A Non Standard Control Strategy for Active Power Filters for Unbalanced Conditions of the Power Mains R.L.A. Ribeiro 3 , F. Profumo 1 , C.B. Jacobina 2 , G. Griva 1 , E.R.C. Da Silva 2 , A.M.N. Lima 2 1 Politecnico di Torino - Dipartimento di Ingegneria Elettrica Industriale C.so Duca Abruzzi, 24 - 10129 TORINO, Italy Fax: ++39(011)564-7199 Email: <profumo@polito.it, rlucio@athena.polito.it> 2 Laborat´ orio de Eletrˆ onica Industrial e Acionamento de M´ aquinas, DEE, UFPB, Brazil 3 Departamento de Eletro-eletrˆ onica, CEFET, Maranh˜ ao, Brazil I. Abstract This paper investigates the utilization of a con- trolstrategyforactivepowersystememployedun- derunbalancedconditionsofthepowermains. The proposed scheme reduces the problems verified in the conventional strategies when the power mains is unbalanced. The system model under unbal- anced condition is derived and a suitable control strategy is presented. This strategy is composed by a parallel association of double sequence cur- rent controller and a resonant current controller. Withthiscontrolschemeafullcompensationofthe current load harmonics is achieved. The feasibility of the control system is validated by experimental results. II. Introduction The development of power electronic equipments, the in- tensive use of static converters and the great number of do- mestic electronic-based applications have deteriorated the quality of the power mains system. These non-linear loads generate current harmonics that can be asymmetric and can cause voltage drops on the supply network impedance resulting in unbalanced conditions. These effects can be worse in the case where the loads change randomly. As a result, conventional solutions like passive filters to reduce the current harmonic pollution are ineffective. Moreover, actually the regulations about the power flow of electrical energy has become strict and this has stimulated the use of active power compensation [1, 2]. The active power com- pensation is normally achieved with the help of switching power converters connected as an active filter to the load. With the great progress of the power electronics, active filters have been focused in a large number of published works [3, 4, 5, 6, 7, 8, 9]. The behavior of the active fil- ters under unbalanced conditions has been already studied and analyzed [10, 11]. Recently, various control schemes applied to control unbalanced three-phase systems con- taining PWM converters have been also introduced [12, 13, 14]. However, theses strategies have been used mostly to control the phase currents of the unbalanced loads. This paper proposes the utilization of a non standard control strategy applied to three-phase shunt active power filter systems operating under unbalanced conditions of the power mains. This unbalanced operation is verified as an asymmetry on the phase voltage amplitudes that can be caused by non-linear loads. III. System Description and Modelling Fig. 1 presents a shunt active filter system composed by the grid source, a three-phase controlled rectifier as a non-linear load and a voltage source inverter (VSI ). The grid source is composed by three balanced voltage sources (e s1 ,e s2 and e s3 ) with their respective internal impedances represented by series association of resistances and induc- tances (r sk and l sk , with k =1, 2, 3). The switching con- verter of the active power filter is connected to the mains by the filter impedances also composed by series associ- ation of resistances and inductances (r f and l f ). Fig. 2 presents the equivalent circuit of the shunt active filter sys- tem. In this circuit the effect due to the non-linear load is introduced by voltage sources u lk by considering that the load can be modelled as three current sources (i lk ). The converter phase voltage are represented by v fk . The impedances r tk and l tk refer to the equivalent impedances of grid and the filter (r tk = r sk + r f and l tk = l sk + l f ). The unbalanced condition of the mains is approximated by the inclusion of different internal impedances for each system phase (r s1 + l s1 = r s2 + l s2 = r s3 + l s3 ). This corresponds to have different phase voltages amplitudes at points v s1 , v s2 and v s3 (see Fig.1). Kirchoff’s laws applied to the equivalent circuit of Fig. 2 leads to e sk − v fk0 − v n0 + u lk = r tk i sk + l tk di sk dt (1) u lk = r f i lk + l f di lk dt (2) 896 U.S. Government work not protected by U.S. copyright 0-7803-7420-7/02/$17.00 © 2002 IEEE U.S. Government work not protected by U.S. copyright