Verification and Implementation of Pseudo-Random-Binary-Sequences for Online Determination of Grid Impedance Spectrum

Renewable energy technologies as Distributed Energy Sources (DER) connected via power inverter on medium or low voltage grid are expected to become a more important part of future generation system. Therefore, it has to be ensured that their integration doesn’t jeopardize the stability and performance of the power system which can only be attained with improved monitoring and control of system parameters on real time basis such as the equivalent grid impedance seen from the grid-tied DER. In order to ensure this, this paper investigates a reliable and precise method for online equivalent grid impedance estimation based on Pseudo-Random-Binary-Sequences (PRBS), which has been extensively used in system identification, communication and information theory. It allows real time computation of complete spectrum of grid impedance at the Point of Common Coupling (PCC) by injecting broad spectrum identification patterns of harmonics and inter-harmonics. The impedance spectrum thus, can be used for filter design, power quality evaluations, grid status determination and inverter tuning. In this paper, a PRBS-based active identification method is tested and implemented on an off-grid system consisting of a 3 phase inverter prototype developed in the lab and has been used to identify the grid impedance spectrum of a 3 phase purely inductive load connected at the output of the inverter. The results obtained confirm that the method estimates the grid impedance spectrum over a significant frequency range with high resolution on real time basis.