Nilesh S. Dhanore
Department of M.Tech (VLSI Design)
Jhulelal Institute of Technology, Lonara, Nagpur (INDIA)
Prof. Sanjeev Sharma
Department of E.T.C
Jhulelal Institute of Technology, Lonara, Nagpur (INDIA)
Underground cables are a viable alternative to overhead transmission lines when proper consideration is given to the many details of using these types of systems. Cables, however, have differing characteristics than overhead lines that must be factored into design, ratings, switching, reactive compensation, operation, maintenance and repair. This paper provides an introduction to the cable system types and presents an overview into considerations for using underground cable systems. The discussion is focused on transmission cables but also has relevance for distribution cable applications.
Keywords— Underground Cable, Cable System Type, Transmission Cable, Cable Specification.
Increasingly, utilities, developers and power producers seeking to build new power transmission systems are required to at least consider underground cables as an alternative to overhead lines. This may be from the standpoint of due diligence when requesting regulatory approval for an overhead line or as part of a hybrid line where a portion of an otherwise overhead circuit must be built underground either for reasons of congestion near substations or other restrictions that would otherwise prevent construction of an overhead circuit (proximity to schools or hospitals, constraints on available rights-of-way, concerns about magnetic and electric fields, etc.). In deciding to use underground cable systems, there are many technical issues to consider. The paper summarizes the many details that should be considered, and the reference section is intended to provide the reader with additional background that may be helpful in gaining further understanding on the various topics.
II. CABLE COMPONENTS AND SYSTEM TYPES
A. Basic Cable Components
Insulated power cables have two basic components; a conductor to carry current and insulation to support the line-to-ground voltage. Conductors are made of stranded copper or aluminum; sometimes larger conductors are arranged in segments to reduce losses. Current carrying capacity increases with conductor size. The predominate insulation materials are laminar taped paper or laminated paper polypropylene (LPP) for dielectric oil-filled cables, or extruded cross-linked polyethylene (XLPE) or ethylene propylene rubber (EPR) for extruded cables. Increased insulation thickness will support higher line-to-ground voltage, although voltage class is designated based on rated line-to-line system voltage. Limiting moisture ingress for all cable types is important; EPR-insulated cable and XLPE distribution cable with tree-retardant additives have been found to be more tolerant to the effects of moisture and do not require a radial moisture barrier (metallic sheath). Conductor and insulation shields on, respectively, the inside and outside of the insulation help control electrical stress. Other components of each cable type are described below.
B. Extruded Dielectric Cable Type
Extruded cables are so called because the insulation material (XLPE or EPR) is shaped by passing heated polymers, which are vulcanized, through a die to provide the required insulation thickness over the conductor. A metallic shield, if present, of tape or, wire of copper or aluminum may be applied over the insulation. A radial moisture barrier (e.g., metallic sheath), if present, may consist of extruded lead, corrugated or foil made of copper or aluminum, or corrugated stainless steel. The metallic shield and sheath carry charging current and fault current. An insulating jacket over the shield or sheath provides electrical isolation of the shield/sheath from local ground and mechanical and corrosion protection to the cable. Three-conductor cables are available up to 138kV, but the vast majority of underground extruded circuits use single-conductor cables. Transmission cables are often 3-5in (75-125mm) in diameter and may be installed directly buried or in, typically, 6-8in (152-203mm) plastic conduits with commercial installations up to 500kV. Most extruded cables are ac, but a very few are dc.
C. Pipe-Type Cable
High-pressure pipe-type (HPPT) cables are so designated because of the carbon steel line pipe used to contain the cables and filling medium under high pressure, which is an integral part of this system type; most new installations use nominal 8in (219mm) pipes with ¼-in (6.35mm) wall, but 5in, 6in, 10in and 12in (141mm, 168mm, 273mm, 324mm) pipes have been used. Paper or LPP insulation is helically taped over the conductor shield to the required thickness, vacuum dried and impregnated with dielectric oil. A metallic skid wire over the high-resistance shield provides mechanical protection while the cables are pulled into the pre-installed pipe and from movement during load cycling. HPPT cables are typically 2-3.5in (50-90mm) in diameter, and all three phases are installed simultaneously in the carbon steel line pipe.
The steel pipe is protected from the surrounding environment and corrosion by an insulated coating of polyethylene tape, fusion bonded epoxy and polymer concrete, or asphalt mastic. A cathodic protection system is important to further protect the pipe from corrosion. For voltages up to 138kV, the pipe can be filled with dry nitrogen gas (high-pressure gas-filled, HPGF), though synthetic dielectric oils (alkylbenze or polybutene) are used for all voltages up to 345kV (high-pressure fluidfilled, HPFF, or oil-filled, HPOF); either system is nominally pressurized to 200psig (1.4MPa).
D. Self-Contained Fluid Filled Cable Type
Self-contained fluid-filled (SCFF) cables use laminar paper or LPP insulation. A fluid channel in the middle of the conductor (or in the interstices between cores of three-conductor cables) permits dielectric oil expansion and contraction under pressure, inhibiting insulation voids from forming. A moisture-impervious metallic sheath, similar to an extruded cable, contains a positive internal pressure (15-75psig, 0.1-0.5MPa), and an insulating jacket is put over the sheath. These cables can be manufactured in very long splice free lengths which make them useful for submarine projects, though use is diminishing worldwide. As with extruded cables, armor wires may be applied over the jacket if designed for submarine projects; most installations are direct buried.
E. Other Cable Types
Mass-impregnated cables are somewhat similar to SCFF cables and are used exclusively for long DC submarine cable installations. Compressed gas-insulated transmission lines have very high current-carrying capacity and are generally limited to very short above ground runs (~1000ft, 300m) within substations due to their high costs, susceptibility to corrosion and large space requirements. Superconducting cables have seen very limited commercial applications, mainly due to the high system costs and ongoing development of cryogenic systems.
Aside from the cable, joints for connecting adjacent cable sections and terminations for connecting cables to other equipment, along with other accessories, are needed to complete the cable system. Link boxes with grounding links, sheath voltage limiters and features to permit cross bonding, plus associated bonding cables and ground continuity conductors, are needed for extruded cable systems. Shipping container-sized dielectric oil pressurization plants (HPGF systems uses a much smaller, nitrogen gas cabinet) and cathodic protection systems are needed for HPPT cables, and fluid reservoirs are needed for SCFF cables. Vaults (e.g., “manholes”) typically, 8ft wide (2.4m) x 7ft (2m) tall x 16-33ft (5-10m) long, depending on voltage and system type, are used for most joints in North America, although direct buried joints are possible and common in other parts of the world. Near terminals, HPPT cables require special trifurcators to separate the three cable phases in the steel line pipe into three separate stainless steel pipes.
- E.C. Bascom, III, D.A. Douglass, et. al., “Hybrid Transmission: Aggressive Use of Underground Cable Sections with Overhead Lines”, CIGRE 1996, SC 21/22-10.
- J.H. Neher, M.H. McGrath, “The Calculation of the Temperature Rise and Load Capability of Cable Systems”, AIEE Insulated Conductors Committee, June 1957.
- IEC-60287, “Calculation of the Continuous Current Rating of Cables (100% Load Factor)”, International Electrotechnical Commission, 1994.
- IEEE Standard 835-1994, “Standard Power Cable Ampacity Tables”.
- IEEE Standard 442-1981, “Guide for Soil Thermal Resistivity Measurements”, 1981, 1996.
- R.C. Rifenberg, “Pipe-Line Design for Pipe-Type Feeders”, AIEE Transactions, Vol. 9, December 1953.
- “Guidelines for Limiting Exposure to Time- Varying Electric, Magnet and Electromagnetic Fields (Up to 300GHz)”, International Commission on Non-Ionizing Radiation Protection, 1998.