Jaan Järvik

AC-DC converter with parametric reactive power compensation

Resonant converters of 50 (60) Hz AC-DC are described, where each half cycle of network voltage the capacitor and inductor of an oscillatory circuit are switched from series into parallel and vice versa. The duration of series and parallel connection and also the transformer ratio are parametrically dependent on load. In the case of short circuit, only the parallel oscillating circuit operates. This restricts sharply the output current. The reactive power of the capacitor and the inductor compensate each other, both in the cases of series and parallel connection. Therefore, the power factor is very high from no load to short circuit. This converter fits very well for supplying arc furnaces, and there is no need for the costly and fast reactive power compensator and filter circuits. The operating principle of the converter, design principles, and a real operating converter rated 5.4 MW supplying a steel-melting arc furnace are described.

EMC education and related problems at Tallinn Technical University

Taking into account the importance of electromagnetic compatibility (EMC), modern electrical education requires more attention to give future electric and electronic engineers with good EMC knowledge. The purpose of this paper is to present the current situation with EMC educational and lab activities, as well as plans for the future at Tallinn University of Technology, Estonia. The paper describes the existing course and contents of creating courses in EMC given in the Department of Electric Machines and Fundamentals of Electrical Engineering. The aim and proposed effect of the courses are outlined.

Multi-electrode arc furnace technology with improved metal processing capability using current driven mixing

This paper presents a finite element solution for three-dimensional isothermal flow for DC arc furnace where low frequency AC component is added. Electrodes of the furnace are connected with independent supply sources. Current densities, magnetic flux densities, force densities and flow of molten metal are investigated in the case of two different furnace constructions. Methodology for supply source control described in this paper can be used in industrial applications for mixing molten metals or other conductive liquids.

Air-core inductive current sensor for fast transients measurements in distribution networks

Observation of fast transients in power networks can provide some of the most valuable information. The transients from partial discharge can provide the information about the power line insulation state; fault-produced fast transients can also provide information about the fault or problematic section location. For measurement of fast transients, there would also need to be a very fast sensor, as the pulsed measurements could require the bandwidth range reaching in the tens of MHz range. In this paper, a sensor for fast transients measurements is presented and tested. While very simple, this sensor can offer very good performance while being inexpensive and robust. The results of performance verification measurements show very good results when testing with high-frequency pulse source, presenting the potential of such sensor.

New theoretical approach to input current shaping in ac-dc power converters

A new method for an input current waveform correction in typical power converters is described. It assumes an implementation of the so-called virtual transformer with a fast-changing voltage ratio. Theory and calculations are presented to confirm the method validity. Practical implementation is left out of the scope of the given paper due to size limitations.

Air-Core Sensors Operation Modes for Partial Discharge Detection and On-line Diagnostics in Medium Voltage Networks

Abstract In this paper, sensors for the application of high voltage networks insulation diagnostics are focused on. For detection and localization of the components with insulation deterioration, observing the partial discharge transients is one of the main methods. However, as the partial discharge traces are transients of extremely short duration, monitoring of such transients is also a challenging task. This paper presents the inductive sensors that would be suitable for the on-line partial discharge monitoring and the selection of operating mode and characteristics are discussed. The issues of resonance and signal processing are analyzed for the targets of reaching highest accuracy of the measurements. Operation criteria and performance limits such as sensitivity and upper bandwidth are particularly discussed. It is presented that in order to reach highest accuracy and sensitivity the sensor should be used in damped mode. Applying the accurate air-core inductive sensors would allow the condition monitoring systems to be cheaper and thus mounted for permanent and continuous condition monitoring in various locations in the power distribution networks.

Suppression of Voltage Flicker by Saturable Reactor Operating under Forced Magnetization

Power quality control in the case of powerful abruptly variable loads, e.g. electric arc furnaces, becomes an actual problem due to the deregulation of electricity markets. A way to mitigate distortions on a power supply network, especially the flicker, is load current stabilization. The saturable reactor is proposed as a solution.

Magnetic current sensor stray components in high frequency operation and their effects

Some of the most severe faults in electrical power networks are caused by faulty insulation or short-circuits. These events are accompanied by traces of partial discharges and traveling waves and recording such pulsed signals accurately is the key for providing effective on-line status monitoring of power lines. Capturing the high-frequency transients is very essential to protection means of local power systems employing power converters and smart-grids [1]. In this paper, operation of an inductive current sensor for on-line measurements is analyzed at high frequencies. Analysis focuses on the aspects of external components used with the sensor but also the connecting components linking the sensor to the measurement circuitry. Some practical aspects are discussed that can help to improve measurement accuracy and effectiveness.

Investigation of molten metal mixing in a DC electric arc furnace with added AC component on different frequencies

This paper describes flowing of molten metal in a novel electric arc furnace. Combined AC and DC components are used in the supply system for the furnace. Finite element method is used to investigate the impact of AC component frequency on the flowing of molten metal.

Magnetic field analysis of permanent magnet low-speed synchronous generators for wind applications

Analysis of both primary and secondary magnetic field of permanent magnet synchronous generator in the axial end zone of air-gap has been presented. The models for study of primary and secondary magnetic field by the method of conformal mapping and using the method of current images have been created and analysed. The influence of axial end effects to primary and secondary magnetic field has been taken into consideration by means of end effect coefficients. The calculated magnetic field distribution in the end zone of air-gap was compared with the results of finite element analysis and with test curves.