Overhead (OH) lines have proven to be the best choice for providing safe, reliable and economical power to our customers. Dominion Virginia Power (Dominion) operates a high-voltage network of approximately 6,100 miles of transmission lines. Of this total, only 55 miles is comprised of underground (UG) cables. At 230kV, there are approximately 2400 miles of OH lines and 36 miles of UG cables.
When considering whether or not to install underground transmission facilities, the following issues need to be evaluated:|
A detailed discussion of the above factors is presented below:
Outage Considerations
In comparing the reliability of OH lines and UG cables, reliability needs to be examined from both short and long timeframes. Short term considerations favor UG cables since they tend to be somewhat immune from most weather-related outage initiators such as weather, trees and lightning. UG cables fall very short of the performance of OH lines in long term views. This is mainly due to their inherently long outage times once problems do occur.
There are several operating conditions that vary dramatically when looking at OH lines and UG cables. OH lines are equipped with protective devices (circuit breakers) that will open whenever they experience electrical problems (faults). If the fault is temporary in nature, i.e., insulator flashover from a lightning strike, the breakers will open and then instantaneously reclose restoring the circuit. With UG cables this automatic reclosing practice is not practiced to avoid potentially severe damage to the expensive systems. This means most any disturbance on the UG circuit will result in an extended outage compared to a "blink" on an OH line.
Fault location on OH lines is a fairly simple process. Equipment located in the substations provides system operators with a location in miles from each end. A crew is dispatched to patrol the line route looking at the structures and wires until the problem is found. This process tends to be defined in terms of minutes to hours as far as duration. Problems on UG cables are far more complicated to diagnose. Dominion relies on highly specialized contractors to bring fairly unique equipment on site to perform electrical tests to locate the fault. Mobilization and actual fault location activities can take up to a week to complete.
Most problems with OH lines can be restored in terms of hours. A damaged structure can usually be replaced in the same day of the damage. Dominion maintains a staff of trained line workers and a fleet of construction equipment that can facilitate OH line repairs. In contrast, UG cables can take up to two weeks to complete repairs even under the ideal conditions.
"Network" vs. "Radial" Installations
There are currently thirteen (13) circuits operating at 230kV in the system. Only five (5) of these circuits are operating in a networked configuration. These cables are subjected to system through flows that can vary tremendously depending on which other lines in the network are in service at any given time. The remaining eight (8) circuits are operated in a radial configuration where load flows are only impacted by local demand.
In the system network, UG cables that are configured in parallel with OH lines present additional technical challenges. Due to the inability to operate UG cables at the high conductor temperatures that OH lines are designed to run, the typical UG installation requires multiple cables to meet the same capacity as the OH line. This adds significantly to the cost and complexity of the UG installation. The electrical characteristics (lower impedance) of these multi-cable circuits will cause them to carry significantly more than a parallel OH line. This severely reduces the ability of the OH line to carry load. There are additional electrical components that can be utilized to help overcome this undesirable effect, however, these units further complicate the system, add significantly to the cost and can potentially reduce the overall system reliability.
Radial UG cable circuit installations in the transmission system usually involve 100 percent redundancy. Typically two circuits are installed each with the capability to serve the entire load of the substation. This is required due to the duration of repairs reasonably anticipated with UG circuits. This required redundancy adds significantly to the overall cost of the installation. On occasion, one spare cable per circuit is used to reduce costs. However, this also reduces restoration efforts.
Line Length Challenges
Long UG cables can have detrimental impacts on the transmission system. UG cables have a significantly higher capacitance in comparison to OH lines. This capacitance causes system voltages to rise in period of light loading. In some cases the voltages rise to levels that require intervention such as removing the cables from the system. Installing additional electrical equipment to compensate for the capacitance can also mitigate this effect.
This capacitance is a function of the circuit length and the number of cables. There are losses (dielectric) associated with this capacitance that are present whenever the cable is energized. Current must flow in the cable to "charge" this capacitance. As the length of cable increases the amount of current that flows in the source end must also increase. In theory a cable can be of a critical length of which the current flowing in it uses up the available capacity just to supply these losses. In practice long cable circuits are equipped with additional components that help compensate for these capacitive losses and will reduce this overall impact of lengthy cables. These components are not required for equivalent length OH lines. There is a significant cost to install this compensation equipment on the cables.
Multiple Cables & Forced Cooling Options
In the detailed design phase of underground projects two options are typically investigated, i.e., circuits with forced cooled cables and circuits of multiple cables. UG cable systems can utilize forced-cooling equipment to help remove heat from the cable systems increasing the capacity of the circuit. Forced cooling equipment can be as simple as fluid-to-air heat exchangers (radiators) or as complex as large refrigeration units.
The cooling requirements dictate what equipment must be utilized. Forced-cooling equipment located in the substations along the UG circuit route would increase noise levels as a result of dielectric fluid pumping equipment and fans cooling the fluid passing through heat exchangers
Forced cooling is typically operated only during peak load periods each day. This approach to increase capacity for short duration might also be employed to enable an UG cable circuit to meet demand in a network contingency situation involving an outage of a line elsewhere in the system. Availability of this equipment when required to operate is critical to network reliability.
Solid dielectric cables systems that utilize cross-linked polyethylene (XLPE) insulation are not typically forced cooled. Multiple cables are operated in parallel to achieve the higher capacities of overhead lines.
This decision as to which approach will be based on several factors such as initial cost, long-term operations and maintenance costs and suitability for the performance required for electric system conditions.
Construction Impacts
The environmental impacts associated with UG cable installation are significantly greater than those of OH line construction. Excavation of a continuous trench approximately 5 –8 feet wide and 5 feet deep is much more disruptive than digging a 4 –6 foot diameter hole, 20 feet deep for each pole foundation located every 700 feet along the route. In some suburban or rural areas, significant clearing and grading may be required to facilitate access for construction of underground facilities.
Impacts to the local community include mainly construction equipment noise and dust from the excavation and back-filling activities. The duration of construction for an UG line is roughly three to six times that for an OH line depending on subsurface conditions encountered.
Easement Requirement for UG Lines
The easement width for underground lines can vary depending on the number of circuits, the design and whether or not it is a network circuit. The width is typically less than that needed for an equivalent overhead line, except in areas where significant grading activities are required to enable the underground line to be installed.
Life Expectancy
When using XLPE (Crossed-linked Polyethylene) underground cable, the life expectancy is roughly half of a typical overhead line. In both cases the company would maintain the lines to extend the useful life in perpetuity. Such maintenance activities would likely include complete replacement of XLPE cables before age-related failures occur.
Costs
In general, the construction and installation of underground transmission lines cost 4-10 times more than an equivalent overhead line. This is mainly due to the time, materials, process, and skilled labor involved. The duration of construction for an UG line is roughly three to six times that for an OH line depending on subsurface conditions encountered. The cables are placed in individual conduit (PVC pipe) for ease of replacement and maintenance. The conduit is incased in a concrete mixture, sometimes a specialized design known as FTB (Fluidized Thermal Backfill) to help dissipate heat and to protect the lines from potential damage and the public from injury.
The entire process of underground cable systems installation requires specialized contractors to perform this skilled work. Skilled labor is specifically needed when connecting (splicing) sections of cable together.
