ETK seminars

NEXT SEMINAR:

May 24th

Shafigh Nategh

Place and time:Stora konferens- rummet, 15:00

Seminar title:Evaluation of Impregnation/insulation Materials for Thermal Management of Electric Machines

Abstract:In intermittent, high-performance electric machine applications such as traction in hybrid electric vehicles, the need for high torque/power densities leads to a high current density in the stator slots. A high current density results in large copper losses and, in turn, high hot-spot temperatures which are a limiting factor. So far many attempts are made to improve thermal behavior of the electric machines. In this regard, a considerable amount of work has been carried out on developing high performance and complex cooling systems that effectively extract losses from the critical parts of the electric machine. Furthermore, a variety of research has been done and published on improvements in the electromagnetic design of different kinds of the electric machines to reduce losses, e.g., the iron losses in the stator and rotor laminations, and eddy current losses in the permanent magnets. Recently, attentions are given to the thermal effects of the materials used in different parts of the electric machines. In this regard, thermal effects of using the steel laminations with different thicknesses and amounts of alloy contents are studied. However, lack of a comprehensive study on the other materials used in the electric machines, is highly apparent. Another component that affects the thermal behavior of an electric machine, is the material used to impregnate the slots and the end windings. Based on the ratio of the end winding to the active winding length, the hot spot temperature builds up in the end winding or the center of the slot which is mainly due to the low thermal conductivity of the impregnation materials filling the stator slots and the end windings. Therefore, using the materials with a higher thermal conductivity reduces the hot-spot temperatures significantly. In this seminar, thermal impact of using two available impregnation materials, and also an alternative material with a higher thermal conductivity will be presented.

COMING SEMINARS:

Sajeesh Babu

Seminar title: MARKOV PROCESS BASICS FOR COMBINED PRIMARY AND CONTROL SYSTEM RELIABILITY

Abstract:The preliminary stages to model the impacts on risk and reliability of distribution station components with a combined perspective including both primary and control equipment are observed. The communication between the primary and control equipment are associated with a number of risks and hence they have an impact on the system performance and reliability. Conventional analysis methods are carried out separately for primary and secondary systems, which might neglect these reliability impacts from the faults and risks within the communication between the equipment. Ample real case justifications are there to imply the requirement for advancement from the conventional evaluation methods employed in distribution system management. Apart from the risks associated with the communication between the primary and control systems becomes even more grave in the futuristic smart grids, as the intelligent components are all associated with enhance communication between control equipment and primary networks components.

The initial study considers both primary and control systems individually as two aggregated units. To induct practical case similarities the two systems are defined to be repairable and capable of performing partial or de-rated operation. The states thus formed are translated to the Markov process; and further to sensitivity analysis to identify the critical states and transition rates. An effort to analyse the specific risks associated with each state and related transition probabilities. Such probabilities are scrutinised against the likelihood and relevance in practical operations. The study observes on both a power service provision case and a restoration process case. Certain states can be considered as a non-system function case while bearing in mind the power provision capability. Still these states might be present in sequential steps through transitions in the restoration process. Markov process is utilised to run exercise on the state space model that form a basis for combined reliability studies. Analysis is conducted over the model to highlight the critical states and system sensitivity to transition rates.

PREVIOUS SEMINARS:

May 17th

Roya Nikjoo

Place and time: Seminarierummet, 15:00

Seminar title:Diagnostics of high voltage power components  (initially high voltage bushings) by utilizing natural occurring transients
Abstract: In order to minimize the risk of failures, online monitoring of power system components would be so important. Natural transients would be helpful in monitoring of the power components by giving this opportunity to perform the monitoring online. We can  utilize them as stimuli for online dielectric responses. Besides the advantage of such transients that makes it possible to perform online diagnosis, they can cause stresses on electrical  insulation which, in some cases, has resulted in failures. Therefore, we can also investigate the effects of transients on insulations to find the possibility of their destruction

May 8th

Ener Salinas

Place and time:Stora konferens- rummet, 15:00

Seminar Title: Some Examples of Applications of  Electromagnetics to Modern Societies
Abstract:Modern society constantly demands more efficient technologies, coincidentally various aspects of electromagnetism are the basis of many of these technologies. In this presentation, various areas of research in electromagnetics, performed as a cooperation with LSBU-Uk,  will be outlined along a frequency scale. Starting from permanent magnet adhesion for climbing robots performing non-destructive tests, followed by improvements in bicycle safety design using electromagnetic induction;  field mitigation in large buildings from 16.7 Hz railway’s magnetic emission and ADSL/VDSL coreless transformer design for telecoms at frequencies 21 kHz-12 MHz, In addition, some experimental demonstrations will also be included in the presentation.

May 3rd

Luis Carlos Castro

Place and time:Stora konferens- rummet, 15:00

Seminar title:GROUNDWALL INSULATION DEGRADATION ASSESSMENT OF STATOR BARS SUBJECTED TO ACCELERATED THERMOELECTRICAL AGEING

Abstract: In Colombia, despite the growth in installed power capacity, diagnostic techniques of groundwall insulation have not developed at the same rate. In fact, over the years little research have been done on the degradation assessment of stator bar groundwall insulation. Thus, Colombia depends strongly on foreign companies for the diagnosis of electrical rotating machines what results in higher maintenance investment. With this background, our contribution will be focused on the understanding of the insulation ageing phenomena as measured by dielectric tests. In addition, the approach will use a chemical analysis as a state‐of‐the‐art diagnosis technique whose application has increased in recent years with good results. This project is part of a broader research which aims to reduce maintenance costs by developing the required technical knowledge to conduct diagnostic, reception and commissioning tests.

April 26th

Christos Kolitsidas

Place and time:Stora konferens- rummet, 15:00

Seminar Title: Introduction to emerging wireless trends and future antenna arrays for base station applications

Abstract: Commercial communications are close to the maximum network capacity for the current demand . A solution to increase the capacity is to employ spatial division (SDMA-Spatial Division Multiple Access) on top of the classic multiplexing techniques. In order to achieve such beam forming capabilities advanced array design techniques needs to be employed. Next generation base station applications are envisioned to offer fully adaptive capabilities according to the network demands. Furthermore we need to support all commercially available frequency bands in one mid-sized aperture for low visual impact. Thus, the ideal array for a future base station is a mid-sized wideband antenna array with wide scanning angle properties for best spatial coverage. In this seminar we will examine the emerging technologies and possible implementations that are suited for next generation base station applications.

April 19th

Per Westerlund

Place and time:Stora konferens- rummet, 15:00

Seminar Title:Condition monitoring with temperature sensors on disconnectors in the electric power transmission grid

Abstract:In order to assess the need for maintenance of disconnectors in electric substations, it is interesting to measure the resistance in the contacts, since a high resistance causes power losses and hence an increase in temperature. As the disconnectors are normally closed and transfer current owing to their often critical positions in the power grid, it is difficult to do a resistance measurement and to carry out preventive maintenance. Hence any condition monitoring that may be carried out on-line is very valuable to the network operator, such as having temperature sensors near the contacts.

The objects of this study are six disconnect switches. Nine temperature sensors have been placed on each, three for each of the three phases, two at the contact between the two moving arms and the respective support and one at the contact between the arms. There are also reference sensors at each substation.

April 12th

Henrik Frid

Place and time:Stora konferens- rummet, 15:00

Seminar Title:Simulation of Microbubbles During the Initial Stages of Breakdown in Cyclohexane

Abstract:The formation of a vapor microbubble has previously been suggested to be the initial mechanism in the process of dielectric failure of dielectric liquids. The microbubble is generated by a rapid, highly localized heating of a volume close to a highly stressed electrode, caused by electric currents in the liquid at high voltages. In this study, a numerical model is presented to investigate the dynamics of the formation,growth and collapse of a single microbubble in a point-plane geometry in cyclohexane. An electro-thermal model is used to find the charge injection and heating of the liquid, and a condition for the formation of a vapor bubble is discussed. Thereafter a Computational Fluid Dynamics (CFD) model of two-phase flow with phase transition is used to study the life of the bubble under a highly divergent electrostatic field in a subcooled liquid. The bubble expansion is also simulated considering the electrohydrodynamic (EHD) processes in the liquid and vapor phases. In addition, it is shown how the electrostatic forces on the dielectric will cause a bubble to detach from the elecectrode.

March 22nd

Lars Jonsson

Place and time:Stora konferens- rummet, 15:00

Seminar Title: Small Antenna bandwidth and an electromagnetic application of variational calculus

Abstract:Variational calculus is an important tool to analyze critical points in applied physics. In this lecture we investigate how electromagnetic energy and radiation in the well-known antenna Q-relation provide detailed information and bounds on the bandwidth given a specific structure. For simplicity we will focus on electrically small antennas of of dipole-type. We derive how bandwidth is connected to the polarizability of the enclosing volume. The presentation will be on the black board, and we will review the derivations and the tools to reach the results.

March 15th

Claes Carrander

Place and time:Stora konferens- rummet, 15:00

Seminar Title:The magnetization current as a tool for transformer core diagnostics

Abstract: The magnetization current and no-load losses are frequently used as indicators of transformer core health. Usually, only time averaged values are measured, but since the core is magnetically nonlinear, it is of interest to study the time dependent magnetization behavior. Small scale experiments suggest that inspection of the magnetization current could help detect some of the more common core problems. Experiments are fairly easy to perform both on small scale models and full scale power transformers, but several issues remain to be addressed: First off, material data measurements are ambiguous, with the three most common methods yielding substantially different results. Secondly, there are issues when scaling down a power transformer to a model two orders of magnitude smaller.

March 8th

Patrick Janus

Place and time:Stora konferens- rummet, 15:00

Seminar Title:Low-cost Partial Discharge Detection system

Abstract:The project is concerning the use of simple off-the-shelf components for detecting Partial Discharges. The simplicity of such a system was then judged to be of vital interest since such equipment could potentially be used in places in the power system where Partial discharge detection up until now has not been cost-efficient.

March 1st

Marley Becerra

Place and time:Stora konferens- rummet, 15:00

Seminar Title: Charge and extension of streamers propagating in air: an inverse problem approach
Abstract: Accurate estimation of the charge and extension of streamers is required to assess the conditions of breakdown in air insulation as well as the properties of leader discharges in long air gaps. However, the approximations available in the literature to calculate the streamer charge are strongly dependent on geometric parameters (e.g. the solid angle) of the streamer zone. This paper presents an alternative approach to estimate both the streamer charge and extension based on the inverse solution of Poisson’s equation. Since streamers in air are known to maintain a constant electric field as they propagate, it is possible to estimate their charge and extension required to keep such a condition. Thus, numerical optimization combined with the finite element solution of Poisson’s equation is used to solve the inverse problem in two dimensions (axisymmetric). Good agreement between the estimations and measured values of the charge and extension of streamers in air has been found. The method has potential advantages to estimate the streamer properties where there is significant background space charge or interfaces with dielectric media along its propagation path.

February 15^th

Baoyou Zhu

Place and time:Stora konferens- rummet, 15:00

Seminar Title: Observations of a distinct class of lightning discharges named Narrow bipolar Events China

Abstract:Narrow bipolar Events(NBEs) refer to a recently studied distinct type of  intra-cloud discharges, which are characterized by extremely strong VHF/HF radiation and energetic VLF/LF narrow bipolar electromagnetic pulse. This report presents observations of NBEs in three different regions in China: (1) 77 NBEs from a local storm in Shanghai were presented. An algorithm was developed to derive the electrical parameters and channel length of NBEs from the single-station narrow bipolar radiation pulse. (2) NBEs over high-latitude regions of Northeast of China during 2009-2010 were detected by a three-station field network, which have not been previously observed in such high-latitude regions. NBEs occurrence observed in high-latitude regions appears to differ significantly from that in lower-latitude regions. (3) A five-station field campaign for NBEs has been carried out in Hefei during the summer of 2012. Characteristics of NBE-producing thunderstorms as revealed from satellite TRMM data will be presented.

February 1^st

Mengni Long

Place and time: Seminarierummet, 15:00, Seminar Title: The dynamic arc model based on high-speed flow and the analysis of practical transmission lines lightning strike characteristic

Abstract:Because the transmission lines in Guangxi (a south province in China) frequently suffer from lightning, we study on a new Jet gas lightning protection arc gap, to study its mechanism and effect in inhibition the over- voltage. The core function of the device is to be a channel leading lightning current into the earth, then quickly extinguishing arc, preventing flashover of insulators, suppressing lightning overvoltage, improving lightning withstand level.We has carried out the following work: first, mechanism and physical properties of AC arc is studied as well as the energy exchange process of AC arc; the improved Mayr arc model is established, for a more precise description of the interaction between high speed flow and arc. Time constants and dissipation constant of arc model are determined. At the same time, pressure distribution, velocity and dynamic development of the high-speed flow is also studied through Computational fluid Dynamics (CFD), modeling flow field, solving the N-S equations in calculation region with finite volume method(FVM) and velocity-pressure coupling method.  Finally, a 110kV line in Guangxi is studied, establishing the lightning model of transmission lines and calculating the parameter accurately, including multi impedance of Tower model, J-MART line model, then, the validity of the model is proved by comparing to experimental measurement. Based on the model, the overvoltage in the lines and the insulators under different lightning conditions is discussed.

December 7th
Ara Bissal

Place and time: Seminarierummet, 15:00
Seminar title: Important design aspects of ultra fast electromechanical actuators
Abstract: The aim of this thesis project is to study the parameters influencing
the Thomson actuator, develop a simulation program to accurately predict
the behavior of the system, build a prototype, validate the results, and
setup a test bench to study different configurations and possible
improvements.

November 19th

Mariana Dalarsson

Place and time: Seminarierummet, 15:00
Seminar title: On-line power transformer diagnostics using microwave
radiation to reconstruct winding conductor locations
Abstract:The project investigates a novel method to detect mechanical
deformations of windings in a power transformer, while the transformer
is in operation. The principle is to insert antennas inside the
transformer tank above and below the winding to radiate and measure
microwave fields that interact with the metallic structure and the
insulation. The responses from the radiated waves are assumed to be
sensitive to any mechanical deformations that could be caused by
electromagnetic forces due to short-circuit currents and possible
manufacturing weaknesses. The analysis of the measured fields and
their relations to material and structure parameters, being the
critical signatures of mechanical deformations, is an inverse
electromagnetic problem that typically is not straightforward to solve
mathematically. The goal of the project is to be able to determine the
locations of the individual winding segments from measurements of the
scattered fields at both ends.

November 2nd
Helin Zhou

Place and time: Stora konferens rummet, 15:00
Seminar title: Derivation of charge density expressions along the tower and the lightning channel when the lightning strikes to a tall tower
Abstract: In this presentation, based on the assumptions of the transmission line model of the lightning strikes to a tall tower and a series point current source placed at the tower top, we derive expressions to calculate the charge density along the tower and the lightning channel. Expressions are found in two different ways, we will show the equivalence between these two methods. Later, for the validation of above expressions,  they are employed to calculate electric fields in the monopole (continuity equation) technique in terms of the retarded current and charge density and their results are compared with those calculated from the traditional dipole (Lorentz condition) technique in terms of the retarded current. Apparently, for the first time, we show the charge density distribution versus height along the tower and the lightning channel.  Alternative explanations are given to the inversion of polarity of the vertical electric field at very close range from the tower base when the tower is struck by lightning.

October 19th
Jesper Magnusson

Place and time: Stora konferens rummet, 15:00
Seminar title: Hybrid switching
Abstract: The project investigates the possibilities of using a combination of power electronics and a mechanical switch to obtain the benefits of both components. Power electronics provide controllability and possibility to interrupt or commutate currents in some micro seconds. The drawback of power electronics are high on-state losses both considering cost and increased complexity since cooling systems are needed. One way to solve it is to put a mechanical switch in parallel to the power electronics to carry the nominal current. The switch never has to interrupt any current, only commutate the current over to the power electronics and hence it can be made lighter and faster than a circuit breaker. The project investigates the development of such a commutation switch considering the issues of fast current commutation and mechanical endurance. With rapidly increasing amounts of distributed generation of green power, the electrical distribution network is experiencing changes in operational conditions. The risk of reversed power-flows causes problem with the voltage levels in the low voltage and medium voltage grids. To reduce the voltage-levels, one way is to regulate the voltages on the distribution transformers. The final objective of this project is to develop a hybrid tap-changer for medium voltage to be used in distribution transformers.

May 11th
Alireza Motevasselian

Place and time: Seminarierummet, 15:00
Seminar title: Cylindrical dielectric resonator antenna (DRA) element excited by a single point coax-fed helix.
Abstract: The feeding network is compact, low cost and easy to fabricate. The design provides a directive end-fire radiation pattern with circular polarization. The design has relatively large 3-dB axial ratio bandwidth and impedance bandwidth. Although it is a single point feed antenna its 3-dB axial ratio bandwidth is wider than most of the other reported single point feed DRAs.

April 27th
Seyed Ali Mousavi

Place and time: Seminarierummet, 15:00
Seminar title: Electromagnetic modeling of power transformers with DC magnetization
Abstract: Existence of superimposed DC currents in power networks may saturate power transformers and cause some considerable adverse effects on power transformers and power networks. The main source of DC currents is geomagnetically induced currents (GIC) which can be created by interaction between solar wind and the Earth’s magnetic field. Countries close to the earth’s poles such as Sweden, Norway, Canada and Russia are more vulnerable to GIC.

April 16th
Kelin Jia

Place and time: Seminarierummet, 15:00
Seminar title: High frequency modeling of AC motors
Abstract: High frequency modeling of the motor is one very important issue in the Adjustable speed drive (ASD)system study in terms of EMI. Accurate simulation of the ASD system is based on the accuracy of the motor modeling. An experimental identification procedure for the high frequency modeling of the motor is proposed. The simulation results from 10 kHz-10 MHz is presented, the simulated impedance is matching the experimental data quite well.

March 23th
Carin Norberg
Place and time: Seminarierummet, 15:00
Seminar title: Brandskydd och säkerhetsfrågor/Fire and safety
Abstract: Brandskydd är viktigt att kunna, inte minst privat hemma men också på jobbet för att öka förståelsen av brandens betydelse. Detta seminarium kommer belysa arbetet före en olycka, under och efter. Mer exakt: Vad du bör veta på din arbetsplatsJag kommer även belysa första hjälpen och vissa av KTH’s säkerhetsregler”

March 16th
Roya Nikjoo

Place and time: Seminarierummet, 15:00
Seminar title:Diagnostics of high voltage power components  (initially high voltage bushings) by utilizing natural occurring transients
Abstract: In order to minimize the risk of failures, online monitoring of power system components would be so important. Natural transients would be helpful in monitoring of the power components by giving this opportunity to perform the monitoring online. We can  utilize them as stimuli for online dielectric responses. Besides the advantage of such transients that makes it possible to perform online diagnosis, they can cause stresses on electrical  insulation which, in some cases, has resulted in failures. Therefore, we can also investigate the effects of transients on insulations to find the possibility of their destruction

January 27th 15:00
Venkatesh Doddapaneni
Place: Seminarierummet
Seminar title: Outgassing of polymers due to electric arcs
Abstract: Polymers have been used to improve the switching performance of electrical switching devices (e.g. breakers, fuses, etc.). This seminar will cover the overview of the field of outgassing (ablation) of polymers related to the “Arc Quenching Materials” project at KTH. More precisely, I will discuss about “why and how the tested polymers will be exposed to plasma arcs” and several key factors like temperature, radiation, pressure, composition of gases, etc. during this process. The project is aimed to cover the key fundamental aspects of the arc-induced ablation of polymers in electrical switching devices.  
 

December 8th 15:00
Mats Bäckström
Place: Stora konferens rummet
Seminar title: Can Intentional Electrical Discharges be Used for HPM Protection?

December 1st 15:00
Nadja Jäverberg
Place: Seminarierummet
Seminar title: Dielectric properties of the poly(ethylene-co-butyl acrylate) filled with alumina nanoparticles.
Abstract: The aim of this study is to evaluate the dielectric properties of the poly(ethylene-co-butyl acrylate) filled with alumina nanoparticles. This study is limited to the properties of general importance for the AC applications. The dielectric permittivity of the nanocomposite materials was studied as a function of filler size, filler content, coating, temperature and air humidity used for conditioning of the samples. The breakdown strength is being evaluated at the writing moment and the final property for evaluation will be electrical endurance. The ultimate goal with this project is to describe the influence of material composition, temperature and air humidity on the dielectric properties and model these dependencies.