Mario Luiso

Power-Quality Monitoring Instrument With FPGA Transducer Compensation

In this paper, the design and the prototype implementation of a power-quality (PQ) measuring and monitoring instrument based on an embedded controller with reconfigurable I/O are described. The instrument performs the calculation of main PQ indexes according to the latest standards, and it can be reconfigured for future standard developments. It is properly implemented to operate in stand-alone mode, and it is interfaced with hardware modules to exchange information and data, locally, with RS-232 connection or, remotely, with Transmission Control Protocol/Internet Protocol communication. The design includes enhancing of transducer performances through the correction of systematic errors with digital filters implemented by a field-programmable gate array module. This instance allows better performance without increasing cost. Finally, the whole accuracy of the proposed measurement system is evaluated by experimental tests that were performed in agreement with the specification of a class-A instrument for some of the main PQ indexes. The tests are performed, including the transducers, although that component is not required by the standard, to evaluate the behavior of compensated transducers. The test results are compared with those obtained by a commercial PQ instrument and show a good agreement.

Real-Time Digital Compensation of Current Transformers Over a Wide Frequency Range

The advance in power electronics causes, over the years, an increasing diffusion of nonlinear loads. Moreover, electrical generation systems are going to include a growing amount of solar and wind energy, characterized by time-variant flow of energy and, very often, with a relevant distortion. These aspects give more and more emphasis to the monitoring of conducted disturbances on power systems over a wide frequency range, and consequently, voltage and current transducers adopted for this aim must have a proper frequency bandwidth. Voltage transformers (VTs) and current transformers (CTs), which are the most installed transducers in electrical power systems, are typically constructed to operate only at the industrial frequency, i.e., 50/60 Hz, but it is clear that their substitution would require an unsustainable cost. Therefore, in this paper, a method for real-time digital compensation of the CT frequency response over a wide range is presented. An implementation of the proposed method is shown, and the compensated CT is characterized and tested in some practical situations, with waveforms affected by typical power quality disturbances.

Advanced Instrument For Field Calibration of Electrical Energy Meters

Calibration of electrical energy meters is coming up as a metrological concern. The latest European Union (EU) directives about measuring instruments adopted for commercial transactions state that their accuracy has to be verified in actual operating conditions. This requirement is leaving all laboratories involved in energy meter calibration in serious difficulty because this kind of metrological verification is, at the moment, still not fully developed or even clarified in all involved aspects. In this paper, a measurement equipment for the calibration of energy meters is presented. Its structure and metrological characterization are discussed. To improve its performances without increasing its costs, two online digital compensation procedures have been realized and are shown: one increases the spectral purity of test signals and one corrects the transducer frequency response. Experimental results relating metrological characterization have shown that the so-realized calibrator is suitable for the onsite calibration of energy meters.

Accuracy Analysis of Algorithms Adopted in Voltage Dip Measurements

This paper analyzes the accuracy of algorithms commonly adopted in instruments devoted to the detection and characterization of voltage dips (which are also called sags). This analysis is particularly interesting, because the results of dip measurements are utilized for calculation of severity levels and site indexes, which are important parameters not only in the assessment of the quality level of power supply but also in the selection of equipment with proper intrinsic immunity. Instruments for dip measurement still have unresolved technical and theoretical issues related to the characterization of their metrological performances, so it can be found that different instruments are significantly in disagreement in some actual measurements. This paper moves a step into the direction of deepening the knowledge about the measurement of voltage dips, pointing out the limits incident to the adoption of the detection algorithms indicated in the standards. It starts with a discussion about parameters that characterize voltage dips, in agreement with the standard. Then, analytical calculations of some systematic deviations in the event characterization, which are introduced by the most diffused dip detection algorithms, in simplified measurement situations, are presented, underlining their remarkable impact. The obtained relations are experimentally verified on a commercial power quality instrument, forecasting its systematic deviations.

Large bandwidth compensation of current transformers

Power quality analyses in the last years has assumed more and more heaviness in industrial environments, due to the presence of non-linear and unbalanced loads: they require measuring instrumentation with large bandwidth. Since voltage and current transducers are the first part of a measurement chain, they must have frequency bandwidth sufficient for analyzing conducted disturbances on power system. Voltage and current transformers (VT and CT), which are the most installed transducers in electrical power system, are typically constructed to operate at industrial frequency, 50/60 Hz, but it is clear that their substitution would require an unsustainable cost. Therefore in this paper the realization and metrological characterization of and electronic device for extension of CT operating frequency bandwidth is presented. It is based on field programmable gate array, which implements in real time a digital compensation technique. The compensated CT continues to be an analog device since the FPGA board is opportunely equipped with analog to digital and digital to analog converters.

Optimization of energy consumption in a railway traction system

Reduction in energy consumption in electrical railway traction systems has become a vital need. The effort spent in providing better quality services implies increasing speed and in turn an increase in energy consumption. Various techniques have been adopted to improve train performances, some regarding vehicle structure, some other related to motor improvements. In this paper a different way to reduce power absorption is presented. It is based on the studies of the traction diagram jointly with the absorption curves. The data are made available by an on-board monitoring system which measures all the electromechanical quantities of the train course and sends them through network connection to a control station. Starting from them it is possible to run an optimization procedure in order to get appropriate rules to drive in energy-efficient way the train. The procedure which get the optimized protocol with and the experimental data related to the train optimized protocol are presented

Fast Hybrid MPPT Technique for Photovoltaic Applications: Numerical and Experimental Validation

In PV applications, under mismatching conditions, it is necessary to adopt a maximum power point tracking (MPPT) technique which is able to regulate not only the voltages of the PV modules of the array but also the DC input voltage of the inverter. Such a technique can be considered a hybrid MPPT (HMPPT) technique since it is neither only distributed on the PV modules of the PV array or only centralized at the input of the inverter. In this paper a new HMPPT technique is presented and discussed. Its main advantages are the high MPPT efficiency and the high speed of tracking which are obtained by means of a fast estimate of the optimal values of PV modules voltages and of the input inverter voltage. The new HMPPT technique is compared with simple HMPPT techniques based on the scan of the power versus voltage inverter input characteristic. The theoretical analysis and the results of numerical simulations are widely discussed. Moreover, a laboratory test system, equipped with PV emulators, has been realized and used in order to experimentally validate the proposed technique.

Survey on Voltage Dip Measurements in Standard Framework

This paper reports the results of the analyses on the accuracy of algorithms commonly adopted in instruments devoted to the detection and characterization of voltage dips (also called sags). This analysis is particularly interesting because the results of dip measurements are used for calculation of severity levels and site indexes that are important parameters in the assessment of quality level of power supply but also for selecting equipment with proper intrinsic immunity. Anyway, instruments for dip measurement still have unresolved technical and theoretical issues related to the characterization of their metrological performance, so it can be found that different instruments are significantly in disagreement in some actual measurements. This paper moves a step into the direction of deepening the knowledge about the measurement of voltage dips, pointing out the limits because of the adoption of the detection algorithms adopted in agreement with standard. It starts with a discussion about parameters that characterize voltage dips. Then, the analytical calculations of systematic deviations in the event characterization, introduced by the most diffused dip detection algorithms, in simplified measurement situations, are presented, underlining their remarkable impact with particular attention to short-dip event. The obtained relations are experimentally verified on a commercial power quality instrument. Brief remarks about the analysis of the main algorithms, alternative to that of the standard, for voltage dip detection are also reported.

Medium Voltage Divider Coupled With an Analog Optical Transmission System

Similar to low-voltage power systems, also in medium voltage networks there is a growing need for power quality (PQ) analyses, required by the increasing diffusion of distributed generation sources equipped with switching converters. This leads to the employment of transducers and measurement techniques able to convert and acquire voltage signals with better accuracy on a wider frequency range than conventional voltage and current instrument transformers. Voltage dividers (VDs) are expected to become the most used transducers in PQ assessments, since, with respect to conventional voltage transformers, they show higher frequency bandwidth and better linearity. The main disadvantage of the VDs is the absence of galvanic insulation between the transducer and the measurement/protection devices. Therefore, in this paper, a prototype of a VD for phase-to-ground connection with an insulating transmission system is presented. It is constituted by a high-impedance resistive-capacitive VD and by an analog electronic insulation interface. Such an electronic interface, inserted between the output of the divider and the input of a measuring instrument, realizes the galvanic insulation by means of an optical transmission system ensuring, simultaneously, wide frequency bandwidth and high accuracy. The design, simulation, and characterization of the realized insulated equipment are presented. Its performance well fit the prescription of the international standards about instrument transformers for PQ measurements.

Low cost smart power metering

Modern trends, including increased electricity requirements and improvements in grid management technologies, are causing the electric industry and regulators to review how the grid is achieved. With this aim, in this paper a low cost solution for the real-time energy management in a smart grid, is presented. It provides several smart meters, which act as slaves, that continuously monitor connected loads communicating with a data aggregator via CAN bus. Through the implemented web server the users can remotely control their consumption using a web browser. To prevent external attacks, a low computational cost protection software, based on Message Authentication Code (MAC), has been implemented. The paper illustrates the hardware architecture, discusses the adopted software solutions, and is completed with an example of energy monitoring for a generic household thanks the web server.