Showing papers by "Trevor Blackburn published in 2012"

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

Abstract: 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.

Partial discharge localization in transformers using UHF detection method

Abstract: The location of a partial discharge (PD) source inside a transformer can be determined from the time differences of arrival (TDOA) between signals that are captured by an array of UHF sensors. The TDOA can be acquired from the received PD waveforms. In this paper, three different methods of acquiring the TDOA from the PD waveforms are discussed. The time difference can be calculated either by taking the first peak of the signal as the arrival instant, or from the cross-correlation of the PD waveforms, or by applying the similarity function to the plots of the PD signals cumulative energy. Computation algorithms for determining the TDOA automatically are introduced so that possible bias from human interpretation is avoided. The presence of noise and its effect on the accuracy of the PD localization will also be presented. Experimental results show the first-peak method has higher accuracy than the two other methods. The application of signal denoising further improves the localization accuracy.

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

Abstract: 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 transformer's windings and core compared to the other method.

Effects of current and voltage harmonics on distribution transformer losses

Abstract: The drive to improve energy efficiency and reduce electrical loading has resulted in energy efficient lighting such as compact fluorescent lamps (CFLs) replacing conventional incandescent lamps However, the presence of such non-linear loads has brought about the injection of voltage and current harmonics into electrical networks As transformers are the interface between the supply and the non-linear loads, the investigation of their effects on transformer losses is of great importance These harmonics can cause excessive loss and abnormal temperature rise in the transformers, thus reducing their operational life span This paper investigates the impact of current and voltage harmonics of the loads on a single-phase 25kVA distribution transformer Harmonic spectra of a range of non-linear loads including CFL, LED tube, PC and fluorescent lamp are obtained A single-phase inverter is used for harmonic generation to simulate power supply harmonics injected into the transformer Open circuit and short circuit tests are conducted on the transformer under the effect of harmonics, and the impacts on core loss are analyzed

Bushing characteristic impacts on on-line Frequency Response Analysis of transformer winding

Abstract: Diagnosis of electrical equipment, particularly n and power transformers has been quite important for quite a long time. Frequency Response Analysis (FRA) has been introduced years ago and has been employed as an off-line diagnosis test since last decade to investigate transformer mechanical integrity. As smart high voltage monitoring concept is under developing, on-line transformer winding deformation diagnosis has been investigated and discussed significantly over the last years. Hence, on-line diagnosis method implementations have become important accordingly. This study has concentrated on issues arising while on-line FRA is going to be employed on transformers. Existing off-line and on-line transformer winding deformation diagnosis methods are discussed and an on-line frequency response analysis measurement setup is proposed. A profound insight to the challenges associated with on-line FRA measurement has been provided. Influence of on-line setup on frequency response trace sensitivity, accuracy and interpretation has been highlighted. In the end, practical study has been performed on a 66 kV interleaved winding connected various types of capacitive bushings to explore the boundary condition for on-line FRA setup design and to address fundamental challenges.

Frequency response analysis vs. flux division measurement in detection of transformer winding internal short circuit

Abstract: Diagnosis of electrical equipment, particularly distribution and power transformers, which are considered as the heart of the electric power grid, has been quite important for quite a long time. Transformer diagnosis methods have been introduced and employed to recognize transformer internal defects since many years ago. Turn to turn and disc to disc short circuit in transformer windings are considered as one of the main problems in transformer active part. Flux Division Measurement (FDM) technique is employed specifically to realize internal short circuit in transformer windings. This test is performed as an on-site, non-destructive, fast and simple method to detect turn to turn or disc to disc short circuits in transformer windings. In this study, a 360 MVA, 230 kV/20 kV five-limb step up transformer has been taken as a case in order to clarify the capability of Frequency Response Analysis (FRA) in internal short circuit detection of transformer. Also, FRA tests were performed to elaborate that it is able to give further detailed information as well. Interpretation of test results on this giant transformer winding shows that FRA method is capable to provide significant information as to the healthy or defected condition of the windings.

Transformer efficiency and de-rating evaluation with non-sinusoidal loads

Abstract: Energy efficiency with all its various aspects has been taken into consideration as the resources are diminishing at various rates. Improving the equipment efficiency is one of the major steps to get closer to ideal efficient use of energy in electric power systems. In this regard, transformer efficiency in distribution networks has been crucially studied over past decade. Advantages and disadvantages of various types of lamps including incandescent, fluorescent, Compact Fluorescent Lamps (CFL) and Light Emitted Diode (LED) from energy point of view have been debated quite effectively in the literature. Since the power utilities and their customers are showing an increased concern about the energy efficiency and power quality aspects, there are some mixed feelings towards recently introduced lamps. In this regard, harmonic distortion level initiated by extensive use of harmonic-generating lamps is considered. Vital electric characteristics of four lamp types will be explored. Then, transformer de-rating under non-sinusoidal condition from standard point of view is investigated. By using power quality analyzer, current harmonic levels generated by the abovementioned lamps are measured. Calculation and simulation of transformer loading while using each light as transformer load is performed, and finally effects of lamps on transformer de-rating is compared.

Dean-Stark vs FDS and KFT methods in moisture content recognition of transformers

Abstract: Transformer is one of the crucial equipments in electric power network. Moisture in paper insulation acts as a catalyst in transformer insulation aging. Experience has shown that humidity will affect electrical strength of paper and pressboard as well as other insulation system materials by reducing degree of polymerization and accelerated aging. Thus, it is essential to determine the extent of moisture content within insulation system before exciting the transformer and also during normal operation. In this regard, chemical tests on oil have been carried out for many years. Also, electrical tests for assessing the transformer condition have been developed and improved in recent years while most introduced methods aim to extract moisture from transformer insulation system. In this study, the results of practically applicable methods in oil sampling (KFT and capacitance sensors), paper sampling (Dean-Stark method) and also dielectric response (RVM, PDC, FDS) are examined in determining insulation system humidity. Test results through various methods are compared to analyze the sensitivity of different approaches. Crucial practical recommendations as well as advantages and disadvantages of each method are discussed. At last, to recommend an accurate and reliable method for practical implementations, practical tests have been performed simultaneously on three distribution transformers and the results of them are discussed.

A case study on FRA capability in detection of mechanical defects within a 400MVA transformer

Abstract: SUMMARY Diagnosis of electrical equipment, particularly distribution and power transformers, which are the heart of generation, transmission and distribution of electrical energy, has been an important issue for a long time. In this study, a 400MVA step-up transformer has been taken as a case study in order to clarify the capability of the Frequency Response Analysis (FRA) method for off-line mechanical fault detection. Detecting mechanical defects of windings is investigated in detail and other diagnostics are performed for a comparison of their efficacy in detection. Mechanical defects can occur due to a variety of disturbances such as short circuit currents, severe explosion of combustible gases, earthquakes, or even unsuitable transportation methods or accidents. Other tests performed were noload loss and no-load current, insulation resistance, dielectric dissipation factor, short circuit impedance, DC resistance, water content analysis using KFT and frequency domain spectroscopy were conducted as diagnostic methods to determine their capabilities to provide information about transformer winding insulation and physical condition. The FRA tests were performed to determine whether or not it is able to give further detailed information on faults. Measurements of transfer function frequency response of the transformer windings were conducted on the HV windings individually. The test results on these very large transformer windings show that the FRA method is capable of providing further information, compared to other more traditional methods, as to the healthy or defective condition and any physical movement of the transformer windings and core.

Frequency Response Analysis to recognize inductance variation in transformer due to internal short circuit

Abstract: Transformer diagnosis methods have been introduced and employed to recognize transformer internal defects since many years ago In terms of mechanical defects in transformers, turn to turn and disc to disc short circuits in transformer windings are considered as one of the main problems in transformer active part On the other hand, Frequency Response Analysis (FRA) is considered as a robust, sensitive and high accurate method to identify transformer mechanical defects This study has concentrated on self-inductance variation recognition due to an internal short circuit in transformer using FRA signature A single phase 11/025 kV, 25 kVA transformer has been taken as test object Frequency response of the test object was recorded for open circuit and short circuit situations Frequency response alteration in low frequencies caused by short circuit is interpreted through mathematical calculations Frequency response trace deviation is employed to recognize self-inductance changes due to the internal short circuit

Application of common transformers faults diagnosis methods on biodegradable oil-filled transformers

Abstract: Due to environmental concerns regarding the use of mineral oil, biodegradable oil is increasingly being used as an alternative dielectric fluid. This paper presents results of experiments performed in the laboratory on the use of biodegradable oils for transformer application. The investigations cover two important diagnostic techniques for insulation assessment: (i) an investigation of partial discharge (PD) activity and characteristics in such oils and (ii) an investigation of the standard hydrocarbon dissolved gas products produced due to transformer faults. The fundamental aim of the investigation was to provide information as to whether the existing analysis techniques of PD fault pattern recognition and dissolved gas analysis methods, developed for mineral oil, are valid when used for insulation assessment with biodegradable transformer oil. The experiments were conducted on test samples to simulate three common types of transformer faults: low-energy PDs, high-energy arcing/sparking, and overheating. For the purpose of comparison, the same tests were performed using a mineral oil (Shell Diala MX) and a biodegradable oil (Envirotemp FR3). Also, each oil type was tested at three different moisture levels and with or without the presence of solid insulating materials (pressboard). PD activity was monitored using the standard IEC60270 phase-resolved analysis method. The fault gases produced were extracted and analysed by standard gas chromatography methods. Test results indicate that the PD phase resolved patterns are, in general, similar for the two oil types and thus existing PD pattern interpretations can be used to distinguish different types of PD faults, e.g. corona versus surface discharges. However, the values of various discharge quantities (PD magnitudes, repetition rate, current, etc) are very different under the same test condition. The quantities and the trend of dissolved gases for faults in biodegradable oil are substantially different compared with mineral oil. For the PD fault, biodegradable oil was found to release only a limited number of gases. For arcing and overheating faults, the main key gas produced is different between the two oils. Also, the presence of cellulosic materials such as pressboard influences the extent of hydrocarbon gases dissolved in the oil. It was found that for biodegradable oil, fewer amounts of gases are produced as the oil gets wetter.

Transformer Frequency Response Analysis: A mathematical approach to interpret mid-frequency oscillations

Abstract: Diagnosis methods have been employed to recognize transformer internal defects since many years ago. Frequency Response Analysis (FRA) has been utilized as an offline diagnosis test since last decade to investigate mechanical integrity of transformer while this technique is quite capable to provide worthy information about electromagnetic characteristics of the windings. This study has concentrated on transformer winding frequency response trace oscillations presented in Bode diagram. Mathematical approach using travelling wave theory is employed to explore frequency response trace behavior. The correlation between winding behavior in frequency domain and time domain is highlighted. Eventually, practical study has been performed on continuous and interleaved 66 kV disc windings to validate mathematical calculation.

UHF sensor array for partial discharge location in transformers

Abstract: The application of the ultra-high frequency (UHF) partial discharge (PD) detection method has proven effective in GIS. This method is now also being applied to detect PDs in oil-filled power transformers. In this paper, the localization of the PD source in transformer is discussed. This is achieved by applying an array of four sensors to capture the PD signals at the same time. To determine the PD location, two methods (first peak and cross-correlation) are used to determine the difference in time of arrival between sensor signals and followed by numerical triangulation. The sensor positions and array size influence the PD waveform patterns captured, and thus the accuracy of the PD localization. Experimental results and analysis indicate the first peak method yields better location accuracy than the cross correlation method. However, the traditional external acoustic detection method performs better than the UHF detection method under the same test configurations.

Partial discharge localization in transformers using monopole and log-spiral UHF sensors

Abstract: The location of a partial discharge (PD) source inside a transformer can be determined from the time differences of arrival (TDOA) between signals that are captured by an array of UHF sensors. From these, the PD location can be found by geometric triangulation which involves solving a set of non-linear equations. This can be achieved using an efficient software realization of the maximum-likelihood estimator. The recorded PD waveforms are affected by the type of sensor used to capture PD signals. In this paper, the accuracy of the PD localization using different sensors is investigated. Two types of sensor, i.e. short monopole and log-spiral are used to capture electromagnetic waves emitted from a PD source. To calculate the TDOA, two methods: first peak and cross-correlation were applied. The localization result shows the monopole sensor produces higher accuracy than the log-spiral and the first peak method achieves better result than the cross-correlation method.

Dielectric response of transformer insulation oils

Abstract: Dielectric diagnosis with polarization and depolarization currents (PDC) which employs the dielectric response of system in time domain is applied to investigate differences between PDC patterns produced by two types of transformer insulating fluids, mineral and bio-degradable oil The mineral oil used is Shell Diala BX, and the biodegradable one is Envirotemp FR3 The latter is ester-based, and a relatively new transformer insulating fluid introduced as an environmentally-friendly alternative to conventional mineral oil The DC conductivity of the two oil types is obtained under the effects of various temperature and moisture levels The experiment is conducted for two levels of moisture when temperature factor is changed from ambient to 90 °C The oil water content measurement is conducted using Karl-FischerTitration Also, an attempt was made to investigate the effects that different testing voltages and time durations have on the polarization and depolarization spectrum A range of DC voltages up to 1000V, and time durations from 1000 to 10000 seconds have been applied Finally, all the PDC patterns obtained under the effect of mentioned factors are evaluated

Measured PD pulses in vegetable-oil-impregnated insulation system

Abstract: Results of PD tests in vegetable oil impregnated insulating system are discussed in this paper. The main objective of the study is to investigate the PD behaviour in the particular insulating systems at elevated temperature. Two different sample arrangements of electrical pressboard and insulating paper were used to simulate insulation defects. The sample was placed in a glass cell and was fully immersed in vegetable oil. The initiated PDs were detected and analysed using a wide band PD detector. The PD waveforms were picked up by means of a HFCT sensor and displayed using a digital oscilloscope. Investigation results show that there are some differences in intrinsic characteristics of PD pulses measured from the two different sample configurations.