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Condition Monitoring:New Initiative in CESC
&
A Case Study on 6 / 11 kV RMU
A Report prepared by CESCs Core Team associated with Singapore Power
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Condition Monitoring: New Initiative in CESC
Condition Monitoring is the process of monitoring some Key parameters of the equipment, such
that a significant change in the value of the parameter can provide information regarding the
deteriorating health of the equipment. It involves either continuous or periodic assessment of the
condition of a plant/equipment component while it is in operation.
The main objectives for the condition monitoring can be listed asa) Prediction of faultsb) Diagnosis of faultsc) Enhanced safety at work place4) Less down time
5) Better inventory management
Singapore Power recommended CESC to adopt condition monitoring of equipment as the key
measure to prevent failure. The same will enable the health of the network elements to be
monitored regularly and its status kept updated.
The CESCs Core Team associated with Singapore Power along with a set up of five engineers
from Mains, Testing and Sub-Station Departments started Condition Monitoring exercise from
April09 and so far has covered condition assessment of all the indoor switchgears of Receiving
Stations, Sub-Stations and Distribution Stations and also all the downstream network elements of
around 100 nos of fault prone 6/11 KV Feeders.
Condition monitoring technique includes detection of partial discharge (PD) using acoustic
technique and Transient Earth Voltage (TEV) measurements using PDL (Partial Discharge
Locator).
Ultraprobe Partial Discharge Locator (PDL)
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A. The Ultraprobe translates the high frequency noise developed due to arcing, tracking and
corona into the audible ranges. Electrical discharges are heard as a cracking or buzzing
sound.
B. The Partial Discharge Locator (PDL1) uses the technique of Transient Earth Voltage
(TEV) measurement: When the PD occurs, electromagnetic waves propagate away from
the discharge site and a very small quantity (pico-coulombs) of electrical charge is
transferred (in a few nanoseconds) capacitively from the high voltage conductor system to
the earthed metal-cladding. At an opening in the metal cladding such as through gaskets,
the electromagnetic waves propagate out into free space. The wave then generates a
transient earth voltage on the metal surface. Two probes are used to accurately locate the
discharge site.
The Condition monitoring exercise will set up condition based maintenance technique for
different groups of equipments that range from purely reactive to reasonably planned. The
deep down root cause analysis of each finding may also suggest changes in design of
equipments and the work practice to a great extent to arrest similar failures in future.
A Case Study on 6/11 kV RMU
We have so far covered online Condition Monitoring on 383 nos of RMUs of different make
in 6/11 kV downstream network where Partial Discharge (PD) were observed at 23
locations with varying degree of measurements starting from 2 dB to a maximum of 42 dB.
Subsequently on thorough examination of such affected equipments with high PD, we
gather significant knowledge particularly on the deficiencies in the design criteria.
Appropriate remedial measures against such defects will not only rectify these definite
potential failures but will also arrest similar occurrences which otherwise would have been
cropped-up in future.
The details of the findings, analysis and remedial actions are mentioned in this report.
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Partial Discharge as Detected in RMUs at Different
Voltage Level till Oct-2009
11KV 6KV
Make Nos
>10dB 10dB
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Findings
Out of total 23 cases 17 were detected Partial discharge in AREVA RMU i.e.74% of total
detection
The balance 6 cases are ABB RMU, where except one all others(5) are having measuredfor very low noise (10dB and maximum 42 dB)
Maximum PD was observed outside the cable chamber of AREVA RMU
Thorough Investigations were carried out at the following 5 installations
Investigation of AREVA RMUs where high PD were detected :
LOCATION PD Measured Voltage levelMake of Cable
Boot
AREVA RMU at Anandapur O/T 41 11 kV 3M
AREVA RMU at Calcutta Green O/T 1 42 11 kV 3M
AREVA RMU 1 at Survey park T/H 5 30 11 kV 3M
AREVA RMU 2 at Survey park T/H 5 42 11 kV 3M
AREVA RMU 1 at Ruby General
Hospital 42 11 kV 3M
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Observations During further investigation:
1. Sound level as detected at these locations are on the high side and in the tune of 30-42
dB and continuous in nature
2. Partial Discharge generation were pin-pointed at the cable termination
3. All these RMUs are provided with 3M make Cable Termination Boots
4. Black marks (tracking)on the outer surface of cable boots and also on the inside wall of the
cable chamber
5. Colour of boots changed to greenish with sticky solutions on the outer surface
6. Inter-phase boots are found touching each other
Photographs
Fig -1 : Fig-2 :
AREVA RMU with 3M Boot Tracking on outer surface of the 3M Boot
Fig-3: Fig-4:
Colour of cable boots changed to greenish with sticky solutions on the outer surface
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Fig-5 : Fig-6:
Black spots on the side wall of Cable Chamber
Fig-7: Fig-8:
Inter-phase boots touching each other in AREVA RMU Cable Chamber
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Comparative studies of AREVA & ABB RMU having 3M Boots
AREVA RMU ABBRMU
HIGH PD DETECTED NO PD DETECTED
Fig-9: Fig-10:
Inter-phase boots touch each other Gap exists between inter-phase boots
Comparative studies of Raychem & 3M Boots on AREVA RMU
Fig-11: Fig-12:
AREVA RMU with Raychem boot AREVA RMU with 3M boot
Analysis
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1. ForABB RMU gap between bushing centre to centre is 107 mm creates an air insulation of
approx 10 12 mm with 3M Boots.
2. For ABB RMU even with 3M make Boots, no abnormal noise was detected in such
arrangement
3. For AREVA RMU gap between bushing center to center is 95 mm, Hence 3M Boots are
touching each other.
4. For AREVA RMU with Raychem make Boots, a small gap exists between the Boots of
different phases.
5. For AREVA RMU with Raychem make Boots no abnormality was detected
6. Connections at the terminating point on bushings are bare (without any insulation covering
i.e. Tape etc.) inside the boots. The charges due to induced voltage surrounding the
termination points accumulate more on the surface of the boots.
7. Since the adjacent cable bushings are also affected by the induced voltages at different
phase potentials, this difference in potential leads to discharges on the surface of two
adjacent boots resulting in carbonizing of the local area. Black spots are produced due to
the concentration of electric charges.
8. The above phenomenon gradually increases the tracking on the boots & generates more
discharges.
9. These types of problems are not noticed in 6kV systems since the induced voltages across
the bushings are comparatively less than 11kV systems to cause discharges.
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Remedial Action Plan
Phase-1: Henceforth All AREVA RMUs are to be commissioned (both for 6 & 11 KV) with
Raychem make Boots instead of 3M make Boots
Phase-2: AREVA RMUs charged at 11kv having detected high PD - All Terminal Protectors
(Cable Boots) of 3M make are being replaced with Raychem make Boots.
Phase-3: 3M make Cable Boots for all existing AREVA RMUs charged at 11KV are to be
replaced with Raychem make Cable Boots.
Periodic repeat measurements on these sites to ascertain the effectiveness of the
above proposals
Inference
Condition monitoring exercise thus carried out above enabled us to diagnose the root cause of
potential failure points in RMUs before occurrence of the actual failure which would have resulted
severe damage and longer interruption of customer service. One most significant aspect of
condition monitoring exercise is the root cause finding and taking appropriate corrective
measures against the potential failure which would otherwise have remained probably
undetected during post failure analysis.
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