Underground distribution substations are also located near to the end-users. Distribution substation transformers change the subtransmission voltage to lower levels for use by end-users. Typical distribution voltages vary from 19,920 volts to 2400 volts.

Underground Distribution Substation

An underground system may consist of these parts:

• Conduits
• Duct Runs
• Manholes
• High-Voltage Underground Cables
• Transformer Vault
• Riser
• Transformers

Conduits:

Duct

Duct 2

Conduits are hollow tubes running from manhole to manhole in an underground transmission or distribution system. They can contain one or more ducts (See Duct Runs). They can be made of plastic (PVC), fibreglass, fibre, tile, concrete, or steel. PVC and fibreglass are most commonly used.

Duct runs:

Duct runs are hollow tubes running from manhole to manhole inside a conduit in an underground system. They are of various sizes usually from 2 to 6 inches in diameter. Electrical cables are run through ducts and the ducts are sized accordingly. The diameter of a duct should be at least 1/2 to 3/4 inch greater than the diameter of the cable(s) installed in the duct. They can be made of plastic (PVC), fibreglass, fibre, tile, concrete, or steel. PVC and fibreglass are most commonly used.

Manhole:

A manhole is the opening in the underground duct system which houses cables splices and which cable men enter to pull in cable and to make splices and tests. Also called a splicing chamber or cable vault.

High-Voltage Underground Cables:

High-Voltage underground cables are constructed in many different ways, but are usually shielded cables. They are made with a conductor, conductor-strand shielding, insulation, semi-conducting insulation shielding, metallic insulation shielding, and a sheath. The sheath can be metallic and may then serve as the metallic insulation shielding and be covered with a nonmetallic jacket to protect the sheath. This sheath helps to reduce or eliminate  inductive reactance. Such cables are commonly used in circuits operating at 2400 volts or higher.

High Voltage Cables

High Voltage Cables 2

Transformer Vault:

Underground Transformer Vault

A  transformer vault is a structure or room in which power transformers, network protectors, voltage regulators, circuit breakers, meters, etc. are housed.

Riser:

A riser is a set of devices that connects an overhead line to an underground line. A riser has a conduit from the ground up the pole where potheads are used to connect to the overhead lines.

Riser 1

Riser 2

Underground Transformer:

An underground transformer is essentially the same as an aboveground transformer, but is constructed for the particular needs of underground installation. Vault type, pad-mounted, submersible, and direct-buried transformers are used in underground systems. Pad-mounted transformers are installed on a concrete pad on the surface near the end-user.

From here the power is distributed to industrial, commercial, and residential customers.