Lab methods for power sys condition monitoring

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Lab methods for power sys condition monitoring

  1. 1. Laboratory Measurements for Power System Condition Monitoring Donald G. Kasten1, Stephen A. Sebo1 and John L. Lauletta2 1 The Ohio State University, Dept. of Electrical and Computer Engineering, Columbus, OH, U.S.A. 2 President and CEO, Exacter, Inc., Columbus, OH, U.S.A. E-mails: kasten.1@osu.edu, sebo.1@osu.edu, jlauletta@exacterinc.comAbstract – About 30 percent of all overhead power distribution Exacter, Inc., has developed a device it is marketing tooutages are caused by failing electrical equipment, e.g., surge detect and analyze the radiated EM noise from distributionarresters, cutouts, insulators, wires, and grounding. Failing components that have a discharge. This Outage-Avoidanceequipment can be located and scheduled for removal prior to an System has been developed using a group of very sensitiveoutage. Replacing failing equipment reduces all reliability indexfigures. Equipment that was identified by the EXACTER® high-frequency detectors, filters, an omni-directionalOutage-Avoidance System as an electromagnetic (EM) noise wideband antenna, a computer, GPS equipment, and data-source was removed from service and tested in the High Voltage communication equipment that operate together to determineLaboratory of The Ohio State University. The purpose of the the location of an arc source. The wide application of thelaboratory effort was to begin the process of identifying the EM system has shown that the system is practical and useful [1].noise emission signatures of a failing component so that the The main objective is to use the Outage-Avoidanceeffectiveness of the field measurement apparatus can be System to survey a particular distribution line, analyzing theimproved. In other words, ultimately develop a Failure Signature radiated EM noise from it, and to determine whether aLibrary to identify what component failed and what type of discharge is producing some of that EM noise, and then tofailure occurred. identify what component it is and where it is located. The nextKeywords – outage-avoidance system, laboratory measurements step of the process is to bring the failed equipment into the laboratory and under a controlled environment test the “defective” components to extract as much information as I. INTRODUCTION possible into the “what and why” of the failure. The common assumption is that different equipment and different arcing It is essential that the electric energy supply system scenarios associated with specific equipment may haveremain operational at all times to meet the electric energy different signatures of the radiated EM noise. Thus as anneeds of the industrial, commercial and residential sectors of ultimate objective, it would be desirable to determine a librarythe system. Equipment failures do occur due to aging of the of failure signatures from the field and laboratoryequipment, installation shortcomings and lack of preventive measurements so as to make the field equipment “smarter”maintenance. Many times low-level discharges are a precursor when identifying a component, and what type of failure thatto arcing followed by a more catastrophic failure later. This component experienced.type of activity occurs on all parts of the electric energy Typical failure locations are all-metal hardware on a towersupply system. If it can be detected early, then the system or pole, especially when there are contamination, oxidation, oroperator can choose when to remove the faulty component so bad contact points, and along insulators, splices, and clamps.as not to jeopardize the integrity of the overall system. If the Mechanical damage and contamination of insulators can alsodischarges and/or arcing were not detected and allowed to be disturbance sources. There are many problems associatedbecome worse and finally result in a complete failure, then the with surge arresters. They are typically related to theoperator no longer has a choice on when and how to remove insulation housing (porcelain or polymer), moisturethe faulty component and parts of the system may be out of penetration (ingress), contamination of gaps, poor contacts,service for the repair interval. The key is to be able to detect ground lead disconnector (detonator), corrosion (e.g., atthe discharges and/or arcing early. grounding), and aging (material changes). The effort described here intends to investigate thespectrum of the EM noise from low-level discharges andarcing and to identify it with the failure of a specific II. DESCRIPTION OF MEASUREMENT EQUIPMENT AND PROCEDUREScomponent on the system. This effort will mainly be directedtowards the electric power distribution systems because thereare many, many more miles of distribution circuits compared Currently the laboratory effort is only associated withwith transmission lines. It should be emphasized that the components that have been identified from field measurementsprinciples applied for the distribution network can also be as emitting a radio frequency EM noise signal. When it can beapplied to the transmission system. coordinated with field personnel, after the identified component is removed from service, it is salvaged and sent to
  2. 2. the laboratory for testing. The majority of the components applied to the component under test and the leakage currenttested were surge arresters. Some of the arresters were part of through the component to ground are monitored and thea fuse-cutout and other arresters were separate. Also tested waveforms recorded using a digital oscilloscope. A precisionwere disconnect switches, dead-end bell insulators, post current-viewing resistor is used to monitor the current.insulators and pin insulators. The most recent tests (not yet The procedure was to test the component at its ratedcompleted at the writing of this paper) involved a string of voltage. For most cases, this meant the line-to-ground voltage230-kV transmission line porcelain suspension insulators. of the component. Surge arresters were tested at the MCOV (Maximum Continuous Operating Voltage) rating value. ThatA. Field Effort value is defined by an IEEE standard as the “maximum designated root-mean-square (rms) value of power frequency The field measurements utilizing the Outage-Avoidance voltage that may be applied continuously between theSystem are fully described in a companion paper [1]. Basically terminals of the arrester [2].”they involve a survey-type process where the detection The voltage applied to the component under test and theequipment is driven within the system with multiple passes, current through the component were monitored as the appliedand the measurements from all passes are correlated so as to voltage was increased from zero to its rated value. In somenarrow down the location of the failed equipment. Various cases where complete failure occurred, it was not possible toother pieces of commercially available hand-held equipment, reach the rated voltage. As indicated in Fig. 1, the appliedsuch as an ultrasonic detector and a portable EM noise voltage was measured using a resistive voltage divider and thedetector, are used to further narrow the focus. leakage current by measuring the voltage across a 50-ohm current-viewing resistor. The instantaneous voltage andB. Laboratory Effort current waveforms displayed on a digital oscilloscope were Tests were conducted in the High Voltage Laboratory of stored since the character of the waveforms changed as theThe Ohio State University (OSU). During all tests, each power failure mode of the component being tested changed.system component identified earlier was energized at rated The EM noise signal emitted from the component undervoltage, or in some cases at an overvoltage. The specific goal test was monitored with the EXACTER equipment and with awas to determine whether the components emitted any EM 1.5 GHz spectrum analyzer. Each device had its own antenna,noise signal that could be detected and recorded. The test the same type for each. The EXACTER device monitors thecircuit used in the High Voltage Laboratory is shown in Fig. 1. EM noise signal, extracts information from that signal and calculates, using a proprietary algorithm, a quantity referred to as “maintenance merit.” When this value exceeds a certain threshold, the equipment extracts from the EM noise signal a spectrum of frequencies ranging from the fundamental up to and including the 50th harmonic [60 Hz to 3 kHz]. This spectrum is updated every second; these spectra are stored for later recall and data analysis. III. TEST RESULTS Many laboratory measurements were conducted in order to determine typical signatures of failing electrical equipment used in the power distribution overhead network. The scope ofFigure 1: Schematic of test circuit used for laboratory measurements the measurements was to test electrical distribution system components under controlled laboratory conditions. The The laboratory effort is used to test the specific components were initially identified by the outage-avoidancecomponent that has been identified as failed or failing. These system. The laboratory measurements checked thetests are only electrical in nature and cannot test the repeatability and consistency of emissions and signals fromcomponent with the mechanical loading that it had component to component as well as a function of time.experienced in the field. Furthermore, the laboratory tests are Most of the tests were conducted under ambientvoltage related as most components being testing are some laboratory conditions when the component tested was dry. Aform of insulation, so no electric current loading is associated few tests were conducted in a fog chamber within the Highwith the high-voltage tests. Also, due to the building- Voltage Laboratory. The procedure was to monitor the appliedcontrolled environment of the laboratory, temperature and voltage and any leakage current with multi-meters as well asweather changes cannot be simulated during the tests. observing the temporal characteristics of the waveforms on an Two high-voltage sources are available. One is up to 50 oscilloscope. The EXACTER Outage-Avoidance System waskV rms, 60 Hz, which was used for most of the components turned on so it could record the low-frequency spectrum. Intested as most components are at the 15 kV level. If necessary, addition, the spectrum analyzer was monitoring the radioa 250-kV rms, 60-Hz source is also available. The voltage frequency EM noise emission.
  3. 3. Fig. 2 shows photographs of two different componentstested. The two components are a 10 kV surge arrester, log#30, and a 15 kV two-unit dead-end-bell, log #31. Their testresults will be presented here and are typical of those recordedfor all equipment tested.Figure 2: Photographs of equipment tested and results presented Fig. 3 shows test results of log #30, the 10 kV surgearrester. The upper graph represents data recorded by theEXACTER equipment over a period of time; this is the Figure 3: Surge arrester log #30 – EXACTER and spectrum analyzerfrequency spectrum from the fundamental 60 Hz up to 3 kHz measurementsextracted by EXACTER from the EM noise emission. Thegraph gives the maximum, median, average and minimum Figs. 5 and 6 show a similar set of results for a dead-endvalues of that spectrum for each frequency of the 798 recorded bell, log #31, which was shown in Fig. 2. The EXACTER-data sets. The number of points indicates how long this determined low-frequency spectrum is shown as well as theparticular noise emission lasted; one data point is recorded full spectrum in Fig. 5. Fig. 6 gives the temporal variation inevery second. It should be noted that in many cases the EM the applied voltage and leakage current for the dead-end bell.noise may be intermittent, so for a given period ofenergization there could be several bursts of noise. The A total of 16 arresters were tested, with ratings of 9 kV,EXACTER equipment does require that the EM noise be 10 kV and 27 kV. Other components tested included dead-endabove a particular level for a certain period of time before bells as shown in Fig. 2, cut-outs (with or without fuses),starting data storage. Similarly, it stops data storage when the switches, suspension insulators, pin insulators and some postEM noise ceases for a given period of time. insulators. Similar data was recorded for all components as The photo shown in the lower section of Fig. 3 is that of illustrated in Figs. 3–6.the spectrum analyzer that “looks” at the entire frequencyspectrum of the noise; for this particular photo the horizontal IV. SPECIAL TESTSaxis is 20 MHz/div. Fig. 4 shows the time domain representations of the same Some special tests were also conducted. (a) A switch withtest results illustrated in Fig. 3. Both sections of Fig. 4 broken insulators was tested for EM noise emissions.represent oscilloscope traces of the monitored applied voltage Conditions of the broken insulator were examined and(top trace), measured leakage current through the current- correlated with the noise emission when the size of the gapviewing resistor (middle trace), and the radiated noise as between the broken parts was changed. (b) Tree branches werereceived by a simple monopole antenna (bottom trace). The brought to the laboratory and contact was established betweenthree traces of the upper section show about 3 cycles (approx. an energized conductor and a grounded tree branch in order to50 ms) of these quantities, whereas the three traces of the simulate the failure conditions during a storm. (c) Aslower section have a time scale of 50 ns/div to more closely mentioned above, tests in a fog chamber were conducted. (d)observe the fast temporal characteristics of the leakage current The changing phase shift between the leakage current and(middle trace) as the arcing occurs. energizing voltage of surge arresters was monitored as the voltage was raised.
  4. 4. Figure 4: Surge arrester log # 30 – scope waveforms at 15 kV; top trace: Figure 6: Dead-end bell insulator log # 31 – scope waveforms at 12.5 kV; topapplied voltage; middle trace: voltage across CVR; bottom trace: spectrum trace: applied voltage; middle trace: voltage across CVR; bottom trace:analyzer input. Horizontal scale: upper: 5 ms/div; lower: 50 ns/div spectrum analyzer input. Horizontal scale: upper: 5 ms/div; lower: 100 ns/div V. OBSERVATIONS AND CONCLUSIONS The objective of the project described in this paper was to characterize the electrical signatures of EM noise emission sources. This noise radiates from those components of the electric power systems that are in the process of partial or complete failure. The ultimate goal is to develop a Failure Signature Library so that the type of component and its failure modes can be identified from the signals observed. ACKNOWLEDGEMENTS The OSU authors wish to acknowledge and thank Exacter, Inc. for its financial support of this effort, and the technical guidance of John Lauletta, President and CEO of Exacter. REFERENCES [1] J.L. Lauletta, S.A. Sebo, “A novel sensing device for power system equipment condition monitoring” (CMD 2010 paper). [2] “IEEE Standard for Metal-Oxide Surge Arresters for AC Power Circuits (> 1 kV),” IEEE Std. 66.22-2005.Figure 5: Dead-end bell insulator log # 31 – EXACTER and spectrumanalyzer measurements

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