Underground Cable Temperature Monitoring in the Real World

Transcription

1 Underground Cable Temperature Monitoring in the Real World

2 Executive Summary Background Brain Storming Possible Causes to Duct Bank Over Heating Mitigating Duct Bank Overheating DTS Installation for Duct Bank Temperature Monitoring October Heat Storm Wrap-Up

3 Background (Jan 2014) Hot conditions were first noticed by Troublemen performing switching operations during early morning hours. A new circuit was needed at the substation, so the ductbank was split in two. (May 2014) While work was being performed, elevated temperatures were again observed. A temperature of 183 F was measured in an empty conduit. Duct from substation cable trench.

4 Brain Storming Possible Causes 1. Neutral Imbalance 2. Load Factor 3. Vault Blower 4. Soil Conditions 5. Over Crowding

5 Possible Causes 1. Neutral Imbalance West Wall inside M from Sub (Line Side) Panther 12kv - Gnd 9 amps Gazelle 12kv - Gnd 16 amps Cougar 12kv - Gnd 12.5 amps Elephant 12kv - Gnd 14.8 amps Jaguar 12kv - Gnd 6 amps Bison 12kv - Gnd 3 amps East Wall inside M (Load Side) Panther 12kv - Gnd 13 amps Elephant 12kv - Gnd 11.5 amps Gazelle12kv - Gnd 14.7amps Bison12kv - Gnd 14amps Cougar 12kv - Gnd 4.4amps Jaguar 12kv - Gnd 10.7amps

6 Possible Causes 2. Load Factor Monthly load factor was calculated for all phases of each circuit. The highest load factor from the three phases was then used as that respective circuit s load factor during simulations. Circuit Name January Monthly Load Factor July Monthly Load Factor Phase A Phase B Phase C Phase A Phase B Phase C Bison Cheetah Cougar Elephant Gazelle Leopard Lynx Panther

7 Possible Causes 3. Vault Blower Not working since Was not the hottest vault.

8 Possible Causes 4. Soil Conditions Hired consultant to measure: Soil thermal resistivity, Ambient temperature, and Moisture content.

9 Possible Causes 4. Soil Conditions Industry Standards ICEA P (WC ) a. Earth thermal resistivity-60, 90 and 120 C-cm/w, designated as 60 RHO through 120 RHO. AEIC CG6-2005, Section 6.1 IEEE (R2006)

10 Possible Causes 4. Soil Conditions Soil around old and new installation was tested for thermal resistivity. Maximum reading around existing conduit was found to be 353 C cm/w. Maximum reading around new conduit was found to be 250 C cm/w. A large amount of trees were present in the area of the duct banks. Large trees are known to dry out the soil, thereby increasing the soil thermal resistivity. Issue 1: Poor Soil Conditions Location Run Comments Thermal Resistivity ( C-cm/W ) Soil Percent Tempera Moisture ture (%) ( C) 1 1 Side Wall Side Wall Bottom of Trench Side Wall Side Wall Side Wall na 2 4 Bottom of Trench Location Run Comments Thermal Resistivity ( C-cm/W ) Soil Percent Tempera Moisture ture (%) ( C) 1 1 North Side Wall East Side Wall South Side Wall South Side Wall East Side Wall North Side Wall South Side Wall East Side Wall North Side Wall

11 Possible Causes 5. Over Crowding of Duct Banks Existing Duct Banks: Original duct banks crossed at two locations First crossing was at a small angle, which allows the highest of mutual heating Second crossing was at 90 degrees, which allows the least amount of mutual heating Transition Duct Bank Six circuits in one duct bank Issue 2: Over Crowding of Duct Banks Issue Existing Duct Banks Transition Duct Bank

12 Possible Causes 5. Over Crowding of Duct Banks In the transition design, 6 circuits were together in one duct bank. High temperature of C Issue 2: Over Crowding of Duct Banks

13 Solution 1: Soil Mitigation with Slurry Backfill With Native Soil Backfill Temperatures are above 90 C With 1-sack slurry Backfill All temperatures are below 90 C

14 Solution 1: Soil Mitigation with Slurry Backfill In order to mitigate the high thermal resistivity of the soil, a slurry backfill was used to replace the soil above the new cable duct bank installations. A thermal resistivity of 60 C cm/w was used for the slurry back fill.

15 Solution 2: Re-Routing of Duct Bank Duct Bank was re-routed to reduce overcrowding. Trees were removed to make room for the duct bank route. New Vault Trees Removed

16 Solution 2: Re-Routing of Duct Bank New duct banks were routed towards the outside, furthest away from the trees and other duct banks.

17 Validation of Solutions How do we know if the separation of duct banks and the use of the slurry backfill addressed the heating issues?

18 Distributed Temperature Sensor (DTS) Fiber Cable: 24 Multi Mode 62.5/125mil Length: ~2600 ft One Continuous Loop Standard SCE fiber cable Zones: Control trench, power cable trench, north duct bank, south duct bank, and fiber well. Conduit: 2 inch Power to control cable trench. 5 inch Control trench to fiber well. 3 1/c 1000 kcmil, TR-XLPE 220 mils V DTS MEER 12kV SWITCHRACK No. 1 CAP BK No. 2 CAP BK FIBER WELL FIBER SPLICE SUBSTATION FENCE

19 Benefits of Distributed Temperature Sensor (DTS) System Thermal Study are an estimate of the thermal characteristics of the cable, ductbank, ambient temperature, soil conditions and loading. Soil thermal properties cannot be known throughout a duct bank run, without taking multiple measurements throughout the run. No other method of determining the actual cable temperature in the middle of the duct bank. Knowing the temperature of the cable: Make better, more accurate decisions in real time. Extend the life of cable by monitoring the cables temperature. Possible increase of current capacity, by loading the cable up to the actual loading limits, rather than an estimate.

20 Duct Bank Performance Monitoring (DTS) The DTS system was implemented to monitor cable temperatures inside the duct banks. Utilizes a fiber optic cable to monitor temperatures along the entire length of the duct bank. DTS and Computer Rack Fiber splice in power cable trench

21 Fiber Route Temperature Profile

22 October Heat Storm - Temperatures Max Ambient Temperature Data in Garden Grove for October 2014

23 Heat Storm Circuit Loading -North For SCE Internal Use Only 23

24 Duct Bank Temperature Performance Validation Measured Duct Bank Model Cable Detail Model Calculated DTS Temperature was as expected. (63 deg. C measured & 65 deg.c calculated.) Improved duct bank design, slurry back fill & tree removal prevent duct bank overheating (Conductor Temperature ~ 68.8*C during heat wave at 103 deg. F) 24

25 Wrap Up - Conclusion Predominant causal factors in the Lampson overheated duct bank condition are the high soil thermal resistivity and the overcrowding of the duct banks. The corrective actions implemented have mitigated both causal factors: Splitting the circuits into two paths (4 circuits and 3 circuits) and cable placement in the duct arrangement to address the circuit density. Adding slurry to address the soil thermal resistivity issue. The DTS system has proven that the integrated solution of splitting the duct bank and adding slurry backfill prevented overheating during the October heat wave.

26 Wrap Up Lessons Learned / Challenges Installing the fiber cable in the same duct as power cables. Damage may occur during installation. Small clearances in duct. Communication fiber cable requirements / standards are higher than needed for monitoring cable temperature. Paying more attention to the contractors, they work fast and may not have the same goals as you do. Cost was small to add fiber cable to an existing power cable pull. Fiber splicing Coordinating non electrical crews. Fiber cables are normal treated as separate installations Setting up computer for DTS Cost is of the project is in the DTS unit.

27 Wrap Up Future Needs Possible installation of fiber cable in highly loaded circuit sections (ie. Substation getaways, highly loaded customers, etc.) Installing fiber cable at the same time as power cable is reasonably low cost.

28 References CYMCAP 6.2, rev2 Duct Bank Modeling Software IEEE 442- Guide for Soil Thermal Resistivity Measurements SENSA-Distributed Temperature Sensor SCE Circuit Loading Data Thermal Images Provided by SCE Troubleman Santa Ana,CA AEIC CG6-2005, Section 6.1 Guide for Establishing The Maximum Operating Temperatures of Extruded Dielectric Insulated Shielded Power Cables IEEE 835 IEEE Standard Power Cable Ampacity Tables ICEA P (WC-50) Ampacities Including Effect of Shield Losses for Single Conductor Solid Dielectric Power Cable 15 kv through 69 kv