled by the excitation of the capacitance or the load impedance. This paper presents a method for calculating the minimum excitation capacitance using the equivalent circuit approach for analyzing the steady state operation of SEIG. A new strategy based on an active power filter (APF) for controlling the current and power quality of the self excited induction generator (SEIG) is also presented in this paper. The shunt active power filter was implemented using a three phase PWM current controlled voltage source inverter (VSI) and connected to the wind generator and loads in order to compensate the current harmonics and reactive power. The PWM-VSI gate control signals are derived from hysteresis band current controller. The proposed active filter proved to play an important role and give good dynamic response and robust behavior upon changes in load parameters. This investigation demonstrated that power average control strategy can facilitate the improvement of the power quality. The proposed control method extracts fundamental (reference) components of the source current for the shunt active power line conditioners for nonlinear and unbalanced loads. The Power average approach additionally maintains the voltage of the capacitor (of the PWM inverter) nearly constant without any external control circuit. The shunt APF in conjunction with the proposed controller performs perfectly under different steady state and transient conditions. The simulation results with nonlinear loads and unbalanced loads have showed the effectiveness of the proposed scheme for harmonic reduction in Wind based Power Generation.
© 2012 Ltd. Selection and/or peerpeer-review review underunder responsibility of TheofTerraGreen Society. 2010Published PublishedbybyElsevier Elsevier Ltd. Selection and/or responsibility [name organizer] Keywords: Shunt Active Filter, Self Excited Induction Generator (SEIG), Wind Energy, PWM Inverter, Renewable Energy.
* Corresponding author. Tel.:+213-550-023-222; fax:+213-41-560-328. E-mail address: [email protected] (A.M BOUZID) / [email protected] (M.BENGHANEM) [email protected] (A.DRAOU).
1876-6102 © 2012 Published by Elsevier Ltd. Selection and/or peer review under responsibility of The TerraGreen Society. doi:10.1016/j.egypro.2012.05.046
350
A.M. Bouzid et al. / Energy Procedia 18 (2012) 349 – 358
1. Introduction
I
n recent years, the Self-Excited Induction Generator (SEIG) has emerged as the best electromechanical energy converter to replace the conventional synchronous generator in isolated power generators driven by renewable energy resources: biogas, micro-hydroelectric, wind etc. The main advantages of the SEIG are: low cost, ruggedness, absence of a separate DC source for excitation brushless rotor construction (squirrel cage construction) and ease of maintenance. The fundamental problem with using the SEIG was its inability to control the terminal voltage and frequency under varying load conditions. The analysis of the SEIG under steady-state conditions and imposed speed is already known [1]-[2]; however there are few papers about transient state operation. Active Power Filters (APF) are often used in applications where low current harmonics are desirable and/or improvement of quality of energy taken from the power grid are needed. With the use of APF, it is possible to draw near perfect sinusoidal currents and voltages from the grid or renewable distributed power sources. Moreover, it will be possible to balance load currents in different phases which itself is important in stand-alone power generation like wind turbines as for the case of unsymmetrical load currents, it could lead to torque pulsation in generator’s shaft and a decrease of reliability. With the use of APF it is also possible to control reactive power and keep unity power factor that is why they are mainly used in industry where DC current is needed e.g. aluminum plants, train power substations, arc welders. The currents taken by household and office consumers have usually high harmonic contents which is related to an increased number of non-linear loads such as rectifiers and capacitors, where the current is drawn at the peak of sinusoidal voltage. At last, it can be said the APF could be used to prevent any kind of harmonic generation (computer’s power supply, energy savings lamp, etc.), to reduce: harmonic contents in the grid, peak value of the current drawn from the grid, the inrush current taken from the grid, and to compensate the neutral line current, and correct the active power factor correction, and thus transformers will not be necessary [3]. 2. Description of the proposed system A schematic of the proposed system is shown in Fig. 1. It consists of a three phase star-connected induction generator driven by an uncontrolled micro hydroelectric turbine. The generator is operated as an load [4]. When SEIG supplies a non-linear load, the load draws a fundamental component of current and harmonic current from the generation systems, which are to be properly controlled. The shunt APF can compensate the harmonic current by continuously tracking the changes in harmonic content. APF’s consists of a voltage fed converter with a PWM current controller and an active filter controller that realizes an almost instantaneous control algorithm shown in Fig.1. As the input power is nearly constant, the output power of the SEIG must be held constant at all consumer loads. Any decrease in load may accelerate the machine and raise the voltage and frequency levels to prohibitively high values, resulting in large stresses on other connected loads.
SEIG Variable load V PWM Fig. 1. Block diagram of the APF with SEIG
APF Controller
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A.M. Bouzid et al. / Energy Procedia 18 (2012) 349 – 358
3. Modeling of the SEIG The dynamic model of the three-phase squirrel cage induction generator is developed by using a stationary d-q axes references frame [5] and the relevant volt ampere equations are as follows: [ ] = [ ][ ] + [ ] [ ] + [ ][ ] (1) Thus, the current derivative can be expressed as: [ ] = [ ] {[ ] [ ][ ] [ ][ ]}
(2)
Where,[ ], [ ] , [ ] , [ ] and [ ] are defined in the Appendix (Section 9). The SEIG operates in the saturation region and its magnetization characteristics are non-linear in nature. Thus, the magnetizing current should be calculated at every step of integration in terms of stator and rotor currents as in [6]: =
(
+
) +
+
(3)
Magnetizing inductance Lm (H)
against , Magnetizing inductance is calculated from the magnetization characteristics plotted as and is obtained by as shown in Fig. 2 for the machine under test. The relation between synchronous speed test. 0.1 0.08 0.06 0.04 0.02
0
5
10 15 20 Magnetizing current Im (A)
25
Fig. 2. Variation of magnetizing inductance as a function of magnetizing current
The developed electromagnetic torque of the SEIG is: = (3 /4)
(4)
The torque balance equation is: = + (2/ )
(5)
The derivative of the rotor speed from (4) is: = (2/ ) /
(6)
4. Process of self-excitation The process of self-excitation can be compared with the resonance phenomenon in an RLC circuit (Elder et al., 1984), (Grantham et al., 1989). In whose transient solution is of the exponential form the solution, K is a constant, and root 1 is a complex quantity, whose real part represents the rate at which the transient decays, and the imaginary part is proportional to the frequency of oscillation. In real circuits, the real part of 1 is negative, meaning that the transient vanishes with time. With the real part
352
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of p1 positive, the transient (voltage) build-up continues until it reaches a stable value with saturation of iron circuit. In other terms, the effect of this saturation is to modify the magnetization reactance , such that the real part of the root 1 becomes zero in which case the response is sinusoidal steady-state corresponding to continuous self-excitation of SEIG. Any current (resulting from the voltage) flowing in a circuit dissipates power in the circuit resistance, and an increasing current dissipates increasing power, which implies some energy source is available to supply the power. The energy source, referred to above is provided by the kinetic energy of the rotor (Grantham et al., 1989). With time varying loads, new steady-state value of the voltage is determined by the self-excitation capacitance value, rotor speed and load [7] [8] [9]. The formula for calculation of minimum capacitance is: =1 (7) 5. Reference current generation using average power method The average power method gives accurate results even if the current is distorted. A PLL based unit vector template is used to obtain fundamental component of mains voltage. To get unit vector templates of voltage, the input voltage is sensed and multiplied by a gain equal to 1/vpk where vpk is the peak amplitude of fundamental supply voltage. These unit vectors are then passed through a PLL for synchronization of signals. Three phase fundamental components are multiplied by vpk to get fundamental mains voltage. The Power average method needs reduced calculation, since it works directly with abcphase voltage and line currents. The elimination of the Clark transformation makes this control strategy simple [10] [12]. The Power average method presents a minimum rms value to draw the same three phase average active power from the source as the original load current. The control strategy principle for the shunt active power filter based on three-level inverter is illustrated in Fig.3.
Compute Average Power
Compute source reference currents
Compute command shunt APF currents
Hysteresis based Current Controller
Gating signals Fig. 3. Block diagram of the proposed shunt active power filter control scheme
6. Analysis and modeling The three phase instantaneous source current can be written as ( )= ( ) ( )
(8)
The instantaneous source voltage is given by ( )= sin
(9)
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If a nonlinear load is applied, then the load current will have a fundamental component and harmonic components, which can be written as ( )=
(
)=
+
(
+
)+
(
+
)
(10)
The reduction of current harmonics in the load current is achieved by injecting equal but opposite current harmonic components at the point of common coupling, thus cancelling the original distortion and improving the power quality. The system comprises an ac source, non-linear load, unbalanced load, the APF and the new control scheme. The components of the system are analyzed separately and integrated to develop the complete model for the simulation [13]. 6.1. Computation of the average power The sensed load currents ( , , ) and bus voltages( the instantaneous power as given by ( )=
( )
( )+
( )
( )+
( )
,
,
) through PLL are used to derive
( )
The three phase instantaneous reactive power in each phase becomes [12]: = = =
(11)
(12)
The instantaneous active and reactive power delivered to a nonlinear load must satisfy (12) and (13). = + = + (13) = , = , , (14) Where - Instantaneous active power supplied by the source - Instantaneous active power supplied by the APF - Instantaneous active fundamental power of the load - Instantaneous harmonic power of the load - Instantaneous reactive power generated by the APF at phase k. In order to ensure that the fundamental active power is supplied to the load from the source, the instantaneous reactive power and harmonic power must be compensated by the APF. When considering is expressed as: the compensation of both harmonic and reactive power, ( )=
( )
( )+
( )
( )+
( )
6.2. Computation of source reference currents
( ) ,
(15)
,
From (14) and (15), the reference compensating currents are determined as:
= = =
(16)
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A.M. Bouzid et al. / Energy Procedia 18 (2012) 349 – 358
6.3. Computation of the command shunt APF currents
,
,
Finally the desired 3-phase references of the APF currents( , , ) are computed by taking the difference between the three phase instantaneous reference source currents ( , , ) and the actual source currents( , , ) as below: = = (17) = 6.4. Hysteresis current controller The actual source currents are monitored instantaneously, and then compared to the reference currents generated by the proposed algorithm. In order to get accurate and instantaneous control, switching of the IGBT devices should be such that the error signal approaches zero, thus providing quick response. For this reason, hysteresis current controller with fixed band is used to derive the switching signals of the three phase IGBT based VSI bridge. The upper device and the lower device in one phase leg of VSI are switched in complementary manner otherwise a dead short circuit will be take place [17]. The APF reference currents ( , , ) , compared with, the sensed source currents ( , , ) , and the error signals are operated by the hysteresis current controller to generate the firing pulses which activate the inverter power switches in a manner that reduces the current error. The switching logic for ‘phase-a’ is ), then upper switch is OFF and lower switch is ON in the phase formulated as follows: If <( ), then upper switch is OFF and lower switch is ON in the phase ‘a’ leg then (SA=1). If >( ‘a’ leg then (SA=0). In the same fashion, the switching of phase-b and c devices can be derived using as the width of hysteresis band. The switching functions SB and SC for phases b and c are determined in a similar manner. 7. Simulation results and analysis The performance of the proposed control strategy is evaluated through simulation using SIMULINK toolbox in the MATLAB. The system parameters values are: Line to line source voltage is 380 V; System frequency (f) is 50 Hz; Source impedance of RS, LS Rc, Lc L, LL respectively; DC voltage (VDC) is 500V;
= 1100
; Power devices used are IGBT/Diode.
200
20
Vds
C ur rent Ids ( A)
Voltage Vds (V)
7.1. Performance of self excited induction generator
0 -200
Ids
0
-20 0
0.2
0.4
0.6
0.8
1
0
0.2
Time (s)
Fig. 4. Simulation results of stator voltage and current of the SEIG with saturation
0.4
0.6 Time (s)
0.8
1
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A.M. Bouzid et al. / Energy Procedia 18 (2012) 349 – 358
=1500 rpm, the generated voltage and current attain their steady state values of 380 Volts and 19 A in 0.8 sec as shown in Fig. 4. 7.2. Shunt active power filter system performance
Ia-rectifier
Iabc-load(A)
The computer simulation results are provided to verify the effectiveness of the proposed control scheme. The unbalanced load RL current before compensation is shown in Fig 5(a) and the six-pulse diode rectifier RL load current or source current before compensation is shown in Fig 5(b). 2 5
0
-5
0
0.05 Time (s)
0
-2
0.1
0
(a)Unbalanced load currents;
0.05 Time (s)
0.1
(b) Non linear load current
Fig. 5. (a) Simulation results of the load currents
5 IFabc (A)
Isabc (A)
5 0 -5 0
0.05 Time (s)
0
-5
0.1
0
(a)Load currents or source current before compensation ;
0.02
0.04 0.06 Time (s)
0.08
(b) Reference currents before APF
Fig. 6. (a) Simulation results of source and reference currents
2 Isabc(A)
IFabc*(A)
5
0
-5
0
0.05 Time (s)
0.1
(a) Reference currents by the power average control algorithm Fig. 7. Simulation results of source and reference currents
0 -2 0
0.05 Time (s)
0.1
(b) Currents source after compensation
0.1
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Fig. 6(a) shows the simulated results of the load currents. The harmonic currents of a nonlinear load and unbalanced load are compensated by the shunt active power filter. The actual reference currents for the three phases are shown in Fig. 6(b). This waveform is obtained from the proposed average power controller. The source current after compensation is illustrated in Fig. 7(b) which indicates that the current becomes sinusoidal. After active filter operation the AC-source current only supplies the active fundamental current to the load. The shunt APLC supplies the compensating current that is shown in Fig. 7(a). The current after compensation shown in Fig. 7(b) would have taken a shape as shown in Fig. 6(b) without APF. It is clearly visible that this waveform is sinusoidal with some high frequency ripples. 7.3. Measurement of the total harmonic distortion measured The total harmonic distortion is measured using the source current waveform and presented in Table 1 both with and without APLC. Table 1. Total harmonic distortion (THD %) of source current
Condition THD Steady state
Source current (Is) Without APLC 23.08%
Source current (Is)With APLC 2.01%
The FFT analysis that was carried out confirms that the active filter brings the THD of the source current down to less than 5% which is in compliance with IEEE-519 standards for harmonics. In general, the THD values for both current and voltage in advanced aircraft electric power system in presence of APF are lower than those for conventional aircraft system [18]. 8. Conclusion This paper has presented the implementation of a cage-rotor IG system completely isolated from the utility grid, in order to supply rural sites or isolated areas. It has been demonstrated that the system is able to feed resistive and inductive loads with regulated current and satisfactory energy quality. In this paper we also discussed the problem of terminal current stabilization of the self excited induction generator (SEIG) in standalone mode from which a new method of stabilization of the current is used to improve the performance characteristics of the SEIG. This investigation demonstrated also that the generalized Power average control strategy can facilitate the improvement of the power quality. Simulation results are included in order to validate the proposed control technique. The shunt APF has been implemented with a three phase PWM current controlled voltage source inverter and is connected to the AC mains in order to compensate the current harmonics and reactive power. It has been shown that the Power average approach additionally maintains the voltage of the capacitor (of the PWM inverter) nearly constant without any external control circuit. Different types of linear and non linear loads for reactive power and current harmonics compensation have been connected to the APF to analyse the steady-state and transient performance of the system. The APF has been proved to remarkably eliminate the harmonic and reactive components of load current resulting in sinusoidal and unity power-factor source currents.
A.M. Bouzid et al. / Energy Procedia 18 (2012) 349 – 358
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Appendix A. Modeling of SEIG The matrices of (1) are defined as follows: ; [ ]=
[ ]=
[ ]=
+ 0
0 + 0
0
; [ ]=
0 0 + 0
0 +
[ ]=
0 0 0
[
0 0 0
Here, suffixes and refer to and axis (in stator reference frame), rotor; refers to magnetizing component. Appendix B. Parameters of SEIG Table. Generator Rating and Parameters
Rated Power Rated Line to Line Voltage Rated line to line Current Rated Frequency Number of poles, P Rated Rotor speed Nn Stator Resistance, Rs Stator Leakage inductance Lls Rotor Resistance, Rr Rotor leakage inductance, Llr Capacitance for excitation C
3.5KW 380 V 14 A 50 Hz 4 1410 rpm 0.003mH 0.003mH 270 μF
]
0 0 0 +
and
0 0 + 0 refer to stator and