Differential voltage underground

The sheath voltage is increased by the sheath current. Thus the sheath voltage exceeds touch voltage, and electroshock risks and cable faults occur. Also, cable temperature is increased by the sheath current, so cable ampacity reduces. In literature, single-point bonding, solid bonding and cross bonding are used to reduce sheath voltage and current.

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Content:

• Unit of competency details
• Analysis techniques of diagnosis high voltage underground cables
• Sheath Voltage Limiters Protect HV Power Cables
• Ground loop (electricity)
• Answers to Frequently Asked Questions about Underground Distribution Lines
• Comparison of Overhead and Underground Cables
• Pilot wire current differential feeder protection

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Unit of competency details

Hurricane Sandy left many electric utility executives, their customers, local and state government leaders and regulators contemplating placing overhead power lines underground. This desire surges into prominence whenever natural disasters cause destruction on the overhead distribution and transmission networks across the country.

In the past, the largest obstacle to placing overhead power lines underground has been the higher cost of installation and maintenance for underground lines. Although overhead power lines are typically more economical, they are susceptible to damage from wind-borne tree branches, debris and high wind and ice-loading conditions from extreme weather. The damages can cause extended power outages that in extreme cases cannot be restored for days or even weeks, as we have seen after Hurricane Sandy.

The cost for repairing the physical damages can be in the billions of dollars. In addition to the intangible impacts, there are considerable direct economic impacts to customers resulting from lost economic activity, food spoilage, looting, etc. Whenever a major weather-related catastrophe occurs or land is being developed, the question of placing overhead power lines underground surges.

Higher initial construction costs. Read More: Is it time for you to consider undergrounding some of your distribution lines? These costs show a potential initial construction cost differential of more than five times for underground lines as opposed to overhead lines for construction in Wisconsin. Costs vary in other regions, but the relative difference between overhead and underground installation costs is similar from state to state.

Technical improvements in cable technology, wire placement, conduit sizing, grounding methods, directional boring techniques and other aspects of undergrounding power lines have advanced the reliability of underground power. They have not lowered their initial construction costs significantly, however, which are mostly associated with trenching through the earth along the entire line route.

Maintenance costs. The present worth of the maintenance costs associated with underground lines is difficult to assess. Many variables are involved, and many assumptions are required to arrive at what would be a guess at best. Predicting the performance of an underground line is difficult, yet the maintenance costs associated with an underground line are significant and one of the major impediments to the more extensive use of underground construction.

Major factors that impact the maintenance costs for underground transmission lines include:. Cable repairs. Underground lines are better protected against weather and other conditions that can impact overhead lines, but they are susceptible to insulation deterioration because of the loading cycles the lines undergo during their lifetimes. If the cables are installed properly, this debilitating process can take years and might be avoided. If and when a fault occurs, however, the cost of finding its location, trenching, cable splicing, and re-embedment is sometimes five to 10 times more expensive than repairing a fault in an overhead line where the conductors are visible, readily accessible and easier to repair.

In addition, easement agreements might require a utility to compensate property owners for disruption in their property use and for property damage caused by the repairs to the underground cables. Line outage durations. The durations of underground line outages vary widely depending on the operating voltage, site conditions, failure, material availability and experience of repair personnel.

The typical repair duration of cross-linked polyethelene XLPE , a solid dielectric type of underground cable, ranges from five to nine days. Outages are longer for lines that use other nonsolid dielectric underground cables such as high-pressure, gas-filled HPGF pipe-type cable, high-pressure, fluid-filled HPFF pipe-type cable, and self-contained, fluid-filled SCFF -type cable. In comparison, a fault or break in an overhead conductor usually can be located almost immediately and repaired within hours or a day or two at most.

During the extended line outages required for underground line repairs, services to customers are disrupted. The length of customer outages can be mitigated using redundant feeders, but the duration of such outages is still longer than those associated with overhead lines, and they have additional costs associated with them. Line modifications. Overhead power lines are easily tapped, rerouted or modified to serve customers; underground lines are more difficult to modify after the cables have been installed.

Such modifications to underground power lines are more expensive because of the inability to readily access lines or relocate sections of lines. Service drops to new residences can be installed within a day or two after the service request is submitted to the utility.

If the utility is requested to provide underground service to the new home, however, the design and construction will take up to a week or two. As the additional construction time, specialty cable costs and excavation costs continue to increase, the issue of who bears these differential costs remains unsolved. Typically the differential costs for new distribution services are paid by the developer according to a regulated tariff.

The developer may then pass those costs to home buyers who purchase property fed by underground power lines. For example, in an Orlando, Fla. For transmission lines, it is difficult to determine how to allocate the differential costs associated with placing them underground to a specific developer, customer class or individual customer. Regulatory agencies usually do not allow utilities to differentiate between underground and overhead services in their rates. Service rates must be the same for each customer classification regardless how the service is provided.

There are signs that regulatory agencies are modifying their approach, however. For example, southeastern Connecticut, a generation resource-limited area, is also one of the wealthiest areas of Connecticut.

A new kV line was required to connect new generation facilities to the New England power grid. Because the bulk power generated would benefit consumers throughout the region, the costs of those new generation facilities and associated overhead transmission tie line were shared by all New England ratepayers.

The differential costs for undergrounding portions of the kV tie line, however, were borne only by the southeastern Connecticut ratepayers. This rate differentiation must be the norm and not the exception. Restrictions enforced by regulatory agencies try to ensure utility customers are not unduly burdened with system improvements that benefit a limited number of customers. In addition, nearly all regulatory agencies base their standard power delivery models using overhead line construction. Any proposed underground line installations that exceed the specified voltage, dollar or line length limit must be justified and approved by the regulatory agency prior to design and construction.

Investor-owned utilities IOUs face additional cost challenges. Unless special exceptions are obtained ahead of time, IOUs are not allowed to include their expenses for works-in-progress into their rate bases. A new power line-whether overhead or underground-cannot be included in the rate base until it is energized and serving customers. In addition, most regulatory agencies require utilities to justify the need and costs of new facilities.

The need for a new power line typically is supported by load growth. The cost of new facilities is justified by who benefits and by performing a typical industry cost comparison. If a new facility cannot be justified to the regulatory agency, the utility must bear the costs or at least the differential costs of designing, constructing and operating the facility.

In the U. Established standard design and construction practices are to place such lines overhead. Unless undergrounding is justified by physical constraints, the utility would be responsible for the differential cost between the overhead and underground installation of the line.

Regulatory reform. The first required change is a redefinition of who is responsible for the differential costs associated with building and maintaining power lines underground and converting overhead lines to underground. Currently for distribution lines, those costs are passed on to the land developers who request underground services who, in turn, pass the costs on to home buyers.

This seems an equitable way to handle the initial construction costs. In addition, for those utilities without underground facilities on their systems, the initial costs for converting overhead lines to underground lines would require additional startup costs associated with staff training, stocking their warehouses with underground materials and equipment, developing new standards, and purchasing new equipment for underground installation and maintenance.

A more equitable approach might be to develop separate rates for customers served underground and overhead. Maintenance costs would be tracked and allocated according to the type of service provided to each customer.

Under the current system, the utilities have the means to recover initial construction costs. Their reluctance to undergrounding distribution lines stems from the higher maintenance costs they have to absorb when underground lines fail.

Independent assessment of differential costs. Another change needed is the development of an independent assessment of the differential costs associated with installing power lines underground. For example, when trying to average the costs of excavating a foot deep trench to a width of 5 feet and include the necessary sloping or appropriate shoring required to prevent cave-ins, it is difficult to provide a realistic average cost that considers the types of soils or rock encountered.

This is because the cost of excavating is determined by the amount to be excavated and what is to be excavated. There is no average subsurface or soil type in the U. The unknowns lead to variable excavating costs that are unrealistic to a U. This calls for local costs to be developed and examined. Other changes already are taking place to consider underground power delivery more seriously.

Engineers and planners are developing lists of costly obstacles to overcome while customers continue to demand underground power delivery. As storms leave behind damages that cost billions of dollars, everyone will focus more intently on the justification for undergrounding.

This change is not revolutionary but reality. The placement of power lines underground typically is driven by the lack of available right of way or aesthetics. Placing lines underground in heavily populated, urban areas is a decision readily justified to regulatory agencies. This concept has been employed by several Florida utilities that needed to construct transmission lines through established, residential communities.

In at least two cases, reasonable agreements were reached by the utility and government agency for sharing the differential costs of placing the transmission lines underground.

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Analysis techniques of diagnosis high voltage underground cables

Once bulk 3 phase AC electricity has been generated it has to be transmitted to the populated areas where it is required and then distributed to the individual users. Almost everyone will be familiar with the sight of a line of overhead electric conductors stretching far off into the distance, supported from tall steel lattice towers or even small wooden poles but does everyone appreciate why overhead conductors are used instead of the buried cables which are invariably used within cities and towns? Electric power, be it generated in traditional power stations or by sources of renewable energy generated by solar or windfarms has got to be transported to the end customer or user, as economically, efficiently and safely as possible with of course, the utmost reliability. In the UK there are basically two means of achieving this, by bare overhead conductors or insulated cables buried underground. Bare overhead conductors are the most popular when lots of electrical energy needs to be carried at high voltage, over long distances, across the open countryside or open spaces.

Sheath Voltage Limiters Protect HV Power Cables

GE's transmission protection solutions deliver the speed and security necessary for advanced transmission line protection. Our specialized technology teams are well versed in transmission protection theory and build protection and control solutions that can be configured to meet a variety of applications from large to small, underground and overhead. Restoring service to urban areas quickly requires fast and accurate fault location. Maintenance crews rely on the advanced functions of the Multilin L90 line current differential system to rapidly pinpoint the fault location. Learn more about this spotlight application. Multilin L90 The L90, a member of the UR family of protection relays, is a transmission-class protection relay providing comprehensive line differential protection for 2 or 3 line ends, subcycle distance and directional earth fault protection. They include full scheme 5-zone distance protection with quad characteristics, plus directional earth fault DEF for single breaker applications. MiCOM P44T Agile The P44T provides protection for railway catenary lines, supplementing a 5 zone distance main protection are a host of backup protection, thermal protection, recording, control, measurement and monitoring features. Wrong phase coupling, panto flash over, train startup and Delta I for high impedance faults protection are also included. With independently settable right and left hand side resistive reaches for each zone the P44T suits routes hosting regenerative braking trains.

Ground loop (electricity)

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Answers to Frequently Asked Questions about Underground Distribution Lines

Current passing through an impedance impresses voltage across that impedance. Even conductors have some, albeit low, value of impedance. Therefore, if a "grounded" 1 object, such as a crane or deenergized and grounded power line, results in a ground fault on a power line, voltage is impressed on that grounded object. The voltage impressed on the grounded object depends largely on the voltage on the line, on the impedance of the faulted conductor, and on the impedance to "true," or "absolute," ground represented by the object. If the impedance of the object causing the fault is relatively large, the voltage impressed on the object is essentially the phase-to-ground system voltage. However, even faults to grounded power lines or to well grounded transmission towers or substation structures which have relatively low values of impedance to ground can result in hazardous voltages.

Comparison of Overhead and Underground Cables

Unit Descriptor. This Competency Standard Unit covers the conducting of high potential testing of underground power cables. It also encompasses the interpreting test results, documenting the actual testing and, recommendations to meet client requirements. Application of the Unit. This Competency Standard Unit is intended to augment formally acquired competencies. It is suitable for employment-based programs under an approved contract of training.

Pilot wire current differential feeder protection

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