Helder de Paula

Increasing long belt-conveyors availability by using fault-resilient medium voltage AC drives

Belt conveyors constitute an efficient and economical way for minerals and heavy load transportation from mines to their destination, they serve vital process units whose success depends on continuous operation. To meet important operational requirements, modern high capacity conveyor systems are equipped with medium voltage variable-speed induction motor drives. The reliability and service continuity of such drives play a critical role for the entire conveyors availability. This work proposes a way to significantly improve the availability of the conveyor systems by using fault-resilient medium voltage AC drives. Based on recent studies and authors experience, it is shown in this paper how the drive conveyor downtime can be dramatically reduced after an internal short-circuit, keeping the overall cost-to-repair to a minimum and avoiding financial losses that can reach several million dollars.

Cable parameter variation due to skin and proximity effects: Determination by means of Finite Element Analysis

Computational simulations involving problems of power quality, electromagnetic transients and electromagnetic compatibility, among others, require the application of time domain cable models appropriate for high-frequency studies, able to represent the cable parameter variation in relation to frequency. Most of the models that satisfy this condition require, to be elaborated, input data regarding the cable resistance and inductance for different frequencies. Since the measurement of such parameters is oftentimes not feasible, the possibility of their determination from analytical or computational methods is highly desirable. Routines of the type ¿cable-line constants¿, commonly featured in commercial simulators, are widely used for this purpose; nevertheless, they present limitations that lead to inaccuracy and restrictions in their applicability. In this context, a methodology for cable parameter determination, based on finite element analysis (FEA), is presented, which overcomes these problems and thus arises as a very attractive tool for cable/line modeling.

Improving the performance of protection schemes in three level IGCT-based neutral point clamped converters

High power IGCT-based converters are currently used in a wide range of drive applications. The protection of such converters against internal faults has been addressed in the technical literature, but the main focus has been on the solutions to disconnect the converter from the mains. In this paper, the short circuit current paths through the power devices in the three level NPC bridge are identified and studied from the perspective of keeping the overall repair cost, after a fault leading to the shoot-through mode, to a minimum. It is shown in this paper that a preferred short-circuit current path that limits the thermal energy in all IGCTs to safe levels exists. Furthermore, it is demonstrated that it is possible to guarantee, through proper busbar geometry and/or device selection, that the short-circuit current stress can always be moved to the power diodes. Simulation results are presented to support all the discussions and conclusions.

A new fault-tolerant realization of the active three-level NPC converter

The three-level (3L) neutral-point-clamped (NPC) voltage-source (VSC) converter has been the standard solution in medium-voltage range for industrial applications. Recently, the NPC converter structure was improved by adding two extra active switches per phase leg, thus overcoming one of its main drawbacks: the unequal power loss among the power semiconductors. The resulting structure became known as active NPC (ANPC) converter. The high availability of these power converters is mandatory, since their maintenance or replacement imply equipment downtimes incompatible with the requirements of the production processes. The fault-tolerant operation of such converters has been addressed in the technical literature, but the main proposed solutions result in more complexity and higher parts count. This work proposes a new fault-tolerant structure for the ANPC IGBT-based converter, using standard devices, with very low additional cost and small complexity. The main goal is to show that the ANPC converter can overcome different and successive failures in its power devices without loss of functionality, before its definitive downtime.

Motor drive systems reliability: Impact of the environment conditions on the electronic component failure rates

This article discusses the actual failure rate of electronic components when these are exposed to harsh environments, which can include vibration, dust, humidity, high temperature and others. It presents a compilation on the physico-chemical effects that can take place when electronic components are stressed and how such effects can reduce the component life. It is also presented a compilation of the results of a thorough and comprehensive research on different standards for reliability prediction, along with a critical and comparative analysis between the methodologies and applications related to each one of them. Furthermore, to provide the reader a broader understanding of the methodology used in these standards, an example employing a typical SV-PWM converter used in motor drive systems, submitted to different environmental stresses, is used as a case study.

Low voltage PV power integration into medium voltage grid using high voltage SiC devices

High voltage high power semiconductor devices are being used for grid integration of renewable energy sources. 1200V, 100A SiC Mosfets, 10 kV SiC Mosfets and 10kV SiC JBS Diodes have proven to be beneficial for high voltage application. High Voltage SiC devices enable high switching frequency operation thus reducing size of passive elements. Scope of this paper focuses on an alternative approach for 0.9 MW PV power plant, which is currently being constructed in Brazil. Use of high power SiC devices for PV power plant for integration into 13.8 kV grid provides higher efficiency, reduction in size and volume.

A Thermal Management Approach to Fault-Resilient Design of Three-Level IGCT-Based NPC Converters

The three-level converter with neutral point clamped (NPC), introduced by Nabae in 1981, has become the most popular converter topology for high and medium-voltage industrial drives, being widely commercialized by major manufacturers worldwide. Responsible for driving key processes in various industries, high availability of these converters is mandatory, since their maintenance or replacement imply downtimes incompatible with the requirements of the production processes. Considering the possibility of failures in the protection schemes, this paper discusses a fault-resilient design for IGCT-based NPC converters based on proper thermal management of the power semiconductors. The main goal is show that the thermal stress caused by the short-circuit currents can be managed, improving the selectivity and tracing of the damaged components, thus reducing the overall converter downtime and the maintenance costs.

Atividade, ação mediada e avaliação escolar

O objetivo deste trabalho e o de contribuir para a construcao de uma concepcao de avaliacao fundamentada na perspectiva socio-historico-cultural, que tem orientado nossa atuacao como professores e como pesquisadores. Para isso, contrastamos dois modelos de avaliacao, denominados como psicometrico e dialogico, tendo como base referencias extraidas das teorias da atividade e da acao mediada. Os modelos sao diferenciados a partir de seu potencial para: (i) explorar tensoes constitutivas da pratica educativa, que podem ser identificadas no processo de avaliacao; (ii) proporcionar a tomada de consciencia, pelos estudantes e professores, das tensoes e desafios da pratica educativa por eles vivenciada; (iii) orientar o planejamento pedagogico concebido pelos professores a partir das contribuicoes dos estudantes. Entendemos essas potencialidades como essenciais a instauracao de praticas de avaliacao comprometidas com a elevacao de professores e estudantes a condicao de sujeitos da experiencia escolar.

Premature wear and recurring bearing failures in an inverter driven induction motor— Part II: The proposed solution

In a previous work, a real case of recurring bearing failures in an induction motor had been extensively studied. As a result, the cause of the problem was revealed to be the circulation of electric current through the bearings. In this way, the present paper deals with the high-frequency leakage currents in ac motors fed by pulsewidth-modulation inverters, contextualized in the analysis of an actual case. From the parameters of an equivalent circuit representing the connecting cable, motor, and inverter return path, a filter, characterized by a common-mode transformer, is designed to mitigate the common-mode quantities and the correlated problems. Both simulated and experimental results show that the proposed solution represents an advantageous alternative in comparison to the traditional RC and RLC filters, since, in addition to its effect on the common-mode quantities, it dissipates a negligible amount of power.

Increasing long belt-conveyors availability by using fault-resilient medium voltage AC drives: Part II — Reliability and maintenance assessment

In the mining industry the profitability goals are often linked to the long belt-conveyors availability. Since the use of the medium voltage variable frequency drives (MV-VFD) in conveying applications is essential to meet various technical requirements, the conveyor service continuity depends on the drive system availability. Although there are at least three established technologies commercially available, the drive solution using three-level (3L) IGCT-based, neutral point clamped (NPC) converters is clearly the most popular for higher powers. In the early parts of this work [1] and [2], it was shown how to design a fault-resilient 3L-NPC IGCT-based converter, in this second part, the reliability, availability and maintenance aspects of MV-VFD are discussed with focus on the benefits provided by the fault-resilient design. The stock forecasting for spare parts considering a multidrive common dc bus used in a real downhill conveyor is performed and analyzed under catastrophic protection failure standpoint.