On-Line Partial Discharge Testing of Transformers Presented by: High Voltage Partial Discharge Ltd Manchester, UK [email protected] Fire at 132 kV Transformer Contents • Partial discharge measurements • Types of transformers • Sensors suitability • Signal detection • On-line PD measurement technique • Data Analysis • Case Studies Factors Affecting On-line PD Testing of HV Transformers • Generally not usual to have PD in main tank or windings as the oil quenches PD activity. • PD more often occurs on HV bushings. • EM signals from PD may emerge on outer surfaces if it has insulated gaskets – e.g. at bushings. • Without insulated gaskets, the transformer acts as a Faraday cage with all EM PD signals held within the transformer. • Location of the origin of a PD site can be made with distributed TEV probes on the LV and HV side, if there are suitable ‘pathways’ for the PD signals to come out of the transformer. • At HV internal Bushing ‘Taps’ (Capacitive Couplers) in HV transformer bushings may be used • Dielectric windows in the main tank for UHF sensors also an option. • The system is online therefore noise and detection of PD from other plant is possible. Sensor Options • High Frequency Current Transformers attached to power cables • TEV Sensors – EM radiation (external coupling) • Bushing Taps • Contact Acoustic • UHF Probes/Antenna – EM radiation (internal coupling) Sensor Discussion • HFCT – – – • TEV – – – • Attached onto feeder cables/earths, neutral points Non-intrusive – provided cable is accessible outside of cable box Wideband (100kHz – 30MHz) Attached at beside insulated gaskets/cable terminations. Detect EM radiation on transformer housing Useful to aid in discrimination Bushing Taps – – – – Capacitive foils Similar to coupling for conventional off-line measurements De-energisation for signal cable/quadrupole attachment required Useful when HFCT attachment not possible – i.e. transformer not cable fed Other Methods (not used by HVPD) • UHF – – – – • Antennas installed inside transformer tank Multiple probes used to locate signals Outside installation possible when Tx has dielectric windows Probes available for on-line installation at oil valve Contact Acoustic – – Probes installed on outside of transformer Multiple probes may be used to locate signals with triangulation HFCT Sensor Point Of Attachment: PD pulses originated inside transformers can propagate outside through conduction (along the MV cables connected to the transformer). HFCT sensor atached onto feeder cables/earths, neutral points, as per option 1 or 2 below. 10 MVA, 35 kV/5kV Transformer 2 63 MVA 110/6.6 kV 1 TEV sensor Point Of Attachment: PD pulses originated inside transformers can propagate outside through irradiation. TEV sensor attached at beside insulated gaskets/cable terminations could detect PD signals/EM radiation on transformer housing. TEV Sensors Online Measurement Technique • Wideband detection – Oscilloscope based system – True wave-shape of PD pulses captured • Four channel device – Three phases + one other sensor – Sensor placement to suite transformer, detected signals • Synchronous capture on all channels – Synchronous waveforms and pulse wave-shapes help locate source PD Sensors HVPD-Longshot™ Unit Hardware Filtering/ Amplification Digitiser/DSO Trigger signal PD Data Analysis Software PDGold and PDReader Software • Data captured synchronously with power cycle on all channels • Impulsive data extracted and saved • Knowledge rules applied to segregate PD and noise • Data then analysed with PDReader Software – Phase plots – Pulse wave shapes – Clustering similar shaped pulses Event recognition of PD pulses • Impulsive events extracted and categorised by PDGold software at data capture time • Optimises data saved – redundant data in between impulsive events not saved. • Even amongst noise pulses, PD data can still be extracted 20 ms Available Waveform Display 0.015 0.01 Chan 1 One power cycle of raw data from HFCT sensor 0.005 0 -0.005 -0.01 -0.015 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 Time (mSec) Chan 1 15 us Segment Waveform Curs 1 Segment Waveform Segment Waveform Ch 1 15 Ch 1 15 5 0 -5 -10 Volts (mV) 10 Volts (mV) 10 5 0 -5 -15 4 6 8 Time uSec 10 12 14 Cable PD Pulse: 16mV, 1160pC 2 0 -4 -15 2 Ch 1 4 -2 -10 0 Curs 2 0 2 4 6 8 Time uSec 10 12 14 Local PD Pulse: 15mV, 24dB 0 2 4 6 8 Time uSec 10 12 Noise Pulse: 6mV 14 Analysis of all PDs 0.015 0.01 One power cycle of raw data from HFCT sensor Chan 1 All PD events are extracted and classified based on wave shape Available Waveform Display 0.005 0 -0.005 -0.01 -0.015 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 Time (mSec) Chan 1 Local Equipment PD Cable PD 800 600 400 Noise Events Noise Magnitude (mV) Extracted PD data against phase 20 15 10 5 200 5 4 3 2 1 0 0 Summary values for cable PD, local PD and noise 0 0 0 Curs 2 6 PD Magnitude (dB) PD Magnitude (pC) 1,000 Curs 1 90 180 270 360 Phase of Pow er Cycle (deg) 90 180 270 Phase of Pow er Cycle (deg) 360 0 90 180 270 Phase of Pow er Cycle (deg) 360 Peak = 1160 pC Peak = 24 dB Peak = 6 mV Count = 12 Count = 11 Count = 37 Activity = 5200 pC/Cycle Activity = 100 mV/Cycle Activity = 200 mV/Cycle Peak PD = magnitude of largest event in power cycle Number/count = number of PDs in power cycle Activity = Sum of all PD pulse magnitudes Case Studies Case Study 1: Premier Power, Northern Ireland 275/18 kV Generator Transformer PD Testing Background • • • Following a failure in the 275 kV cable termination in the 21BAT transformer, the cable and sealing end were replaced. The commissioning of the new equipment on 21BAT prompted these measurements which were carried out online, but with the control of the voltage using the generator exciter control. The test involved exciting the transformer and cable up to working voltage with no measurable PD activity and to continue the test for 24 hours as per the IEC recommendation. Case Study 1: Premier Power, Northern Ireland 275/18 kV Generator Transformer PD Testing Sensor Connections • HFCT sensor on earth of 275 kV cable • TEV probes at bottom of transformer bushings/cable sealing end chambers to detect internal PD (insulated gaskets required). • With TEV probes on the LV and HV sides of the transformer Time-Of-Flight measurements can aid location of the PD events. Sealing end Chamber HFCT HV Link EarthTransformer Strap Earth Strap chamber Link to Transformer Transformer windings windings TEV probe HV Cable PD Magnitude (mV) Case Study 1: Premier Power, Northern Ireland 275/18 kV Generator Transformer PD Testing PD data for multi Channels Peak PD activity across the phase at 25% excitation 8 6 HV Side - Blue Phase TEV=red colour, HV Side - Yellow Phase TEV =blue colour, HV Side - Yellow Phase CT =green colour 4 2 0 -2 LV side TEV =purple colour -4 -6 Peak Activity = 8mV -8 0 50 100 150 200 Phase (Degrees) 250 300 350 Peak PD activity across the phase at 60% excitation PD data for multi Channels 600 HV Side - Blue Phase TEV=red colour, HV Side - Yellow Phase TEV =blue colour, HV Side - Yellow Phase CT =green colour 500 400 300 200 100 0 LV side TEV =purple colour -100 -200 Peak Activity = 550mV -300 -400 -500 -600 0 50 100 150 200 Phase (Degrees) 250 300 350 Large PD events observed from the blue phase on the HV side of the transformer. Case Study 1: 275/13.8kV Gen. Transformer PD Testing Time Of Flight Measurements for PD Location 0.07 0.06 0.05 0.04 0.03 0.02 0.01 0 -0.01 -0.02 -0.03 -0.04 -0.05 -0.06 -0.07 -0.08 Chan 1 0 0.035 0.03 0.025 0.02 0.015 0.01 0.005 0 -0.005 -0.01 -0.015 -0.02 -0.025 -0.03 -0.035 -0.04 Chan 2 Chan 3 Chan 4 Available Waveform Display 0.35 0.3 0.25 0.2 0.15 0.1 0.05 0 -0.05 -0.1 -0.15 -0.2 -0.25 -0.3 -0.35 0.1 0.12 0.14 Chan 1 0.16 Chan 2 0.18 0.2 Time (uSec) Chan 3 Curs 1 0.22 0.24 Chan 4 0.26 0.28 Curs 2 HV Blue Phase PD pulses at 60% excitation Blue Phase TEV Signal First - timed difference in cursors is 15 nSec. HV Blue Phase CT = blue colour, HV Yellow Phase TEV = green colour, HV Blue Phase TEV = red colour, LV side TEV = yellow colour . Signals were timed coming from the various sensors. Results shows that the Blue Phase TEV signal (red colour) on the HV side leads the Blue Phase CT (blue colour) signal by about 15 nSec. This represents a distance of 5 m travelling at the speed of light in air. All other signals trailed the Blue Phase TEV in time, showing that the signals could not originate in the other phases. The TEV probe on the LV side of the transformer also showed that it could not originate on the LV side of the system. The timings show clearly that this was a PD event originating in the HV side of the blue phase cable sealing end. Case Study 1: 275/13.8kV Generator Transformer PD Testing HV Cable Sealing end Chamber HV Link Transformer chamber Link to Transformer windings Support Insulator 275kV Cable The PD was located at the support insulators on the blue phase connecting link between the HV cable sealing end and the transformer output bushing. The support insulators were removed and then renewed which resulted in a discharge free operation for the HV side of the blue phase. Case Study 1: Premier Power, Ballylumford, N.Ireland 275/13.8kV Generator Transformer PD Testing Recommendations to the Customer • Regular PD testing and monitoring of the HV and MV systems should continue, to ensure that no further PD related failures can occur without warning. • If the manual method of monitoring the system presently employed is becoming to expensive in manpower, then a monitoring scheme should be considered which allows continuous data to be recorded. • To enable a reduction in staff time for the diagnostics, consideration should be made to install a network and internet link , so that measurements can be done remotely. Hence both the monitoring and spot testing can be carried out remotely, with the massive time savings which this offers. Part of such a scheme would be the installation of BNC cables for signals and LAN connection to the monitor. • Consideration should be given to installation of a unit to remotely monitor and spot test both ‘pairs’ of transformers (4 in total). A multiplexer unit can be used to give more channels for logging each transformer to could cover the monitoring of all 4x transformers. CASE STUDY 2: Customer: Hovensa, U.S. Virgin Islands In-service, online PD testing of 34.5/5kV 10MVA transformer Background • • • Following a failure of a 34.5/5kV 10MVA transformer PWT4902, the decision was made by to carry out Partial Discharge (PD) testing on the ‘sister’ transformer PWT4901. On-line partial discharge testing was carried out by engineers from HVPD Ltd. As the tests were carried out on-line whilst the transformer was connected to the network, it was necessary to make measurements at both the transformer and the 34.5 kV Substation end of the cable in order to take into account possible radiation of PD signals from other items of plant such as the connecting cables and switchgear PD Propagation from Transformer on to Network Power Transformer 10 MVA 34.5/4.160 kV GIS Switchgear CASE STUDY 2: Customer: Hovensa, US Virgin Islands In-service, online PD testing of 34.5/5kV 10MVA transformer NGR A B A B C Legend B 34.5 kV Bushing C 5 kV Bushing TEV PD Sensor 34.5 kV HFCT PD Sensor 5 kV Substation 44 (˜ 700ft, 213m) Substation 48 (˜ 66ft, 20m) HVPD Longshot On-line PD Test Unit Sensor Connections at PWT4901 Transformer CASE STUDY 2: Customer: Hovensa, US Virgin Islands In-service, online PD testing of 34.5/5kV 10MVA transformer Sensor Connections at Substation End Sensor Connections at PWT4901 Transformer HFCT and TEV sensor connections on 34.5kV cable cores CASE STUDY 2: Customer: Hovensa, US Virgin Islands In-service, online PD testing of 34.5/5kV 10MVA transformer • A series of tests were carried out on the transformer PWT4901 which involved the testing at the transformer’s HV and LV sides and also at the 34.5kV Substation 44. • From these tests PD activity was found on the circuit and isolated to the transformer side of the cable. Further tests at the transformer revealed the likely source of the PD was in the Phase B of the 34.5kV cable termination, transformer bushing or end winding. • To isolate the PD source to either the cable or transformer components, the 34.5kV cables on phases A, B and C were disconnected from the transformer and tested under working voltage supplied from the substation. • No PD was detected on this test and thus through process of elimination it was concluded that the PD pulses originate from the transformer. CASE STUDY 2: Customer: Hovensa, US Virgin Islands Noise PD Waveform Noise Waveform PD CASE STUDY 2: Customer: Hovensa, US Virgin Islands Separated PD and Noise against Power Cycle Waveform Measured at Transformer HFCT B Phase with HFCT on LV Neutral CASE STUDY 2: Customer: Hovensa, US Virgin Islands Power Transformer 10 MVA 34.5/4.160 kV GIS Switchgear Comparison of pulse rise time distributions at both cable ends Substation End Transformer End Case Study 2: Online PD testing of 34.5/5kV 10MVA transformer PWT4901 – B Phase PD Data from Online Test (Purple Colour) PD Across 60Hz Power Cycle Pulse Risetime Graph PD Waveform Shape (expanded trace below) Case Study 2: Online PD testing of 34.5/5kV 10MVA transformer Recommendations to the Customer • The peak levels of partial discharge detected were 500 pC which is a reasonably moderate level for MV cable accessories and transformer bushings. • Having said this however, each item of HV plant must be considered on an individual case basis and the more critical an asset’s presence on the owner’s network is, the more seriously any PD activity on it must be taken. The PD detected is not thought to be an imminent threat but it should definitely be regularly monitored/tested. The presence of PD, even at relatively moderate levels, does make the risk of failure higher than that on a discharge free component. • Regular On-line testing and monitoring of PD activity should be made to ensure the PD in Phase B does not escalate to an unacceptable level Case Study 3: On-line Partial Discharge (PD) Testing of 110kV Transformer Cable Terminations for a Refinery Client Slovakia 2009-2010 Case Study 3 – 110 kV Transformer Cable Termination Condition Assessment Following the failure of 2x 110 kV transformer cable terminations (see photo below right) at the Customer’s refinery in Slovakia, it was decided to carry out online partial discharge (PD) testing on the other cable terminations of same type. The testing was carried out to indicate if there were any incipient faults which were likely to cause failure in the near future. The terminations were tested using the HVPD Longshot™ Test Unit and HFCT/TEV sensors on May 13th 2009. Case Study 3 – 110 kV Cable Terminations May 2009 – PD Test Results Significant levels of PD activity of up to 400 pC were detected in Phase L2 of T109 Transformer during the testing in May 2009. HFCT L1 Earth Strap HFCT L2 Earth Strap Synchronous View, Larger L2 HFCT Signals HFCT L3 Earth Strap TEV Mounted on L2 Casing Case Study 3 – Replacement of Terminations and Forensic Analysis • In August 2010, the customer replaced the Phase L2 termination on the T109 Transformer, this was 15 months after the initial tests were made in May 2009. • Investigation showed evidence of tracking between the stress cone and the XLPE cable. The ‘early warning’, between detection of PD and the subsequent repair in this case, was greater than 15 months (and it had still not failed!). Case Study 4 - Online PD Testing on 25 kV Locomotive Voltage Transformers (VT’s) Client: Location: Alstom Transport Manchester & London, UK Helsinki, Finland Date: April to December 2006 Fleet-wide online and offline PD Testing of 25 kV Voltage Transformers (VTs) further to explosion of 3x VTs Case Study 4 - Online PD Testing on 25 kV Loco VTs Fleet-wide On-line PD Survey of 106 in-service VTs for PD activity Lab Testing to measure pC to mV Calibrations On-line HFCT and TEV Connections Available Waveform Display 0.1 0 -0.1 Chan 1 0.2 Chan 2 Example of Internal PD pulse detected by TEV sensor (green) and HFCT sensor (red) 0.3 0 -0.2 -0.3 0 1 2 3 4 Time (uSec) Chan 1 Chan 2 Curs 1 Curs 2 Case Study 4 - Online PD Testing on 25kV Loco VTs Fleet-wide On-line PD Survey of 106 in-service VTs for PD activity 1000 Peak (mV) 800 600 400 200 86 14 55 85 71 93 1109 955 45 334 1502 371 89 90 98 50 1101 4 152 61 40 1604 760 25 72 47 30 35 1907 813 39 67 68 54 21 99 24 46 568 77 74 41 18 42 28 88 29 92 69 10 44 23 36 17 78 32 20 63 82 70 73 83 94 84 13 1 313 51 16 48 577 60 66 0 VT Number The PD Survey showed 4x VTs had high levels of internal PD activity – these were removed from operation immediately Conclusions • On-Line PD Testing of HV Transformers is possible by using High Frequency Current Transformers (HFCT’s) connected around the cables in the cable boxes of the transformer. • On-line PD site location is possible using ‘Time-of-Flight’ measurements using distributed TEV and HFCT probes and bushing tap sensors. • If the transformer is fully enclosed with steel gaskets then the Transient Earth Voltage (TEV) PD signals will not pass out of the transformer as it acts like a Faraday Cage, keeping all signals inside and thus it is not possible to do on-line testing with TEV sensors on this type of transformer • It is now becoming popular to include internal Capacitive Coupler Bushing ‘Taps’ in HV Transformer bushings and also UHF Coupler ‘Windows’ within the main tank to ‘look into’ the transformer to detect PD in the both the bushings and main tank/windings. Recommendations for HV Transformer Partial Discharge Monitoring • Regular PD testing and monitoring of the HV transformers should be made to ensure trouble free service operation of the high voltage components. Monthly testing seems as frequent as operationally convenient, but the risks of service trouble is always higher after commissioning, so perhaps a sliding scale could be applied, with more frequent measurements at the start, becoming less frequent as the system beds down without incident. • There is a real challenge with making On-line PD measurements at HV voltages of 110kV+ as we need to measure very small events in generally large noise. Any new capability which will help look for PD activity which is small (i.e. below 100pC) should be used. Transformer owners should make sure that new methods of analysing PD data and removing noise are implemented. Training in PD measurements for the staff is a vital part in keeping the high voltage systems discharge free. • Some thought could be given to fitting permanent PD sensors inside the transformers and/or their cable boxes. This would have the advantage that they can be calibrated, and are placed inside the Faraday cage of the transformer, and hence are free of interfering noise external to the transformer. Such systems in the form of internal UHF Capacitive Couplers are routinely fitted into GIS switchgear at manufacture and this technology has recently been applied successfully to HV transformers.