Abstract – Voltage source inverters in micro-grid applications need the effective control of the filter output voltages. Harmonic compensation is a mandatory request to eliminate voltage distortion due to non-linear and unbalanced loads. The use of resonant controllers allows achieving high quality voltage regulation. The proposed paper investigates a closed form tuning strategy for controllers’ parameters selection based on the symmetrical frequency behavior of the whole resonant controller structure. Symmetrically tuned controllers allow achieving the largest bandwidth around the resonance frequency in a multi- resonance controller structure. Index Terms – Resonant controllers, integral resonant control, harmonic compensator, four-leg inverter, closed form control tuning, power generation systems. I. INTRODUCTION Over the years, power inverters have found several applications in numerous either grid-tied or stand-alone systems. More stringent regulations on harmonic distortion and power quality require even more complex control architectures, that conventional PI and PID controllers are not able to effectively provide. Compensation of distorted loads is mandatory in both grid connected and off-grid systems. In micro-grid applications the regulation of the inverter output voltage is required to provide constant voltage constant frequency to the loads. As in grid-tied applications, also in stand-alone systems the ability of the inverter control structure to regulate to zero the injected DC-component is a mandatory requirement. However, the feedback signals to control the DC- component are different in grid-tied configurations with respect to off-grid applications. In grid-connected systems, the feedback is usually closed on the inverter current, either grid side or inverter side, and its DC-current component is forced to be zero. In stand-alone application, main control loop is closed on the filtered output voltages and the DC-voltage component must be forced to be zero. The ability of resonant controllers to track sinusoidal references and saving computational load have made this controller topology widely used when sinusoidal current or voltage must be regulated at inverter output. However, they are totally blind from DC-component point of view (definitely unsuitable for regulating to zero the injected DC-component), as it is shown in Section III.A of the present paper. Several studies have been proposed so far concerning the use of Resonant Controller (RC) structures in both grid-tied and off-grid applications. The benefits in adopting this kind of controllers are well known and deeply explained as in [1-5]. Moreover, RCs have been combined in both single loop and double loop control schemes to accomplish voltage and current regulation when needed [6-9]. Usually in a single control loop, the control action is performed only on the inverter output voltage or current. When a double control loop is implemented, the outer loop is the VSI output voltage whereas the inner loop regulates the supplied current. Different combinations of transfer functions as proportional- resonant and integral-resonant have been analyzed in [8]. In [8] a single loop control structure with integral compensation and current limiting feature is finally proposed for a 50 Hz stand-alone generation system. Also in [9] the single loop architecture is preferred and applied to a 400 Hz ground power unit. Resonant controllers’ application has been extended also to electric drives. Both induction motor and permanent-magnet base drives have been considered to be controlled by RCs as shown in [13- 15]. Some basic criteria for RCs tuning have been preliminary depicted in [17] where the resonant control structure is compared with the H-infinity algorithm. Controllers gain and width are mainly selected by the designer using a trial and error procedure which is usually verified through root-locus and Bode diagrams. After that, the controller is digitally implemented and the performances are verified by simulations and experimental tests. In [18] a different approach based on the Nyquist diagram is proposed for Proportional+Resonant (PR) and Vector Proportional+Integral (VPI) controllers. However, the mathematical-analytical tuning approach for RCs is usually missing; for this reason, the aim of the paper is to provide a closed form tuning procedure to be used to select the parameters of each controller being part of a Multi Resonant Control System. According to the proposed approach, the control designer is able to select the desired controller gain with respect to the harmonic to be regulated, and the controller width will be automatically selected by the tuning algorithm. When two or more resonant controllers are combined together, the achieved tuned controller structure results in a symmetrical form in terms of magnitude and phase distribution around each controller resonance frequency. The symmetrical tuning assures the maximum distance in term of frequency between two adjacent resonant controllers reducing their reciprocal interaction. The extension of the tuning procedure from the pure Resonant (R) structure to the more suitable Integral+Resonant (I+R) scheme, which is able to compensate the DC-component, is also shown and experimentally verified. Symmetrical Tuning for Resonant Controllers in Inverter based Micro-Grid Applications A. Lidozzi, G. Lo Calzo, L. Solero, F. Crescimbini University of ROMA TRE, Department of Engineering Via della Vasca Navale 79, 00146 Roma (Italy). lidozzi@uniroma3.it 755 978-1-4799-0336-8/13/$31.00 ©2013 IEEE