I. Cadirci

Performance optimization of induction motors during Voltage-controlled soft starting

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.

Soft starting of large induction motors at constant current with minimised starting torque pulsations

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 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.

Multi-DSP and -FPGA-Based Fully Digital Control System for Cascaded Multilevel Converters Used in FACTS Applications

In this paper, a fully digital controller based on multiple digital signal processor (DSP) and field-programmable gate array (FPGA) boards has been proposed for parallel-operated cascaded multilevel converters (CMC) used in flexible AC transmission system (FACTS) applications. The proposed system is composed of a DSP-based master controller in combination with a multiple number of slave DSP boards, FPGA boards, microcontrollers, a programmable logic controller (PLC), an industrial computer, and their peripherals in interaction. Intercommunication of these digital controllers is achieved mainly through fiber-optic links, via synchronous serial data link wherever a high-speed, full duplex communication is needed, and via asynchronous serial communication interface wherever relatively slow communication speed is required. The proposed fully-digital control system has been implemented on a sample 11-level CMC-based 154-kV, +/-50-MVAr transmission type static synchronous compensator (T-STATCOM). Field test results have shown that the proposed fully digital control system provides good transient response and steady-state characteristics for the oveall system including protection and monitoring functions.

Various induction generator schemes for wind-electricity generation

Abstract In this paper, various generator types and control systems employed in wind energy conversion systems have been reviewed with particular emphasis on systems including induction generators. Advantages and disadvantages of these systems and relative generator sizes are also discussed. Variable shaft speed schemes employing induction generators have been studied more closely and the important aspects of the findings are emphasized. Throughout the paper, the importance of maximizing the conversion efficiency is stressed. The parameters controlling the energy conversion are identified. Simple methods have been developed to determine the optimum parameters for a constant speed synchronous generator wind energy conversion system and a variable speed squirrel-cage induction generator wind energy conversion system. This last configuration is shown to have advantages from the viewpoint of energy production as well as some other aspects including simplicity and robustness.

Design and Implementation of a 154-kV

In this research work, the design and implementation of a 154-kV ± 50-Mvar transmission static synchronous compensator (T-STATCOM) have been carried out primarily for the purposes of reactive power compensation and terminal voltage regulation and secondarily for power system stability. The implemented T-STATCOM consists of five 10.5-kV ±12-Mvar cascaded multilevel converter (CMC) modules operating in parallel. The power stage of each CMC is composed of five series-connected H-bridges (HBs) in each phase, thus resulting in 21-level line-to-line voltages. Due to modularity and flexibility of implemented HBs, each CMC module has reached a power density of 250 kvar/m3, thus making the mobility of the system implementable. DC-link capacitor voltages of HBs are perfectly balanced by means of the modified selective swapping algorithm proposed. The field tests carried out at full load in the 154-kV transformer substation where T-STATCOM is installed have shown that the steady-state and transient responses of the system are quite satisfactory.

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This paper describes a thyristorized static switch which is designed and implemented specifically for frequent direct-on-line starting of medium voltage large induction motors. The circuit configuration chosen utilizes back-to-back connected series thyristor strings on two of the supply lines. Both the starting current and the torque pulsations of the induction motor are minimized by a controlled, nonsimultaneous switching of motor phases to the supply. This reduces the electrical stresses on the power system and the mechanical stresses on the shaft as compared to conventional switching elements. The optimum switching strategy is found by the use of a hybrid mathematical model in ABC/dqO form. The validity of the results obtained for medium voltage, squirrel-cage induction machine are verified by the use of a physical simulator consisting of a low voltage, small induction machine and a shaft torque measuring system. The system has been in operation successfully in an iron and steel plant since 1991.

50-Mvar Transmission STATCOM Based on 21-Level Cascaded Multilevel Converter

The iron and steel industry has been growing increasingly in Turkey in the last decade. Today, its electricity demand is nearly one tenth of the installed generation capability of 40 GW in the country. In this paper, power quality (PQ) investigations based on the arc furnace installations of the iron and steel plants using field measurements according to the international standard IEC 61000-4-30 are documented. Interharmonics and voltage flicker problems occurring both at the common-coupling points of those plants and at the arc furnace and static VAr compensator (SVC) systems of the plants themselves are determined with the use of GPS receiver synchronization modules attached to the mobile PQ measurement systems. It has been observed that flicker and interharmonic problems are dominant at the points of common couplings where arc furnace installations are supplied. Based on the field measurements obtained with collaborative work of five arc furnace plants, it is possible to say that contemporary SVC systems cause interharmonic amplification problems around the second harmonic, and novel methods are required to solve this problem.

A solid state direct on line starter for medium voltage induction motors with minimized current and torque pulsations

The variable-frequency operation of the coreless induction melting furnace (IMF) has been investigated in detail both theoretically and experimentally. The time variations of the operational impedance value of the IMF, owing to the time-varying resonance frequency and crucible conditions, have been derived for typical melting cycles by using a variable parallel RLC model derived from extensive field measurements. The work coil and its crucible are supplied at variable frequency from a multipulse rectifier and a current-fed load-resonant inverter cascade. Uncharacteristic current harmonics caused by cross-modulation phenomenon across the alternating current ac-dc-ac link have been derived theoretically and verified by field tests. These theoretical results and extensive field measurements on a sample IMF installation have shown the following: 1) The major power quality problem of an IMF as a load on the power system is the injection of various characteristic and uncharacteristic current harmonics to the grid; 2) line currents on the supply side are nearly balanced; 3) flicker contribution is below the limits specified in the standards; 4) reactive power demand is relatively low and the variations in the magnitude and rate of change of active power are not drastic; and 5) no voltage sag and swell phenomena arising from furnace operation occur.

Electrical Power Quality of Iron and Steel Industry in Turkey

The variable-frequency operation of the coreless induction melting furnace (IMF) has been investigated in detail both theoretically and experimentally. The time variations of the operational impedance value of the IMF, owing to the time-varying resonance frequency and crucible conditions, have been derived for typical melting cycles by using a variable parallel RLC model derived from extensive field measurements. The work coil and its crucible are supplied at variable frequency from a multipulse rectifier and a current-fed load-resonant inverter cascade. Uncharacteristic current harmonics caused by cross-modulation phenomenon across the alternating current ac-dc-ac link have been derived theoretically and verified by field tests. These theoretical results and extensive field measurements on a sample IMF installation have shown the following: 1) The major power quality problem of an IMF as a load on the power system is the injection of various characteristic and uncharacteristic current harmonics to the grid; 2) line currents on the supply side are nearly balanced; 3) flicker contribution is below the limits specified in the standards; 4) reactive power demand is relatively low and the variations in the magnitude and rate of change of active power are not drastic; and 5) no voltage sag and swell phenomena arising from furnace operation occur.

Medium frequency induction melting furnace as a load on the power system

This paper is devoted to the design and implementation of a multipurpose platform (MPP) for power system monitoring and analysis. This MPP is a novel device, which combines the abilities of a power quality (PQ) analyzer, an event logger, a synchronized phasor measurement unit, and an interarea oscillation identifier, all in one device. The multiple functions of the proposed MPP can serve the needs of the power system operators (SOs) as a wide-area monitoring system to observe the states and stability of the power system, and as a PQ analyzer to monitor the PQ events and parameters, permanently. Furthermore, the algorithms of flicker and harmonic current contributions at a point of common coupling can be embedded on this MPP, to determine the individual contributions of different loads supplied from the same bus. The operational features of the developed MPP have been tested on the Turkish electricity transmission system and its interfaces with the distribution system by installing 450 MPPs and integrating them with a monitoring and control center. The proposed all-in-one MPP can therefore meet the multiple requirements of the power SOs to serve the needs of modern electricity markets and convert an ordinary electricity system to a smart grid.