The Transmission Underground Cables Interest Group (TUCIG) of CEA Technologies Inc. (CEATI) provides a forum for the exchange of information on transmission cable asset management and the establishment and promotion of techniques and tools for improving the management of fluid-filled and XLPE high-voltage underground cable systems.
Underground cables offer a number of distinct advantages over traditional overhead designs, both technically and in simple aesthetics. Because of their basic design, underground systems can often have a lesser environmental impact, without a need for vegetation clearances and with reduced rights-of-way. This approach also means that customer property values are kept intact, as the lack of visible towers is inherently pleasing to the eye. The simpler routing of underground cable systems, due to less stringent rights-of-way requirements and tower placement, also means that the cost of basic equipment is reduced, which is certainly a benefit to asset managers. For these reasons, underground systems are often ideal for urban environments.
Transmission cables are without doubt critical arteries of the electrical power grid; it is necessary that they be highly reliable. Certain older transmission cables, including high-pressure fluid-filled pipe type (HPFF) cables or low-pressure self contained fluid-filled (SCFF) cables, have been in service for sixty years or longer. The reliability of both types of cables has been, and continues to be, exceptional. However, there are several important issues with these cables that will need to be addressed in the coming years. There are concerns regarding the potential impact on the environment from fluid leaks, concerns about increasing maintenance costs as the older circuits continue in service, concerns about the loss of expertise to carry out this maintenance due to demographic shifts, and concerns over a possible reduction in the reliability of the cables with time.
The alternative to fluid-filled cable systems is polymer insulated cables. Some of the first extruded transmission cables, using polyethylene as the insulation, were installed in the 1960s but operated at an electrical stress level of less than half that used in HPFF and SCFF cables. Cross-linked polyethylene (XLPE), because of its superior mechanical properties, replaced thermoplastic polyethylene as the insulation of choice and is now used almost exclusively in extruded transmission cables, particularly at higher transmission voltages up to 500 kV. There was an early reluctance to use extruded transmission cables, partly because the “older technology” fluid-filled cable systems were highly reliable, partly due to poor experience in many utilities with extruded medium voltage cables due to water treeing, and partly due to the lack of economic incentives to switch to a relatively “unproven” technology. The increased reliability of extruded medium voltage cables in the 1980s, the realization that transmission cables had to be sealed to prevent the ingress of moisture, and the operational experience with installed extruded cables gradually overcame this reluctance in the 1990s. Today, XLPE transmission cable systems are in widespread use and have a major environmental advantage in the absence of oil leaks. However, there are still concerns about the high stress levels used in the new extruded cable designs and the long-term performance of extruded cables particularly when they are operated at high temperatures for long periods.
The scope of the TUCIG covers all the above-mentioned issues for both fluid-filled and extruded cable systems. A sampling of the group’s projects currently underway includes:
Cable Health Index (CHI)
It is very important for utilities to assess the condition of their cable circuits, particularly their older circuits. Currently, no methods exist for quantifying re-use, refurbishment or replacement needs. The cable health index will provide a clear, concise, verifiable and repeatable methodology for this assessment. The index will consider electrical (outage records, loading history), maintenance (oil testing, insulation condition), and corrosion protection (stray currents) issues, environmental (installation location, sheath grounding), safety and other risk issues, as well as maintenance costs and investment issues, so that a set of numerical values for each cable circuit can be derived. The CHI, including precise grading and weighting factors, is a technical assessment of cable circuits and can be used on both fluid-filled and extruded cable systems that will allow for comparison to be made between different circuits so that cable assets can be managed accordingly.
Maintenance Survey
A survey of the TUCIG participant utilities was carried out to determine the different maintenance procedures (function tests, visual checks, infrared inspections, etc.), and the frequency at which they are carried out on transmission cable circuits, in order to establish industry best practices. Utility procedures can vary according to the age or importance of the circuit, and utilities often have different “default” frequencies at which many procedures are carried out on “normal” circuits that are not suspected of having any serious problems. Certain utilities carry out routine inspections every three months, while others perform these inspections annually.
PD testing of Fluid-Filled Cable Systems
Partial Discharge testing has long been recognized as a technique for detecting potentially harmful defects in medium voltage cables and in extruded transmission cables. This technique is now being applied by a number of utilities on problematic HPFF and SCFF cables suspected of having some form of degradation or damage. In order to interpret the test data, conventional tests such as dissolved gas analysis (DGA) have also been performed to establish whether there may be a correlation with results from PD testing, and as to whether PD testing can reliably indicate the presence of contaminants and fault gases (such as H2 and C2H2).
Mitigation of Stray Currents on Underground Cable Systems
Corrosion of underground cable systems is a major concern for utilities. It can lead to environmentally harmful oil leaks and the loss of insulation integrity in fluid-filled cables and the ingress of moisture into extruded cable systems, potentially leading to failure. A key focus of this research concerns DC stray currents from Light Rail and Transit systems, as these can create corrosion problems in nearby underground transmission systems. This project is being initiated to evaluate the methods, equipment, materials, and programs that will eliminate or mitigate corrosion problems in the underground transmission systems.
Cable Selection Guide
Utilities have an increasing number of choices in the types of transmission cables available to replace existing circuits or when planning to install new ones. The choice includes HPFF and SCFF cables and different designs of extruded cables (for example, high and low stress designs). There are advantages and disadvantages for each cable type and design, which makes the final choice a complex decision for utilities. The Cable Selection Guide will list and discuss the technical and economic issues (test equipment, trade-offs, installation and maintenance issues) and help utilities make the best choice for their particular application. The guide will give an impartial assessment of the types of cable designs available and will provide selection criteria for a number of different scenarios that will include environmental and logistical factors.
Joining Together to Fund Underground Cable Research
As deregulation continues to impact the electricity industry, utilities are often forced to cut back on research spending to lower their expenditures. Simply put, the costs associated with research and technology development are very often prohibitive for individual utilities. The collaborative nature of the CEA Technologies Inc. (CEATI) program and the Transmission Underground Cables Interest Group helps utilities undertake projects that respond to their strategic goals at a fraction of the cost of doing so independently.
CEATI operates a number of interest groups consisting of participants from electrical utilities from all over the world. Each interest group focuses on particular technical issues and carries out projects critical to the participating utilities. The Transmission Underground Cable Interest Group (TUCIG) is made up of thirteen utilities from the USA, Canada and Europe.
Along with the projects listed above, the TUCIG also has a number of internal projects underway to collect information on failure statistics for each different type of cable, new developments in cable installation techniques, cable diagnostics and new materials. Group meetings provide excellent opportunities for participants to discuss day-to-day operating problems and concerns.
Participation is open to all electrical utilities, independent system operators, regional transmission operators, independent market operators, and government agencies that have an interest in underground system issues. The group identifies areas of common concern and defines research objectives to solve those concerns. Individual group participants can select to co-fund projects of interest that will be of benefit to their organizations. This flexible, user-driven, responsive and collaborative approach provides substantial cost-benefit advantages to the participants.
About the Author
Mark Michailuk, B.Eng., currently oversees the Transmission Underground Cables Interest Group of CEA Technologies Inc. (CEATI). A graduate of McGill University in Montreal, as well as L’Institut en génie de l’énergie électrique, his primary area of focus is power systems analysis.
Underground cables offer a number of distinct advantages over traditional overhead designs, both technically and in simple aesthetics. Because of their basic design, underground systems can often have a lesser environmental impact, without a need for vegetation clearances and with reduced rights-of-way. This approach also means that customer property values are kept intact, as the lack of visible towers is inherently pleasing to the eye. The simpler routing of underground cable systems, due to less stringent rights-of-way requirements and tower placement, also means that the cost of basic equipment is reduced, which is certainly a benefit to asset managers. For these reasons, underground systems are often ideal for urban environments.
Transmission cables are without doubt critical arteries of the electrical power grid; it is necessary that they be highly reliable. Certain older transmission cables, including high-pressure fluid-filled pipe type (HPFF) cables or low-pressure self contained fluid-filled (SCFF) cables, have been in service for sixty years or longer. The reliability of both types of cables has been, and continues to be, exceptional. However, there are several important issues with these cables that will need to be addressed in the coming years. There are concerns regarding the potential impact on the environment from fluid leaks, concerns about increasing maintenance costs as the older circuits continue in service, concerns about the loss of expertise to carry out this maintenance due to demographic shifts, and concerns over a possible reduction in the reliability of the cables with time.
The alternative to fluid-filled cable systems is polymer insulated cables. Some of the first extruded transmission cables, using polyethylene as the insulation, were installed in the 1960s but operated at an electrical stress level of less than half that used in HPFF and SCFF cables. Cross-linked polyethylene (XLPE), because of its superior mechanical properties, replaced thermoplastic polyethylene as the insulation of choice and is now used almost exclusively in extruded transmission cables, particularly at higher transmission voltages up to 500 kV. There was an early reluctance to use extruded transmission cables, partly because the “older technology” fluid-filled cable systems were highly reliable, partly due to poor experience in many utilities with extruded medium voltage cables due to water treeing, and partly due to the lack of economic incentives to switch to a relatively “unproven” technology. The increased reliability of extruded medium voltage cables in the 1980s, the realization that transmission cables had to be sealed to prevent the ingress of moisture, and the operational experience with installed extruded cables gradually overcame this reluctance in the 1990s. Today, XLPE transmission cable systems are in widespread use and have a major environmental advantage in the absence of oil leaks. However, there are still concerns about the high stress levels used in the new extruded cable designs and the long-term performance of extruded cables particularly when they are operated at high temperatures for long periods.
The scope of the TUCIG covers all the above-mentioned issues for both fluid-filled and extruded cable systems. A sampling of the group’s projects currently underway includes:
Cable Health Index (CHI)
It is very important for utilities to assess the condition of their cable circuits, particularly their older circuits. Currently, no methods exist for quantifying re-use, refurbishment or replacement needs. The cable health index will provide a clear, concise, verifiable and repeatable methodology for this assessment. The index will consider electrical (outage records, loading history), maintenance (oil testing, insulation condition), and corrosion protection (stray currents) issues, environmental (installation location, sheath grounding), safety and other risk issues, as well as maintenance costs and investment issues, so that a set of numerical values for each cable circuit can be derived. The CHI, including precise grading and weighting factors, is a technical assessment of cable circuits and can be used on both fluid-filled and extruded cable systems that will allow for comparison to be made between different circuits so that cable assets can be managed accordingly.
Maintenance Survey
A survey of the TUCIG participant utilities was carried out to determine the different maintenance procedures (function tests, visual checks, infrared inspections, etc.), and the frequency at which they are carried out on transmission cable circuits, in order to establish industry best practices. Utility procedures can vary according to the age or importance of the circuit, and utilities often have different “default” frequencies at which many procedures are carried out on “normal” circuits that are not suspected of having any serious problems. Certain utilities carry out routine inspections every three months, while others perform these inspections annually.
PD testing of Fluid-Filled Cable Systems
Partial Discharge testing has long been recognized as a technique for detecting potentially harmful defects in medium voltage cables and in extruded transmission cables. This technique is now being applied by a number of utilities on problematic HPFF and SCFF cables suspected of having some form of degradation or damage. In order to interpret the test data, conventional tests such as dissolved gas analysis (DGA) have also been performed to establish whether there may be a correlation with results from PD testing, and as to whether PD testing can reliably indicate the presence of contaminants and fault gases (such as H2 and C2H2).
Mitigation of Stray Currents on Underground Cable Systems
Corrosion of underground cable systems is a major concern for utilities. It can lead to environmentally harmful oil leaks and the loss of insulation integrity in fluid-filled cables and the ingress of moisture into extruded cable systems, potentially leading to failure. A key focus of this research concerns DC stray currents from Light Rail and Transit systems, as these can create corrosion problems in nearby underground transmission systems. This project is being initiated to evaluate the methods, equipment, materials, and programs that will eliminate or mitigate corrosion problems in the underground transmission systems.
Cable Selection Guide
Utilities have an increasing number of choices in the types of transmission cables available to replace existing circuits or when planning to install new ones. The choice includes HPFF and SCFF cables and different designs of extruded cables (for example, high and low stress designs). There are advantages and disadvantages for each cable type and design, which makes the final choice a complex decision for utilities. The Cable Selection Guide will list and discuss the technical and economic issues (test equipment, trade-offs, installation and maintenance issues) and help utilities make the best choice for their particular application. The guide will give an impartial assessment of the types of cable designs available and will provide selection criteria for a number of different scenarios that will include environmental and logistical factors.
Joining Together to Fund Underground Cable Research
As deregulation continues to impact the electricity industry, utilities are often forced to cut back on research spending to lower their expenditures. Simply put, the costs associated with research and technology development are very often prohibitive for individual utilities. The collaborative nature of the CEA Technologies Inc. (CEATI) program and the Transmission Underground Cables Interest Group helps utilities undertake projects that respond to their strategic goals at a fraction of the cost of doing so independently.
CEATI operates a number of interest groups consisting of participants from electrical utilities from all over the world. Each interest group focuses on particular technical issues and carries out projects critical to the participating utilities. The Transmission Underground Cable Interest Group (TUCIG) is made up of thirteen utilities from the USA, Canada and Europe.
Along with the projects listed above, the TUCIG also has a number of internal projects underway to collect information on failure statistics for each different type of cable, new developments in cable installation techniques, cable diagnostics and new materials. Group meetings provide excellent opportunities for participants to discuss day-to-day operating problems and concerns.
Participation is open to all electrical utilities, independent system operators, regional transmission operators, independent market operators, and government agencies that have an interest in underground system issues. The group identifies areas of common concern and defines research objectives to solve those concerns. Individual group participants can select to co-fund projects of interest that will be of benefit to their organizations. This flexible, user-driven, responsive and collaborative approach provides substantial cost-benefit advantages to the participants.
About the Author
Mark Michailuk, B.Eng., currently oversees the Transmission Underground Cables Interest Group of CEA Technologies Inc. (CEATI). A graduate of McGill University in Montreal, as well as L’Institut en génie de l’énergie électrique, his primary area of focus is power systems analysis.
