The focus of this article is on the design, selection, and testing of transmission cables within specific geographic regions. Design considerations on a regional basis include temperature constraints, voltage and voltage stress limits, capacity and ampacity, backfill requirements, cooling and bonding. The material presented here covers land-based transmission cables only. Submarine cables, URD or UD cables are not included.
The North American power system consisting of the US and Canada differs in several ways from most other parts of the world. The United States is only minimally nationalized while Canada is highly nationalized. In spite of the diversity, a highly reliable power system has been developed by a number of very strong interconnections, power pools and reliability councils.
The earliest underground transmission cable installations in the late 1920's used self-contained oil-filled cables and the dominance of high pressure oil-filled pipe type cables (HPGF) in the United States started in 1932. Voltage levels have recently included 230 kV and 525 kV. In Canada, there is no such dominance, as the highest voltage cable is 345 kV and 525 kV are of the SCFF design. Interest in SF6 insulated underground transmission systems has abated for the foreseeable future.
Extruded solid dielectric transmission cables were introduced into commercial use in the late 60Ős employing HMWPE. This was soon displaced by XLPE and to a lesser extent EPR's. The maximum voltage level for extruded dielectrics is 230 kV.
PPP has lower dielectric losses and significantly higher dielectric strength permitting dramatic reductions in insulating thickness allowing the selection of smaller diameter-sized pipe with appreciable new installed cost savings. It also offers the possibility of reconducting existing pipes with higher voltage or increased conductor size cables or both. Utilities in Boston and New York/New Jersey areas have made installations of PPP cable in recent years.
AEIC specifications have generally been the standard for both paper and extruded cables. Traditionally, the practice has been to fix the insulation thickness for the various voltage classes with a ceiling on the maximum electrical stress set by the minimum conductor size.
Transmission HVSD cables over 69 kV, use a hermetic seal over the core to protect it from water exposure. Extruded lead, extruded or seam welded aluminum or seam welded copper sheath provide both the hermetic barrier and a path for short circuit currents. Water swellable semiconductive tapes provide a longitudinal blockage.Details of cable accessories, sheath bonding, installation practices, thermal considerations including forced cooling and EMF issues are provided elsewhere.
The high voltage transmission and distribution system in England, Scotland and Whales were owned by the State Central Electricity Generating Board (CEGB) and the fourteen area Boards between 1948 and 1989. In England and Whales, the CEGB was responsible for the generation and transmission of electricity and the Area Boards for its distribution. The transmission and generation functions of the CEGB were split during the privatization of the industry by act of parliament (The Electricity Act 1989). The high voltage transmission system is now owned by the National Grid Company plc (NGC). The Area Boards were transformed into publicly owned Regional Electricity Companies (RECs).
The organization of the electricity supply in Scotland is different with two companies being responsible for both transmission and distribution. As only a small part of the 275 and 400 kV transmission system consists of underground cable, the underlying requirement for cable is that it matches the rating capability of the overhead line system. This results in underground cable systems capable of carrying 1600-3000A (1100-2080 MVA at 400 kV) in winter. The temperature climate in the UK coupled with only limited air conditioning produces a summer load which is significantly lower than the winter load curve. The NGC peak demand on a typical winter day is 46 GW compared with only 34 GW on a typical summer day.
The requirements for high current ratings has led to the need for large conductor, up 2600 mm (5200kcmil), cables which to date have been met by the use of self-contained fluid filled cables (SCFF). Even with these large conductor sizes, it has often been necessary to employ separate cooling pipes to achieve the required rating.
As at the end of 1991, the approximate amounts of underground cable and overhead lines installed in the UK are shown in Table 1. For the range of cables under review, detailed national construction specifications, such as the AEIC specifications, do not exist in the UK. Each utility company will usually have their own purchasing specification but these cover general matters rather than detailed dimensional requirements. In the UK, the practice has been to specify performance requirements in the form of type approval tests. Therefore, designs vary to some extent between manufacturers. However, for mature products such as the SCFF cable, these differences are now much reduced.
Two main types of cable systems now being installed are the SCFF and XLPE cables. Some 66 and 132 kV self-contained gas filled cable is still being made but this is principally for the diversion or extension of existing circuits.
Recently commercial use has started with cables insulated with Polypropylene/Paper Laminate (PPL). In self-contained cables the main advantage over paper insulated cables is the increased current rating achieved by the substantial reduction in dielectric losses. This becomes significant at 275 kV but is more cost effective at 400 kV. This increase in current rating can be employed by using a smaller conductor size for a given rating or by extending the current rating range of the cables. It is particularly effective in this latter role as it can avoid the use of two cables per phase or avoid the necessity of an expensive water cooling system.
Corrugated aluminum is the preferred construction as this type of sheathing does not require reinforcement and provides additional resistance to crushing. These cables are being increasingly used for operating voltages up to 132 kV but as yet not for commercial installations at higher voltages. A short length of 275 kV cable has been installed at a NGC site to obtain operational experience. 66 and 132 kV cables are designed on the basis of a constant insulation thickness.
It is normal practice to apply a water swellable tape over the insulation to provide a bedding for the metallic sheath and a barrier to moisture in case of accidental damage to the sheath. Continuous current ratings for both types of cables are based on a maximum conductor temperature of 90 degrees Celsius for the full range of voltages. Emergency ratings based on higher temperatures are not normally used in the UK. The high current ratings of the associated 275 and 400 kV overhead line circuits have resulted in the wide use of intensively cooled underground cable systems at these voltages. Separate water pipe cooling is the most widely used system and the only type used for directly buried systems. The most common form is the use of four high density polyethylene pipes associated with the three single core cables.
The following systems have been used in tunnel installations:
Water cooling in troughs: The cables are installed in a concrete trough through which water is circulated.
Integral pipe cooling: The cable is installed in an oversize polymeric pipe through which water is circulated.
Forced air cooling: The cables are installed in a tunnel and subjected to a high flow of cooling air.
The performance of the SCFF cable in service has been excellent. Some failures were experienced on 275 and 400 kV stop joints in the early 1970s. This led to improved designs which have given satisfactory service. Over the years attention has been paid to the mechanical aspects of accessory design resulting in modern systems being virtually free of fluid leakage.
The introduction of XLPE cables has gone smoothly. The only problems experienced have been the failure of a few joints when used for the first time. It is considered that the move to XLPE cables at 66 and 132 kV will continue. The principal reasons for this are; environmental concerns regarding the possibility of leakage of fluid from the system and not being a pressurized system, maintenance costs are lower.
The same situation applies at 275 and 400 kV and it is anticipated that initial commercial installations will be made in the not too distant future. At 400 kV particularly, it is considered that a main requirement is the development of suitable accessories, particularly practical designs of straight joints. It is considered that it will be several years before there is the same confidence in XLPE systems as there exists for SCFF cable systems. In the meantime growing use of SCFF cables insulated with PPL is anticipated.
In France, a state owned company, Electricite de France (EdF), is largely responsible for generation, transmission and distribution of electricity power. The high voltage and extra high voltage systems operate at nominal voltages of 63 kV, 90 kV, 225 kV and 400 kV; some transmission lines operate at 150 kV but this network is not very large. The EdF transmission system data is shown in Table 2.
In the EdF network there are mass impregnated cable, self-contained oil-filled cables, pipe-type cables and extruded dielectric cables (LDPE-HDPE-XLPE). For many years new circuits are only installed with polymeric cables; for the time being, these polymeric cables represent more than 50 per cent of the in-service cables in the EdF network.
Two French standards are used for the design and tests of cables; these are functional and not constructive. These standards are C 33252 for 63 kV and 90 kV cables and C 33253 for 150 kV-225 kV and 400 kV cables. The thermal rating is defined by the procedures in IEC 287.
According to the electrical stress of the cable, several materials can be used. For cables in the 63-90 kV range, LDPE, HDPE and XLPE have been used. XLPE has been commonly used since 1986. For 225 kV cables, LDPE-HDPE are the insulations of choice. XLPE has been used for stand-by cables for 2 years. Some XLPE cables for fixed links were installed in 1994. For 400 kV cables, LDPE has been used.
Approval tests are carried out on cable samples and accessories in the EdF laboratories according to the EdF technical specifications. As an example, the main tests for 400 kV cables and accessories are the short-term tests, impulse test consisting of 10 impulses of each polarity at 1425 kV with the test temperature equal to the operating temperature plus 5 degrees Celsius and an ac voltage test at 500 kV for 24 hours at room temperature.
The long-term test is a 6000 hour test at 400 kV (between the conductor and metallic screen) and with daily thermal cycles (8 hours heating - 16 hours cooling). The cables are generally installed in concrete prefabricated troughs; These troughs are filled with sand after pulling of the cables and are laid at 1.40 m under the ground level.
If we except external damage and water penetration, the number of breakdowns is very low and significantly lower than the objective which is less 0.2 fault per 100 km of circuit-year. For example, for the 225 kV cables, the failure rate is 0.06 fault per 100 km of circuit year. In France more than 50 per cent of the in-service cables in the field 63 kV-400 kV are polymeric cables (LDPE-HDPE and XLPE). For many years Electricite de France has installed only cables with synthetic extruded insulation and their reliability in service is excellent.
Australia is a large land mass of 2.97 million square miles with a population of some 17 million people scattered around the periphery of the continent. The total installed generating capacity is 34,500 MW but the majority of the population and 60 per cent of the power demand is divided between the two main cities of Sydney and Melbourne.
Most electricity generation is by coal fired power stations near major coal fields and there are some hydro-electric power stations. There is no nuclear power generation in Australia. There are some major inter-connections of transmission lines to allow for load to be transferred between the centers of population of South Australia, Victoria and New South Whales. Naturally, with such vast distances most of the transmission supply is by overhead transmission lines typically at voltages of 275,330 and 500 kV.
Due to the topography of Melbourne, Perth and Adelaide there are comparatively few extra high voltage underground systems installed. The major EHV underground systems are in Sydney at 132 kV, and in Brisbane at 110 kV. Due to the fact that each state within Australia developed its own standards for electricity supply, we have varying transmission and distribution voltages. Virtually all underground transmission cables installed prior to 1984 were of paper insulated oil-filled low pressure type. Most of these circuits were installed more than 20 years ago.
The use of XLPE for underground high voltage cable up to 33 kV commenced in 1974 but the first 66 kV XLPE cable was not installed until 1984. However, now the current policy for all supply authorities is to adopt only XLPE cable for up to and including 220 kV. It is likely that this policy will extend to 275 and 330 kV in the near future.
Victoria and South Australia predominantly use 66 kV as the transmission and sub-transmission voltage. Tasmania and Queensland have adopted 110 kV and Western Australia and New South Whales use 132 kV. All 66, 110 and 132 kV XLPE cable currently used in Australia is provided with a lead alloy sheath together (when required) with additional copper screen wires over the lead to provide increase earth fault rating. This design is chosen due to the need to have metal moisture barrier protection and flexibility of installation.
For the recent installation of 220 kV cable in Victoria, due to the weight and size of the cable, additional protection was required, and a stainless steel sheath design was chosen. This cable comprises 1200 mm squared copper conductor, XLPE insulated to 27 mm (1.06 inch.) wall thickness and is provided with a copper wire screen and an overall sheath of hermetically sealed corrugated stainless steel and protective plastic over sheath combination of PVC and high density polyethylene.
On the 220 kV project, a monitoring system was also installed to enable continuous observation of the temperature and pressure of each of the 39 joints involved in the project and also the cable temperature and ambient conditions at locations of specific interest. This information is relayed by means of optical fibre cable to the terminal substations and from there to the Supply Authority Head Office and also to the manufacturer's Technical Department for evaluation. This data is proving invaluable to the Supply Authority as it enables the real time response to overloads and ambient condition variations to be observed thus enabling the full transmission capability of the cable to be realized. The data logging system is also accumulating a full history of each and every joint hence allowing any changing trends to be noticed and any maintenance required to be planned well in advance.
The use of aluminum as a sheathing material for medium voltage cables was suspended in Australia more than 5 years ago and it is unlikely that any cable with an aluminum sheathing would now be accepted in Australia. Furthermore, due to environmental concerns, it is also unlikely that approval would be given to an extension of the oil-filled cables which are nearing the end of their reliable service life may well have to be replaced by XLPE cable.
In China, there has been a tremendous growth in demand for underground cable at transmission voltages of typically 110 and 220 kV. Here installation conditions are often quite severe and in order to reduce costs, it was found preferable to use the longest cable lengths possible. In the past, most of the solid dielectric cables were installed with an aluminum sheath but now the trend is to adopt stainless steel or in some cases where the cable can be adequately protected, a lead sheath with a copper wire screen. On a recent 110 kV cable installation in China, cables were required to pass through a complicated bridge structure which required the design of special dilation equipment to overcome bridge movement. Cable lengths were more than 1.1 km (3600 ft) and upon reaching the land portion were required to be direct buried. The provision of a stainless steel sheath provided adequate protection and yet suitable flexibility for the most arduous handling conditions on site.
A similar type of cable has been used on another project in China where again the cable lengths were up to 1200 m (3937 ft) between joints to reduce installation costs. In this instance, the cable is in an area with very severe flooding with possible corrosive effluent and once again stainless steel sheath cable was chosen to provide the best protection.
Brazil is a Federal Republic consisting of 26 states and a federal district. Power transmission, both overhead and underground, is carried out by state utilities under the coordination of a Federal Government company, Eletrobras. The main Brazilian cities have underground transmission: Sao Paulo, Rio de Janeiro, Belo Horizonte, Salvador and Florianopolis. Voltages of 69 kV up to 345 kV are used in such installations. Apart from these cities, there are insulated cable lines installed in oil plants, steel plants and hydroelectric power stations, which use voltages from 69 kV to 220 kV. Therefore, underground transmission lines (UTL) in Brazil are essentially urban. As each city is within the concession area of a state utility, specifications for cables, accessories, installation and construction are different.
Underground transmission lines were first installed in Brazil in the fifties. Up to 1985 only self-contained low pressure cables were used (SCOF cables), except in the case of some minor lines using PIPE and XLPE cables, installed in hydroelectric power stations, substations and steel plants. Initially, 138 kV SCOF cables were installed in the city of Rio de Janeiro, while 88 kV cables were installed in Sao Paulo, using copper wires and lead sheaths. The metallic sheath was reinforced with copper tapes to withstand the inner pressure and jacketed with Neoprene. In the seventies, 138 kV tension was used in Sao Paulo and in other main cities, except in Salvador which uses 69 kV.
Cables then began to use conci-type conductors (annular sectors) and thermoplastic jackets (PVC and LDPE). With the development of aluminum welding processes, particularly MIG welding, aluminum was also used as a conductor, especially because of its low cost and light weight. As from 1975, aluminum sheaths were used is SCOF cables, applied by continuous extrusion and then corrugated. The low cost of aluminum, as well as its light weight and high short-circuit current-carrying rating, when compared with lead, were attractive features in its use.
Short-circuit currents are about 30 kA for 138 kV and 40 kA for 345 kV, both for one minute. To start with aluminum sheaths on SCOF cables were used for 230 kV and 345 kV. Later aluminum sheaths were also used on 138 kV cables.
In the early eighties, the first installations with 69 kV extruded cables were made. These cables use both EPR and XLPE insulation and the maximum gradient, which was 4 MV/m for both, evolved to 6 MV/m at present. In 1985, the first installation of a 138 kV UTL was made with extruded cables. XLPE cables for 138 kV also have a blocked conductor, a 6 MV/m maximum gradient and a radial and longitudinal water-proof barrier. The radial barrier consists of polyethenated aluminum tape applied onto the HDPE outer protection.
Approximately 62 km of 69 km extruded cables are installed, 53 km of which are EPR-insulated; and 46 km of 138 kV extruded cables, 40 km of which are EPR-insulated. Power utilities are willing to install 69 kV and 138 kV extruded cables whenever possible.For type tests, special and routine tests, SCOF cables and accessories should comply with IEC Norm 141-1 some cases, extruded cables comply with the IEC Norm 840 and in other cases with the buyers' norms which are generally based on IEC 840, AEIC CS-6 and AEIC CS-7. Accessories are tested with cables in the type test.
In Brazil, there are two types of high voltage underground installations: directly buried and in duct banks. In both cases, the double circuit (one being a stand-by) is commonly used. With all lines, a pilot cable for communication and safety (protection) is installed. In the case of self-contained oil-filled cable, the pilot cable also indicates oil pressure.
The first installations in Brazil used a "Trifolium" configuration, in other words a triangular configuration with cables close to one another. Shielding was multi-earthed. Installation was always made with two circuits, of which one was a stand-by. In the late sixties and early seventies, installations were made using 4 cables, of which one was a spare. These 4 cables were transported, and the multi-earthed shielding as well. This system with four cables, besides being uneconomical due to the increase in the conductor sections, proved operationally disadvantageous and ceased to be used in the seventies. Since then, the Trifolium installation has been used for high loading up to 120 MVA in 138 kV or 80 MVA in 69 kV, or the cross-bonded earthing system for the shielding is used for higher loading. The cables are installed on a plateau within a distance of 150 to 300 mm between them. In both cases, two circuits are used, one being a stand-by.
Tokyo Electric Power Company Inc. (TEPCO) is one of ten utilities in Japan, that generates and supplies electrical power to a region of 39,000 square km around the Tokyo area. Tokyo is the center of government and economical activity in Japan with the highest summer peak load recorded on September 4, 1993 of 54,100 MW and the peak winter load recorded on February 1, 1994 of 46,150 MW. In the 12 month period ending March 1994 TEPCO sold 230,100 GWh of electric energy.
The main transmission voltage levels are 66 kV, 275 kV and 500 kV with 154 kv used for regional dispatching. The Tokyo metropolitan area is supplied from 500 kV overhead transmission lines. Recently, XLPE cable is being more widely used at each voltage level instead of OF cables and other types, as XLPE has the merits of environmental compatibility, better electrical properties, stronger fire resistance, easier handling and simpler maintenance. The first long distance underground line (9.6 km), the Minami-Ikegami Line, was installed in 1989. A second circuit was added along the same route in 1991.
The conventional cable laying methods are ducts (non-filled) and tunnels. When an underground transmission cable is designed, many circuits are planned for the one route. The number of circuits chosen is based on a three to ten year plan. As a rule, ducts and tunnels are buried under the road, not the sidewalk. The route selection is based upon the road conditions, the environment of the area and the presence of other utilities (i.e. water, gas, sewage, telephone and subways). Duct systems are usually installed for 6.6 to 66 kV cables up to 20 ducts. For installations of more than 21 ducts the tunnel method is employed and for 154 kV and 275 kV a cooling system is often employed.
To achieve a large current capacity, insulated wire stranded cable (66 kV, 154 kV and 275 kV) and indirect water cooling methods (275 kV) are used. The insulated wire stranded cable is made up of copper oxide insulated strands to reduce the ac resistance. Indirect cooling in the trough within a tunnel is carried out by snaking the cables in a fire proof trough and installing cooling in the trough. Cooling stations are located at 5 km intervals.
In-service failures of OF cables have been minimal. Of those failures that have occurred, 47 per cent were due to external damage and 53 per cent to defective workmanship and other reasons. Oil leakage caused by deterioration of the lead sheath manufactured prior to 1962 occurred many times at problematical points of concentrated crookedness caused by thermal movements at manholes or bridges. Even though E-alloy lead sheaths were adopted, cracking and separation still occurred. Aluminum sheaths were adopted in 1975.
Gas-in-oil analysis has been successfully used to locate acetylene gas in 74 joints. In total 9539 joints were tested that had been in service for more than 20 years. Cables installed in sand-filled FRP troughs often broke and emerged from the trough because of thermal movement. It was found that the sand dried out over a period of time. To counter act these problems an optical fibre distributed temperature sensor was installed to monitor the temperature profile. Water is sprayed on the sand according to the trough temperature measurement.
Failures in 66 kV XLPE insulated cables have occurred during dc withstand testing (8) and in actual service (48). Of these 27 were located in the cable and 29 in joints. The cables failures were attributed to manufacturing defects, external damage and to workmanship problems. Joint failures were due to workmanship related problems.
H.E. Orton is with Consulting Engineers International Limited, located in Vancouver, BC, and R. Samm is with EPRI. This article was taken in part from a presentation made at T&D World Expo 97. ET