Direct current control: A novel control strategy for harmonic and reactive compensation with active filters under unbalanced operation J.M. Carrasco, E. Galvan, M. Perales Dept of Electrical Engineering University of Seville, Spain carrasco,galvan,perales @gte.esi.us.es G. Escobar, A.M. Stankovi´ c Dept Electrical and Computer Eng Northeastern University, USA gescobar,astankov @ece.neu.edu P. Mattavelli Dept of Electrical Engineering University of Padova, Italy mattavelli@light.dei.unipd.it Abstract—In this paper we present a control technique for the compensa- tion of selected harmonics in a line using an active filter. We treat the cases where both, voltage source and load current, are under unbalanced oper- ation. Our approach does not belong to the class of high gain controllers (advocated in most conventional approaches to reduce the tracking errors) so we avoid the problems entailed by the excitation of unmodeled dynam- ics. Moreover, the performance of the system is not limited by the low con- trol bandwidth since we account for the information of the references time derivative (as dictated by classical control theory) and compensate for the unavoidable delay associated to the digital implementation. Keywords— Active filters, reactive power control, dissipative systems, adaptive control, nonlinear systems. I. I NTRODUCTION Flexible AC Transmission systems (FACTS) [4] based on voltage–sourced inverters, such as Statcom’s (Static syn- chronous compensator), are finding increased utilization in power systems because of their ability to provide improved performance compared with conventional thyristor–based con- verters. Moreover, at the distribution level, especially in the medium voltage applications, the PWM switching frequency of D–Statcom’s (Distribution Statcom) can be increased to a few kHz (1–5kHz). Then the compensation of low order harmonics is becoming possible to comply with harmonic standards (such as IEEE 519), emulating the typical active filter behavior. For such purpose, however, conventional techniques [5] used for ac- tive power filters seem to reduce the achievable performance mainly due to the limited control bandwidth, delays in digital implementation [6]. In this paper we present a control technique for the compen- sation of selected harmonics in a line using an active filter. The cases where both, voltage source and load current, are distorted and under unbalanced operation are treated. The proposed ap- proach is intended to alleviate the inconveniences mentioned above. Instrumental in our approach is the use of some resonant type filters to reconstruct a linear combination of the currents reference and their time derivatives which do not introduce any phase delay in the loop. The idea behind the proposed filters is to extract the harmonic component of a given distorted signal, by tuning their resonant frequency at the value of the harmonic selected. Due to the tracking nature of the problem, the need of the information provided by the time derivative of the cur- rents reference is imminent to guarantee perfect tracking. The information of current reference and its time derivative is then introduced, in the form of a linear combination, into a current control loop that forces the injected current from the active filter to track its reference. The current control algorithm is actually a switching control strategy based on the minimization of a cost function (in terms of the currents tracking error) to generate the sequence of control vectors, i.e., the switch positions. We refer to this technique as Direct Current Control (DCC) since the con- trol vectors to be applied are chosen to directly control both cur- rents without the need of an auxiliary PWM. A one–step–ahead modification is also considered in the criteria to compensate for the lower switching frequency limit imposed by physical con- straints. The paper is composed as follows. In Section 1 we give a brief description of the system model, and we state the control objective. In Section 2 a description of the filters, used to ex- tract the harmonic components of a distorted signal, is given, as well as a description of their principle of operation and a brief justification. At the end of the section the algorithm to generate the reference function is discussed. In section 3 the switching control strategy is presented and a discussion of implementation is given. As a result, a bank of filters operating in the refer- ence signal, constructed in the previous section, is obtained to generate the input signal for the switching strategy. In the last section, some simulation results are presented which exhibit the improvements in performance obtained with the proposed algo- rithm with respect to the conventional sliding mode. Experi- mental result are in progress and they will be presented in the final version of the paper. II. PROBLEM FORMULATION A three–phase three–wire of VSI working as D–Statcom (or as PWM rectifier) is depicted in Fig. 1. The system is mainly designed to compensate reactive power, unbalance and harmon- ics in distribution lines. The need for compensation arises from non-linear and distorting loads that we represent as a current source . This main task is performed by injecting to the line the compensating currents to force the currents on each line to be proportional to the positive sequence fundamental harmonic component of the respective source line voltage . To accomplish this, the capacitor is charged to a certain voltage value and kept at this value during the normal operation by the controller. Switching and other losses are aggregated and modeled as an unknown con- stant resistive element . Since both the load current and the source voltages are allowed to be distorted and unbalanced, the amplitudes and phase angles could take arbitrary values. The dynamics of this system in terms of – coordinates can be expressed by the following model (1) (2) IECON'01: The 27th Annual Conference of the IEEE Industrial Electronics Society 0-7803-7108-9/01/$10.00 (C)2001 IEEE 1138