In this article we will discuss about:- 1. Meaning of Substations 2. Classification of Substations 3. Selection and Location of Site 4. Main Electrical Connections 5.Graphical Symbols for Various Types of Apparatus and Circuit Elements on Substation Main Connection Diagram.
Meaning of Substations:
Substations serve as sources of energy supply for the local areas of distribution in which these 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 distribution and provide facilities for switching.
Some substations are simply switching stations where different connections between various transmission lines are made, others are converting substations which either convert ac into dc or vice versa or convert frequency from higher to lower or vice versa. Substations have some additional functions. They provide points where safety devices may be installed to disconnect equipment or circuit in the event of fault. Voltage on the outgoing distribution feeders can be regulated at a substation.
A substation is convenient place for installing synchronous condensers at the end of the transmission line for the purpose of improving power factor and make measurements to check the operation of the various parts of the power system. Street lighting equipment as well as switching controls for street lights can be installed in a substation.
Classification of Substations:
ADVERTISEMENTS:
The substations may be classified in numerous ways such as on the basis of:
A. Nature of duties
B. Service rendered
C. Operating voltage
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D. Importance, and
E. Design.
A. Classification of Substations on the Basis of Nature of Duties:
The substations, on the basis of nature of duties, may be classified into the following three categories:
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1. Step-Up or Primary Substations:
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 power can be transmitted over long distances to the load centres economically.
2. 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.
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3. Step-Down or Distribution Substations:
Such substations are located at the load centres, where the sub- transmission/primary distribution voltage is stepped down to secondary distribution voltage (415/240 V). These are the substations which feed the consumers through distribution network and service lines.
B. Classification of Substations on the Basis of Service Rendered:
The substations, according to service rendered are:
ADVERTISEMENTS:
1. 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. At such substations 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.
C. Classification of Substations on the Basis of Operating Voltage:
The substations, according to operating voltage, may be categorised as:
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.
3. Ultra High Voltage Substations (UHV Substations) operating on voltage above 400 kV.
D. 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. These 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 such substations results in the failure of supply for whole of the town.
E. Classification of Substations on the Basis of Design:
1. Indoor Type Substations:
In such substations the apparatus is installed within the substation building. Such substations are usually for a voltage up to 11 kV 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.
2. Outdoor Substations:
These substations are further subdivided into:
(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 transformers of capacity up to 25 kVA, 125 kVA and above 125 kVA (but up to 250 kVA) respectively.
(b) Foundation Mounted Substations:
For transformers of capacity above 250 kVA the transformers are too heavy for pole mounting. Such substations are usually for voltages of 33,000 volts and above.
Selection and Location of Site for a Substation:
The following factors are considered while making site selection for a substation:
1. 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 transmission, should be located as close to the generating stations as possible to minimize the transmission losses.
Similarly a step-down substation should be located nearer to the load centre to reduce transmission losses, cost of distribution system and better reliability of supply.
2. Availability of Suitable and Sufficient Land:
The land proposed for a substation should be normally level and open from all sides. It should not be water logged particularly in rainy season. The site selected for a substation should be such that approach of transmission lines and their take off can be easily possible without any obstruction.
According to the latest practice the land required for various types of substations is given below:
The places nearer to 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 construction. It is better, therefore, to select the site along-side on existing road to facilitate an easier and cheaper transportation.
4. Atmospheric Pollution:
Atmosphere around factories, which may produce metal corroding gases, air fumes, conductive dust etc., and nearer to sea coasts, where air may be more humid and may be salt laden, is detrimental to the proper running of power system and therefore substations should not be located near factories or sea coast.
5. 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.
6. Drainage Facility:
The site selected for the proposed substation 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.
Main Electrical Connections:
The electric power in substations and switchgear installations is received 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, and
(2) The secondary (or control) circuit arrangements.
Primary Circuit Main Connections:
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 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 lines.
The single line diagram represents the electrical circuit arrangement of any electrical power installation in its basic form. It is the diagram that is used by the designer in selecting the electrical equipment for any given installation, in drawing up the protective relay, control and signalling-alarm system schemes and their elementary and wiring diagrams, and also in working up the constructional and mechanical design of the switchgear structures. In the everyday operation of electrical power installations single line diagrams, in the form of operative diagrams, play an important role in the performance of nearly all circuit switching operations.
A single line diagram will indicate all the main elements of any given installation such as generators, bus-bars, power transformers, circuit breakers, isolators, series and shunt capacitors, fuses, CTs and PTs, line trap units, diode or thyristor rectifiers, static VAR sources, harmonic filters, surge arresters etc.
The components in series with the main circuit of power flow are:
Bus-bars, power transformers, circuit breakers, isolators, fuses, CTs, line trap units, series capacitors, series reactors, diode or thyristor rectifiers etc.
The components in shunt circuits connected between phase and ground are:
Shunt capacitors; shunt reactors, static VAR sources, harmonic filters, PTs, lightning or surge arresters etc.
Special types 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 substations.
Connections may be divided as incoming, (power feeder connections) tie (lines interconnecting two substations or switchgear installations, each of which is fed through its own incoming feeder connection), outgoing (feeder connections for feeding other subsequent substations or switchgear installations), power transformers (connections made in a given substation), voltage transformers (connecting for control and metering).
The relative locations of circuit breakers, isolators and bus-bars follow the general practice and particular switching needs and maintenance and protection needs. Circuit breaker is connected between the bus-bar and each incoming and outgoing circuit. Isolator is provided on each side of the circuit breaker. CTs are provided for measurement and protection. CTs may be necessary on both sides of the circuit breaker so that protection zones overlap and cover the circuit breaker.
PTs are generally connected to bus-bars and on incoming line side. Lightning or surge arresters are connected phase to ground at the incoming line as the first apparatus and also at the terminal of transformer, terminal of capacitor bank, terminal of shunt reactor, terminal of generator, terminal of large motor to divert switching/lightning surges to ground. 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) Number of incoming and outgoing lines,
(iv) Number of transformers,
(v) Safety to equipment,
(vi) Safety to operating personnel, and
(vii) Future extension requirement.
The main connection diagram drawn for a substation shows the arrangements of all the circuits with its main bus-bars.
For simplicity and to facilitate reading, all the electrical connections of a substation can be represented by a single line diagram. It is understood that all the phases are connected identically.
Graphical Symbols for Various Types of Apparatus and Circuit Elements on Substation Main Connection Diagram:
The main elements of the installations; circuit breakers, isolators, fuses, instrument transformers, power transformers etc., are represented by standard graphical symbols, as given below, on the single line diagrams-
