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On-line Measurement of Inward - Outward Impedances for Stability Assessment

Image of equivalent circuit and bode plot for inward, outward impedance
Fig.1. Calculated Inward - Outward impedances (Eq.2) compared with those obtained
using linearization (conventional approach - superscript IMU)
The tremendous increase in the employment of power electronics in the production, transfer, and consumption of energy not only enables a sustainable future, it undoubtedly brings major energy savings and stimulating improvements to people's quality of life. However, these gains are not for "free". This trend is considerably changing the nature of the sources and the loads in the electrical grid, altering their mild properties, and inflicting low-frequency dynamic interactions that did not exist in the conventional power system before. To be able to understand, analyze, design, and dynamically control the existing and future power systems, it is unarguably required to develop concepts and tools that offer better insights into the system-level behavior and stability of the grid.

This paper presents an alternative on-line stability monitoring method, where by perturbing the converter's duty-cycle, source output impedance and load input admittance can be simultaneously measured (Eq.1). Using these measurements, and combining them with the un-terminated transfer functions of the dc-dc converter, equivalent system output impedance (and admittance) can be calculated out (Eq.2)., offering a great insight into system stability (observed at both, input and output side of the converter) - Fig.1. Additionally, using all terminal impedances, Nyquist plots for both sides can also be plotted offering better insight into system stability margins.

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