J. Rebolledo

Reduced Switching-Frequency-Modulation Algorithm for High-Power Multilevel Inverters

Multilevel inverters have emerged as attractive high-power medium-voltage power-conversion systems. They are mainly controlled with high-frequency pulsewidth-modulation methods. This is not suitable for very high-power application due to significant switching losses. This paper presents an adaptive duty-cycle modulation algorithm that reduces the switching frequency to a minimum necessary to fulfill the dynamic requirements of the system. Switching losses are, therefore, strongly reduced. This is achieved by using the slope of the voltage reference to adapt the modulation period to ensure that only one-step change between two voltage levels. Simulation and experimental results are presented for a nine-level cascaded inverter. Voltage waveforms obtained for variable frequencies and amplitudes show similar switching patterns, with a reduced and near-constant number of commutations per cycle, regardless of the reference frequency and amplitude.

A Reduced Switching Frequency Modulation Algorithm for High Power Multilevel Inverters

Multilevel inverters have emerged as the state of the art power conversion systems for high power medium voltage applications. Many topologies and modulation methods are commercially available. This paper presents a new adaptive duty cycle modulation algorithm, that reduces the switching frequency and consequently the switching losses. This can be important for high power applications, where high frequency modulation methods like PWM are not suitable. Results are shown for a nine level asymmetric cascaded inverter. Output voltage waveforms obtained for references with variable frequencies and amplitudes show a similar switching pattern, with a reduced and near constant number of commutations per cycle, regardless the reference frequency and amplitude. The proposed modulation is compared to a multiple carrier PWM method, achieving the same fundamental reference tracking performance with a reduced number of commutations

Operation of high power cycloconverter-fed gearless drives under abnormal conditions

This paper presents the behavior of large power mills (more than 10 MW each) that are fed by cycloconverters. Special attention is dedicated to the protection philosophy to avoid operational interruptions that can produce huge production losses and equipment damage. Several failure possibilities are considered: control failure, complete power outage, if a thyristor fails and cannot block reverse voltage, etc. Simulation studies and field measurements show that large currents and torque can occur under certain circumstances and that motor foundations must be able to support them. As a general conclusion, it can be said that proper protective design effectively avoids equipment damage

Design and Evaluation Criteria for High Power Drives

The need to increase the amount of mineral being processed has originated the use of high power converters (HPC), many of them in the Megawatt range. There are several design aspects which must fulfill the operational requirements, which mean technical challenges for designers, manufacturers and operators. This paper presents the general requirements for high power drives applied to the most used industrial topologies (cycloconverters, current source inverters, neutral point clamped and H-Bridge cascade), and the design aspect for these converters. This paper includes evaluation criteria and some practical experiences.

Cycloconverter behavior for a grinding mill drive under firing pulses fault conditions

Malfunction of the electronic control system or defects in the electronic equipment among other causes can lead to different kind of faults in high-power cycloconverter-fed mill drives. This paper presents a study of the behavior of a cycloconverter for a grinding mill drive under firing pulses fault conditions. For this purpose, a controlled three-phase, six-pulse cycloconverter system, operating from a 50 Hz input, was simulated. Several firing pulses fault conditions are considered: synchronism failure, firing pulses failure and loss of blocking capability. Simulation and experimental results support the understanding of the different failures modes of the cycloconverter and the trouble-shooting in industrial equipment. The application of these modern tools, simulations and real-time controlled cycloconverter model, allows the improvement of reliability and availability of industrial grinding circuits with better procedures and instrumentation system for the real time surveillance, for trouble-shooting, fault detection and recognition of abnormal operating conditions.

High-power LCI grinding mill drive under faulty conditions

Mining plants present harsh operating conditions because locations at high altitude in the mountains, with large temperature variations and dusty environment, that can affect the integrity of the electronics producing loss of reliability with undesirable effects on safety and production. This paper presents a problem and corresponding solution related with a mining issue, where a 6.6 kV fed load-commutated-inverter (LCI) drive is employed for driving a 6500 Hp-grinding mill. In the last time, the drive has shown a variety of faulty conditions causing several downtimes and the concern of the staff. The drive has a control system with hybrid technology comprising analogue and digital components, which was installed in year 1991 with a history of few operational problems. Recent failures have shown a different and complex nature. Because the employed technology has not an intelligent diagnostics system integrated to the distributed control system (DCS) of the plant, it is a challenging task to understand the origin of the problems. A study was carried out in order to find the causes of the failures. A procedure based on a theoretical and experimental approach was developed in order to establish hypothesis for finding the roots of the problem. Among other factors, the study indicated that a lost of firing pulses of the machine converter was the most likely reason for a commutation failure. Simulated results and measurements confirmed that hypothesis. Therefore, overcurrents caused by failure in the pulses pattern of machine side converter were detected and a focalised search in specific printed boards allowed the finding of defects and remedial actions could be taken.

A Novel Noninvasive Failure-Detection System for High-Power Converters Based on SCRs

In modern high-power industrial processes, multipulse cycloconverters are fundamental to feed synchronous motors. Given the importance of these critical processes, particularly for mining industry, the reliability of the high-power drive is critical. Having a noninvasive system capable of detecting and isolating faults in the thyristors is quite valuable, and it will reduce the economic losses caused by the long repair times of the converter. Currently, it is difficult and, in some cases, almost impossible to determine which of the SCRs experience an overcurrent or those which do not trigger properly, causing a malfunction. In this paper, we present a new method of online detection which is capable of delivering information about the SCR involved in a failure and the nature of it. This new method takes only a few input and output measurements (noninvasive feature) already available in the most of industrial cycloconverters and performs a timing analysis of these measurements. The algorithms of the method estimate and analyze the switching states of all thyristors, providing an impressive way of detecting the cause and consequences of the failure in the cycloconverter.

Hysteresis current control of a vector controlled induction motor and DTC: an assessment

This paper presents the speed control of an induction motor using nonlinear current control in rotating coordinates. Two hysteresis comparators and the position of the motor flux are used to generate directly the gate pulses for the power transistors of the inverter. This field oriented control method is compared with direct torque control and it is concluded that both control methods, although being conceptually different, have very similar features in terms of structure and performance. Experimental results confirm the high quality of the control reached with this control strategy.

Interharmonic currents assessment in high-power cycloconverter-fed drives

Large drives fed by Cycloconverters (CCVs) are extensively applied in industrial and traction drives. For 12-pulse configurations, multi-winding transformers are employed. For high-power applications, three separated three-phase transformers with two secondary windings with star- and delta connection are used. In this paper, a study and evaluation of the interharmonics currents behavior injected by CCV in the transformer windings is presented. Field measurements and simulations support the analysis. Results supports further improvements concerning design, engineering and operation issues.

Up-Rating of Electrical Drives in Mining Installations

Up-rating of electrical drives employed in mining installations, like mills and conveyors, is gaining interest for providing additional production capacity, process optimization in an effective way considering time, efficiency and economic factors while keeping reliability. This paper presents a review of the technical issues to be faced, concerning conceptual analysis, decision-making and different technical aspects, as well as the interaction between end users, consultants and manufacturers. After analysis of new operating conditions and trade-offs, decision making has to keep the margin factors for safety and reliability. For illustrating the main issues related with an uprating of an electrical drive, an example for a grinding line with a gearless mill drive is shown.