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This paper describes the design, implementation, and performance of a medium-size distribution-type static synchronous compensator (D-STATCOM) with the simplest two-level three-leg voltage-source converter (VSC) topology. Reactive-power control is achieved by phase-shift-angle control, and VSC harmonics are eliminated by selective harmonic elimination method (SHEM). VSC has been designed at the highest low-voltage level of 1 kV and connected to a medium-voltage (MV) bus through a low-pass input filter and [DELTA]/Y-connected MV/1-kV coupling transformer. At the MV side of D-STATCOM, line-current harmonics are minimized to comply with the IEEE Std. 519-1992 for the weakest supply conditions by applying 8-angle TLN2 elimination technique. This necessitates switching the water-cooled high-voltage insulated-gate bipolar transistor (HV-IGBT) modules at 850 Hz, thus eliminating 5th, 7th, 11th, 13th, 17th, 19th, 23rd, and 25th voltage harmonics at the input of VSC. By carefully designing the laminated bus system and selecting minimum stray-inductance dc-link capacitors directly mountable on the laminated bus, stray inductance of the commutation path is brought to a nearly absolute minimum of 60 nH, thus maximizing the utilization of wire-bond single-side cooled HV IGBTs and eliminating the need for RCD clamping snubbers. The performance of SHEM, together with the phase-shift-angle control, has been tested in the field on a 0-1780-kVAr capacitive 6.3-kV VSC-based D-STATCOM (-750/+ 900 kVAr VSC) prototype. Field-test results show that SHEM, together with phase-shift-angle control, leads to optimum switching frequency and device utilization for HV IGBTs and high system performance at the expense of slower response as compared to the other known control techniques. Index Terms--Power quality, selective harmonic elimination, static VAR compensator, voltage-source converter (VSC).

This paper deals with the development and implementation of a current-source-converter-based static synchronous compensator (CSC-STATCOM) applied to the volt-amperereactive (VAR) compensation problem of coal mining excavators. It is composed of a [+ or -] 750-kVAR full-bridge CSC with selective harmonic elimination, a low-pass input filter tuned to 200 Hz, and a [DELTA]/Y-connected coupling transformer for connection to medium-voltage load bus. Each power semiconductor switch is composed of an asymmetrical integrated gate commutated thyristor (IGCT) connected in series with a reverse-blocking diode and switched at 500 Hz to eliminate 5th, 7th, 11th, and 13th current harmonics produced by the CSC. Operating principles, power stage, design of dc link, and input filter are also described in this paper. It has been verified by field tests that the developed STATCOM follows rapid fluctuations in nearly symmetrical lagging and leading VAR consumption of electric excavators, resulting in nearly unity power factor on monthly basis, and the harmonic current spectra in the lines of CSC-STATCOM at the point of common coupling comply with the IEEE Std. 519-1992. Index Terms--Current-source converter (CSC), harmonic elimination, power quality, static VAR compensator (SVC).

In this work, performance analyses of slip energy recovery drives during and at recovery after instantaneous power-supply failure in one or more supply phases have been carried out. Transient slip energy recovery drive currents and voltages under such fault conditions have been obtained theoretically using mathematical models which take into account the three-phase, single-phase, and disconnected power-supply conditions. A detailed set of theoretical and experimental results is given to clarify the expected behavior of the drive under instantaneous power-supply failure of a few cycles duration and at recovery. Insertion of an external bypass resistor into the dc link by the use of a static switch has been discussed to suppress the induced overvoltages and, hence, overcurrents in the rotor windings and converters, after recovery of the supply. Index Terms--AC motor drives, power-supply failure, slip energy recovery.

This paper describes the solution of power quality problems arising from 12-pulse smelter converters in the ETI Aluminum Works, Seydisehir, Turkey. The design work is based on practical data collected in the field as well as on the results of detailed simulation studies obtained by EMTDC/PSCAD, SPICE, and MATLAB. The main design objective is to meet both present regulations and harmonic standards by using robust, reliable, and cost-effective systems with a long life expectancy. The shunt-connected passive reactive power compensation system is composed of a damped 11th harmonic filter and a make-up filter tuned to the 5th harmonic with optional damping resistor. Such a system exhibits nearly transient-free switching and series-resonance proof characteristics owing to damping resistors. A close correlation has been obtained between theoretical results and field data. Index Terms--Harmonic analysis, metals industry, power quality, reactive power compensation.

This paper deals with the performance optimization of medium/high-power induction motors during soft starting by eliminating the supply frequency torque pulsations, and by keeping the line current constant at the preset value. Starting torque pulsations are eliminated by triggering back-to-back-connected thyristors at proper points on the first supply voltage cycle. Line current during starting can be kept constant at any preset value by a simple strategy composed of successive cosinusoidal and constant function segments of the triggering angle. These strategies are implemented by the use of an 8-b microcontroller. Transient performance analyses of the system are carried out by means of a hybrid ABC/dq machine model which takes into account the three-phase, two-phase, and disconnected modes of operation in terms of actual stator variables. Experimental results obtained on a custom-design test bed are found to be in good agreement with the theoretical ones. Index Terms--Induction motor, soft starter, torque pulsations.

Design and implementation of a unified, relocatable static var compensator (SVC) meeting the reactive power compensation needs of various coal enterprises of Turkey for open-cast lignite mining are presented. Design and sizing of SVCs are based on mains data for all possible connection points, load bus data recorded in the field, monthly electric bills, and simulations carried out on PSCAD/EMTDC, SPICE, and MATLAB. In addition to standard ac/dc converter and motor tools used in the modeling of electric excavators, actual load characteristics obtained from data collected in the field by means of a data acquisition system have been processed in PSCAD/EMTDC to find out reactive power demand of each load bus, and to optimize sizing of the SVC within the power-factor penalty limits imposed by electricity authorities. The SVC component ratings have been based upon the suitability to all possible loads under worst operating conditions, thus making sure that they can be operated for different loading conditions, minimizing stock requirements, and inspection and maintenance costs. The implemented systems are flexible and extendable in order to meet the future needs of coal enterprises. Index Terms--Reactive power control, real-time data simulation, relocatable, thyristor applications.

Self-excitation of induction motors compensated by permanently connected capacitors is investigated in this paper. Theoretical analyses of self-excitation phenomenon are carried out by using some simplified equivalent circuits, and a hybrid mathematical model in ABC/dq axes, respectively, in steady state and transient state. An unusual operating condition about water pumping stations is reported, in which water within the pipeline may drive the motor in the reverse direction at speeds higher than synchronous, when a supply interruption coincides with a check-valve failure. In order to prevent the motor from dangerous overvoltages due to self-excitation, it is recommended to connect a simple static protection circuit consisting of a resistor in series with a thyristor switch between any two lines of the motor. Critical resistance boundaries, which will lead to loss of excitation, and demagnetization of the rotor core are determined separately as a function of operating speed. A suitable resistance value chosen in the Safe Design Area constitutes a reliable protection mechanism against self-excitation. Index Terms--Compensation, induction motors (IMs), self-excitation.

This paper describes the application of a single-stage unity-power-factor buck-type pulsewidth modulation (PWM) rectifier to medium--and high-power variable-speed dc motor drives. The advantages of the developed system are low harmonic distortion in ac supply currents (complying with IEEE Std. 519 and IEC 555), nearly unity power factor over a wide operating shaft speed range, and nearly level armature current and voltage waveforms. These properties of output voltage and current quantities of the converter eliminate entirely any failure risk in current commutation even for oldest motor designs, and further motor problems such as accelerated aging in motor insulation, and mechanical failure due to circulating bearing currents. The design criteria and operating features and characteristics of the buck-type PWM rectifier employed in a unity-power-factor dc motor drive are discussed in the paper. The performance of the resulting system has been tested on 25-65-kW 100-150-A 10-20-kHz dc motor drives, used in various processes of an iron and steel plant. Index Terms--DC motor drives, power quality, pulsewidth modulation, rectifiers.

The performance of a voltage-controlled large induction motor soft starter has been improved, resulting in nearly perfect current and torque profiles. The performance analysis of the soft-starter motor-load combination has been carried out in the dynamic state using a hybrid induction machine model which takes account of disconnected two-phase and three-phase operational modes of the machine. Some simple control strategies have been proposed to keep the current constant at any preset value during starting, and to eliminate supply-frequency starting torque pulsations. These are shown to be very effective in the elimination of reclosing transient torque and current components at any speed at the reconnection instant to the supply after an interruption, or at the instant of bus transfer. The proposed strategies have been verified experimentally on a laboratory machine using a special torque measurement system in the dynamic state. Index Terms--Induction motor, soft starter, torque pulsations.

In this paper, the effects of transformer replacement on the electric arc furnace power system of an iron and steel plant have been investigated. The new operating strategy has been determined according to stable arc and maximum productivity requirements. Based on these new operating conditions, the sufficiency of the existing flicker compensation system in terms of the load balancing, power-factor correction, and flicker have been examined. All investigations made in this work are supported by real-time measurements. Index Terms--Arc furnace, flicker, reactive power compensation, steel, transformer.