DTCR Functional Diagram
3.9 CASE STUDIES FOR UNDERGROUND CABLE CIRCUITS
Section 3.9 describes uprating projects recently con-ducted at utilities. It is hoped that these case studies help to illustrate the general application of uprating tech-niques described in this chapter.
3.9.1 CenterPoint Energy
Description of Circuit and Summary of Rating Constraints and Utility Goals
A 138-kV HPFF underground transmission line was constructed in 1969 from CenterPoint Energy’s Polk substation (located at the intersection of Polk and La Branch Streets in Houston, Texas) to CenterPoint’s Garrott Substation (located at the intersection of Gar-rott Street and Blodgett). The total length of this 138-kV underground transmission line is approximately 2.37 miles (12,500 ft). A 2500-kcmil, compact segmental copper, 138-kV HPFF cable with 505 mils of insulating tapes was used to construct the Polk – Garrott transmis-sion line.
In March 2001, a loop feed to a new CenterPoint sub-station, Midtown Subsub-station, was constructed by tap-ping into the existing Polk to Garrott underground transmission line. This loop feed to the Midtown Sub-station (located on La Branch Street between Taum and Drew Streets) segregated the Polk to Garrott under-ground transmission line into two parts with the lengths of 0.96 and 1.41 miles. CenterPoint wanted to evaluate the power transfer capabilities of the Polk-Midtown-Garrott and Polk-Downtown 138-kV underground transmission lines in light of these changes and to opti-mize the current-carrying capacity of the circuits. The ampacity audit was based on an evaluation of recent and historical data including:
• Long-term load current, ambient soil temperatures, and cable pipe temperatures during summer operat-ing conditions
• Distributed fiber optic temperature sensing (DFOTS) measurements performed on the circuits in February 2002 for hot spot identification
• As-built plan and profile drawings for the two cable circuits
• Cable manufacturing data
CenterPoint Energy also wanted to evaluate the condi-tion of the thirty-two-year-old Polk-Garrott HPFF cables in coordination with the ampacity analysis. Con-sequently, two investigations were performed for this purpose. First, Detroit Edison (DECo) performed dis-solved gas analysis (DGA) and laboratory testing of cable paper tape samples obtained during construction of the loop feed to the new Midtown Substation. Power Delivery Consultants (PDC) also performed cable dissi-pation factor measurements at rated voltage using EPRI-developed instrumentation. A previous EPRI project (Transmission Cable Life Evaluation and Man-agement) indicated that cable tape physical property measurements, DGA, and dissipation factor measure-ments are the best diagnostic tests to determine cable loss-of-life.
The primary focus of the DTCR project was to examine the ratings on the Polk-Midtown-Garrott circuit. This circuit consists of two segments: 5,060 ft from Polk to Midtown Substation, and 7,440 ft from Midtown Sub-station to Garrott SubSub-station. DTS measurements showed that the hotspot for the Polk-Midtown-Garrott underground line is a crossing with the Polk-Downtown 138-kV underground transmission line at the intersec-tion of Polk and La Branch (just outside of the Polk Substation). The cable used for the Polk to Downtown 138-kV line is identical to the Polk to Garrott line. The depth of cover over the Polk-Midtown-Garrott line is approximately 11 ft-2 in. at the hot spot location, and
the vertical clearance to the Polk-Downtown line (above it) is approximately 3 ft. CenterPoint placed a thermo-couple on the Polk to Garrott cable pipe near the inter-section. Initially, the thermocouple temperature was monitored with a data logger, but this thermocouple is now connected directly to CenterPoint's SCADA sys-tem. A thermocouple was also placed on the Polk-Downtown circuit at the location of the crossing and connected to SCADA.
In 2000, CenterPoint Energy (Houston Lighting &
Power) began this investigation (completed in December 2002 – EPRI Report 1007539) to increase the circuit capacity on the high-pressure fluid-filled (HPFF) pipe-type cable connecting the Polk and Garrott Substations.
Various studies were performed to evaluate uprating possibilities for this circuit, including the application of distributed fiber optic temperature sensing (DFOTS).
Results of DFOTS revealed that a hot spot existed where another pipe-type cable (CenterPoint’s Polk-Downtown circuit) crossed over the Polk-Garrott circuit. Although an overall increase in ampacity was found for the general cable circuit, a net decrease in ampacity resulted from the modeling of the mutual heating of the two cable cir-cuits where they cross. This was anticipated prior to beginning the project, so DTCR was implemented on the Polk-Garrott circuit in an effort to optimize available circuit capacity.
The principal goal of the project was to investigate an optimized circuit rating in light of the interference tem-perature effects detected by DFOTS and experienced by the crossing pipes. A secondary objective was to demon-strate that, under normal loading patterns, the maxi-mum normal temperature (85°C) of the conductor would infrequently be exceeded.
Results of Uprating and Benefits to Utility
The following conclusions may be reached from review-ing the application of DTCR at CenterPoint:
• The DTCR modifications and subsequent data anal-ysis showed that CenterPoint Energy’s Polk-Mid-town-Garrott and Polk-Downtown pipe-cable circuits can benefit from dynamic ratings. DTCR’s predicted load pattern assumes a typical 24-hour loss factor cycle consistent with static ratings. However, the emergency loading is a function of pre-load con-ditions, and DTCR accurately calculates the conduc-tor pre-load temperature based on hisconduc-torical loading patterns. Also, DTCR uses the present loading to predict time to temperature overload (TTO).
• While a detailed ampacity study indicated there was effectively a reduction in the established book rating of 3.4%, applying DTCR allowed for a net increase in the normal rating of 20.7%, based on considering a
“dynamic” rating using summer 2000 load data for a
“quasi-real-time” dynamic rating analysis.
• In summary, the real-time dynamic ratings of the Polk-Midtown-Garrott and Polk-Downtown under-ground transmission lines are expected to be signifi-cantly higher than the static ratings that were calculated under a separate project, assuming there is similar load pattern variability to that observed dur-ing the summer 2000.
The following conclusions are a result of the cable con-dition assessment testing performed on the Polk-Gar-rott 138 kV underground transmission line.
• DGA testing of pipe fluid samples and laboratory testing of a cable sample indicated that Polk-Garrott 138-kV cable offers an exceedingly long life, which is characteristic of HPFF cables.
• Results of the dissipation factor measurements con-firmed that the Polk-Garrott 138-kV HPFF cables do not show any signs of insulation deterioration after more than 30 years of operation.
3.9.2 United Illuminating Company
Description of Circuit and Summary of Rating Constraints and Utility Goals
In 1989, United Illuminating Company (UI) performed an engineering evaluation on the ampacity of the exist-ing 1.4-mile-long UI 115-kV high-pressure gas-filled cable Circuits 1710 and 1730, which connect UI’s Pequonnock Substation to the Seaview Tap in Bridge-port, Connecticut, where the lines transition to over-head conductors. A 1600-ft section under Bridgeport Harbor, where the cables were buried approximately 25 ft under high-resistivity sediments, appeared to limit the overall circuit rating. UI’s construction records indi-cated that the cable configuration under the harbor con-sists of three pipes in a 5-ft trench, with a spare (empty) pipe centered between the two cabled pipes.
Results of the 1989 thermal tests showed that soil resis-tivity ranged widely, from 90º to 250º C-cm/Watt. This range of values produced a degree of uncertainty in the ratings. In addition, the degree of siltation and the actual pipe positions since the cable pipes were installed in 1961 was unknown. Because of the uncertainty of the pipes’ locations, 1989 soil tests were done at least 50 ft away from the expected pipe position to avoid possibly damaging the pipes with the soil-coring equipment. The uncertainty of some parameters from the 1989 study, the limiting of the entire circuit’s ampacity by the har-bor section, combined with UI’s interest in increasing the total power transfer on the circuit, precipitated UI in undertaking a more thorough ampacity evaluation of
the harbor section. An additional goal was to consider means for increasing ampacity on the circuit.
Power Delivery Consultants, Inc. (PDC) was contracted in 2001 to perform a very detailed evaluation of the Bridgeport Harbor portion of UI’s 1710 and 1730 lines using sophisticated modeling and state-of-the-art tech-nology to gather information about the installation and environment. The evaluation included several technolo-gies:
• Gyroscopic testing on the empty cable pipe to develop accurate cable pipe plan and profile informa-tion for the harbor crossing.
• Hydroscopic surveying of the harbor bottom to eval-uate the degree of siltation over the cable pipes since they were installed in 1961 and, ultimately, to deter-mine the cable depth of cover.
• Distributed temperature sensing (DTS) using EPRI’s DTS equipment and fiber optic cable installed in the spare cable pipe
• Continuous thermocouple temperature monitoring using installed thermocouples and data loggers
• Updated soil sample testing to characterize the soils at the depths of interest
• Forced air ventilation to characterize possible uprat-ing by removuprat-ing heat from the cabled pipes
An EPRI report (1007534) documents the results of these evaluations and a detailed ampacity study to determine the actual capacity of UI’s 1710 and 1730 lines under both normal and emergency ampacity con-ditions, and describes possible approaches for increasing the ampacity on the lines.
In 2002, UI implemented two of the recommendations of the ampacity study:
• Applied forced-air cooling on the parallel cable pipe.
• Implemented DTCR to monitor conductor tempera-ture and evaluate real-time temperatempera-tures on the cir-cuit as the result of circir-cuit loading.
Results of Uprating and Benefits to Utility
The initial ampacity study resulted in several recom-mendations to mitigate the rating limits on UI’s cable circuits. These are summarized in Table 3.9-1. Two of the options, forced air cooling on a parallel empty cable pipe and dynamic ratings, were later implemented.
The forced-air cooling equipment is shown in Figure 3.9-1. The actual cost to install the forced-air cooling equipment was substantially higher than the initial
esti-mate, largely due to some complications associated with the civil works of the installation.
The following conclusions and recommendations were reached based upon implementing DTCR on United Illuminating Company’s 1710/1730 circuits:
• DTCR shows that the maximum normal operating temperature of paper-insulated pipe-type cables (e.g., 85°C) is rarely exceeded, even though there are fre-quent occasions when the actual circuit loading exceeds the normal book ratings.
• The combined effects of circuit loading on 1710 and mutual heating from circuit loading on 1730 some-times result in the conductor temperature on the 1730 circuit exceeding its maximum normal temperature.
As was indicated in an earlier study, this results from the typically increased load levels on the 1710 and 1730 circuits, the increased daily loss factors, and the fact that loads on the 1730 circuit are increasingly approaching the loads on the 1710 lines.
• UI may want to consider performing a long-term loss-of-life evaluation on the 1710 and 1730 circuits to see how the calculated operating temperatures on the circuits have influenced accumulated loss-of-life.
Despite the occasional incursions above rated tem-perature, the evaluation would likely show that dur-ing a typical calendar year, the below 85°C operatdur-ing temperatures for most of that time indicate that less than a calendar year of life has been consumed. This would provide increased confidence to UI that this occasional high-temperature operation should not adversely affect the future operation of the circuit. If pursued, this additional work would apply paper-insulated pipe cable aging characteristics developed during EPRI work on accelerated aging at Waltz Mill.
• From the standpoint of evaluating normal ratings, the book ratings were previously found to be 743 A (on the 1710 circuit). Real-time ratings show a range of improvements (10–25%) depending on the condi-tions (Table 3.9-2).
• If the 1710 and 1730 circuits are ever decommis-sioned, an evaluation of the paper insulation aging should be performed to determine if the typically higher operating temperatures on the 1710 cables show increased aging over the 1730 cables, since both circuits are made of similar vintage cable and have Table 3.9-1 Summary of United Illuminating Pipe Cable Uprating Methods
Forced Air Cooling
Recondi-tioning
Fluid
Filling Circulation
Forced Cooling
Reconduc-toring XLPE
Water Cooling
Dynamic Rating Location Harbor only Harbor only Circuit Circuit Circuit Circuit Circuit Harbor only Circuit Maximum
estimated increase
4.0%a
a. Based on testing in September-October 2001; small additional increase possible.
5.6%b
b. Land rating becomes limiting.
<3.0% 7.8% 25–50% 18.0% c
c. Not recommended for pipe-type retrofit with conventional technology.
5.6%d
d. Not recommended; air-cooling could achieve same result.
e
e. Rating improvements by dynamic ratings vary depending on circumstances.
Additional
mainte-nance
Minimal None Moderate Moderate High None None Moderate None
Environ-mental
Con-cerns
Low None High High High None None Moderate None
Estimated
Cost $100k $1.2M $1.5M $200kf
f. Increase in cost minimal after fluid filling.
$1.5Mg
g. Assumes fluid filling already done.
$3M c $500kd $200k
Figure 3.9-1 Blower assembly for forced-air cooling on
• Further work on DTCR may be warranted to pro-vide for a cyclic rating factor on emergency ratings of longer than 24 hours to fully take advantage of the
“dynamic normal rating” concept.
3.10 SUMMARY OF UPRATING AND