Abstract
This paper proposes a nonlinear decoupled current control scheme for a grid-connected inverter with LCL filter. Decoupling the active and reactive current control channels is one of the main demands in the control of inverters. For inverters with an L filter, the decoupling can be achieved by a proper feed-forward of grid voltages. However, the coupling of channels is a complex issue for converters with LCL filters. The resonance mode of the LCL filter may cause instability, which adds more complexity to the analysis. In this paper, state equations of the system are provided, which highlight the coupling between active and reactive currents injected into the grid. Accordingly, a non-linear control scheme is proposed which effectively decouples the channels and dampens the resonant modes of the LCL filter. The stability of the proposed control method is verified by the Lyapunov criterion. Independency of the system stability to the grid-impedance is another feature of the proposed approach. Moreover, only grid-side currents are needed for implementation of the proposed scheme, avoiding the need for additional current sensors for the output capacitor and grid-side inductor. For accurate modelling of the inverter, the computation and PWM sampling delays are included in the controller design. Finally, various case studies are provided that verify the performance of the proposed approach and the stability of the system.
Highlights
Voltage source converters (VSCs) are widely used in modern power systems for the integration of renewable energies, distributed power generation, energy storage systems, electric vehicle chargers, high and medium voltage transmission systems, hybrid AC/DC microgrids, motor derive systems, DC homes, etc. [1,2]
In feedback-based methods, in addition to using the state variable of grid-side current, other state variables are used in the control loop, which is known as the multi loop method
This paper proposes a non-linear control method for a grid-connected inverter with an LCL filter that satisfies both requirements
Summary
Voltage source converters (VSCs) are widely used in modern power systems for the integration of renewable energies, distributed power generation, energy storage systems, electric vehicle chargers, high and medium voltage transmission systems, hybrid AC/DC microgrids, motor derive systems, DC homes, etc. [1,2]. Apart from significant benefits coming from the usage of VSCs, their switching-based operation yields to harmonically polluted output voltages, which creates harmonic currents injected into the grid. International standards have provided restrictions on the harmonic content, which are normally satisfied by adding harmonic filters in the output of the inverters. For this purpose, standard types of L and LCL filters are well defined in the literature [3]. Using an LCL-filter is more common in high power applications, in which the switching frequency is lowered to reduce switching losses and VSC cost. The existing solutions for both challenges are provided
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