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Electrical Insulation Conference (EIC), 2011

Date 5-8 June 2011

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Displaying Results 1 - 25 of 112
  • 2011 Golden Omega Award

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  • 30th Electrical insulation conference

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  • To: Attendees of the 2011 EIC

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  • To: Attendees of the 2011 EIC

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  • Electrical insulation conference: Past general conference chairs

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  • Table of contents

    Page(s): 1 - 13
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  • Condition assessment of cables and transformers in an aging infrastructure (and aging talent base)

    Page(s): 1 - 9
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (1283 KB) |  | HTML iconHTML  

    The rapid industrialization that followed World War 11 required electricity to power and sustain this ensuing growth. As a result, infrastructure for electric power was put in place at an accelerating pace in subsequent decades, resulting in aging equipment. The challenges presented in taking better care of aging infrastructure include technical, lack of commensurate efforts and availability of sufficient financial resources. The gradual erosion of expertise and long general lack of hands-on electrical power equipment education in the US does not help either nor does the significantly reduced manufacturing power equipment base, as we address the aging equipment. The West Europeans and Japanese are in a better position toward this end, due to relatively robust electrical power equipment manufacturing, exporting strengths and active equipment - related graduate education. The prevailing utility business climate brought about by deregulation, competition and increasing fiscal restraints dictate that maximum and trouble-free use is made of aging infrastructure. This places focus on reliable and cost-effective diagnostic tests. While such tests have been always addressed and improved over decades, the need has never been greater, as of now. This paper covers the condition and life assessment of laminar dielectric transmission cables, power transformers and extruded cable terminations containing a dielectric fluid through Dissolved Gas Analysis (DGA). Several successful cases showing the extent of insulation damage from minor to severe as revealed by DGA-prompted equipment openings are presented. All the cable systems were re-built. This demonstrates that the techniques and lessons learned from one product can be profitably applied to another, despite lack of markedly different dielectric materials, designs and operating conditions. The general lack of hands-on equipment-related graduate education has been addressed, and ameliorative measures emphasized to aid in taking b- - etter care of aging equipment. View full abstract»

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  • Stator end winding vibration and temperature rise monitoring

    Page(s): 10 - 14
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (561 KB) |  | HTML iconHTML  

    In most turbine generators, excessive vibration and thermal cycling weaken the insulation on the windings causing stator bar to fracture and leading to partial discharge (PD), which ultimately results in fire. Therefore, PD becomes extremely dangerous for large hydrogen cooled turbines because it might lead to explosion in the plant. These risks can be mitigated by using VibroFibre™ fiber optic vibration sensors and the Integrated Signal Conditioning (ISC) system as condition monitoring solution. This solution prevents unplanned down time by obtaining a warning against arcing and fire resulting from the measurement and trending of vibration amplitudes in the stator end windings. The presence of an alarm stops the machine for repair before a disaster occurs, resulting in a life extension of critical power plant assets by at least 200%. Inside power generators, there are high voltages and strong electromagnetic fields, and such hostile environment makes the use of ordinary piezoelectric vibration sensors impossible. In this case, fiber optic vibration sensors are the most effective solution. Yet, until now, only single channel fiber optic sensors have been available. They are hard to manage and require one full week of down time to install. Using a bundle of fiber optic vibration sensors and an ISC unit, the architecture can be simplified. Moreover, one-day installation can be achieved by gluing the sensors onto the end cap with high temperature epoxy. This results in instantaneous return on investment. View full abstract»

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  • Characteristics of noise and interphasal PD pulses in operating stator windings

    Page(s): 15 - 19
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    Online partial discharge measurement in machines requires effective electrical noise suppression to reduce the risk of false indications of stator winding insulation problems. Many noise suppression techniques have been proposed and implemented over the years. This paper examines the method of suppressing noise using 3 phase synchronous (i.e. simultaneous) detection. Known noise pulses were recorded on three phases and they were contrasted with pulses from phase-to-ground PD in the stator slot or phase-to-phase PD between coils in the stator endwinding. Experiments indicated that interphasal PD produces strong pulses on 2 of the 3 phases. In contrast, phase-to-ground PD and noise caused by sparking in one phase of a generator output bus create a dominant pulse in one phase only. With all three types of situations, there may be significant cross-coupling to the other phase(s). View full abstract»

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  • Update on partial discharge screening tests for stator bars & coils

    Page(s): 20 - 22
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (422 KB) |  | HTML iconHTML  

    In 1992 a paper examined evidence of a relationship between measured partial discharge and subsequent voltage endurance of certain stator bars. The PD test method utilizes a higher frequency range than the conventional 30 kHz to 400 kHz and may be more appropriate for the detection of internal voids. More recently there have been other instances where these PD measurements have been useful for identifying stator bars or coils which may have short times to failure on voltage endurance. This paper updates the evidence. View full abstract»

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  • Developments relating to litz wire in the insulation systems of generators for renewable energies

    Page(s): 27 - 31
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (1597 KB) |  | HTML iconHTML  

    As new designs for generators applied in the renewable energy market appear and rated power continues to increase, litz-wire winding technology has brought its advantages to machine and process efficiency. The division into many strands increases flexibility, facilitating the processing of larger and special windings, and reduces high-frequency winding resistance, reducing losses. The use of litz wire, however, impacts the insulation system. The litz-wire profile locally enhances electric field strength, with implications for the mainwall and turn insulations. Conductor mica tape can provide appropriate turn insulation, but influences wire flexibility, coil consolidation, and finally resin penetration during impregnation. During vacuum-pressure impregnation, resin should penetrate both the region between turns and the litz wire itself, without draining during curing. This can impose new requirements on the impregnation process and on the choice of resin, which must deal with low pressure and lack of catalyst inside the litz wire. The paper reports on considerations and investigations of such aspects in real generator designs. View full abstract»

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  • Impact of rotor bar bridges in torque development in integral horsepower machines in PWM inverter environments

    Page(s): 32 - 35
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (1934 KB) |  | HTML iconHTML  

    The impact of rotor bar bridges in closed-slot induction motor rotors has been simulated using Finite Element Analysis (FEA) and single phase equivalent circuit methods in 60 Hz sinusoidal environments. In this paper we will discuss a study on a 455HP, 90Hz, 380 Vac machine utilizing a rotor with a deep bridge, then the impact on output torque once the diameter of the rotor was turned down, reducing the bridge depth. The research performed allowed for optimizing optimal bridge depth and air gap sizing based upon machine design for peak torque development. View full abstract»

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  • A method for the measurement of leakage current due to surface discharge at the oil-pressboard interface

    Page(s): 41 - 44
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (438 KB) |  | HTML iconHTML  

    Surface tracking is the conducting path formed on the surface of an insulation material as a result of surface currents. Tracking has commonly been associated with solid insulation in air such as overhead line insulators but is now recognized to occur at the oil-pressboard insulation interface inside large transformers. The surface current is effectively the drift of charge under the action of an electric field which may be either the general electric field or a locally enhanced field developed around a space charge. A needle-bar configuration has been developed to promote surface tracking under controlled conditions at the oil-pressboard interface without surface flash-over or breakdown. This method is used to study the leakage current associated with surface discharge applied from a high voltage source on the oil-pressboard interface. This paper outlines the experimental set-up and presents results which compare the measured current against the corresponding partial discharges. Subsequently a mechanism describing the process by which leakage current is formed is proposed. View full abstract»

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  • Design of the insulating supports in medium voltage dry-type transformers

    Page(s): 45 - 48
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (822 KB) |  | HTML iconHTML  

    In this work, a study of the parameters that affect the breakdown voltage in dry-type transformer is performed. The effect of the high operation temperatures of the transformer on the voltage breakdown in the insulating supports is investigated and the electric field distribution on different profiles of these supports is modeled using Finite Element Method (FEM). A design of the insulating support that can help to reduce its length without increasing the possibility of breakdown is discussed. View full abstract»

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  • Applicability of the dielectric response methods of diagnostics of power transformers: Hungarian experiences

    Page(s): 49 - 53
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (603 KB) |  | HTML iconHTML  

    The actual life of transformers strongly depends on the real operating conditions and the residual life of these units can spread in a fairly broad scale. The key factor is the presence of moisture and ageing products because they strongly influence the condition of oil-paper insulation. Therefore recent attention has been directed to methods of determining moisture content and ageing products in the transformer. There are two fundamental dielectric processes in transformer insulation: the conduction which is characterized by conductivity, and the polarization characterized by the polarization spectrum. Both processes are in very strict relation to the structure of the material. Any changing of the structure caused by the deterioration of insulation brings a typical alteration of the dielectric processes and the dielectric parameters of insulation as well. Thus the shape of polarization spectrum is characteristic not only to the insulation material, but to its deteriorating processes to its condition as well. Especially the long-time constant range of the polarization spectrum gives a lot of information about the deteriorating processes of insulation. The Hungarian results - one detail has been only shown in this paper - could improve the interpretation of results of spectrum methods; it is useful in evaluating the condition of the oil-paper insulation and allows observation of the progress of the aging and moistening process in oil-paper insulation. View full abstract»

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  • On-load tap changer's dynamic resistance measurement: Settings and interpretation

    Page(s): 54 - 58
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (602 KB) |  | HTML iconHTML  

    An on-load tap changer (OLTC) is prone to many different defects and degradation mechanisms due to its continuous operation under load condition. A high load current and high operation temperature accelerate the process of contact wear. Contacts erode due to the fact that under certain conditions the contacts develop a resistive layer due to hot insulation oil. As a major cause of OLTC failure, these defects might take place after some years of operation and their frequency increases with the operation time. Regular maintenance is applied due to degradation of the insulation oil. Dynamic resistance measurement (DRM) can be conducted during this maintenance to measure the resistance of the OLTC when it operates. The measurement results provide important information for assessing the actual condition of the OLTC. This paper aims to analyze the effect of test parameters on DRM results from service-aged OLTCs and describes the implementation of DRM in OLTC condition assessment. Two test parameters were selected for observation: the test current amplitude and the circuit resistance. Their amplitude was varied during laboratory experiments as well as on-site after regular maintenance. To quantify the various measurement results, an analytical tool using 4 performance parameters has been developed. Finally for interpretation, with the help of a three dimensional classification system which is presented in this paper, the actual condition of OLTCs can be identified during maintenance activities. View full abstract»

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  • Partial discharge localization in transformers using UHF sensors

    Page(s): 64 - 68
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (385 KB) |  | HTML iconHTML  

    The location of the partial discharge (PD) source inside a transformer can be determined by using the time differences between signals that are captured by an array of three UHF sensors. However, other unwanted signals are also picked up by the sensors and may result in a false PD location. This paper discusses the application of multivariate denoising to the PD signals. Three methods are compared to determine the time difference between the received signals and locate the PD source. With the first peak method, the time difference is calculated by taking the first peak of the signal as the arrival instant. In the second method, the arrival time is determined from the knee point of the cumulative energy curve. For the third method, the time difference is determined by applying the similarity function to the plot of the cumulative energy of the PD signals. The last method is fully automated. However, experimental results show the first peak method tends to be the most accurate. View full abstract»

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  • Insulation diagnosis for 220kV oil-immersed current transformer by frequency dielectric spectroscopy

    Page(s): 69 - 73
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (564 KB) |  | HTML iconHTML  

    Current transformer (CT) is used for protection of major components and to provide continuity of operation in power system. Failure of CT may lead to an energy explosion and result in phase to ground fault that will hinder the operation of a substation. Therefore insulation diagnosis is of great importance for evaluation of CT. As a non-destructive dielectric testing tool, Frequency Dielectric Spectroscopy (FDS) is widely used by power utility for assessments of the condition of electrical equipments such as transformer and cable, but seldom used on CT. Due to the similarity of transformer's insulation, the insulation condition of oil-paper insulation CT may be evaluated by FDS. In this paper, with the frequency range from 10-2~103Hz, FDS has been investigated on insulation condition assessment for several 220kV oil-immersed CTs. Complex permittivity and dissipation factor data are obtained as a function of frequency for each CT. To verify the validity of FDS, high voltage dissipation factors from 10kV to 145kV have been measured by Schering Bridge and compared with the FDS result at power frequency. Results indicate that FDS evaluation is a reliable tool for determination of moisture content in papers and of oil conductivities. View full abstract»

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  • Rating methods for cables installed in unventilated and partially ventilated surface troughs

    Page(s): 74 - 78
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (1268 KB) |  | HTML iconHTML  

    Cables installed in surface troughs are a critical part of many transmission circuits. This paper presents a method for calculating the rating of such circuits using computational fluid dynamics techniques, applied to two common trough designs. One design has solid concrete lids while for the second some lids are replaced by grilles to encourage natural air circulation. For the case of covered troughs with solid concrete lids, the IEC 60287 method was found to be conservative owing to its failure to consider the thermal impact of the ground. Good agreement was observed between the CFD model and an improved analytical formulation by Anders. Modeling of partially ventilated troughs, which at present cannot be rated analytically, shows continuous rating increases of up to 31% over the covered design owing to greater air circulation. View full abstract»

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  • Practical issues of partial discharge testing of low-voltage cables

    Page(s): 83 - 87
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (740 KB) |  | HTML iconHTML  

    Demand for low voltage cable diagnostics, arising from lifetime extension programs of power plants, grows recently. This paper reports on investigations of the authors with partial discharge testing of low voltage control, measurement and signal cables. The construction of low voltage cables differs in many ways from the middle voltage and high voltage cables, on which partial discharge diagnostics has been widely investigated and highly developed. The lack of semiconducting layers makes them less lossy on higher frequencies than MV and HV ones. These results in a different behavior of the propagation of PD induced impulses. Besides, without smoothing layers, these cables are not necessarily PD free at their test voltage. Due to the issues mentioned above, the bandwidth and impulse resolution of the measuring equipment were addressed as well as the discrimination between discharge impulses formed in the insulation bulk and on the conductor surfaces. Measurements were carried out on single and multiple conductor cables by equipments with different bandwidth. The results of the investigation are presented and discussed in this paper. View full abstract»

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  • Dependencies of the PD- and tan(δ)-characteristics on the temperature and ageing status of MV PILC cables

    Page(s): 88 - 92
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    Diagnostic tools and methods for the state evaluation of MV (medium voltage) and HV (high voltage) power cables are mostly based on single key parameters like the partial discharge (PD) levels (inception voltage and intensity) or the values of the dissipation factor tan(δ). The main goals are the determination of the functionality and actual condition of the equipment. Additionally, the prediction of the rest life-time of power components could help to improve the investment planning and the maintenance strategy. In the field of cables and especially in case of PILC (Paper Insulated Lead Covered) cables, the requirements on the measurement technology and diagnostic systems are rather complex. In many cases the attention is directed to the methodology and the accuracy of the measurement equipment itself or to the value or spectrum of the measurement frequencies. However, the reliability of these methods is also very dependent on the experience of the test engineers and the knowledge and interpretation of all measured parameters. Therefore, besides the electrical parameters, in particular the temperature and humidity during the measurement, the equipment's dimensions, cable lengths and noise levels besides others must be taken into account. In this article, the principal dependencies of two major electrical criteria, which are the tan(δ)- and the PD-characteristics on the cable temperature and the cable age or the operational history of the cables, are presented. The strong correlation on e.g. the temperature makes clear, that the interpretation of a single criteria and a subsequent prognoses of the cable status are nearly impossible without further information. The derived results would be a subject of an immense scattering depending on seasonal influences, the load status during the disconnection of the analyzed lines, time constants, cable length, etc. View full abstract»

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  • Partial discharge characteristics of electrical trees in XLPE power cables

    Page(s): 93 - 96
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (577 KB) |  | HTML iconHTML  

    This paper studies the initiation process and the PD characteristics of electrical trees, and proposes a new method to recognize the growth state of electrical trees. The experiment result show that the electrical trees PD energy mainly concentrated in the -40°~90° and 120°~250° of 5MHz~7.5MHz, -20°~90° and 150~250° of 7.5MHz~10MHz. With the dense increase of the electrical trees, the more PD energy will be found in the frequency 7.5MHz~10MHz. At the first development stage of electrical trees, the PD energy mainly located at the frequency 5MHz~7.5MHz, when electrical trees develop to the next stage, the more PD energy will be found at the frequency 7.5MHz~10MHz. View full abstract»

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  • Implementation of statistical principles to reflect the long term performance of composite insulators

    Page(s): 97 - 101
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (501 KB) |  | HTML iconHTML  

    There are numerous advantages of polymeric composite insulators over the traditional ceramic ones. However composite insulators, being comprised of organic elements, age more rapidly over time. Environmental conditions control both the manner in which composite insulators age, and the effective stresses that insulator strings are exposed to in service. The current work presents a statistical methodology to reflect the reliability of a network consisting of composite insulators. This accounts for the working environment and its impact on both the ageing of the insulator and risk of flashover. This is achieved by combining previously established statistical dimensioning principles according to the site's pollution severity and a four-state Markov ageing model. The risk of flashover is estimated according to the site's pollution severity using statistical principles. Then by choosing appropriate transition probabilities between the states of Markov's ageing process the population of insulators at risk of flashover because of ageing is estimated. The combination of the risk of flashover due to the pollution severity and due to ageing of the insulators results in an estimation of risk of flashover for a given number of insulators in a given point in time. The resulting forecast of flashover performance could be used to assist asset management decisions as well as optimising condition monitoring of composite insulators. View full abstract»

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  • Effect of insulator configuration on ageing deterioration of polymer insulators

    Page(s): 102 - 105
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    According to our accelerated ageing test results of 275-kV full scale insulator strings coated with the same silicone rubber, conventional porcelain long-rod insulators showed better anti-ageing performance compared with transmission line polymer insulators. As the reason for such results two factors are considered. One is difference in core diameter, and the other is difference in shed shape. In order to clarify the reason for such results, we started comparative accelerated ageing tests on five different specimens, coated with the same room temperature vulcanized silicone rubber, with almost the same shed shapes but three different core diameters and with almost the same core diameters but three different shed shapes. As of 380 cycles of accelerated ageing test, any discernible deterioration was not observed on any specimens, although partial discharges had been observed at the energized end core surface of the transmission line polymer insulator after 200 cycles. It takes so long to get any significant deterioration by the accelerated ageing test. Since it is considered that the main cause of surface deterioration of polymer insulators is partial discharges, we investigated the partial discharge inception voltages of these specimens under contaminated and wet conditions. The transmission line polymer insulator showed the lowest partial discharge inception voltage. Very large ratio of shed to core diameter is considered to be the cause of easy occurrence of partial discharges due to concentrated voltages allotted at the core surfaces. We prepared three specimens to improve the voltage distribution of the transmission line polymer insulator and evaluated. Promising results were obtained. View full abstract»

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  • Effect of voltage type and polarity on the gas temperature of an arc over an ice surface

    Page(s): 106 - 110
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    The rotational temperatures of arcs propagating over an ice surface, under DC-positive, DC-negative and AC applied voltages, have been measured and compared. The spectra of the emitted light during the arc propagation were collected and analyzed using a spectroscopic system. A strong diatomic molecular band of OH was observed from the captured spectra with two distinct peaks at about 307 nm and 309 nm. The rotational temperature was derived from the relative intensities of two groups of rotational lines corresponding to the peaks of the OH (A-X) (0, 0) vibrational band. From the comparison of arc temperatures under different applied voltages, it was found that the DC-positive arc is hotter than the DC-negative one. The arc temperature under AC applied voltage was found to be close to that of DC-negative voltage. View full abstract»

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