Large capacity high voltage power transformers are one of the most expensive items of equipment in an electrical power network. Power utilities can ill-afford breakdown of transformers, especially, in a deregulated scenario. The consequences of such a failure are well known. Under these circumstances, utilities have figured-out that condition-based monitoring and diagnosis is worth pursuing, in spite of increased expenditure. Thus, monitoring and diagnosis is an integral part of operation and maintenance. Mechanical forces generated during short-circuits is the main cause leading to displacement/deformation of windings. Frequency response measurements have attained worldwide acceptance as a highly sensitive monitoring tool for detecting occurrence of such events. This is evident from the fact that customized commercial equipment are available (popularly called FRA or SFRA instruments), and with recent introduction of an IEEE draft trial-use guide for application and interpretation of frequency response analysis. Once a damage is detected, the next task is to identify its location along the winding and, if possible, determine its extent of severity. Understandably, these two tasks are best achieved, without disassembling the transformer and should ideally be based on off-line and on-site terminal measurements. In this regard, literature analysis reveals that recent research efforts have successfully demonstrated possibilities of using frequency response data for localization of discrete change in windings. This is indeed noteworthy, in spite of one major drawback. This pertains to excessive computing time needed to synthesize large-sized ladder-network, which automatically limits its practical use. Keeping these issues in mind, a research was initiated to find alternatives. The primary objective of this thesis is to examine the use of- • Complexnetwork-function-lociforlocalizationofadiscretechangeinasingle,isolatedtransformerwinding,basedonterminalmeasurements It goes without saying that the proposed method should be non-invasive, simple, time-efficient and overcome drawbacks in the earlier approach. A brief summary of the proposed method follows- This thesis presents a different approach to tackle the problem of localization of winding deformation in a transformer. Within the context of this thesis, winding deformation means, a discrete and specific change imposed at a particular position on the winding. The proposed method is based on the principle of pre-computing and plotting the complex network-function-loci (e.g. driving-point-impedance) at a selected frequency, for a meaningful range of values for each element (increasing and decreasing) of the ladder network. This loci diagram is called the nomogram. After introducing a discrete change (to simulate a deformation), the driving-point-impedance (amplitude and phase) is measured again .By plotting this single measurement on the nomogram, it is straightforward to estimate the location and identify the extent of change. In contrast to the earlier approach (wherein the entire ladder-network had to be synthesized for every new measurement), the proposed method overcomes the drawbacks, is non-iterative and yields reasonably accurate localization. Experimental results on a model coil and two actual transformer windings (continuous-disc and interleaved-disc) were encouraging and demonstrate its potential. Further details are presented in the thesis.

No description available.

Transformers are essential for modern power distribution system. They are efficient and enable the voltages to be transformed up for transportation of electricity and back down for consumer use. The reliability of the transformer is affected by faults and fatigue of the copper. The bigger faults a transformer is subjected to, the shorter life time it will have due to damage to the winding and other parts of the transformer. This thesis investigates if it is possible to characterise the noise made from the windings during a short circuit fault or lightning strike, to see if it can be replicated and if transformers can be diagnosed with the help from the noise. Faults creates forces on the windings, these forces can be so great that the windings collide with each other and in worst case breaks. The sound sources that are interesting are the thermal expansion due to the current, the radial forces and the axial forces acting on the coil. Simulations were made in Comsol multiphysics to see how the currents and forces behaved in a winding. A simplified microphone circuit was built and tested to see if it could detect the noise made from the collisions in the coil. Two microphone types and amplifier circuits were tested to see which ones was most suited for the experiments. The microphone circuit was used to record the sound made from the coils when being compressed. An experiment with a capacitor bank sending a large current pulse through different coils and the noise made from the coil was recorded with a microphone circuit connected to an oscilloscope. The currents are recorded by a computer using the program Picoscope. The capacitor bank was charged to different voltages to get different current amplitudes. A microphone circuit was built and tested so it could detect the sound from the collision. Sound occured with a current pulse with an amplitude of 2 kA, and permanent deformation occured when the amplitude of the current pulse was 4.5 kA. The frequency content of the impact was within the audible spectrum. Possibly even higher frequencies than 20 kHz was present during the fault. The microphone had a bandwidth between 20-20000 Hz, which limits the frequencies that is picked up by the microphone. / Transformatorer är viktiga för det morderna elsystemet. Transformatorer är effektiva och tillåter spänningen att transformeras upp för transport vilket minimerar förlusterna i lendningarna och sedan ned igen för kunder. Tillförlitligheten av transformatorer blir påverkad av fel, felen kan orsaka utslitning av kopparen i lindningarna. Ju större fel som transformatorn blir utsatt för, ju kortare livstid får transformatorn på grund av skada på lindningarna eller andra delar av transformatorn. Denna avhandling undersöker om det är möjligt att karaterisera ljudet från transformator lindningar under ett kortslutningsfel eller blixtnedslag. Felströmmarna skapar krafter på lindningarna, dessa krafter kan vara så stora att lindningarna kolliderar med varandra och i värsta fall går lindningarna sönder. Några intressanta ljudkällor är termisk expansion av kopparen från strömmarna, den radiella kraften på spolen och den axiella kraften på spolen. Simuleringar gjordes i Comsol Multiphysics för att se hur strömmar och krafterna beter sig i transformator lindningarna. En mikrofonkrets byggdes och testades för att se om den kunde detektera ljudet från kollisionerna i spolen. Två olika mikrofontyper och två olika förstärkartyper testades för att se vilka som passade bäst för experimenten. Mikrofonkretsen användes för att spela in ljudet från spolarna. Experimenten gjordes med med hjälp av en kondensatorbank som genererar en strömpuls genom de olika lindningarna/spolarna och ljudet från spolarnas kompression var inspelade av mikrofonkretsen som var inkopplad till ett oscilloskop. Strömpulsen var sparad på datorn med programmet Picoscope. Kondensatorbanken var uppladdad med olika spänningar för att få strömpulser med olika amplituder. Experimenten gjordes på fyra olika spolar, en två varvs spole, en 19 varvs spole, en 40 varvs tätlindad spole och en deformerad 19 varvs spole, för att se om det var skillnad mellan kollisionerna. Ljud uppstod vid en strömpuls med amplituden 2 kA och permanent deformation uppstod vid 4.5 kA. Frekvensinnehållet från kollisionen befann sig inom det hörbara spektrumet. Det fanns möjligen frekvenser över 20 kHz, men mikrofonen hade en bandbredd mellan 20-20000 Hz vilket begränsar de upptagna frekvenserna.

Transients encountered by transformers in power stations during normal operation can have complex oscillatory overvoltages containing a large spectrum of frequency components. These transients can coincide with the natural frequencies of the transformers windings, leading to voltages that can be greater or more severe than the current factory proof tests. This may lead to insulation breakdown and catastrophic failures. Existing lumped parameter RLCG transformer models have been proven to be less accurate for very fast transient overvoltages (VFTO) with frequencies over 1 MHz. A white box model for transient analysis of transformer windings has been developed using Multiconductor Transmission Line (MTL) Theory. This model enables the simulation of natural frequencies of the transformer windings up to frequencies of several MHz, and can be used to compute voltages between turns by representing each turn as a separate transmission line. Both continuous and interleaved disk windings have been modelled and a comparison and validation of the results is presented. / May 2017

Hydropower is the most important power balancing resource in the Swedish electrical power system, regulating the power supply to match the load. Consequently, several hydropower plants have periods of stand-by operation where the power production is absent but where several devices within a plant are still active. Such a device is the step-up power transformer, which during stand-by operation still generates no-load energy losses. These losses can accumulate to a considerable amount of energy and costs during the long technical lifetime of the apparatus. One option to minimize these no-load energy losses is by turning the transformer off when its generating unit is in stand-by operation. However, when this transformer operational change has been explained to experts in the field, the most common response has been that a more frequent reenergizing of a transformer leads to higher risks for errors or transformer breakdowns. This study aimed to analytically investigate three effects from this operational change. First, the potential of fatigue failure for the windings due to the increased sequences of inrush current. Secondly, the thermal cycling as a consequence of change in present losses. Lastly, the energy and economic saving potentials for hydropower plants where this operational adjustment is applied. The study used both established as well as analytical tools explicitly created for this study. These were then applied on currently active transformers in different plant categories in Fortum’s hydropower fleet.  The study primarily showed three things. Firstly, risk of fatigue failure due to the increased presence of inrush currents did not affect the transformer’s technical lifetime. Secondly, the thermal cycling changes were slightly larger with absent no-load losses during stand-by operation. The average temperature for the transformer decreased, which in general is seen as a positive indicator for a longer insulation lifetime and thus the transformer’s technical lifetime. Finally, the created frameworks showed the potential of saving energy and money for all plant categories, where the potential grew with the installed production capacity and the stand-by operation timeshare. Despite the simplifications made to describe the complex reality of a transformer operating in a hydropower plant, this thesis contributes to lay a foundation for future investigation of an easy adjustment to avoid unnecessary energy losses and costs for transformers in hydropower plants.