condition monitoring - new initiative in cesc

8/13/2019 Condition Monitoring - New Initiative in CESC

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Condition Monitoring:New Initiative in CESC

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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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condition monitoring - new initiative in cesc

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