Search

Asymmetric multilevel converters can optimize the number of levels by using H-bridges scaled in the power of three. The shortcoming of this topology is that the H-bridges are not interchangeable, and then, under certain faulty conditions, the converter cannot operate. A reconfiguration system based on bidirectional electronic valves has been designed for three-phase cascaded H-bridge inverters. Once a fault is detected in any of the insulated gate bipolar transistors of any H-bridge, the control is capable to reconfigure the hardware keeping the higher power bridges in operation. In this way, the faulty phase can continue working at the same voltage level by adjusting its gating signals. Some simulations and experiments with a 27-level inverter, to show the operation of the system under a faulty condition, are displayed. Index Terms--Fault tolerance, multilevel systems, power conversion.

An active power filter and static var compensator with active power generation capability has been implemented using a 27-level inverter. Each phase of this inverter is composed of three 'H' bridges, all of them connected to the same dc link and their outputs connected through output transformers scaled in the power of three. The filter can compensate load currents with a high harmonic content and a low power factor, resulting in sinusoidal currents from the source. To take advantage of this compensator, the dc link, instead of a capacitor, uses a battery pack, which is charged from a photovoltaic array connected to the batteries through a maximum power point tracker. This combined topology make it possible to produce active power and even to feed the loads during prolonged voltage outages. Simulation results for this application are shown, and some experiments with a 3-kVA device are displayed. Index Terms--Active filters, multilevel systems, solar power generation, static var compensators (SVCs).

This paper presents a new procedure to analyze power distribution systems that energize nonlinear loads. The algorithm uses the singular values of the transfer matrix that relates the output variables and the system perturbation. The algorithm is especially developed to analyze the compensation performance of shunt and series active power filters. The algorithm is proved by simulation in a multibus industrial power distribution system and programmed in Matlab. Index Terms--Active power filter, harmonics, resonance.

A new topology for active power filters (APF) using an 81-level converter is analyzed. Each phase of the converter is composed of four three-state converters, all of them connected to the same capacitor dc link voltage and their output connected in series through output transformers. The main advantages of this kind of converter are the negligible harmonic distortion obtained and the very low switching frequency operation. The single-phase equivalent circuit is analyzed and their governing equations derived. The dc link voltage control, based on manipulating the converter's voltage phase, is analyzed together with the circuit's characteristics that determine the capability to draw or deliver active and reactive current. Simulation results for this application are compared with conventional pulsewidth-modulated (PWM) converters, showing that this filter can compensate load current harmonics, keeping better-quality sinusoidal currents from the source. The simulated configuration uses a 1-F ultracapacitor in the dc link, making it possible to store energy and deliver it during short voltage dips. This is achieved by applying a modulation control to maintain a stable ac voltage during dc voltage drops. A prototype of the filter was implemented and tested, and the obtained current waveforms showed to be as good as expected. Index Terms--Active filters, multilevel systems.

Multilevel inverters with a large number of steps (more than 50 levels) can generate high quality voltage waveforms, good enough to be considered as suitable voltage template generators. Many levels or steps can follow a voltage reference with accuracy, and with the advantage that the generated voltage can be modulated in amplitude instead of pulse-width modulation. The main disadvantage of this type of topology is the large number of power supplies and semiconductors required to obtain these multistep voltage waveforms. This paper is focussed on minimizing the number of power supplies and semiconductors for a given number of levels. Different combinations of topologies are presented, and the corresponding mathematical relations have been derived. This paper shows optimized curves to obtain the relation between a minimum number of power semiconductors required for a given number of levels. Experimental results obtained from an optimized prototype, capable of generatng 81 levels of voltage with only four power supplies and 16 transistors per phase, are shown. Index Terms--Amplitude (AM), pulse-width modulation (PWM).

This paper proposes a simple linear feedback-based control technique that ensures zero-neutral-point (NP) potential in three-level voltage-source inverters. The technique works even under dynamic load conditions and enables the decoupled control of the direct and in-quadrature load currents. The technique is based on the relative-gain-array (RGA) approach, which has been successfully used in the control of chemical-engineering processes. The resulting scheme allows the use of outer control loops to set the direct and in-quadrature load currents as required for the specific application (e.g., adjustable speed drives). Experimental results prove the theoretical considerations and show the simplicity of the proposed control scheme. Index Terms--Decoupling of systems, inverters, MIMO systems, multilevel systems.

A special 27-level four-quadrant rectifier for subway applications is analyzed. The arrangement uses only three H-bridges per phase, common dc bus, and independent input transformers for each H-Bridge. The transformers allow galvanic isolation and power escalation to obtain high-quality voltage waveforms, with total harmonic distortion of less than 1%. Some advantages of this 27-level rectifier are: 1) only one of the three H-bridges, called the main converter, manages more than 80% of the total active power in each phase and 2) it switches at fundamental frequency, reducing the switching losses at a minimum value. The rectifier analyzed in this paper is a current-controlled voltage-source type, with a conventional feedback control loop. Some simulations in a rectifier substation, including power reversal at full load are displayed (750 Vdc, 1200 A). The rectifier shows the ability to produce clean ac and dc waveforms without any ripple, and fast reversal of power. Some experimental results with a small prototype, showing voltage and current waveforms, are finally displayed. Index Terms--Multilevel systems, rectifiers.

Grinding mill drives play an important role in the mining industry. Almost 60% of the electrical energy consumed by modern concentrator plants goes to grinding mill drives. Different technologies are used in grinding systems presenting special characteristics from the electrical and process points of view. This paper presents a technical evaluation and practical experience of two different technologies used in high-power grinding mill drives. The analysis is focused on the static power converter and associated control scheme required to drive the motor. Cycloconverters and load-commutated inverters are analyzed in terms of power grid interaction, motor interaction, and the required control scheme. The evaluation is supported with practical results obtained in different concentrator plant facilities. Index Terms--Cycloconverters, gearless motor drives, load-commutated inverter.

This paper presents an overview analysis and a discussion of field experience with different drive configurations used in shovel applications. The analysis includes different drive topologies, shovel duty cycles for a typical truck pass loading, and energy regeneration capabilities. Special attention is given to the interaction between the high power shovel drives and the grid: harmonics injection, reactive power requirements, and voltage regulation. This paper presents operational experiences obtained in copper mine industries. The main characteristics of the newest shovels with active front end (AFE) at the line side are also included. Finally, a critical evaluation of the different topologies is presented, showing that the shovel drive operation strongly affects the behavior of the power distribution system and that the introduction of the AFE technology produces an important performance improvement. Index Terms--AC drives, active front end (AFE), dc drives, mining, shovels.