Electrical

Equipment or Instruments Used in Substations

Description:

Substations play a crucial role in the efficient and reliable distribution of electrical power. These complex installations require various types of equipment to control, protect, and manage the flow of electricity. The equipment used in substations ensures the safe transmission of power from high-voltage transmission lines to lower-voltage distribution lines, providing electricity to homes, businesses, and industries. Understanding the classification of equipment used in substations is essential for maintaining reliable power supply networks.




Equipment Used in Substations & Their Classification

Normally, the following equipment is used in substations;

  1. Bus border
  2. Isolators
  3. Insulators
  4. Circuit breakers
  5. Power transformers
  6. Instrument transformer
  7. Safety relays
  8. Fuses
  9. Load interrupter switch
  10. Surge arresters
  11. Reactors
  12. Capacitors
  13. Power line carrier current equipment
  14. Control cables
  15. Switchboards
  16. Control room
  17. Indicating and measurement instruments
  18. Neutral ground
  19. Line trip
  20. Earth switches
  21. Storage battery room
  22. Voltage regulator
  23. Galvanized steel structure

A brief description of these instruments used in a substation along with their types is as follows;

Bus Bars

The conductors which are used in a substation, are known as bus bars. All incoming and outgoing circuits are connected to the bus bars. In fact, a bus bar is a type of conductor, rod, or tube which is connected with the main supply and through which various branch circuits are also carried. The bar can either be flexible or rigid. A flexible bus bar is manufactured through ACSR (Aluminum Conductor Steel Reinforced) conductors and it is mounted supported through strain insulators. The rigid bus bars are constructed from tubes and they are supported through post insulators. The rigid bus bars are generally used in small substations.



Types of Bus Bars

According to connection, shape, and material, there are the following types of bus bars;

(a). Main Bus Bar

A bus bar that is being fed from an H.T line (i.e., which is connected to a high-tension line) is known as the main bus bar. In this bus bar, various measuring instruments and security relays, etc. are fitted   

(b). Distributor or Transfer Bus Bar

A bus bar through which an L.T line is connected is known as a distributor bus bar. Apart from different security and measurement instruments, selector switches, etc., are also mounted on this bus bar.

(ii). Types of Bus Bars with Respect to Shape

Generally, a rectangular-shaped bus bar is used in the substations, however, apart from it, round tube-shaped, round solid, and square-shaped tube conductors are also used as bus bars.

(iii). Types of Bus Bars with Respect to Material

The bus bars are generally made from copper or aluminum. In small substations, mostly hard-drawn solid copper conductor’s rigid bus bars are used, however, in high ratings or capacities substations, bus bars consisting of copper tubes are used. However, during the past decade, the use of aluminum as an electric conductor has increased significantly as compared to copper.  

As compared to the copper, aluminum bus bars have the following advantages;

  1. Aluminum bus bars are lightly weighted
  2. Their price for equivalent current carrying capacity is low i.e., they are cheap

iii. They have a high conductivity (the capacity to pass current is known as conductivity)

  1. They are rust-free

However, a silver coating is also done on the aluminum bus bar for enduring and lasting electrical connections.

The length of bus bars generally tends to be 5 or 6 meters; however, its size depends on the voltage drop occurring in the substation’s maximum current carrying capacity, power losses, temperature, and skin effect in a situation of AC (in the situation of AC, the flow of current through the surface or outer portion of the conductor rather than its center, is called skin effect). The normal sizes of bus bars are as follows;

40 x 4 mm (160 mm2); 40 x 5 mm (200 mm2);

50 x 5 mm (250 mm2); 50 x 6 mm (300 mm2);

60 x 8 mm (480 mm2); 80 x 8 mm (640 mm2);

100 x 10 mm (1000 mm2)



Insulators

The insulators are used to provide support and insulation to the transmission and distribution line conductors and bus bars. Their basic job is to completely thwart the flow of current (going from pole to the ground) by providing high insulation between the pole and conductor. They are constructed from porcelain. They can be mounted either vertically or horizontally depending on the situation. The following types of insulators are used on a substation;

(i). Post Type Insulator

Post-type insulators are used for the installation of a bus bar. A post insulator consists of a porcelain-made body, cast iron cap, and a cast iron flanged base, as has been illustrated in figure 3.7. A rim or collar mounted on something, which keeps it holding on that spot, is called a flange. For example, the rim or collar jutted out on a railway wheel, which holds it on the railway line and does not let it come down the track. Buckle screws are being made inside the hole of an insulator’s cap. The bus bar is fitted directly with the cap through nuts and bolts or the bus bar is installed with the help of a bus bar clamp. Post insulators are normally available as round, oval, and square-shaped flanged bases and install the bus bar through one, two, or four bolts. An auxiliary earthing bolt also exists along with every base.

Figure 3.7 – post insulator

Equipment or Instruments Used in Substations

Figure 3.8 – bushing insulator

Equipment or Instruments Used in Substations

(ii). Bushing Type Insulator

A bushing insulator consists of a porcelain shell body (i.e., a rigid porcelain outer shell), wherein two washers exist to install the bus bar or rod onto the shell. After making two holes in the flange, which is being fitted in the middle of the insulator, it is mounted through a bolt and this bolt also provides a point for earthing. If the current rating is higher than 2000 amperes, then bushing is designed in such a style that the main bus bars directly pass through them. Red, yellow and blue color paint is coated on the phase so that the phases of the main bus bars can be identified. A bushing insulator is also known as a through insulator. In figure 3.8, a bushing insulator has been illustrated.



(iii). Pin Type Insulator

As a pin type insulator is fastened or mounted on a cross–arm fitted on the pole, through a pin, therefore it is known as a pin type insulator. There is a groove on the upper part of the insulator, wherefrom the conductor is passed, and tied with the same material from which it has been made.

Pin-type insulators are normally designed for up to 11 KV on the transmission line and substations up to 11 KV, 33 KV, and 66 KV. However, in case they are designed larger than 11 KV, they prove costly. They are mostly used with the substation’s outgoing circuits. In figure 3.9, a pin-type insulator has been depicted.

Figure 3.9 – pin insulator

Equipment or Instruments Used in Substations

(iv). Suspension Type Insulator

Suspension type insulators or disk type insulators are mounted on high voltage lines. One insulator is used for up to 11 KV. As line voltage keeps increasing, these insulators are joined together through a metal link to form a chain, which is called an insulator string. The conductor is connected with the lowest part of this string (i.e., with the lowest unit of this insulator’s string) whereas the other end of this string (i.e., upper end) is tied firmly with the cross–arm fitted along the pole or tower, as has been illustrated in figure 3.10. Every unit disc is being designed for 11 KV and several discs are joined together in a series according to the working voltage.

(v). Strain Type Insulator

A huge tension or strain exists on the point from where the line starts or where the line ends or where some kind of a bend occurs. To avoid this excessive strain, strain insulators are used at the point of start, ending, or bends of the line. This has been illustrated in figure 3.11. The strain insulators are similar in appearance to the suspension-type insulators; however, their mechanical strength is almost 50 percent higher than the suspension insulators. Remember that in a strained string, there is at least one auxiliary insulator as compared to the suspension string.

Figure 3.10 – Suspension type insulator

Equipment or Instruments Used in Substations

Figure 3.11 – Strain type of insulator

Equipment or Instruments Used in Substations




Isolator

The instrument or gadget which is used to isolate or turn OFF a whole circuit or a part of it to look after or repair a power circuit (transmission or distribution line) and other related equipment on a no-load, is called an isolator. In other words, a mechanical switch that is used for turning off the circuit on a no-load (so that sparking could be averted), is called an isolator. It is also known as a disconnecting switch. It is always fitted along with a circuit breaker. To turn off or remove a circuit, the first circuit breaker is turned off and then the isolator is opened to isolate the circuit completely.  

Types of Isolators

Types of Isolator with Respect to Function

With respect to function, there are the following two types of an isolator;

(a). horizontal break type isolator

(b). Vertical break type isolator

©. Vertical pantograph type isolator

(a). Horizontal Break Type Isolator

The isolators, which open in a horizontal style, are called horizontal break type isolators. These isolators are used for all types of outdoor purposes (e.g., for circuit breaker isolation, etc.). In figure 3.12, this type of isolator has been illustrated.

Figure – 3.12

Equipment or Instruments Used in Substations

Figure – 3.13

Equipment or Instruments Used in Substations

Figure – 3.14

Equipment or Instruments Used in Substations

(b). Vertical Break Type Isolator

The isolators, which open vertically, are called vertical break type isolators. As the blades of these isolators cannot bend, therefore it has the simplest of designs. In figure 3013, this type of isolator has been illustrated.

©. Vertical Pantograph Type Isolator

A vertical style circuit breaking isolator, the rotating insulator column of which rotates around its axis during the opening, is called a vertical pantograph type isolator. This type of isolator has been illustrated vide figure 3.14. When pantograph blades fall in a vertical style, isolation occurs between the line terminal and the terminal above the pantograph.

(ii). Types of Isolator with Respect to Pole

(a). Single pole isolator

(b). Three pole isolator

The isolators being used in power systems normally consist of a three-pole structure. Three-pole isolator has three similar poles. Every pole consists of two or three insulator posts, on which conducting parts are mounted, which consist of conducting copper or aluminum rod, fixed and moving connects. In the opening state, conducting rod moves on one side and provides isolation. The simultaneous operation of three poles becomes possible through the mechanical interlocking of these three poles. Moreover, a common operating mechanism also exists for these three poles.



Circuit Breakers

An electric instrument, which is used to turn ON or OFF a circuit in a secure manner during normal conditions (i.e., for the make and break) and in abnormal conditions (e.g., in the situation of a short circuit, etc.) disconnecting it from the supply system through an automatic system, is called a circuit breaker.

The installation of a circuit breaker at every switching point of the substation is necessary. These are fitted on the supporting structure.

The circuit breakers are installed for performing the following functions;

(i). To pass full load current through within it

(ii). Opening or closing of the circuit on no-load

(iii). Making or breaking of the circuit on normal operating current

(iv). Automatically breaking itself on the designated quantities of the short circuit currents (i.e., turning off automatically in a situation of a short circuit)  

Types of Circuit Breakers

Basically, a circuit breaker consists of a set of moveable contacts and fixed contacts. When moveable contacts disconnect from the fixed contacts through an operating mechanism, a flame or arc generates between them which puts off through a suitable insulating medium (e.g., die-electric oil, vacuum, SulphurHexa Fluoride gas, etc.). Remember that a breaker is named according to the insulating medium which is being used to extinguish the arc (spark) in the breaker. Thus, according to arc extinguishing, there are the following types of the circuit breaker;

(i). Oil Circuit Breaker

In this type of circuit breaker, oil is used to put out the arc resulting from the opening of the contacts within the circuit breaker. There are further two types of oil circuit breakers which are known as low oil circuit breakers and bulk oil circuit breakers. In figure 3.15, the oil circuit breaker has been illustrated.

(ii). Air Circuit Breaker

The circuit breaker wherein a pressure air is used to put out an arc is known as an air circuit breaker. There are further two types of this circuit breaker which are known as air brake circuit breaker and air blast circuit breaker.

With respect to the arc, the air blast circuit breaker has further three types according to the air direction, named as excel blast air circuit breaker, cross blast air circuit breaker, and radial blast circuit breaker.

Figure 3.15 – Circuit Breaker

Equipment or Instruments Used in Substations

(iii). Vacuum Circuit Breaker

A circuit breaker, wherein the existing vacuum, functions as an arc extinguisher is known as a vacuum circuit. Remember that vacuum is a top-quality dielectric and an excellent medium for putting out the arc. The usage of this type of circuit breaker is quite common for medium voltages.

(iv). Bas Circuit Breaker

A circuit breaker, in which SulphurHexa Oxide (SF6) is used for arc extinguishing and the provision of insulation, is called a gas circuit breaker or SF6 circuit breaker. As a result of its multiple advantages, the application of these types of circuit breakers is increasing very rapidly.



Power Transformer

A transformer is an idle instrument or equipment, which increases or decreases the voltage level on the principle of mutual induction, without changing the frequency. Power transformers are also installed for changing the voltage levels on the substations. Mostly, these transformers are three-phase. However, three single-phase transformers can also be connected in the form of a bank to build a three-phase transformer. There is also a three-phase ON or OFF loop tape changer on the low voltage side of the power transformer., through which the voltage level is changed by means of changing the transformer’s tapping. Instead of placing a three-phase transformer directly on a flat concrete slab, it is placed above a one-to-one-and-a-half-meter high concrete constructed foundation through installing an iron railing or line, so that the object of getting air can be achieved, besides easily shuffling its place as and whenever required. This has been illustrated in figure 3.16. For cooling the power transformers, gigantic fans are installed alongside them, and to tackle any sort of fault, all types of internal and external protection are being provided.

Figure 3.16 – power transformer

A transformer is an essential part of any substation. It consists of two insulated coils above a laminated core. The core along with its winding is placed in an enclosed metal container and then transformer oil is filled into the container. Oil besides, providing insulation between windings and containers existing on the core, also emits winding heat (i.e., oil carries out two functions of insulation and cooling). For the emission of heat, metal tubes are fitted along both sides of the transformer’s container. After getting heated, oil spreads and circulates within the tubes and containers. When hot oil spells onto the tubes from the container, oil heat discharges outside into the open space as a result of striking of air with the tubes. The winding terminals are jutted out by means of passing through the porcelain oil-filled or condenser-type bushings.

Types of Transformers

There are the following types of transformers according to the voltage function, core structure, and cooling;

(i). Step-up Transformer

A transformer that is used to convert low voltages to high voltages, is called a step-up transformer. These types of transformers are installed on generating stations or primary substations, the function of which is to convert 11 KV to 132 KV or 220 KV, or 500 KV.

(ii). Step Down Transformer

A transformer that converts high voltage to low voltage, is known as a step-down transformer. These transformers are mostly used for distribution objectives. The H.T side or primary winding of these transformers is normally connected to 132 KV, 220 KV, or 500 KV. 

(2). Types of Transformers with Respect to Construction of Core

 From the perspective of core construction, a transformer has the following types; 

(i). Core Type Transformer

The transformer, having a core on the inner side while winding on the outer side (i.e., winding has been done around the transformer core) and there is just one route for magnetic flux, is called a core type transformer. In this type of transformer, cylindrical coils are wound/looped on a rectangular core, as has been illustrated vide figure 3.17.

Equipment or Instruments Used in Substations

(iii). Shell Type Transformer

A transformer having a core on the outer side while winding on the inner side, with two paths for magnetic flux, is called a shell-type transformer. This has been illustrated in figure 3.18.

Figure 3.18- section of a shell-type transformer

Equipment or Instruments Used in Substations



(3). Types of Transformer w.r.t Cooling

(i). Oil immersed self-cooled

(ii). Oil Immersed Combination of self-Cooled and Air Blast

(iii). Oil Immersed Water Cooled

(iv). Oil Immersed Forced Oil Cooled

(v). Oil Immersed Combination of Self-Cooled and Water-cooled

(vi). Oil-Forced Air-Forced Cooled

(vii). Forced Oil Water Cooled

(viii). Forced Oil Self-Cooled

Instrument Transformers

The transformers which are used to enhance the range of different AC measurement instruments (e.g., ampere meters, wattmeters, etc.) and to reduce the current or voltage of different protective relays, are known as instrument transformers. In other words, the transformers which are used to reduce current and voltage value for control, measurement, and protection, is called instrument transformer. These transformers are mostly used to increase the range of ampere meters and voltmeters already installed on the AC circuit. Thus, a low-range AC meter can measure up to a vast range with the help of an instrument transformer. 

According to core construction, there are following two types of the instrument transformer;

(i). Current transformer

(ii). The voltage or potential transformer

(i). Current Transformer 

The transformers which reduce high currents to low currents for measurement, protection, and control and provide it to the measurement instruments and security relays, are called a current transformers. As such, with the help of these transformers, high currents can be measured, high currents can be controlled, and measuring instruments can also be protected against high currents. 

Current transformer’s (which is commonly known as C.T) primary winding consists of one or two turns of a thick wire, whereas its secondary winding consists of many turns of a thin or narrow wire. The primary winding is fixed on that circuit series, the current of which is desired to be measured. Whereas, the secondary winding is connected to an ampere meter or relay. Remember that the current transformer tends to be step-up with respect to the voltage, however, it steps down the current (because its primary winding consists of one or turns of a thick wire whereas secondary winding consists of more turns of a thin wire. As a result, its voltage increase, however current reduces). Moreover, the secondary winding of the current transformer should never be opened under any circumstances. As compared to a power transformer, the current ratio of a current transistor is high (e.g., 500A/5A) and volt-ampere capacity relatively less (e.g., 50VA). Through the primary winding of the current transformer, its rated current passes (the current for which it has been designed, is called its rated current), whereas normally a 5V current transmits through its secondary wiring. In figure 3.19, a current transformer (column 3) installed on an outdoor substation has been illustrated.

Figure 3.19- 245 KV hybrid substation with SF6 insulated double bushers and breakers, other components being conventional Bay spacing 13 m. 

Equipment or Instruments Used in Substations

There are actually two types of a current transformer. Measuring current transformer and protective transformer. Measuring current transformers are used for measurement purposes whereas, protective transformers are used for protection and control purposes. Measuring current transformers are used for reducing the line current by 1 ampere of up to 5 amperes through the application of ampere meters, KVA meters, and KWH meters. Whereas, protective transformers are used for overcurrent protection, earth fault protection, differential protection, impedance protection, etc. 




(ii). Potential or Voltage Transformer

A transformer, which reduces high voltage for measurement, protection, and control and provides it to measuring instruments and protective relays, is called a potential transformer (P.T) or voltage transformer (V.T). Through this transformer, low-ranged ordinary measuring instruments (voltmeter and wattmeter, etc.) become capable of high voltage measurement. Moreover, these transformers also secure measuring equipment against a high voltage. The volt-ampere capacity of this transformer is low (i.e., 100VA) and its voltage ratio is relatively high (i.e., 132KV/120V). 

The primary winding of a potential transformer is connected with a high voltage circuit or main busbar of the switchgear, whereas secondary winding is connected to measuring instruments and protective relays etc. when primary winding is provided with the required high voltages, only 120V is produced in its secondary. Potential transformers are different from each other, based on primary and secondary rated voltage, number of phases, and cooling system. 

Remember that potential transformers are used to feed the potential coils and relays of the indicating and metering instruments having a voltage of more than 400 volts.

Protective Relays 

A low power device that is used to activate or energize any high-power device, is called a relay. In other words, a relay is an automatic device, which closes its contacts when the value of its actuating quantity (current or voltage) exceeds its secured value. For example, an overload relay is being set on the secondary of a current transformer installed on a line. When the current value exceeds the relay settings as a result of overload, then open contacts of the relay tend to close, due to which the breaker’s coil energizes (because the relay provides a signal to the nearest circuit breaker on the closing of its contacts as a result of a fault) and the breaker trips. Therefore, the passing of a trip command to the circuit breaker on the transmission and distribution power system is a function of the protective relay. These relays protect against prospective loss to the feeders and equipment in the event of a fault. In fact, relays are a type of measuring instrument, along which auxiliary contacts are being installed and operated (i.e., close) when the quantity of current or voltage passing through it, is either higher or lower than the preset value. When these contacts operate, they tend to move the mechanical mechanism of the switch or circuit breaker, owing to which the circuit breaker trips and disconnects the faulty circuit from the supply. 

The following different types of relays are used on substations for protection and measurement objectives.

(i). Types of Relays w.r.t Input

(a). Current Relay

(b). Voltage Relay

©. Impedance Relay

(d). Reactance Relay

(e). Frequency Relay

(ii). Types of Relay w.r.t Function

(a). Main Relay

(b). Auxiliary Relay

©. Signal Relay

(iii). Types of Relays w.r.t Construction & Principle of Operation

(a). Electromagnetic Relay

(b). Electrodynamic Relay

©. Moving Coil Type relay

(d). Thermal Relay

(e). Induction Relay

(f). Physio Electric Relay

(g). Static Relay

(iv) Types of Relays w.r.t Purpose or Application or Performance Characteristics

(a). Under Voltage, Under Current, Under Power Relay

(b). Over Voltage, Over Current, Over Power Relay

©. Directional or Reverse Current Relay

(d). Directional or Reverse Power Relay

(e). Differential Relay

(f). Distance Relay

(g). Earth Fault Relay

(h). Gas Pressure Relay

(i). Under Frequency Relay



(v). Types of Relay w.r.t Time of Operation

(a). Instantaneous Relay

(b). Definite Time Lag Relay

©. Inverse Time Lag Relay

(d). Inverse Definite Time Lag Relay

Fuses

A protective instrument that disconnects the overflow of excessive current resulting from some kind of a fault in an electrical circuit, is called a fuse. The L.T and H.T type fuses are commonly used for the protection of different types of circuits and equipment in the substations. Along with other security appliances (e.g., breakers and relays), the use of fuses is preferred on the high voltage side. In this way, appliances or equipment are securely protected. Fuses, apart from protecting the transformer from the system, also safeguard other auxiliary equipment installed on a substation. As soon as a current higher than the capacity of a fuse-protected appliance passes through it, the wire fixed in the fuse melts and breaks down. As a result, the circuit opens or breaks. As such, the flow of current suspends and the instruments get protected. 

There are the following types of fuses with respect to voltage;

(A). Low Voltage Fuses

(i). Open Type Rewireable Fuses

(ii). Semi-enclosed type Rewireable Fuses

(iii). Cartridge Type Fuses

(a). D-Type Cartridge Fuses

(b). link Type Cartridge or HRC Fuses

(B). High Voltage Fuses

(i). Cartridge Type High Voltage Fuses

(ii). Liquid Type Fuses

(iii). Time Delay Fuses

(iv). Metal Clad Type Fuses

Load Interrupter Switch

During normal working conditions (i.e., on a normal load) load interrupter switches are used for the opening and closing of the high voltage circuits. When this switch is opened, an arc generates through its contacts, for the extinguishing of which, hydrogen gas is normally used. A lever arm is used to open or close such switches, which tends to be a hand-operated mechanism. 

Surge Arresters or Lightning Arresters

The lightning arrester is used to protect the system against excessive voltages resulting from lightning or switching, which are mostly connected along power lines and transformers. Whenever voltage quantity increases on a line owing to lightning, these arresters discharge such high voltages towards the earth (that’s turning them towards the earth) and protect precious equipment against high voltages or overvoltages resulting from lightning or switching surges. When a lightning arrester operates, a flashover occurs on its gap as a result of a high voltage, and high voltages shift towards the earth immediately, whereas normal voltages cannot transmit through the arresters. If viewed from an overhead transmission line, a lightning arrester tends to be the first apparatus of any substation, which is installed between a line and the earth. 



Types of Lightning Arresters

Normally following types of lightning arresters are used on substations;

(i). Rod Gap arrester (ii). Horn Gap Arrester (iii). Metal Oxide Arrester (iv). Multi-Gap arrester (v). Expulsion Type Arrester (vi). Valve Type or Thyrite Arrester (vii). Electrolytic Arrester

Reactor

An inductive coil having too many turns, which has a very high inductance with low resistance, and which limits an infinite current flowing through the circuit during a short circuit, is known as a reactor. 

Types of Reactors

Types of Reactors w.r.t Construction

(i). Unshielded or Dry Type Reactors

(ii). Magnetically Shielded or Oil Immersed Reactor

Types of Reactors w.r.t Connection

There are two types of reactors with respect to connection;

(i). Shunt Reactors

Shunt reactors are used to compensate for reactive power resulting from line capacitance during low loads along high voltage transmission lines. 

(ii). Series Reactors

The series reactors are used to reduce short circuit current or starting current.

Capacitors

Capacitors normally reduce the reactive component of a leaking power factor. Besides, capacitors are also used for building connections and filtering purposes. 

(i). Series Capacitors

These capacitors are fitted on a series of long Extra High Voltage (EHV) AC lines, to compensate for line reactance. Thus, the power factor of the line improves.

(ii). Shunt Capacitors

Shunt capacitors are mounted near load points on the receiving substations and distribution substations to improve the power factor. In other words, a reactive load of an inductive line can be compensated through the application of a shunt capacitor.  

(iii). Coupling Capacitors

A coupling capacitor is used for establishing a connection between equipment existing on a high voltage line and power line carrier current (PLCC). 

PLCC Equipment   

These types of equipment are installed for communication, telemetry, tele-control, relaying, or supervisory purposes. The room where such equipment is installed is known as a carrier room. This equipment is very carefully connected to the high voltage power circuit. 

In other words, PLCC equipment transmits or receives a high-frequency signal through power lines or transmission lines for the following purposes; 

(i). Voice communication (ii). Data transmission (iii). Protection signals (iv). Control signals

Control Cables

Control cables and a conduit system is required for automatic controls. The Control system normally operates on 110 volt and 220 volts and the cables which are used for this purpose, are multi-core cables, wherein 10, 37, or 61 conductors exist. These cables are normally spread on underground ducts from the control room to the main junction box. Whereas, conduits (pipes) are used to carry cables from the main junction box to the corresponding load points. 



Switch Boards

The switchboard installed on a substation consists of meters, relays, and control apparatus. This has been illustrated in figure 3.20 (a). The most important meters are fitted on the upper part whereas less important meters are installed on the lower part of the switchboard. Control equipment is generally fixed on the center part of the switchboard so that it can easily be operated. A control disc is also used alongside a switchboard for convenience, as has been illustrated in figure (b). 

Control Room

Switchboard, carrier current equipment, batteries and alarm system, etc. are installed in the control room. Further, the indication lights of different instruments and switching devices are also installed in the control room, which provides a piece of information about their turning ON or OFF. Moreover, informative charts and maps regarding the power system and substation, also keep hanging in the control room. 

Figure 3.20

Equipment or Instruments Used in Substations

Indicating and Measuring Instruments

Ammeters, voltmeters, KWh meters, and KVarh meters are also fitted in the substations so that currents flowing through the circuit and power loads can be supervised, controlled, and measured. 

Neutral Ground Resistor 

The neutral ground resistors can be used to limit the earth fault current. According to this method, neutral is connected with the earth through one or more than one resistors. This method of limiting current through a resistor is called resistance earthing. 

Line Trip

A line trip is used to prevent high-frequency signals from entering other protective zones. They are fitted on a series of every incoming line to a substation.

Earth Switches

An earth switch is a type of switch, which is mounted between every line conductor and the earth, so that charge existing on a conductor, could discharge towards the earth. These switches are usually installed on the isolators’ frames existing in the substations. In normal conditions, the earth switch remains open. When a line is disconnected by turning OFF the breaker and opening the isolator, the earth switch is closed during that time. In this way, the charge existing on the line (which occurs due to the line capacitance), gets earthed. Remember that when a line is opened, some quantity of voltage exists on it, as a result of which capacitance existing between the earth and the line charges up. Such voltages are extremely high in a high voltage system. Before overhauling the line, these voltages are discharged towards the earth by closing the earth switch. 

Storage Battery Room

In case the main power supply turns OFF, a storage battery is used to continue supply to some important electric equipment. For this purpose, a battery room exists at every substation, where a number of storage batteries tend to remain available. These batteries supply DC power. 

Normally lead-acid and alkaline type batteries are used.   



Voltage Regulator

A device, through which the voltage quantity can be increased or decreased according to needs for distribution purposes, is known as a voltage regulator. A voltage regulator is actually an autotransformer, the secondary coil of which is configured in such a way that the overall voltage generated on it or part of this overall voltage, can be added to or deducted from the supply line voltage. These regulators are installed at substations on poles or platforms of the distribution feeder and the desired change can be made in voltage by changing the transformation ratio automatically without closing the unit. 

There are following two types of a voltage regulators;

(i). Induction Regulator

It is used to change the voltage of a 5 KV or low rating circuit

(ii). Tap Changing Under Load Regulator

 It is used along with high voltage or high-powered circuits or lines. 

Galvanized Steel Structure

A galvanized steel structure is a bolted or welded structure, which is constructed from angles, channels, or pipes and its function is to provide support to towers and other equipment installed at the substation. Apart from providing support to different instruments, a galvanized steel structure also provides support to insulators. The structure design should be simple and economically cheap. 

A substation generally consists of the following structure;

(1). Towers for providing support to the incoming and outgoing transmission lines

(2). Towers and beams etc. for providing support to the strain insulators and flexible busbars. They are used for installing isolators, surge arresters, and other instruments. 

(3). Towers and beams for providing support to rigid tubular busbars and post insulators

(4). Supporting structures for providing support to CTs, PTs, isolators, circuit breakers, line trips, etc. 

(5). Auxiliary structures for providing support to the cooling water system and firefighting system



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