SUBSTATIONS

A.Nagaraja Sekhar

Functions of a Substation1 - Supply of required electrical power. 2 Maximum possible coverage of the supply network2 - Maximum possible coverage of the supply network. 3 - Maximum security of supply. 4 - Shortest possible fault-duration. 5 - Optimum efficiency of plants and the network. 6 - Supply of electrical power within targeted frequency limits, (49.5

Hz and 50.5 Hz).)7 - Supply of electrical power within specified voltage limits. 8 - Supply of electrical energy to the consumers at the lowest cost.

Classification Of substations:The substations may be classifiin numerous ways such as on the basis of y

(i) Nature of duties(ii) Service rendered(ii) Service rendered(iii) Operating voltage(iv) Importance and ( ) i(v) Design

The substations, on the basis of nature-of-duties, may be classified i h f ll i h iinto the following three categories:

Step-up or primary substations.p p p y

Such substations are usually associated with generating stations.

The generated voltage, which is usually low (3.3, 6.6, 11 or 33 kV), is stepped up to primary transmission voltage so that huge blocks of

b t itt d l di t t th l d tpower can be transmitted over long distances to the load centres economically.

Primary grid substations.

Such substations are located at suitable load centres along the primary transmission lines.

In these substations, the primary transmission voltage is stepped down to different suitable secondary voltages.

The secondary transmission lines are carried over to the secondary substations situated at the load centres where the voltage is further stepped down to sub-transmission or primary distribution voltages.

1. Step-down or distribution substations.

Such substations are located at the load centres, where the sub-transmission/primary distribution voltage is stepped down to

d di t ib ti lt (400/230V) th thsecondary distribution voltage (400/230V). these are the substations which feed the consumers through distribution network and service lines.

2 Classification of Substations on the basis of service rendered The2 Classification of Substations on the basis of service rendered. The substations, according to service rendered are:

1 T f b t ti T f i t ll d h1. Transformer substations. Transformers are installed on such substations to transform the power from one voltage level to another level as per needs.

2. Switching substations. Such substations are meant for switching operation of power lines without transforming the voltage. Atoperation of power lines without transforming the voltage. At each substation different connections are made between various transmission lines.

3. Converting substations. Such substations are meant for either converting ac to dc or vice-versa or converting frequency from higher to lower or vice-versa.

Classification of Substations on the basis of operating voltage:p g g

The substations, according to operating voltage, may be categorized asas

1. High voltage substations (HV Substations): involving voltages between 11 KV and 66 kV

2. Extra High Voltage Substations (EHV Substations). Involving voltages between 132 kV and 400 kV.g

3. Ultra High Voltage Substations (UHV Substations). Operating on Voltage above 400 kVVoltage above 400 kV.

Classification of Substations on the basis of importance.

1. Grid Substations. These are the substations from where bulk power is transmitted from one point to another point in the grid. p p p gThese are important because any disturbance in these substations may cause the failure of the grid.

2. Town Substations. These substations step-down the voltages at 33/11 kV for further distribution in the towns and any failure in

h b t ti lt i th f il f l f h l f thsuch substations results in the failure of supply for whole of the town.

Classification of Substations on the basis of Design.

1 Indoor Type Substations1. Indoor Type Substations.In such Substations the apparatus is installed within the substation building. Such substations are usually for a voltage upto 11kV but can be erected for the 33 kV and 66 kV when the surrounding atmosphere is contaminated with impurities such as metal corroding gases and fumes, conductive dust etc.g g ,

2. Outdoor Substations. These substations are further subdivided into:

(a) Pole mounted substations Such substations are erected for(a) Pole mounted substations. Such substations are erected for distribution of power in localities. Single stout pole or H-pole and 4 pole structures with suitable platforms are employed for

f f i 25 kVA 125 kVA d b 125transformers of capacity up to 25 kVA, 125 kVA and above 125 kVA (but upto 250 kVA) respectively.

(b) Foundation mounted substations: for transformers of capacity ( ) p yabove 250 kVA the transformers are too heavy for pole mounting. Such substations are usually for voltages of 33,000 volts and above

The outdoor substations have the following main advantages over g gindoor substations(i) All the equipment is within view and therefore the fault locatis easieris easier.(ii) The extension of the installation is easier, if required(iii) The time required in erection of such substations is lesser.i i i i i(iv) The smaller amount of building materials (steel-concrete) is

required.(v) The construction work required is comparatively smaller a( ) q p ycost of the switchgear installation is low.(vi) There is practically no danger of a fault which appears at opoint being carried over to another point in the installation becapoint being carried over to another point in the installation becathe apparatus of the adjoining connectios can be spaced liberawithout any appreciable increase in costs.( ii) R i i k i(vii) Repairing work is easy.

The disadvantages of outdoor installations in comparison of indoor installations are:(i) The various switching operations with isolators as well as supervision and maintenance of the apparatus is to be performed insupervision and maintenance of the apparatus is to be performed in the open air during all kinds of weather.(ii) More space is required for the substation.(iii) Protection devices are required to be installed for protection(iii) Protection devices are required to be installed for protection against lighting surges.(iv) The length of control cables required is more.(v) The influence of rapid fluctuation in ambient temperature and dust and dirt deposits upon the outdoor substation equipment makes it necessary to install apparatus specially designed for outdoorit necessary to install apparatus specially designed for outdoor service and, therefore, more costly.Notwithstanding the disadvantages, outdoor substations are very widely used in power systemswidely used in power systems.

SELECTION AND LOCATION OF SITE FOR A SUBSTATIONSELECTION AND LOCATION OF SITE FOR A SUBSTATION

T The following factors are considered while making site selection for a substationfor a substation1. Type of substation. The category of substation is important for its location.

For example a step-up substation, which is generally a point where power from various sources (generating machines or generating stations) is pooled and stepped up for long distance g g ) p pp p gtransmission, should be located close to the generating stations as possible to minimize the transmission losses, cost of distribution system and better reliability of supplysystem and better reliability of supply.2. Availability of suitable and sufficient land.

The land proposed for s substation should be normally level and open from all sides. It should not be water logged particularly in rainy season. The site selected for s substation should be such that

According to the latest practice the land required for various types of substations are given below.

Type of substation Area required

(a) 400 kV substation 50 acres

(b) 220 kV substation 25 acres

(c) 132 kV Substation 10 acres( )

The places nearer to the aerodrome, shooting practice grounds etc., should be avoided.

3. Communication facility. Suitable communication facility is desirable at a proposed substation both during and after its p p g

construction. It is better, therefore, to select the site along side on existing road to facilitate an easier and cheaper transportation.

4. Availability of essential amenities to the staff. The site should be such where staff can be provided essential amenities

like school, hospital, drinking water, housing etc.

5. Drainage facility. The site selected for the proposed b t ti h ld h d i tsubstation should have proper drainage arrangement or

possibility of making effective drainage arrangement to avoid pollution of air and growth of micro-organisms detrimental to

equipment and health.

Power transformers are connected between two voltage levels.The main connection scheme is drawn keeping in view the following factors:(i) General bus-bar arrangement(ii) Operating voltage(iii) N b f i i d i li(iii) Number of incoming and outgoing lines(iv) Number of transformers(v) Safety of equipment(vi) Safety to operating personnel(vi) Safety to operating personnel (vii) Future extension requirement.The main connection diagram drawn for substation shows the arrangement of all the circuits with the main bus-bars.all the circuits with the main bus bars.For simplicity and to facilitate reading, all the electrical connections of a substation can be represented by a single line diagram.

A single line diagram will indicate all the main elements of any given installation such as generators bus bars power transformersinstallation such as generators, bus-bars, power transformers, circuit breakers, isolators, series and shunt capacitors, fuses, CTs and PTs, diode or thyristor rectifiers, static VAR sources, harmonic filters, surge arresters etc.

Connections may be divided as incoming (power feed connections) tie (lines interconnecting two substations or switchgear installations, each of which is fed through its own incoming feeder connections),each of which is fed through its own incoming feeder connections), outgoing (feeder connections for feeding other subsequent substations or switchgear installations), power transformers ( ti d i i b t ti ) lt t f(connections made in given substation), voltage transformers (connected for control and metering).

The components in series with the main circuit of power flow are:Bus-bars, power transformers, circuit breakers, isolators, fuses, CTs, series capacitors, series reactors, diode or thyristor rectifiers etc.series capacitors, series reactors, diode or thyristor rectifiers etc.

The components in the shunt circuits connected between phase and dground are:

Shunt capacitors, shunt reactors, static VAR sources, harmonic filters, pTs, lightning or surge arresters etc.

Special type of apparatus such as isolators (or disconnectingSpecial type of apparatus such as isolators (or disconnecting switches), circuit breakers, instrument transformers etc. are used for interconnecting high voltage power lines (overhead or cable), with the main bus bars in the substationsthe main bus-bars in the substations.

Circuit breaker is connected between the bus-bar and each incoming d i i iand outgoing circuit.

Isolator is provided on each side of the circuit breaker.CTs are provided for measurement and protection.p p

PTs are generally connected to bus-bars and on incoming line sidePTs are generally connected to bus-bars and on incoming line side.

Lightning or surge arrester are connected phase to ground at the i i li th fi t t d l t th t i l fincoming line as the first apparatus and also at the terminal of transformer, terminal of the capacitor bank, terminal of shunt reactor, terminal of generator, terminal of large motor to divert g gswitching/lightning surges to ground.

Main Electrical ConnectionsThe electrical power in substations and switchgear installations is received and distributed by means of the main bus bars to which thereceived and distributed by means of the main bus-bars to which the equipment is connected according to some given main circuit scheme.It is necessary to distinguish two fundamental types of power station and substation circuit arrangements.(1) The main connections of primary (or power) circuits.( ) p y ( p )The secondary (or control) circuit arrangements.

Primary circuit main connections are represented by either single line or three line schematic or elementary diagrams.

The first type of diagram shows all three-phases of each circuit as aThe first type of diagram shows all three phases of each circuit as a single line while the second type of diagram shows each phase of every circuit as a separate line, i.e., shows each three-phase circuit as three linesthree lines.

The single line diagram represents the electrical circuit arrangement of any electrical power installation in its basic form.