Impedance in voltage-current relations description of the power system PCC - experimental investigations of the accuracy of the LTI system model

Abstract

The paper presents experimental verification of the quality of LTI power grid model valid for the frequency range of 8 kHz. The single PCC was investigated so the paper is the local, University grid investigations oriented, however concerns two general important problems: one is the absolute accuracy of real grid impedance measurements to which no direct reference data are accessible, the second is the validity of the LTI model, which is used for description of the complex structure composed of nonlinear and switched power installations. The LTI models form very interesting and useful approximation of the complex reality and have meaningful time and frequency domain interpretation. When used for power grids they deal with their Norton/Thevenin equivalent models. Two components of such a model: impedance and system side sources describe the grid and are discussed in the paper. Rich experimental data were referenced to the structural model of the grid so the paper gives evaluative information on the quality of ad-choc grid modeling. Interesting, incremental method for the estimation of absolute impedance errors was proposed and used for improvement of the implemented DSP. Time varying nature of the investigated grid impedance was presented, and system errors with implementation of LTI model to such a time-varying PCC description are discussed.

Smart grids instrumentation-obtaining subgrid impedance information

“…The impedance measuring conditions and DSP parameters are set in the controlling fields, and in the answer, a renewed PCC impedance pattern and voltage harmonics PCC phasors are drawn each few seconds. The chosen set of recent harmonics observations (new in‐last out) is presented on the screen in the form of the polar plot with logarithmic level scale (Figure ) .…”

“…The chosen set of recent harmonics observations (new in-last out) is presented on the screen in the form of the polar plot with logarithmic level scale ( Figure 2) [4].…”

“…The fundamentals of linear model grid impedance determination were given in [4,7,8,27,28] the papers are easily accessible in the netthat is why only comments to the method are given here. The author's method is invasive, with the use of short-time current impulse grid excitationsas such, the impedance data are calculated on request when needed.…”

“…The impedance frequency contour of the grid can serve as the source for its linear Thevenin model or the grid descriptor, which allows for obtaining its correct numerical model. Such a model of the grid can be used in simulation packages , mainly for power quality control and improvement.…”

“…With the use of digital signal processing (DSP) and wide band excitations the grid impedance frequency contour can be found basing on two (excited and non‐excited) PCC states observations. Such a method is implemented in the Grid Impedance/Harmonics Analyzer, (GIHA) developed at the Warsaw University of Technology — the only closed power grid impedance measuring instrumentation used in the research today .…”