A brief overview of 132kv sub-station/grid-station

Substation is an assembly of apparatus, which transform the characteristics of electrical energy from one form to another, say from one voltage level to another level, hence a substation is an Intermediate link between the Generating station and consumers.

"The most important part of electric power system is the Grid Station. A 132kv Grid Station is that part of power system where high voltage of 132kv is received, stepped down to 11kv and then given to different feeders that feed distribution lines. This whole process is carried out by different parts and equipment".

Parts of Grid Station:

The main parts of the grid station are the switch yard, 11kv control room, 132kv control room, battery room. All of them are discussed below one by one along with their respective switching, protection and control equipment.

1. Switch yard

Switch yard is divided into three parts i.e. line bay, bus bar and transformer bay.

Line bay:

Line bay consists of

• Wave trap: ?? it traps the waves associated with 132kv line, which are used for communication purposes.
• Isolator:??? an isolator is installed which isolates the grid from the line. Isolators are “no load” switches and are operated under no-load conditions. Its operating under load condition is very dangerous as disconnecting high voltages will create a large flash.
• Earth switch:??? an earth switch is also installed with the isolator, with the help of which we ground any static charges before working/maintenance.
• Line Circuit breaker:??? a high rating SF6 C/B is installed which operates automatically under abnormal conditions and disconnect the grid from the line to avoid any damages. It can also be operated manually and unlike isolators the can be operated under load conditions as they have quenching media in them.
• Line CT: ?? after circuit breaker a current transformer is installed which calculates current and give indication to the C/B when over current fault occurs hence acts as a protective device as well as measuring device. Its CT ratio is generally 600/5A.
• Isolator: ?? again an isolator is installed.

Bus Bar.

• After line bay it comes 132kv bus bar. It acts as a node point between line bay and transformer bay. Bus bar helps in getting connections to several units (transformers) from a single 132kv line. It can be used for incoming connections as well as outgoing ones.

• PT:??? A potential transformer is also connected in parallel to 132kv bus bar through an isolator, which measures voltage as well as give indication to over voltage relay under over voltage conditions.

Transformer bay:

Transformer bay consists of

• Transformer Isolator:?? after bus bar another isolator is used which isolates transformer from the line. Operating it in ON load condition is very dangerous as high voltages produce arc while connecting/disconnecting and they don’t have any quenching media.
• CT:??? again a current transformer is installed which calculates current and give indication to the transformer C/B when over current fault occurs hence acts as a protective device as well as measuring device. Its CT ratio is generally 200/5A.
• Transformer Circuit breaker:??? again an SF6 circuit breaker is installed which operates automatically under abnormal conditions and disconnect transformer from the bus bar to avoid any damages. It can also be operated manually and unlike isolators the can be operated under load conditions as they have quenching media in them.
• Lightning arrester (132kv):??? lightning arresters are devices which arrests and ground any thunder storms falling on the power transformer and secure it from any harm. They are installed on each phase at a relatively more height. They are grounded and have a non-linear resistor which act as insulator for132kv voltage and as a conductor for higher voltages like that of thunder storms and lightning bolts. In this way they ground these high voltages and provide safety to the transformer and whole system.
• Power transformer:??? then the power transformer is installed which steps down 132kv to 11kv. (power transformer is discussed in detail in the next section)
• Lightning arresters (11kv):??? again on the LT side of power transformer small lightning arresters are used. And the purpose is again the safety of transformer from thunder lightning.
• Cables (11kv):??? these cables give the transformer output to the 11kv incoming panel of the respective transformer.

Power transformer

These are the step down transformers with Delta circuit configuration on input side (HT) and Star circuit configuration on output side (LT). This transformer steps down 132Kv to 11Kv. An input of 132kv is given to the transformer primary coil through three input wires i.e. Red(R), Yellow(Y) and Blue (B) and then 11kv is taken from secondary coil. The transformers use the magnetic induction process to step down the 132Kv to 11Kv due to specific turn ratio in coils.

The main parts of a power transformer are its windings and bushes. Some other parts, also known as accessories are:

• Conservator oil:??? also called mineral oil. It helps in insulation of windings as well as in cooling.
• Conservator tank: ?? it is fixed at an upper position and is filled about 70% to 80% of mineral oil.
• Radiator tubes: ?? the circulation of transformer mineral oil takes place through these tubes, as heated oil goes up and cooled oil goes down, hence continue the oil circulation for better cooling.
• Cooling fans:??? these fans also help in transformer cooling. They are fitted below or at the side of radiator tubes and provide cool air.
• Bucchloz relay: ?? this relay is fitted near conservator tank and is also called float relay and gas detecting relay. This relay detects the gases that are produced in the transformer under abnormal conditions and operates to trip the transformer, hence avoid further damages.
• Diaphragm:??? diaphragm is a cap shaped object and fixed on the top of power transformer, it acts as emergency exit for transformer inner oil and gases.
• Wheels: transformer has small wheels upon which it can be rolled from one place to another. As they hold the transformer at a height from the ground or foundation so air flow is possible beneath transformer.
• Oil temperature indicator: ?? this indicator detects the temperature of transformer oil. If the transformer oil get heated to such a temperature that can harm transformer, the oil temperature indicator operates. It has two settings, 1st is alarm and the other is tripping.
• Winding temperature indicator:??? this indicator supervise winding temperature. Like oil temperature indicator it also have two settings, one is alarm and the other is tripping the transformer.
• Tap changer:??? this is an instrument connected with the power transformer that is used to vary the transformer output. It actually changes the number of turns in the secondary coil of transformer.
• Dehydrating Breather:??? it is a tube on the side of transformer connected to conservator tank. It extracts un-moisture air from the outside to fill the vacuum in the tank. To extract water particles from the air, silica gel is used which is filled in the breather tube. Note that vacuum is created when some gases are expelled through the same tube.

Transformer cooling

• When the transformers operate continuously, as a result the core, coils, main tank and the transformer oil heats up to a high temperature which can affect the operation, maintenance, and output voltage of the transformer. In grid stations transformers are kept cool through three different processes. That are:

1. ONAN: ?? It stands for oil natural air natural. In this method fans are OFF and the transformer is cooled by natural air, same is the case of oil which circulates by spontaneous motion of hot and cold oil.
2. ONAF:??? It stands for oil natural air forced. This method is applied under a bit higher temperature. Oil circulation takes place by itself while the air is provided by radiator fans.
3. OFAF:??? It stands for oil forced air forced. This method is applied under highest temperatures. Fans are ON that provide more air and the circulation of oil is also speedup by oil circulation pump.11Kv RoomThe outputs of each power transformer is than brought into 11Kv Room through power cables. The 11Kv room has incoming panels for each transformer, different outgoing panels for each incoming panel and a bus coupler. All of them are explained below.

• 11kv Incoming Panels:

These panels are fitted in the 11Kv room, receiving 11Kv from each transformer as each transformer has its own incoming panel. The incoming have a number of parts like 11Kv C/B, bus bar, relays and power meter etc. that are included in each of the incoming panel.

• 11kv Outgoing panels:

The output of incoming panel is given to several outgoing panels through a rigid bus bar. They also have different parts like current transformer, vacuum circuit breaker, relays and different meters and indicators.

• Bus bar coupler:

As the name “bus bar coupler” indicates that it couples the bus bars of two incomings. It also known as bus coupler. It couples one 11kv bus bar to another when one of the transformer is faulty so that the supply towards the consumers is not disturbed. The advantage is the uniform (un-interrupted) supply. It is installed between the two incoming panels in the 11kv control room.

It is possible that a transformer gets damaged and is out of work, so to transfer the load of one transformer to other in grid station the bus coupler connects the incoming of damaged transformer to the incoming of the other working transformer.

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132kv control room

As the name control room indicates that it is the room from where the whole grid is controlled. Each and every circuit breaker, current transformer, potential transformer, power transformer and its accessories, yard lightning and other equipment are controlled from the control room of the grid station. This room also has a number of panels like control panels, relay panel, transformer auxiliary panel, AC DC panel and rectifier/battery charger. Each of them is discussed below.

132kv line protection panel:

This panel is especially for 132kv line. It consists of distance relay, over current and earth fault relay and DC supervision relay.

• Distance relay:??? this relay shows the zone where the fault occur as well as the phase in which occur i.e. A, B & C.
• Over current and earth fault relay: ?? this relay does tripping as well as shows whether the fault (over current) was due to phase to phase short circuit or phase to ground one.
• DC supervision relay:??? DC supervision relay shows whether the system (panel and relays) is healthy or not.
• 132kv line control panel:??? This control panel controls the line bay i.e. the system from the tower to the bus bar.

• Transformer control panel:?? This panel controls the system inside bus bar. Transformer CT, PT, circuit breaker etc comes under the umbrella of this panel. It gives indications of different faults and problems.

• Transformer auxiliary control panel:??? This panel controls various operations related to the working and protection of the power transformer. In 132kv control room it is installed one for each power transformer. It has
• Two meters: one shows the tap position and the other shows temperature of transformer in .
• Two selector type switches: one for cooling fans ON OFF and other for tap changer.

It also gives different indications like

• Cooling fans automatic ON OFF
• Cooling fans manual ON OFF
• Fans supply fault
• Winding temperature alarm
• Fan motor circuit breaker trip

It also have an Emergency stop press button which is used in emergency.

Also an automatic voltage regulator is installed in the transformer auxiliary control panel.

• AC DC panel:

This is another important panel in the 132kv control room. It controls AC and DC supply of the whole grid. It has two parts one for AC and other for DC.

• AC panel: ?? AC panel consists of three meters for R, Y & B phases, a voltmeter and a KWH meter. It controls AC supply of
• Switch yard lights
• Tap changer
• 132kv breaker motor
• Battery charger
• Transformer cooling fans
• Control panels.

It also controls AC supply (220 v) of the 11kv incoming and outgoing panels.

• DC panel:??? It also have a DC ammeter and a DC voltmeter to show current and voltage. It controls DC supply of
• control room building lights (DC)
• switch yard DC lights
• 11kv & 132kv breaker motors
• 11kv switch-gear protection
• DC emergency bus
• Control and relay panels.

It also have battery alarm and under/over voltage relay.

Rectifier:

A rectifier is an instrument/circuit that changes AC into DC. In the grid station it is used for several purposes i.e.

• It charges batteries
• Provide DC supply to the load
• Shares load with battery during overload condition.

The rectifier used in grid has a voltmeter and an ammeter for incoming AC and another voltmeter and ammeter for outgoing DC.

It also have control switches like

• ON/OFF
• Auto/manual
• Boost/float (for boost? and normal charging of battery)
• Load/battery. Relay panel:

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This panel controls different relays like overcurrent relay, distance relay, differential relay, oil temperature and winding temperature relay and many more that are discussed as under.

• Over current relay:

As the name indicates these relays keeps the transformer safe from over current which may cause serious problem to the transformer. As we know that some time the consuming of electric power increase to a dangerous level that causes a huge current due to over loading. The relay trips and gives indication of the type and area of tripping.

• Differential relay:

These relays operate on the fault between transformer input and output. The fault may be due to low voltage, high voltage or may be due to earth fault.

• Oil temperature relay:

These relays as the name indicates supervise the oil temperature. As we have discussed earlier that as the transformer operates continuously, the temperature of oil increases, this temperature may sometime increase to a dangerous situation as a result this relay operates.

• Winding temperature relay:

These relays as the name indicates supervise the winding temperature. As we have discussed earlier that as the transformer operates continuously, the transformer winding heats up. This temperature may sometime increase to a dangerous situation which cause this relay to operates.

• T/F buchholz Relay:

This relay operates as the buckle action occurs. Buckle reaction is discussed before. As we know that buckle action occur when dangerous gasses produce in main tank so this situation is also an alarming situation and it must also be noticed as well so this job is done through this relay.

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4. Battery room

Battery bank is the conserved source of 110v dc voltage.

It gives supply to the protection system of the grid when supply from the rectifier stops.

In the grid station’s battery room/bank, there are generally 55 cells of 2v dc each. These cells are connected in series to get 110v. The batteries are charged by the rectifier.

These are lead acid batteries having specific gravity of 1.220 and are of 150 A.H capacity.

A battery have the following components:

• Container/jar: it is generally made of plastic and gives support to the electrolyte and plates.
• Electrolyte: a solution of 35% sulphuric acid (H2SO4) and 65% water (H2O) is filled in the container.
• Plates: two dis-similar plates are dipped in the electrolyte, one made of “lead oxide” act as cathode and other made of “lead” act as anode.
• Connection leads: the electrodes are connected with the connection leads.
• Vent cap: Every battery have a vent cap that is used to evolve any gas bubbles produced during conduction in electrolyte.