International Conference on Control, Engineering & Information Technology (CEIT’14) Proceedings -Copyright IPCO-2014 ISSN 2356-5608 Advanced Nonlinear Control of Three Phase Shunt Active Power Filter Y. Abouelmahjoub1, F.Z. Chaoui1, A. Abouloifa2, F. Giri3, M. Kissaoui1 Abstract— The problem of controlling three-phase shunt active power filter (TPSAPF) is addressed in this paper in presence of nonlinear loads of rectifier type. The control objective is twofold: (i) compensation of harmonic currents and the reactive power absorbed by the nonlinear load; (ii) regulation the DC bus voltage of the converter. A nonlinear controller which has a cascade structure comprises two loops, an inner-loop which is formed by two non-linear regulators is designed, using the Backstepping technique, to provide the harmonic compensation and an outer-loop is designed to regulate the DC bus voltage of the converter. Controller performances are illustrated by numerical simulation in Matlab/Simulink environment. currents and reactive power absorbed by the non-linear load. The outer-loop is designed to regulate the DC bus voltage of the converter. This theoretical result is confirmed by numerical simulations. I. INTRODUCTION II. ACTIVE POWER FILTER MODELING Nowadays, with the wide use of nonlinear loads and electronic equipment in distribution systems, the problem of power quality (PQ) has become increasingly serious. This fact has lead to more stringent requirements regarding PQ which include the search for solutions for such problems [1]. A. Active Power Filter Topology A reduced-part three-phase shunt active power filter under study has the structure of Figure 1. It also has two-Leg less than full-bridge configuration. In the DC side it has two identical capacitors values of energy storage C f . In the AC The paper is organized as follows: The system includes the electric network and the DC/AC converter is modeled in section 2, the control problem is formulated in section 3 which also includes the design, the stability analysis in section 4. Performances controller are illustrated by simulation in section 5. A general conclusion ends the paper in the last section. In the case of harmonic pollution, the solution can consider the use of passive filters however these filters have the disadvantage of having a fixed compensation and can generate resonance problems. In this way, the active power filter (APF) appears as the best dynamic solution for harmonic compensation [2]. In concrete, active power filters are devices designed to improve the power quality in distribution networks. In order to reduce the injection of nonsinusoidal load currents, shunt active power filters can be connected in parallel with the nonlinear loads. side the TPSAPF is connected in parallel with a nonlinear load to the electric network which is modeled by three sinusoidal AC voltage sources that form a directly balanced three-phase system. The circuit operates according to the well known Pulse Width Modulation principle (PWM),[7],[8],[9],[10]. Finally the supply network contains three voltages that form a direct balanced three-phase system is given by: Sin (ωn t ) vn1 ( t ) 2π vn2 ( t ) = E Sin ωn t − 3 v (t ) n3 4π Sin ωn t − 3 (1) where E and ωn denote respectively the amplitude and the frequency of system ( vn1 ( t ) , vn2 ( t ) , vn3 ( t ) ).The resulting In recent years, three-phase four-wire shunt active power filters have appeared as an effective method to solve the problem caused by harmonic and unbalanced currents as well as to compensate load reactive power. In these filters, three topologies for current-controlled voltage source inverter are commonly used, namely the Four-Leg Full-Bridge (FLFB) topology, the Three-Leg Split-Capacitor (TLSC) topology and the four-leg split-capacitor (FLSC) topology. These topologies were presented in the early 90s [3], and the research work has been done on only those topologies. Indeed many publications on their control have appeared ever since [4]-[6]. current in three-phase load is given by its Fourier expansion: Sin ( hωn t + ϕ h ) iL1 ( t ) ∞ 2π I h Sin hωn t + ϕ h − iL2 ( t ) = 3 i ( t ) h =1 L3 4π Sin hωn t + ϕ h − 3 In this paper, a control strategy is developed for other topology named: Three-phase shunt active power filter or Two-Leg Split-capacitor (Two LSC) in the presence of nonlinear loads. A nonlinear controller that has a cascade structure has two nested loops, an inner loop which is formed by two non-linear regulators is designed, using the Backstepping technique, to ensure compensation of harmonic ∑ 95 (2) International Conference on Control, Engineering & Information Technology (CEIT’14) Proceedings -Copyright IPCO-2014 ISSN 2356-5608 ˜ N vn1 in1 iL1 Lo vn2 in2 iL2 Lo in3 iL3 Lo vn3 S2 u2 i1 Cf S1 u v1 The switching function µk of the inverter is defined by: Non Linear Load +1 −1 µk = is ON If Sk is OFF and and Sk + 2 is OFF Sk + 2 is ON Rf Lf 1 Rf Lf vf2 Rf vf3 S4 Sk k ∈ {1,2} if1 if2 if3 vf1 i2 Cf If M Figure 1. (5a) v f 2 = v f 2M + vMN (5b) v f 3 = v f 3M + vMN (5c) Lf S3 v2 v f 1 = v f 1M + vMN ( ) as v f 1 , v f 2 , v f 3 is a symmetric system then Three-phase shunt APF B. Active Power Filter Modeling vf 1 + vf 2 + vf 3 = 0 (5d) The modeling of the three-phase shunt APF is performed by applying the usual Kirchhoff’s laws. Doing so, one easily gets: 1 µ + µ2 vMN = − vo + v2 + 1 vo 3 2 (5e) (5f) Lf Lf Lf Cf di f 1 dt di f 2 dt di f 3 dt + R f i f 1 = v f 1 − vn1 (3a) 2 µ − µ2 vf 1 = 1 6 + R f i f 2 = v f 2 − vn2 (3b) v µ + µ2 vf 2 = 1 vo − d 3 6 (5g) + R f i f 3 = v f 3 − vn3 (3c) v 2 µ − µ1 vf 3 = 2 vo + d 6 6 (5h) dv1 = i1 dt (3d) dv C f 2 = i2 dt (3e) (4a) v f 2M = v2 (4b) di f 2 dt di f 3 dt 1 + µ2 = ( v1 + v2 ) 2 1 + µ1 1 + µ2 i1 = − if 1 − 2 2 Substituting (4d-e) and (5f-h) in (3) one obtains the instantaneous model of the filter: dt 1 + µ1 v f 1M = ( v1 + v2 ) 2 v f 3M with vo = v1 + v2 and vd = v1 − v2 di f 1 The three-phase shunt active power filter undergoes the equations: (4c) if 3 1 − µ1 1 − µ2 i2 = if 1 + if 3 2 2 vd vo + 6 =− =− =− 2 µ − µ2 if 1 + 1 6 Lf Lf Rf µ + µ2 if 2 − 1 6 Lf Lf Rf v v vo − d − n2 3L f L f 2 µ − µ1 v v if 3 + 2 vo + d − n3 6 Lf Lf 6 Lf Lf Rf (4d) µ1i f 1 + µ2 i f 3 dvo = − dt Cf (4e) if 1 + if 3 dvd = − Cf dt 96 v v vo + d − n1 6 L Lf f (6a) (6b) (6c) (6d) (6e) International Conference on Control, Engineering & Information Technology (CEIT’14) Proceedings -Copyright IPCO-2014 ISSN 2356-5608 x4 vn3 x The binary control signals µ 1 , µ 2 are produced by a PWM generator and take its values in the finite set {-1, 1}, the switched converter model (6) is a variable structure system and therefore inadequate for the design of a continuous control laws. To overcome this difficulty we resorted to using the averaged model where the different state variables and control laws are replaced by their average values over cutting intervals [11]. Since equation (6b) is a linear combination of the equations (6a) and (6c) then the obtained averaged model of the three-phase shunt APF is the following: x1 = − Rf Lf x1 + v U1 1 x3 + x4 − n1 Lf 6 Lf Lf x x3 x1 x2 x3 vn1 Regulator 1 Regulator 2 x*2 U2 (u1, u2) x*1 U1 (u1, u2) + iL1 iL3 + vn3 x4 x u2 - u1 - Three-phase Shunt Active Power Filter x2 (7a) vn1 x1 x + β- Regulator 3 Rf v U 1 x2 = − x2 + 2 x3 + x4 − n3 Lf Lf 6 Lf Lf u x +u x x3 = − 1 1 2 2 Cf x +x x4 = − 1 2 Cf (7b) + * y ( )2 (7c) y ( ) 2 x32 ( x3*2 (7d) Figure 2. The structure of the controller A. Current inner loop design with 2u − u U 1 = 1 2 and 6 2u − u U2 = 2 1 6 The currents in1 , in2 and in3 delivered by the supply network must be a sinusoidal signals in phase respectively with the network voltage vn1 , vn2 and vn3 . Indeed the currents x1 , x2 injected by three-phase shunt active power filter should follow the best as possible respectively the reference x*1 , x*2 defined by: where x1 , x2 , x3 , x4 , u1 and u2 respectively denote the average values over cutting periods of the signals i f 1 , i f 3 , vo , vd , µ1 and µ2 . The system (7) is clearly nonlinear because of the product between the state variables x1 , x2 , x3 , x4 and the control inputs u1 and u2 . It is assumed that all signals are measurable. x1* = iL1 − β vn1 x* = i − β v n3 2 L3 III. CONTROLLER DESIGN (8) Let’s introduce the tracking errors on the currents x1 , x2 : The control objective of the three-phase shunt APF is to compensate the harmonics and reactive power required by the nonlinear load. The controller synthesis is carried out by cascading two loops. First an inner current loop which is formed by two non-linear regulators is designed to compensate the reactive and harmonic power. Second an external loop is constructed to ensure control of the voltage in the DC side of TPSAPF. The controller has the structure of Figure 2. The currents i f 1 , i f 3 are controlled in the system and due to symmetry of the system i f 2 is being controlled. * e x − x e= 1= 1 1 * e2 x2 − x2 (9) Given (7a) and (7b), time derivative of errors e1 , e2 are written as follows: Rf U v 1 x1 + 1 x3 + x4 − n1 − x*1 − Lf 6 Lf Lf e1 L f (10) = e R U2 vn3 1 f 2 * − x + x + x − − x2 Lf 2 Lf 3 6 Lf 4 Lf Since the controls U1 and U 2 appear in (10) after a single derivation of the tracking errors, then the control laws 97 International Conference on Control, Engineering & Information Technology (CEIT’14) Proceedings -Copyright IPCO-2014 ISSN 2356-5608 of the currents controller will be elaborated in a single step using Lyapunov function [12]. V= 1 T 1 e e = e12 + e22 2 2 ( ) 2 4π 3E 2 ϕ ϕ k = ko + k1 L f ωn I1Cos 1 + Sin 1 , ko = 3 3 Cf 2 2 (11) k1 = Its dynamic is given by: V = e1e1 + e2 e2 (12) 3E Cf q ( t ) = q1 ( t ) + q2 ( t ) + q3 ( t ) The choice V = −c1e12 − c2 e22 ensuring the asymptotic stability of (10) with respect to the Lyapunov function (11) where c1 0 and c2 0 are a design parameters. Indeed this choice would imply: where q1 ( t ) , q2 ( t ) and q3 ( t ) are given in the appendix 1. e1 −c1e1 = e2 −c2 e2 law so that the squared voltage y = x32 The ratio β stands as a control input in the first-order system (16). The problem at hand is to design for β a tuning (13) * Bearing in mind the fact that β reference signal y = and its derivative should be available (Proposition 1), a filtered PI control law is considered: Comparing (13) and (10) gives the following: R x +v − U 1 1 f 1 n1 = U 2 x3 R x + v − f 2 n3 x4 + L f x1* − L f c1e1 6 x4 * + L f x2 − L f c2 e2 6 tracks a given x3*2 . (14) β = e5 = b ( c3 e3 + c4 e4 ) b+s (17) One deduce the expressions of effective control laws u1 and t u2 with u1 4U 1 + 2U 2 = u2 2U 1 + 4U 2 0 (15) At this point, the regulator parameters ( c3 , c4 , b ) are any positive real constants. The next analysis will make it clear how these should be chosen for the control objectives to be achieved. Proposition 1: Consider the three-phase shunt APF of Fig.1 described by its averaged model (7) with the control laws (15). If ∫ e3 = y* − y , e4 = e3 dτ diL is measurable, the signal β and its first dt time-derivative are available then the inner loop system is globally asymptotically stable and its dynamic is described by (13). IV. ANALYSIS OF THE CLOSED-LOOP CONTROL SYSTEM In the following Theorem, it is shown that the control objectives are achieved (in the mean). B. Voltage outer loop design Theorem 1 (main result). Consider the three-phase shunt active power filter shown by Figure 1 represented by its average model (7), together with the control system consisting of the inner-loop regulator (15) and the outer-loop regulator (17). Then the closed-loop system has the following properties: The purpose of the outer loop is to generate a tuning law for the ratio β in a way that makes the output voltage x3 regulated to a given reference value x3* . To this end, the relation between β and x3 must be made clear. 1) Let the reference signal y* be any positive constant signal. Replacing in (7c) the effective laws u1 and u2 defined by their expressions (15) while replacing U1 and U2 by (14) an supposing negligible parasitic resistance R f , one obtains: There exists a positive real ε * such that if ε ≤ ε * then, the tracking errors e1 , e2 , e3 , e4 and the ratio β are harmonics signals that continuously depends on y = k β − k1 I1Cos (ϕ1 ) + q ( t ) (16) e1 = e1 ( t,ε ) , β = β ( t ,ε ) with 98 e2 = e2 ( t,ε ) , e3 = e3 ( t ,ε ) , ε = 1 / ωn , i.e. e4 = e4 ( t,ε ) , International Conference on Control, Engineering & Information Technology (CEIT’14) Proceedings -Copyright IPCO-2014 ISSN 2356-5608 2) Furthermore, when ε → 0 , errors ( e1 ,e2 ,e3 ) vanishes and e4 , β converges: T P = ( 0 0 k1 I1Cos (ϕ1 ) 0 0 ) lim e1 ( t ,ε ) = 0 , lim e2 ( t,ε ) = 0 , lim e3 ( t,ε ) = 0 , ε →0 ε →0 lim e4 ( t,ε ) = c4 k ε →0 In order to get stability results regarding the system of interest (19), it is sufficient (thanks to averaging theory) to analyze the following averaged system: ε →0 k1 I1Cos (ϕ1 ) , lim β ( t ,ε ) = k1 I1Cos (ϕ1 ) ε →0 (20) Z = ε AZ + ε P k (21) To this end, notice that (21) has a unique equilibrium at: Proof. First, notice that (17) guarantees that β and its first time-derivative are available. Then, by Proposition 1, the errors e1 , e2 undergoes (13). This together with (16) and (17) show that the augmented state vector: Er = ( e1 e2 e3 e4 Z* = 0 0 0 k1 I1Cos (ϕ1 ) c4 k T k1 I1Cos (ϕ1 ) k (22) On the other hand, as (21) is linear, the stability properties of its equilibrium are fully determined by the statematrix A More specifically, the equilibrium Z * will be globally exponentially stable if the matrix A is Hurwitz. To this end, we note that the eigen values are zeros the following (18) characteristic polynomial: T e5 ) undergoes the following state equation: E r = AEr + P ( t ) where det ( λ I − A ) = a5 λ 5 + a4 λ 4 + a3 λ 3 + a2 λ 2 + a1λ + a0 −c1 0 A= 0 0 0 0 −c2 0 0 0 0 0 0 1 bc3 0 0 0 0 bc4 0 0 −k 0 −b Where a5 = 1, a4 = b + c1 + c2 , a3 = c1c2 + b ( c1 + c2 ) + kbc3 a2 = bc1c2 + kbc3 ( c1 + c2 ) + kbc4 , a1 = kbc1c2 c3 + kb ( c1 + c2 ) c4 , a0 = kbc1c2 c4 T P ( t ) = ( 0 0 k1 I1Cos (ϕ1 ) − q ( t ) 0 0 ) Putting The stability of the above system will now be analyzed using the averaging theory. Now introduce the time-scale change τ = ωn t . It is readily seen from (18) that Z( τ ) ≡ Er ( τ / ωn ) undergoes the differential equation: dZ (τ ) dτ = ε AZ (τ ) + ε P (τ ) (19) where The application of algebraic Routh criterion implies that the system is stable at the condition that: T τ P (τ ) = 0 0 k1 I1Cos (ϕ1 ) − q 0 0 ωn a4 > 0 , b1 > 0 , Now, let us introduce the average functions: 1 ε →0 2π Z = lim 1 P = lim ε →0 2π a4 a3 − a5 a2 a a −a a , b2 = 4 1 5 0 , b3 = 0 , a4 a4 b1 a2 − a4 b2 b1a0 − a4 b3 b4 = , b5 = , b6 = 0 , b1 b1 b b −b b b b −b b b7 = 4 2 1 5 , b8 = 0 , b9 = 0 , b10 = 7 5 8 4 , b4 b7 b11 = 0 , b12 = 0 b1 = b4 > 0 , b7 > 0 , b10 > 0 The equilibrium Z * of the linear system (21) is actually globally exponentially stable. Applying Theorem 4.10 in [13], one concludes that there exists a ε * > 0 such that for ε < ε * , the differential equation (19) has a harmonic solution Z = Z (t , ε ) that continuously depends on ε . Moreover, one 2π ∫ Z (τ )dτ 0 2π ∫ P (τ )dτ has 0 lim Z (t , ε ) = Z * . This, together with (22), yields in ε →0 particular that: It follows from (19) that : 99 International Conference on Control, Engineering & Information Technology (CEIT’14) Proceedings -Copyright IPCO-2014 ISSN 2356-5608 lim e1 ( t ,ε ) = 0 , lim e2 ( t,ε ) = 0 , lim e3 ( t,ε ) = 0 , ε →0 ε →0 lim e4 ( t,ε ) = sinusoidal and in phase with the network voltages vn1 which shows that the power factor correction is achieved. Fig. 7 shows the control laws u1 and u2 in inner-loop. Fig. 8 shows the waveform of the load current iL1 we observe that the wave current are almost square in shape so it is rich in harmonics. The compensation current i f 1 delivered by the three-phase ε →0 k1 I1Cos (ϕ1 ) c4 k ε →0 Then, using (17) one gets lim β ( t ,ε ) = shunt active power filter is shown in Figure 9. Fig. 10 shows that the network currents in1 is sinusoidal and therefore testifies the good performances of the inner loop. k1 I1Cos (ϕ1 ) k ε →0 1520 The Theorem is thus established. 1500 V. NUMERICAL SIMULATIONS 1480 In order to simulate the behavior of the three-phase shunt active power filter shown in Figure 1, the chosen nonlinear load is a three-phase full bridge rectifier which supplies an inductive load comprising a resistor RL and an inductor LL , the coil Lo is inserted to the input of three-phase Rectifier diL . The performances of the proposed dt controller are now numerically evaluated using a TPSAPF based system with the following characteristics: 1460 1440 1420 1400 1380 0 0.1 0.2 Figure 3. Bridge to limit the 0.3 Time(s) 0.4 0.5 0.6 Output voltage vo 1400.4 1400.2 TABLE I. PARAMETERS SIMULATION VALUES 1400 Parameters Network Symbol E f Rf Three-phase active shunt power filter Lf Cf Currents regulators Voltage regulator 1399.8 220 2 V 1399.6 50 Hz 1399.4 0.2 80 mΩ 3 mH 9000 µF Lo c1 = c2 10000 0.4 c3 2 × 10 c3 b 6 × 10 1000 0.24 0.26 0.28 0.3 Figure 4. Ripple in the Output voltage vo 0.8 LL 0.22 Time(s) 20 Ω 500 mH 5 mH RL Rectifier Values 0.6 −6 0.2 −5 0 0 0.1 0.2 Figure 5. The objective of the simulation is to illustrate the behavior of the controller in response to progressive changes of the voltage reference x3* . More specifically, the voltage reference goes from 1400V to 1500V. The performances of the controller are illustrated by Figs 3 to 10. As predicted by Theorem 1, the output voltage vo converges, in the mean, to its reference value with a good accuracy Figure 3, a zoom is made in Figure 4 it is observed that the voltage ripples oscillates at the frequency 2ωn , but their amplitude are too small compared to the average value of the signals, confirming thus Theorem 1. Fig. 5 shows that the ratio β always takes (in the mean) a constant value after transitional periods. Fig. 6 clearly shows that the network current in1 is 100 0.3 Time(s) 0.4 0.5 0.6 Control signal β 80 (vn1) / 5 in1 60 40 20 0 -20 -40 -60 -80 0 0.05 0.1 Time(s) 0.15 Figure 6. PFC checking: in1 in phase with vn1 0.2 International Conference on Control, Engineering & Information Technology (CEIT’14) Proceedings -Copyright IPCO-2014 ISSN 2356-5608 1 APPENDIX u1 u2 0.5 2L f β E q1 ( t ) = 0 -1 0 2L f β 4π iL 3 ( t ) − I1Sin ωn t + ϕ1 − Cf 3 + -0.5 0.02 0.04 0.06 0.08 2L f β 0.1 Time(s) + vn 3 ( t ) Cf Figure 7. Inner-loop Control 2 30 − − 0 -10 − -20 -30 0.1 0.11 0.12 0.13 Time(s) 0.14 0.15 0.16 Cf Cf Cf 2 L f c1e1 Cf − q2 ( t ) = 0 -5 -10 0.11 0.12 0.13 Time(s) 0.14 0.15 0.16 + 0.15 iL 3 ( t ) ( Cf Cf 4L f β − VI. CONCLUSION − Cf vn1 ( t ) 4 L f e1β Cf Cf − + 2L f Cf 4L f Cf 4L f β Cf iL1 ( t ) ( dt + dt − ( iL1 ( t ) iL1 ( t ) − I1Sin (ωnt + ϕ1 ) Cf ) − 4L f β 2 v 2 L f e2 Cf 4π Sin 2ωnt + 2ϕ1 − 3 2 L f β EI1ωn dt Cf 2 L f β e1 dvn3 ( t ) Cf diL1 ( t ) ( iL1 ( t ) − I1Sin (ωnt + ϕ1 ) ) d iL1 ( t ) − I1Sin ( ωnt + ϕ1 ) dt L f I12ωn vn1 ( t ) − Sin ( 2ωnt + ϕ1 ) + 2 L f e1 diL 3 ( t ) Cf ) ) iL1 ( t ) + 4 L f β EI1ωn Figure 10. Network current in1 101 Cf vn3 ( t ) Cf 4 L f e1 0.16 In this paper, a nonlinear controller is proposed for the three-phase shunt active power filters. It aims to compensate the harmonic current and reactive power caused by non-linear loads. The problem is processed using a non-linear controller with two cascaded loops. The inner loop which is formed by two non-linear regulators is designed, using the Backstepping technique, to compensate the current harmonics. The outer loop is designed to regulate the DC bus voltage of the inverter. The simulation results show that it performs perfectly the objectives of pursuit. In addition, it is robust against changes in setpoint DC bus voltage. 4 L f c2 e2 4 L f e1 diL1 ( t ) 4 L f e1β dvn1 ( t ) x4 βx + iL1 ( t ) − 4 vn1 ( t ) − Cf Cf Cf dt Cf dt -20 0.14 Cf vn 3 ( t ) + 2 EI1 4 Cos ( 2ωnt + ϕ1 ) − vn1 ( t ) iL1 ( t ) − I1Sin (ωnt + ϕ1 ) Cf Cf -10 0.13 Time(s) 2 L f c1e1β + + 0.12 vn3 ( t ) 8L f β d ( vn3 ( t ) iL 3 ( t ) ) 16 L f dvn23 ( t ) + − Cf dt Cf dt ( − 0.11 Cf 2 4π β E2 vn3 ( t ) iL1 ( t ) − I1Sin (ωnt + ϕ1 ) − Cos 2ωnt − Cf Cf 3 10 -30 0.1 Cf 4 L f e2 β iL 3 ( t ) + − 30 0 4 L f e2 2β E 2 2 EI1 4π Cos ( 2ωnt ) + Cos 2ωnt + ϕ1 − Cf Cf 3 Figure 9. Compensation current i f 1 20 − 4 L f e2 diL 3 ( t ) − C dt f − -15 -20 0.1 4π Sin 2ωn t − 3 iL 3 ( t ) − 4 L f c2 e2 β 10 5 dt dt dt dvn1 ( t ) dt d ( iL1 ( t ) − I1Sin (ωn t + ϕ1 ) ) 4 L f e2 β dvn3 ( t ) 4 L f diL23 ( t ) Figure 8. Load current iL1 15 2 L f β E ωn 20 10 4π I1ωn Sin 2ωn t + 2ϕ1 − 3 Cf 4π Sin 2ωnt + ϕ1 − 3 ) n1 iL1 ( t ) − (t ) dvn1 ( t ) dt dvn1 ( t ) dt 2 L f e2 β Cf vn1 ( t ) International Conference on Control, Engineering & Information Technology (CEIT’14) Proceedings -Copyright IPCO-2014 ISSN 2356-5608 REFERENCES q3 ( t ) = + 4π d iL 3 ( t ) − I1Sin ωn t + ϕ1 − 3 vn1 ( t ) dt 2L f β Cf L f β 2 E 2ωn − 4π Sin 2ωn t − 3 Cf − 4 L f c1e1β Cf vn1 ( t ) + 2 L f c2 e2 Cf [2] 2 L f c1e1 iL1 ( t ) + C f iL1 ( t ) − 2 L f c2 e2 β Cf [3] vn1 ( t ) [4] EI 4π + 1 Cos 2ωn t + ϕ1 − 3 Cf − − [5] 2 4π vn1 ( t ) iL 3 ( t ) − I1Sin ωn t + ϕ1 − 3 Cf β E2 Cf 4π Cos 2ωn t − 3 − + − 2β E 2 8π Cos 2ωn t − Cf 3 2 L f e2 diL1 ( t ) Cf dt 2 L f β e1 Cf 2L f Cf − vn3 ( t ) − iL 3 ( t ) Cf Cf x4 β x4 + C iL 3 ( t ) − C vn3 ( t ) f f 2 L f β e2 dvn1 ( t ) Lf [6] 8π 2 EI1 + C Cos 2ωn t + ϕ1 − 3 f 4π d iL 3 ( t ) − I1Sin ωn t + ϕ1 − 3 4 vn 3 ( t ) − Cf dt − dt − I12ωn Sin 2ωn t 2 L f e1 Cf 4π + 2ϕ1 − 3 d ( iL1 ( t ) − I1Sin (ωn t + ϕ1 ) ) dt − − 2 L f c2 e2 e1 Cf 4 L f e2 e2 Cf + − [7] [8] [9] [10] iL 3 ( t ) 4e1 vn1 ( t ) Cf 2 x e 4 L f e1e1 2 L f e1e2 4 L f c1e1 4e − 1 vn3 ( t ) + 4 1 − − + Cf Cf Cf Cf Cf + [1] [11] [12] [13] 2 L f e2 e1 xe 2e2 2e vn1 ( t ) − 2 vn3 ( t ) + 4 2 − Cf Cf Cf Cf 2 L f c1e1e2 Cf + 4 L f c2 e22 Cf 102 J. 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