Zbigniew Staroszczyk

A method for real-time, wide-band identification of the source impedance in power systems

In this paper, the principal problems of distribution system identification and accurate measurements of distribution system impedance frequency contour are discussed. Some new results of field identification are presented. The focus of this paper is on the effective method of signal processing adapted to real signals which appear in the power systems. The principal problem of accurate identification is in the system nonstationariness. Its influence on accuracy is discussed and illustrated with experimental results. The proposed method of impedance identification based on short 40-ms system observations is relatively insensitive to power system nonstationariness. The presented discussion confirms that, with the proper signal processing, 1-s-long system observations are quite sufficient to describe the system with an accuracy to satisfy practical demands.

Problems in real-time wide band identification of power systems

In the paper principal problem of distribution system identification-distribution system impedance frequency contour accurate measurements, are discussed and some new results of field identification are presented. The focus of the paper is on the effective method of signal processing adapted to real signals which appear in the power systems. The principal problem of accurate identification is in the system nonstationarity-its influence on results is discussed and illustrated with experiment results. Presented discussion confirms that with the proper signal processing only 1 s long system observations are sufficient to describe the system with the accuracy satisfying practical demands.

Real-time power system linear model identification: instrumentation and algorithms

In the paper the solution of the measuring system for power system linear model identification is presented. Power signal properties which influence signal processing methods are discussed. It is pointed out how important is the system impedance information to system description, especially if the system is loaded with nonsinusoidal currents. Two methods of impedance measurements, assuring good quality of impedance estimation for the non-stationary power system are discussed. The presentation is illustrated with experimental data from real system measurements.

Time-dependent power systems impedance-interpretation and measuring problems

Problems of interpretation and measurement of the time-local impedance of a single phase power system loaded with switched and nonlinear loads are discussed. Simulations and measurements are presented for an active method of system identification for which narrow current spikes serve as system testing signals. Two practical realizations of harmonic generating devices are discussed and compared. The fact that even a highly harmonics polluting user cannot disturb too much the system voltage waveforms, makes the time-local linear system model useful if switched or even nonlinear loads are installed in the system. The problem of linear model approximation of a real power system is considered.

High accuracy harmonics identification and power measurements in power systems

The paper deals with the amplitude, phase, frequency and reactive power measurements for voltage/current power signals affected by the a/d converter finite word length noise. The version of the interpolating method of FFT spectrum processing was proposed with the focus on method efficiency. The problem was: how to process the corrupted signals to obtain accurate measurements and how to assure the final measurement accuracy for every quantity when a/d converter word size is superimposed. Some results of simulation were presented which confirm theoretical expectations and allow to relate presented data to the different user requirements.<<ETX>>

Power system nonstationarity and accurate power system identification procedures

Power system nonstationarity is the main source of errors in the system analysis and description with the use of DSP. Accurate and advanced DSP has to be used for power system identification. It is based on high resolution DFT, well known from its ideal accuracy for coherently sampled signals. As the power system is nonstationary such conditions are violated. In this paper the principal sources of errors in real power system DSP measurements are presented. The DSP methods of identification, insensitive to power system nonstationarity are proposed and verified with simulations and experiments. Few examples of results of real system identification are shown to inform the reader on actually assured identification accuracy.

Power system time variance-LPTV model implementation and identification problems

In power system harmonic studies the power system nonlinearity is sometimes observed. It mainly manifests as a time variance of power system properties as the nonlinear system components working points are set by relatively stable system voltage waveforms. The LPTV (linear periodically time variant) power system model is a natural extention of the time invariant model, and better describes the power system voltage/current relations. As the LPTV model deals with parametric system description it is more difficult in implementation and identification. Two approaches to LPTV system description are presented in the paper. The interpolating methods are discussed which allow for efficient model identification in nonstationary power system conditions.

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

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.

Combined, experimental data supported simulations in development of power grid impedance identification methods

The development of invasive methods for power grid investigations is not easy as has to be made in comfortable conditions (laboratory equipped with instrumentation and enough computing power), while grid signals and experiments are accessible in field conditions. Ad hock planned invasive experiments during research development are dangerous to the power system and the software/hardware developer. The paper is focused on development of invasive methods of grid impedance determination with the use of experiment data supported simulations. Simulations form standard tool replacing difficult to access reality, however can not deliver enough detailed description of signals which exist in real power grids, and which affect impedance identification/determination DSP. In the paper the combined method for generating valuable signals, which describe the excited real grid, is proposed. The method needs passive (non-invasive) grid observations and enriches them with testing signals of the simulated grid. In such the approach the simulations are responsible only for impedance information, while observations deliver full information on the real grid signal properties. The usefulness of the method for impedance devoted DSP improvement was illustrated with the examples of a grid exciter development.

Experimental investigations of the quality of power system small-scale transformer modeling

The paper presents problems with experimental verification of the quality of small scale transformer model which is aimed to describe its “harmonics” properties in the kHz frequency range. Two small, three-phase power transformers (220/380V, 5 kVA) were fully accessible for experimental observations (primary and secondary side voltages and currents) with the use of the low cost, low voltage isolating/conditioning circuits. As they are not the operating power system transformers, they could be extracted from the grid and their frequency properties were relatively easily experimentally investigated in energized and deenergized states. That way relatively full knowledge on transformers properties was collected, what allowed for use the correctly parameterized transformer model in the laboratory/university subgrid modeling. The accuracy of the grid model in different load configurations was used for verifying the correctness of the transformer model. In the paper only single phase connections of transformer are discussed. Interesting data on their three phase behavior were collected but are not presented here due lack of space and its not too clear current physical explanation.