distribution substations

Distribution substations serve a wide range of private and public customers in distributing electric power. They can be shareholder, cooperatively, privately, and government owned. All substations contain power transformers and the voltage-regulating apparatus required for converting the high incoming subtransmission voltages to lower primary system voltages and maintaining them within specified voltage tolerances. Those voltages, typically 11 to 15 kV, are then sent to distribution transformers and load substations for serving regional and local customers.

Substations serve many purposes, including connecting generators, transmission or distribution lines, and loads to each other and generally stepping higher voltages down to lower voltages to meet specific customer requirements. They can also interconnect and switch alternative sources of power and control system voltage and power flow. Power factor can be corrected and overvoltage can be regulated by substations. In addition, instruments in substations measure power, detect faults, and monitor and record system operational information.

The basic equipment in substations includes transformers, circuit breakers, disconnect switches, bus bars, shunt reactors, power factor correction capacitors, lightning arresters, instrumentation, control devices, and other protective apparatus related to the specific functions in the power station.

Circuit breakers and other switching equipment in a substation can be organized to separate a bus, part of a transformer, or a control device from other equipment. The common system switching arrangements are shown in the one-line diagrams in below Figure . In these diagrams connections are indicated by arrowheads, switches by offset lines, and circuit breakers by boxes.


The single-bus switching system in above Figure (a) is bus protected by the circuit breakers on the incoming and outgoing lines. The double-bus system in above Figure (b) has two main buses, but only one is normally in operation; the other is a reserve bus. The ring bus above Figure (e) has the bus arranged in a loop with breakers placed so that the opening of one breaker does not interrupt the power through the substation.

A typical distribution system consists of :

  • Subtransmission circuits, which carry voltages ranging from 12.47 to 245 kV (of these, 69, 115, and 138 kV are most common) for delivering electrical energy to the various distribution substations.
  • Three-phase primary circuits or feeders, which typically operate in the range of 4.16 to 34.5 kV (11 to 15 kV being most common) for supplying the load in designated areas.
  • Distribution transformers rated from 10 to 2500 kVA, installed on poles, on aboveground pads, or in underground vaults near customers. These transformers convert primary voltage to useful voltages for practical applications.
  • Secondary circuits at useful voltage levels, which carry the energy from the distribution transformers along highways, streets, or rights-of-way. These can be either single- or three-phase lines.
  • Service dropsand service laterals,which deliver energy from the secondary circuits to the user’s service entrance equipment.

Power is switched from the substation transformers as shown in below Figure. to separate distribution buses. In some systems the buses distribute power to two separate sets of distribution lines at two different voltages. Smaller transformers connected to the bus step the power down to a standard single-phase line voltage of about 7.2 kV for residential and rural loads, while power from larger transformers can leave in another direction at the higher three-phase voltages to serve large industrial and commercial loads.


  • Substation transformers have laminated steel cores and are built with isolated primary and secondary windings to permit the transfer of power from the primary side to the secondary side at different voltages. These transformers typically range in size from small units rated for 1 MVA to large units rated for 2000 MVA.

Most of these transformers are insulated and cooled with oil, making them vulnerable to fire. Adequate precautions must be taken to minimize the possibility of fire and to extinguish any fires that occur as rapidly as possible. In addition to the installation of fire extinguishers, they are located at safe distances from other equipment and positioned in pits to contain any oil leakage. Additionally, fire walls might be built between them.

  • Substation circuit breakers capable of interrupting the highest fault currents are installed in substations. They are typically rated for 20 to 50 times the normal current and are built to withstand high voltage surges that occur after interruption. Switches rated only for normal load interruption are called load-break switches.
  • Disconnect switches have isolation and connection capability but lack current interruption capability.
  • Bus bars make connections between substation equipment. Flexible conductor buses connect insulators, but rigid buses, typically hollow aluminum alloy tubes, are installed on insulators in air or in gas-enclosed cylindrical pipes.
  • Shunt reactors compensate for line capacitance in long lines, and shunt capacitors compensate for the inductive components of the load current.
  • Current and potential transformers are used to measure currents and voltages, and they provide low-level currents and voltages at ground potential for control and protection.
  • Control and protective devices include protective relays that can detect faults rapidly in substation equipment and lines, identify their locations, and provide appropriate signals for opening circuit breakers. They also include equipment for controlling voltage and current and selecting optimum system configurations for the load conditions. Included in this category are fault-logging and metering instruments, internal and external communications equipment, and auxiliary power supplies.
  • Solid-state digital instruments containing microprocessors have replaced many of the earlier-generation analog moving-coil instruments. Most substations are fully automated yet have provision for manual override. Essential status information is transmitted via communications channels to the central office dispatcher and can be displayed on video terminals.


Power can leave a typical substation in sets of three wires, each headed down the distribution network in a different direction. Three wires at the top of the poles are required for three-phase power, and a fourth or neutral/ground wire is usually positioned lower down on the utility pole.

Homes and small businesses (offices and stores) usually need only one of the three phases, so those requirements are met by tapping single-phase power from the three-phase transmission lines for distribution on individual conductors, at about 7.2 kV. The second wire, positioned lower on the utility pole, is the neutral/ground wire. In some locations two single-phase conductors and a neutral/ground wire are carried on the same pole. One of those phases serves nearby homes and offices, while the second phase continues on as an individual conductor to serve more distant loads.

In most newer residential subdivisions the single-phase power line is brought down from a pole near the entrance to the subdivision to pad-mounted transformers for underground distribution to homes. However, underground service has been provided for cities for many years in an effort to eliminate the jumble of poles and wires.

Voltage regulators are located along the routes of both overhead and underground power lines to regulate the voltage on the line, preventing undervoltage or overvoltage conditions. These regulators contain switches that allow them to be disconnected for maintenance. Regulator voltage is also typically about 7.2 kV.

Substation voltage is controlled with tap changers on the distribution substation transformers, but some require separate voltage-regulating transformers, individual feeder-voltage regulators, or induction voltage regulators. Most distribution substations perform metering, relaying, and power control automatically. The main units of equipment to be controlled are the feeder circuit breakers if the substation includes them. Metering is required to provide consumption data for billing customers if the power provider does not own the distribution system.

The American National Standards Institute (ANSI) has defined the voltage range for single-phase residential users as 114/228 V to 126/252 V at the user’s service entrance and 110/220 V to 126/252 V where it is being used. The difference in these values recognizes that there will be a voltage drop in the consumer’s system. Nominal voltage in the United States and Canada is 120/240 V, 60 Hz.

Dips in voltage large enough to cause incandescent lamps to flicker are expected to be limited to 4 to 6 percent if they occur infrequently and 3 to 4 percent if they occur several times an hour. Frequent dips caused by the start-up of large electrical machines such as motors or elevators should be limited to 1.5 or 2 percen


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