On-Line Partial Discharge Testing of Transformers
Presented by:
High Voltage Partial Discharge
Ltd
Manchester, UK
info@hvpd.co.uk
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.