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WEEK 13: DISTRIBUTION: SUBSTATION


Sections: Functions | Types | Switching Schemes | Components

Functions of Substations

 Functions Substations serve as sources of energy-supply for the local ares of distribution in which they are located. Their main functions are to receive energy transmitted at high voltage from the generating stations, reduce the voltage to a value appropriate for local use and provide facilities for switching. they also provide points where safety devices maybe installed to disconnect circuits or equipment in the event of trouble. Voltage on the outgoing distribution feeders can be regulated at a substation. It is also a convenient place to make measurements to check the operation of various parts of the system. Street lighting equipment as ell as on-and-off control for street lights can be installed in a substation.

Design Considerations

Factors Many factors influence the proper selection of the type of substation for a given application. Factors such as voltage levels, load capacity, environmental considerations, site space limitations, and transmission-line right-of-way requirements are imperative.

Sites for substations are generally selected so that the stations will be as near as possible to the load center of the distribution areas which they are intended to serve. Availability of land, cost, local zoning laws, future load growth and taxes are just a few of the many factors which must be considered before a site is ultimately chosen.

In addition, design criteria such as (1) Bus layout, (2) Choice of Insulation Level (3) High level of service continuity (4) Expansion (5) flexibility of operations, and (6) Low initial and ultimate cost. Selection of optimum voltage levels depend on the load requirements and transmission-line distances involved. The substations used in distribution systems operate at voltage class from 13.8 to 69kV. Transmission substations serving bulk power sources operate at voltages from 69 to 765kV. The choice of insulation levels and coordination practices affects cost considerably. A drop of one level in BIL can reduce the equipment cost by the thousand. A substation must be reliable, economical, safe and as simple in design as possible. Means should be provided for maintaining lines, breaker, and switches with no service interruptions or hazard to personnel. The physical orientation of the transmission-line routes often dictates the substations location and bus arrangement. For reliability, the substation design should prevent total substation shutdown caused by breaker failure or bus faults and should be such as to permit rapid restoration of service after a fault occurs. The layout must permit future additions and extensions without interrupting service.


Sections: Functions | Types | Switching Schemes | Components

Types of Substations

Classifications and Types Substations have step-down transformers, high voltage switches and oil circuit breakers and lightning arresters located just outside the substation building within which are located the distribution and street lighting facilities. Substations can be classified in a number of ways.

Classification of Substations:

Service

Alternating Current
AC to DC

Mounting

Indoor / Outdoor

Function

Tap
Distributing
Industrial
Sectionalizing
Transmission-Line
Power-Factor Correction
Frequency Changer
Portable/Unit

Type of Apparatus

Transformer
Rotary Converter
Motor Generator
Frequency Changer
Synchronous Condenser
Power Rectifier

Control

Manual
Semiautomatic
Automatic
Supervisory

 Indoor Substations are entirely enclosed in buildings. All rotating equipment, such as frequency changers, rotary converters, motor-generator sets, and synchronous condensers, together with all control equipment for such machines, must be placed indoors. Outdoor Substations are built entirely in the open. Unit Substations, also known as Portable Substations, have all their equipment assembled into a factory-built, metal-clad unit, usually enclosed by a fence, and supply only a few feeders. Tap Substations, employed for transmitting large amounts of power over long distances, where at advantageous points along the high-tension lines, stations step down the voltage from which numerous small loads may be supplied. Its equipment may comprise high- and low-tension switching equipment, transformers, and lightning arresters. Distribution Substation are used to transform electrical energy to a potential that is low enough for general distribution and utilization. In many cases, all high-tension equipment is placed outdoors, while low-tension equipment is placed indoors. Industrial Substation are used when fairly large blocks of power are required by industrial plants. Its simplest form would comprise only of switching equipment, there being no voltage transformation. If the latter is needed, transformer equipment is included. Sectionalizing Substations are used in very long high-voltage large-capacity lines, particularly when several circuits are run in parallel, thus it is necessary to split the lines into sections, in order that proper protection to the line and service be obtained. It will generally comprise only of switching equipment. In long lines, it may also serve to supply power-factor-correction. Transmission-Line Supply Substation steps up the generator voltage for high-voltage transmission lines. Power-Factor Correction Substations controls the voltage and the power factor of the line through the use of synchronous condensers. It will include switching equipment, transformers, and all equipment necessary for the operation of the condensers. Frequency Changer Substations used to interconnect two systems of different frequencies, such as 25 hz and 60 hz.

Control of Substations

Manual Control is applicable only to small-capacity substations. All the disconnecting switches, circuit breakers, and other equipment are manually controlled.

Semiautomatic Control Majority of stations fall under this class. Control is obtained from a switchboard through mechanically or electrically remote methods, but all control is dependent upon the operator in charge at the substation.

Automatic Control substations are entirely unattended, started up and shut down automatically.

Supervisory Control The operation of the substation is under direct control of an operator located at a considerable distance away.

 Distribution Substation Design: Bus Design


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