Alex-Sander Amavel Luiz

Minimum reactive power filter design for high power converters

This work proposes and evaluates an optimized filter design for high power converter with sinusoidal voltage and current. The inherent low switching frequency of this class of converters complicates the filter design and generally results in large filter components. The proposed design culminates in minimum filter reactive power, size and cost. Simulation and experimental results support those ideas.

Improving power quality in mining industries with a three-level active front end

This paper deals with the power quality improvements that can be achieved in mining industries by the employment of a high power active front end. A three-level neutral-point-clamped rectifier with low harmonic distortion and high power factor is chosen alternatively to diode rectifiers. A constraint on this class of converters is its lower switching frequency that results in low order harmonics. This is often solved employing passive filters. In this paper an appropriate choice of modulator and filter topology leads to significant simplifications on the filter design and reduces both weight and cost of its elements. The Selective Harmonic Elimination is presented and though it is generally considered as a steady-state technique it was designed to work in transient conditions within a closed loop current control. The system is submitted to tests in order to assess its operational performance through simulation results. Experimental data from a prototype is also presented.

Operation of a high gain bidirectional DC-DC converter for photovoltaic on-grid systems

The aim of this paper is to present a dc-dc converter with high-stepping ratio for photovoltaic (PV) systems operating in parallel arrangement. The converter utilizes a resonant LC circuit to continue provide an increasing voltage at the output terminals by rotating the polarity of the capacitor using a thyristor based H-bridge. Compared with the existing converters, the proposed circuit reach higher voltage gains with low switching frequency using simple circuitry. It works efficiently with minimal switching loss even dealing with high currents. The prototype has shown the usefulness of the converter for low input voltage applications, mainly when a string of photovoltaic panels is connected in parallel, with the possibility to power in a commercial inverter.

A simple torque estimator for in-service efficiency determination of induction motors

Monitoring energy consumption and condition of electric motors is important for economical purposes and safe operation of drives and processes in industry. Those activities are especially valid in processes requiring high availability such as those founded in mining and metals industry. Methods able to implement these functions without process interruption and/or requiring minimum changes on the drive configuration are extremely welcome. The air-gap torque estimator is at the core of the main efficiency estimation methods used today and therefore it is crucial to its successful implementation. Most of the torque estimation algorithms require two or three voltage and current measurement to be implemented. Unfortunately, this number of sensors is not always available for all the motors used in industrial processes. Therefore, a solution employing a reduced number of sensors is necessary. In order to meet this requirement, this paper presents a new and simple technique to estimate torque that is able to be used in algorithms of efficiency estimation of in-service induction motors. The technique requires data acquisition of current and voltage of only one phase for its implementation. Sensors for measuring these variables are usually available in the Motor Control Center — MCC — cabinet for monitoring and protection purposes. From one phase current and voltage samples, instantaneous values of torque, angular velocity, power and efficiency are calculated. Simulation and experimental results are used to check the validity and performance of the proposed technique.

A new design of selective harmonic elimination for adjustable speed operation of AC motors in mining industry

This paper presents an appropriate design of selective harmonic elimination for a high performance adjustable speed operation of ac motors in mining industry. These motors are normally old machines and/or fed by long cable and require pure sinusoidal voltages. A passive filter is employed between the machine and a three-level NPC converter. Owing to its higher frequency band free of harmonics the selective harmonic elimination was chosen to command the converter. Therefore it is required the operation of selective harmonic elimination in variable fundamental frequency conditions. It then brings the noneliminated harmonics to the low frequency region. A new design of selective harmonic elimination consists of a technical criterion of changing the set of pulse pattern in a more economical way in order to maintain the low frequency harmonic content far from the fundamental frequency range still keeping high system efficiency, fast acceleration and less torque pulsation and feasible digital implementation. The technical criterion also simplifies the passive filter design letting enough space to allocate cutoff and resonant frequencies inside the band free of harmonics. All the discussion of this work is supported by simulation and experimental results of a prototype which is aimed to confirm the feasibility of the above ideas.

An alternative five level NPC converter for medium voltage AC drives and technical issues

This paper presents a study of an alternative five level inverter for medium voltage ac drives based on neutral point clamped concepts. The main advantage of this topology is its higher voltage operation compared to the commercial three level structures but with less mounting complexity when it is compared to a traditional five level neutral-point-clamped converters. Besides the converter is suitable for various low frequency modulation techniques which are able to reduce switching losses and thus increase the converter efficiency. In this line a detailed explanation of the topology is presented along with a cost comparison among the traditional multilevel topologies based on topological analysis. A description of a five level modulation process for this topology is also presented and in this line a discussion of modulation issues in very low switching frequency is also addressed. The differences and similarities of modulation technique in this study from a classical phase-disposition pulse width modulator are discussed. An important feature of this modulator is its ability to improve the converter harmonic spectrum. All this work is supported by results which are aimed to confirm these ideas.

Study and implementation of a high gain bidirectional dc-dc converter for photovoltaic on-grid systems

The aim of this paper is to present a bidirectional dc-dc converter with high-stepping ratio for photovoltaic systems operating in parallel arrangement operating with energy storage systems. The converter utilizes a resonant LC circuit to continue provide an increasing voltage at the output terminals by rotating the polarity of the capacitor using a thyristor based H-bridge. Compared with the existing converters, the proposed circuit reach higher voltage gains with low switching frequency using simple circuitry. It works efficiently with minimal switching loss even dealing with high currents. The prototype has shown the usefulness of the converter for low input voltage applications, mainly when a string of photovoltaic panels is connected in parallel, with the possibility to feed in a commercial inverter and battery banks comprising a dc bus bar.

Bidirectional power converter with adaptive controller applied in direct-current microgrid voltage regulation

Recently, many researches have been done related with the integration of distributed generation in the distribution system. However, in large scale applications, there are several challenges in terms of control design. A simple example is the possibility to operate in island mode or connected to the main power. The operation in island mode requires that power sources should be inserted inside the microgrid structure. The use of dc microgrid is increasing because of advantages in the integration of dc charges, dc loads and storage systems through electronic converters. Between theses advantages, the elimination of frequency are reactive power control are highlighted. However, this simplification does not exempt the microgrid from stability problems, in order to achieve different voltage levels for several power devices. These converters introduce destabilizing effects on the system, leading the microgrid to show significant oscillations in the dc bus voltage. The converter control is the primary concern in operating a microgrid. This work presents a dc microgrid operating in island mode. An adaptive controller is designed to control the balancing of power through a dc-dc bidirectional converter. This converter is connected to a storage device, controlling the dc bus voltage. Simulation results allow us to validate the proposed controller under variations in the availability of energy sources and during load changes.

Cavitation detection in centrifugal pumps driven by frequency inverter fed motors

This paper evaluates the ability of the Load Torque Signature Analysis (LTSA), which is a mechanical fault detection method, to detect cavitation phenomenon in centrifugal pumps driven by induction motors supplied by frequency inverters. The LTSA method is described and some modifications are proposed in order to improve its performance in noise polluted applications such as frequency inverter induction motor drives. These improvements are checked by means of simulations. Finally, LTSA is applied to a laboratory hydraulic plant in order to validate the method. Experimental results are plotted and compared to previous results obtained from the same system, but with motor supplied directly from the mains.

An assessment of harmonic filter for indirect field oriented induction motor drives in mining industry

This paper presents a study of the technical feasibility of using harmonic filters with indirect field oriented control — iFOC — in medium voltage induction motor drives such as that in mining industry. The filter consists of a passive second order LC structure connected between a NPC three level converter and the machine. In general, the purpose of the filter is to attenuate the harmonic components generated by the converter in order to make possible to supply old insulation induction machines. Because of the poor insulation technology, these machines do not withstand high dv/dt as those generated by PWM converters. Despite its effectiveness in eliminating harmonic components, the filter introduces challenges to the implementation of the iFOC control because the motor and the converter currents are different. Practical implementation of the iFOC requires careful choice of the controlled variables and location of the current sensors. This work shows that no additional sensor besides that used in regular implementation is required. Only current and speed measurement is employed here, and no voltage measurement is required. Performance comparisons are performed for the system with and without the filter, also at low speeds by means of computer simulations. The results show the feasibility of the iFOC control even if the filter is included.