This workforce solution is funded by the IHUM Consortium which is 100% financed through a $15, 000 grant from the U. S. Department of Labor’s Employment & Training Administration. The product was created by the grantee and does not necessarily reflect the official position of the U. S. Department of Labor. The Department of Labor makes no guarantees, warranties, or assuran This work is licensed under the Creative Commons Attribution 4. 0 International License. To view a copy of this license, visit http: //creativecommons. org/licenses/by/4. 0/. Underground Residential Distribution Systems ELE 362
Underground Residential Distribution Systems Many residential subdivisions require all utilities be installed underground. The electricity is provide via underground residential distribution (URD) systems The source for URD systems typically comes from overhead primary distribution lines The URD system is connected to the overhead system at a “riser” pole (aka – “dip” pole)
Three phase “riser” poles; it’s likely these are feeders exiting a substation located nearby
Types of URD Systems and Components There are three types, or designs, of URD Systems: • Radial Feed • Loop Feed • Double Feed URD systems consist of a variety of components such as: • Underground cable (primary and secondary) • Pad mounted transformers • Pad mounted switchgear • Protective devices
Radial Feed System A radial feed URD system means one end of the system is connected to a distribution primary source while the other end is connected to a component but does not return to the source One challenge with a radial feed is when a cable faults (goes bad) there is no way to provide electricity to customers located down stream of the fault In cases such as this customers located downstream of the fault will remain without power until repairs have been made
This diagram shows a radial feed URD system. The source is noted at the cable pole with a fused switch. The triangles indicate transformers serving customers. 3 2 This diagram also shows a fault (noted with an “X”) between transformers #2 and #3 that will impact two transformers. The cable coming from transformer #2 going to transformer #3 can be isolated and power restored to transformers #1 and #2. 1 4
Loop Feed System A loop feed URD system has both ends of the system connected to the primary. Loop feed URD systems can be configured in two manners. One configuration would have the entire loop system connected together. The other option would be to have a “normal open” point; this configuration is known as a “open loop system” With an open loop system the open point can be located in a pad mount transformer, switch cabinet or on the riser pole The advantage of an open loop system is in the event an underground cable or piece of equipment fails, the URD system can be reconfigured by switching and likely restore power to most if not all the customers
This diagram is of a single phase open loop system. The advantage of this configuration is if a cable fails, by conducting switching power can be restored to all customers. Note the open point
Double Feed System Some customers, such as hospitals, require a reliable source of power and an alternative is available; a double feed design In a double feed design the URD system is connected to two systems or sources. Each system is energized from a different source If one source fails service can be switched to the other source. This can occur either automatically or manually
This diagram shows a double feed configuration (Circuits A and B). The customer is receiving power from Circuit A; in the event something were to occur on the system the customer’s switch could be operated and they would then be receiving power from Circuit B. Circuit A Circuit B
URD System Components The components found in a URD system can vary depending on the voltage, type of installation and whether the power source is overhead or underground URD systems can typically be divided into three sections: • Primary Connection(s) • Transformer(s) • Secondary Connection(s)
URD - Overhead Source When a URD system is fed from an overhead primary source, a connection is made at a riser pole The device that actually makes the connection is a termination; also referred to as a pothead One end of the termination is connected to the overhead primary and the other end is connected to the URD primary cable
When a URD system is fed from an underground source the connection is typically made using a pad mounted switch cabinet There are two categories of switch cabinets and pad mounted transformers; they are referred to as dead front or live front Dead front means there are no exposed primary connections when the cabinet is opened Live front means there are exposed primary connections when the cabinet is opened The cabinet show in this picture is a dead front unit
The cabinet shown in this picture has exposed primary connections once the doors are opened This would be a live front cabinet
This is a picture of a typical single phase pad mount transformer This would be a dead front transformer
URD Cables and Terminations Because URD cable is installed underground it must be resistant to moisture, corrosion and other problems common to underground installations Many different sizes and types of cable are available and can be used depending on system requirements URD cable sizes are specified in circular mils or wire gauge URD cable can be classified as primary or secondary
This is a picture of URD primary cable and a single stranded conductor in the center. The conductor is surrounded by a layer of semiconducting material. Outside of the semiconductor is a layer of polyethylene or synthetic rubber insulation followed by another layer of semiconductor The outermost layer on most URD cable is a concentric neutral which helps protect the cable and provides a grounded neutral for the length of the cable
This is a another picture of URD primary cable with a single stranded conductor in the center. The only difference with this cable and the one shown on the previous slide is the outermost layer on this cable has a protective jacket. The cable in the previous slide has the concentric neutral exposed as the outermost layer.
For URD systems requiring three phase power it’s necessary to install three separate cables (one for each phase). However, some companies use a primary cable that has three separate phases encased inside a single jacket.
Secondary URD cable generally has two, three or four conductors depending on system or customer requirements. The cable shown below has three conductors. The conductor with the yellow stripe would be used as the neutral. This type of cable would be used for single phase installations such as home. The cable show below has four conductors. The conductor with the yellow stripe would be used as the neutral. This type of cable would be used for three phase installations.
URD cable terminations are used to connect cables to other pieces of equipment and have mechanical or compression connections. The URD cable termination shown below is typically used to transition from an overhead line to an underground system. These terminations are often located at “riser” poles. The URD cable termination shown below is called a load break elbow. These terminations are used to connect a URD cable to a transformer or switch cabinet. Note: the white bands on the elbow indicate it has load break capability.
Shown below is a single phase, pad mount transformer. The secondary voltage is likely 120/240…. . but…. . always read the nameplate. Nameplate Shown below is a three phase switch cabinet. Note the two elbows located on the left side. What are they? ?
Below are pictures of three phase pad mount transformers. Which unit is dead front and which unit is live front? Figure 1 Figure 2
URD Equipment Installation Most URD cable, both primary and secondary, is designed to be buried directly in the ground. However, there are times when the ground conditions or job specifications require the cable to be installed in conduit. This can occur when there is a lot or rocks or debris in the trench that could damage the cable. Also, when burying cable underneath a parking lot, street, highway or railroad it’s preferable to put it in conduit. Installing a cable in conduit is also helpful in the event there’s a cable fault or a need to upgrade the cable. Rather than digging another trench the old cable can be removed from the conduit and a new one installed.
In some instances installing conduit can be rather complex while in other cases it’s pretty simple.
Equipment Operation When a problem such as a faulted cable or defective transformer is discovered in a URD system it may be necessary to de-energize part of the system in order to make repairs. De-energizing part of a URD system could involve interrupting service to customers. Therefore, URD systems are often designed and operated so work can be done while minimizing the impact to customers. The following example is of an open looped URD system and the procedure used to isolate a faulted portion of cable.
Note the open point