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  • 1.
    Danylov, Daniil
    et al.
    KTH, School of Electrical Engineering and Computer Science (EECS), Electrical Engineering.
    Frei, Yanick Patrick
    KTH, School of Electrical Engineering and Computer Science (EECS), Electrical Engineering.
    Colombo, Leonardo
    KTH, School of Electrical Engineering and Computer Science (EECS), Electrical Engineering.
    Montalba Mesa, Raimundo
    KTH, School of Electrical Engineering and Computer Science (EECS), Electrical Engineering.
    Liu, Zhanzhan
    KTH, School of Electrical Engineering and Computer Science (EECS), Electrical Engineering.
    Janus, Patrick
    KTH, School of Electrical Engineering and Computer Science (EECS), Electrical Engineering, Electromagnetic Engineering.
    Morozovska, Kateryna
    KTH, School of Electrical Engineering and Computer Science (EECS), Electrical Engineering, Electromagnetic Engineering.
    An experimental assessment of the intermittent heating and cooling operation effect on power transformer insulation2021Report (Other academic)
    Abstract [en]

    In the future grid with a high share of renewables, grid flexibility is becoming an important issue to solve. Dynamic Transformer Rating (DTR) is a technology that allows to unlock additional grid capacity and provide extra flexibility to power system operation. However, since DTR is a fairly novel operation method, the effect of this type of operation on the rate of insulation ageing has not been studied in detail yet. Dynamic change of load and insulation temperature can introduce new extreme to the operation and lower the insulation lifetime. Therefore, this work studies the impact of dynamic rating on transformer insulation when exposed to different heating cycles.

    This work proposes an experimental evaluation of the impact that different heating cycles can have on insulation degradation. The setup consists of the control loop, heating element and paper-oil samples. The main focus is on the moisture exchange between insulating paper and oil when the samples are exposed to different temperature cycles. Moisture has been measured for both oil and paper and compared to a control group.

    This report presents published data from the first performed lab test. The experiment has to be repeated several times to increase the validity of the results. Consequently, the experiment has been documented carefully in order to ensure its replicability.

    It is shown that the limited interaction of the inner paper layers with the oil has forced the moisture to stay there. Moreover,  part of the samples exposed to the highest temperatures is observed to be damaged. 

    Download full text (pdf)
    An experimental assessment of the intermittent heating and cooling operation effect on power transformer insulation
  • 2.
    Ghaffarian Niasar, Mohamad
    et al.
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Taylor, Nathaniel
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Janus, Patrick
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Wang, Xiaolei
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Edin, Hans
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Kiiza, Respicius Clemence
    DIT Dar Es Salaam Institute of Technology, Tanzania .
    Partial Discharges in a Cavity Embedded in Oil-Impregnated Paper: Effect of Electrical and Thermal Aging2015In: IEEE transactions on dielectrics and electrical insulation, ISSN 1070-9878, E-ISSN 1558-4135, Vol. 22, no 2, p. 1071-1079, article id 7076808Article in journal (Refereed)
    Abstract [en]

    In this paper, the rapid aging due to Partial Discharge (PD) activity was investigated in unaged and thermally aged oil-impregnated paper with a disk-shaped cavity between the sheets of paper. The PD inception voltage and the voltage for instantaneous breakdown were measured, and the time to breakdown was measured by applying voltages less than the instantaneous breakdown voltage. A comparison between time to breakdown of unaged and thermally aged paper shows no significant difference between thermally aged and unaged paper at sustaining PD activity. The time to breakdown as a function of applied electric field was plotted and compared for both cases showing points scattered around a line in log-log scale. Changes of the PD parameters, such as PD magnitude and PD repetition rate were analyzed from the beginning of PD activity up to the moment of breakdown. The results show that the number and magnitude of PD increase at the beginning of aging until they reach to a maximum value, then both quantities decrease slowly over time until the final puncture breakdown occurs. The results emphasize the importance of PD monitoring on real equipment with oil-impregnated paper as insulation system, such as power transformers, since a focus on the number and magnitude of PD at just the present time may mislead the interpretation. Dielectric spectroscopy measurements performed on unaged and thermally aged paper showed an increasing trend of epsilon '' after the sheets of paper were exposed to thermal aging for longer time. Dielectric spectroscopy measurements performed on samples before and after exposure to PD activity showed a big change of epsilon '', This change can be attributed to byproducts and ions produced by PD activity.

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