Gheorghe-Eugen Subtirelu

Accurate estimation of photovoltaic (PV) energy production is essential for ensuring the optimal design, operation, and economic viability of solar installations. However, discrepancies often arise between predicted and actual energy yields, particularly in regions characterized by variable meteorological conditions. The paper presents a synthesis of methods for estimating energy production of photovoltaic systems. For two types of residential systems located in Craiova, a comparative analysis is presented between the estimated values obtained using the most efficient methods and the actual measured values. The research combines simulation-based predictions with real operational data collected over a defined monitoring period to evaluate the accuracy of commonly used estimation models. To estimate solar radiation and energy production, some of the most common specialized software for renewable energy sources were used. The findings reveal that estimation tools tend to overpredict energy output during periods of high temperature and unstable atmospheric conditions, while performing more accurately during stable seasonal intervals. These results highlight the importance of localized calibration of prediction models and the integration of detailed climatic data for improving accuracy. The study contributes valuable insights for system designers, operators, and policymakers seeking to enhance PV system reliability and advance renewable energy planning in Romania and similar climatic regions.

It is known that the use of synchronous motors in variable speed drives to simultaneously generate mechanical power and reactive power is among the most effective methods of improving the power factor in electrical distribution networks. Also, the adoption of automatic excitation control has expanded the application field of synchronous motors to handle high-load shock drives. Modern digital control systems designed for controlling the excitation of synchronous motors have revitalized the significance of these machines. This paper presents a synchronous motor excitation control system that provides a fully digital solution for the design of equipment to supply excitation and control the operation of high power synchronous motors. This system can be easily parameterized for low and medium voltage synchronous motor drives of various rated powers, with asynchronous starting capability. An application of the system to the realization of an equipment for excitation power supply and operation control of high power synchronous motors is also presented. The equipment was realized in prototype version and after successful laboratory tests it was successfully produced in series and used to build electric drives for ore mills using asynchronous motors of 2.93 MVA.

This article describes a control method called "direct power control" designed specifically for parallel active power filters. The purpose of this method is to attenuate harmonics in the supply current and offset reactive power problems. The main goal is to bring the active power and reactive power back to the reference values through hysteresis control. The output of the hysteresis controller is combined with the switching table to regulate active and reactive power in real time by determining the optimal switching configuration of the inverter. This study proposes a novel switching table design based on analyzing the impact of the inverter switching vector on the instantaneous reactive and active power derivatives. The goal is to reduce the number of commutations by eliminating zero vectors while maintaining the required DC bus voltage using anti-windup techniques based on PI controllers.

Today's electrical networks, as part of the national power system, must meet a number of requirements imposed on power transmission and distribution equipment. The main requirements are: continuity of electricity supply to consumers, operational safety, quality of electricity supplied, development of networks, economic viability of investments, environmental impact requirements. In this context, the testing of the technical performance of the various elements that make up the electrical networks, both at the design stage and, in particular, at the certification stage, is of great importance and is regulated by internationally recognised technical standards and norms. The performance of tests under laboratory conditions requires the existence of specialised installations and equipment capable of generating the electrical parameters to which the various elements of electrical networks are subjected, for the purpose of performance testing and certification. The paper presents an installation for carrying out thermal tests on the current paths of electrical power equipment with a fully digital control system based on virtual instrumentation, which replaces the physical control panel made up of electronic modules and analogue devices. This enables the transition from primarily manual local control by the operator to remote control and eventually full automation of the thermal test process for medium and low voltage switchgear. By taking advantage of virtual instrumentation, the solution can be easily implemented and enhanced by integrating process control and computerisation applications.

This paper is focused on an adaptive controller for the direct current (DC) voltage in three-phase three-wire shunt active power filters systems. Although the controller structure of the proportional-integral (PI) type is determined off-line and does not change, the prescribed DC voltage and the controller parameters are calculated in an adaptation block, depending on the non-active power to be compensated. The adaptive control is based on the design expressions for the DC circuit of the shunt active power filter found by the authors, based on the detailed analysis of its operation, during the active filtering. The performances of the proposed adaptive control and its advantages compared to the classical control (where the prescribed DC voltage and the controller parameters are constant) were first determined on the virtual model of a laboratory setup. Then, the adaptive control was implemented on the dSPACE 1103 control board, which allowed the experimental determinations that prove and support the results obtained on the virtual model.

The aim of this paper is the implementation of the control algorithm of an active DC traction substation using the dSpace DS1103 DSP control board. The adopted current control method is the indirect current control, which assumes that the current regulator controls the power grid current instead of the active filter output current. The active character of the traction substation is given, on one hand, by the active filter which compensates the non-sinusoidal current absorbed by the traction rectifier and on the other hand, when the trains are braking the active filter works as a power inverter absorbing power from the DC line and injecting active power to the power grid. The experimental model of the active substation uses a DC machine as the train(s) absorbs or generates electrical power. For this operating modes, the DC machine operates as motor as the train(s) absorbs power and as generator as the train(s) generates power. The mechanical load (the train load and inertia) is replaced by a synchronous machine coupled to the power grid which can operate as a synchronous motor or generator. Given the hardware and software resources of the DS1103, the entire experimental model can be controlled by means of a virtual control panel which controls the system and the transducers data sampling.

Main objective of this paper is to provide a practical tool for the analysis and study of power quality and deforming regime. The LabVIEW graphical programming environment is used for designing virtual instrument to achieve the main two objectives. First goal is synthesizing a distorted signal by adding a number of sinusoidal harmonics which composing it. An accurate voltage signal and the current one are synthesized. Each harmonic amplitude, frequency and phase can be changed by simulation, noting the effects on the shape and parameters of the distorted signal obtained. For each signal distortion voltage and current are calculated following parameters: components level of the harmonic number including 0 (DC), 1 (fundamental), 2 (second harmonic), n (nth harmonic), up to and including the highest harmonic; the detected fundamental frequency resulting from searching the frequency domain; (THD) total harmonic distortion up to and including the highest harmonic. They are also calculated: active power, reactive power, apparent power and distorted power, power factor, each by component and the total value. The second objective of virtual instrument is power harmonic analysis for the experimental instantaneous input data of three-phase power consumer. These values instantaneous currents and voltages are obtained by data acquisition and are found saved in a text-based measurement file (.txt). This virtual instrument can be used at the same time in power quality education and research.

In this paper, using Lab VIEW graphical programming environment are made a number of virtual instruments which are used at the demo/study of low power active filters with operational amplifiers (op-amp). There are analyzed the active first order low pass, high pass and band pass filters. The results are included in two papers. The first one contains the simulation of active filters. The different values of passive components (feedback or input resistor and capacitors) around the op-amp can be selected with numerical controls for each type of filter. The effect is seen using the Bode characteristic (frequency response curve for filter). The AC (alternating current) frequency characteristic is best described in term of a Bode plot where the gain is plotted on a log scale on the vertical axis and the frequency is plotted on a log scale on the horizontal axis. The op-amp ideal frequency response curve (Open Loop) it is also displayed in the chart. The values of Open Loop Gain and Frequency Cut-Off are also set with numerical control. The validity of the program is done by comparing simulation results with data obtained by calculation using the theory. The program is designed in a modular manner and it can be used successfully in education and training courses.

The main objective of this paper is to solve a practical and current problem, by taking advantage of the virtual instrumentation in testing electrical machines. The abilities of virtual instrumentation are used to data acquisition, measurement and analyze the values of no-load testing's parameters for three-phase induction motor. The virtual measurement system bench is designed and consist from two principal components: the hardware components (six LEM transducers for measuring three voltages and three currents; elements for signal conditioning and power transducers; USB multifunction Input / Output module; a personal computer) and the software components (operating system for the computer; drivers for the acquisition and manipulation of data; virtual instrument for calculation and graphical presentation of results). The LabVIEW graphical programming environment is used for designing virtual instrument. This virtual measurement system bench is an easy to use device which can be used in engineering education laboratories from universities or in electrical machines testing workbenches; it is capable of data acquisition, storage or memorization on different media, visualization of different graphs or analysis on-line or off-line of the results obtained. The virtual measurement system described in the paper can work independently (in the Simulation mode or Real time Acquisition mode) or integrated as part of a future complex virtual system for measurement and analysis in the domain of electrical machines testing workbenches.

The paper deals with a hardware solution for the study of low power active filter with Bode plotting. By using a DAQ card and an experimental platform it shows how LabVIEW graphical programming environment can be used for manual or automated testing and analysis of the first order active filter with operational amplifier (op-amp). The hardware platform can provide a template to build real op-amp circuits to better understand (together with a previous one paper for developing the software simulation and demonstrations) the op-amp filters principles. The main contribution for this paper refers to a software / hardware solution for obtaining a manual / automat Bode plotting. For plotting the frequency response curve of a filters, it is often necessary to measure the output signal over a range of input frequency. The hardware experiments allows a range of test voltages with variable frequency to be input and the filters response measured, displayed and analyzed. The frequency scan range, scan rate and number of test points can be selected from front panel controls. Another contribution is a practical tool for the analysis of the response filter to various input signals (sine / square / triangular wave) and calculating the filter gain depending on the amplitude measured in units of peak, peak to peak or roots mean square signal level.