Palo Verde to Westwing Line 2 and Palo Verde to Rudd Double Line Outage

Probability Analysis SRP

Faustino L. Quintanilla flquinta@srpnet.com <mailto:flquinta@srpnet.com> Executive Summary This report details the mitigating factors and a double contingency outage analysis of the Palo Verde to Westwing line 2 and Palo Verde to Rudd line. This outage is considered of such low probability of occurrence and recurrence, that it warrants submittal to the WECC Phase I Probabilistic Based Reliability Criteria (PBRC) Performance Category Evaluation (PCE) Process. Under this process, a project Robust Line Design Features may be used to adjust from a Category C to a Category D. This report follows the Probability Reliability Evaluation Work Group (RPEWG) recommended steps provided in Appendix I, Figure 11 RPEWG Recommended Analysis Steps. Analysis of the Palo Verde to Westwing Line 2 and Palo Verde to Rudd line double contingency (N-2) qualifies to be moved to Category D based on the following statistical analysis and mitigating factors:

1) Robust Line Design Features mitigates the risk factors and contribute to a very low probability of a multiple line outage.

2) In the 7 years of accurately recorded outage history in electronic format, there has never been a double contingency outage of the Palo Verde to Westwing line 2 and Palo Verde to Rudd line. Evidence suggests that since both lines were in service, this outage has not ever occurred.

3) Westwing, Rudd and Palo Verde switchyard use breaker and a half arrangement. 4) As a result of the Rudd line installation in 2003, the Palo Verde to Westwing line

1 and 2 outage is no longer the most critical outage. 5) There are no Electric High Voltage (EHV) line crossings of the Palo Verde to

Westwing Line 2 and Palo Verde to Rudd 500kV lines. 6) The Robust design features are overhead ground wires, lines are built 130 feet

apart (centerline to centerline) with towers designed to fail in the middle. The failure and fall of one tower does not jeopardize the continued safe operation of the other tower.

7) Palo Verde to Westwing Line 2 and Palo Verde to Rudd line are located outside the areas of consideration for air traffic. The elevation of the lines is beyond and beneath the criteria FAA defines for consideration as an obstacle or hazard.

8) The isokeraunic level near Palo Verde and Rudd is one of the lowest in the Western US, ranging from 1.0 strike per square mile per year near Palo Verde to 2.5 strikes per square mile per year near Rudd switchyard.

9) The risk of earthquakes in Maricopa County is the lowest in the Western US. 10) The risks of flood, snow, and fire are negligible.

11) The foundations are over designed in the range of 137 to 199% for PV-WW lines and in the range of 101 to 151% for PV-RD.

12) The lattice tower design is conservative for weather related loads. 13) Lines are designed with state of the art spacer dampers to control conductor

motion. 14) The insulation level exceeds EPRI’s guidelines. 15) Electronic protection is provided by redundant microprocessor based technology

with communication via fiber optics and digital microwave systems on independent paths. A third microprocessor based relay system operating in current differential scheme is provided for backup protection.

16) SRP aggressively maintains the lines with twice yearly patrols, bird guard systems in place, an insulator-washing program, and a spacer damper replacement program.

In summary, based on the Robust Line Design feature analysis, excellent design and maintenance practices; it is recommended that this N-2 outage be moved to Category D (Extreme Events) with no other conditions or requirements.

Description of the Palo Verde to Westwing Lines 2 and Palo Verde to Rudd Path.

SRP (Salt River Project) is a major multipurpose reclamation project comprising two principal operating groups: the Salt River Project Agricultural improvement and Power district, a political subdivision of the state of Arizona; and the Salt River Valley Users’ Association, a private corporation. The district provides electricity to more than 933,000 customers in the Phoenix area. It operates or participates in eight major power plants and numerous other generating stations, including biomass, solar, thermal, nuclear, and hydroelectric sources. The District serves a 2,900-square-mile area spanning portions of Maricopa County (the metropolitan Phoenix area), Gila, and Pinal counties in central Arizona.

The two 500kV transmission lines discussed in this report connect the Palo Verde Nuclear Generating Station near Wintersburg to the Westwing Receiving station, north-northeast of Sun City West, or Rudd Receiving station south of Avondale (west of metropolitan Phoenix area). The common corridor line length is 10.9 miles. The lines are located to the west of the Phoenix metropolitan area. Below is the photograph of the Palo Verde yard looking west bound. The Westwing lines 1 and 2 are to the far right and Rudd line is adjacent to the Westwing lines.

Below, are two photographs of the Palo Verde to Westwing lines and the Palo Verde to Rudd towers.

The photo below depicts a tower on Palo Verde to Rudd Line north of I-10 crossing and diverging from the Westwing lines.

The photo above depicts a tower on Palo Verde to Rudd near the Palo Verde substation.

From the Palo Verde substation the lines proceed east for 2.2 miles east and then northeast for 8.7 miles as shown in the diagram below:

The along the common corridor of the Palo Verde to Rudd line and the Palo Verde to Westwing Line 2 do not cross any Electric High Voltage (EHV) lines.

The new Palo Verde to Rudd line is in the same corridor of the Palo Verde to Westwing lines for several miles outside of Palo Verde switchyard. The Palo Verde to Rudd line is the rightmost line.

Here is the photo of the Palo Verde to Westwing lines and Palo Verde to Rudd line at the Hassayampa River. The Palo Verde to Rudd line is the leftmost line.

An unusual January 2010 storm event resulted in high water flow within the normally dry Hassayampa River channel. The flood waters altered the main river channel causing partial reveal of foundations and exposed one of the tower foundations of the Palo Verde to Rudd line. The foundation was temporarily backfilled, while a more permanent solution is currently being designed with the cooperation primarily of Maricopa County Flood Control and the Army Corps of Engineers. The permanent solution will be implemented within the next 24 months.

Here is the photo of the Palo Verde to Westwing lines and Palo Verde to Rudd line at the Interstate 10(I-10) highway crossing. The Palo Verde to Rudd line is the leftmost line. Interstate 10 (I-10) is on the backside of the towers shown and receded below grade that is shown.

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Mitigating Factors I. Aircraft Operations in the Area of the Palo Verde to Westwing 500kV Lines: The Palo Verde to Westwing line 2 and Palo Verde to Rudd 500kV lines were reviewed by a licensed pilot for hazard location and elevation. The pilot found that these lines do not qualify as a hazard to Public Use or Military as defined by the FAA. The four areas of consideration to determine if it were to qualify as a hazard are: location relative to Public Airports, Military Airports, En-route operations and Aerial application operations. The review was based on current FAA Standards. Public Use Airport – The closest public airport to the transmission line is the Buckeye Municipal Airport. It is located approximately 3.2 nautical miles (nm.) south east of the line. The airport is a VFR (visual flight rules) airport with a runway length of 5500 ft. The runway fits the requirement for obstacle clearance considerations because it is a visual runway. Obstacle clearance criteria are based on the runway classification and five other areas of consideration. The 500kV lines are located outside the areas of consideration and the elevation of the lines is below all criteria the FAA defines for consideration as an obstacle or hazard. Military Airport - Luke Air Force base is located approximately 9 nm. south of the 500kV lines. The Air Force base has even more stringent obstacle clearance criteria. Due to the distance from the Air Force base, the 500KV lines are both beyond and beneath any area of consideration. En-route – The transmission lines are well below any criteria to be considered as an obstacle to an en-route IFR (Instrument Flight Rules) airway and are not located in any common corridor for visual operation. Aerial Application – The line is located to the west and north of the White Tank Mountains. This area is not a developed agricultural area and no aerial application activities have been observed in this area. II. Analysis of natural hazards on the Palo Verde to Rudd 500kV line Right of Way and

Hassayampa area locations. SRP’s Water Resource operation group prepared an evaluation of the natural hazards that could impact the two transmission lines from Palo Verde Nuclear Generating Station near Wintersburg to the Interstate 10 (1-10) crossing, when Palo Verde to Rudd line diverges away for the Palo Verde to Westwing lines. А. Lightning and Thunderstorms Thunderstorms are most numerous during the summer from late June to September. Lightning flash density tends to decrease from east to west over the Phoenix metro area.

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(A study by the National Severe Storms Laboratory (1995), using data from 1984 and 1987-93, shows the following flash densities (see Appendix 1, Figures 1 and Figure 2): Westwing area 2.5 strikes per square mile per year Hassayampa area 1.5 strikes per square mile per year Palo Verde NGS area 1.0 strikes per square mile per year In contrast, the flash density over north Scottsdale, AZ in the far northeast Valley is 4.0 strikes per square mile per year. Luke Air Force Base is located 17 miles east of the Hassayampa site, 10 miles south of Westwing and 30 miles east-northeast of Palo Verde NGS. Luke AFB has an average of 26 days per year when thunder is heard (with a minimum of 13 and a maximum of 40 days) during the 1964-2001 period. B. Damaging Winds and Dust Storms High winds often occur with summer thunderstorms. Potentially damaging winds (50 or more knots) from micro bursts in the West Valley are rare. Luke AFB records (1946-96) indicate that a daily peak wind gust between 50 and 64 knots (58-74 mph) occurs only once in three years. August is the month of greatest likelihood. Wind gusts of 65 knots (75 mph) or more happen only during the summer and fall, July through October, with each month having a recurrence interval of once in 33 years. The maximum peak gust reported at Luke AFB is 88 knots (101 mph) on August 29, 1996. Thunderstorm winds often stir up dirt from the Valley floor. Luke AFB reported an average of 17.5 hours with blowing dust per year from 1973 through 1996. The majority of the hours were in July, August and September. However, SRP has no recorded transmission outages due to dust storm flashover. A microburst study was done by James Walter, Water Resource Operations, using weather balloon data from PAB for July-August, 1999-2010. The 50-year return period microburst wind speed for the Phoenix metro area (including the Hassayampa Valley) is 103.6 mph. This is compares closely with the 101 mph wind reported at Luke AFB and 106 mph at Deer Valley Airport (September 22, 2007). C. Tornadoes

Tornadoes are a very rare event in the Phoenix area. Tornadoes not reaching the ground are the most common ones, although still rare. One or two tornadoes per year may be reported in the Valley as a whole. Damage potential of any Valley tornado is small, estimated at F0 to F1 on the Fujita tornado intensity scale. The National Weather Service’s Storm Prediction Center estimates that from 1950 through 2001, 61-80 percent of tornadoes reported in Maricopa County were weak (F0-F1) while only 1 to 20 percent were strong (F2 or F4).

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D. Earthquakes The American Society of Civil Engineers Manual 7 (ASCE7-2005, Figure 22-1) identifies this portion of the PL-RUD transmission line within one of the lowest potential risks areas from earthquake damage in the western United States (a maximum expected ground motion spectral response acceleration between 16 and 20 percent of gravity force (g) for a 0.2 second period). International Building Code classification (IBC 2006, Table 1613.5.2) would describe subsurface soil conditions along this portion of the line as Class C (very dense soil and soft rock). The US Geological Survey’s Earthquake Ground Motion Parameters software (version 5.0.9a) can be used to analyze both information sources. The resulting adjusted maximum spectral response factors for design (SDS = 0.135g, SD1 = 0.074g) place this portion of the transmission line in the IBC’s lowest risk seismic design category based on short-period response acceleration (Category A) and the second lowest risk seismic design category for longer (1-second period) response acceleration (Category B, assuming occupancy category III for a public utility facility).

E. Flood and Fire Hazards No significant threat from fire or flood exists. Vegetation over this low desert area is sparse and low, generally less than 6 feet high. There are a few areas with slightly taller trees or bushes (10-15 feet high) and they pose no danger to the power plant or transmission line towers in case of range fire. Refer to Figure 3 and 4 of Appendix I for typical vegetation under the Palo Verde to Westwing lines. A service hydrologist at the Phoenix National Weather Service office surveyed the two crossover sites in 2003. The Westwing site is about 3 miles west of the center of the Aqua Fria river channel. Flow in the Aqua Fria is controlled by releases from Lake Pleasant through the new Waddell Dam. During the flooding of February 1980, a peak release of 66,600 cubic feet per second (cfs) was recorded. This release has a recurrence interval of 47 years. A photomap of that episode shows that the western edge of the flood flow was at least a mile from the Westwing station and over 2 miles from the site. A dry wash, which connects to the Aqua Fria several miles downstream, was noted between two of the towers closest to Westwing station. Signs of previous high flow were well below the base of the towers. The Hassayampa site is about 3 miles east of the Hassayampa River channel centerline. Peak flow on the Hassayampa was 47,500 cfs in 1970. F. Snow and Ice Hazard No threat exists from snow or ice accumulation. No measurable snow was reported at Luke AFB from 1951 through 1991. No freezing drizzle or rain has ever been reported on the desert floor of Maricopa County.

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III. Design A. Foundations

I. Palo Verde to Westwing Lines

A series of inspection reports by the project's geotechnical consultant (SH&B) from 1979 was found in the Palo Verde to Westwing files. These reports noted that the first 41 tower foundation borings were field-tested and it was concluded that the subsurface conditions were equal to or better than anticipated to resist uplift forces. A review of the geotechnical engineer's report design charts was done and concluded that the footings were designed in accordance with their recommendations for ultimate load capacity. The second approach was to compare foundations for Towers 1 thru 71 of this line with the foundation design for Towers 1003 thru 1075 of the Mead-Phoenix 500kV transmission line, which are in the same vicinity. It is recognized, that the design for Palo Verde-Westwing lines I and II was based on soils information and design methodologies from pre-1978, with the design actually based on 1950's and 1960's methods. Conversely, the Mead-Phoenix foundation design was based on a combination of the latest EPRI computer foundation design methods and a statistically based soils analysis using a much larger database than the earlier work had available. Design information from both sets of lines was compared in adjacent sections (foundation in uplift was the worst case) to determine their relative uplift capacity. For this set of towers, the PV-WG foundations were determined to be over-designed in the range of 137% to 199%, depending upon soils type and foundation size. This means that on the average, the tower foundations for the PV-WG transmission lines should carry 159% higher load than required by structure design. This value includes all overload and safety factors.

II. Palo Verde to Rudd Lines

Foundation design calculations are not available for the PL-RUD transmission line. Review of file drawings indicate that the foundations were generally designed to match adjoining tower foundation dimensions found on the PL-WG lines since tower structures are similar. Existing geotechnical reports and foundation design charts for the PL-WG 500kV lines were reviewed for this analysis. Investigation work was performed and reports were prepared by Sergent, Hauskins and Beckwith in 1978 and 1984. A combined total of 14 subsurface borings were completed during both investigations. This data is supplemented with two borings performed by SRP in early 2010 at PL-RUD transmission structures near the Hassayampa River for a river scour hydrologic evaluation.

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The 1978 SHB report provides foundation design zones and recommendations for pier depth along this line segment. General geologic maps were also reviewed to better define changes in near-surface strata. For this review blow count data within the SHB design zones (Zones 8 through 11) were statistically re-evaluated and correlated to foundation engineering design properties.

The analysis indicates that all towers meet or exceed factored loads under current codes. Tangent tower foundation capacities are 101% to 151% of factored loadings while angle tower foundation capacities are 101% to 224% of factored loadings. On the average, tower foundations on the PL-RUD transmission line should carry 131% higher loads than required by factored loading conditions (including all overload and safety factors).

III. Hassayampa River

An unusual January 2010 storm event resulted in high water flow within the normally dry Hassayampa River channel. The flood waters altered the main river channel causing partial reveal of foundations and exposed one of the tower foundations of the Palo Verde to Rudd line. The foundation was temporarily backfilled, while a more permanent solution is currently being designed with the cooperation primarily of Maricopa County Flood Control and the Army Corps of Engineers. The permanent solution will be implemented within the next 24 months.

B. Weather Related Loads A loads review of the Lattice Towers on the Palo Verde to Westwing Line 2 and Palo Verde to Rudd lines was conducted. The towers were designed for NESC Light Load District, Grade B Construction. This is effectively the minimum design criterion for defining weather related loads. However these loads are "deterministic" and not loads based on actual statistically evaluated weather events. To quantify the general structural reliability of the tangent structures, SRP compared the capacity of these structures as compared to statistically based loads.

I. Palo Verde to Westwing Lines

In general, structures with line angle loads are controlled by line tension, not weather loads, so only tangent structures (5T2 and 5T3 Towers) were evaluated as being most vulnerable to a weather related loads. This evaluation assumed that developed wind loads on the towers were based on a review of the tower drawings. Wind areas were based on rough approximations of quantity of angles in various sections of the tower. Wind loads on conductors were based on actual spans and elevations on structures. NESC shape factors were used for lattice towers. A shape factor of 1.2 was used for the static (dia < .5")

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Wind loads were developed per the methods shown in the 2002 edition of the NESC for Rule 250C application of wind loads (height effect and gust response considered). Line tensions for winds greater than 60mph were approximated. The angles on the tangent structures were 2 degrees are less. Based on a comparison of approximated tower capacities with actual loads along the lines:

The MINIMUM Tower capacity is 98mph wind gust The AVERAGE Tower capacity is 100mph wind gust

Based on 90mph being the 50 year extreme wind event, the respective Return Periods (RP) are

Minimum Return Period = 135 years Average Return Period = 150 years

The probability of occurrence of a wind of any RP magnitude is 1/RP in any year (i.e.: 1/50 probability of a 90mph wind in any year). Therefore the lattice tower design is conservative for weather related loads. Figure 5 of Appendix I shows a typical tower footing.

II. Palo Verde to Rudd Line

Analysis of tower capacity was conducted using Power Line System’s TOWER program for nonlinear truss analysis of steel lattice structures and PLS-CADD for analysis of conductor span behavior. For the purposes of stability assessment, structure T-21 was selected as the most heavily loaded 5T3 tower on the PL-RUD line. Stability of T-21 was studied for NESC Light loading in accordance with Rule 250B and NESC Extreme Wind loading in accordance with Rule 250C. The model performance under these loading conditions indicates that NESC Light loading is the governing load case representing the maximum structural capacity of T-21 and therefore all 5T3 structures in the PL-RUD line. The NESC Light loading case utilizes an overload factor (OLF) of 2.5 on the applied wind load of 9 psf. Therefore, the actual wind load applied under NESC Light loading conditions is 22.5 psf (93 mph) which is approximately representative of the 100 year wind event (97 mph) for the Phoenix Metropolitan area. Under NESC Light loading, the maximum member usage is 105% in the lower portion of the tower portal and the conductor bridge. Based on the location of high stress members within the upper reaches of the structure it is unlikely that a collapse would make contact with it the adjacent towers of PL-WW2. It should be noted that a perfectly hinged failure which is a very conservative representation of the more likely collapse mechanisms resulting in larger clearances than those available.

C. Design – Lattice Towers and Insulators

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The Palo Verde to Westwing Line 2 and Palo Verde to Rudd 500kV lines are constructed on lattice towers, typical spans of 1300 feet, 130 feet of centerline-to-centerline separation between lines, each circuit constructed with 3-1780kCM 84/19 ACSR “Chukar” conductors per phase. The Palo Verde to Westwing Line 2 static wire consists of 27 No. 8 alumoweld overhead ground wires for lightning protection. Also, the Palo Verde to Rudd static wire consists of 132mm Focus optical ground wire (OPGW) overhead for lightning protection The two lines are located in the Arizona Sonoran Desert west of the Phoenix metropolitan area. The region is a relatively low isokeraunic level, approximately 30, that translates into a GFD (Ground Flash Density) of approximately 5.0 flashes per square mile per year. The Design Criteria required tower footing resistance values to be 15 ohms or less and typical values were about 5 ohms. The low footing resistance combined with a good shielding angle and two overhead ground wire provide excellent lightning performance. The vegetation in the Arizona Sonoran Desert, where the two Palo Verder to Westwing lines are located, does not grow of sufficient height to present potential outage problems due to direct flashover or from fires. If there was a brush fire of sufficient magnitude, SRP would likely elect to de-energize the line during a fire to minimize any likelihood of a fault, but the possibility of a fire is extremely unlikely due to the very small amount of vegetation that could contribute to any wild fire. SRP’s twice yearly helicopter and ground patrols identify any and all vegetation problems for corrective action. The transmission lines are in a region of low contamination, V-String Insulator Assemblies and 27 porcelain insulators per leg or polymer insulation with corona ring equivalent. This insulation level exceeds EPRI’s “Transmission Line Reference Book, 345kV and Above” criteria from Table 10.2.1 of 24 insulator units. The three additional units provide supplemental insulation in the event of damage or punctured units. The Palo Verde to Westwing Line 2 was designed with state of the art spacer-dampers to control conductor motion. Approximately 15 years ago SRP removed numerous serviced aged units, tested the units and compared performance with new units. Based on test results, the spacer-dampers have many years of service remaining. But the twice-yearly helicopter and ground patrols identify damaged units for replacement. SRP’s aggressive maintenance program identifies damaged insulators, conductor and spacer dampers prior to potential line outages. SRP stocks replacement, open body type spacer dampers, or repair material for these 500kV lines and has the trained personnel for either hot-line maintenance or under outage conditions. The Palo Verde to Rudd line utilizes the open body type spacer dampers, replacement type spacer-dampers of the Palo Verde to Westwing Line 2, to control conductor motion. The spacer-dampers have been in service for 7 years. The twice-yearly helicopter and ground patrols identify damaged units for replacement. SRP has an aggressive maintenance program that identifies damaged insulators, conductor and spacer dampers prior to potential line outages. SRP stocks replacement or repair material for these

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500kV lines and has the trained personnel for either hot-line maintenance or under outage conditions. D. Protective Relaying The protection for the Palo Verde to Westwing Lines 2 and Palo Verde to Rudd is provided by redundant microprocessor based technology, permissive overreaching relay schemes, communicating via fiber optics and digital microwave systems on independent paths. Independent dual channel transfer trip systems are provided over the redundant communication channels. In addition, a third microprocessor based relay system, operating in a current differential scheme is provided for back up protection. This scheme utilizes relay-to-relay communication over one of the redundant communication paths. The microprocessor technology of this equipment incorporates self testing and monitoring capabilities for identifying critical problems, removing tripping functions from service, and alarming to the dispatching offices via the EMS system, where the trouble alarms receive a high priority. The use of total redundant systems, allows for one scheme to be out of service to correct problems, without compromising the protection of the line. Situations can be identified and corrected before incorrect operations occur on an unfaulted line, decreasing the likelihood of a simultaneous double line outage. The Palo Verde to Westwing line 2 and Palo Verde to Rudd line are not equipped with single-pole nor high speed reclosing relays. In the North West, where very long paths are present, generation is remote, and the isokeraunic level is high, single pole and high speed reclosing is advantageous. In the Valley of the Sun, where SRP is located, load and generation are closely located. Our generation is not “remote”. Therefore tripping a large amount of generation is not a recommended procedure. The reason SRP does not employ these relays and single pole reclosing is because of SSR problems discussed in the “Transmission Line Reclosing – turbine generator duties and stability considerations” article by P.G. Brown and R. Quay presented at the 29th conference of Relay Engineers in 1976. IV. Maintenance The Palo Verde to Westwing line 2 and Palo Verde to Rudd line are built 130 ft. apart (centerline to centerline). This is sufficient to ensure that one tower failing for any reason would not fall into or jeopardize the continued safe operation of the other. Probably the biggest risk to these lines is vandalism, which almost always occurs from shooting of insulators. SRP’s practice of performing an aerial patrol in the spring and fall of each year and then performing a ground patrol once a year should be sufficient to find any vandalism areas and to make necessary repairs before they result in an outage. There are several areas where insulator contamination from birds has been a problem, even resulting in an outage to one of the lines years ago. The largest birds capable of coming in contact with the lines are small and incapable of causing a phase to phase fault. A picture of a typical large bird is shown in Appendix I, Figure 6. SRP has been

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proactive in constructing protective devices and placing them above insulators at several locations on these two lines. These protective devices contain the bird droppings and keep them from falling on the insulators. The protective devices are shown in Appendix I, Figures 7 and 8. The devices are flexible enough that winds blow the bulk of the material away before the weight is high enough to be of any concern structurally. Pacific Gas and Electric has used these bird guards successfully for a number of years. SRP’s solution is taken from PG&E’s lead in this area. SRP also owns two separate power washers used for washing insulators when contamination is sufficient to warrant it. Insulator washing, with the tools and training available to SRP, can be done with the lines energized. Changing of insulators, repair of gunshot conductor, replacement of other hardware are all maintenance activities that SRP performs with the lines energized. Also, our company has the equipment and trained personnel to perform live line bare hand maintenance. Many companies don't have this capability. There are several deteriorating items on these lines that SRP monitors and replaces as needed. One of these is the spacer dampers, which SRP changes as they deteriorate and fail. At this point, the failure rate has been low enough that a major project to replace all of them has not been necessary. However, the insulators on the Palo Verde to Rudd line are polymer. SRP does not replace energized polymer insulators or associated line hardware. Currently, there is no polymer insulator tester on the market to quantify the insulation availability, as required by energized or barehand work practices. Conductor and other wire repairs can be done while energized, but polymer insulators or associated line hardware work requires a line outage. SRP has also been proactive in the identification and marking for potential aircraft hazards. One location that was identified was where the two lines cross Interstate 10 west of the town of Buckeye. It is conceivable that a low flying airplane could snag one of the shield wires and pull it across both lines. The lines at this location are less than 170 ft. above ground, which is well below the FAA minimum height for airplanes. However, since this is a known potential route for small airplanes, SRP has marked the shield wires at this location with aircraft warning balls. SRP will continue to maintain these balls and replace them if and when they fail.

V. Substation Configuration As can be seen from the two one line diagrams in this section, both the Rudd substation and the Palo Verde substation are breaker and a half arrangement. At either substation, if there is a breaker out for maintenance, it would require both (1) a false trip on one line and (2) a stuck breaker on the other line to result in a loss of both lines because of substation configuration.

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System Configuration Palo Verde – Westwing Line 2 and Palo Verde – Rudd Double Line Outage Reclassification The Palo Verde/Hassayampa Hub area is a major electrical energy trading hub in the United States. There are currently Independent Power Producer generators at the Palo Verde hub in addition to the existing Palo Verde Nuclear Generating Station. This year there are no new 230kV & 500kV transmission system enhancements or generator interconnectors synchronizing to the transmission system. Current Generators (existing) Total (Net) Palo Verde Nuclear 4032MW Arlington Valley 600MW Pinnacle West Red Hawk 1040MW Sempra Mesquite 1260MW Harquahala 1140MW Panda Gila River(network) 2334MW* Total Generation Value 10406MW *The TECO Panda Gila River interconnection is not a radial connection to the Palo Verde – Hassayampa hub. This interconnection has a 230kV tie into the Gila Bend – Liberty circuit. This generation interconnection has a reduced interaction, 0.5 to 1 as compared to the direct interconnectors, at the Palo Verde – Hassayampa hub, as measured in enhancing generating capability and requirement of curtailment. There are currently seven 500kV circuits emanating out of the Palo Verde/Hassayampa hub; Palo Verde – Westwing #1 & #2, Hassayampa – Jojoba – Kyrene, Hassayampa – Pinal West (SEV – Pinal Central) and Palo Verde to Rudd lines are part of Palo Verde East Path. Palo Verde – Devers and Hassayampa – North Gila are part of Palo Verde West – not rated as path, it is a part of EOR. Most recent operating studies (SGC) were performed in 2008 to determine the amount of generation that could be safely interconnected to the Palo Verde/Hassayampa hub without Remedial Action Scheme (RAS) and with RAS. The conclusions of this study were that there are no thermal or transient stability limitations to the generation injection in the PV/HAA hub. The all 10406MW of generation may operate at one time. The amount of MVARs that are produced or absorbed by the Palo Verde nuclear units determines the amount of generation that has to be used in the RAS for safe operation. The most limiting condition studied was with the Palo Verde 500kV bus at 525kV and the Palo Verde units absorbing 800MVARs. These constraints come from the operators of the Palo Verde Nuclear Generating Station. The 2008 Summer Operating Study proved that no generation interconnecting at Palo Verde/Hassayampa hub needs to be armed to trip even at 800MVAr bucking conditions.

The simultaneous outage of the Palo Verde – Westwing #2 & Palo Verde –Rudd circuits is initiated by a single line to ground fault at the Palo Verde bus. This is a Category C outage per the WECC Table W-1. Category B, three phase faults applied at the Palo Verde bus with a single circuit contingency outage are more restrictive than this Category C outage. The three standards: transient voltage dip, minimum frequency or post transient voltage deviation, were monitored at Palo Verde, Kyrene and North Gila. The 2008 Summer Operating Study proved that the worst impact on transient stability behavior has Hassayampa to North Gila 500kV line outage (18.3% at Palo Verde 500kV bus). The table below shows that with any one of the circuits emanating from Palo Verde initially out of service, a subsequent SLG fault on the Palo Verde 500kV Bus which would take both PV-WW #2 & PV-Rudd 500kV lines out of service will not exceed the 30% voltage dip criterion during the first swing after the fault. SINGLE-LINE-TO-GROUND FAULT AT THE PALO VERDE 500KV BUS TRIPPING THE PALO VERDE/WESTWING #2 & PALO VERDE/RUDD 500KV LINES (all cases are well below the required 30% voltage drop criteria)

500kV LINE IOS VOLTAGE DROP (WORST CASE)

HASSAYAMPA/NORTH GILA 11% PALO VERDE/DEVERS 10.5% KYRENE/JOJOBA 11.9% PALO VERDE/WESTWING 1 5.4% HASSAYAMPA/JOJOBA 12% HASSAYAMPA/PINAL W. 9.3% IOS = Initially out of Service

1. 2008 SUMMER 2. PV/HAA GEN NET=9906MW W/ PV = -422MVAR, HAA = -378MVAR (NET –800MVAR). 3. SLG FAULT ON PV 500KV BUS; WITH PV-WWG #2 & PV-RUDD CKTS OUT. 4. MONITORED VOLTAGES: ARIZONA, CALIFORNIA, UTAH, AND NORTHWEST

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Appendix I Figure 1 and Figure 2 Caption: Flash density in for US from 1997-2007. The lighting density of flashes per square mile per year is shown by scale for figures below. Figure 2 shows flash density from NSSL 1995 and values in gray exceed 8. 500 kV lines in red; 345 kV lines in black. Salt River Project service areas outlined in black.

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Figure 3. Tallest tree underneath the Palo Verde to Rudd 500kV Lines (left most line).

Figure 4. Tallest tree underneath the Palo Verde to Westwing line 2 (center line).

Figure 5. Typical tower footing.

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Figure 6. A hawk.

Figures 7 and 8. Protective bird guards on the Palo Verde to Westwing line 2.

A-4

Figure 9. A typical spacer damper on Palo Verde to Westwing Line 2.

Figure 10. A typical spacer damper on Palo Verde to Rudd. (Replacement spacer

damper for Palo Verde to Westwing Line 2)

A-5

Figure 11. RPEWG Recommended Analysis Steps. Seven step Process for PBRC adjustment: 1. Provide Complete Project Description, and why it is being considered for PBRC

adjusted rating, including supportive data: a. Overview of terminations b. Physical Layout and Transmission Construction c. Substation Configurations d. Protective Relaying e. Isochronic Level f. Aircraft Hazard g. Fire Hazard

2. Identify the Statistical Base to be used:

a. Historical b. Similar Lines c. Mileage d. Terrain e. Climate

3. Determine Uncorrected of Mean Time Between Failure (MTBF)

• All events should be counted and considered, and then select events and

circumstances can be removed on a case-by-case basis. 4. Provide a Corrected Estimate of MTBF (based on Project Robustness Features)

• Consider various robustness features introduced to reduce the risk of outage. For examples see reference [1].

5. Complete Exposure Analysis. (Refer to example) 6. Illustrate the Consequences of Outage. (Refer to example) 7. Conclude the how the adjustment meets the PBRC criteria. (Refer to example) Reference [1] Robust Line Design Features, RPEWG working paper, 5/28/02.

Appendix II Palo Verde/Hassayampa Station Site 500kV Transmission Lines. Failure Mode analysis of 500kV 5T2 and 5T3 Transmission Tower (Power Engineers Report)

Foundation Review: Palo Verde to Rudd 500kV Transmission Line Lattice Towers T1 through T36 (SRP Engineering Service Memorandum) Palo Verde to Rudd 500kV Transmission Line Tangent 5T3 Lattice Tower Review (SRP Engineering Service Memorandum)

MEMORANDUM Date: August 10, 2010

File No.: 13-2-5

To: Tino Quintanilla, Senior Engineer, Transmission System Planning

From: Peter Kandaris, Senior Principal Engineer, ESP&E

Re: Foundation Review: Palo Verde to Rudd 500kV Transmission Line Lattice Towers T1 through T36

In response to your request we have completed foundation capacity analyses for Towers 1 through 36 of the Palo Verde to Rudd 500kV transmission line. We have also reviewed existing general geologic and geohydrologic hazard data on this line segment. The enclosed summary report provides a synopsis of our findings.

INTRODUCTION

The Palo Verde to Rudd 500kV transmission line (PL-RUD) originates near the northeast corner of the Palo Verde Switchyard (Section 34, T1NR6W) in western Maricopa County, Arizona. Tower 1 is just east of the switchyard property line and runs parallel to and on the south side of the existing Palo Verde to Westwing 500kV transmission lines (PL-WG). The line extends generally eastward for less than two miles to Tower 8 and then turns to the northwest for a little more than seven miles to Tower 36 (Section 2, T1NR5W) just north of Interstate 10 in Buckeye Arizona. At this location the PL-WG transmission lines continue to the northwest while the PL-RUD line turns east-southeast.

The PL-RUD line (jointly owned by the Salt River Project and Arizona Public Service) was completed in 2003 as part of the Southwest Valley 500kV Transmission Project. The project was managed by Arizona Public Service and constructed by PAR Electrical Contractors, Inc. All angle structure foundations supporting four-legged lattice steel towers utilize partially embedded stub angles encased within reinforced concrete drilled shafts. Tangent towers utilize full length stub angles encased within unreinfored concrete drilled shafts. A summary of the structures and footing descriptions are included in Appendix A.

FOUNDATION CAPACITY ANALYSIS

Foundation design calculations are not available for the PL-RUD transmission line. Review of file drawings indicate that the foundations were generally designed to match adjoining tower foundation dimensions found on the PL-WG lines since tower structures are similar. The foundations at Tower 2 and Tower 36 differ from this assumption: Tower 2 and the adjoining PL-WG tower angle structure foundations were redesigned and constructed new during the 2003 work: Tower 36 is a large angle structure that adjoins tangent towers and was redesigned for the higher load conditions.

Existing geotechnical reports and foundation design charts for the PL-WG 500kV lines were reviewed for this analysis. Investigation work was performed and reports were prepared by Sergent, Hauskins and Beckwith in 1978 and 1984. A combined total of 14 subsurface borings were completed during both investigations. No laboratory tests were done on

2

samples. Standard penetration blow counts were obtained near surface and every five feet thereafter within each boring. This data is supplemented with two borings performed by SRP in early 2010 at PL-RUD transmission structures near the Hassayampa River for a river scour hydrologic evaluation. Boring depths for all investigations range from 15 to 30 feet.

The 1978 SHB report provides foundation design zones and recommendations for pier depth along this line segment. General geologic maps were also reviewed to better define changes in near-surface strata. For this review blow count data within the SHB design zones (Zones 8 through 11) were statistically re-evaluated and correlated to foundation engineering design properties. Low-bound 90% confidence interval limits were utilized to develop design parameters. Calculations are provided in Appendix B.

Design foundation uplift capacity values were developed based on the foundation geometry and the low-bound soils properties (it was determined that uplift loads controlled foundation design for all towers). These capacities are found to be match reasonably well when compared to the original SHB design charts (Note: the SHB design charts include safety factors while the SRP approach utilize a reliability-based statistical approach that results in higher predicted capacity estimates).

SRP also performed PLSCADD evaluations of general tower types utilized on this line segment to determine foundation uplift loading based on current code requirements. This structural analysis should be considered conservative (done at initial tensions). A specific load analysis was performed for Tower 2 under final tensions since it was determined that this tower’s foundations are relatively shallow and small when compared to the actual loads and other tower foundations. A summary of the results are included in Appendix B.

The analysis indicates that all towers meet or exceed factored loads under current codes. Tangent tower foundation capacities are 101% to 151% of factored loadings while angle tower foundation capacities are 101% to 224% of factored loadings. On the average, tower foundations on the PL-RUD transmission line should carry 131% higher loads than required by factored loading conditions (including all overload and safety factors). A summary of the load analysis is included in Appendix A.

GEOLOGIC AND GEOHYDROLOGIC HAZARD ANALYSIS

An unusual January 2010 storm event resulted in high water flow within the normally dry Hassayampa River channel. The flood waters altered the main river channel causing partial reveal of foundations for Towers 144 of both PL-WG lines. The river bank also eroded and approached the edge of foundations for Tower 28 of the PL-RUD line. Review of the tower drawings indicate that the PL-WG lines were designed to include the bank loss that occurred during the flood event, but PL-RUD Tower 28 design did not assume sufficient ground loss when compared to the actual new adjacent channel (only 5 feet for the +7-foot embankment). Studies by SRP have concluded that the bank has shifted over 300 feet to the southwest since the original line was installed in 1979 from periodic flows.

SRP contracted with WEST Consultants, Inc., to perform detailed hydrologic analyses of current and future river flow conditions. The study determined that only these three towers could be impacted by Hassayampa River flood waters within a 50-year time frame. Based on this report, SRP has determined that it will remediate and armor the riverbed around the affected towers on all three lines. The embankment will be returned to its original elevation in the proximity of the towers and armoring will be installed to prevent future erosion and potential scour of the

3

foundations from atypical floods. Preliminary remediation designs are currently being prepared. It is expected that remediation will be performed within the next 24 months.

Subsidence and earth fissures have been reported in western Maricopa County for many decades. A review of current subsidence maps available from the Arizona Department of Water Resources shows the nearest subsiding basin at least three miles to the east of this segment of the PL-RUD transmission line. Ground subsidence is not expected to impact the transmission line.

In 2001 the Arizona Geological Survey discovered a lone earth fissure to the southeast of the Palo Verde Nuclear Generating Station. This fissure appears to be unrelated to any subsiding basin and is contained within the very strongly cemented subsurface soils of the region. The fissure most likely relates to its proximity to the edge of a steep underground rock interface within surface alluvium. The 1300-foot long north-south running fissure (mostly buried by surface sediments, but visible on aerial photographs) is approximately 1200 feet southwest of PV-RUD Tower 6 and 1000 feet southeast of Tower 7. Fissure growth appears very slow and to the south, away from the transmission line alignment. There are no indications of other fissure activity and this feature is not considered a hazard to transmission line foundations. Additionally, Towers 6 and 7 are founded on very stable alluvial fan deposits that overlie near-surface bedrock.

CLOSURE

Please contact me if you have any questions or need additional information.

REFERENCES

Sergent, Hauskins and Beckwith (1978) “Westwing Substation to Palo Verde Plant Site: 500kV Transmission Line,” Project E78-45 (PL-WG I), April 21.

Sergent, Hauskins and Beckwith (1984) “Westwing Substation to Palo Verde Plant Site: 500kV Transmission Line,” Project E84-24 (PL-WG II), April 13.

Demsey, K. A. (1989) “Geologic Map of Quaternary and Upper Tertiary Alluvium in the Phoenix South 30’x60’ Quadrangle, Arizona,” Arizona Geological Survey Open File Report (OFR 89-7), revised August 1990.

Harris, R.C. (2001) “A new earth fissure near Wintersburg, Maricopa Co, AZ,” Arizona Geological Survey Open File Report (OFR 01-10), November.

Arizona Department of Water Resources (2008) “Land Subsidence in the Buckeye Area, Western Maricopa County Based on ADWR EnviSat Time-Series InSAR Data, “ time period of analysis: 1.2 years 02/10/2007 to 04/05/2008.

Arizona Geological Survey (2009) “Earth Fissure Map of Wintersburg Study Area: Maricopa County, Arizona,” Digital Map Series - Earth Fissure Map 10 (DM-EF-10), February.

WEST Consultants, Inc. (2010) “500 kV Electric Transmission Structures Hassayampa River Hydrologic Engineering Services,” July.

MEMORANDUM August 17, 2010

Report No: CE-573

To: Tino Quintanilla, Senior Engineer, Transmission System Planning

From: Zack Heim, Engineer, ESP&E

Re: Palo Verde to Rudd 500kV Transmission Line Tangent 5T3 Lattice Tower Review

In response to your request, Electric Systems Engineering (ESE) has conducted a review of the stability of the 5T3 500kV tangent lattice towers located between Palo Verde Switchyard and T-36 on the Palo Verde to Rudd 500kV Transmission Line (PL-RUD). A series of two analyses were conducted to evaluate the potential for contact between a failing 5T3 structure on the PV-Rudd 500kV Line and the adjacent Palo Verde to Westwing-2 500kV Transmission Line (PL-WW2) as well as the potential for a cascading failure. Neither analysis indicates that the potential failure modes of the 5T3 tower will pose a significant risk to the adjacent PL-WW2 structures as discussed herein.

INTRODUCTION

The Palo Verde to Rudd 500kV transmission line (PL-RUD) originates near the northeast corner of the Palo Verde Switchyard (Section 34, T1NR6W) in eastern Maricopa County, Arizona. Tower 1 is just east of the switchyard property line and runs parallel on the south side of the existing Palo Verde to Westwing 500kV transmission lines. The line extends generally eastward for approximately two miles to Tower 8, then turns to the northeast for a little more than seven miles to Tower 36 (Section 2, T1NR5W). At this location the adjoining transmission lines continue to the northeast while the PL-RUD line turns east-southeast.

The PL-RUD line (jointly owned by the Salt River Project and Arizona Public Service) was completed in 2003 as part of the Southwest Valley 500kV Transmission Project. The project was managed by Arizona Public Service and constructed by PAR Electrical Contractors, Inc. Structures T-1 to T-36 are latticed steel tower structures and the remaining structures from P-37 to P-192A are single shaft tubular steel poles. The latticed steel towers utilized for PL-RUD were constructed in accordance with 500kV lattice structure designs originally developed for APS by Bethlehem Steel Corporation circa 1978. The line is currently configured to support a three conductor bundle of 1780 kCM “Chuckar” ACSR and two 132mm Focas optical ground wires (OPGW).

The PL-RUD line was constructed at an offset of 130 feet from the existing PL-WW2, thus providing a minimum 36 foot clearance between adjacent structures.

TOWER CAPACITY ANALYSIS

Analysis of tower capacity was conducted using Power Line System’s TOWER program for non-linear truss analysis of steel lattice structures and PLS-CADD for analysis of conductor span

behavior. For the purposes of stability assessment, structure T-21 was selected as the most heavily loaded 5T3 tower on the PL-RUD line. Structure T-21 is a 124 foot 5T3 tower configured with a 36 foot body extension atop 30 foot leg extensions on all quadrants. T-21 supports a back span of 1,500 feet and a span ahead of 1,480 feet at a maximum initial design tension of 12,500 lbs.

Stability of T-21 was studied for NESC Light loading in accordance with Rule 250B and NESC Extreme Wind loading in accordance with Rule 250C. The model performance under these loading conditions indicates that NESC Light loading is the governing load case representing the maximum structural capacity of T-21 and therefore all 5T3 structures in the PL-RUD line. The NESC Light loading case utilizes an overload factor (OLF) of 2.5 on the applied wind load of 9 psf. Therefore, the actual wind load applied under NESC Light loading conditions is 22.5 psf (93 mph) which is approximately representative of the 100 year wind event (97 mph) for the Phoenix Metropolitan area. Under NESC Light loading, the maximum member usage is 105% in the lower portion of the tower portal and the conductor bridge. A complete summary of load case parameters and structural analyses are provided in Appendix A.

Based on the location of high stress members within the upper reaches of the structure it is unlikely that a collapse would make contact with it the adjacent towers of PL-WW2 as shown in Figure 1. It should be noted that Figure 1 demonstrates a perfectly hinged failure which is a very conservative representation of the more likely collapse mechanisms resulting in larger clearances than those shown.

Figure 1 Graphical representation of perfectly hinged failure

CASCADE ANALYSIS

One concern surrounding the initial failure of a tangent structure is the potential for a cascading failures of adjacent towers which become overloaded due to impact and additional line loads. An analysis was conducted to determine the likelihood of a cascading failure in the event that a 5T3 structure failed as shown in Figure 1. Cascade loading was modeled using PLS-CADD to simulate the overturning of the bridge section on the initially failed structure and falling directly adjacent to the failed tower base as shown in Figure 2. Through finite element span analysis, it was determined that phase conductors would not restrain the bridge section of the initially failed

tower from impacting the ground while both OPGW’s would function to arrest the bridge section’s fall. However, analysis results indicate that at least one of the OPGW’s would likely exceed their ultimate tensile strength in the process and fail to fully support the falling bridge section.

Figure 2 Isometric view of cascade failure model

The implications of an initial tower failure on the adjacent structures was evaluated using a series of two structural evaluations, prior to and after failure of the OPGW’s. Since it is impossible to know the exact failure behavior of the initial tower several assumptions regarding timing and weather loading were made. Evaluation of the 5T3 tower indicates that a failure would only occur under loading in excess of the 100-year wind event. Based on this, it is reasonable to expect that a peak wind gust of such high magnitude would only last several seconds and occupy a width approximately equal to one span (ASCE 2010). It is also reasonable to expect that the collapse process of the initial tower will consume a similar amount of time, thus the behavior of adjacent towers was evaluated for a lesser wind event of 12.5 psf (70mph) with an emergency OLF of 1.0.

Conductor span behavior during failure of the initial tower indicates that at least one of the OPGW’s would fail during the collapse process. Therefore, adjacent structures were analyzed for loading with all phase conductors intact and both OPGW’s tensioned in the span ahead at the ultimate tensile strength of 16,500 lbs and in the span behind to a typical line tension of 2,500 lbs. This configuration was then analyzed for a 12.5 psf (70 mph) wind event. Results of this analysis indicate that adjacent towers would likely suffer extensive damage to the ground wire peak portion of the tower, with no impact on the remainder of the structure. Under this loading condition, member usages in excess of 300% were observed in the ground wire peaks while usages of 110% were observed in isolated locations within the portal. The disparity of member usages in the peak compared to the portal imply that member failures in the peak would likely reduce load to the extent that slightly overloaded members in the portal would remain intact. Similar results were observed by evaluating the adjacent structure under the condition depicted in Figure 2 in which one OPGW has failed with the other left intact at a

tension of 11,400 lbs. Consequently, there does not appear to be any risk that a cascading failure could occur to the extent that it would endanger the PL-WW2 line. Structure and span analyses associated with a cascade failure are presented in Appendix B.

DISCUSSION

All analyses performed indicate that the tangent lattice structures on the PL-RUD line meet NESC district loading requirements which are approximately equivalent to the 100-year wind event for the Phoenix metropolitan area. In the case of a weather event which exceeds the NESC design loads, structural models indicate that failures would occur in the upper portions of the line structures, placing the adjacent PL-WW2 line at a very low risk for collateral damage. Similarly, there appears to be an exceptionally low risk of any form of cascading failures that would endanger additional structures on the PL-RUD or PL-WW2 lines.

REFERENCES

American Society of Civil Engineers (ASCE) (2010). Guidelines for Electrical Transmission Line Structural Loading, ASCE Manual No. 74, American Society of Civil Engineers, Reston ,VA

Please feel free to contact me with any questions.

Thank you

_______________________Zack Heim

Enclosures: Appendix A: Single tower failure calculations Appendix B: Cascade failure calculations Appendix C: APS 5T3 structure drawings

Cc: E. Dizadarevic, POB100 P. Kandaris, XCT317 F. Dobbins, XCT317

RPEWG Evaluation Palo Verde to Westwing Line 2 and

Palo Verde to Rudd Double 500kV Line Outage Seven Step Process for PBRC Adjustment

 

BACKGROUND Salt River Project (“SRP”) requested a PBRC Adjustment of the Palo Verde to Westwing Line 2 (“PV-WW Line 2”) and Palo Verde to Rudd (“PV-RD”) 500kV double line outage from the NERC/WECC Category C to Category D. The report submitted by SRP includes significant detail describing the PV-WW Line 2 and PV-RD lines and was augmented with consultant and SRP reports for specific elements of the report. SRP followed the Seven Step Process for the Adjustment and provided additional detail beyond the defined steps. Evaluation of the Seven Step Process: 1. Provide Complete Project Description, and why it is being considered for PBRC adjusted

rating, including supportive data: a. Overview of terminations b. Physical Layout and Transmission Construction c. Substation Configurations d. Protective Relaying e. Isochronic Level f. Aircraft Hazard g. Fire Hazard

This step was met. A complete project description, including the suggested a-g supportive data, was addressed in the report. Many photographs accompanied the discussion. The report states that the double PV-WW Line 2 and PV-RD “... outage is considered of such low probability of occurrence and recurrence, that it warrants submittal to the WECC Phase I Probabilistic Based Reliability Criteria (PBRC) Performance Category Evaluation (PCE) Process...” 2. Identify the Statistical Base to be used:

a. Historical b. Similar Lines c. Mileage d. Terrain e. Climate

The outage history of PV-WW Line 2 and PV-RD circuits has been excellent. There has not been any double PV-WW Line 2 and Palo Verde to Rudd circuit outage in the history of the lines. The Palo Verde to Westwing Line 2 has had 4 outages since 1997 and the other line has not had any outage. Given the non-existent PV-WW Line 2 and PV-RD double line outage data, SRP chose to meet this step by focusing on the Robust Line Design Features. 3. Determine Uncorrected of Mean Time Between Failure (MTBF)

• All events should be counted and considered, and then select events and

circumstances can be removed on a case-by-case basis.

Given the non-existent PV-WW Line 2 and PV-RD double line outage data, SRP chose to meet this step by focusing on the Robust Line Design Features. 4. Provide a Corrected Estimate of MTBF (based on Project Robustness Features)

• Consider various robustness features introduced to reduce the risk of outage. For

examples see reference [1]. Robust line design minimizes the opportunity for initial transverse tower failure to strike the second line. A detailed study of the transmission towers conducted by Power Engineers, Inc. and SRP Engineering Services was included in the report. Based on extensive analysis it was concluded that the initial transverse tower failure does not strike the adjacent line’s tower, however any towers that are “hauled down” as secondary failures could impact the tower of the other line. The timing between the sequences of tower failures is estimated to be 1 to 3 seconds. A) The Power Engineers, Inc., analysis used standard modeling assumptions for member performance, including nonlinear P-delta affects. The failure scenarios were based on member performance relative to the finite element modeling assumptions. A critical parameter in the modeling assumptions is that the tower was originally detailed to minimize member connection eccentricities. Therefore, it is recommended that the structural details of the 5T2 and 5T3 towers be reviewed by a experienced Detailer and/or Professional Tower Design Engineer to determine that these towers satisfy standard detailing practices that will result in minimizing connection eccentricities and validates member connection modeling assumptions. B) SRP Engineering Services Analysis used the NESC light loading, the maximum member usage of 105% in the lower portion of the tower portal and the conductor bridge. Based on the location of high stress members within the upper reaches of the structure it is unlikely that a collapse would make contact with it the adjacent towers of PL-WW2.

Also, there are no overhead Electric High Voltage (EHV) crossings along the common corridor. The justification for protective relying could be improved by model line testing to reduce risk of sympathetic tripping. 5. Complete Exposure Analysis. (Refer to example) SRP operations estimate that the combined scheduled and unscheduled outage time when the PV-WW Line 2 and PV-RD double would be the critical outage will be 150 to 200 hrs per year total for all lines combined. 6. Illustrate the Consequences of Outage (Refer to example)

The outage of the Palo Verde to Westwing Line 2 and Palo Verde Rudd results:

• Palo Verde to Westwing Line 1 to 111% of the emergency rating • Pinal West 500/345kV transformer to 108% of the emergency rating

WECC voltage dip criteria will not be violated. SRP’s operating studies show that with any one of the circuits emanating from Palo Verde initially out of service, a subsequent SLG fault on the Palo Verde 500kV Bus which would take both PV-WW #2 & PV-Rudd 500kV lines out of service will not exceed the 30% voltage dip criterion during the first swing after the fault. 7. Conclude how the adjustment meets the PBRC criteria (refer to example) There are one points of reference used by RPEWG in evaluating whether or not a PBRC Adjustment request should be approved:

1. Outage or MTBF performance. The historical performance of the PV-WW Line 2 and PV-RD has been excellent. There has been no double line outage in the history of the PV-WW and PV-RD lines. The PV-WW Line 2 circuit has had four outages since 1996 and the second circuit has had zero outages. Thus, a MTBF analysis using only PV-WW data was impossible.

1. A review of the Robust Line Design Features of the line. RPEWG used its Robust Line Design Features working paper to evaluate the robustness of the line design. The analysis compared thirteen factors in examining the line robust features. The conclusion from this evaluation was that the PV-WW Line 2 and PV-RD lines meet or exceed the Robust Line Design Features required for PBRC adjustment. RECOMMENDATION: RPEWG members vote to approve and recommend that PCC also approve SRP’s request for PBRC Adjustment for the double PV-WW Line 2 and PV-RD line outage from Category C to Category D.

RPEWG Evaluation Palo Verde to Westwing Line 2 and

Palo Verde to Rudd Double 500kV Lines Robust Line Design Features

 

BACKGROUND Salt River Project (“SRP”) requested that the RPEWG approve an adjustment of the Palo Verde to Westwing Line 2 (“PV-WW Line 2”) and Palo Verde to Rudd (“PV-RD”) 500kV double line outage from the NERC/WECC Category C to Category D. The report submitted by SRP contains significant detail describing the case presented for approval. SRP’s analysis followed the “Seven Step Process For PBRC Adjustment” to the extent permitted by line characteristics and historical outage information. As permitted in Step 4 of the Seven Step Process for PBRC Adjustment, RPEWG approval can be evaluated in terms of the robustness of its line design. The RPEWG used the Robust Line Design Features as the standard to compare the PV-WW Line 2 and PV-RD lines for a robust line design. The evaluation follows. ANALYSIS Compliance is examined in the context of the eight risk factors (R1-R8) outlined in the Robust Line Design Features. Reference is made to the pertinent sections of the report measures taken to achieve robustness.

R1 Risk of fire affecting both lines • Sparse low vegetation (photos provided) in low desert terrain (page M-3). • Twice yearly helicopter patrols to identify and take corrective action of vegetation

problems (page M-8). • SRP likely to de-energize line during a fire (page M-7). • Risk of fire extremely unlikely (page M-3)

R2 Risk of one tower falling into another line

• 130 foot line separation (page M-6) • Overdesign of tower foundations 101-151% (pages M-5 to M-6 and photo of footing) • Separate documents providing tower failure mode analysis by Power Engineering and

SRP Engineering Services. This concludes that an initial failure of a tower is not likely to jeopardize the parallel line.

• Average tower capacity is 100 mph wind gust. Average return period of 90 mph gust is 150 years (pages M-5 to M-6)

• The Power Engineers, Inc., analysis used standard modeling assumptions for member performance, including nonlinear P-delta affects. The failure scenarios were based on member performance relative to the finite element modeling assumptions. A critical parameter in the modeling assumptions is that the tower was originally detailed to minimize member connection eccentricities. Therefore, it is recommended that the structural details of the 5T2 and 5T3 towers be reviewed by an experienced Detailer and/or Professional Tower Design Engineer to determine that these towers satisfy standard detailing practices that will result in minimizing connection eccentricities and validates member connection modeling assumptions.

• SRP Engineering Services analysis of 5T3 tower capacity was conducted using Power Line System’s TOWER program for nonlinear truss analysis of steel lattice structures and PLS-CADD for analysis of conductor span behavior. Under NESC Light loading, the maximum member usage is 105% in the lower portion of the tower portal and the conductor bridge.

R3 Risk of a conductor from one line being dragged into another line

• [see R5] R4 Risk of lightning strikes tripping both lines

• Low isokeraunic level one of lowest in Western US • Estimated lightning flash densities of 1 to 2.5 strikes per square mile per year • 13-40 days per year when thunder is heard (page M-2) • Both lines equipped with shield wires (page M-2)

R5 Risk of an aircraft flying into both lines

• Lines do not qualify as a hazard to Public Use or Military as defined by FAA (page M-1).

• Closest public airport 3.2 miles (Buckeye, page M-1) • Shield wire marker balls used to near Buckeye in area of known potential route for small

airplanes (page M-9). • Lines well below FAA minimum height for aircraft (page M-9)

R6 Risk of station related problems resulting in loss of two lines for a single event

• Breaker and half design at both Palo Verde and Westwing (pages M-10,11) R7 Risk of snow or earth slides

• No threat from snow or ice accumulations (page M-3). R8 Risk of loss of two lines due to an overhead crossing

• No overhead crossings by Electric High Voltage (EHV) lines ADDITIONAL FACTORS ADDRESSED IN REPORT

R9 Earthquakes

• Damage due to earthquake highly unlikely. Lowest western US Category B (page M-2) R10 Flood

• Detailed description of distance from river channels and flood reaches (page M-3 to M-5) • Closest flood encroachment not less than 2 miles (pages M-3 to M-5)

R11 Protective Relaying

• No high-speed reclosure due to avoid possible generator shaft impacts (page M-9) • Redundant microprocessor relay technology (pages M-7 to M-8) • Alarming allows quick identification and correction of problems (page (M-8) • The justification would be further improved by information on model line testing to

reduce risk of sympathetic tripping. R12 Faults Caused by Birds

• Largest birds capable in coming in contact with wires are small and incapable of causing a phase to phase fault (page M-8 and picture provided)

R13 Maintenance • Damage caused by vandalism (probably the biggest risk) is mitigated by spring and fall

aerial patrol and one per year ground patrol (page M-8) • Areas of insulator contamination by birds are protected by protective devices above the

insulator (page M-8) • Insulator washing, with tools and training available to SRP, can be done with the lines

energized (page M-8) • Changing insulators depending on insulator type and associated hardware, repair of

gunshot conductor, replacement of other hardware are done with the lines energized (page M-8)

• Deteriorating items, such as spacer dampers, change as they deteriorate and fail (page M-9)

SUMMARY of Robust Line Design Element Risk of Evaluation R1 Fire affecting both lines Very low risk

R2 One tower falling into another line

Low risk (suggest expert review of joint detail in addition to finite element analysis to strengthen justification)

R3 Conductor from one line being dragged into another line See R5

R4 Lightning strikes tripping both lines Very low risk

R5 R5 Aircraft flying into both lines Very low risk Very low risk

R6 Station related problems resulting in loss of tow lines for a single event Very low risk

R7 Snow or earth slides Very low risk

R8 Loss of two lines due to an overhead crossing Very low risk

Other Information provided R9 Earthquakes Very low risk R10 Flood Very low risk

R11 Protective relaying

Low risk (suggest model line testing of relays for very low risk to further reduce risk of sympathetic tripping)

R12 Faults caused by birds Mitigated – very low risk R13 Maintenance Aggressive practice

RPEWG EVALUATION The above analysis confirms that the PV-WW Line 2 and PV-RD lines meet or exceed the Robust Line Design Features required for PBRC adjustment.