Document Sample

World Academy of Science, Engineering and Technology 70 2010 On the AC-Side Interface Filter in Three-Phase Shunt Active Power Filter Systems Mihaela Popescu, Alexandru Bitoleanu, and Mircea Dobriceanu The active filtering performance in terms of harmonic Abstract—The proper selection of the AC-side passive filter interconnecting the voltage source converter to the power supply is distortion factor of the supply current is substantial influenced essential to obtain satisfactory performances of an active power filter by the passive components associated with the VSC structure, system. The use of the LCL-type filter has the advantage of i.e. the interface filter to interconnect the converter to the eliminating the high frequency switching harmonics in the current power supply and the DC capacitor. injected into the power supply. This paper is mainly focused on analyzing the influence of the interface filter parameters on the active The main goal of the interface filter is to reduce the filtering performances. Some design aspects are pointed out. Thus, switching harmonic distortion of the injected current as much the design of the AC interface filter starts from transfer functions by as possible to prevent the switching harmonics from imposing the filter performance which refers to the significant current propagating into the power supply. On the other hand, the flow attenuation of the switching harmonics without affecting the of harmonics to be compensated must be allowed. harmonics to be compensated. A Matlab/Simulink model of the entire Typically, the passive interface filter is either of first-order active filtering system including a concrete nonlinear load has been L type or of third-order LCL (inductor-capacitor-inductor) developed to examine the system performances. It is shown that a type. gamma LC filter could accomplish the attenuation requirement of the In practice, the simple L filter is not able to ensure the current provided by converter. Moreover, the existence of an optimal value of the grid-side inductance which minimizes the total harmonic active filter dynamics nor to provide sufficient attenuation of distortion factor of the power supply current is pointed out. the current ripple due to the converter switching. An efficient Nevertheless, a small converter-side inductance and a damping alternative is to connect the active filter to the power supply resistance in series with the filter capacitance are absolutely needed through an LCL filter as shown in Fig. 1. in order to keep the ripple and oscillations of the current at the As both high and low frequency characteristics of the LCL- converter side within acceptable limits. The effect of change in the type filters are superior compared to those of L type, the third LCL-filter parameters is evaluated. It is concluded that good active order filters have been increased in popularity in recent years. filtering performances can be achieved with small values of the The proper design of the LCL filter is a sensitive action capacitance and converter-side inductance. especially when the VSC control is based on hysteresis controllers which lead to a variable switching frequency. Keywords—Active power filter, LCL filter, Matlab/Simulink modeling, Passive filters, Transfer function. There are a lot of approaches to tuning a LCL filter, most of them related to interconnecting the VSC to the power network I. INTRODUCTION in an active rectification structure. In [1], the LCL filter design is achieved together with the control of the active rectifier C ERTAINLY , the superior performances of the shunt active power filters (SAPF) based on voltage source converters using proportional-plus-integral based control strategies for the DC voltage and the AC current. In accordance with [2], the (VSCs) made them the most adopted solution in improving total inductance of the filter should be selected at first to meet power quality of nonlinear loads at the point of common the requirement of current ripple and the converter-side coupling to the power supply. inductance should be much higher than the grid-side inductance. Some methods using the resonance frequency and the attenuation of the line current amplitude at the switching Mihaela Popescu is with Faculty of Electromechanical, Environmental and Industrial Informatics Engineering, University of Craiova, 200440 Craiova frequency as the main design parameters are presented in Romania (phone: +40 251 435 255; fax: +40 251 435 255; e-mail: literature [3], [4], [5]. To avoid stability problems, passive mpopescu@em.ucv.ro). additional damping resistors or active damping solutions have A. Bitoleanu is with Faculty of Electromechanical, Environmental and already been studied and tested [1], [4]. The current control of Industrial Informatics Engineering, University of Craiova, 200440 Craiova Romania (phone: +40 251 435 255; fax: +40 251 435 255; e-mail: the voltage source inverter connected to the power supply abitoleanu@em.ucv.ro).. through an LCL-filter based on state estimators has been M. Dobriceanu is with Faculty of Electromechanical, Environmental and presented in [6]. Industrial Informatics Engineering, University of Craiova, 200440 Craiova Romania (phone: +40 251 435 255; fax: +40 251 435 255; e-mail: mdobriceanu@em.ucv.ro). This work was supported by Romanian Ministry of Education and Research, Grant 21-010/2007. 988 World Academy of Science, Engineering and Technology 70 2010 allows expressing the following equations in the domain of ua ia iLa Laplace transform. ub ib iLb Nonlinear uc ic iLc load I 1 (s ) = I 2 (s ) + I c (s ) , (1) Current Reference current U 1 (s ) − U c (s ) = sL1 I 1 (s ) , (2) controller calculation Voltage u*DC U c (s ) − U 2 (s ) = sL2 I 2 (s ) , (3) controller ica Inverter I c (s ) = sC f U c (s ) , (4) control icb Power supply Converter icc side L2 L1 L2 L1 side C uDC i2 i1 u2 ic u1 Cf Cf Fig. 1 Structure of a three phase shunt active power filter system with LCL interface filter Fig. 2 Equivalent single phase diagram of the idealized LCL filter Design considerations on the LCL interface filter between As the design process of the passive filter depends on the the active power filter and the power supply have been very attenuation needed to reduce the high switching frequency little treated in literature. components of the currents provided by SVC, the transfer Different strategies to generate the reference currents for the function between converter-side current and grid-side current current controllers have been analyzed in [7] and the is expressed on the basis of (1)-(4). advantages of the Fourier method have been pointed out for imposed values of the LCL filter. Contribution [8] had the practical goal to replace the I (s ) 1− U 2 (s) ( 1 + s 2 L1C f) H (s ) = 2 U 1 (s) = . (5) I 1 (s ) existing L type interface filter of a four-wire active power filter U ( s) with an LCL filter in order to diminish the ripple of the current 1 + s 2 L2 C f − 2 U 1 (s) injected into power network. Small inductors in parallel with damping resistors were selected on the supply- side while the capacitors selection took into consideration the desired Assuming the harmonic frequencies domain, then U2(s)=0 resonant frequency. and the transfer function becomes: Reference [9] proves that the performances of a passively H (s ) = damped LCL filter can be close to those of an active damped 1 . (6) LCL filter and that the additional losses are not significant. L2 C f s 2 + 1 Some transfer functions from the output voltage of a single- phase inverter to the currents and condenser voltage of the The previous expression shows that the attenuation of LCL filter have been expressed and interpreted in [10]. The harmonic currents does not depend on the converter side total inductance of the passive filter has been chosen inductance of the interface filter. respecting the rated values of DC voltage and maximal current The Bode plot in Fig. 3 illustrates the maximum magnitude through inverter. Then, the effect of distribution of the total at natural frequency inductance over the grid-side and converter-side are analyzed, while the capacitance is adjusted to maintain an imposed value 1 of the resonance frequency. fn = (7) This paper investigates the influence of the LCL filter 2π L2 C parameters on the active filtering performance based on Matlab/Simulink time-domain simulations. and an attenuation of -40dB/decade over the cutoff frequency, II. INTERFACE FILTER TRANSFER FUNCTIONS The equivalent single phase of the idealized LCL filter (Fig. f cutoff = 2 f n . (8) 2), where the series resistances of the inductors are neglected, 989 World Academy of Science, Engineering and Technology 70 2010 Magnitude (dB) Magnitude (dB) Frequency (Hz) Frequency (Hz) Fig. 3 Frequency response from converter-side current to Fig. 4 Frequency responses from VSC output voltage to grid-side grid-side current current for LCL-filter (solid line) and L-filter (dashed line) Additional transfer functions of the LCL-type filter are addressed and interpreted in literature, such as I1(s)/U1(s), III. INTERFACE FILTER PARAMETERS I2(s)/U1(s), Ic(s)/U1(s), Uc(s)/U1(s) [1], [8], [10]. In designing the inductors and capacitor values, the Among these transfer functions, the admittance transfer ratio harmonic filtering requirements must particularly be taken into Y21(s)= I2(s)/U1(s) gives information on the advantages of the consideration, i.e. the interfacing filter has to reject the LCL filter compared to the ordinary first order L filter switching harmonics without affecting the harmonics to be configuration and can be taken into consideration in LCL filter compensated. For this reason, the cutoff frequency must be design. It is expressed as below the switching frequency in order to obtain a rejection current slope of -40dB/decade. Thus, the following condition I 2 (s ) 1− U 2 (s) ( 1 + s 2 L1C f ) is obtained: Y21 (s ) = U 1 ( s) = sC f U 1 (s ) ( )( 1 + s 2 L1C f 1 + s 2 L2 C f − 1 . ) (9) fs ≥ 2 fn = 1 , (11) π ⋅ 2 L2 C In the harmonic frequencies domain, where U2(s)=0, the expression (9) becomes: where fs is the switching frequency. Besides, the minimum frequency which determinates the filter pass band must exceed the highest harmonic frequency to sC f Y21 (s ) = = (1 + s )( ) be compensated. Hence, taking into account the superior pass 2 L1C f 1 + s 2 L2 C f − 1 (10) band limit of f n 2 which has no influence on the input 1 = signal, the second condition is expressed: L1 L2 C f s 3 + (L1 + L2 )s 1 fN ≤ , (12) The associated frequency response is shown in Fig. 4 2π ⋅ 2 L2 C compared to L filter frequency characteristic. It is shown that, where fN is the lowest frequency among the harmonic if the total inductance is the same, both filters have the same frequencies to be rejected by the interface filter. behaviour at low frequencies, while the LCL-filter is better at Conditions (11) and (12) can be written together as follows: high frequencies area since the harmonic admittance decreases by 60 dB/decade for the LCL-filter compared to 20 dB/decade 1 1 for the L-filter. ≤ L2 C ≤ . (13) 2π 2 f s2 8π f N 22 For instance, by imposing fs = 10 kHz and fN = 2 kHz, expression (13) becomes: 0.51 ⋅ 10 −9 (rad s )−2 ≤ L2 C ≤ 3.17 ⋅ 10 −9 (rad s )−2 . (14) Since the product L2C domain is large and several combinations of L2 and C values fulfil the previously 990 World Academy of Science, Engineering and Technology 70 2010 expressed conditions, a detailed analysis of the whole active 100 filtering system is essential to find an optimal solution. ua /4 (V); iLa (A) In addition, although a gamma filter composed of L2 and C 50 should be enough to accomplish the current supply ripple attenuation, the practical implementation of this solution 0 makes evident unacceptable high current spike and excessive rate of current change at the VSC-side, as shown in section IV. -50 Consequently, the converter-side inductance is indispensable and a damping solution is required. -100 0.04 0.045 0.05 0.055 0.06 0.065 0.07 0.075 0.08 IV. ANALYSIS OF ACTIVE FILTERING PERFORMANCES Time (seconds) Several computer simulations have been carried out under Fig. 6 Waveforms of supply voltage (thin line) and distorted load Matlab/Simulink environment in order to examine the current (thick line) of THD =44% influence of the interface filter parameters on the active filtering performances. Thus, the system in Fig. 1 has been In order to avoid the superposition effect of the DC-bus implemented into the Simulink model shown in Fig. 5. voltage control on the system performances, the assumption of The nonlinear load taken into consideration is an an ideal DC voltage regulator has been made, so that the DC uncontrolled rectifier supplying a PWM inverter. The distorted voltage can be considered constant. A value of 710V has been load current of THD = 44% and the supply voltage are shown taken into consideration for this case study. in Fig. 6. The shunt active power filter is charged to compensate both The control strategy to generate compensation commands is the load current harmonics and the reactive power. based on the Fourier analysis of the distorted load current to The filtering performances have been appreciated in terms extract its fundamental component. of tracking of the compensating currents compared to the To generate gating signals for the switching devices of the desired values, the total harmonic distortion factor of the active power filter the hysteresis-based current (1 A) control supply current, the ripple current at the converter-side as well has been implemented. as of the active filtering effectiveness factor defined as: Fig. 5 Matlab/Simulink model of the active filtering system THD L AFE = 100 , (15) THD S 991 World Academy of Science, Engineering and Technology 70 2010 where THDL and THDS denote the total harmonic distortion 100 factor of the supply current without and with the APF system. ua /4 (V); ia (A) Since the attenuation of the converter side current ripple 50 through the interface filter is essential, the influence of parameters L2 and Cf has been examined at the beginning. 0 Thus, as illustrated in Fig. 7, keeping a constant value of filter capacitance, there is an optimal inductance which minimizes -50 the power supply harmonic distortion. Moreover, for both Cf=0.6µF and Cf= 3µF taken into consideration, the optimal -100 0.06 0.065 0.07 0.075 0.08 value of L2 is of about 4mH and the corresponding minimum Time (seconds) THD is of about 6.4%. Fig. 9 Waveforms of supply voltage and current for L2= 4mΗ and Cf = 0.6µF Supply current THD (%) 15 600 500 400 iDC (A) 10 300 200 5 100 0 0 -100 1 2 3 4 5 6 7 8 0.06 0.065 0.07 0.075 0.08 600 Inductance (mH) 400 Fig. 7 Supply current THD versus inductance L2 for Cf = 0.6µF (solid i1 (A) line) and Cf = 3µF (dashed line) 200 0 The current injected into the power supply by the gamma -200 L2C filter tracks the reference current provided by the current control loop (Fig. 8). As a result, the compensation task is -400 accomplished, i.e. the supply current is almost sinusoidal and -600 0.06 0.065 0.07 0.075 0.08 its fundamental is in phase with the voltage supply (Fig. 9). Time (seconds) 30 Fig. 10 DC-side current and VSC output current 20 for L2= 4mΗ and Cf = 0.6µF 400 10 i2, iref (A) 0 200 -10 i1, i2 (A) 0 -20 -30 -200 0.06 0.065 0.07 0.075 0.08 Time (seconds) -400 Fig. 8 Waveforms of current to be compensated (in black) and 0.04 0.045 0.05 0.055 0.06 current i2 (in gray) for L2= 4mΗ and Cf = 0.6µF Time (seconds) Fig. 11 Input (in gray) and output (in black) currents of the LCL filter Although the active filtering effectiveness factor introduced for L2= 4mΗ, Cf = 0.6µF and L1= 0.1mΗ by (15) is high (of about 6.8), the lack of converter-side inductance leads to unacceptable spikes in both VSC input and To reduce the current ripple and oscillations to acceptable output currents (Fig. 10). values, damping resistances are placed in series with the filter As it can be seen in Fig. 11, the simple addition of the capacitors (Fig. 12). Thus for instance, by inserting a damping inductance L1 is not satisfactory due to the significant resistance of 5Ω an important diminution of current oscillations of the current at the converter-side. oscillations is obtained as illustrated in Fig. 13. The price is however the increasing of system losses and a little decreasing of AFE factor value to about 6. 992 World Academy of Science, Engineering and Technology 70 2010 V. CONCLUSIONS i2 L2 L1 Power supply i1 Converter side The use of the LCL-type filter to interface the three phase side SAPFs based on VSCs to the power supply is an efficient solution due the ability of eliminating high order switching Rd current harmonics. As the tuning process based on transfer functions is rather flexible, time-domain simulations have been Cf carried out to find optimal parameters of the interface filter in order to improve the active filtering performances. Although Fig. 12 LCL interface filter with damping resistors the lack of converter-side inductance seems to have no 100 influence on the current tracking performance, a small converter-side inductance and a damping resistance in series 50 with the filter capacitance are absolutely needed in order to keep the ripple and oscillations of the current at the converter- i1, i2 (A) 0 side within acceptable limits. The existence of an optimal power supply-side inductance -50 which minimizes the total harmonic distortion factor of power supply current is pointed out. -100 It is concluded that good active filtering performances can 0.04 0.045 0.05 0.055 0.06 Time (seconds) be achieved with a smaller capacitance and smaller converter- side inductance compared to the corresponding values in Fig. 13 Input (in gray) and output (in black) currents of the LCL filter literature [8], [9], [10]. for L2= 4mΗ, Cf = 0.6µF, L1= 0.1mH and Rd= 5Ω The harmonic spectra of the current through inductances L1 REFERENCES and L2 point out even better the efficiency of the interface filter [1] M. Liserre, F. Blaabjerg, and S. Hansen, “Design and control of an LCL-filter-based three-phase active rectifier,” IEEE Trans. Ind. Appl., (Fig. 14). vol. 41, no. 5, pp. 1281-1291, 2005. 10 [2] Y. Lang, D. Xu, Hadianamrei S.R, and H. Ma, “A novel design method a) Harmonic current (A) of lcl type utility interface for three-phase voltage source rectifier,” in 8 Proc. of 36th IEEE Power Electronics Specialists Conference, pp. 313- 317, 2005. 6 [3] P. Peltoniemi, R. Pollanen, M. Niemela, and J. Pyrhonen, “Comparison 4 of the effect of output filters on the total harmonic distortion of line current in voltage source line converter - Simulation study,” 2006, 5 s., 2 ICREPQ 2006, Mallorca, Espanja, 2006. [4] M. Liserre, A. Dell’Aquila, and F. Blaabjerg, “Genetic algorithm-based 0 design of the active damping for an LCL-filter three-phase active 0 50 100 150 200 250 300 350 400 450 500 rectifier,” IEEE Trans. Power Electronics, vol. 19, no. 1, pp. 76-86, 10 2004. Harmonic current (A) b) [5] H.R. Karshenas and H. Saghafi, “Performance investigation of LCL 8 filters in grid connected converters, in Proc. IEEE PES Transmission 6 and Distribution Conference and Exposition, pp.1-6, 2006. [6] E.J. Bueno, F. Espinosa, F.J. Rodriguez, J. Ureila, and S. Cobreces, 4 “Current control of voltage source converters connected to the grid through an LCL-filter, in Proc of 35rh Annual IEEE Power Electronics 2 Specialisls Conference, vol.1, pp. 68 – 73, 2004. [7] M. Lindgren and J. Svensson, “Control of a voltage source converter 0 connected to the grid through an LCL-filter—application to active 0 50 100 150 200 250 300 350 400 450 500 filtering,” in Proc. of the IEEE PESC’98, vol. 1, pp. 229–235, 1998. Harmonic order [8] S. Pettersson, M. Salo, and H. Tuusa, “Applying an LCL-filter to a four- Fig. 14 Harmonic spectra of the current through inductance L1 (a) wire active power filter,” in Proc.Power Electr. Specialists Conf.- and inductance L2 (b) Pesc’06, pp.1413-1419, 2006. [9] M. Routimo and H. Tuusa, “LCL type supply filter for active power filter –Comparison of an active and a passive method for resonance Thus, in addition to the harmonics to be compensated, the damping,” in Proc. Power Electronics Specialists Conference, pp. current through the converter-side inductance contains 2939–2945, 2007. superior harmonics over the 100th order. Their weight related [10] B. Bolsens, K. De Brabandere, J. Van den Keybus, J. Driesen, and R. to the fundamental component required to compensate the Belmans, “Model-based generation of low distortion currents in grid- coupled PWM-inverters using an LCL output filter,” IEEE Trans Power reactive power gets near to 15% (Fig. 14a). Concurrently, the Electronics, vol. 21, No. 4, pp.1032-1040, July 2006. same high order harmonics of the current through L2 are strongly attenuated so that, practically, there is no harmonic over 150th order (Fig. 14b). 993

DOCUMENT INFO

Shared By:

Categories:

Tags:
DC side, Power Supply, Overvoltage protection, power factor, Italian English, Power switch, Maximum efficiency, DC Voltage, DC Current, Diode Bridge

Stats:

views: | 44 |

posted: | 3/28/2011 |

language: | English |

pages: | 6 |

OTHER DOCS BY nikeborome

How are you planning on using Docstoc?
BUSINESS
PERSONAL

By registering with docstoc.com you agree to our
privacy policy and
terms of service, and to receive content and offer notifications.

Docstoc is the premier online destination to start and grow small businesses. It hosts the best quality and widest selection of professional documents (over 20 million) and resources including expert videos, articles and productivity tools to make every small business better.

Search or Browse for any specific document or resource you need for your business. Or explore our curated resources for Starting a Business, Growing a Business or for Professional Development.

Feel free to Contact Us with any questions you might have.