Abstract
A novel nonlinear current-limiting controller for three-phase grid-tied droop-controlled inverters that is capable of offering voltage support during balanced and unbalanced grid voltage drops is proposed in this paper. The proposed controller introduces a unified structure under both normal and abnormal grid conditions operating as a droop controller or following the recent fault-ride-through requirement to provide voltage support. In the case of unbalanced faults, the inverter can further inject or absorb the required negative sequence real and reactive power to eliminate the negative sequence voltage at the point of common coupling (PCC) whilst ensuring at all times boundedness for the grid current. To accomplish this task, a novel and easily implementable method for dividing the available current into the two sequences (positive and negative) is proposed, suitably adapting the proposed controller parameters. Furthermore, nonlinear input-to-state stability theory is used to guarantee that the total grid current remains limited below its given maximum value under both normal and abnormal grid conditions. Asymptotic stability for any equilibrium point of the closed-loop system in the bounded operating range is also analytically proven for first time using interconnected-systems stability analysis irrespective of the system parameters. The proposed control concept is verified using an OPAL-RT real-time digital simulation system for a three-phase inverter connected to the grid.
Highlights
In the recent years, the smart inverter concept has attracted a lot of attention since its adaptability and plug and play properties enable the seamless integration of distributed energy resources (DERs) into the generation smart grid [1]
The main novelty of this paper is that a unified control structure is proposed that achieves: (i) voltage and frequency support under normal grid conditions and compliance with the “voltage support concept” for balanced and unbalanced faulty grid conditions, with an inherent current limitation; (ii) a non-dynamic function for dividing the total current into the two sequences according to the grid conditions, during unbalanced grid faults and (iii) a rigorous stability analysis for the closed-loop system, regardless of the system parameters
Compared to the current controllers that limit the inverter current on both sequences by limiting their reference values [11,18], or to the methods employing saturation units which can lead to instability under faults or power step changes, as showcased in [10,12,13], here a droop controller is proposed and grid current boundedness is guaranteed from the input-to-state stability (ISS) property of the closed-loop system
Summary
The smart inverter concept has attracted a lot of attention since its adaptability and plug and play properties enable the seamless integration of distributed energy resources (DERs) into the generation smart grid [1]. The main novelty of this paper is that a unified control structure is proposed that achieves: (i) voltage and frequency support (droop control) under normal grid conditions and compliance with the “voltage support concept” for balanced and unbalanced faulty grid conditions, with an inherent current limitation; (ii) a non-dynamic function for dividing the total current into the two sequences according to the grid conditions, during unbalanced grid faults and (iii) a rigorous stability analysis for the closed-loop system, regardless of the system parameters. Compared to the current controllers that limit the inverter current on both sequences by limiting their reference values [11,18], or to the methods employing saturation units which can lead to instability under faults or power step changes, as showcased in [10,12,13], here a droop controller is proposed and grid current boundedness is guaranteed from the input-to-state stability (ISS) property of the closed-loop system. Extended real-time simulation results are provided in order to validate the performance of the proposed control approach
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