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  • 1.
    Boricic, Alexandar
    et al.
    KTH.
    Laban, Danilo
    KTH.
    Moedim, Bruno
    KTH.
    Conde Lopez, Almudena
    KTH.
    Molinari, Benjamin
    KTH.
    Riaz, Zain
    KTH.
    Nikjoo, Roya
    Megger Sweden AB, Danderyd, Sweden..
    Dynamic Resistance Measurements and Result Interpretation for Various On-Load Tap Changers2019In: 2019 IEEE MILAN POWERTECH, Institute of Electrical and Electronics Engineers (IEEE) , 2019Conference paper (Refereed)
    Abstract [en]

    Dynamic Resistance Measurement (DRM) is introduced in the last decade and provides more information about the condition of the OLTC than static measurements. However, there is little information available on how to interpret the results from the DRM for different types of tap changers and different faults. In this paper, different faults are introduced in a typical tap changer and it is then subjected to DRM tests. The test results are presented and interpreted. DRM measurements are performed on resistor-type and vacuum-type OLTCs and their DRM behavior is simulated in Simulink to compare with the measured results for further studies. Results demonstrate the possibility to determine and locate the likely cause of a problem in an OLTC, thus improving fault identification and further optimizing maintenance and reliability of a transformer.

  • 2.
    Nikjoo, Roya
    et al.
    KTH, School of Electrical Engineering and Computer Science (EECS), Electrical Engineering, Electromagnetic Engineering.
    Estebsari, A.
    Nazari, Mohammad
    KTH.
    Non-parametric Regression Model for Continuous-time Day Ahead Load Forecasting with Bernstein Polynomial2019In: Proceedings - 2019 IEEE International Conference on Environment and Electrical Engineering and 2019 IEEE Industrial and Commercial Power Systems Europe, EEEIC/I and CPS Europe 2019, Institute of Electrical and Electronics Engineers (IEEE), 2019, article id 8783908Conference paper (Refereed)
    Abstract [en]

    Growing perception of diverse generation resources and demand response operation of power system with high uncertainty has increased the attention to a more dynamic and accurate day-ahead load prediction. In this paper, we develop an stochastic model for short term load forecasting based on the Gaussian process, in which the non parametric estimator of the regression functions are obtained by using Bernstein polynomials. One of the major features of this model is its ability to predict a continuous load at any time of the day with a regression function. We use the historical data for training and the constrained marginal likelihood problem is optimized for finding the hyperparameters of the model. Real data sets from California ISO were used for training and testing the model. The results are compared to the day ahead piecewise constant load and the real time load. The common error measures are employed to infer the deviation of the load forecast from the real data.

  • 3.
    Nikjoo, Roya
    et al.
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Taylor, Nathaniel
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Edin, Hans
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Dielectric response measurement by impulse stimulus on AC: Measurement considerations, and laboratory testing on a bushing2017In: IEEE transactions on dielectrics and electrical insulation, ISSN 1070-9878, E-ISSN 1558-4135, Vol. 24, no 1, p. 511-518, article id 7873509Article in journal (Refereed)
    Abstract [en]

    Dielectric response (DR) measurement is commonly used for condition assessment of insulation systems of power components. A method for measurement of dielectric response using the stimulus of natural transients such as lightning and switching impulses was proposed in earlier work. Its desirable features include the ability to make measurements online over a range of frequencies, without requiring a voltage source. This article presents a laboratory demonstration of the method on a 150 kV service-aged transformer bushing, where the stimulus was a standard lightning impulse voltage superimposed on an AC voltage. Several aspects of the measurement and data processing that affect the results are studied experimentally and numerically. The results are compared with low-voltage frequency domain spectroscopy (FDS). Reasonable accuracy for monitoring changes in DR can be achieved by suitable choice of measurement circuit and data acquisition. The study suggests an approach for online monitoring of dielectric properties of power transformer bushings, and diagnostics of defects that affect the high frequency region of DR, such as moisture content.

  • 4.
    Nikjoo, Roya
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Dielectric Response and Partial Discharge Diagnostics of Insulation Systems by Utilizing High Voltage Impulses2016Doctoral thesis, comprehensive summary (Other academic)
    Abstract [en]

    In this thesis, power system transients are considered as an opportunity for development of on-line diagnostics of power components and specifically the insulation systems of power transformers and bushings.

    A new technique for on-line dielectric response measurement of power transformer bushings is proposed which utilizes natural transients in the power system, such as lightning and switching surges, as stimuli. Laboratory investigations are done on implementation of the proposed technique. Measurement considerations, data acquisition and processing involved in achievement of reasonable accuracy in the Dielectric Response (DR) are presented. Capability of the technique in tracking of the degradation signatures such as moisture content in the insulation has been evaluated and it has shown a good level of accuracy by being compared to the Frequency Domain Spectroscopy (FDS). 

    The proposed technique is tested on the service-aged 150 kV bushings and feasibility of the technique for monitoring of dielectric properties of power transformer bushings has been assessed; the results are promising for the technique to be used in the real application. 

    Partial Discharges (PD) behavior under transients has been also studied for different materials in this project. PD behavior of different defects, at different insulation condition, responding to the overvoltage transients in form of superimposed impulses on ac voltages was investigated and it was perceived how their distinctive response and the interpretation of  that, can be useful for their identification.

    Besides the conventional materials, surface ac PD properties of modified paper with silica and zinc oxide nanoparticles under the superimposed impulses have been assessed in this project. Proper type and optimum concentration level of nanoparticles in the paper are the factors that lead to the improvement of PD behavior in the modified paper under overvoltage transients.

    Download full text (pdf)
    fulltext
  • 5.
    Nikjoo, Roya
    et al.
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Taylor, Nathaniel
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Edin, Hans
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Effect of High Voltage Impulses on Partial Discharge Characteristics of Oil-Impregnated Paper for Online Diagnostics2016Conference paper (Refereed)
    Abstract [en]

    An important signature of degradation is usually linked to the onset of Partial Discharge (PD) activity within the insulation. The change in the PD pattern can concern the insulation degradation level. High voltage transients such as lightning and switching impulses are inevitable phenomena that happen in power systems. They can impose electrical stresses on the insulation system of power components such as power transformers and their bushings which may lead to their failure. In this paper, the effect of HV impulses on the change of surface and cavity discharges of oil-impregnated paper has been investigated. With resemblance to the real situation, superimposed impulse on the AC voltage has been applied to the samples. The probability of initiating partial discharges in oil-impregnated paper due to the impulses has been studied. The PD initiation depends on the status of the insulation, the impulse magnitude and the AC phase angle at which the impulse occurs. Behavior of the partial discharges before and after the impulse on the healthy samples has been compared to the samples with aging defects such as papers with moisture content or carbonization. The possible physical phenomenon behind each behavior has been discussed. The results show the correlation of PD characteristics with the insulation degradation level and HV impulses. Recording the voltage signals with incident of high voltage transients can be used for assessing the insulation condition of power transformers and their bushings. These measurements can be done through test tap of the bushings to be used for the online diagnostics.

  • 6.
    Nikjoo, Roya
    et al.
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Taylor, Nathaniel
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Edin, Hans
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Effect of superimposed impulses on AC partial discharge characteristics of oil-impregnated paper2016In: IEEE transactions on dielectrics and electrical insulation, ISSN 1070-9878, E-ISSN 1558-4135, Vol. 23, no 6, p. 3602-3611, article id 7823414Article in journal (Refereed)
    Abstract [en]

    The effect of HV impulses on surface and cavity partial discharges (PD) of oil-impregnated paper (OIP) is investigated in this work, by subjecting specimens to impulse voltages superimposed on a continuous AC voltage. The superposition of transient and AC voltage is relevant to insulation systems in power-system equipment, which is exposed to transients such as lightning and switching impulses during its operation. For several different PD-generating defects in oil-impregnated paper, the PD behaviors before and after the impulse are compared. The influence of moisture content is studied by using OIP with moisture content <0.5%, 3.0% and 5.5%. The results show a distinctive behavior of PD due to the impulse for each defect and insulation condition, either by change in the PD rate and magnitude or by causing PD inception or extinction. Recordings of the voltage signals and associated PD measurements during and after the incidence of a high voltage transient are of potential use for condition assessment of the insulation of power transformers and their bushings, as a form of online diagnostics.

  • 7.
    Nikjoo, Roya
    et al.
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Taylor, Nathaniel
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Kiiza, Respicius Clemence
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Edin, Hans
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Dielectric Response of Oil-impregnated Paper by Utilizing Lightning and SwitchingTransients2015In: IEEE transactions on dielectrics and electrical insulation, ISSN 1070-9878, E-ISSN 1558-4135, Vol. 22, no 1, p. 335-344, article id 07033403Article in journal (Refereed)
    Abstract [en]

    On-line monitoring of insulation systems in power components such as power transformer bushings has significant value for their reliability. Dielectric spectroscopy is known as a powerful tool in condition assessment of power components' solid insulation. However, it is difficult to apply it on-line. Therefore, a new approach of intended use for on-line diagnosis of oil-impregnated paper, as a major insulation component of power transformers and bushings, is presented. In this technique, natural transients such as lightning and switching impulses are used as stimuli for on-line dielectric response measurements. The wide frequency range of these transients is their advantage as stimuli for dielectric spectroscopy. The influence of moisture content and temperature on the dielectric response of oil-impregnated paper has been investigated for performance evaluation of the proposed technique. It is shown that this technique can track the changes caused by moisture content and temperature in the dielectric response of oil-impregnated paper. The results of both low and high voltage study cases have been verified by comparison with Frequency Domain Spectroscopy (FDS). According to the well-matched results with FDS, the proposed technique can offer a valuable approach for frequent monitoring of dielectric properties of oil-impregnated paper insulated systems such as those commonly encountered in power transformer bushings. However, the validity range of the results depends on the bandwidth of the applied transients and other discussed measurement considerations.

  • 8.
    Nikjoo, Roya
    et al.
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Taylor, Nathaniel
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Edin, Hans
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Effect of High Voltage Impulses on Surface Discharge Characteristics of Polyethylene2015Conference paper (Refereed)
  • 9.
    Nikjoo, Roya
    et al.
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Taylor, Nathaniel
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Edin, Hans
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Effect of High Voltage Transients on Surface Discharge Characteristics of Oil-Paper Insulation2015In: 2015 IEEE Electrical Insulation Conference (EIC), IEEE , 2015, p. 69-72Conference paper (Refereed)
    Abstract [en]

    Condition assessment of power components such as power transformers and their bushings depends on the interpretation of data obtained from the monitoring systems. Various defects in their insulation would behave differently under electrical stresses. Surface discharge on the oil-paper interface is one of the defects which can be responsible for the failure of those components. Power system transients can impose stresses on the insulation of the power transformers and their bushings. This study investigates the effect of transients such as lightning impulses on the surface discharge characteristics of oil-impregnated paper. Two configurations, needle and rod (IEC-b), are used here. Two distinctive situations occur on paper due to these different configurations. The corresponding effect is shown in the form of variation in the partial discharge numbers and phase-resolved patterns. In the needle electrode configuration, the paper remains un-aged and the applied impulses cause a prompt change in the total number of discharges which decays fast. However, with the rod electrode, the paper gets carbonized and the applied impulses cause a decrease in the total number of discharges. The influence of moisture content in oil-impregnated paper was also studied. For this case, the total number of discharges considerably increase after applying impulse.

  • 10.
    Nikjoo, Roya
    et al.
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Taylor, Nathaniel
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Edin, Hans
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Insulation condition assessment of power transformer bushings by utilizing high voltage lightning impulses2015In: 2015 IEEE Eindhoven PowerTech, PowerTech 2015, IEEE conference proceedings, 2015Conference paper (Refereed)
    Abstract [en]

    High voltage bushing forms an essential part of a power transformer and its insulation condition is of high importance in a transformer's reliability. Transformer failure statistics indicate considerable contribution from bushing failures, which makes the monitoring of power transformer bushings essential. Several off-line and on-line methods are available for condition assessment of these components. However, most of the on-line methods cannot provide as much information as off-line methods because of the performance limitations. Therefore, condition assessments of these components are mostly done through several on-line methods and decision-making must be done according to majority agreements between different techniques. This paper investigates the possibility of assessing the insulation condition of power transformer bushing through utilizing high voltage transients. They are used as stimuli to get the dielectric response of the insulation system. The investigation is done on two service-aged 150 kV power transformer bushings. The results are verified by comparison with Frequency Domain Spectroscopy (FDS). The proposed application seems promising for monitoring of dielectric properties of power transformer bushings.

  • 11.
    Kiiza, Respicius Clemence
    et al.
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Ghaffarian Niasar, Mohamad
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Nikjoo, Roya
    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.
    The effect of PD by-products on the dielectric frequency response of oil-paper insulation comprising of a small cavity2015In: IEEE transactions on dielectrics and electrical insulation, ISSN 1070-9878, E-ISSN 1558-4135, Vol. 22, no 5, p. 2923-2930Article in journal (Other academic)
    Abstract [en]

    This work investigates the effect of alternating current partial discharge (AC PD) byproducts on the dielectric frequency response (DFR) of oil-paper insulation in the frequency range of 1.0 mHz to 1.0 kHz. The investigations were done by utilizing experimental results in combination with a Finite Element Method (FEM) model. The results presented in this paper show that AC PD by-products in the oil-paper insulation can result in a dissipation factor curve whose magnitudes increase with increasing volume of the aged zone. The change in a dissipation factor curve due to PD byproducts was observed to occur mainly in the middle and low frequency regions. The results indicate that the dissipation factor curves in the middle and low frequency regions depend on the level of PD by-products in oil and paper, respectively.

  • 12.
    Kiiza, Respicius Clemence
    et al.
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Ghaffarian Niasar, Mohamad
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Nikjoo, Roya
    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.
    Change in Partial Discharge Activity as Related to Degradation Level in Oil-Impregnated Paper Insulation: Effect of High Voltage Impulses2014In: IEEE transactions on dielectrics and electrical insulation, ISSN 1070-9878, E-ISSN 1558-4135, Vol. 21, no 3, p. 1243-1250Article in journal (Refereed)
    Abstract [en]

    High voltage (HV) transients in electrical power systems are mainly caused by lightning strikes and switching operations. The dielectric properties of a particular electrical insulation at the instant of interaction with a high voltage transient would determine a level of the degradation of an electrical insulation. This work investigates how high voltage impulses would change Phase Resolved Partial Discharge (PRPD) patterns in test samples consisting of a cavity deliberately introduced between the sheets of oil-impregnated paper used in power transformer bushings. It also investigates how the change in PRPD patterns is related to the degradation level of oil-impregnated paper. In order to accomplish these objectives, the experiments were set in such a way that the effect of HV impulses and an early stage PD activity at an elevated AC stress, HV impulses in combination with a prolonged PD activity at an elevated AC stress, and a prolonged PD activity alone at an elevated AC stress could be investigated separately. The experimental results presented in this paper indicate that HV impulses below the impulse breakdown stress following an early stage AC PD activity would not cause a significant change in PRPD patterns, and would not damage oil-impregnated paper to a level which can be noticed by a visual observation. On the other hand, a prolonged AC PD activity alone can cause the change in PRPD patterns, but cannot quickly damage the oil-impregnated paper as it would do when it is combined with HV impulses. However, the combination of both, HV impulses and a prolonged AC PD activity caused a high drop in the PD parameters (total PD charge and a repetition rate). The study found that the decrease of the PD parameters can be considered as a sign of severe degradation of oil-impregnated paper.

  • 13.
    Nikjoo, Roya
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Diagnostics of Oil-Impregnated Paper Insulation Systems by Utilizing Lightning and Switching Transients2014Licentiate thesis, comprehensive summary (Other academic)
    Abstract [en]

    Development of the power gridtowards a more reliable and smarter system requires frequent on-line monitoring of the power components. Power transformers and their bushings are particularly important components in a power transmission system and their insulation degradation may lead to catastrophic failures. Time consuming and costly replacement of these components raise the importance of their frequent monitoring. A fault in a power transformer bushing can also involve in the failure of the transformer. Therefore, on-line diagnostics of power transformers and their bushings is of great interest. Several methods exist for diagnostics of these components. However, some of them can only be done off-line in maintenance periods, and the existing on-line methods generally provide less information, especially on the internal solid insulation parts.

    In this project, a new technique for on-line diagnostics of the power transformer and the bushing insulation is proposed. In this technique, natural transients happening in the power system such as lightning and switching surges can be used as stimuli for on-line dielectric response measurements. This technique can provide information on insulation close to what Dielectric Spectroscopy offers in off-line measurements. The wide-ranging frequency content of power system transients is their advantage for being usedas stimuli when measuring the Dielectric Response. The response can have particular signatures due to different types of defects in the insulation varying with frequency.

    Oil-impregnated paper as a major insulation component in power transformer and its bushing has been investigated in this project. Moisture content and temperature, as two important degradation factors in this type of insulation, have been studied to evaluate the performance of the proposed technique in the diagnostics of the oil-impregnated paper. The results are verified with the dielectric response obtained through commercial instruments. The results show that the proposed technique has the ability to track the changes in dielectric response due to the moisture content and temperature. Measurements were done at both highvoltage (40kV) and low voltage (10V) levels, and the corresponding circuit models to achieve reasonable accuracy for the results are discussed.

    Moving on from the material samples, a further study was done on three service-aged 150 kV bushings to investigate the feasibility of the technique on the diagnostics of power transformer bushings. Their dielectric response measured by the transient stimuli showed good agreement with their response obtained by the commercial instruments. The effect of the transformer winding on the transient response of the bushing is a further aspect of the real conditions for on-line diagnostics. This has been investigated through the simulation of transient models for transformers and bushings, and possible solutions for distinguishing the responses are presented.

    The proposed new on-line diagnostics technique by utilizing natural transients can provide information about the insulation system in a certain range of frequency without interrupting the operation or requiring an external voltage source. However, the validity range of the results depends on the bandwidth of the applied transients and other measurement considerations. This approach can be valuable in frequent monitoring of dielectric properties of the power transformers and their bushings as a complement to the other available on-line techniques.

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    Thesis
  • 14.
    Nikjoo, Roya
    et al.
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Taylor, Nathaniel
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Kiiza, Respicius Clemence
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Edin, Hans
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Dielectric Response of Aged Transformer Bushings Utilizing Power System Transients2013In: 2013 4th IEEE/PES Innovative Smart Grid Technologies Europe, ISGT Europe 2013, IEEE , 2013, p. 6695398-Conference paper (Refereed)
    Abstract [en]

    The reliability of power transformer bushings is of great importance. Failures may lead to severe consequences. Frequent diagnostics by off-line methods is not feasible as the interruption is costly. Therefore it is of interest to have the possibility to perform on-line diagnostics. Transformer bushings can become aged during their operation periods. Several phenomena such as partial discharges, thermal effects and humidity may change the strength of the insulation in power transformer bushings. There are several methods which provide diagnostics of such components, but some of them can only be done offline in maintenance periods and some online methods would lack useful information about the insulation characteristics. In this paper, a new method is investigated, which uses natural transients such as switching and lightning impulses as stimuli to get the on-line dielectric response of the insulation system. Also, laboratory investigations are reported on three in-service aged 132 kV power-transformer bushings. The dielectric response measured under excitation by transient voltage is compared with results from low-voltage dielectric spectroscopy.

  • 15.
    Wang, Xiaolei
    et al.
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Kiiza, Respicius Clemence
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Ghaffarian Niasar, Mohamad
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Nikjoo, Roya
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Taylor, Nathaniel
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Edin, Hans
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Effect of Dielectric Material on Decay of Surface Charge Deposited by Corona Discharge2013In: Proceedings of the 18th International Symposium on High Voltage Engineering (ISH), 2013, p. 753-758Conference paper (Refereed)
    Abstract [en]

    This work investigates the effect of different dielectric barriers on coronadischarge in the needle-plane geometry, where a layer of dielectric material was placedover the surface of the plane electrode. Different dielectric materials used werepolytetrafluoroethylene (PTFE), polyethylene (PE), polycarbonate (PC), polyvinylchloride(PVC), epoxy and pressboard. The discharges were generated by the application ofperiodic negative step voltage pulses. The results show that the evolution of coronapulses over time depends on the dielectric properties of the barriers. Unlike pressboardwith a higher conductivity, the dielectric materials with a lower conductivity such as PTFEand PE would have more surface charges deposited on its surface during the firstcharging period of 100 ms, and the charges would not disappear during the relaxationperiod of 10 s, therefore, the discharge activities can be reduced significantly after thefirst charging period.

    Download full text (pdf)
    fulltext
  • 16.
    Kiiza, Respicius Clemence
    et al.
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Ghaffarian Niasar, Mohamad
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Nikjoo, Roya
    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.
    Effect of High Voltage Impulses on Surface Discharge at the Oil-Paper Interface2013In: 23rd Nordic Insulation Symposium (NORD-IS 13), 2013, p. 108-111Conference paper (Refereed)
    Abstract [en]

    High voltage transients in electrical power systems are mainly caused by lightning strikes and switching operations. The high voltage transients above the basic insulation level (BIL) would degrade the electrical insulation of the equipment. In this work, the effect of high voltage impulses on the trend of the surface discharge at the oil-paper interface was investigated. The partial discharge patterns were recorded by the Insulation Condition Monitoring (ICM) system. The HV impulses were generated by a standard 1.2/50 μs HV impulse generator. The measurement results presented in this paper seem to show that surface discharge would cause complete breakdown (continuous flashovers) at the oil-paper interface much faster when the test sample is first aged by HV impulses compared to when it is unaged.

  • 17.
    Nikjoo, Roya
    et al.
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Taylor, Nathaniel
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Kiiza, Respicius Clemence
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Edin, Hans
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Ghaffarian, Mohamad
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Wang, Xiaolei
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Insulation Condition Diagnostics of Oil-impregnated Paper by Utilizing Power System Transients2013Conference paper (Refereed)
    Abstract [en]

    Transients of varied spectral content, such as lightning and switching impulses, are inevitable phenomena that happen in power systems. These natural transients can be used as the stimuli for online dielectric spectroscopy for monitoring power components. The wide range of frequency in those impulses makes it possible to evaluate the dielectric response of their insulation. This paper presents a new technique for online diagnostic of oil-impregnated paper by utilizing those natural transients. Paper with different moisture levels was used to investigate the effectiveness of the proposed technique.The presence of water in the impregnated paper can be seen within those range of frequencies existed in high voltage transients. However, the validity of the results depends on the bandwidth of the applied impulse. Change in frequency response due to different moisture content in oil impregnated paper can be detected by this method. The results have been verified with results from low-voltage frequency-domain dielectric spectroscopy (FDS).

  • 18.
    Kiiza, Respicius Clemence
    et al.
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Nikjoo, Roya
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Edin, Hans
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Changes in phase resolved partial discharge patterns in a cavity embedded in paper insulation due to AC voltages andhigh voltage impulses2012In: proceedings 10th NORDAC 2012, 2012, p. 93-103Conference paper (Refereed)
    Abstract [en]

    Electrical insulation materials such as pressboards, oils and paper are important components inelectrical power equipment. The breakdown of insulation materials will cause severe failure of the equipment. Therefore, it is important to have condition monitoring methods in insulation materials of power equipment. A phase resolved partial discharge (PRPD) measurement is one of them.

    In this work, the effect of high voltage impulses on PRPD patterns was investigated in test samples consisted of a cavity deliberately introduced between oil-impregnated paper used in power transformer bushings. In the first study, the test sample was highly stressed by 50 high voltage impulses. In the second study, the test sample was aged at elevated AC voltage during almost 5 hours, but with four high voltage impulses injected at selected times during the experiment. In the third study the test sample was aged by a PD activity at elevated AC voltage up to about 18 hours. The PRPD patterns were periodically recorded by an Insulation Condition Monitoring (ICM) system. In order to investigate how different electrical stresses affect the dielectric properties of the paper insulation, the fourth study about the dielectric spectroscopy (DS) measurements were performed. The results show that high voltage impulses can have a large impact on the behavior of PRPD patterns if the test sample is aged by partial discharges, but a smaller effect when the test sample is unaged.

  • 19.
    Kiiza, Respicius Clemence
    et al.
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Ghaffarian Niasar, Mohamad
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Nikjoo, Roya
    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.
    Ahmed, Z.
    ABB.
    Comparison of phase resolved partial discharge patterns in small test samples, bushing specimen and aged transformer bushing2012In: 2012 Annual Report Conference on Electrical Insulation and Dielectric Phenomena (CEIDP), IEEE , 2012, p. 88-91Conference paper (Refereed)
    Abstract [en]

    The transformer bushings are one of the key devices in electrical power networks. With time, the electrical insulations in the transformer bushings become aged. One effect of a long time ageing would be due to Partial Discharge (PD) defects. In this work, the phase resolved partial discharge (PRPD) pattern of the defects naturally present in an aged 36 kV Oil Impregnated Paper (OIP) bushing was compared with the PRPD patterns of defects deliberately introduced in simple test samples and in a bushing specimen. The results presented in this paper show that it might be possible to predict the type of the defects in aged transformer bushings through comparing measured PRPD patterns with PRPD patterns from defects deliberately introduced in simple test samples and in bushing specimens.

  • 20.
    Nikjoo, Roya
    et al.
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Taylor, Nathaniel
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Ghaffarian Niasar, Mohamad
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Edin, Hans
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Dielectric response measurement of power transformer bushing by utilizing high voltage transients2012In: 2012 Annual Report Conference on Electrical Insulation and Dielectric Phenomena (CEIDP), IEEE , 2012, p. 503-506Conference paper (Refereed)
    Abstract [en]

    Insulation has a significant role in most of power components failures. Therefore, proper diagnostics of insulation condition which can be done non-destructively and online in the field is of interest. In this paper, a new insulation diagnostic technique is proposed which utilizes natural transients in power system for online dielectric response measurement. In the proposed technique, lightning and switching impulses with wide range of frequency are used as stimuli for dielectric spectroscopy. Oil impregnated papers and condenser bushing are the objects under investigation and dielectric response of them is derived at those transients. Dielectric responses obtained by this method are verified by the responses measured by insulation diagnostic systems IDAX and LCR meter.

  • 21.
    Kiiza, Respicius Clemence
    et al.
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Nikjoo, Roya
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Ghaffarian Niasar, Mohamad
    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.
    Effect of high voltage impulses on partial discharge activity in a cavity embedded in paper insulation2012In: 2012 Annual Report Conference on Electrical Insulation and Dielectric Phenomena (CEIDP), IEEE , 2012, p. 133-136Conference paper (Refereed)
    Abstract [en]

    High voltage impulses in electrical power systems are mainly caused by lightning strikes and switching operations. The dielectric properties of a particular insulation at the instant of interaction with a high voltage impulse would determine the level of the degradation of the electrical insulation caused by the high voltage impulse. In this work, the effect of high voltage impulses on the partial discharge (PD) parameters was investigated. The investigation was done in test samples consisting of a cavity deliberately introduced between oil-impregnated papers used in power transformer bushings. The measurement results presented in this paper show that high voltage impulses can have a large impact on the behavior of the PD parameters if the test sample is aged by partial discharges, but a smaller impact when the test sample is unaged.

  • 22.
    Ghaffarian Niasar, Mohamad
    et al.
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Kiiza, Respicius Clemence
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Wang, Xiaolei
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Nikjoo, Roya
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Edin, Hans
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Effect of temperature on surface discharge in oil2012In: 2012 Annual Report Conference on Electrical Insulation and Dielectric Phenomena (CEIDP), IEEE , 2012, p. 96-99Conference paper (Refereed)
    Abstract [en]

    In this paper surface discharge along the oil-pressboard interface was investigated. Surface discharges were measured both in time domain by means of an oscilloscope and as phase resolve patterns. The effect of temperature from 20°C to 90°C on surface discharge inception, extinction and surface flashover voltage were investigated. Trend of number and maximum magnitude of PD was compared for 20°C and 90°C over time. The effect of voltage rise and surface flashover on number and maximum magnitude of PD were investigated. The results show that PD inception voltage is lower for higher temperature but surface flashover voltage is higher for higher temperature. However for 90°C the final damage due to surface discharge is much severe compared to 20°C.

  • 23.
    Nikjoo, Roya
    et al.
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Edin, Hans
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Effect of transformer winding on the transient response of power transformer bushings2012In: Lightning Protection (ICLP), 2012 International Conference on, IEEE , 2012, p. 6344346-Conference paper (Refereed)
    Abstract [en]

    This paper deals with the response of power transformer bushings under the effect of transformer windings during electromagnetic transients such as lightning. In transient domain, investigating the characteristic of each power component separately in the network wouldn't be adequate. Each component can make an influence on the transient current and voltage that may be measured from another one. This paper proposes to find the effect of transformer windings on the transient response of the bushing, which contains valuable information on bushing's condition. It may result helpful in diagnostics of such an important power component to avoid catastrophic failure. Natural transients would be helpful in monitoring of the bushing by utilizing them as stimuli for online dielectric response measurement. Transient response of the lumped model of the bushing in EMTP is compared with its finite element model (FEM) in COMSOL Multiphysics to validate the robustness of modeling approach. According to the frequency component of the lightning, transient electromagnetic wave module of COMSOL is applied. Then, the effect of the transformer winding on the transient response of the bushing is investigated in EMTP.

  • 24.
    Nikjoo, Roya
    et al.
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Taylor, Nathaniel
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Edin, Hans
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Dielectric Response Measurement with Impulse Stimulus on AC: Measurement Considerations, and Laboratory Testing on a BushingManuscript (preprint) (Other academic)
  • 25.
    Nikjoo, Roya
    et al.
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Taylor, Nathaniel
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Edin, Hans
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Effect of Superimposed Impulses on AC Partial Discharge Characteristics of Oil-Impregnated PaperManuscript (preprint) (Other academic)
  • 26.
    Kiiza, Respicius Clemence
    et al.
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Ghaffarian Niasar, Mohamad
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Nikjoo, Roya
    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.
    Partial discharge patterns in a cavity embedded in oil-impregnated papers: Part 1: Effect ofhigh voltage impulsesArticle in journal (Other academic)
  • 27.
    Nikjoo, Roya
    et al.
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Hollertz, Rebecca
    KTH, School of Chemical Science and Engineering (CHE), Fibre and Polymer Technology.
    Wåhlander, Martin
    KTH, School of Chemical Science and Engineering (CHE), Fibre and Polymer Technology.
    Taylor, Nathaniel
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Edin, Hans
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Wågberg, Lars
    KTH, School of Chemical Science and Engineering (CHE), Fibre and Polymer Technology.
    Surface Discharge Characteristics of Oil-impregnated Paper with SiO2 and ZnO nanoparticles under AC with Superimposed ImpulseManuscript (preprint) (Other academic)
  • 28.
    Kiiza, Respicius Clemence
    et al.
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Ghaffarian Niasar, Mohamad
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Nikjoo, Roya
    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.
    The effect of PD by-products on the dielectric frequency response of oil impregnated paper insulation comprising of a small cavityManuscript (preprint) (Other academic)
1 - 28 of 28
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