Mehdi Bagheri

Advanced transformer winding deformation diagnosis: moving from off-line to on-line

On-line monitoring and diagnosis of transformers have been investigated and discussed significantly in last decade. This study has concentrated on issues arising while on-line transformer winding deformation diagnosis is going to be applied on transformers with various kinds of techniques. From technical perspective, before replacing off-line methods by on-line methods and eventually by intelligent approaches, practical challenges must be addressed and overcome. Hence, available off-line transformer winding deformation diagnosis methods are discussed precisely. Mathematical calculation in on-line short circuit impedance measurement is investigated. On-line transformer transfer function measurement setup is presented. A profound insight to the problems pertaining on-line transformer winding deformation recognition methods, characterizes existing online methods, explains the concepts behind online measurements and striving to open the discussion doors towards challenges are discussed. In the end a 400 MVA step up transformer has been taken as a case in order to clarify the capability of Frequency Response Analysis (FRA) method in fault detection while short circuit impedance could only demonstrate some rough understanding about transformer condition.

Frequency response analysis and short-circuit impedance measurement in detection of winding deformation within power transformers

Power transformers are in service under different environmental, electrical, and mechanical conditions [1] and may be subject to enormous hazards during the course of operation [2], [3]. They are commonly considered to be the heart of the transmission and distribution sectors of electric power systems; monitoring their condition and diagnosing faults are important parts of the maintenance function [4]. Utility engineers strive to keep power transformers in service and to prevent even shortterm outages. Failure of a transformer can cause extensive damage to equipment owned by consumers or the utility [5].

Influence of temperature and moisture content on frequency response analysis of transformer winding

Frequency Response Analysis (FRA) has been in use since the last decade as a sensitive method for detecting transformer internal defects. This study is focused on the influence of temperature and moisture migration on the FRA response of transformer winding. It also discusses the feasibility of FRA capability in moisture diffusion recognition in transformer paper insulation. To conduct this study, a single phase model transformer involving concentric LV and HV windings and a 20/0.4 kV, 1.6 MVA three-phase two windings transformer are taken as test objects. Experiments are carried out at different temperature and moisture conditions. FRA spectra are then recorded and analyzed. FRA spectra deviations as well as total capacitance variations due to the temperature and moisture changes in the test objects are calculated. Karl-Fischer Titration (KFT) is utilized to monitor the moisture migration within oil and paper insulations. Furthermore, a mathematical model is used to simulate one of the test object windings and verify the experimental result, and also clarify the main reason of FRA spectrum deviation in this circumstance. Finally, statistical indices in FRA evaluation are calculated to explore their capability in FRA spectrum interpretation once the moisture content of paper insulation is changed.

FRA vs. short circuit impedance measurement in detection of mechanical defects within large power transformer

Power transformers are supposed to be and remain in service in various environmental circumstances under different electrical and mechanical stresses. Base on failure history in power transformers obtained from four corners of the globe one of the major problems in transformers is mechanical defect. A number of monitoring and diagnostic methods have been introduced to recognize transformer active part displacement and winding deformation. Frequency response analyses and short circuit impedance measurement have been employed as two common diagnosis methods in large power transformer winding deformation recognition. On the other hand, researchers are expressing an increased concern about power transformer condition monitoring in the smart grid context. Hence, all of off-line methods need to move towards on-line applications. One of the challenges is finding reasonably accurate method which can provide sufficient information about transformer winding condition. In this study, mechanical defects of windings and their causes are investigated in detail. Frequency response analyses and short circuit impedance measurement as two popular methods in transformer winding deformation diagnosis will be employed to get insight into transformer active part condition. A large power transformer has been taken as a case in order to put the capability and sensitivity of abovementioned methods into test. Onsite test results on this giant transformer winding show that frequency response analyses method is capable to provide far more information as to the healthy or defected condition and physical movements of the transformers windings and core compared to the other method.

Transformer frequency response analysis: mathematical and practical approach to interpret mid-frequency oscillations

Frequency Response Analysis (FRA) has been utilized as an off-line diagnosis test since last decade to investigate mechanical integrity of transformer. FRA data typically reports as a spectrum in Bode diagram over determined frequency band. To evaluate FRA data, determined frequency band may be conveniently divided into three bands, namely low-, medium- and high- frequency bands. It is well-known low-frequency band dominated by core, mid-frequency region dominated by winding structure and high-frequency band influenced by measurement connection leads. This study has concentrated on midfrequency oscillations of transformer winding frequency response spectrum presented in Bode diagram. Mathematical approach using travelling wave theory is employed to explore frequency response trace behavior. Practical studies on a prepared glassy transformer as well as two 66 kV, 25 MVA continuous and interleaved disc windings have been performed to validate mathematical calculation. In addition, two 245 kV, 45 MVA and 66 MVA power transformers have been examined to study mid-frequency oscillations and compare the result with mathematical evaluation.

Practical challenges in online transformer winding deformation diagnostics

Monitoring and diagnosis of transformers have been investigated and discussed for many years. While transformer routine tests indicate electrical and also insulation weakness or defects, Frequency Response Analysis (FRA) as a modern test can indicate mechanical defects in transformer active part. This paper introduces the concepts behind online measurements in frequency domain and is striving to open the discussion doors towards advanced investigations which are all aligned with smart grid concept. This study has also concentrated partly on issues rising while online transfer function measurement techniques are going to be applied on transformers with various kinds of winding connections and accessories. In the end, crucial issues from theoretical and also practical perspectives for online transfer function implementation are discussed.

Impulse voltage distribution in intershield disk winding VS interleaved and continuous disk winding in power transformer

Strike of a lightning stroke to the high voltage terminal of a power transformer results in an impulse stress on its windings. The magnitude of the series capacitance of the windings has an important effect on the impulse voltage distribution and the stresses along the windings. Increasing the series capacitances of the windings, the impulse voltage distribution becomes more linear and the stress on the winding is reduced. So interleaved windings rather than disk windings are used in power transformer windings to increase the series capacitances. In this paper, the electrostatic shielding, used in disk windings to increase the windings series capacitance, and for linearization of its impulse voltage distribution is introduced and its impact on the modification of impulse voltage distribution is compared with the disk winding without shielding.

Influence of Electrostatic Shielding of Disc Winding on Increasing the Series Capacitance in Transformer

Strike of a lightning stroke to the high voltage terminal of a power transformer results in an impulse stress on its windings. The magnitude of the series capacitance of the windings has an important effect on the impulse voltage distribution and the stresses along the windings. Increasing the series capacitances of the windings, the impulse voltage distribution becomes more linear and the stress on the winding is reduced. So interleaved windings rather than disc windings are used in power transformer windings to increase the series capacitances. In this paper, the electrostatic shielding, used in disc windings to increase the windings series capacitance, and for linearization of its impulse voltage distribution is introduced and its impact on the modification of impulse voltage distribution is compared with the disc winding without shielding.

A study of enhanced performance of VUV/UV process for the degradation of micropollutants from contaminated water

VUV/UV is a chemical-free and straightforward solution for the degradation of emerging contaminants from water sources. The objective of this work was to investigate the feasibility of VUV/UV advanced oxidation process for the effective degradation of a target micropollutant, atrazine, under continuous flow operation of 0.5-6.5L/min. To provide an in-depth understanding of process, a comprehensive computational fluid dynamics (CFD) model, incorporating flow hydrodynamics, 185nm VUV and 254nm UV radiation propagation along with a complete kinetic scheme, was developed and validated experimentally. The experimental degradation rates and CFD predicted values showed great consistency with less than 2.9% average absolute relative deviation (AARD). Utilizing the verified model, energy-efficiency of the VUV/UV process under a wide range of reactor configurations was assessed in terms of electrical energy-per-order (EEO), OH concentration as well as delivered UV and VUV dose distributions. Thereby, the extent of mixing and circulation zones was found as key parameter controlling the treatment economy and energy-efficiency of the VUV/UV process. Utilizing a CFD-driven baffle design strategy, an improved VUV/UV process with up to 72% reduction in the total electrical energy requirement of atrazine degradation was introduced and verified experimentally.

Rank-based ant system method for non-linear QSPR analysis: QSPR studies of the solubility parameter

The solubility parameter (δ) plays a unique role in the development of stable pharmaceutical formulations for assessing phase segregation during product synthesis. Understanding this parameter helps to determine how a drug substance will behave when processed or when dosed in vivo. The aim of this work was to develop a novel comprehensive yet rapid and accurate Quantitative Structure–Property Relationship (QSPR) method based on the rank-based ant system feature selection. The method was coupled with the multiple linear regression and support vector regression and applied to the assessment of solubility parameters for a diverse dataset of 1804 chemical compounds. The models were validated by solubility prediction of 360 test set compounds which were not used in building models. The developed models have high prediction power characterized by r 2 values 0.75 and 0.82, and RMSE values 1.96 and 1.65 (J/(cm3))0.5 for the external test set. Various validation techniques and comparison results with the novel optimized support vector regression indicate that the developed models can be used to determine the solubility parameters for a diverse set of chemicals with an acceptable accuracy. The developed models can be beneficial for designing new chemical materials with desired solubility parameter values.