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4.2 DistributionInfrastructure and Assets
4.2 DISTRIBUTION INFRASTRUCTURE AND ASSETS
Electricity distribution infrastructure takes many forms—from whole buildings to poles and wires. In addition, there are other assets such as vehicles that play an important role within distribution infrastructure operations.
What is Distribution Infrastructure?
Distribution infrastructure includes substations, transmission lines, poles, transformers, vaults and switchgear. Each element of infrastructure has an important role to play in ensuring that electricity makes its way to the end consumer as efficiently and safely as possible.
Substations
Substations serve as vital nodes between electricity generation, transmission, and distribution systems.
Substations are where voltage is transformed either from low-to-high, or high-to-low among other important functions. They commonly serve as an offramp from the electricity superhighway. Electricity may pass through several substations at different voltage levels as it flows from the generator to the end user.
Substations can be found at various inter-connections.
Generation Stations
Substations can be found at generation stations, where electricity may need to be “stepped up” to a level at which it can be moved across transmission lines.
Transmission or Distribution Grids
Substations can be found within either a transmission or distribution grid, where a change in voltage levels is needed.
Between Grids
Substations can be found between the transmission grid and the distribution grid, where electricity needs to be “stepped down” to a level at which it can be moved across distribution lines and used.
Between the Grid and the Consumer
Substations can be found between the grid and a large industrial or commercial electricity customer.
Substation Operation
How a Substation Operates
Substations may be owned and operated by an electricity utility or by large industrial or commercial customers. They are generally unattended and remotely operated. The basic operation of a typical substation is as follows.
1. From Generation to the Substation: The substation is connected to the high-voltage transmission system. 2. Stepping Down the Voltage: Electricity travels through a station power transformer, which reduces the transmission voltage to a lower distribution-level voltage. 3. The Main Circuit Breaker: The distribution-level voltage electricity is fed through station distribution switchgear, which is analagous to the electric switch box found in a home. 4. Distribution Switchgear: From the distribution switchgear, multiple distribution circuits, or feeders, exit the station to supply the overhead and underground distribution system which feeds homes and businesses. 5. Distribution Feeders: Each circuit breaker feeds electricity into individual distribution feeders, which carry it to end users via the distribution grid of wires and cables. An individual feeder may connect directly to a combination of industrial, commercial, and institutional customers, and through additional transformers, it will also connect to individual residential customers.
Summary
Substations regulate voltage and efficiently move electricity down various pathways, each appropriate to the needs of the end users connected to it. They also provide protective gateways, allowing the flow of electricity to be stopped along a particular pathway if necessary.
Substations are also primary hubs for the monitoring and control of the distribution system. Most substations have a programmable remote terminal unit (RTU), which is the point of connection back to the utility control room, enabling both data acquisition and control measures.
Lines and Cables
Some of the most visible infrastructure owned and maintained by an electricity distribution utility consists of the power lines that serve the local community. In high-density areas, such as an urban downtown and many residential areas, much of this infrastructure is buried underground.
Distribution lines are made of copper or aluminum and are supported on insulators mounted on poles (in the case of overhead lines). Distribution insulators provide separation between the energized distribution lines and the structures that carry them.
Buried Cables and Overhead Lines
The term “lines” is typically used to describe this infrastructure when it is overhead, and the term “cables” is used to describe it when it is underground. Cables are typically installed in concrete-encased “duct banks,” which are accessed via maintenance holes. There are benefits and drawbacks to each approach.
Buried
Pros: Eliminates tree contact and other forms of interference, reduces the risk of fire, and improves reliability (fewer outages due to vehicle accidents, animal contact, and weather). Relatively easy to build during the initial construction of a new development or subdivision.
Cons: Installing or replacing underground lines after initial road or subdivision construction can be 5 to 10 times more expensive than above-ground wires. They are also more difficult to maintain, as most of the infrastructure is not readily visible or accessible.
Overhead
Pros: Less expensive, more visible, and easier to maintain.
Cons: More subject to tree, animal, vegetation, weather, and other interference.
Maintenance and Upkeep
A core responsibility for electricity utilities is to make sure the grid remains intact, including periodically replacing poles, wires, and other critical components as they age. Utilities must constantly monitor and remove any potential sources of interference, through activities such as tree trimming and vegetation control.
Poles
Overhead wires are fixed to poles. In Canada, poles are typically constructed from the wood of tall trees. However, concrete, and composite materials are becoming more widely used under certain circumstances.
Utility Poles
The standard utility pole ranges 12 to 20 metres in height and typically lasts about 50 years before needing to be replaced. Poles are usually buried more than two metres in depth, but the higher the pole, the deeper it must be buried.
Composite poles
Composite poles are made from a fibreglass-like material and are more expensive than their wood counterparts. They are easier to handle, especially in larger sizes and tight locations, but are more difficult to climb. Composite poles are expected to last longer and are sometimes used in difficult-to-serve regions, or where woodpeckers have damaged wooden poles.
Spacing
Poles are typically set anywhere from 25 to 150 metres apart and provide the structure upon which insulators and distribution lines are mounted.
Which Lines Are Electricity Lines?
Utility poles have multiple uses. Besides carrying primary distribution lines, utility poles also carry lines used for telecommunications, including phone and cable TV lines. These multi-use arrangements are handled through contracts among the companies involved, and some poles are, in fact, owned by telecommunications companies.
Typically, the primary distribution conductors or lines are in the top-third portion of the pole. The middle third is used for distribution transformers and secondary wires. The lower third is used for telecommunications lines—both for utility communications use and for other telecommunications service providers.
Vehicles
Vehicles are another important component of distribution utility operations.
Utilities deploy an extensive array of vehicles, each with a specialized purpose. Today’s utility vehicles are highly advanced to enable safe, efficient and precise work.
Bucket Trucks
Bucket trucks are an iconic utility vehicle and have greatly improved the capability of electricity workers to do their jobs.
Today’s bucket trucks provide far easier and safer options than working directly on a pole. Different bucket trucks are designed to reach different heights and manage different weights of materials. Their booms can typically reach 12 to 24 metres, with specialized trucks available with significantly longer reaches to service the transmission system.
Vehicle Fleet
There are many other kinds of vehicles deployed by modern electrical utilities.
• Pickup trucks are multi-purpose vehicles for transporting people and equipment on and off road.
• Step vans often include an enclosed area allowing for specialized work to be done inside the van.
Various sizes are deployed for substation work, cable work, and fault finding.
• Pole-handling backhoes are equipped with a pole claw for safe and efficient distribution pole installations.
• A backyard bucket is used in smaller locations such as urban backyards, at heights of up to 14 metres.
• Vacuum trucks are equipped with pressurized water jets to create holes and are then able to vacuum up the resulting sludge.
• Cable handling vehicles, such as trailers and trucks, are used for large cable pulling and installation.
• Specialized computer and communication vehicles are designed for managing the distribution system’s intricate connections to telecommunications and computing systems.
• Electrical utilities use a range of trailers, including pole-carrying trailers, cable trailers, overhead tension stringing trailers and others.
• Drones are becoming increasingly popular as a tool for job planning, as well as assessing outages or equipment malfunctions.
• Helicopters are mostly used in transmission work, both to inspect and access towers and lines.
As part of their commitment to environmental sustainability, electrical utilities continue to electrify many components of their vehicle fleets.
Transformers
Distribution transformers scale the voltage of the electricity to customer needs. Transformers are typically filled with an insulating fluid such as mineral oil, which provides protection and cooling for the internal electrical components. Two types of transformers are found within the distribution system: pad mounted transformers and pole mounted transformers.
Pad Mounted Transformers
A pad mounted transformer is ground mounted on a concrete base. Since all energized connection points are securely enclosed in a grounded metal housing, a pad mounted transformer can be installed in relatively close proximity to homes or businesses.
Pad mounted transformers are used with underground distribution lines, to step down the primary voltage on the line to the lower secondary voltage supplied to customers. A single transformer may serve one large building or many homes.
Pole Mounted Transformers
Pole mounted transformers are typically cylindrical and are usually placed below the level of the overhead primary conductors or wires. They vary in physical sizes and voltage levels and often weigh several hundred kilograms each. They can be used to connect residential or commercial customers and can supply multiple customers. These transformers add flexibility to the grid—making it easy to add or remove customers—and the transformers themselves can also be easily changed or moved.
The only people who should ever have direct contact with a transformer are utility employees. Through signage and public education, utilities work to dissuade people from climbing on or otherwise coming into direct contact with this equipment.
Vaults
Vaults are rooms or structures that allow utility workers easy access to equipment.
Customer Vaults
Customer vaults are most often located at or above ground level providing access to subterranean equipment, such as switchgear, transformers, cables, and service connections. Vaults can vary in size and are often placed within customer buildings to allow for higher reliability and safe, easy access. Vaults need to be maintained regularly to prevent equipment degradation and damage caused by moisture, dirt, and salt.
Maintenance Chambers
Maintenance chambers are where primary cables are placed and can be “pulled” to connect customers. They are typically four metres long, two metres wide and over two metres high, and are found two metres or more under street level. One or two workers will enter a maintenance chamber at a time while being monitored by colleagues above ground for safety.
Distribution Switches and Switchgear
Switchgear allows electricity to be controlled either manually or remotely. It can be used to reroute electricity—much like changing tracks on a train system—as required for safety or power-restoration purposes. Distribution switchgear is typically pad mounted in a large green box similar to a pad mounted transformer. Switchgear is sometimes located in a substation, a vault or is mounted on poles.
It is now common for leading utilities to install digital switches, paving the way for an automated and remote-controlled grid, and providing improved power reliability and restoration. Digital switches also help to manage increasingly multi-directional flows of electricity on the grid.
Knowledge Check
Substations serve as vital nodes within an electricity distribution system. Lines and cables are visible infrastructure, with lines being found overhead and cables buried underground. Utility poles are also used for telecommunications infrastructure, in addition to electricity distribution. Transformers scale the voltage of electricity to meet customers’ needs. Vaults allow utility workers easy access to equipment, and switchgear is used to reroute electricity as required.
