This paper presents a small-signal analysis of a stand-alone ac system composed by two or more voltage source inverters connected in parallel. Each inverter is controlled using a modified frequency and voltage droop method, which consists of the conventional frequency and voltage droop method including a further loop that feeds the power deviation from the nominal value back into the inverter phase. The proposed small-signal model of the system is presented, which is validated by simulation results. A root locus of the system is presented, which shows that the higher is the gain of further loop, the higher is the damping of the system. In order to show the viability of the proposed controller and proposed model, the results obtained from the model are compared with simulations and experimental results.
-This paper proposes the application of an accurate, fast and stable method to be used by the control systems in the selective compensation of harmonic currents in three phase systems susceptible to frequency changes and phase jumps. This application is based on a MSOGI-FLL (Multiple Second Order Generalized Integrators -Frequency Locked Loop) method, which is composed of multiple double adaptive filters called Dual Second Order Generalized Integrators (DSOGI) coupled to the frequency detector entitled Frequency Locked Loop (FLL) which keeps the set synchronized, even in abnormal situations.
This work presents the small signal stability of sinusoidal PWM inverters connected in parallel in a standalone system, that is, without the presence of an infinite bus. The inverters have no interconnection control, so that only local variables are referred. The control strategy of each inverter unit is based on the Pxω and QxV droops and it has an additional loop δxP designed to increase the damping. This loop feeds the deviation of the active power of inverter from its nominal value back into the reference voltage phase, resulting in a control action that produces small shifts in the inverter voltage phase. A small signal model in state space form is proposed, which corresponds to the system linearization around an arbitrary equilibrium point. The proposed model is validated by simulation and experimental results.Root locus analyses are also presented to show the effect of the additional loop in the system damping. The proposed small signal model, main contribution of the presente work, allows the knowledge in advance of the stability and power sharing of an islanded microgrid composed by PWM sinusoidal inverters connected in parallel.
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