Dc magnetization due to the geomagnetically induced currents (GICs) and HVDC (High Voltage Direct Current) systems may cause core saturation and result in serious destruction in the transformer performance as well as the power system stability. Based on susceptibility, transformers are classified into different groups. For instance, a three-phase, three-limb transformer is considered less vulnerable to effects of GIC compared to a single-phase or a three-phase, five-limb transformer. However, our study shows that such classifications do not apply to the dc magnetization caused by converter modulation. In this article, we introduce the concept of common mode and differential mode to distinguish dc-bias caused by different mechanisms. Main focus is given on differential mode dc current since it has rarely been reported in any literature. The differential mode dc current was demonstrated by system simulations of classic three-level voltage source converters as well as modular multilevel converter. Detailed experimental investigations were made on a three-phase, three-limb transformer, where the loss impact as well as reactive power consumption were studied. The test shows a significant difference in stray loss between the two modes in three-phase power transformers. Finally, we discuss the effect of delta winding on dc-bias of different modes.

Partial discharge measurements have been used for studying shrinkage voids in the insulation of mass-impregnated high voltage DC subsea power cables. Three 4.5-m long cable samples were subjected to ac partial discharge measurements at different ambient temperatures, both under isothermal conditions and after load current turn-offs. Two distinctly different phase resolved partial discharge patterns were observed, suggesting that two different "types" of cavities were created. The type causing by far the most powerful discharges appeared a few hours into the cooling period after load turn-off, which coincides in time with when load cycling breakdowns during type testing usually occur. Current loading has a significant effect on the cavities, as it leaves them -at least temporary -with a different dielectric strength afterwards. The mechanisms behind this change are not identified, but the associated time constants are of the order of days and weeks. Consequently, accurately determining the limitations of such cables with regard to their ability to withstand polarity reversals and rapid load changes may be more complicated than previously assumed.

Geomagnetically induced currents (GICs) and the converter modulation effect are the two main causes for dc magnetisation in power transformers. It is well known that a small dcbias can saturate a large transformer and thereby generate high unbalanced magnetising currents, imposing a serious risk of excessive power losses and local overheating. Magnetising currents due to GIC phenomena have been studied extensively, whereas studies on converter related dc-bias are few. In particular, a discussion on loss characteristics related to converter modulation lacks. In this study, the dc-bias of common mode and differential mode in a three-phase, three-limb transformer is investigated experimentally. Additionally, to interpret the physical phenomena, the system was modelled using the finite element method. The results revealed that the power losses are significantly influenced by the dc current direction, arrangement of the structural parts, and the method of winding connection.This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited.

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SUMMARY Overhead lines in areas with sensitive populations of large birds raise demands for protection against bird electrocution (mortal current transfer through the bird bodies), particularly for threatened species like the eagle owl. For the decision making of what protection measures to apply, it is important to assess both their environmental benefits and the measures of technical consequences. One of the popular retrofit protection methods is insulation of the aluminium conductors at pin insulated medium voltage overhead line poles. In the present work, we have performed accelerated corrosion tests on aluminium conductors using four different bird protection systems available on the market. The tests were performed in laboratory environment for marine corrosivity indices 4 and 8, and simulated time periods up to 24 years. The results show that the systems influence the degradation of the aluminium conductors, leading to life‐time expectancy reductions between 12 and 22% for the investigated corrosivity indices. Copyright © 2013 John Wiley & Sons, Ltd.