Published in IET Power Electronics Received on 11th February 2009 Revised on 10th June 2009 doi: 10.1049/iet-pel.2009.0048 ISSN 1755-4535 Exponentially convergent estimator to improve performance of voltage source converters A.E. Leon 1 J.A. Solsona 1 M.I. Valla 2 1 Departamento de Ingenierı ´a Ele ´ctrica y de Computadoras, Instituto de Investigaciones en Ingenierı ´a Ele ´ctrica (IIIE) ‘Alfredo Desages’, Universidad Nacional del Sur (UNS), Avenida Alem 1253, Bahı ´a Blanca, P.O. 8000, Argentina 2 Laboratorio de Electro ´nica Industrial, Control e Instrumentacio ´n (LEICI) and CONICET, Facultad de Ingenierı ´a, Universidad Nacional de La Plata, La Plata, P.O. 1900, Argentina E-mail: andreseleon@gmail.com Abstract: In this study, the authors introduce an exponentially convergent estimator of load power to improve the performance of voltage source converter (VSC) applications. This estimator, based on a non-linear reduced-order observer, is used to estimate the load power and its time derivative. The estimated signals are feedforwarded to build a non-linear controller in order to improve the whole system performance. The main advantage of the proposed estimator is that it presents linear error dynamics which allows to design the observer by using linear techniques. In this way, the observer transient performance (i.e. convergence rate) is clearly established in the whole VSC operation range. The feedback controller is based on input–output feedback linearisation considering the energy as output so that the DC-voltage internal dynamics is avoided and linear control strategies can be also used in the controller design. Several tests – including performance in presence of switching frequency, parameter uncertainties and disturbances in the AC input voltage – validate the proposed technique. 1 Introduction Voltage source converters (VSCs) are used in many industrial applications. These converters present unity power factor, sinusoidal input current, accurate DC-voltage control, bi- directional power flow and low total harmonics distortion index. Among others, these features are very useful in AC– DC–AC power conversion [1, 2], adjustable speed drives [3], active and hybrid power filters [4, 5] and electric vehicles [6]. VSCs are also being used in high power applications as basic cells of several technologies used in flexible AC transmission systems [7, 8] and renewable energy systems [9–12]. This illustrates the importance of designing controllers to obtain high-performance VSCs in current and even future applications. Researchers have developed several strategies in order to control VSCs. Controllers based on Jacobian linearisation around an equilibrium point have been introduced in [13–15]. Among non-linear approaches, fuzzy logic control [16, 17], sliding mode technique [18], passivity-based control [19] and direct power control [20] have been applied. When DC voltage is selected as output, a non-linear unstable internal dynamics appears (see [21]). This drawback is overcome through current inner loops using feedback linearisation which has currents as outputs. Then, a slower outer DC-voltage loop is designed via a PI-based cascade control [21–24]. Some other authors have proposed converter energy as output in feedback controller design to accomplish a minimum-phase output and faster DC-voltage control [25–28]. The design of controllers to attain high performance depends strongly on the kind of load model. In some cases, a known load (for instance, a resistance, an impedance or a current source) is considered [14, 29–31]. However, in applications where the VSC is connected to another VSC, it is convenient to model the load as a power load [32]. 668 IET Power Electron., 2010, Vol. 3, Iss. 5, pp. 668–680 & The Institution of Engineering and Technology 2010 doi: 10.1049/iet-pel.2009.0048 www.ietdl.org