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2. Riego en Madrid: los disturbios del Teatro Príncipe
6.8.1 Contact Resistance
Contact resistance is the most important factor in determining the condition of the terminations/joints because, being performed at the time of commissioning or after repair.
These tests can identify internal arcing, bad electrical contacts, hot spots, partial discharge, or overheating of conductors. The “health” of the connections is reflective of the health of the cable itself.
Results of the contact resistance analyses are applied to Table 6.1 to arrive at an appropriate mitigating action.
Table 6.1 Contact Resistance
This test is done during installation. If any concern identified then it is rectified before putting the cable into service. Therefore this test results are not consider for condition assessment while the
cable is in service.
Results Score Remarks/Action
Less than 50 µΩ in all the phases
- The connection is healthy and can be put into service
> 50 µΩ but less than 100 µΩ in any particular phase or all the phases
- The connection can be put into service with caution
> 100 µΩ in any particular phase or all the phases
- The connection must be
replaced
6.8.2 Cable Condition Indicator 1 – Thermography
Thermography is important factor in determining the condition of the exposed terminations because, being performed periodically it may be the first indication of a problem. These tests can identify internal arcing, bad electrical contacts, hot spots, partial discharge, or overheating of conductors. The “health” of the connections is reflective of the health of the cable itself.
Table 6.2 Thermography
This test is done on exposed terminations during cable in-service at every 24 months interval under normal condition. This test results are considered for condition assessment while the cable
is in service.
Results Score Action The hot spot temperature
difference between phases is
3 Normal. The monitoring
frequency of 24 months can be
less than 5 degree centigrade maintained.
The hot spot temperature difference between phases is >
5 degree centigrade but less than 10 degree centigrade
2 The monitoring frequency
should be revised to 12 months.
The hot spot temperature difference between phases is >
10 degree centigrade but less than 20 degree centigrade
1 The monitoring frequency
should be revised to 3 months.
The hot spot temperature difference between phases is >
20 degree centigrade
0 Remove the cable from
service and perform tan delta tests immediately.
6.8.3 Cable Condition Indicator 2 – Tan Delta Test
Table 6.3 Tan delta
This test is done on cables at regular interval of 24 months under normal condition. This test results are considered for condition assessment while the cable is in service.
Results Score Action XLPE:
tan δ (2 U0) < 1.2 E-3 and [tan δ (2 U0) - tan δ (U0)] < 0.6 E-3 PILC:
tan δ (50Hz) < 2.3 E-3
3 Normal. The monitoring
frequency of 24 months can be maintained.
2 The monitoring frequency
should be revised to 6 months.
XLPE:
1 The monitoring frequency
should be revised to 3 months.
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Results Score Action XLPE:
service and perform Tier 2 tests (whichever applicable) immediately.
6.8.4 Cable Condition Indicator 3 – Insulation resistance test
Table 6.4 Insulation resistance
This test is done on cables at regular interval of 24 months for MV cables and 60 months for LV cables under normal condition. This test results are considered for condition assessment while the
cable is in service.
Results Score Action XLPE:
DAR value ≥ 1.6 PILC:
PI value ≥ 3.0
3 Normal. The monitoring
periodicity of 24 months for MV cables and 60 months for LV cables can be maintained.
XLPE:
1.1 < DAR value < 1.5 PILC:
1.5 < PI value < 3.0
2 The monitoring periodicity should be revised to 6 months for MV cables and 24 months for LV cables.
XLPE:
1.0 < DAR value < 1.1 PILC:
1.0< PI value < 1.5
1 The monitoring periodicity should be revised to 3 months for MV cables and 12 months for LV cables.
service and perform Tier 2 tests (if applicable) immediately for MV cable.
Replace LV cables.
6.8.5 Cable Condition Indicator 4 – Operation and Maintenance Performance
Operation and maintenance (O&M) history may indicate overall cable condition. O&M history factors that may apply are:
• Sustained overloading.
• Abnormal temperatures indicated by infrared scanning.
• Nearby lightning strikes or through-faults.
• Abnormally high partial discharge detected.
• Increase in breakdown maintenance or difficulty in acquiring spare parts.
• Anomalies determined by physical inspection
• Previous failures on this cable.
• Failures or problems on cable of similar design, construction, or age operating in a similar environment.
Qualified personnel should make a subjective determination of scoring that encompasses as many operation and maintenance factors as possible under this Indicator. Results of the O&M history are analyzed and applied to Table 6.5 to arrive at an appropriate Condition Indicator Score.
Table 6.5 Operation and Maintenance Performance Scoring
Results Score Action Operation and Maintenance
are normal
3 - Some abnormal operating
conditions experienced and/ or additional maintenance above conditions experienced and/ or additional maintenance above normal occurring
0 -
6.8.6 Cable Condition Indicator 5 – Age
Cable age is an important factor to consider when identifying candidates for cables replacement. Age is one indicator of remaining life and upgrade potential to current state-of-the-art materials. During the life of the cable, the insulating properties of materials which are used for electrical insulation, especially XLPE, deteriorate. Although actual service life varies
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widely depending on the manufacturer’s design, quality of assembly, materials used, operating history, current operating conditions, and maintenance history, the average expected life for a large population of cables is statistically about 40 years. Apply the cable age to Table 6.6 to arrive at the Condition Indicator Score.
Table 6.6 Age Scoring
Results Score Action XLPE:
Between 11 to 20 years PILC:
Between 21 to 30 years
2 -
XLPE:
Between 21 to 30 years PILC:
Between 31 to 40 years
1 -
Enter the condition indicator scores from the tables above into the Cable Condition Assessment Summary form at the end of this Chapter. Multiply each condition indicator score by the Weighting Factor, and sum the Total Scores to arrive at the Tier 1 Cable Condition Index. The value of the individual weighting factor of the Tier 1 Condition Indicator is determined by the expert. The sum of all the weighting factors should be equal to 3.33.
Table 6.7 Tier 1 Cable Condition Index
No Condition Indicator Score Weighting
Factor
Total Score
1 Thermography 0.70
2 Tan Delta 0.80
3 Insulation Resistance 1.00
4 Operation and Maintenance
Performance 0.53
No Condition Indicator Score Weighting Factor
Total Score
5 Age 0.30
Tier 1 Cable Condition Index (Sum of Individual Total score)
(Condition Index should be between 0 and 10)
6.8.8 Tier 1 – Cable Data Quality Indicator
The Cable Data Quality Indicator reflects the quality of the test and measurement results used to evaluate the cable condition under Tier 1. The more current and complete the tests and measurements, the higher the rating for this indicator. The normal testing frequency is defined as the organization’s recommended frequency for performing the specific test or inspection. Qualified personnel should make a subjective determination of scoring that encompasses as many factors as possible under this indicator. Results are analyzed and applied to Table 6.8 to arrive at a Cable Data Quality Indicator Score.
Table 6.8 Cable Data Quality Indicator Scoring
Results Score Adjustment Action All Tier 1 testing equipment were calibrated
within the recommended calibration frequency AND results are reliable.
Subtract 0 -
One or more of the Tier 1 testing equipment were calibrated between 0 and 6 months past the recommended calibration frequency.
Subtract 0.5 -
One or more of the Tier 1 testing equipment were calibrated between 6 and 12 months past the recommended calibration frequency.
Subtract 1.0 -
One or more of the Tier 1 testing equipment were calibrated more than 12 months past the recommended calibration frequency.
Subtract 1.5 -
The Tier 1 Cable Condition Index is adjusted by the Cable Data Quality Indicator Score to attain the final Tier 1 Cable Condition Index Value as shown in Table 6.9.
Table 6.9 Final Tier 1 Cable Condition Index Value
No Condition Indicator Score Weighting Factor
Total Score
1 Thermography 0.70
2 Tan Delta 0.80
3 Insulation Resistance 1.00
4 Operation and Maintenance History 0.53
5 Age 0.30
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No Condition Indicator Score Weighting Factor
Total Score Tier 1 Cable Condition Index
(Sum of Individual Total score)
(Condition Index should be between 0 and 10) 6 Cable Data Quality Indicator Score Adjustment
(value can be 0, -0.5 ,-1.0 or -1.5)
Final Tier 1 Cable Condition Index Value (Condition Index should be between 0 and 10)
Based on the final Tier 1 Cable Condition Index Value the suggested recommendations on the testing frequency of Tier 1 and proposal for Tier 2 tests are mentioned below in Table 6.9.
Table 6.10 Cable Tier 1 Condition-Based Alternatives
Final Tier 1 Cable Condition Index Value Suggested Course of Action
≥ 7.0 and ≤ 10.0 (Good) Maintain the normal frequency of Tier 1 test.
≥ 3.0 and < 7.0 (Fair) Revise frequency of Tier 1 tests to 6 months interval. Make arrangements for Tier 2 tests.
≥ 0.0 and < 3.0 (Poor) Perform Tier 2 tests immediately.