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International Journal of Science, Engineering and Technology Research (IJSETR) Volume 1, Issue 1, July 2012 High Voltage Underground Electric Transmission System Hnin War, Thet Tin Abstract— This paper contains information about electric transmission lines which are installed underground, rather than overhead on poles or towers. Underground lines have different technical requirements than overhead lines and have different physical, environmental, and construction needs; underground lines generally cost more than overhead lines. And, this paper describes types of underground electric transmission lines, the impacts of siting, how they are built, how they differ overhead lines and cable layout and system design. Index Terms—Cross-linked Polyethylene (XLPE), International Electrotechnical Commission (IEC), Right-of-way (ROW), Underground (UG) I. INTRODUCTION Much new distribution is being placed underground, especially for taps in suburban residential areas. Whether urban, suburban, or even rural, all parts of a distribution circuit can be underground, including the main feeder. Underground cables carry the load current without overheating and also without producing excessive voltage. Underground line installation will be coordinated with the installation master plan to avoid conflict with construction of future facilities. Lines will normally be installed adjacent to roadways in urban, housing, or industrial plant areas, but may be routed as required to meet the project objectives. A careful study will be made of all underground utilities in order to ensure a minimum of interference between electrical lines and other underground utilities, whether existing, being constructed, or proposed as a definite future construction project. Electrical lines will be at least six feet from any steam or hot water lines, except at crossings where a one-foot separation from such lines is adequate[1]. II. OVERHEAD VS. UNDERGROUND Electric energy utilizes overhead and underground means to deliver power. Overhead networks comprise relatively low-cost insulators and conductors mounted on poles made of various materials like wood, steel or concrete. Other overhead equipments are installed on some of these poles which make it more cost effective to repair and maintain. On the other hand, Manuscript received Oct 15, 2011. Hnin War, Department of Electrical Power Engineering, Mandalay Technological University, (e-mail: [email protected]). Mandalay City, Myanmar, +95401669483. Thet Tin, Department of Electrical Power Engineering, Mandalay Technological University, Myanmar, +952010693). this direct exposure has a down side of being highly susceptible to malfunctioning due to environmental and manmade breakdowns. Undergrounding consists of maintenance holes which are commonly referred to as manholes which are tubes used to connect underground utilities to the surface. Manholes are widely used in sewer systems, electrical and communication systems. These manholes are situated at regular intervals along the utility path, to allow easy access to maintenance workers. Rubber and other insulated conductors (cables) are installed and sliced inside these underground cavities. Therefore, fault detection and repairs can be a very costly exercise. The biggest advantage of underground installation is that it is less exposed and susceptible to external factors than overhead. However, somehow or other, this advantage can be offset by the time and effort spent in locating and repairing faults should they take place. The nature of the installation design and the complexity will be determined by the obligatory standards used for the installation [2]. III. TYPES OF UNDERGROUND TRANSMISSION CABLES There are two main types of underground transmission lines currently in use. One type is constructed in a pipe with fluid or gas pumped or circulated through and around the cable in order to manage heat and insulate the cables. The other type is a solid dielectric cable which requires no fluids or gas and is a more recent technological advancement. The common types of underground cable construction include high-pressure, fluid-filled pipe (HPFF), high-pressure, gas-filled pipe (HPGF), self-contained fluid-filled (SCFF) and solid cable, cross-linked polyethylene (XLPE)[3]. A. High-Pressure, Fluid-Filled Pipe-Type Cable A high-pressure, fluid-filled (HPFF) pipe-type of underground transmission line consists of a steel pipe that contains three high-voltage conductors. Each conductor is made of copper or aluminum; insulated with high-quality, oil-impregnated kraft paper insulation; and covered with metal shielding (usually lead) and skid wires (for protection during construction). The conductor and its wrappings are often referred to as “cables.” The cables are surrounded by dielectric oil that is maintained at 200 pounds per square inch (psi). This fluid acts as an insulator and does not conduct electricity. The steel pipe protects the conductors from mechanical damage and water infiltration and prevents fluid leaks. The pipe is protected from the chemical and electrical environment of the soil by means of a coating and cathodic 1 All Rights Reserved © 2012 IJSETR International Journal of Science, Engineering and Technology Research (IJSETR) Volume 1, Issue 1, July 2012 protection. The pressurized dielectric fluid prevents electrical discharges in the conductors’ insulation. An electrical discharge can cause the line to fail. The fluid also transfers heat away from the conductors. The fluid is usually static and removes heat by conduction. In some situations the fluid is pumped through the pipe and cooled through the use of a heat exchanger. B. High-Pressure, Gas-Filled Pipe-Type Cable The high-pressure, gas-filled (HPGF) pipe-type of underground transmission line is a variation of the HPFF pipe-type described above. Instead of a dielectric fluid, pressurized nitrogen gas insulates the conductors. Nitrogen gas is less effective than dielectric fluids at suppressing electrical discharges and cooling. To compensate for this, the conductors’ insulation is about 20 percent thicker than the insulation in fluid-filled pipes. Thicker insulation and a warmer pipe reduce the amount of current the line can safely and efficiently carry. C. Self-Contained, Fluid-Filled Pipe-Type Cable The self-contained, fluid-filled (SCFF) pipe-type of underground transmission line is often the choice for underwater transmission lines. The conductors are hollow and filled with an insulating fluid that is pressurized to 25 to 50 psi. In addition, the three cables are independent of each other. They are not placed together in a pipe. Each cable consists of the fluid-filled conductor insulated with high-quality kraft paper and protected by a lead-bronze or aluminum sheath and a plastic jacket. The fluid reduces the chance of electrical discharge and line failure. The sheath helps pressurize the conductor’s fluid and the plastic jacket keeps the water out. D. Cross-Linked Polyethylene The cross-linked polyethylene (XLPE) underground transmission line is often called “solid dielectric”. The XLPE type is becoming the national standard for underground electric transmission lines. This type of line relies on high-quality manufacturing controls to eliminate any contaminants or voids in the insulation that could lead to electrical discharges and breakdown of the line from electrical stress. The solid dielectric material replaces the pressurized liquid or gas of other types of cable. This type of construction has three independent cables. They are not housed together in a pipe, but are set in concrete ducts or buried side-by-side directly in specially prepared soil. Each cable consists of a copper or aluminum conductor, a semi-conducting shield, cross-linked polyethylene insulation, and an outer covering consisting of another semi-conducting shield, a metallic sheath, and a plastic jacket. The insulation is about twice as thick as the oil insulation used in other types of cable. IV. INSTALLATION METHODS Cable installation is one of the costliest items in an underground cable system project. The underground cable shall be installed with proper methods and equipment to minimize cable damage during installation which may cause higher installation and maintenance cost. Although some utilities have their own installation crews, many utilities rely on contractors to install cables. The three main installations can be classified into eight installation methods are shown in below [4]. A. Direct Burial A direct burial installation is defined as installing cable without pulling into a pipe or duct, but by pulling or laying cables in an open trench and having the earth in direct contact with the cable jacket or sheath after backfilling. To avoid the excavation by a stranger, it is recommended that utility apply this method only in the specific areas, e.g., in substation. B. Semi-Direct Burial Semi-direct burial installation is similar to direct burial but the cables are pulled into the conduits. Conduits provide mechanical protection to cables. To avoid the excavation by a stranger, some utilities apply this method in only the specific areas, e.g. in substation and foot path. C. Concrete Troughs Concrete troughs installation is similar to direct burial but the cables are pulled into the concrete troughs. Concrete troughs provide mechanical protection to cables. D. Concrete Encased The cables are laid in conduits which are arranged in rectangular formation and covered by reinforced concrete to be named as double protection. A group of conduits can also be called “Duct Bank”. Conduit spacing shall be 25 cm. E. Trenches Trenches are commonly used in substations. The trenches are generally accessible from the surface for the ease of extension or maintenance. Cables are laid in reinforced concrete trench and backfilled with sand. The trench can be easily accessed by opening the top cover. Some utilities apply this method only in the specific areas, e.g., in substation. F. Horizontal Directional Drilling (HDD) HDD process begins with boring a small, horizontal hole (pilot hole) under the obstacle (e.g., a highway or river) that needs to be crossed. This is done with a continuous string of steel drill rod. When the bore head and rod emerge on the opposite side of the crossing, a special cutter called a back reamer will be attached and pulled back through the pilot hole. The reamer bores through the pilot hole, enlarging it so that a pipe can be pulled through. The pipe is usually pulled through from the side of the crossing opposite to the drill rig. Utilities usually apply this method for crossing obstacles such as a river or a road. G. Pipe Jacking Pipe Jacking is a method of constructing a pipeline under the ground. The technique involves pushing a pipe through the ground with the thrust provided by hydraulic jacks. The excavation of soil takes place at the same time the pipe moves into the ground. This provides a structurally flexible and watertight finished conduit. This method is commonly adopted for underground cables. Nowadays, it is the main method used for laying pipe to preserve the natural 2 All Rights Reserved © 2012 IJSETR International Journal of Science, Engineering and Technology Research (IJSETR) Volume 1, Issue 1, July 2012 environment and maintain pleasant living conditions. Some utilities use reinforced concrete pipes with internal diameter from 1.0- 1.8 m as a jacking pipe with small conduits arranged inside. This is the preferred choice for constructing the main line because it helps to minimize environmental effects. H. Tunnels Burial installation has the advantage of relatively low construction and installation cost due to the limited civil works. However, direct burial is not realistic in case of a high number of circuits (20 or more) or circuits with high transmission capacities or in countries with high average temperatures and dry soil, the installation of direct burial is not realistic any more. With burial installation, big conductor cross-section would be required because of higher temperature compared with installation in tunnel. That means more expensive cable. In addition, cables installed in tunnels can be inspected and monitored easier. For all these reasons, more and more utilities opt to install cables in tunnel. In some cases cables are buried on a long section and installed in a tunnel, for example at the entrance of substation where there are many joints or splicing. V. ACCESSORIES The following structures are called “accessories,” because they are a part of all types of underground electric transmission lines. They are also responsible for 90 percent of all underground line failures [5]. A. Splices Splices join separate pieces of conductor. Splices are needed because there is a limit to the amount of cable that can be put onto a spool for shipping and there is a limit to the amount of tension a cable can withstand as it is pulled through a pipe. The length that works best also depends on the number of bends and dips in the line. XLPE type lines need a splice every 900 to 2000 feet. Pipe-type lines need a splice at least every 3,500 feet. Pipe-type lines require a concrete work vault to hold each splice. While the connecting piece is only about 7.5 inches long, the materials needed to reduce the electrical stress results in a joint about 63 inches long (over 5 feet). This means that the vault must be at least 135 inches long (over 11 feet). Typically, the hole dug for a vault is over 15 to 20 feet long and about twice as wide as the trench for the pipe. There are typically one or two “chimneys” or vents to the surface that are closed with steel covers. The strength of the concrete must withstand the load of overhead traffic. Vaults are generally not located under sidewalks or within 125 feet of street intersections. About two-thirds of XLPE splices are in permanent concrete vaults and one-third are constructed in temporary vaults. Temporary vaults are not lined with concrete but filled in after construction. When XLPE is installed in concrete ducts, all splices are constructed in permanent, concrete vaults. B. Terminations Underground lines terminate (connect to overhead lines or to substations) by means of “risers,” which are fastened to above ground structures. Three spread arms (the “spreaderhead”) carry the underground line aboveground and separate the three conductors so that they meet electric code requirements for the spacing of overhead conductors. If spreaderheads are placed underground, they are called “trifurcators” and may be placed in a concrete vault. Porcelain insulators or housings, contain the actual connections between the in-earth and in-air portions of the line. These housings are often called “potheads.” Terminations keep moisture out of the underground system. Lightning arrestors are placed close to the terminations to protect the underground cable from over-voltage damage that can be caused by nearby lightning strikes. The insulating material is very sensitive to large voltage changes and cannot be repaired. Typically a completely new cable is pulled into placed a lightning damaged cable. C. Pressurizing Source For HPFF systems, a pressurizing plant maintains fluid pressure in the pipe. The pressurizing plant is located at one end of the line, usually within a substation fence. It includes a reservoir that holds reserve fluid. An HPGF system does not use a pressurizing plant, but rather a regulator and nitrogen cylinder. These are located in a “gas-cabinet” that contains high-pressure and low-pressure alarms and a regulator. Sometimes, a longer SCFF line that varies in elevation has equipment approximately every mile at splice points to maintain equal pressure throughout its length. The XLPE system does not require any pressurization. VI. SIZING IMPACTS The impacts of underground transmission lines differ from those of overhead transmission lines during construction and afterwards. Underground lines generally cause greater soil disturbance due to trenching requirements, while overhead lines disturb the soil primarily at the location of the transmission poles. Trenching an underground line through farmlands, forests, wetlands, and other natural areas causes significant land disturbances.The ROW for underground transmission lines must be kept clear of trees and bushes, while small trees and bushes are allowed within the ROW under overhead lines. Post-construction issues such as aesthetics, concerns regarding electric and magnetic fields (EMF), and property values are usually less of an issue for underground lines. Underground lines are not visible after construction and have less impact on property values and aesthetics. A. Construction Impacts in Suburban and Urban Area The construction impacts of underground lines are temporary and, for the most part, reversible. They include dirt, dust, noise, and traffic disruption. Increased particles in the air can cause health problems for people who live or work nearby. Particularly sensitive persons include the very young, the very old, and those with health problems, such as asthma. If the ROW is in a residential area, construction hours and the amount of equipment running simultaneously may need to be limited to reduce noise levels. In commercial or industrial 3 All Rights Reserved © 2012 IJSETR International Journal of Science, Engineering and Technology Research (IJSETR) Volume 1, Issue 1, July 2012 areas, special measures may be needed to keep access to businesses open or to control traffic during rush hours. B. Construction Impacts in Farmland and Natural Areas Most underground transmission is constructed in urban areas. In non-urban areas, soil compaction, erosion, and mixing are serious problems, in addition to dust and noise. During construction, special methods are needed to avoid mixing the topsoil and lower soil horizons and to minimize erosion. The special soils often placed around an underground line may slightly change the responsiveness of surface soils to farming practices. Trees or large shrubs would not be allowed within the ROW due to potential problems with roots. Some herbaceous vegetation and agricultural crops may be allowed to return to the ROW. VII. CABLE LAYOUT AND SYSTEM DESIGN The dimensioning of a high voltage cable system is always based on the specifications and demands of the project at hand. The required detail for calculation are the type of cable insulation, nominal and maximum operating voltage, short-circuit current with statement of the effect time, transmission capacity or nominal current, operating mode: permanent operation or partial load operation (load factors) and ambient conditions: type of installation, ambient temperature ( including external effects ) and special thermal resistance of the ground. The calculation of the admissible load currents (ampacity) and the cable temperatures is performed in accordance with the IEC publication 60287. 1) Electrical Field In initial approximation, the main insulation of a high voltage XLPE cable can be regarded as a homogenous cylinder. Its field distribution or voltage gradient is therefore represented by a homogenoius radial field. The value of the voltage gradient at a point x within the insulation can therefore be calculated as: Ex U0 r rx ln a ri (kV/mm) where, U0 = operating voltage (kV) rx = radius at position x (mm) ra = external radius above the insulation (mm) ri = radius of the internal field delimiter (mm) The electrical field strength is highest at the inner semiconductor and lowest above the insulation (below the external semiconductor, rx = ra ). 2) Capacity, Charging Current The operating capacity depends on the type of insulation and its geometry. The following formula applies for all radial field cables: Cb 5.56ε r (µF/km) D ln d where, εr = relative permittivity (XLPE=2, 4) D = diameter over main insulation (mm) d = diameter over inner semiconductor (mm) Single-core high voltage XLPE cables represent an extended capacitance with a homogenous radial field distribution. Thus a capacitive charging current to earth results in the following formula: I C U 0ωCb (A/km) where, U0 = operating voltage (kV) ω = angular frequency (1/s) Cb = operating capacity (µF/km) 3) Inductance, Inductive Reactance The operating inductance in general depends on the relation between the conductor axis spacing and the external conductor diameter. Practically, two cases have to be considered: Laying formation: trefoil The operating inductance for all three phases calculates as: a (H/km) L m 2 104 ln 0.779rL where, a = phase axis distance (mm) rL = diameter of conductor over inner semiconducting layar (mm) Laying formation: flat The mean operating inductance for the three phases calculates as a (H/km) L m 2 104 ln 0.779r L where, a' = phase axis distance (mm) rL = diameter of conductor over inner semiconducting layer (mm) The inductive reactance of the cable system calculates for both cases as: X = ωL (Ω/km) where, ω = angular frequency (1/s) 4) Current-dependent losses The current-dependent losses consist of ohmic conductor losses, losses through skin effect, losses through proximity effect and losses in the metal sheath. (i) Ohmic conductor losses The ohmic losses depend on material and temperature. For the calculation of the ohmic losses I2 R, the conductor resistance stated for 20°C (R0) must be converted to the operating temperature θ of the cable: R = R0 (1+α (θ-20°C)) (Ω/km) where, α = 0.0393 for copper and 0.0403 for aluminium The conductor cross-section and admissible DC resistance at 20°C (R0) correspond to the standards series pursuant to IEC 60228. (ii) Losses through skin effect The losses caused by the skin effect, meaning the displacement of the current against the conductor surface, rise approximately quadratic with the frequency. This effect can be reduced with suitable conductor constructions, e.g. segmented conductors. (iii) Losses through proximity effect The proximity effect detects the additional losses caused by magnet fields of parallel conductors through eddy currents and current displacement effects in the conductor and cable 4 All Rights Reserved © 2012 IJSETR International Journal of Science, Engineering and Technology Research (IJSETR) Volume 1, Issue 1, July 2012 sheath. In practice, their influence is of less importance, because three-conductor cables are only installed up to medium cross-sections and single-conductor cables with large cross-sections with sufficient axis space. The resistance increase through proximity effects relating to the conductor resistance is therefore mainly below 10%. (iv) Losses in the metal sheath High voltage cables are equipped with metal sheaths or screens that must be earthed adequately. Sheath losses occur through circulating currents in the system, eddy currents in the cable sheath (only applicable for tubular types) and resulting sheath currents caused by induced sheath voltage ( in unbalanced earthing methods. 5) Calculation of the induced voltage The induced voltage Ui within a cable system depends on the mutual inductance between core and sheath, the conductor current and finally on the cable length: Ui = XM I L (V) where ,XM = mutual inductance between core and sheath (Ω/km) I = conductor current per phase (A) L = cable length Two cases must be considered for the determination of the maximum occurring voltage and for the dimensioning of the surge arresters: I = IN Normal operating current (A) I = IC Three-pole short-circuit current (A) The mutual inductance between core and sheath calculates from the following formula: XM = ω LM (Ω/km) where, ω = angular frequency (1/s) LM = the mutual inductivity between core and sheath (H/km). The mutual inductivity between core and sheath L calculates as follows: For installation in trefoil formation: 2a (H/km) L M 2 107 ln dM For installation in flat formation: 2 3 2a (H/km) L M 2 10 7 ln d M 6) Short-Circuit current capacity The short-circuit currents can be converted with the following formula: I k,x 1 I k,1s tc tangential (belts etc.) forces. The amplitude of a dynamic force in general is calculated applying the following formula: 2 10 7 I s Fs (kN/m) a 2 where, a = phase axis distance (mm) Is κ 2Ic where, Is = impulse short-circuit current (kA) κ = surge factor (usually defined as 1.8) Ic = short-circuit current (kA) VIII. CONCLUSION The electrical transfer from one place to another with standard regulation is one of the major problems in the field of electric power engineering. On the other hand, the choice of size is also important in order to enough the amount of current that flows on the line due to the transfer of power. To get standard regulation, it has become mandatory for most modern cities worldwide to use underground power cables. This demonstrates that undergrounding has become feasible economically and technologically. Using underground cables for transmission system, the many advantages are obtained such as minimal visual impact and lightning problems, high level of personal and public safety, no fallen lines, not affected by ice, snow, rain, wind, dust, smoke or fog, land use minimized, high reliability and availability. ACKNOWLEDGMENT The author would acknowledge to Dr. Khin Thuzar Soe, Associate Professor and Head, Department of Electrical Power Engineering, Mandalay Technological University for her kindly permission and suggestion throughout the preparation of paper. Especially, the author would like to express deeply gratitude to her supervisor, U Thet Tin, Lecturer, Department of Electrical Power Engineering, Mandalay Technological University for his valuable advice, encouragement help throughout the period of study. The author is deeply grateful to her parents who specially offered strong moral and physical support, care and kindness. REFERENCES [1] [2] where, Ik,x = short-circuit current during x seconds (kA) tc = duration of short-circuit (s) Ik,1s = short-circuit current during one second (kA) 7) Dynamics forces Single-core cables have to be fixed in their position at certain intervals. The calculation of dynamic forces for cable systems is important for the determination of the fixing interval and the layout of the fixing devices. It has to be distinguished between radial (e.g. clamps, spacers) and [3] [4] [5] Brown, R., “Undergrounding Assessment Phase 1 Final Report Literature Review and Analysis of Electric Distribution Overhead to Underground Conversion”: Raleigh, 2007. Pansini, A.J. and Smalling, K.D., ‘Undergrounding Electric Lines’. Second ed, Lilburn: The Fairmont Press, Inc.1993. “Underground Transmission Systems Reference Book,1992 Edition”. “Power Cable Handbook”, Harmonisation of Power Distribution Systems in the Lower Mekong Subregion, 2011 Edition. “Educational and Project Materials supplied by Power Delivery Consultants, Inc. of Ballston Lake. New York”. 5 All Rights Reserved © 2012 IJSETR International Journal of Science, Engineering and Technology Research (IJSETR) Volume 1, Issue 1, July 2012 First Author personal profile which contains their education details, their publications, research work, membership, achievements, with photo that will be maximum 200-400 words. Second Author personal profile which contains their education details, their publications, research work, membership, achievements, with photo that will be maximum 200-400 words. Third Author personal profile which contains their education details, their publications, research work, membership, achievements, with photo that will be maximum 200-400 words. 6 All Rights Reserved © 2012 IJSETR