662 Cigre Guia de Dp Por Metodos No Convencionales

662

Guidelines for partial discharge detection using conventional (IEC 60270) and unconventional methods

Working Group D1.37

August 2016

GUIDELINES FOR PARTIAL DISCHARGE DETECTION USING CONVENTIONAL (IEC 60270) AND UNCONVENTIONAL METHODS WG D1.37

Members

E. Gulski, Convenor (CH), W. Koltunowicz, Secretary (AT), T. Ariaans (NL), G. Behrmann (CH), R. Jongen (CH), F. Garnacho (ES), S. Kornhuber (DE), S. Ohtsuka (JP), F. Petzold (DE), M. Sanchez‐Uran (ES), K. Siodla (PL), S. Tenbohlen (DE)

Copyright © 2016 “All rights to this Technical Brochure are retained by CIGRE. It is strictly prohibited to reproduce or provide this publication in any form or by any means to any third party. Only CIGRE Collective Members companies are allowed to store their copy on their internal intranet or other company network provided access is restricted to their own employees. No part of this publication may be reproduced or utilized without permission from CIGRE”.

Disclaimer notice “CIGRE gives no warranty or assurance about the contents of this publication, nor does it accept any responsibility, as to the accuracy or exhaustiveness of the information. All implied warranties and conditions are excluded to the maximum extent permitted by law”.

ISBN: 978-2-85873-365-1

Guidelines for PD Detection

Guidelines for Partial Discharge Detection using Conventional (IEC 60270) and Unconventional Methods Contents LIST OF FIGURES............................................................................................................................................. 4 LIST OF TABLES............................................................................................................................................... 7 EXECUTIVE SUMMARY ................................................................................................................................... 8 1

INTRODUCTION ...................................................................................................................................... 9

2

GAS-INSULATED SWITCHGEAR (GIS) .................................................................................................... 12 2.1 Typical PD sources in GIS......................................................................................................................................... 12 2.2 Conventional electrical PD Measurement according to IEC 60270 ................................................................. 13 2.3 Unconventional PD measurement: acoustic methods ........................................................................................... 14 2.4 Unconventional PD measurement: UHF method.................................................................................................... 19 2.5 Case Studies ............................................................................................................................................................... 25 2.5.1 Case Study 1: External signals trigger alarms on UHF PD monitoring system ......................................... 25 2.5.2 Case Study 2: TOF location of PD recorded by UHF PD monitoring system ............................................ 27 2.5.3 Case Study 3: TOF location of PD signals in a cable bushing..................................................................... 29 2.5.4 Case Study 4: External EMI recorded by UHF PD monitoring system ....................................................... 30 2.6 Advantages and disadvantages of different methods ...................................................................................... 32 2.7 Summary and important aspects ............................................................................................................................ 33

3

POWER CABLE SYSTEMS ....................................................................................................................... 34 3.1 Typical PD sources in power cable systems .......................................................................................................... 34 3.2 PD pulse propagation............................................................................................................................................... 34 3.3 Conventional electrical PD measurement according to IEC 60270 ................................................................. 36 3.4 Unconventional HF PD-measurement with coupling capacitor .......................................................................... 38 3.4.1 Single-ended measurement and PD origin localization ................................................................................ 38 3.4.2 Double-ended measurement and PD origin localization .............................................................................. 40 3.5 Application of wideband HFCT for UHF PD detection at joints and terminations ........................................ 44 3.5.1 Application of HFCT at ground straps of terminations and joints ............................................................... 44 3.5.2 HFCT application at cross bonded joints ......................................................................................................... 45 3.6 System calibration with sensitivity and performance check .............................................................................. 46 3.6.1 Conventional electrical PD measurement according to IEC 60270 ............................................................ 46 3.6.2 HF PD-measurement with coupling capacitor ................................................................................................. 47 3.6.3 Sensitivity check of on-line PD measurements applied to MV grids........................................................... 55

Page 2

Guidelines for PD Detection

3.6.4 Sensitivity and performance check of unconventional PD system ............................................................... 57 3.7 Case studies ................................................................................................................................................................ 61 3.7.1 Case Study 1: Maintenance test of a 66 kV power cable with single-ended PD detection and PD localisation....................................................................................................................................................... 61 3.7.2 Case Study 2: After-laying test of a 10 kV power cable with single-ended PD detection and PD localisation ............................................................................................................................................................. 62 3.7.3 Case Study 3: Double-ended PD measurement on medium voltage cable .............................................. 63 3.7.4 Case Study 4: After-laying test 220 kV cable with single-ended PD measurements at both ends..... 64 3.7.5 Case Study 5: Continuous PD monitoring experience in a MV cable system ........................................... 65 3.7.6 Case Study 6: Comparison of insulation defect at a 66 kV GIS cable joint detected using HFCT and HF antenna .................................................................................................................................................... 67 3.7.7 Case Study 7: PD testing of short 220 kV XLPE cable system .................................................................... 69 3.8 Advantages and disadvantages of different methods ...................................................................................... 71 3.9 Summary and important aspects ............................................................................................................................ 72 4

POWER TRANSFORMERS ...................................................................................................................... 73 4.1 Types of partial discharges ..................................................................................................................................... 73 4.1.1 Pulse shape and frequency content .................................................................................................................. 73 4.2 Electrical PD measurement according to IEC 60270 .......................................................................................... 75 4.2.1 Signal attenuation ................................................................................................................................................ 77 4.3 UHF PD-measurement ............................................................................................................................................... 79 4.3.1 Signal attenuation ................................................................................................................................................ 80 4.3.2 Sensor sensitivity ................................................................................................................................................... 81 4.3.3 Recommendation for standardized valves for retrofit of UHF sensors ...................................................... 84 4.3.4 Recommendation for a dielectric window for installation of UHF sensors ................................................ 85 4.3.5 Interpretation ........................................................................................................................................................ 86 4.4 Acoustic PD-measurement......................................................................................................................................... 86 4.4.1 Signal attenuation ................................................................................................................................................ 86 4.4.2 PD localization ...................................................................................................................................................... 87 4.5 Case studies ................................................................................................................................................................ 89 4.5.1 Case Study 1: On-line PD measurement in a single-phase power transformer 220 kV/132 kV, 80 MVA.................................................................................................................................................................. 89 4.5.2 Case Study 2: Combination of conventional and UHF PD detection methods.......................................... 91 4.5.3 Case Study 3: Combination of conventional and UHF partial discharge detection methods ............... 95 4.5.4 Case Study 4: Combination of conventional method with acoustic method ........................................... 100 4.5.5 Case Study 5: Localization of PD by acoustic and UHF-measurement ................................................... 102 4.5.6 Case Study 6: Monitoring by UHF PD-measurement.................................................................................. 105 4.6 Advantages and disadvantages of different methods ................................................................................... 108 4.7 Summary and important aspects ......................................................................................................................... 108

5

REFERENCES ........................................................................................................................................ 110

Page 3

Guidelines for PD Detection

LIST OF FIGURES Figure 1: Typical PD sources in GIS along with UHF & acoustic sensors, typical interference sources ................... 12 Figure 2: IEC60270 standard test circuit [2] ...................................................................................................................... 13 Figure 3: Manual location of PD using commercially available acoustic receiver ...................................................... 15 Figure 4: piezoelectric acoustic energy sensors for picking up PD signals in GIS ....................................................... 15 Figure 5: Sensitivity of acoustic PD detection in a 300 kV GIS [5] ................................................................................ 16 Figure 6: Acoustic PD patterns; amplitude vs. time at left, PRPD on the right ............................................................. 16 Figure 7: acoustic (ultrasonic) PD pulse and enclosure reflections .................................................................................. 18 Figure 8: Narrowband vs. broadband application of the UHF PD method ................................................................. 19 Figure 9: Example equipment set-up for making on-site PD measurements using the UHF method ........................ 20 Figure 10: Typical time-domain GIS PD signal; decay time is approx. 600 ns .......................................................... 21 Figure 11: Frequency response of a simple straight section of GIS (0 – 2000 MHz)................................................ 21 Figure 12: Examples of UHF PD sensors.............................................................................................................................. 22 Figure 13: PD sensor sensitivity shown using real PD signal (MOVING particle), high-quality PRPD plot (r)........ 23 Figure 14: External UHF couplers for use at GIS spacer casting inlet .......................................................................... 23 Figure 15: Principle of time-of-flight (TOF) localization .................................................................................................. 24 Figure 16: CIGRÉ sensitivity verification: lab part 1 (l), on-site part 2 (r) ................................................................... 25 Figure 17: PD monitoring system display showing 120° offset in signals, Y-phase pattern for comparison ........ 26 Figure 18: Single-line diagram showing PD sensor locations (l), tof measurement (r)................................................ 26 Figure 19: Method for verifying TOF path delay time and direction using an externally injected pulse ............. 27 Figure 20: TOF path delay verified with injected pulse; also note the order of arrival reverses .......................... 27 Figure 21: PD monitoring display showing patters suggesting a defect in solid insulation....................................... 28 Figure 22: PRPD taken at the same sensor using UHF method (zero-span).................................................................. 28 Figure 23: Location of sensors used for TOF measurement (L); photo of sensors –Q56Y & PD30Y (R) ................. 28 Figure 24: Plot of TOF of Fig. 21: red trace is the signal from the lower sensor (PD30Y) ....................................... 29 Figure 25: UHF PD monitoring system display showing PRPD suggesting surface discharge activity ..................... 29 Figure 26: TOF set-up to locate cable bushing pd, photo of external sensor #1 (r) ................................................. 30 Figure 27: Plot of time-of-arrival of TOF diagramed in figure 26 above: approx. 38 ns ...................................... 30 Figure 28: UHF PD Monitoring system displaying signals strongly suggesting external EMI.................................... 31 Figure 29: Measurements of external EMI picked up by a UHF PD monitoring system............................................. 31 Figure 30: low-level pulsed emi interfering with cigré sensitivity verification check .................................................. 32 Figure 31: causal emi, spectrum (left), oscilloscope plot showing digital modulation (right) .................................... 32 Figure 32: Frequency-dependent phase velocity and attenuation parameters used for cable model calculation [31] ......................................................................................................................................................................... 35 Figure 33: Test circuit for measurement with a coupling capacitor at the cable termination ................................... 36 Figure 34: Measured and Simulated PD magnitude as function of travelling distance, normalized to 500 pC for the first value ............................................................................................................................................................... 37 Figure 35: Typical example of a PD pattern measured during an on-site cable test ............................................... 38 Figure 36: Measurement setup for single sided PD measurement ................................................................................. 39 Figure 37: PD source location by tdr analysis of the two travelling waves A and B with speed v ......................... 39 Figure 38: Near-end PD fault location with reflection from far end on a 500 m cable ........................................... 40 Figure 39: Setup for double-ENDED PD measurement with localization CAPABILITY ................................................ 41 Figure 40: Setup for the double-ended PD measurement with fault localization functionality ................................ 41 Figure 41: Normalized PD attenuation for double- and Single-ended measurement (10 km). ............................... 42 Figure 42: Normalized PD amplitude attenuation with a fault location at 2 km from the left end. ....................... 43 Figure 43: Double-end PD measurement data with PD origin at 220 m from Detector A and 424 m from Detector B .................................................................................................................................................................................. 43

Page 4

Guidelines for PD Detection

Figure 44: H-field at the Middle of the ‘Joint’ Section ..................................................................................................... 44 Figure 45: H-field at the End of the ‘Joint’ Section ........................................................................................................... 45 Figure 46: Model for application of hfct to a cross-bonded joint ................................................................................. 46 Figure 47: Current Level in a Cross Bonding Link .............................................................................................................. 46 Figure 48: Example of an on-site PD calibration, injecting 100 pC pulses every 10 ms .......................................... 47 Figure 49: pulse signal Measurement compared with responses calculated for different travel lengths .............. 48 Figure 50: Example of a calibration with a pulse of 200 pC on a 10,500 m long cable Unconventional PD measurement with HFCT sensors ..................................................................................................................................... 49 Figure 51: Low Frequency Response of an HFCT .............................................................................................................. 50 Figure 52: Wideband Frequency Response of an HFCT ................................................................................................. 51 Figure 53: 12 ns Square Pulse with 17 MHz HFCT (magenta) and wideband HFCT (blue) ..................................... 51 Figure 54: E-Field Coupling of Three Different HFCTs ..................................................................................................... 52 Figure 55: Input reflection of the tunnel test jig ................................................................................................................. 54 Figure 56: Tunnel test jig shown with HFCT installed for testing ..................................................................................... 54 Figure 57: Frequency Response of a well designed and shielded HFCT: green = Transfer FUNCTION; blue = return loss ..................................................................................................................................................................... 55 Figure 58: On line test to determine signal attenuation behaviour at increasing frequency ................................... 56 Figure 59: Signal attenuation versus sinusoidal frequency at transformers substations 2 and 3............................. 57 Figure 60: Setup of multi-channel PD measurement system (optical communication) ................................................. 58 Figure 61: Propagation of calibration pulses along 20 km XLPE cable line [35] ....................................................... 59 Figure 62: Propagation of calibration pulses along 20 km XLPE cable line ............................................................... 59 Figure 63: Spectra of calibration pulses ............................................................................................................................. 60 Figure 64: PD patterns of wire test ...................................................................................................................................... 60 Figure 65: PD patterns observed at 1.5x Uo during maintenance testing of a 30-year cable ............................... 61 Figure 66: PD mapping as made up to 1.5x Uo during maintenance testing of a 30-year old cable .................. 62 Figure 67: PD results of an after-laying testing of a 10 kV 2.1 km long XLPE cable section .................................. 62 Figure 68: Investigation of the joint at 955 m having PD up to 800 pC at 2xUo....................................................... 63 Figure 69: Double sided measurement on a 644 m long medium voltage cable ....................................................... 63 Figure 70: On-site testing of a 220 kV 13.3 km long XLPE cable circuit. .................................................................... 64 Figure 71: PD patterns as observed during the voltage withstand testing of a 220 kV XLPE cable ..................... 64 Figure 72: PD mapping made up to 1.3x U0 during on-site testing of a 220 kV 13.3 km long cable circuit. ......................................................................................................................................................................................... 65 Figure 73: Diagram and signals from continuous PD monitoring of a 45 kV cable system ...................................... 65 Figure 74: PD mapping of the cable system monitored................................................................................................... 66 Figure 75: frequency spectra of PD at each monitoring point of the cable system ................................................... 66 Figure 76: details of the PD source located in splice CB1 ............................................................................................... 66 Figure 77: Photograph showing PD measurement via HFCT and two HF horn antennas ........................................... 67 Figure 78: Typical simultaneous measurement result of the two HF antennas and HFCT .......................................... 68 Figure 79: Relationship between signals measured with HFCT and 2 horn antennas ................................................ 68 Figure 80: Measurement results when punched panels were not removed .................................................................. 69 Figure 81: Three-phase PRPD patterns and their equivalent 3PARD diagram ........................................................... 69 Figure 82: PRPD patterns of the PD signal at different frequencies after separation .............................................. 70 Figure 83: Findings at the GIS cable termination inspection after the failure............................................................. 70 Figure 84: Typical positive and negative PD current pulse waveforms for each degassing treatment ................. 75 Figure 85: Test circuit for measurement at a tapping of a bushing [2] ........................................................................ 76 Figure 86: Typical example for a PRPD pattern generated in a PD measuring system ........................................... 77 Figure 87: Experimental set up showing moveable PD signal source............................................................................ 78 Figure 88: Signal power dependency on location and measuring FREQUENCY ........................................................ 78 Figure 89: dependency of MEASURED APPARENT CHARGE ON LOCATION AND MEASURING FREQUENCY .............................................................................................................................................................................. 79 Page 5

Guidelines for PD Detection

Figure 90: Principle of UHF PD measurement in comparison to electrical measurement ........................................... 80 Figure 91: POSITION DEPENDENCE ON UHF SIGNAL ATTENUATION INSIDE A 210 MVA TRANSFORMER ...... 81 Figure 92: examples of uhf pd sensors for hv transformers ............................................................................................ 82 Figure 93: Typical antenna factor of a UHF PD sensor measured in GTEM cell ......................................................... 82 Figure 94: Directivity characteristic of a typical cone-shaped UHF probe simulated at 500 MHz ........................ 83 Figure 95: dependency of antenna factor on insertion depth for UHF PD sensor in oil-filled GTEM cell .............. 83 Figure 96: Potential valve types for retrofit of UHF sensors; valves with straight-through opening/duct ............. 84 Figure 97: Oil valves without straight opening/duct; retrofit of valve-type uhf sensors not possible .................... 84 Figure 98: UHF Plate Sensor with stainless steel flange and dielectric window ......................................................... 85 Figure 99: Example for dimensions of stainless steel flange and dielectric window.................................................. 85 Figure 100: Typical example of a transformer PRpD pattern obtained using uhf techniques ................................. 86 Figure 101: Illustration of the structure-borne path problem .......................................................................................... 86 Figure 102: ACOUSTIC PD SIGNAL WITH KNOWN ARRIVAL TIME HIGHLIGHTED PLUS STRUCTUREPATH component’ ..................................................................................................................................................................... 87 Figure 103: External acoustic sensors on a transformer tank with a PD inside using Cartesian coordinates ........ 87 Figure 104: comparison between pure acoustic and UHF-triggered acoustic PD acquisition [59].......................... 88 Figure 105: a) sensor PD_S1 in the 220 kV bushing, b) sensor PD_S2 in the 132 kV bushing, c) sensor PD_S3. HFCT placed at the tank earth connection ........................................................................................................... 89 Figure 106: Internal defect detected. Insulation damage due to heating ................................................................... 91 Figure 107: 15/7 MVA, 66/22 kV transformer ............................................................................................................... 92 Figure 108: long-term On-line DGA Results of 15/7 MVA, 66/22 kV transformer.................................................. 92 Figure 109: Tap sensor for PD measurements ................................................................................................................... 93 Figure 110: UHF antenna installed in oil drain valve ....................................................................................................... 93 Figure 111: Three phase PD trend of 15/7 MVA, 66/22 kV transformer .................................................................. 93 Figure 112: Three phase synchronous PRPD patterns from 15/7 MVA, 66/22 kV transformer ............................. 94 Figure 113: Equivalent 3PARD diagram ............................................................................................................................. 94 Figure 114: Individual PRPD patterns of the selected clusters ........................................................................................ 94 Figure 115: Frequency sweep diagram .............................................................................................................................. 94 Figure 116: PRPD pattern ...................................................................................................................................................... 94 Figure 117: 130/130/100 MVA – 230/115/48 kV Transformer .............................................................................. 95 Figure 118: PD tap sensor ..................................................................................................................................................... 95 Figure 119: UHF sensor placed between phases V and W ............................................................................................ 95 Figure 120 Three phase PD trend from the 130/130/100 MVA – 230/115/48 kV Transformer ...................... 96 Figure 121: Separation of PD sources using 3PARD from the 130/130/100 MVA – 230/115/48 kV Transformer ............................................................................................................................................................................... 96 Figure 122: On-line frequency sweep diagram ................................................................................................................ 97 Figure 123: On-line frequency sweep diagram ................................................................................................................ 97 Figure 124: Trend diagram and PRPD patterns of the signal detected in the UHF range ....................................... 97 Figure 125: PD sources separation combining the conventional and unconventional measurements ...................... 98 Figure 126: Increase of the amplitude of the PD pulses detected at the phases V and W ..................................... 98 Figure 127: Acoustic PD localization .................................................................................................................................... 99 Figure 128: PD source localization in the vicinity of the phase V .................................................................................. 99 Figure 129: Dismantled bushing of phase V ...................................................................................................................... 99 Figure 130: Endoscopic inspection ........................................................................................................................................ 99 Figure 131: PD tracks at phase V ..................................................................................................................................... 100 Figure 132: PD tracks at phase W ................................................................................................................................... 100 Figure 133: MVA 150/20.8 kV power transformer and PD decoupling at the bushing tap................................ 101 Figure 134: Conventional PD measurements at U= 85 kV in phase V (upper pattern) and in phase W (lower pattern) ...................................................................................................................................................................... 101 Figure 135: Acoustic measurements and defects location ............................................................................................ 102 Page 6

Guidelines for PD Detection

Figure 136: 333 MVA transformer showing positions of UHF sensors and acoustic sensors [67] ......................... 103 Figure 137: Measured propagation time differences between three UHF probes ................................................. 104 Figure 138: Deteriorated paper insulation on leads at the tap changer.................................................................. 105 Figure 139: UHF PRPD patterns 1 – 3 [69] .................................................................................................................... 106 Figure 140: Number of PD per minute and correlation coefficient [69].................................................................... 107 Figure 141: results of recognition algorithm of determined patterns 1 - 3 [69] .................................................... 107

LIST OF TABLES TABLE 1: EVALUATION OF DIFFERENT PD MEASURING TECHNIQUES ........................................................................ 11 Table 2: Acoustic propagation velocity for different materials in GIS ......................................................................... 18 Table 3: Comparison of UHF and conventional (IEC 60270) methods (GIS) ............................................................... 32 Table 4: Typical Dimensions of Joints .................................................................................................................................. 44 Table 5: dB to Rt conversion ................................................................................................................................................... 55 Table 6: Comparison of conventional (IEC 60270) and unconventional PD detection methods for cables ........... 71 Table 7: Raw PD patterns acquired by means of three sensors (row 1); PD patterns after clustering processing (rows 2, 3, 4) ........................................................................................................................................................ 90 Table 8: Fault gases concentrations from lab tests ........................................................................................................... 98 Table 9: Comparison of UHF and conventional IEC 60270 measurement method .................................................. 108

Page 7

Guidelines for PD Detection

EXECUTIVE SUMMARY Detection and evaluation of partial discharges (PD) belong to fundamental measurements applied to HV components. Due to on-going development of their application in both laboratory and field conditions, there is a continuing need to support this process with guidelines for PD detection and testing, describing new and established methods. As a result, PD measurement has become a worldwide-accepted method for insulation diagnosis and a required part of the acceptance testing for most HV assets. Based on the absence or presence of PD activity caused by insulation defects discharging during routine tests, on-site tests, or periodic in-service inspections throughout the service life-time, conclusions may be made about the actual condition of the dielectric insulation system. Following the studies published in 2010 - CIGRÉ Technical Brochure 444 - this new publication continues the discussion on the application of conventional and unconventional partial discharge detection methods. In particular, the individual chapters of this brochure discuss several aspects of applying PD detection to different types of HV components: 



Evaluation of quantities to correlate conventional (IEC 60270) PD detection to unconventional methods: a) definition of quantities and procedures to consistently correlate standardized PD [pC] and unconventional (HF) instrument reading(s), b) overview (case studies) of best-practice methods to apply and evaluate PD measurements for testing purposes of different components, c) effects of advanced noise suppression and signal processing techniques on the reading(s) sensitivity, Methods to determine the sensor sensitivity: a) evaluation procedures of parameters to describe the sensors’ sensitivity, b) frequency spectrum (magnitude, power spectrum), c) impedance of sensors versus frequency / effective height, etc.

are evaluated by means of practical examples (case studies) in both laboratory and field. In chapter 1, the general aspects and background information of conventional and unconventional PD detection as applied to different power components are presented. In chapter 2, PD defects typical for GIS and their PD pulse characteristics are evaluated for conventional and unconventional detection methods. In chapter 3, specific aspects of using different PD detection methods as well as interpretation techniques for power cables are described. In the case of unconventional detection methods, solutions are shown to demonstrate the importance of sensitivity and performance checks. In chapter 4, PD detection as used for power transformers is discussed in the scope of PD pulse shape, signal attenuation, the sensitivity of using UHF sensors. In addition, a small section covering acoustic PD detection has been included. Finally, an extensive reference list covering the most recent overview of international publications is included.

Page 8

Guidelines for PD Detection

1 INTRODUCTION PD measurements are recognized worldwide for insulation diagnosis and are usually a required part of the acceptance testing procedure for most HV equipment assets. Based on the absence or presence of PD activity caused by insulation defects discharging during routine tests, on-site tests, or periodic in-service inspections throughout the service life-time, conclusions may be drawn about the actual condition of the dielectric insulation system. To use one example, on-site PD tests can be applied to prove two main characteristics of a system of HV components: 1. HV system component quality and integrity of the system components:  As part of commissioning on-site: a check for possible damage after completed factory tests due to transportation, storage and installation.  To assure that no critical defects in the insulation system were caused during transport from the manufacturer to the site and during the erection on-site. Typically, the system components are tested in the factory including the main parts and any prefabricated accessories. However, the effect of transportation and the correctness of the final assembling can only be tested after completing the installation in the field.  After on-site repair: to spot mistakes in workmanship and to demonstrate that the equipment has been successfully repaired and that all dangerous defects in the insulation system have been eliminated. 2. Availability /reliability of the HV component:  For diagnostic purposes: to estimate actual condition of the service aged component by checking the insulation fingerprint in order to note any insulation system degradation after a period of service operation e.g. 40 or 50 years.  By providing reference value (‘finger print’) for diagnostic tools (voltage test including partial discharge) for later tests to demonstrate whether the insulation system is still free from dangerous defects and that the lifetime expectation is sufficient high. To improve the sensitivity of PD measurements, different techniques/methods have been developed and they are in use for both laboratory and on-site applications. All these methods can be divided into conventional and unconventional PD measuring methods. History shows that PD measurement technology has been proven as an excellent method for quality control of HV insulation for over 40 years. Due to on-going aging of the insulation systems of HV components in service, on-site PD testing and diagnostic methods have attracted increased interest for application in condition monitoring. Since conventional PD measuring systems used in the controlled factory environment are not usually suitable for on-site application, specialized PD detection and measurement methods have been introduced. PD signals can be detected by so-called unconventional PD measuring methods and systems which utilize different physical characteristics and properties of the PD processes. In general, electric methods are based on the measurements of electrical signals in the radio frequencies (RF) ranges e.g. HF, VHF, UHF, etc. Many technical studies and papers have demonstrated that there is virtually NO correlation between the apparent charge [pC] values recorded from standardized conventional measurements and the values recorded by unconventional measurements. This is especially true when measuring with the RF method. Measurements using RF techniques are based on the detection of electromagnetic waves emitted by the PD event. There is no correlation to standardized PD measurements because the PD level recorded depends on the type of PD, the location, the sensor type and the size and geometry of the physical object in which the electromagnetic wave is measured. In particular, the measured values of a PD signal strongly depend on the insulation defect's type and geometry and on the location of the sensor in relation to the PD source.

Page 9

Guidelines for PD Detection

Since it is impossible to measure a PD event directly, remote sensors will always record values that are influenced by the fact that PD pulse propagates over some distance from its point of origin. The following effects are normal:  

The increasing impact of pulse propagation path on the received PD waveform e.g. reduced rise time and increased pulse width, attenuation, etc. The superposition between different signals, e.g. multiple PD sources, crosstalk, external interferences.

Due to the fact that there is a general interest in the application of these “nonstandard” methods because of their advantages for use in the field, this guide has been prepared to provide recommendations for implementing them. This guide discusses the current state of the art in the field of PD measurements with unconventional methods. Included in the following guide, the reader will find actual results obtained for different types of HV components, with 'best practice' solutions being discussed and explained. There are several methods for generating high voltage ac (HVAC) which can be applied for on-site insulation withstand testing and for high voltage testing in combination with PD measurements, see TABLE 1. Therefore, combining PD detection with the standard on-site HVAC test provides much more comprehensive information about the state of the insulation system, especially the possibility to discover and locate defects within the insulation system. It is also known that there is no general relationship between the PD intensity and the breakdown probability. Therefore PD measurement is an indirect method which delivers an indication of the extent of degradation and the potential danger posed by the weaknesses identified. With conventional PD measurement (IEC 60270), the apparent charge is measured in pC. It is the integrated current pulse, caused by a PD, which flows through the test circuit. The conventional method allows a precise calibration, but requires a sufficiently high signal-to-noise ratio (SNR) in the measurement circuit to easily resolve the PD signal in question. The standardized method of IEC 60270 and HVAC test protocol is well established for factory and laboratory testing, but is often not appropriate for on-site testing. In the case of field-testing where very high background noise levels are present, unconventional methods which are capable of high SNR have been proven to be the best way to obtain meaningful PD measurements. Several unconventional PD detection methods based on acoustic and electromagnetic phenomenon have been used for some time for PD detection on power cables, transformers, GIS, and generators. Up until now, there have not been accepted procedures and guidelines for these 'unconventional´ methods as has been the case for conventional methods for many decades. There are many open questions including: calibration or sensitivity verification procedures, techniques for noise suppression, and methods of fault location. The authors of this guide believe that now is the time to prepare guidelines and international recommendations for these unconventional PD detection methods in order to ensure reproducible and comparable PD measurements on high voltage equipment between users throughout the industry.

Page 10

Guidelines for PD Detection TABLE 1: EVALUATION OF DIFFERENT PD MEASURING TECHNIQUES Conventional PD measurements (IEC 60270)

Unconventional PD measurements

Advantages

Disadvantages

Advantages

Disadvantages

1. Calibrated readings of apparent charge value in [pC] 2. Well-known method since 1960s 3. Reference PRPD patterns are widely available for different discharge defects and for different components 4. Standardized procedures for type-, factory- and on-site tests

1. EMI (electromagnetic interference) makes on-site application difficult 2. Size and complexity of the object influence the sensitivity of the measurements. 3. Localization of the PD source not possible 4. In the case of large (distributed) test object there is strong influence of defect position on measured PD value 5. Reduced sensitivity in the case that the coupling capacitor capacitance is low compared to that of test object

1. Better rejection of EMI on-site 2. Distributed measurements and synchronous PRPD pattern evaluation possible 3. In most cases applicable for on-line monitoring 4. PRPD patterns very similar to those obtained with conventional measurements

1. Calibrated readings of apparent charge value in [pC] not possible 2. Standardized testing procedure not available for all components 3. Strong influence of defect position on measured PD value, in particular for measurements in the VHF and UHF frequency ranges 4. Minimum number of sensors necessary to fulfil the CIGRE sensitivity recommendation

Transformers: a) Apparent charge levels not sufficient for diagnostic purposes b) Difficult application for transformer bushing without measuring tap

HF measurement for cables (use of coupling capacitors and HF coupling devices): a) Applicable off-line b) Localization of PD source possible on complete power cables using Time Domain Reflectometry (TDR)

HF measurement for cables (use of coupling capacitors and HF coupling devices): a) Not applicable on-line b) PD measurement in long cable requires measurements at both cable ends

Power Cables: a) EM interference dependent on type of HV source used on-site b) PD measurement in long cable requires measurements at both cable ends GIS: Sufficient on-site PD sensitivity fulfilled only with encapsulated voltage test set-ups (typically for GIS of lower voltage ratings)

HF/VHF measurement (use of HFCTs): a) Applicable on-line and off-line b) Sensors can be installed during operation c) PD localization possible on cable accessories

UHF measurement (Transformers, GIS): a) High signal-to-noise ratio possible b) Widely accepted for GIS testing & monitoring c) UHF sensor can be installed online for transformers Acoustic measurement (Transformers, GIS): a) Applicable on-line and off-line b) Used for PD source localization on transformers On GIS used for PD source localization and PD defect recognition; no restriction on sensor positioning

Page 11

HF/VHF measurement (use of HFCTs): a) Sensitivity and possibility of localization depends on measurement frequency selection b) Sensor installation not always possible c) PD localization in cable accessories only possible if the sensor can be installed d) Type and position of the sensor influences the sensitivity e) Comparison of measurements with different detection systems not possible UHF measurement (Transformers, GIS): a) Standardized procedure for acceptance test and sensitivity check available for GIS only b) Pre-installed internal sensors are common practice for GIS c) External sensors can sometimes be applied for GIS, but have limited sensitivity to PD and higher susceptibility to external EMI d) Signal processing necessary for noise suppression Acoustic measurement (Transformers, GIS): a) Limited sensitivity (except on GIS)

Guidelines for PD Detection

2 GAS-INSULATED SWITCHGEAR (GIS) 2.1

Typical PD sources in GIS

Figure 1 contains a pictorial summary of typical PD sources and insulation defect types which may occur in GIS and produce radio-frequency (VHF and UHF) signals.

FIGURE 1: TYPICAL PD SOURCES IN GIS ALONG WITH UHF & ACOUSTIC SENSORS, TYPICAL INTERFERENCE SOURCES

Similarly to the case with other complex insulating systems, two requirements must be fulfilled for PD to occur [1]:  

The electric field strength E must exceed the withstand level of the insulation material in the immediate vicinity of the defect. Free electrons must be available to initiate and sustain the PD (the processes governing this availability are extremely complex and lead to the stochastic, non-regular behavior of PD signals).

As shown in Figure 1, typical PD sources in GIS consist of the following main types: 





Moving or ‘hopping’ particles, typically metallic and in the millimeter range, e.g. a tiny fragment of aluminum (from the enclosure) chiseled off by screw-threading during assembly. It is well-known that such particles move under the influence of the electric field. The signals they generate are relatively easy to detect by both conventional electrical and unconventional (acoustic & UHF) methods. This relative ease of detectability is good because moving particles are considered a relatively high risk because of their unpredictability in GIS [2], [3]. Floating potential discharges result when a conducting (metallic) object, e.g. a field electrode or contact component, has no galvanic connection to one of the HV poles - the inner conductor or the enclosure. This forms a capacitive voltage divider which charges up under the influence of the electric field. Since the gap across the missing contact is usually very small ( 1 GHz. The local maximum caused by the gate valve is shifted to lower frequencies. The worst case installation at Pos. 1 increases the AF in the entire frequency range, for example at the local maximum at 400 MHz. Additionally, new peaks at 1.9 GHz / 2.1 GHz and frequencies f > 2.6 GHz occur [51]. 4.3.3 Recommendation for standardized valves for retrofit of UHF sensors Most available UHF sensors are designed for standardized DN50 or DN80 gate valves. The sensors also fit to guillotine and ball valves. In some cases an adapter flange may be required. Other valve types without a straight-through opening, e.g. globe, butterfly, and diaphragm valves are not supported.

FIGURE 96: POTENTIAL VALVE TYPES FOR RETROFIT OF UHF SENSORS; VALVES WITH STRAIGHT-THROUGH OPENING/DUCT

FIGURE 97: OIL VALVES WITHOUT STRAIGHT OPENING/DUCT; RETROFIT OF VALVE-TYPE UHF SENSORS NOT POSSIBLE

Page 84

Guidelines for PD Detection

4.3.4 Recommendation for a dielectric window for installation of UHF sensors On new transformers, UHF sensors can be installed directly into the tank wall; such UHF plate sensors do not need oil valves for installation. They can be installed at a stainless steel flange with a dielectric window at the transformer tank wall according to Figure 98 [52].

FIGURE 98: UHF PLATE SENSOR WITH STAINLESS STEEL FLANGE AND DIELECTRIC WINDOW

FIGURE 99: EXAMPLE FOR DIMENSIONS OF STAINLESS STEEL FLANGE AND DIELECTRIC WINDOW

Following the specification for transformer tanks the dielectric window should fulfill the following requirements:  

Resistance to mineral transformer oil and natural/synthetic ester. Vacuum stability of 0.15 mbar for a minimum holding time of 72 h. The leakage rate should not exceed 5 mbar/s after stop of the vacuum pump.

Page 85

Guidelines for PD Detection

 

Pressure: 5 bar. Temperature: 120 °C

4.3.5 Interpretation PD pattern analysis of UHF PD measurement is comparable to the methodology employed in electrical PD measurements, but there is no correlation of the received signal strength – i.e. the antenna output signal voltage or energy - to the apparent charge measured with the electrical method. Therefore in UHF PRPD patterns, the y-axis is usually scaled in UHF signal amplitude [dBm, dBuV, mV] or UHF signal energy [pJ]. Both values show the same qualitative correlations. In Figure 100 a typical example of a UHF PRPD is shown. 30

14

10

10

8 6

5

4

number of PD

amplitude / mV

12

2 50

100

150 200 250 phase / degree

300

350

1

FIGURE 100: TYPICAL EXAMPLE OF A TRANSFORMER PRPD PATTERN OBTAINED USING UHF TECHNIQUES

4.4

Acoustic PD-measurement

In addition to the measurable electrical signals described above, partial discharges in oil appear also generate acoustic impulses which propagate as waves in the ultra-sonic range (20-1000 MHz) [53]. 4.4.1 Signal attenuation For performing acoustic PD measurements on transformers, piezoelectric sensors are employed, mounted on the outside of the transformer tank. The velocity of propagation of acoustic signals in oil for operational temperatures between 50°C and 80°C may e.g. vary from around 1240 m/s to 1300 m/s [54]. Basically, insulation materials exhibit a low-pass character [55] and acoustic attenuation increases approximately as the square of the frequency f2 [56]. PD longitudinal/ transversal



transversal

oil

reflection

oil path steel path

.

tank sensor FIGURE 101: ILLUSTRATION OF THE STRUCTURE-BORNE PATH PROBLEM

As shown in Figure 101, because the acoustic sensor is not directly located normal to the PD source, the propagation path (simplified in the drawing as 'oil path', may possibly include e.g. pressboard and other winding components as well) and steel [57]. Page 86

Guidelines for PD Detection

Figure 101 illustrates the so-called structure-borne path problem which can cause erroneous early arrival times of the acoustic signal. The mechanical waves (vibrations) encountering the transformer housing create an alternative propagation path via the tank wall whose acoustic propagation velocity is much higher than that of oil. Calculations based on using the arrival times of signals which have travelled partly over structure-borne paths but assuming an average acoustic propagation velocity valid for oil for all signals will lead to incorrect estimates of the distance between the PD source and the sensor.

FIGURE 102: ACOUSTIC PD SIGNAL WITH KNOWN ARRIVAL TIME HIGHLIGHTED PLUS STRUCTURE-PATH COMPONENT’

Systematic investigations of this matter were carried out [58]. An acoustic sensor mounted on a steel plate was gradually moved from a position perpendicular to an acoustic source in oil to positions involving a growing proportion of the acoustic signal path via a metal plate (Figure 101). Depending on the angle Ψ, three regions can be distinguished where different wave types are predominantly stimulated. For PD location using an average sound velocity (propagation speed in oil), only signal portions of the direct path should be denoted as 'PD-signal'. The structure-borne component can thus be treated as 'interference’ instead (Figure 102) [59]. 4.4.2 PD localization Regarding localization of the PD source, two main approaches can be found: (i) on the one hand, alterations of the signal amplitude or deformations of the signals shape along the propagation path can provide hints for a source location, (ii) on the other hand, measured arrival times are used to calculate the origin of signals (often referred to as ‘triangulation’). S3 (x s3 , y s3 , z s3 )

S (x s4 , y s44, z s4 ) Si (x si , y si , z si )

Di

D3 D4 PD (x, y, z)

D2

S2 (x s2 , y s2 , z s2 )

D1

S1 (x s1 , y s1 , z s1 ) FIGURE 103: EXTERNAL ACOUSTIC SENSORS ON A TRANSFORMER TANK WITH A PD INSIDE USING CARTESIAN COORDINATES

Figure 103 shows a schematic view of a transformer tank to which i acoustic sensors are attached, a PD source inside, and with the resulting distances Di from the sensors Si to the PD source. Such arrangements form the geometric basis for the required mathematical formulations. The appropriate nonlinear observation equations are in the simplest case characterized by sphere functions which intersect at the PD origin.

Page 87

Guidelines for PD Detection

Depending on whether mixed-acoustic (i.e. triggering with the electric or electromagnetic PD signal) or allacoustic measurements are used, the number of unknowns is three (space coordinates (x, y, z) of the volume containing the PD source) or four (an additionally unknown temporal origin) respectively. Hence an exact spatial location of the PD source in the determined case requires at least three or four usable acoustic sensor signal arrival times. Often the SNR is too low in order to determine the arrival times with sufficient accuracy. In these cases, increasing the SNR by suppressing noise using signal averaging is usually a very effective method. This implies some prerequisites: (i) the signals are repetitive and it is possible to get a jitter-free trigger on them, (ii) signal and noise are uncorrelated and (iii) the noise is supposed to be white (white noise should feature a fairly constant spectral density in the frequency range investigated). During an averaging process, the noise included in the signals tends towards its statistic mean value which is zero, if the noise characteristic is indeed white. The repetitive part of the signal is superimposed constructively and remains unaffected. The theoretical maximum signal-noise-ratio gain is N0.5 where N is the number of superimpositions [59].

FIGURE 104: COMPARISON BETWEEN PURE ACOUSTIC AND UHF-TRIGGERED ACOUSTIC PD ACQUISITION [59]

The black trace is from a purely acoustic-based measurement of a single 132 pC PD pulse, while the orange trace is comprised of 500 superimpositions of acoustic signals with a maximum amplitude of only 9 pC, but obtained using UHF triggering; both signals come from the same experimental arrangement [59]

To be successful with acoustic averaging, a stable trigger of an actual signal related to the PD with a higher sensitivity/SNR than the acoustic signals is required. Such a combination of two PD signals of different type and sensitivity is classically used in test laboratories of transformer manufacturers, where the sensitive/higher SNR electric PD signal is combined with the acoustic signals to locate PD. The same holds for a UHF-acoustic combination, since comparative investigation on the sensitivity revealed a higher UHF sensitivity, especially for hidden defects. Figure 104 shows a comparison between an acoustic PD single pulse signal with an apparent charge of 132 pC and a UHF-triggered, averaged acoustic PD signal with 500 superpositions of maximum 9 pC. Although both signals were recorded within the same experimental arrangement (identical PD source, sensor and sensor position and an equal signal amplification), the single pure acoustic PD pulse acquisition showed no clearly observable information. However, the averaged acoustic PD signal with 500 superimpositions revealed an obviously visible pulse. The UHF PD signals which were used to trigger the averaging process were detectable for discharge levels all the way down to 5 pC apparent charge. Regarding the experimental arrangement, an attempt was made to model the acoustic propagation processes realistically. For that purpose, the configuration involved a disc-winding package at high voltage surrounded by two pressboard cylinders with a stimulated PD at the inner side, immersed in an oil-filled transformer tank.

Page 88

Guidelines for PD Detection

4.5

Case studies

4.5.1

Case Study 1: On-line PD measurement in a single-phase power transformer 220 kV/132 kV, 80 MVA The main challenges involved in on-line PD measurement are noise suppression, locating the PD source(s) within the transformer, and the pulse clustering of each PD source that can appear in the power transformer. In order to analyse the insulation condition of a single-phase power transformer 220 kV/132 kV, 80 MVA on-line PD measurements were performed [60], [61]. PD sensors were installed as follows: 



At both bushings on the 220 kV and 132 kV sides, in order to acquire PD signals by means of coupling impedances. These coupling impedances were set up for each capacitive bushing in order to get a band-pass filter response (400 kHz-20 MHz) suitable for acquisition of PD signals (Figure 1-a & 1-b. Sensors PD_S1 & PD_S2). At the earth connection of the tank via an HFCT sensor with a band-pass filter response of 100 kHz to 20 MHz (see figure 1-c. Sensor PD_S3).

FIGURE 105: A) SENSOR PD_S1 IN THE 220 kV BUSHING, B) SENSOR PD_S2 IN THE 132 kV BUSHING, C) SENSOR PD_S3. HFCT PLACED AT THE TANK EARTH CONNECTION

A measuring system with a bandwidth of 300 kHz to 30 MHz using an automatic filtering noise suppression tool was applied. The noise suppression tool is developed on the basis of the wavelet transform plus additional statistical treatment. The effectiveness of the noise suppression using wavelet transform is clearly demonstrated by observation of the residual background noise after filtering. It is so low that high-definition phase-resolved PD patterns can be seen clearly above the base-line level (PRPD patterns in the first row of Table 7). Although the major portion of background noise was rejected by means of the wavelet transform, the PRPD pattern recorded is the overlapping of several PD sources. Consequently, correct insulation diagnosis cannot be made by simple visual inspection of the PRPD pattern. These situations are very typical for on-line PD measurements. In recent years the technique of clustering the PD signals based on their impulse wave-shape has been being refined and is very appropriate to separate the overlapped PD sources. A generic damped sinusoidal wave-shape model, defined by the equation below, was applied to each recorded current PD pulse i(t) . Three characteristic parameters (f, , and ) of each pulse’s wave-shape were used (oscillation frequency, f, and two time constants  and  associated to the PD pulse envelope) to perform the PD clustering (Table 7). i(t )  sin(2    f  t   ) 

 (t  t 0 )

e

Page 89

1  e  ( t  t 0 )

4.2

Guidelines for PD Detection

where t0 and  parameters are the time and phase displacements for the instant t=0. In this PD record, three different PD clusters can be observed (Table 7):   

Cluster #1, PD pattern related to an internal defect, which evidences the existence of an internal defect in the transformer insulation. Cluster #2, PD pattern unrelated to internal insulation defects (corona PD, surface PD signals). Cluster #3, pattern related to e.g. signals generated by power electronic systems. TABLE 7: RAW PD PATTERNS ACQUIRED BY MEANS OF THREE SENSORS (ROW 1); PD PATTERNS AFTER CLUSTERING PROCESSING (ROWS 2, 3, 4) Sensor PD_S1

Sensor PD_S2

Sensor PD_S3

f [7,7 MHz; 11,2 MHz] α [1,5 106 s-1; 77,4 106 s-1] β [2,4 106 s-1; 1,4 106 s-1 ]

f [7,4 MHz; 11,4 MHz] α [5,4 106 s-1; 64,1 106 s-1 m]

f [9,0 MHz; 11,4 MHz] α [1,9 106 s-1; 85,7 106 s-1] β [2,1 106 s-1; 90,2 106 s-1 m]

f [5,1 MHz; 6,4 MHz] α [0,5 106 s-1; 34,1 106 s-1] β [1,6 106 s-1; 138,0 106 s-1 ]

f [4,9 MHz;6,7 MHz] α [0,5 106 s-1; 5,97 106 s-1] β [0,9 106 s-1; 54,8 106 s-1]

-

-

-

-

Overlaped PD patterns

Cluster #1

β [2,1 106 s-1; 29,8 106 s-1]

Cluster #2

f [2,0 MHz; 7,2 MHz] α [0, 58 106 s-1; 86,5 106 s-1] β [0,4 106 s-1; 117,0 106 s-1 ]

Cluster #3

Page 90

f [18,4 MHz; 25,5 MHz] α [1,8 106 s-1; 160,0 106 s-1 ] β [3,1 106 s-1; 130,0 106 s-1]

Guidelines for PD Detection

The three sensors detected cluster #1 which results from an internal insulation defect within the transformer. The highest sensitivity was achieved with the sensors connected to the capacitive taps. The three sensors also detected cluster #2 which is related to corona and surface PDs. Only the HFCT sensor, connected to the tank earth (PD_S3), was able to detect the pulses related to the disturbances generated by external power electronics. After opening of the transformer, traces of damage due to insulation overheating was observed (see Figure 106).

FIGURE 106: INTERNAL DEFECT DETECTED. INSULATION DAMAGE DUE TO HEATING

4.5.2 Case Study 2: Combination of conventional and UHF PD detection methods The transformer investigated was a 5/7 MVA ONAN/ONAF 66000/22000 V Dyn1 unit constructed in 1998 (Figure 107). It was initially put into service in an urban environment where it operated predominantly on a fixed tap and without any problems until removed from service in 2007 as part of a major upgrade project. Later in 2007, the transformer was relocated to a rural environment where it was used to replace a failed transformer. The unit was not heavily loaded and gave no indication of any problems until February 2010, when the result of the annual dissolved gas analysis (DGA) run on an oil sample indicated 5400 ppm of hydrogen. The dissolved gas ratios indicated that PD was taking place [62]. In order to better monitor what was occurring, an on-line DGA monitor was installed. Initially the monitor recorded a lower concentration of H2, but it was quickly realized that this was actually a false reading due to the fact that the detection limit for a dissolved hydrogen level of 3000 ppm had been exceeded and, counterintuitively, this was causing the monitor too read low. The transformer was then partially de-gassed. The results for the next three years are shown in Figure 108. As can be seen, the hydrogen and methane levels showed a steady increase, which seemed to indicate that PD activity was ongoing.

Page 91

Guidelines for PD Detection

FIGURE 107: 15/7 MVA, 66/22 kV TRANSFORMER

FIGURE 108: LONG-TERM ON-LINE DGA RESULTS OF 15/7 MVA, 66/22 kV TRANSFORMER

Two methods were recommended for PD measurements: the conventional method (according to IEC 60270) with sensors installed at the bushing taps (Figure 109), and an unconventional ultra-high frequency (UHF) method with a sensor placed inside the transformer tank (Figure 110). With the conventional method, the PD signal from each tap was synchronously acquired by a three channel acquisition unit. The center frequency of the digital band pass filter of the acquisition unit being selected to reach the best signal-to-noise ratio. To obtain more detailed information about the type and location of the insulation PD defects, unconventional UHF PD measurements in the frequency range between 100 MHz and 2 GHz with an antenna installed inside the oil drain valve were performed. The presence of external noise in this frequency range is low, and external radio or mobile phone signals are easily recognized and eliminated from the measurements. PD activity inside the bushing insulation and close to the end winding area is mostly detected with the conventional method, while the rest of the tank is covered by the UHF antenna. Even if the PD signal is measured in a different frequency range, the PRPD patterns obtained exhibit strong similarity and thus make recognition of the defect type easier. The signal detected by the UHF antenna was synchronized with the signal detected at the bushing taps. Furthermore, the pulses measured in the UHF range, mostly coming from internal PD activity, can trigger the

Page 92

Guidelines for PD Detection

start of conventional measurements. Thus a better separation between internal and external PD pulses can be obtained.

FIGURE 109: TAP SENSOR FOR PD MEASUREMENTS

FIGURE 110: UHF ANTENNA INSTALLED IN OIL DRAIN VALVE

The three-phase PD trend is presented in Figure 111. At each point indicated in the trend diagram, PRPD patterns and 3PARD [63] are available. Figure 112 depicts the PRPD patterns of the PD signal acquired by the three-channel synchronous system. They are complex patterns with several overlapping PD sources. In order to separate clusters of different PD sources, a synchronous multi-channel PD evaluation technique was applied. The 3PARD diagram (Figure 113) visualizes the relationship between amplitudes of a single PD pulse in one phase and its crosstalk-generated signals in the other two phases. By repetition of this procedure for a large number of PD pulses, PD sources within the test object as well as external noise sources appear as clearly distinguishable concentrations or ‘clouds’ of pixels in the 3PARD diagram. By examining individual clusters in the 3PARD diagram, a separation between noise and PD phenomena is possible [63], [64]. Figure 114 shows the transformation back to PRPD pattern from clusters 1 and 2. The patterns of clusters 1 and 3 appear to be generated by bubbles and surface discharge, with the highest amplitude in phase B (cluster 1) and phase A (cluster 3). Similar PRPD patterns were reported in [65]. The shape and phase position of the patterns of the clusters 2 (phase A) and 6 (phase C) may indicate partial discharge activity inside voids within the insulation system. The clusters 4 and 5 are generated by external interference.

FIGURE 111: THREE PHASE PD TREND OF 15/7 MVA, 66/22 kV TRANSFORMER

Page 93

Guidelines for PD Detection

FIGURE 112: THREE PHASE SYNCHRONOUS PRPD PATTERNS FROM 15/7 MVA, 66/22 kV TRANSFORMER

Two spectra of the signal were obtained by performing a frequency sweep (Figure 115). The upper spectrum is built up based on the maximum peak amplitudes of the time domain signal acquired at each value of the frequency during the sweep. The lower spectrum corresponds to their minimum amplitudes. Internal PD activity is always visible on the upper spectrum while external interferences (corona discharge, radio waves, GSM) are visible on both spectra.

Phase C

FIGURE 113: EQUIVALENT 3PARD DIAGRAM

Phase B

Phase A

FIGURE 114: INDIVIDUAL PRPD PATTERNS OF THE SELECTED CLUSTERS

Internal PD activity was identified in the frequency range from 450 MHz to 650 MHz. The PRPD pattern corresponding to a center frequency of 600 MHz is presented in Figure 116. The signal was synchronized with a 50 Hz signal taken from the measuring tap of phase A. It can be seen in Figure 114 that the phase of the voltage where the PDs occur is the one which is characteristic to internal discharges. Furthermore, it indicates a possible location of the PD activity, namely, in the vicinity of phase A. The PRPD patterns of frequencies between 1 GHz and 1.4 GHz were checked and no internal PD activity was found.

FIGURE 115: FREQUENCY SWEEP DIAGRAM

FIGURE 116: PRPD PATTERN

Page 94

Guidelines for PD Detection

4.5.3

Case Study 3: Combination of conventional and UHF partial discharge detection methods The transformer in question was a 130/130/100 MVA – 230/115/48 kV unit constructed in 1973 (Figure 117). The PD monitoring system was installed in 2013 on the 230 kV RBP bushings. For each bushing, one capacitive tap PD sensor with multiple redundant protection was installed (Figure 118) and connected synchronously to an acquisition unit. A UHF antenna was installed within the upper part of the transformer tank (Figure 119) and was first connected to the UHF down-converter and later to the acquisition unit.

FIGURE 117: 130/130/100 MVA – 230/115/48 kV TRANSFORMER

FIGURE 118: PD TAP SENSOR

FIGURE 119: UHF SENSOR PLACED BETWEEN PHASES V AND W

The three-phase PD trend is presented in Figure 120. For each point of the trend, PRPD patterns and 3PARD diagrams are available. Figure 121 depicts the PRPD patterns of the PD signal acquired in April 2014. The patterns are complex, with several PD sources overlapping each other. In order to separate clusters of different PD sources, a synchronous multi-channel PD evaluation technique (3PARD) is applied [63]. The back transformation to PRPD patterns of the clusters 1 and 2 is presented in Figure 121. The pattern of cluster 1 indicates the presence of PD activity inside gas cavities. The highest amplitude of the signal is detected at the phase V, but signal cross-talk to phases U and W is also visible. The PRPD pattern of cluster 2 appears to be generated by PD surface discharge in the vicinity of the phase W. The other clusters visible in the 3PARD diagram are generated by external interference.

Page 95

Guidelines for PD Detection

3PARD diagram

3-phase PRPD patterns

FIGURE 120 THREE PHASE PD TREND FROM THE 130/130/100 MVA – 230/115/48 kV TRANSFORMER

FIGURE 121: SEPARATION OF PD SOURCES USING 3PARD FROM THE 130/130/100 MVA – 230/115/48 kV TRANSFORMER

In order to gain information about the frequency content of the acquired PD signal, a frequency sweep is performed. Both off-line and on-line sweeps are presented in Figure 122 and Figure 123, respectively. An offline frequency sweep was performed during the installation of the monitoring system, while the transformer was de-energized. Such an off-line spectrum provides important information about the sources of interference produced by other equipment in the substation and vicinity. These sources must be discarded when the analysis of the on-line detected PD signal is performed. The increasing upward trend line of the UHF PD signal and PRPD pattern of the signal at 560 MHz are shown in Figure 124. An on-line frequency sweep was performed and internal PD activity was identified in the frequency range from 450 to 650 MHz (Figure 123). The signal detected during the on-line monitoring was synchronized with a voltage signal taken in phase U. The PRPD patterns at frequencies above 1 GHz were also investigated and no PD activity was identified.

Page 96

Guidelines for PD Detection

FIGURE 122: ON-LINE FREQUENCY SWEEP DIAGRAM

FIGURE 123: ON-LINE FREQUENCY SWEEP DIAGRAM

FIGURE 124: TREND DIAGRAM AND PRPD PATTERNS OF THE SIGNAL DETECTED IN THE UHF RANGE

Three independent sources of the received PD signal are necessary to build a 3PARD diagram. In Figure 125, such a diagram was built up using the UHF signal together with the two conventional signals measured at the bushings of phase V and W. After the cluster separation, the PD activity was confirmed and the PRPD patterns were identified from the clusters surrounded by the red rectangles shown in Figure 125. They have similar shape and voltage phase position to those presented in Figure 121. Detecting the PD activity in the UHF range is a good indication that the PD source is located inside the transformer tank and not inside the bushing.

Page 97

Guidelines for PD Detection

FIGURE 125: PD SOURCES SEPARATION COMBINING THE CONVENTIONAL AND UNCONVENTIONAL MEASUREMENTS

PD activity was detected at phases V and W of the transformer by conventional and UHF measurements. The evolution of the PD activity is presented in Figure 126. It can be noticed that the amplitude of the PD pulses increased by a factor of 3 over time. The operating conditions of the transformer were similar and did not influence the readings of the PD pulses amplitude. Dissolved Gas Analysis (DGA) was also performed (Table 8). The increase of the H2 and CH4 concentrations confirms the presence of the PD activity. The increase of the CO concentration indicates paper deterioration, probably as an effect of the PD. TABLE 8: FAULT GASES CONCENTRATIONS FROM LAB TESTS Sample date

H2

CO

CO2

CH4

C2H2

C2H4

C2H6

N2

O2

15-04-2014

433

416

3016

115

9.1

92

15.4

33680

1100

15-05-2014

966

835

5952

226

21.4

179

31.5

60120

860

12-06-2014

1212

808

5797

225

20.5

171

30.4

65440

1390

Phase V

Phase W

FIGURE 126: INCREASE OF THE AMPLITUDE OF THE PD PULSES DETECTED AT THE PHASES V AND W

In order to obtain a more precise localization of the PD source, acoustic PD measurements were performed on the1st of July. Figure 127 shows a preliminary position of the sensors. After the repositioning of acoustic sensors (S1…S4), the coordinates of the PD source were determined (Figure 128 – red dot). According to the results of the acoustic method, the PD activity takes place at the exit leads of the HV winding of phase V.

Page 98

Guidelines for PD Detection

For the next step, the transformer was de-energized and the bushing of phase V was dismantled (Figure 129). Internal inspection was performed using an endoscope (Figure 130) and the PD activity around the phases V and W was confirmed – Figure 131 and Figure 132 respectively.

FIGURE 127: ACOUSTIC PD LOCALIZATION

FIGURE 129: DISMANTLED BUSHING OF PHASE V

FIGURE 128: PD SOURCE LOCALIZATION IN THE VICINITY OF THE PHASE V

FIGURE 130: ENDOSCOPIC INSPECTION

Page 99

Guidelines for PD Detection

FIGURE 131: PD TRACKS AT PHASE V

FIGURE 132: PD TRACKS AT PHASE W

4.5.4 Case Study 4: Combination of conventional method with acoustic method A 16 MVA, 150/20.8 kV power transformer of dimensions 4.25 m x 1.36 m x 2.15 m was subjected to dielectric routine tests (Figure 133) [66]. The PD level has been measured using the conventional method by connecting a synchronous three channel PD acquisition unit to the capacitive taps of the transformer HV bushings (Figure 133). After calibration, the measurements were performed at a center frequency of 500 kHz with a 650 kHz bandwidth. The testing voltage has been linearly increased, and at 85 kV, a PD cluster characterized by about 4 nC of apparent charge has been detected in phase W (Figure 134). The same type of pattern but of lower apparent charge magnitude was measured in the other two phases. Due to the high PD level on phase W, it is assumed that the PD is located near or inside the winding of phase W and the PD activity on phase U and V is only measured due to inductive and capacitive coupling between the phases. For more accurate localization of the PD source, acoustic measurements were undertaken. For the acoustic measurements, four acoustic emission sensors with resonant frequency of 150 kHz were placed in different location on the exterior of the transformer tank. Using the difference in arrival time of the acoustic PD signal at multiple sensors, the appropriate software computes the location of the PD source [66], [59]. These time differences are the only available data from the acoustic method, and their accuracy determines the precision with which the PD defect(s) can be located. Higher accuracy of a defect’s localization can be obtained by triggering the acoustic signals with conventional PD signal. This means that the acquisition of the acoustic signals is started only if the electrical PD signal is detected. Through stepwise repetition of the measurement, the acoustic sensors were placed progressively closer to the assumed location of the defect. The signal acquired by each sensor is presented in Figure 135. Arrival times and voltage amplitudes indicate a defect location in the upper part of the low voltage side of the winding of phase W. The calculation of the defect location resulted in the coordinates x = 2.6 m, y = 1.1 m and z = 1.4 m. This location is related to the position of a shield electrode in the low voltage side of the winding of phase W. All three low voltage phases were energized, one after the other, and acoustic PD was only detected when phase W was energized. After opening the transformer tank, a foreign copper wire was found at the indicated location.

Page 100

Guidelines for PD Detection

FIGURE 133: MVA 150/20.8 kV POWER TRANSFORMER AND PD DECOUPLING AT THE BUSHING TAP

FIGURE 134: CONVENTIONAL PD MEASUREMENTS AT U= 85 kV IN PHASE V (UPPER PATTERN) AND IN PHASE W (LOWER PATTERN)

Page 101

Guidelines for PD Detection

FIGURE 135: ACOUSTIC MEASUREMENTS AND DEFECTS LOCATION

The drawing on the left shows the circular intersections which result from the automatically computed arrival times for all three sensors; these circles indicate the probably location of the PD source.

4.5.5 Case Study 5: Localization of PD by acoustic and UHF-measurement Because of increasing gas-in-oil values, a 333 MVA, 400/220 kV single-phase autotransformer was tested onsite and on-line for PD [67]. The high noise level at site strongly disturbed the conventional PD measurements made according to IEC 60270 at frequencies lower than 1 MHz. In this case the main source of the noise was the 400 kV bus bar above the transformer, which was producing audible corona discharges. Consequently, UHF PD measurements for PD detection in combination with acoustic measurements for PD location were performed in order to get reliable results. In this case, the transformer contained three oil filling valves and thus three identical UHF Sensors were able to be installed, one in each valve. Figure 136 shows the positions of the UHF sensors (UHF 1 – UHF 3). Two sensors are opposite to each other at the top of both front ends of the tank and the third (UHF 3) is located at the bottom in the middle of the transformer side.

Page 102

Guidelines for PD Detection

FIGURE 136: 333 MVA TRANSFORMER SHOWING POSITIONS OF UHF SENSORS AND ACOUSTIC SENSORS [67]

First, the so-called dual port performance check was done [68]. Artificial UHF impulses were injected at each sensor with a signal generator (60 V at 50 Ω). It was not possible to detect the artificial impulses at any combination of emitting and receiving sensor. The manufacturer’s design drafts explain this strong damping of the UHF signals: tubes are installed behind the oil filling valves in order to direct the oil flow around the winding. According to the unsuccessful dual port performance check, it could be stated that the sensors are electromagnetically decoupled from each other and might also be shielded against UHF pulses from internal PDs. A further explanation might be that the maximum signal generator output voltage of 60 V is not sufficient to transmit UHF waves through that particular transformer. Nevertheless, UHF signals from internal sources were detectable at all three sensors at nominal voltage, i.e. the internal PD was producing UHF signals with higher energy content than the applied artificial impulses. It can be concluded that the Dual Port performance check is thus just a worst-case estimation of the sensitivity. But even though the performance check was not successful, sensitive UHF measurements were still possible. Frequency analysis of the signals measured from the installed UHF probes revealed the internal shielding characteristic of the tank, see Figure 137. The signals exhibit frequency content of up to 1 GHz, as emitted by a broadband emitter of UHF waves, similar to and typical of internal PD in oil. External disturbing sources would have been narrow band, e.g. at around 500 MHz for digital video broadcasting or around 900 MHz or 1800 MHz for GSM I and II, respectively, all of which are modulated carrier frequencies. In Figure 137 the unamplified measured signals of the UHF probes are shown with their frequency analyses (FFT).

Page 103

Guidelines for PD Detection

FIGURE 137: MEASURED PROPAGATION TIME DIFFERENCES BETWEEN THREE UHF PROBES

Shown are the time-domain signals from the 3 UHF probes (unamplified) and their associated frequency-domain spectra. The latter exhibit distinctly broadband characteristics, indicating PD, without external EMI.

Propagation time differences in the range of nanoseconds (ns) are recognizable between the signals. Taking the propagation time differences caused by different lengths of measuring cables into account, a first estimation of the geometric PD location pointed to the on-load tap changer (OLTC) on the left hand side of the transformer. That is supported by the measured UHF amplitudes of the three UHF probes. The probe nearest to the tap changer (probe UHF 2) show the highest output reading of 10 mV, whereas the other probes did not reach more than 5 mV. Therefore probe UHF 2 was used for triggering and determining the starting time in order to calculate the propagation time differences. PD also produces acoustic waves, which are measured with piezo-electric sensors installed on the outer tank wall. Their measurement frequency range is between 50 and 200 kHz. Due to comparatively high acoustic signal attenuation within the solid and liquid insulation material along with intervening structures inside the transformer, sensitive acoustic measurements are hard to achieve [57]. Additionally, acoustic signals of PD might be obscured by higher amplitude signals produced by ambient mechanical noise and inherent noises within the transformer itself (core noise). Summarizing, exclusive acoustic PD measurement is only useful to a limited extent. To increase the sensitivity of acoustic measurements the method is combined with the more sensitive UHF measuring method. UHF signals are used as trigger signals in order to activate the acoustic measurement during the occurrence of UHF PD signals. By using averaged signals (averaging in time domain), the acoustic PD pulses add up constructively whereas the white background noise is averaged to zero. Thus the signal-to-noise ratio (SNR) of the acoustic signals is increased by implementing this technique. The UHF measuring method is based on picking up electromagnetic waves which radiate in all directions from the PD source, at approximately two-thirds of speed of light, inside the transformer. Therefore, for the purpose of acoustic location, the UHF signals are detected at essentially the same time the PDs occur,

Page 104

Guidelines for PD Detection

enabling the use of the UHF signals as a trigger for the acoustic measurements. Conversely, the speed of acoustic waves is 1400 m/s [54], producing transit times within the range of milliseconds. Geometrical distances between sensors and the source of PD (calculated from the propagation times of the individual acoustic sensors) result in a spherical region inside the transformer. With at least three acoustic sensors and the corresponding propagation times, it is possible to calculate the intersection of the spheres and thus to determine the PD location. It must be assumed that the acoustic waves travel directly along the line-of-sight path from the PD source through the oil and through the steel tank to the sensor without any reflections. However, the location process has also to deal with acoustic waves travelling faster through the tank wall (whose propagation velocity is much higher) than through the oil. The propagation times of the acoustic signals can be computed objectively with the help of the Hinkley criterion [67]. It is based on the signal energy of the measured signal and results in an absolute minimum for the signal starting point. Figure 136 depicts the positions of the acoustic sensors used (A1 – A6). As an example, Figure 137 shows the measured and averaged acoustic signals of the acoustic sensors A5 and A6. Averaging was performed with approx. 100 signals. Although averaging was used, the SNR was quite low, thus the determination of the propagation times was only possible by application of the Hinkley criteria. The respective propagation times are tA5 = 1.03 ms and tA6 = 2.07 ms. Based on the determined propagation times, the supposed position of the PD source is located in the vicinity of the tap changer (Figure 138) [68]. Geometrical inaccuracy is within the range of approx. 40 cm on all space axes. This inaccuracy is caused by using different combinations of propagation time differences and different location methods [59].

FIGURE 138: DETERIORATED PAPER INSULATION ON LEADS AT THE TAP CHANGER

After transportation of the transformer to a factory, the location result was confirmed by an IEC triggered acoustic measurement in the test area, so the transformer was detanked for inspection and repair. The visual inspection of the tap leads at the tap changer region confirmed the location results and revealed deteriorated paper insulation, see Figure 138. After repair of the affected leads, the transformer passed the acceptance test without any indication of PD activity and was put back into service. 4.5.6 Case Study 6: Monitoring by UHF PD-measurement Online monitoring of power transformers, which supports established diagnosis methods, is steadily gaining acceptance. Continuous measurement using trend analysis allows the detection and tracing of undesirable changes at an early state. For PD, ultra-high frequency monitoring represents an advantageous technique, because the measurement is done inside the tank and is thus much less sensitive to external noise. Additionally, it is applicable to transformers in service. The considerable amount of data generated requires appropriate evaluation; partially automated analysis is inevitable. A 50-year-old unit generator transformer with a rated voltage of 110/10 kV and a rated power of 120 MVA was monitored [69]. An online UHF PD measurement system recorded data. UHF PD signals were measured with approximately 35 dB amplification and a bandwidth of 9 MHz at a center frequency of 505 MHz. The noise level of the system was between 1.5 and 2 mV. Therefore, all PD below 2 mV were discarded. Phase L1 was used for phase correlation. Due to the fact the generating unit is only in operation on demand, the transformer is not continuously in service. Measurements

Page 105

Guidelines for PD Detection

are available for approximately 65 days from 2009 until 2012. This case study presents an approach using phase-resolved PD pattern analysis. Typical patterns from known PD sources are reduced to an abstracted shape which uniquely characterizes the form of the PD source. This so called template is compared to measured PRPD patterns gained from the monitoring data. Comparison between pattern and template is calculated by 2dimensional normalized cross-correlation algorithm. Source tracking over time is evaluated using continuous correlation. By introducing a set of templates for correlation, the progress of individual PD sources is determined. Cross-correlation is an algorithm used for pattern recognition within an image. The higher the similarity between two images, the higher is their correlation factor. In this contribution, the normalized cross-correlation is used providing values of correlation coefficients between -1 and +1 for each matrix element. Thus, cross-correlations of different images become comparable. A correlation coefficient of 1 indicates an exact match of the template (this never occurs in practical pattern analysis). -1 represents an area where image intersection and template are opposed (negative image, also never occurs in practical pattern analysis). Figure 139 shows three typical PD patterns of this transformer.

FIGURE 139: UHF PRPD PATTERNS 1 – 3 [69]

Patterns should be traceable over time. Therefore, the constant PD data stream is divided into segments with constant duration. For each segment, the PRPD pattern is generated and then cross-correlated with a template. Determining an adequate duration period depends on the behavior of the source over time. Pattern 1 from Figure 139 shows high volatility. Therefore, duration is set to 1 minute. Correlation is calculated with the small template from Figure 139. The maximum value of the correlation matrix represents the correlation coefficient for the time segment. An example is plotted in Figure 140. The correlation coefficient is shown in red, aligned to the left axis. For comparison, the number of PDs per minute is also plotted (black, right axis).

Page 106

Guidelines for PD Detection

FIGURE 140: NUMBER OF PD PER MINUTE AND CORRELATION COEFFICIENT [69]

Figure 141 shows a graph of the frequency of occurrence for each pattern. The colors indicate each time the pattern types shown in Figure 139 were detected by cross-correlation. Pattern 1 is only present at 15% of the time showing intermitted behavior. Pattern 2 has a higher rate of appearance and can be detected at 40% of the measurement time. Pattern 3 is the dominating source which can be detected 60% of the time over the entire period.

FIGURE 141: RESULTS OF RECOGNITION ALGORITHM OF DETERMINED PATTERNS 1 - 3 [69]

PD data can be evaluated using normalized cross-correlation for PRPD pattern recognition. Therefore, the characteristic shape of a pattern is defined by a template matrix. The data is segmented into constant periods of time which are used to generate PRPD patterns. Each pattern is cross-correlated with the template. The result of a correlation is a matrix whose coefficients define the similarity between template and pattern. The quality of correlation depends on several parameters. Filtering can improve correlation outcome; results strongly depend on the chosen filter method and its parameterization. If preconditions are met, a threshold level can be defined. The maximum value of the resulting matrix is used as trigger indicating the presence of the determined pattern. Its x-coordinate represents the phase angle the pattern occurs. Combination of both allows long term tracking of PD and thus its behavior over time. Comparison of pattern shape and phase angle with known PD sources from literature can be made. In this contribution, only the relative phase can be considered due to the UHF measurement. Nevertheless, the cross-correlation method presented here can be applied to any PD measurement method which arrays the PD signal pulses against the phase angle of the applied voltage. The essential benefit of the method presented is its application on large PD datasets, e.g. from monitoring systems. In the case presented, 65 days of monitoring data was evaluated. Therefore, three PRPD patterns Page 107

Guidelines for PD Detection

being typical for the determined transformer are evaluated. Using cross-correlation, it is possible to track patterns over the monitoring period in terms of their appearance and their phase positions.

4.6

Advantages and disadvantages of different methods

In Table 9, UHF and conventional (IEC 60270) measurement methods are compared with respect to specifically defined and chosen topics. TABLE 9: COMPARISON OF UHF AND CONVENTIONAL IEC 60270 MEASUREMENT METHOD

IEC 60270

UHF

1) Actual PD source level

pC

not known

mV

Not known

2) Attenuation of coupling path

Low pass filtering of winding & ratio of internal capacitances

Not known

Damping of EM waves

Not known (but very low)

3) Sensor sensitivity

Ratio coupling capacitor specimen capacitance

Calibrated

Antenna factor

Not yet calibrated

Installed sensor

Sensitivity check in preparation by WG A2/D1.51

< 100 pC of apparent charge

@1,2xUm/√3

-

-

4) Acceptance test level

4.7

Summary and important aspects

1) It is emphasized that when making PD measurements using unconventional methods, it is not possible to quantitatively correlate or calibrate the received signal level in terms of PD charge(pC or nC). 2) As also shown through practical measurements on a winding model, conventional PD measurment according to IEC 60270 may also be affected owing to damping (attenuation) caused by the low pass behavior of the winding. Therefore the calibration routine may not eliminate all influences. It is beneficial if lower frequencies are used during the conventional PD measurement, which is also noted in IEC 60270 and IEC 60076-3. Nevertheless it is well known that conventional PD measurement is also often done at higher frequencies (e.g. to improve SNR); in such cases it is important that the low pass filtering effect of the windings should not be neglegted. 3) It is well known that PD events in insulation oil exhibit steep rise-times and thus their frequency content extends to very high frequencies. In actual research results (by using state-of-the-art extra high frequecy measuring devices) the rise time was measured down to several 10 ps. 4) Unconventional UHF PD measurement is beneficial because the transformer vessel acts as a Faraday shield for ultra high frequencies, so that external corona noise and other sources of external electromagnetic intereference (EMI) is filtered out, although some external signal can still enter via the bushings. UHF PD measurements on transformers can often result in high SNR values, whereby damping and sensor effects must always be considered. Page 108

Guidelines for PD Detection

5) Unconventional acoustic PD measurement is well known for localization of PD event using several sensors and measuring the time difference between the acoustical signals. However, the SNR may in different cases not be sufficiently high enough to be able to accurately estimate the exact arrival times of the incoming signals. By using averaging techniques together with a reference signal (UHF or conventional PD measurement signal) for a trigger reference, the SNR of the acoustical signals can be significantly improved and thus their arrival times can be more accurately estimated, leading in turn to more accurate localization of the discharge site. 6. Again, it is good to follow general best practise procedures when carrying out all PD measurements, i.e. extensively documenting test set-ups and taking detailed notes and so on. This is especially important when employing unconventional methods because the test set-ups are not industry standardized, but typically assembled according to the needs of the user and the requirements of the equipment at hand. In this chapter, an overview about PD-measurements on transformers employing both conventional and unconventional methods is given and several practical examples are presented showing the application of different measuring systems working on different measurement principles. The progress in PD measurements on transformers is currently under discussion in CIGRE WG D1.29. The attempt has been made to define possible criteria how to distinguish between dangerous and less dangerous PD-sources in oil-impregnated electrical insulation system of power transformers. Despite a large number of practical examples showing the identification and localization of PD-sources, unambiguous identification of PDsources in the electrical insulation system of transformers together with a reliable estimate of the actual risk they present remains a topic for further research.

Page 109

Guidelines for PD Detection

5 REFERENCES [1] L. Niemeyer, “A generalized approach to partial discharge modelling,” IEEE Trans. Diel Elec Insul., Vols. 2, pp. 510-28, 1995. [2] U. Schichler et al., Risk Assessment on Defects in GIS Based on PD Diagnostics (TB525), CIGRÉ AG D1.03, 2013. [3] CIGRE WG 15.03, “Effects of Particles on GIS and the Evaluation of Relevant Diagnostic Tools,” in CIGRE Session, Report 15-103, Paris, 1994. [4] H. Okubo, N. Hayakawa and A. Matsushita, “The Relationship Between Partial Discharge Current Pulse Waveforms and Physical Mechanisms,” IEEE Electrical Insulation Magazine, Vols. Vol. 18 No. 3 pp. 38-45, 2002. [5] E. Gulski et al., “Guidelines for Unconventional Partial Discharge Measurements (TB444),” CIGRÉ WG D1.33, Paris, 2010. [6] S. Boggs, “Electromagnetic Techniques for Fault and Partial Discharge Location in Gas-Insulated Cables and Substations,” IEEE Trans. on Power Apparatus and Systems, Vols. PAS-101, No. 7, pp. 1935-1941, 1982. [7] B. Hampton and R. Meats, “Diagnostic Measurements at UHF in Gas Insulated Substations,” IEE Proceedings C: Generation, Transmission and Distribution, Vols. 135, Issue 2, pp. 137 – 145, 1988. [8] J. Pearson, 0. Farish, B. Hampton, M. Judd, D. Templeton, B. Pryor and I. Welch, “Partial Discharge Diagnostics for Gas Insulated Substations,” IEEE Transactions on Dielectrics & Electrical Insulation, vol. 2 No. 5, 1995. [9] M. Judd, O. Farish and B. Hampton, “The Excitation of UHF Signals by Partial Discharges in GIS,” IEEE Transactions on Dielectrics, Vols. 3, pp.213-228, 1996. [10] CIGRE JWG 33/23.12, “Insulation Co-ordination of GIS: Return of Experience, On Site Tests and Diagnostic Technique,” Electra No.176, pp. 67-97, February 1998. [11] CIGRE WG 15.03, “Diagnostic Methods for Gas Insulating Systems,” in Report 15/23-01, CIGRE Session, Paris, 1992. [12] A. Reid and M. Judd, “Ultra-wide bandwidth measurement of partial discharge currents in SF6,” Applied Physics, 2012. [13] IEC60270, “High-voltage test techniques – partial discharge measurements,” 2000. [14] R. Pietsch, W. Hauschild et al., “High-Voltage On-Site Testing with Partial Discharge Measurements (TB502),” CIGRÉ WG D1.33, Paris, 2012. [15] A. Bargigia, W. Koltunowicz and A. Pigini, “Detection of Partial Discharges in Gas Insulated Substations,” IEEE Transactions on Power Delivery, Vols. 7, No. 3, pp. 1239-1249, 1992.

Page 110

Guidelines for PD Detection

[16] M. Leijon and A. Vlastos, “Pattern Recognition of Free Metallic Particles Motion Modes in GIS,” in IEEE Inter. Symp. Elec. Insul., Boston, 1988. [17] L. Lundgaard, “Partial Discharge - Part XIII: Acoustic Partial Discharge Detection - Fundamental Considerations,” IEEE Elec Insul Mag, Vols. Vol.8, No.4, 1992. [18] L. Lundgaard, “Partial Discharge - Part XIV: Practical Application,” IEEE Elec Insul Mag., Vols. 8, No.5, 1992. [19] L. Lundgaard, “Particles in GIS Characterization from Acoustic Signatures,” IEEE Transactions on Dielectrics & Electrical Insulation, vol. 8 No.6, 2001. [20] IEC62478, High-voltage test techniques – measurement of partial discharge by electromagnetic and acoustic methods (publication pending). [21] CIGRÉ TF 15/33.03.05, “Sensitivity Verification for the UHF and Acoustic Method,” Electra, No. 183, pp. 75-87, April 1999. [22] S. Hoek and S. Neuhold, "Tuned Medium-Band UHF PD Measurement Method for GIS," in Cigre Session, Paris, France, 2012. [23] Agilent, “Spectrum Analyzer Basics, Application Note 150,” Agilent Technologies Inc.,5952-0292, CA, USA, 2004-2006. [24] M. Hikita, S. Ohtsuka, T. Hoshino, S. Maruyama, G. Ueta and S. Okabe, “Propagation properties of PDinduced electromagnetic wave in GIS model tank with T branch structure,” IEEE Trans Dielectrics & Electrical Insulation, Vols. 18, no 1, pp 256 – 263, 2011. [25] M. Hikita, S. Ohtsuka, J. Wada, S. Okabe, T. Hoshino and S. Maruyama, “Study of partial discharge radiated electromagnetic wave propagation characteristics in actual 154 kV model GIS,” IEEE Transactions Dielectrics & Electrical Insulation, Vols. 19, No. 1, pp. 8-17, 2012. [26] S. Neuhold, T. Heizmann, R. Bräunlich, D. Köchli, U. Riechert and C. Dehne, “Experience with UHF PD detection in GIS using external capacitive sensors in windows and disk-insulators,” in XVth International Symposium on High Voltage Engineering (ISH), Ljubliana, 2007. [27] S. Hoek, U. Riechert, T. Strehl, S. Tenbohlen and K. Feser, “A New Procedure for Partial Discharge Localization in Gas-Insulated Switchgear in Frequency Domain,” in 14th ISH, Beijing, China, 2005. [28] R. Pietsch, “Sinnvoller Einsatz von UHF-Teilentladungsmessungen bei Entwicklungs- und VorOrt-Prüfungen,” in Haefely Symposium, Stuttgart, 2000. [29] F. Wester, Condition assessment of power cables using partial discharge diagnosis at damped AC voltages, Dissertation TU Delft, Optima Grafische Communicatie, ISBN:90-8559-019-1, 2004. [30] CIGRE WG 21.09, “After Laying Tests,” Electra, no. 166, 1996. [31] M. Wild, S. Tenbohlen, E. Gulski and R. Jongen, “Power cable modeling for PD pulse propagation and sensitivity,” in IEEE Electrical Insulation Conference (EIC), 2013. [32] M. Wild, S. Tenbohlen, E. Gulski, R. Jongen and F. d. Vries, “Practical aspects of PD localization for long

Page 111

Guidelines for PD Detection

length power cables,” in IEEE Electrical Insulation Conference (EIC), 2013. [33] F. Garnacho, M. Sánchez-Urán, J. Ortego, F. Álvarez and A. González, “Control of insulation condition of smart grids by means of continuous PD monitoring,” in CIRED, Paper 1103, Stockholm, Sweden, 2013. [34] Chan, Duffy, Hiivala and Wasik, “PD Testing of Solid Dielectric Cable,” IEEE Electrical Insulation Magazine, Vols. 7, No.5, 1991. [35] R. Plath et al., “PD Measurements on Extra High Voltage Cable Accessories During Commissioning Exeptionally Long 400 kV XLPE Insulated Cable System,” in International Symposium on High Voltage Engineering (ISH), 2007. [36] F. Garnacho, M. Sánchez-Urán, J. Ortego, F. Álvarez, D. Prieto, J. Vallejo and M. Jiménez, “Experiences of PD Measurements on HV Cable Systems Installed,” in CIGRE Session, Paper B1-206, Paris, 2014. [37] J. Fuhr, “Procedure for Identification and Localization of Dangerous PD Sources in Power Transformers,” IEEE Trans. on Dielectrics and Electrical Insulation, pp. 1005-1014, October 2005. [38] S. Ohtsuka et al., “Measurement of PD Current Waveforms in SF6 Gas with a Super High Frequency Wide Band Measuerement System,” in 16th International Symposium on High Voltage Engineering (ISH), 2011. [39] G. Cleary and M. Judd, “UHF and current pulse measurements of partial discharge activity in mineral oil,” IEE Proc.-Sci. Meas. Technol., vol. 153, no. No.2, 2006. [40] S. Ohtsuka et al., “PD Current Pulse Waveforms of Environmental Friendly Gases Measured with SHF_PDPW System and the Applicability of the UHF Method,” in IEEE International Conference on Condition Monitoring and Diagnosis (CMD), Indonesia, Bali, 2012. [41] M. Fukuzaki, S. Ohtsuka et al., “Frequency Bandwidth Dependence of PD current Waveforms in Transformer Oil Measured with the SHF_PDPW system,” in IEEE International Conference on Condition Monitoring and Diagnosis (CMD), paper G-22, Indonesia, Bali, 2012. [42] M. Fukuzaki, S. Ohtsuka et al., "Sophisticated Measurement of PD Current Pulse in Insulation Oil and the Effects of Degassing Treatment on the Waveforms with the SHF_PDPW System," in 18th International Symposium on High Voltage Engineering (ISH), PD-44, 2013. [43] S. Okabe, G. Ueta and H. Wada, “Partial discharge signal propagation characteristics inside the winding of gas-filled power transformer - study using the equivalent circuit of the winding model,” IEEE Transactions on Dielectrics and Electrical Insulation, pp. 1668-1677, 2011. [44] M. Siegel and S. Tenbohlen, “Comparison Between Electrical and UHF PD Measurement Concerning Calibration and Sensitivity for Power Transformers,” in CMD 2014, International Conference on Condition Monitoring and Diagnosis (CMD), Paper No. OB1-02, Jeju, Korea, 2014. [45] M. Judd, “Partial Discharge Monitoring for Power Transformers using UHF Sensors Part 2: Field Experience,” IEEE Electrical Insulation Magazine, vol. 21, no. 3, 2005. [46] S. Coenen, Measurement of Partial Discharges in Power Transformers Using Electromagnetic Signals, PhD Thesis, Universität Stuttgart, Germany, 2012. [47] A. Pfeffer, S. Tenbohlen and S. Kornhuber, “Influence of PD Location and Frequency Ranges on measured

Page 112

Guidelines for PD Detection

Apparent Charges,” in International Symposium on High Voltage Engineering (ISH), Hannover, Germany, 2011. [48] S. Okabe, G. Ueta, “Partial discharge criterion in AC test of oil-immersed transformer and gas-filled transformer in terms of harmful partial discharge level and signal transmission rate,” IEEE Transactions on Dielectrics and Electrical Insulation, pp. 1431-1469, 2012. [49] S. Coenen, M. Siegel, G. Luna, S. Tenbohlen, “Parameters influencing Partial Discharge Measurements and their Impact on Diagnosis, Monitoring and Acceptance Tests of Power Transformers,” in Cigre Colloquium, Paris, France, 2016. [50] S. Coenen, S. Tenbohlen, S. Markalous and T. Strehl, “Fundamental Characteristics of UHF PD Probes and the Radiation Behaviour in Power Transformers,” in ISH, Cape Town, South Africa, 2009. [51] M. Siegel, M. Beltle and S. Tenbohlen, “Characterization of UHF PD Sensor for Power Transformers using an Oil-filled GTEM Cell,” IEEE Transactions on Dielectrics and Electrical Insulation, p. (to be published), June 2016. [52] M. Siegel, S. Tenbohlen and S. Kornhuber, “Langzeitüberwachung von Leistungstransformatoren: Teilentladungsmonitoring von Leistungstransformatoren mit der UHF-Methode,” Elektrizitätswirtschaft (ew), Heft 25, Jg. 111 2012. [53] C57.127 IEEE Guide for the Detection and Location of Acoustic Emissions from Partial Discharges in OilImmersed Power Transformers and Reactors, New York, USA: The Institute of Electrical and Electronics Engineers, 2007. [54] E. Howells and E. Norton, “Parameters affecting the velocity of sound in oil,” IEEE Trans. Power App. Syst., Vol. 103, pp. 1111-1115, 1984. [55] M. Beyer, H. Borsi and M. Hartje, “Some aspects about possiblities and limitations of acoustic PD measurements in insulating fluids,” in 5th International Symposium on High Voltage Engineering (ISH), Braunschweig, Germany, 1987. [56] E. Großmann, Akustische Teilentladungsmessung zur Überwachung und Diagnose von Öl/Papier-isolierten Hochspannungsgeräten, PhD Thesis Universität Stuttgart, Germany, 2002. [57] S. M. Markalous, S. Tenbohlen and K. Feser, “Detection and Location of Partial Discharges in Power Transformers Using Acoustic and Electromagnetic Signals,” IEEE Trans. Dielectr. Electr. Insul., Vol. 15, pp. 1576-1583, 2008. [58] L. E. Lundgaard, W. Hansen and K. Dursun, “Location of discharges in power transformers using external acoustic sensors,” in Intl. Symposium on High Voltage Engineering (ISH), New Orleans, USA, 1989. [59] S. M. Markalous, Detection and Location of Partial Discharges in Power Transformers Using Acoustic and Electromagnetic Signals, Stuttgart: PhD Dissertation, Universität Stuttgart, 2006. [60] F. Garnacho and M. Sánchez-Urán, “PD Monitoring System for HV Substations,” in CIGRE Session, Paris, France, 2014. [61] F. Garnacho, M. Sánchez-Urán, J. Ortego, F. Álvarez, O. Perpiñan, E. Puelles, R. Moreno, D. Prieto and D. Ramos, “New Procedure to Determine Insulation Condition of High Voltage Equipment by Means of PD

Page 113

Guidelines for PD Detection

Measurements in Service,” in Cigre Session 2012, Paper D1-309, Paris, 2012. [62] L. Badicu, W. Koltunowicz, A. Piccolo, A. McGuigan and C. Feely, “Monitoring of a Distribution Transformer at Winchelsea Substation,” in Proceedings of TechCon 2014, Sydney, Australia, 2014. [63] W. Koltunowicz and R.Plath, “Synchronous Multi-Channel PD Measurements,” IEEE Transactions on Dielectrics and Electrical Insulation, Vols. Vol. 15, No. 6, p. 1715-1723, 2008. [64] K. Rethmeier, M. Krüger, A. Kraetge, R. Plath, W. Koltunowicz, A. Obralic and W. Kalkner, “Experience in on-site partial discharge measurements and prospects for PD monitoring,” in Proceedings of CMD 2008,, Beijing, China, 2008. [65] A. Carlson, J. Fuhr, G. Schemel and F. Wegscheider, Testing of Power Transformers – Routine tests, Type Tests and Special Tests, Zürich, Switzerland: 1st Edition, published by Pro Print, 2003. [66] S. Hoek, A. Kraetge, O. Kessler and U. Broniecki, “Time-based partial discharge localization in power transformers by combining acoustic and different electrical methods,” in International conference on Condition Monitoring and Diagnosis (CMD), Bali, Indonesia, 2012. [67] S. Coenen and S. Tenbohlen, "Location of PD Sources in Power Transformers by UHF and Acoustic Measurements," IEEE Transactions on Dielectrics and Electrical Insulation (TDEI), no. Vol. 19, Issue 6, pp. pp. 1934-1940, 2012. [68] S. Coenen, S. Tenbohlen, S. M. Markalous and T. Strehl, "Performance Check and Sensitivity Verification for UHF PD Measurements on Power Transformers," in Proceedings 15th Int’l. Symposium on High Voltage Engineering (ISH), Ljubljana, Slovenia, 2007. [69] A. Müller, M. Beltle, M. Siegel and S. Tenbohlen, “Assessment of UHF PD Monitoring Data by Means of Pattern Recognition,” in 18th International Symposium on High Voltage Engineering, ISH 2013, Paper OF405, Seoul, South-Korea, 2013.

Page 114