to these spikes, damaging thus the control performance. This problem is worse in case of harmonics where the noise presence is still higher and the control signals themselves have a higher frequency.This paper deals with the implementation of parallel virtual admittances at harmonic frequencies. The virtual admittance approach provides advantages if compared with virtual impedance, as it does not introduce any derivative term in the controller. As the admittance is a virtual element this work takes advantage of this and considers a synchronous generation systems with different admittances at different frequencies. By means of this approach once the grid harmonic distortion is being analyzed, several harmonic virtual admittance loops can be implemented in the control of DC-AC converters, in order to attenuate the grid voltage distortion. These virtual admittances operate in parallel with the main voltage loop at fundamental frequency, in order to inject the required harmonic content at a specific frequency to enhance the voltage profile.On this paper the scope will be focused just in the harmonic attenuation. As it will be proven, being able to inject harmonics to the grid by this synchronous power control makes it possible to inject active and reactive power and also attenuate harmonic distortion appearing at the point of common coupling (PCC).
II. FUNDAMENTAL AND HARMONIC CURRENT CONTROL LOOPIn this chapter the control strategy applied is based on the enhanced emulation of a synchronous generator where the synchronization loop is inherently implemented in the control equations [5]. The main mechanical characteristics of the synchronous generators, collected by the swing equations, are the mechanical inertia and the damping effect. Following the electrical characteristic of the synchronous generator is enclosed in the virtual admittance block, which is described in detail in [6].These blocks are depicted in Fig. 1.Abstract -The appearance of harmonics in the grid voltage is an issue for the generation systems and grid connected consumers. This paper presents the basic control strategy to be implemented in Synchronous Power Control (SPC) based power converters, which contributes to reduce the harmonic content in the voltage by means of injecting different harmonic current to the grid using a separated virtual admittance algorithm. This control strategy is capable of generating such a current reference that attenuates the harmonic values at the point of connection of the converter.