Abstract
Whereas power-electronics-based power systems are expected to enable more integration of renewable energy sources, they could pose crucial challenges including stability issues due to the Thévenin impedance seen by inverters. Such problems could be mitigated by enabling the inverters to estimate the grid impedance by including a grid impedance estimation technique into their control loop. However, one aspect which has been overlooked thus far is that the accuracy of such grid impedance estimation techniques is significantly affected by various grid conditions. For instance, the unbalance in three-phase power systems causes unwanted oscillations at double the fundamental frequency in the inverters control loops. Therefore, this paper proposes a simple and reliable online estimation of the grid impedance under unbalanced conditions. The technique is based on wide-band impedance estimation incorporated into the control loop of the positive sequence of a three-phase grid-connected inverter equipped with a positive- and negative-sequence control (PNSC) strategy. Additionally, complex curve fitting is utilized to obtain parametric models of the grid impedance. To demonstrate the efficacy of the proposed grid impedance estimation technique, extensive case studies are performed. These include: (1) unbalanced operations of both resistive-inductive (RL) and resistive-inductive-capacitive (RLC) models of the grid, (2) background harmonics, and (3) asymmetrical impedances of the network.
Highlights
Potential stability issues [1,2] could be caused by a lack of grid impedance information in power-electronics-based power systems
The simulation has been implemented in MATLAB/Simulink environment, where piecewise linear electrical circuit simulation (PLECS) toolbox components are used for accurate modeling of power electronics components
A discrete-time control and estimation model is used for the estimation results in order to consider real sample measurements and to highlight the limitations of implementing a grid impedance estimation technique using pseudo-random binary sequence (PRBS)
Summary
Potential stability issues [1,2] could be caused by a lack of grid impedance information in power-electronics-based power systems. The estimation algorithm is implemented into the control loop of grid-connected inverter equipped with PNSC strategy; takes into account both resistive-inductive (RL) and resistive-inductive-capacitive (RLC) models of the low-voltage distribution network; considers asymmetrical line impedances of the distribution network; assesses the estimation performance in the presence of background harmonics in the grid voltage; evaluates the effects of the initialization parameters of the curve-fitting algorithm on the accuracy of the obtained parametric model These parameters are the PRBS frequency resolution ( f res ), the number of selected data points (N), and the minimum frequency of selected data points to be fitted ( f min ).
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