Post on 26-Oct-2015
description
Diagnostic of PD in Cables
PD activity → incipient faults in cables. • PDM is best indicator of insulation
degradation being caused by cavities, electrical trees and other such defects, etc • Provides ‘Early Warning’ → insulation fault. • XLPE more susceptible to PD than paper
insulation.
PD in Cables can be caused by
Interfacial tracking in joints – stress cones. Surface discharge at cable termination. Discharge in Bulk XLPE insulation. Joints/splices – termination can sustain >> 100 pC for weeks. Bulk XLPE does sustain ~ 100 pC in hours → BD. Once PD occurs in XLPE insulation, an electrical tree initiates and can progresses very rapidly
On-line PD Testing
Provides quick ‘Look-See’ tests on large number of feeders in a power network → identify → locate. Monitors and evaluate PD levels, cumulative activity and provides trends to compare with other and past data for asset management. Considered the most cost effective diagnostic technique that helps to avoid unplanned outages.
OLPD System Acquired at KSU is based on 4-phase approach
Phase 1: On-line PD Screening
with handheld PD ‘surveying’ devices
Phase 2: On-line PD Testing
with PD diagnostic ‘spot testers’
Phase 3: PD Location/Mapping
with on-line PD cable mapping
Phase 4: On-line PD Monitoring
with portable on-line PD monitors
Long experience on cable network shows → 5-20% of cable circuits have high PD in prescreening (Ph-I) → quick → cheap. PD surveyor: → hand held.
• Ultrasonic / acoustic sensor. • HFCT → BW is 20 MHz. • TEV → BW is 100 MHz
Provides LED based PD level → Identifies 5 to 10% of cables feeders that may experience failures.
Phase-I: Surveyor
Phase-II: ‘Longshot’ – Spot Tester
Utilizes HVPD – Longshot unit with software. It measures / records PD activity synchronously on four channels fed from the sensors attached on the cables on PC based 400 MHz CRO. Sensors used are:
• HFCTs. • TEV • RF Antanae.
Most prominent challenge in OLPD is to differentiate and isolate PD pulses from high Electromagnetic interference (Noise) prevalent in the field. Noise sources are:
• Frequency converters / thyristor firings. • Variable speed drives. • Surface discharges on external Insulation. • Radio frequency interference. • Cross talk from neighboring equipment.
PD pulses undergo attenuation and dispersion during their travel in the cable and their rise time / fall time values change.
To isolate attenuated PD pulses from the noise pulses, state of the art filtering techniques are required.
Long shot + set of filters, it is possible to: • Differentiate PD signals from noise. • Establish location of PD. • Device based on Windows PC + On board
LAN Port.
Online PD Monitoring Sensors
HFCT – To capture PD travelling along the length of cable
TEV – To capture locally induced PD signals inside
switchgears RF Antenna – To capture external noise and interference
Online PD Monitoring Sensors
High Frequency Current Transformer –HFCT
Frequency response of HFCT
Transient earth voltage sensor
Earthing Requirements
There are two prerequisites for conducting a successful online PD measurements.
There must be independent access to either the earth-strap or the core of the cable at the switchgear/transformer. There must be an insulated gland between the cable earth and the switchgear earth.
Laboratory Experimental Setup
Long cable with proper terminations One end connected to HV transformer Other end open Toroid is used at the end to relief electric field stress around sharp edges of conductor. HFCT around core or earth TEV near the termination
Laboratory Experimental Setup
Long Cable
HV Transformer with primary and secondary circuit
breakers
AC Conductor
Insulation Insulation screen
Metallic shield
Sheath Toroid
Measuring instrument
C1 C2 Filter with 50 Ohm termination
Stress cone
HFCT
Laboratory Experimental Setup D
C
B
A
HFCT
HFCT
A
HFCTD
HFCT
B C
500m500m500m
JointsStart of PD pulse
500m
Joints
HFCT HFCT
HFCT
HFCT
Cable PD Source Localization
Single-ended PD site location method
PD pulse train as seen from the measurement end
Location from measurement end (% of Cable
Length) = 100*(1-ΔT/L)
Cable PD Source Localization
Segment Waveform
Time us14131211109876543210
Volts
(mV)
50
0
-50 Reflected pulse Main pulse
Cable Length = 100m Time Difference = 1.2μs Defect Location = 0% of the length (approx.)
Cable PD Source Localization
Segment Waveform
Time us14131211109876543210
Volts
(mV)
10
5
0
-5
-10
Main pulse Reflected pulse
Cable Length = 100m Time Difference = 1.19μs Defect Location = 0% of the length (approx)
Cable PD Source Localization
Main pulse
Reflected pulse
Cable Length = 1500m Time Difference = 19.2μs Defect Location = 0% of the length (approx)
Cable PD Source Localization
Segment Waveform
Time us14131211109876543210
Volts
(mV)
6
4
2
0
-2
-4
-6Main pulse
Reflected pulse
Cable Length = 1500m Time Difference = 6.4μs Defect Location = 66.67% of the length (approx)
Comparison of HVPD and Conventional System
a) Knife cut in the insulation at position # 3 b) Mechanical damage-sharp cut extending from cable’s
sheath down to its insulation c) Test on a field aged water treed XLPE cable.
Defect Type
OLPD Conventional PD System
PDIV (kVrms)
qm (pC) PDIV (kVrms)
qm (pC)
(a)
(b)
(c)
8.0
8.7
13.0
125
268
281
8.0
8.7
13.0
136
320
350
qm=10∼25% ↓
5. Using OLPD to Detect PD from Cable Defects (1/6)
Surface discharge at termination.
4.1 kV (1150 pC)
6.5 kV (790 pC)
PD Pulse Shapes
Surface Discharge
Segment Waveform
Time us14131211109876543210
Volts
(mV)
200150100
500
-50-100-150-200
Segment Waveform
Time us14131211109876543210
Volts
(mV)
201510
50
-5-10-15-20
Cut in Insulation
Electrical Tree
Cavity Discharge
Segment Waveform
Time us14131211109876543210
Volts
(mV)
604020
0-20-40-60
Segment Waveform
Time us14131211109876543210
Volts
(mV)
30
20
10
0
-10
-20
-30