Marian-Stefan Nicolae

Real-time analysis using discrete wavelet transform in power systems

The paper is concerned with the analysis based on the Discrete Wavelet Transform (DWT) of (non)stationary regimes in power systems under real-time operation constraints. Different techniques (some original, other provided by Matlab) are employed to perform decompositions using trees with 10 levels for a signal in stationary regime and respectively in a complex non-stationary regime, when most of the signals parameters are affected (shape, periodicity, magnitude etc). Power indices are calculated in both regimes with different methods, correlating the results with the energetic evaluations. The abilities of different methods to detect faults and respectively to deal with severe asymmetries are analyzed in various situations. Considering the results and other important criteria (run time, memory consumptions, perfect reconstruction), practical recommendations are made with respect to the selection of the fastest reliable DWT algorithm depending on the operating conditions.

Wavelet packet transform, a reliable and fast method to obtain the fundamental components required for active filtering in power plants

The paper focuses on the joint analysis (using Wavelet Packet Transform (WPT)/Discrete Wavelet Transform (DWT) and respectively Fast Fourier Transform (FFT)) of voltages and currents acquired from a generator operating in the Power Plant Turceni, Romania, as a major stage in designing the components of an active power filter (APF). When fast methods, providing accurate results are used, fundamental active currents can be calculated and subtracted from the load currents. Afterward the results are to be inverted in order to obtain the reference currents for the APF. Firstly brief information about the algorithms implementing DWT/WPT, respectively FFT are given. Afterward a study of errors associated to the evaluation of the three major RMS (for fundamental frequency, for distortions and total) is presented. The errors were yielded when analyzing signals with known characteristics (sinusoidal and polluted by a single harmonic), by using 2 interpolation methods (IM): spline and respectively shape-preserving piecewise cubic interpolation. Different numbers of calculation points and disjoint ranges of harmonic orders were used. Comparisons were made between results yielded by the same method but using different simulation parameters, respectively FFT versus DWT (WPT). Two sets of data from the power plant were analyzed. The 1-st one had a reduced harmonic content, whilst the 2-nd one presented significant multi-harmonic distortions. The errors in evaluating the total RMS were computed considering a reference calculated with a Riemann sum. For this PQ index, DWT(WPT) records smaller errors in all studied cases. The differences between the values provided by FFT and respectively by DWT(WPT) are very small (<;0.21% in absolute values), proving the reliability of both methods. Spline generated better results. For similar memory consumptions, FFT performed very fast, while WPT achieved very good runtimes (2.98×10-2 sec/period for the calculation of the fundamentals RMS). Smaller mean errors were generated by WPT when evaluating the fundamentals RMS when the fundamental signal was slightly deviated from sinusoidal.

Instantaneous power theory applied to power conditioning under distorted mains voltages: A MATLAB/simulink approach

The paper presents some problems and solutions for instantaneous power theory applied to active power conditioning under distorted mains voltages in a MATLAB/Simulink approach. Simulation of a real system is made, namely a transformer which feeds a controlled rectifier bridge in an excitation system from a power system generator group, based on measurements acquired with a dedicated data acquisition system. Replacing the old auxiliary excitation generator eliminates the disadvantage of using electrical machines with the mass in motion inertia and wear over time, but with the price of facing two important problems: the line voltage notching due to thyristor switching and also the distorting of line current. The MATLAB/Simulink model allows for testing the instantaneous power theory implementation and dealing with problems arising with distortion of mains voltages, respectively observing its effectiveness based on the results.

Direct-Flux Vector Control of induction motor for light traction

The paper presents a Direct-Flux Vector Control (DFVC) scheme suitable for induction motors employed in light traction applications. In such applications, deep flux weakening is required, and the maximum torque production must be obtained under current and voltage constraints. The mathematical model of the control scheme is discussed and the implementation issues are analysed. Experimental results are presented for a small power induction motor, showing the feasibility of the proposed control, with a particular focus on the robust flux-weakening operation.

A control strategy for an induction motor used for vehicular traction and/or positioning systems with variable speeds

The paper presents a control strategy employed to control an induction motor (IM) used in vehicular traction applications with variable speed. This strategy is based on the Direct-Flux Vector Control (DFVC) method. In such applications, deep flux weakening is required, and the maximum torque production must be obtained under current and voltage constraints. Information on the mathematical model and control scheme for DFVC is provided. The reference frame is given by the stator flux, all the calculations being made with respect to this frame. The proposed control scheme is also implementing a limitation of the maximum values of current and voltage, according to specific control laws - MTPV (Maximum Torque per Volt) and MTPA (Maximum Torque per Ampere). Experimental results are presented for a small power IM, accelerating in idle running, with an inertial load and for the steady state respectively. They are showing the efficiency of the proposed solution and a robust operation in flux-weakening conditions. The controller is belonging to the dSPACE family, being able to provide very fast floating point results. The proposed strategy can be used for controlling other types of motors, as the Interior Permanent-Magnet (IPM), Synchronous Reluctance (SyR) and wound rotor synchronous motors, considering the modifications imposed by the magnetic model (load angle, flux orientation a.s.o.).

Induction versus interior permanent magnet synchronous motors for positioning systems using direct flux vector control

The development of a control strategy suitable for Permanent Magnet (PM) and Induction Motor (IM) drives is considered. The proposed scheme is based on the Direct Flux Vector Control (DFVC) method, adapted for positioning systems (over azimuth and elevation). A comparative study of IM and Interior PM motors performances is presented.

Preprocessing steps required by denoising of quasi-stationary electric signals in power plants

Special preprocessing techniques for denoising quasi-stationary signals (SN) acquired from the secondary winding of the excitation transformer in a power plant are addressed. Sequences of 30 periods were analyzed. SN were polluted by white Gaussian noise and “average” signals (AS) of one period length were deduced. Because vectors of samples were handled, corrective measures (CM) were applied firstly. They were conceived to diminish the numerical errors generated by fractional number of samples per period. Unlike the quasi-sinusoidal voltages, the currents contain almost horizontal segments changing their magnitude in sharp transitions (ST) due to thyristors. Therefore different criteria and thresholds were used to mark the “not enough stationary” periods as terms not involved in computing AS. Areas of individual periods and, only for voltages, maximum deviations allowed for magnitudes were used. Having AS, 3 sets of periods (1-st, middle and last) were afterward used (now applying CM over AS). Different (currents vs voltages) thresholds based techniques were afterward conceived and used in order to provide immunity against transients and ST effects to the estimated noises (EN=SN-AS). Single and multi-level Wavelet based denoising techniques were finally applied to obtain denoised signals (SD). Root Mean Square Deviations associated to the pair (SD,AS), powers of noises and Signal to Noise Ratios, evaluated in different ways, were used to select the most suitable denoising techniques.

Study on consumers influence over the supplying network in a smart grid monitoring and state estimation

The paper analyzes the behavior of a three-phase consumer, consisting of several personal computers connected to a transformers secondary phases. The modalities of monitoring and recording of electrical quantities are presented in this work. The structure of the consumer is described. The recorded waveforms are analyzed. The Fast Fourier Transform (FFT) analysis allowed for the decomposition of the electric signals - voltages and currents - in harmonic components, which are presented both graphically and tabular. The quality indices are computed, underlining the capacitive nature of the consumer, firstly on each phase, then globally. There are presented some conclusions related to the variation of the electric quantities due to the load variation. In order to estimate the states, Discrete Wavelet Transform (DWT) is used. Some indices related to the power quality for the acquired signals have been determined and are presented in a tabular form. The analyzed consumer introduced strong distorted and non-symmetrical regimes, affecting the network. This is leading to the conclusion that some intelligent recording systems need to be designed and realized, in order to correctly estimate the different states in real time.

An improved control technique of a very precise positioning system used in hail suppression systems

A control strategy for positioning systems used in hail suppression is presented in this paper. The proposed position control solution is designed to obtain a precise control of the launch pad position over the azimuth and elevation, resulting in controlling a system with two degrees of freedom. The proposed strategy has been simulated starting from a permanent magnet synchronous motor that was modeled by means of an inversed magnetic model. The control algorithm initially takes into consideration a regulation of the main motors electrical quantities: torque, speed and position. Then a mechanical model for the system was developed, accounting for the experimentally determined static torque, total reduction factor (worm gear and launch pad), total inertia and efficiency. The simulation work has been developed in Matlab Simulink, and it considers two different systems, each one formed by one motor, along with its worm gear and the corresponding mechanical parts. The simulation results are provided for both positioning systems, for the azimuth and elevation. The obtained results demonstrate that the proposed control solution is able to meet the application requirements and can be implemented with a real-time processor.

Performance comparison between active and passive power filters for a static excitation system in a power generator group: A MATLAB/Simulink approach

This paper presents a comparison of the performances related to compensation and transient response of active and passive power filters when operation relies on distorted mains voltages. MATLAB/Simulink models are used. Measurements acquired with a dedicated data acquisition system provided the parameters required by the real system simulation, namely a static excitation system of a power generator group. The simulated system consists of a transformer which feeds a controlled rectifier bridge. Replacing the old auxiliary excitation generator raises two important problems: the line voltage notching due to thyristors switching and additional line current distortion. Theoretical aspects of line notching and passive and active solutions for compensation and their limitations are also discussed.