Underground System Design TADP 547

Transcription

1 Underground System Design TADP 547 Transmission Cable Installation Presentation 5.4 Instructor: Frank Frentzas

2 Transmission Design and Installation When planning an underground power cables system the challenge is to design a system that can withstand both the electrical and physical parameters. To avoid installation problems and achieve a reliable long-term cable system, installation conditions and methods must be an integral part of the design phase. This session will cover important design requirements and options that should be considered when installing an underground transmission circuit.

3 Cable Installation Considerations Cable Selection Route Selection Construction methods Maintenance requirements

4 Cable Selection Typically based on voltage level, route, and length of circuit. Most common installations in North America have been HPFF pipe type. More recently, XLPE cables are now becoming the more popular choice. HPFF cables have been installed since early 1930 s and have a very reliable track record. XLPE cable installations do not have the long track record that fluid filled cables have. However, they have proved reliable over the short term.

5 Cable Selection (cont.) Each utility has different systems so most choose a cable they are comfortable with and only introduce new types slowly in short lengths to gain experience. Current LPFF cable use is limited and tends to be confined to extra high voltage (EHV) circuits and underwater (submarine) installations. For short lengths, XLPE cables are generally preferred since no hydraulic equipment is required - compared to fluid filled systems.

6 Route Selection When selecting a route you must look at the following: Environmental concerns Land permits Soil conditions Existing utilities Construction requirements

7 Environmental Concerns/Permits When choosing a route, the entire area needs to be surveyed to avoid (if possible) wetlands, waterways, archeological sites, and sensitive areas such as schools, playgrounds, and parks. These areas require special permits and the permitting process may take several years before being granted - which can impact cost and schedule. Most underground utilities are installed in public roadways and permits may be an issue if the street was recently paved, traffic flow to businesses or residence in the area is obstructed, and noise restrictions exist.

8 Soil Conditions Depending on type of soil being excavated, soil conditions can effect installation cost and schedule. Amount of pavement or rock to be removed or, if in soft soil, extensive shoring required. Soil resistivity can effect cable rating so a geotechnical study should be performed and soil conditions corrected with thermal backfill, when necessary. Whenever possible, engineers/designers should avoid difficult routes or excavating areas.

9 Existing Utilities Locating other underground utilities can help with route design by selecting depth required to avoid conflict, or by selecting different route if street is too congested. External heat sources (such as distribution circuits or steam pipes found in urban areas) should be avoided or a 10 minimum clearance maintained to prevent possible thermal de-rating of the installed line. Although thermal barriers or mitigation is possible, it is generally a very costly alternative.

10 Construction Requirements Manhole installations: Manholes are installed at locations where cables need to be spliced. Manhole spacing determined by maximum cable length on a reel, along with weight and size of reels that can be transported to job site. Maximum pulling tension and sidewall pressure allowed.

11 Manhole Installations Manholes are either precast or casted-in-place and precast are more economical. However, precast manholes require heavy equipment to transport and install and contractors must ensure adequate clearance exists to move equipment into position to unload and install manholes. In urban congested areas, cast-in-place might be only viable option. For pipe-type systems, manholes from feet long can be spaced between 2,500 to 3,500 feet apart.

12 Manhole Installations (cont.) Manholes for LPFF are ft. long and spaced about 800 to 1,600 feet apart. For XLPE cable systems, manholes typically longer to allow for joint construction approximately ft. and spaced 2,000 to 3,000 feet apart. Manholes usually installed first so that they can be moved if other utilities are in the way. As moving manholes may change cable cut lengths, engineers and designers typically add 5% to cable lengths to account for such changes.

13 Precast Manhole Installation

14 Cast-in-Place Manhole Installation

15 Conduit Installation Engineers/designers should ensure contractor s realize transmission cable conduit installation is critical and that strict quality control is required during installation. Transmission cables are typically large, heavy, and require a large radii. Duct systems also need to be robust to withstand high installation loads required for longer pulls. Very important that duct alignment, especially at joints, is kept straight to avoid cable damage (which can lead to failures) during installation.

16 Conduit Installation (cont.) Spacing and size determined by ampacity and cable design requirements - spare conduits usually installed. Duct banks typically have minimum 30 cover and are encased with high strength 3,000-3,500 psi thermal backfill for mechanical protection. Ducts typically made of PVC are Schedule 40 thickness. (When encased in concrete, thinner ducts tend to become oval to a point where cable cannot be pulled through.) Other types of ducts used are polyethylene (PE) and fiberglass conduits - in 10 and 20 ft. sections.

17 Conduit Installation (cont.)

18 Pipe-Type Installation When installing pipe for pipe-type cables, trench excavation is similar to installing a duct system. However, the actual installation practice is different. Before pipe is installed, engineer/designer must provide a weld procedure and ensure welders are qualified by performing test welds and having qualified testing agency approve them - welds must meet American Petroleum Institute (API) Standard Pipe installations typically require an open trench of at least 200 feet to allow for adjusting alignment of pipe, if needed.

19 Pipe-Type Installation (cont.) Pipe is either welded in trench or on top and lower into trench or, in some cases, pulled with a winch line so pipe can be fed underneath existing utilities. Pipe sections typically 20 and 40 ft. long with flared ends to provide smooth transition between pipes when welded. Joint inside surface must be smooth to avoid damage to cable during pulling. To prevent weld beads from entering pipe during welding, a backer or chill ring is used. A portable bending machine can be used to bend pipes to the desire radius in the field

20 Pipe-Type Installation (cont.) Typical bending radius for 8 pipe is 50 ft. and for vertical grade changes it should be limited to 500 ft. A larger radius requires a lower pulling tension. All welds tested using magnetic particle, dye penetrant, ultrasound, or radio graphical (X-rays) test methods. Most common methods used today have been X-Ray and dye penetrant tests. After welds tested and approved a corrosion protected coating is installed over welded area.

21 Pipe-Type Installation (cont.) Current corrosion coatings are polymeric heat shrink sleeves to cover weld area. Pipe coating are tested for damage and imperfections that may occur during production or installation prior to backfilling the trench. After backfilling, a manhole-to-manhole coating resistance test is performed to make sure no damage occurred during the backfill process. The pipe must be kept dry during installation so 5 psi dry nitrogen pressure is applied to each completed section.

22 Typical Pipe Trench

23 Pipe-Type Installation (cont.) When pipe termination point is reached a trifurcating joint is used to go from single pipe to three individual pipes (typically made of stainless steel or other nonferrous/non-magnetic metal) for termination ends. After pipe installed and tested, inside is cleaned with swabs, mandrelled, and pressure and vacuum tested. Mandrel checks roundness of pipe to assure proper diameter is maintained. AEIC specifies size and shape of mandrels for various pipe sizes. Typically, the mandrel is about ¼ smaller then pipe diameter.

24 Pipe-Type Installation (cont.) Vacuum process performed to dry pipe prior to pulling cable. Each pipe section vacuumed until desired reading obtained - typically 150 microns with 100 micron rise in one hour. If unable to achieve desired reading, continued vacuum process for another 4 to 8 hours and repeat test. Once required vacuum reading achieved apply 10 psi dry nitrogen pressure and maintained until cable pulling operation begins.

25 Cable Pulling - HPFF When pipe is ready the cable pulling process can begin. Weather a key factor for pulling HPFF cable since pipetype cables have no protective sheath. If rain or snow forecast, the pulling operation must be rescheduled. Cable and pipe must be kept dry at all times. In a pipe-type installation, all three cables are pulled in at the same time. You must, therefore, verify there is sufficient space to set up reel trailer(s) and/or reels. All pulling tensions must be within cable s design parameters.

26 Cable Pulling HPFF (cont.) Once cable has been pulled into a section both pipe ends (including cable) are sealed to protect against moisture until the splicing process has been completed. Install end (or night) caps at each end to seal cable and pipe section. Evacuate section to keep cable and pipe dry. When desired vacuum achieved (typically 250 microns or less) a positive dry Nitrogen pressure of 5 psi is applied. Always follow safety procedures when bleeding or using Nitrogen in manholes or confined spaces.

27 Cable Pulling Cable Reel Trailer

28 HPFF Splicing Prior to initiating the splicing operation, protective covers or temporary housing is applied to the manholes to prevent moisture/ water from entering manholes. Splicing area must be clean and dry to avoid possible contamination during splicing. Manhole or splicing area must be climate controlled during entire splicing process - maintain F and relative humidity of 35% or below. Once splicing completed and joint casing installed a final system vacuum is performed. Terminations are spliced in a similar process - protective shelter built around termination area to keep it dry/clean.

29 HPFF Splice

30 HPFF Splicing (cont.) Based on line length, multiple vacuum pumps can be used to achieve required vacuum reading (250 microns) sooner. Line ready for filling with dielectric fluid when final vacuum level achieved. Filling from lowest line elevation, with vacuum pump at highest point, is generally preferred. Multiple tanker trucks used to fill line at gallons per minute. When line is filled the pressurizing process begins. Line pressure raised in 25 psi steps every 8-12 hours until a normal operating pressure of 200 psi achieved.

31 HPFF Splicing (cont.) After line reaches normal operating pressure it should be maintained for 24 hours prior to applying any test or operating voltages. For HPFF cables, AEIC Standards specify DC Hi-Pot voltage test level and time duration - typically 3.5 times rated voltage for 15 minutes on each cable phase. In some cases the DC Hi-Pot test is followed by a 24 hour system voltage, no load, electrical soak test. (Some utilities may require a longer soak time.)

32 Solid Dielectric Cable Installation To verify conduit integrity a mandrel is pulled through duct. The cable is ready for pulling after duct has been deemed acceptable. Temperatures must be above 20 F to avoid cable damage. Unlike HPFF cables, solid dielectric cables are pulled one phase at a time. A pulling lubricant is used to reduce friction. Positioning of cable reels and rigging manhole with rollers required to maintain minimum bending radius and allowable pulling tensions. Once cable is installed each end is typically sealed with a heat shrink cap to prevent moisture from entering cable.

33 Solid Dielectric Cable Pulling Cable Reel Trailer

34 Solid Dielectric Cable Pulling (cont.) 345 kv XLPE Cable Installation

35 Solid Dielectric Cable Pulling (cont.) 345 kv XLPE Cable Installation

36 Solid Dielectric Cable Splicing: When splicing solid dielectric cables, splice shelters or manholes must be clean and protected from weather conditions and humidity. To prevent future cable failure, splicing should be performed in a clean, dry, and controlled space to avoid contaminants. Termination splicing is handled in the same manner - a protective shelter is built over the termination structure to keep the area clean and protected from weather conditions.

37 Solid Dielectric Cable Splicing Cable splicing in a manhole

38 Solid Dielectric Termination Splice (cont.)

39 LPFF or SCOF Installations LPFF cable installations are similar to solid dielectric cables. However, the fluid/hydraulic system must be addressed for LPFF cables systems. Reel or portable pressure reservoirs used to maintain fluid pressure on the cable during pulling and splicing. After completion, joints and terminations are evacuated and filled with a degassed dielectric fluid.

40 General To provide for maintenance to the system, it will be important to secure an easement or access for all cable types. Typical underground line easement widths of ft. allow equipment to access the site for conduit/pipe or cable repairs. Clearance between other utilities is also important since there must be enough room between utilities for future maintenance and repairs to the pipe or conduit system.