Megger (United Kingdom)

This paper describes experimental verification of single-phase variable inductor prototypes rated at 2.4 kVAr (75 mH, 10 A, 230 V, 24 kg) and 12 kVAr (43 mH, 30 A, 400 V, 78 kg). Results for much smaller devices rated at 1-10 Var are also shown. The variability of inductance of the main AC winding was achieved by adding a DC winding, wound through the apertures placed symmetrically in the core such that both windings were magnetically orthogonal. Local saturation caused by the magnetic field due to DC winding creates virtual air gaps. This method is capable of lowering the main inductance by over 90% if no real air gap is present, but for industrial applications the range of variability is likely to be closer to 20-30% range, depending on the size of the real air gap in the magnetic circuit. The amount of variation/control can be improved by using more than one control winding. Very good repeatability of performance was observed, with relatively small total harmonic distortion introduced due to control. The experimental findings agree with a generalised analytical model.

Magnetic losses in a ferromagnetic lamination can be separated into three contributions. Bertotti theoretically assessed this distribution at the end of the twentieth century in the statistical theory of losses (STLs), triggering significant progress in understanding the dissipation mechanisms. Recent studies have shown the possibility of reconstructing a hysteresis cycle from the high-frequency Barkhausen noise signal. Applying STL to the Barkhausen noise cycles has never been done before. Still, it could help establish a parallel with the measurement of the magnetization cycle versus frequency and the energy loss. However, STL analysis in its ultimate description requires sinusoidal flux density, while Barkhausen noise measurements are usually done with a constant excitation slope. Multiple magnetic flux density control methods were described in the literature and are reviewed in this article. However, the Barkhausen noise context, requiring high-frequency sampling during the magnetization cycle, is more constraining. Therefore, specific performance criteria were considered, followed by numerical tests to determine the most adapted method to a Barkhausen STL description. Eventually, the proportional iterative learning control (P-ILC) gave the highest satisfaction rate and was chosen for experimental tests. Some of these experimental results are provided in this article discussion together with suggestions for convergence speed improvement. It is, for instance, recommended to increase the gain near saturation, where the system response is poor.

The aim of this article is to present an approach to the analysis of the phenomena of magnetization of materials in a rotating magnetic field. This approach is based on examination of the rotating vectors, specifically on the time variation of the moduli of vectors of the magnetic field strength H and the magnetic flux density B, as well as on time variation of their angles during rotation and the lead/lag between them. For the first time, the power loss separation based on Bertotti's statistical model under rotational magnetization is represented completely in the time domain. The main benefit of the presented analysis is the observation of time variation of power loss components, especially excess power loss and the corresponding excess magnetic field in the material. Two typical electrical steels are analyzed: conventional grain-oriented M140-27 (CGO) and non-oriented M400-50A (NO).

Power swing which is primarily caused by sudden addition or removal of load in a transmission line, consequent to an enormous interruption in power system, which if not blocked, could cause wrong operation to the distance relay which may lead to tripping the healthy part of the transmission line. In the absence of power swing function it may result in severe damage to the machines or cascading tripping in the grid resulting in blackouts. To prevent such scenarios, intelligent electronic devices (IEDs) have power swing block (PSB) detection and trip logics incorporated with distance schemes. This logic provides severe challenges to T & C engineers, where the conventional methods of using several states and multiple configuration steps while testing the IEDs complicates the process of testing. Further, the variations in logics due to manufacturer specific algorithm add to the complications. To address such complications, a new testing philosophy has been developed in, where PSB, PSD and PST functionality testing methodology and detailed discussion on the oscillography file is explained in this paper.

System reliability and improving System Average Interruption Duration Index (SAIDI) and System Average Interruption Frequency Index (SAIFI) performance are of utmost importance for utility companies today. The first step to systematically achieve a reliable distribution network which contains a high percentage of Medium Voltage underground cables is Very Low Frequency (VLF) testing. VLF testing on cable networks identifies critical defects during a scheduled test when the circuit is de-energized intentionally. At the same time the n-l principle ensures continued supply to customers. The next step beyond VLF testing is diagnostics testing, namely offline Partial Discharge (PD) analysis in the field. There are various technologies available, but real-world comparisons have shown significant differences in performance between the different types of excitation voltages. This paper will illustrate those differences and give recommendations for best testing practices.

The continuously increasing installation complexity for medium voltage power cables, results in many weak spots and cable failures due to bad workmanship. This is mainly caused by the large variety of cable accessories as well as the reduction in insulation thickness and therefore in the margin of error throughout the entire cable installation process. The necessity for both commissioning testing of new cables as well as diagnostic measurements as part of a condition based maintenance is just a logical given fact. Nowadays cable testing as well as condition monitoring are well proven measurement methods for MV power cables which are guided by IEEE 400 [1] and IEC 60502-2 [2] standards.

Instrument current and voltage transformers remain the basic apparatus used in measurements of energy in electrical grid. Increasing importance of distributed energy sources necessitates more precise measurements of consumed as well as generated electrical energy. Both types occur in low voltage network where the main focus is on current transformers for electronic-type electrical energy meters. These current transformers should fulfil many requirements for precise electrical energy measurements. This paper presents a modern system for measurement of various metrological features of current transformers.

Journal of Applied Mathematics and Computational Mechanics, Prace Naukowe Instytutu Matematyki i Informatyki, Politechnika Częstochowska, Scientific Research of the Institute of Mathematics and Computer Science, Czestochowa University of Technology

The increased usage of the phase shifting transformers in various fields within power systems has introduced new challenges in performing on-site maintenance testing. These special transformers were used as converter transformers, rectifier transformers, harmonic mitigating transformers, zigzag transformers and quadrature boosters with unconventional vector groups, having an additional phase shift such as +/-7.50, +/-150 combined with normal `Delta and Star' configurations. The challenges now faced by the increased use of these transformers are testing them for accuracy on the voltage ratio and phase shift at site during commissioning and maintenance. Different test methods were performed / studied and were compared. A prototype of a three-phase converter transformer with variable ratio taps and vector groups was constructed and tested. The prototype transformer is constructed to have variable parameters for test trials on different vector groups and ratios. The three phase converter transformer prototype will have options to regulate the output voltage on varying input voltage using the tap changer and also can vary the phase shift by varying the vector group combinations. On-site testing of ratio and phase shift will be done on different types of traction rectifier transformers. The approach for factory testing and onsite testing of the special transformers will be different. The effect on the phase shift values when the ratio taps are changed will be the focus and the results compared. The study on the importance of phase shift on these special transformers is presented. The test results of different test methods using modern portable test equipment and conventional classical methods were compared and presented. The results of the ratio testing and the phase shift measurement variation during vector group variation and tap change were presented. (6 pages)