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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.

In this paper, a new voltage sag compensator for powering critical loads in electric distribution systems is discussed. The proposed scheme employs a pulsewidth-modulation ac-ac converter (four insulated gate bipolar transistors per phase) along with an autotransformer. During a disturbance such as voltage sag, the proposed scheme supplies the missing voltage and helps in maintaining rated voltage at the terminals of the critical load. The approach does not employ any energy storage components such as bulk capacitors/inductors and provides fast response at low cost. Under normal conditions the approach works in bypass mode, delivering utility power directly to the load; this method of control allows the transformer to be rated only for transient conditions, thus reducing its required size. A four-step switching technique to drive the ac-ac converter is employed to realize snubberless operation. A design example is presented, and simulation results are shown for a three-phase 230-V 5-kVA system. Experimental results on a single-phase unit are discussed. The proposed approach can be easily integrated into a distribution transformer supplying critical loads. Index Terms--AC chopper, critical loads, duty cycle, four-step switching, insulated gate bipolar transistor (IGBT), pulsewidth-modulation (PWM) ac-ac converter, voltage sags.

A high-performance reactive-power compensator is presented and analyzed in this paper. The var compensator consists of a three-phase current-regulated pulse width modulated voltage-source inverter connected to a self-controlled dc bus. Reactive-power compensation is achieved by forcing the inverter output current to follow a reactive sinusoidal reference waveform at a constant switching frequency. The main advantages of this scheme are that it reduces the stresses on the switching devices (as compared with other current regulated techniques), and it has a fast response time, which allows almost instantaneous reactive current control, and low harmonic distortion in the line currents. In particular, this paper discusses the proposed scheme in terms of principles of operation, power and control system design, and the analysis under transient operating conditions. Simulated results obtained with the Spice simulating package for steady-state and transient operating conditions are presented and validated on an experimental unit.

Synchronous front-end PWM rectifiers are compatible with variable-frequency inverters because they generate a near sinusoidal input current with a controllable power factor and allow for the instantaneous reversal of power flow. Of the numerous publications in this area, however, none has dealt with the problems caused by input line voltage unbalance. As this paper shows, such problems include a significant distortion in the input current waveforms, considerable increase in the dc capacitor ripple current and voltage, and more importantly, the generation of subharmonic components in the inverter output voltages. The objective of this paper is to analyze the effects of input line voltage unbalance starting with the distortion in the input line currents and extending to the dc bus components and inverter output line voltages and currents. Analytical results are next used to obtain system design curves as a function of input voltage unbalance for all major system components. Finally, key predicted results are verified on a 2-kVA prototype unit.