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Power transformers are a key element in the transmission and distribution of electrical energy and as such need to be highly reliable and efficient. In power system networks, transformer core saturation can cause system voltage disturbances or transformer damage or accelerate insulation ageing. Low frequency switching transients such as ferroresonance and inrush currents, and increasingly what is now known as geomagnetic induce currents (GIC), are the most common phenomena to cause transformer core saturation. This thesis describes extensive simulation studies carried out on GIC and switching ferroresonant transient phenomena. Two types of transformer model were developed to study core saturation problems; one is the mathematical transformer magnetic circuit model, and the other the ATPDraw transformer model. Using the mathematical transformer magnetic circuit model, the influence of the transformer core structure on the magnetising current has been successfully identified and so have the transformers' responses to GIC events. By using the ATPDraw transformer model, the AC system network behaviours under the influence of the DC bias caused by GIC events have been successfully analysed using various simulation case studies. The effects of the winding connection, the core structure, and the network parameters including system impedances and transformer loading conditions on the magnetising currents of the transformers are summarised. Transient interaction among transformers and other system components during energisation and de-energisation operations are becoming increasingly important. One case study on switching ferroresonant transients was modelled using the available transformer test report data and the design data of the main components of the distribution network. The results were closely matched with field test results, which verified the simulation methodology. The simulation results helped establish the fundamental understanding of GIC and ferroresonance events in the power networks; among all the influential parameters identified, transformer core structure is the most important one. In summary, the five-limb core is easier to saturate than the three-limb transformer under the same GIC events; the smaller the side yoke area of the five-limb core, the easier it will be to saturate. More importantly, under GIC events a transformer core could become saturated irrespective of the loading condition of the transformer.

This thesis demonstrates a method for transformer core diagnostics. The method uses the no-load current of the transformer as an indicator, and gives different characteristic signatures for different types of faults or defects. Using the no-load current for the diagnostic gives high sensitivity. The method is therefore able to detect defects that are too small to have an impact on the losses. In addition to different types of fault, the method can in some cases also distinguish between faults in different locations within the core. Both single-phase and three-phase transformers can be diagnosed using this method, and the measurements can be easily performed at any facility capable of measuring the no-load loss. There are, however, some phenomena that occur in large transformers, and in transformers with high rated voltages. Examples include capacitive resonance and magnetic remanence. This thesis proposes and demonstrates techniques for compensating for these phenomena. With these compensating techniques, the repeatability of the measurements is high. It is shown that units with the same core steel tend to have very similar no-load behavior. The diagnostics can then be performed either by comparing the transformer to another unit, or to simulations. The thesis presents one possible simulation method, and demonstrates the agreement with measurements. This topological simulation method includes both the electric circuit and an accurate model of the magnetic hysteresis. It is therefore also suitable for other, related, studies in addition to core diagnostics. Possible subjects include ferroresonance, inrush, DC magnetization of transformers, and transformer core optimization. The thesis also demonstrates that, for three-phase transformers, it is possible to compare the phases to each other. This technique makes it possible to diagnose a transformer even without a previous measurement to compare to, and without the data required to make a simulation. / <p>QC 20170607</p>

Transformers emit a characteristic humming noise due to magnetostriction which is the continuous change in dimensions during magnetization. The noise is amplified if the induced frequencies match the core’s natural frequencies, consequently avoiding geometries that create resonance is critical in order to fulfill customer sound level requirements. In this thesis, a high voltage direct current transformer core with two main limbs and two return limbs is studied. Using finite element analysis (FEA), the core can be modeled and analyzed in a computer environment. The main contributors of noise are the first bending and longitudinal resonance modes. Data for how these modes change with geometric alterations is collected and stored through parametric studies. An analytical expression is then constructed through Rayleigh’s energy method with added coefficients that can be correlated to FEA datasets achieving a verified model via data-fitting. A satisfactory model is created for both resonance modes. / Transformatorer avger ett karaktäristiskt surrande ljud. Ljudet uppkommer på grund av magnetostriktion vilket är förändringar i geometri som uppkommer då kärnan kontinuerligt magnetiseras. Ljudet förstärks om induktionsfrekvenserna matchar kärnans naturliga frekvenser, så att undvika kärngeometrier som skapar resonans är viktigt för att klara ljudnivåkrav som kunden har. I denna studie betraktas resonansfenomenet i en högspänningstransformator för likström (HVDC) med två lindade ben och två sidoben. Med avstamp i en finita elementanalys (FEA) kan kärnan modelleras och analyseras i en datormiljö. Data för hur resonansmoderna som bidrar mest till ljud förändras med förändringar av geometriska parametrar samlas genom parametriska studier. I detta fall analyseras de första böj- och longitudinella resonansmoderna. Ett analytiskt uttryck skapas sedan med hjälp av Rayleigh’s energimetod där coefficienter anpassas efter FEA-datan. Detta leder slutligen till en verifierad modell som fungerar väl för uppskattning av de båda relevanta resonansmoderna.