What is a switchyard ?

It is a switching station which has the following credits :

Main link between Generating plant and Transmission system, which has large influence on the security of the supply.

Step-up and /or Step-down the voltage levels depending upon the Network Node.

Figure : Switchyard

SWITCHYARD OPERATION ACTIVITIES

A switching station may also be known as a switchyard, and these are commonly located directly adjacent to or nearby a power station. In this case the generators from the power station supply their power into the yard onto the Generator Bus on one side of the yard, and the transmission lines take their power from a Feeder Bus on the other side of the yard.

As mentioned elsewhere, switchyard is handling bulk power and its operation and Maintenance has become critical. Any ambiguity in the operation of the switchyard may lead to such disasters like grid failure, station outages crippling not only the normal life of people but also the very economy of the country. Even in less serious situations such as cascade tripping of Auto Transformers due to unplanned over loading has caused under utilization of our generating capacity many times. The operation of switchyard calls for a very alert staff that shall have to sense the abnormalities in time and prompt to concern timely to enable normalcy of the system. The following are some of the identified activities of 400 KV switchyard operations.

1. Identifying of faulty equipment, safe isolation of equipment without disturbing other system as much as possible, raising job cards, arranging shutdowns, trial charging and normalization of 400 KV SWYD. And 132KV Swyd, associated equipment like CBs, Isolators, A/Ts, T/Ts, Shunt Reactors, ACDBs, DCDBs, Battery, Charges PLCC equipment, Swyd. Compressors and lighting.

2. Daily inspection of indoor/outdoor swyd equipment, checking of oil leakages, temperatures and any other abnormalities like sparks etc. SF6 gas pressures, compressed air pressures, running period of compressors, availability status of mulsifier system, swyd. And station P.A. system and PLCC communication system etc. monitoring of physical conditions of swyd equipment.

3. Analyzing and locating of fault leading to feeder/Transformer trip, reporting emergencies to the higher authorities, coordinating with other agencies like AP Transco/Genco, PGCIL in clearing faults and normalization of system.

4. Close monitoring of grid parameters, coordinating with CPCC, SRLDC, OS (SR), OS (ED), LDC (APSEB), Shift charge Engineer & Desk Engineers for smooth operation of grid system, timely action to ensure continuity of power supply.

5. Quick arrangement of startup power supply in case of grid failures, station outages. 6. Continuous monitoring of system parameters like voltage, frequency, line and

Transformer, loading unit generations, MVAR and MW net exports etc. recording and corrective action where the abnormality found.

7. Preparing of daily power generation / export/import energy reports, exchanging data with CPCC, OS (ED), OS (SR), collection of generation details from other power projects and storing.

8. Assisting the shift in charge in transmitting the flash report, availability report, unit trip/synchronization messages, shutdown messages, generation back down messages,

modification of availability declarations, feed back to shift in charge, the deviation if any in total generation with respect to the declaration.

SWITCHYARD EQUIPMENTTo perform switchyard operation activities perfectly, operation staff should have good

knowledge about the equipment provided in switchyard as well as in control room. They should be familiar with the control system adopted here and a good understanding about the procedures to be followed during the emergencies, outage requirements and charging. Brief description about switchyard equipment is given below.

Equipments commonly found in switchyard:-

1. Lightening arrestor

2. Current transformer

3. Voltage transformer

4. Bus bar and clamp fittings

5. Isolaters

6. Circuit breaker

7. Wave traps

8. Earthing switch

9. Power transformers

10. Support structure

1 .SURGE/LIGHTINING ARRESTER:

A lightning arrester is a device used on electrical power systems and telecommunications systems to protect the insulation and conductors of the system from the damaging effects of lightning. The typical lightning arrester has a high-voltage terminal and a ground terminal. When a lightning surge (or switching surge, which is very similar) travels along the power line to the arrester, the current from the surge is diverted through the arrestor, in most cases to earth.

A lightning arrester is a device used on electrical power systems to protect the insulation on the system from the damaging effect of lightning. Metal Oxide Varistors (MOVs) have been used for power system protection since the mid 1970s. The typical lightning arrester also known as surge arrester has a high voltage terminal and a ground terminal. When a lightning surge or switching surge travels down the power system to the arrester, the current from the surge is diverted around the protected insulation in most cases to earth.

Surge Arresters are provided to ground the over voltage surges caused by switching and lighting surges. Surge Arresters provide leakage path to the ground whenever the system Voltage rises above the specified value. They are equipped with surge monitors, which measure the leakage currents and a counter to record the number of surges taken place.

Components:-

A potential target for a lightning strike, such as a television antenna, is attached to the terminal labeled A in the photograph. Terminal E is attached to a long rod buried in the ground. Ordinarily no current will flow between the antenna and the ground because there is extremely high resistance between B and C, and also between C and D. The voltage of a lightning strike, however, is many times higher than that needed to move electrons through the two air gaps. The result is that electrons go through the lightning arrester rather than traveling on to the television set and destroying it. A lightning arrester may be a spark gap or may have a block of a semiconducting material such as silicon carbide or zinc oxide.

Fig: Lightning arrester

See also:- Lightning rod Surge arrester Thyristor switched capacitor

2 . CURRENT TRANSFORMER (CT): Current transformers are used for the instrumentation, Protection or metering of power

systems.

Purpose:-

- To step-down the high magnitude of current to a safe value to incorporate Measuring and Protection logics.

Current transformer (CT) is used for measurement of electric currents. A current transformer (CT) is a type of transformer that is used to measure AC Current. It produces an alternating current (AC) in its secondary which is proportional to the AC current in its primary. Current transformers, together with voltage transformers (VTs) or potential transformers (PTs), which are designed for measurement, are known as an Instrument transformer.

The main tasks of instrument transformers are:

− To transform currents or voltages from a usually high value to a value easy to handle for relays and instruments.

− To insulate the metering circuit from the primary high voltage system.

− To provide possibilities of standardizing the instruments and relays to a few rated currents and voltages.

When the current to be measured is too high to measure directly or the system voltage of the circuit is too high, a current transformer can be used to provide an isolated lower current in its secondary which is proportional to the current in the primary circuit. The induced secondary current is then suitable for measuring instruments or processing in electronic equipment. Current transformers have very little effect on the primary circuit.

Current transformers are the current sensing units of the power system. The output of the current transformers are used in electronic equipment and are widely used for metering and protective relays in the electrical power industry.

Construction:-

Current transformers typically consist of a silicon steel ring core wound with many turns of copper wire as shown in the illustration to the right. The conductor carrying the primary current is passed through the ring. The CT's primary therefore consists of a single 'turn'. The primary

'winding' may be a permanent part of the current transformer, i.e. a heavy copper bar to carry current through the core. Window-type current transformers (aka zero sequence current transformers, or ZSCT) are also common, which can have circuit cables run through the middle of an opening in the core to provide a single-turn primary winding. To assist accuracy, the primary conductor should be centered in the aperture.

Function:-

The alternating current in the primary produces an alternating magnetic field in the core, which then induces an alternating current in the secondary. The primary circuit is largely unaffected by the insertion of the CT. Accurate current transformers need close coupling between the primary and secondary to ensure that the secondary current is proportional to the primary current over a wide current range. The current in the secondary is the current in the primary (assuming a single turn primary) divided by the number of turns of the secondary. In the illustration on the right, 'I' is the current in the primary, 'B' is the magnetic field, 'N' is the number of turns on the secondary, and 'A' is an AC ammeter.

Use:-

Current transformers are used extensively for measuring current and monitoring the operation of the power grid. Along with voltage leads, revenue-grade CTs drive the electrical utility's watt-hour meter on virtually every building with three-phase service and single-phase services greater than 200 amperes.

High-voltage current transformers are mounted on porcelain or polymer insulators to isolate them from ground. Some CT configurations slip around the bushing of a high-voltage transformer or circuit breaker, which automatically centers the conductor inside the CT window.

Current transformers can be mounted on the low voltage or high voltage leads of a power transformer. Sometimes a section of a bus bar can be removed to replace a current transformer.

Fig:Current transforme

3. CAPACITIVE VOLTAGE TRANSFORMER (CVT):

Voltage transformer are required for the operation of many types of instrumentation and relay protective systems. They measure voltage in conjunction with CT , they measure power.

Purpose : -

- To step-down the high magnitude of voltage to safe value to incorporate measuring and protection logics.

A capacitor voltage transformer (CVT or CCVT), is a transformer used in power systems to step down extra high voltage signals and provide a low voltage signal, for metering or operating a protective relay. CVTs step-down the system voltage to sufficiently low value (110 V) for measuring, protection and synchronizing circuits. CVT has a H.F. terminal point for receiving & transmitting the high frequency signals for carrier protection and communication.

Components:-

In its most basic form, the device consists of three parts: two capacitors across which the transmission line signal is split, an inductive element to tune the device to the line frequency, and a voltage transformer to isolate and further step down the voltage for metering devices or protective relay.

The tuning of the divider to the line frequency makes the overall division ratio less sensitive to changes in the burden of the connected metering or protection devices.[1] The device has at least four terminals: a terminal for connection to the high voltage signal, a ground terminal, and two secondary terminals which connect to the instrumentation or protective relay.

In practice, capacitor C1 is often constructed as a stack of smaller capacitors connected in series. This provides a large voltage drop across C1 and a relatively small voltage drop across C2. As the majority of the voltage drop is on C1, this reduces the required insulation level of the voltage transformer. This makes CVTs more economical than the wound voltage transformers under high voltage (over 100kV), as the latter one requires more winding and materials.

"Capacitive voltage transformers exist and are used by utilities for high-voltage (greater than 66 kV) metering. They have a capacitive voltage divider but also have a dual-winding transformer to couple the divided voltage to the metering circuit. They tend to have lower allowable burdens than a wound transformer but can be made economically at higher voltage ratings. Another difference is that even though they decrease voltage, they do not increase current as found in wound electromagnetic transformers - an ampere drawn by the load is an ampere drawn from the primary circuit. And of course they can only reduce voltage, not increase".

The above is a part of a Wikipedia write-up. I might sum up the definition of a capacitor voltage transformer as a step down transformer with a convenient node of a series-connected capacitor network connected in series with the primary winding. The free end of the capacitor and the free end of the transformer primary constitute the primary terminals. This device is presently used as a potential transformer to monitor high voltages. Of course ordinary step down transformer does not employ series capacitor in the primary.

CVT Voltage Frequency Response:-

With the rated load at the voltage transformer secondary side, The output voltage of CVT initially decrease a little bit, then reaches the resonance peak at around 800 Hz. Then it decreases drastically and remains almost level out after 2000hz.

CVT Current Frequency Response:-

 The C2 current is linear with frequency. The frequency response for voltage transformer current has a resonance peak at around 800 Hz. C2 current is substantially larger than voltage transformer current.

Other Applications:-

The CVT is also useful in communication systems. CVTs in combination with wave traps are used for filtering high-frequency communication signals from power frequency. This forms a carrier communication network throughout the transmission network.

Fig:Current transformer

4. BUS & CLAMP FITTING:-

Bus bar is a thick strip of copper or aluminum that conducts electricity.

Bus bar are used to carry very large currents, or to distribute current to multiple devices with in switchgear or equipment.

In electric power distribution, a busbar (also bus bar, buss bar or bussbar) is a metallic strip or bar, typically housed inside switchgear, panel boards, and busway enclosures for local high current power distribution. They are also used to connect high voltage equipment at electrical switchyards, and low voltage equipment in battery banks. They are generally uninsulated, and have sufficient stiffness to be supported in air by insulated pillars. These features allow sufficient cooling of the conductors, and the ability to tap in at various points without creating a new joint.

Design and placement:-

Busbars are produced in a variety of shapes such as flat strips, solid bars and rods typically copper, brass or aluminium in solid or hollow tubes. Some of these shapes allow heat to dissipate more efficiently due to their high surface area to cross-sectional area ratio. The skin effect makes 50–60 Hz AC busbars more than about 8 millimetres (0.31 in) thickness inefficient, so hollow or flat shapes are prevalent in higher current applications. A hollow section also has higher stiffness than a solid rod of equivalent current-carrying capacity, which allows a greater span between busbar supports in outdoor electrical switchyards.

A busbar must be sufficiently rigid to support its own weight, and forces imposed by mechanical vibration and possibly earthquakes, as well as accumulated precipitation in outdoor exposures. In addition, thermal expansion from temperature changes induced by ohmic heating and ambient temperature variations, and magnetic forces induced by large currents must be considered.

Distribution boards split the electrical supply into separate circuits at one location. Busways, or bus ducts, are long busbars with a protective cover. Rather than branching from the main supply at one location, they allow new circuits to branch off anywhere along the route of the busway.

Fig:Bus bar

A busbar may either be supported on insulators, or else insulation may completely surround it. Busbars are protected from accidental contact either by a metal earthed enclosure or by elevation out of normal reach. Power neutral busbars may also be insulated because it is not guaranteed that the potential between power neutral and safety grounding is always zero. Earthing (safety grounding) busbars are typically bare and bolted directly onto any metal chassis of their enclosure. Busbars may be enclosed in a metal housing, in the form of bus duct or busway, segregated-phase bus, or isolated-phase bus.

Busbars may be connected to each other and to electrical apparatus by bolted, clamped, or welded connections. Often, joints between high-current bus sections have precisely-machined matching surfaces that are silver-plated to reduce the contact resistance. At extra high voltages (more than 300 kV) in outdoor buses, corona discharge around the connections becomes a source of radio-frequency interference and power loss, so special connection fittings designed for these voltages are used.

Advantage and Disadvantage of Bus Bar:

Bus bars reduce system costs, improve reliability, increase capacitance, and eliminate wiring errors. They also lower inductance and increase capacitance. Plus, the physical structure of bus bars offers unique features in mechanical design. For example, complete power distribution subsystems can also act as structural members of a total system. Multilayer bus bars offer a structural integrity that wiring methods just can’t match.

Bus Bars Reduce System Costs

A laminated bus bar will lower manufacturing costs by decreasing assembly time as well as internal material handling costs. Various conductors are terminated at customer specified locations to eliminate the guesswork usually associated with assembly operating procedures. A reduced parts count will reduce ordering, material handling and inventory costs.

Bus Bars Fabrication Allows Flexibility

Fabricated bus bars fit your specific needs and are customized for maximum efficiency.

Bus Bars Improve Reliability

Laminated bus bars can help your organization build quality into processes. The reduction of wiring errors results in fewer reworks, lower service costs and lower quality costs.

Laminated Bus Bars Increase Capacitance

Increased capacitance results in decreasing characteristic impedance. This will ultimately lead to greater effective signal suppression and noise elimination. Keeping the dielectrics thin and using dielectrics with a high relative K factor will increase capacitance.

Bus Bars Eliminate Wiring Errors

By replacing a standard cable harnesses with bus bars, the possibility for miss-wirings is eliminated. Wiring harnesses have high failure rates relative to bus bars, which have virtually none. These problems are very costly to repair. Adding bus bars to your systems is effective insurance.

Bus Bars Lower Inductance

Any conductor carrying current will develop an electromagnetic field. The use of thin parallel conductors with a thin dielectric laminated together minimizes the effect of inductance on electrical circuits. Magnetic flux cancellation is maximized when opposing potentials are laminated together. Laminated bus bars have been designed to reduce the proximity effect in many semiconductor applications as well as applications that involve high electromagnetic interference (EMI) such as SiC or GaN high frequency circuits.

Bus Bars Lower Impedance

Increasing the capacitance and reducing the inductance is a determining factor in eliminating noise. Keeping the dielectric thickness to a minimum will accomplish the highly desired low impedance.

Bus Bars Provide Denser Packaging

The use of wide, thin conductors laminated together led to decreased space requirements. Laminated bus bars have helped decrease total system size and cost.

Bus Bars Provide Wider Variety of Interconnection Methods

The flexibility of bus bars have allowed an unlimited number of interconnection styles to choose from. Bushings, embossments, and faston® tabs are most commonly used. Wire harnesses,

solderable connectors, and pressed-in fittings are also integrated into the design, making a bus bar compatible with virtually any type of interface.

Bus Bars Improve Thermal Characteristics

The wide, thin conductors are favorable to allowing better airflow in systems. As package sizes decrease, the cost of removing heat from systems is greatly increased. A bus bar not only reduces the overall size of system package required, but it can also improve airflow with its sleek design.

5. ISOLATER:-  

Isolator is a Disconnected switch and to be operated at no load.

 Isolator switch is used to ensure that an electrical circuit is completely de-energized for service or maintenance.

Interlocked with Breakers and Earth switch's.

Isolator is amechanical switch which isolates a part of circuit from system as when required. 

In electrical engineering, a disconnector, disconnect switch or isolator switch is used to ensure that an electrical circuit is completely de-energized for service or maintenance. Such switches are often found in electrical distribution and industrial applications, where machinery must have its source of driving power removed for adjustment or repair. High-voltage isolation switches are used in electrical substations to allow isolation of apparatus such as circuit breakers, transformers, and transmission lines, for maintenance. The disconnector is usually not intended for normal control of the circuit, but only for safety isolation. Disconnectors can be operated either manually or automatically.

Unlike load switches and circuit breakers, disconnectors lack a mechanism for suppression of electric arcs, which occurs when conductors carrying high currents are electrically interrupted. Thus, they are off-load devices, intended to be opened only after current has been interrupted by some other control device. Safety regulations of the utility must prevent any attempt to open the disconnector while it supplies a circuit. Standards in some countries for safety may require either local motor isolators or lockable overloads (which can be padlocked).

Disconnectors have provisions for a lockout-tagout so that inadvertent operation is not possible. In high-voltage or complex systems, these locks may be part of a trapped-key interlock system to ensure proper sequence of operation. In some designs, the isolator switch has the additional ability to earth the isolated circuit thereby providing additional safety. Such an arrangement would apply to circuits which inter-connect power distribution systems where both ends of the circuit need to be isolated.

A switch disconnector combines the properties of the disconnector and the load switch, so it provides the safety isolation function while being able to make and break nominal currents.

Fig:Isolator

Unlike a CB an IS has no protection capability  and is used to physically disconnect any circuit when repairs etc are being done. In a substation switchyard an IS switch would be used to physically disconnect any incoming HV lines to allow work on the transmission line to be performed.

Circuit breaker always trip the circuit but open contacts of breaker cannot be visible physically from outside of the breaker and that is why it is recommended not to touch any electrical circuit just by switching off the circuit breaker. So for better safety there must be some arrangement so that one can see open condition of the section of the circuit before touching it. Isolator is a mechanical switch which isolates a part of circuit from system as when required. Electrical isolators separate a part of the system from rest for safe maintenance works. So definition of isolator can be rewritten as Isolator is a manually operated mechanical switch which separates a part of the electrical power. Isolators are used to open a circuit under no load. Its main purpose is to isolate one portion of the circuit from the other and is not intended to be

opened while current is flowing in the line. Isolators are generally used on both ends of the breaker in order that repair or replacement of circuit breaker can be done without and danger.

An isolator differs from a switch in that it is intended to be opened when the circuit is not carrying current.An isolator is a device used for isolating a circuit or equipment from a source of power.An isolator is a mechanical switching device that, in the open position, allows for isolation of the input and output of a device.Types of Isolation:

The isolators depend upon the type and capacity of lines/location at which the substation is present. An isolator is just a disconnecting switch. So there are less complexities involved when increasing the voltage range (since the isolator unlike circuit breakers do not quench the arcing). Moreover these operate in off-load conditions - when the currents are zero.Depending on the line voltages - there may be low voltage(upto 11 kV), medium voltage(11kV to 66kV), high voltage (66kV to 330 kV) , extra high voltage (above 330 kV) lines which require different isolator designs. 

With the advent of sophisticated circuit breakers capable of isolating high power lines, use isolators is reduced to lower power lines where the need of isolation is not sufficiently met alone by circuit breakers.

Depending upon the application, single-break, double-break, bus and line isolators are used.

Double Break Isolator

Single Break Isolator

Pantograph type Isolator.

Depending upon the position in power system, the isolators can be categorized as

Bus side isolator – the isolator is directly connected with main bus

Line side isolator – the isolator is situated at line side of any feeder

Transfer bus side isolator – the isolator is directly connected with transfer bus.

Isolater Advantege & Disadvantage:

However, isolators have limited flexibility and their physical requirements can call for significant planning. The enclosed machinery in an isolator system needs to be highly reliable in order to avoid frequent maintenance and the isolator design should provide easy access through

glove ports or half-suits to enable adjustments during production. In addition, isolators used in continuous manual operations usually need to be configured to allow for worker comfort.

6. CIRCUIT BREAKER:-

Circuit breaker is an automatically-operated electrical switch designed to protect an electrical  Circuit from damage caused by overload of electricity or short circuit .

A circuit breaker function is to detect a fault condition and, by interrupting continuity, to

immediately discontinue electrical flow.

A circuit breaker is an automatically operated electrical switch designed to protect an electrical circuit from damage caused by excess current, typically resulting from an overload or short circuit. Its basic function is to interrupt current flow after a fault is detected. Unlike a fuse, which operates once and then must be replaced, a circuit breaker can be reset (either manually or automatically) to resume normal operation. Circuit breakers are made in varying sizes, from small devices that protect low-current circuits or individual household appliance, up to large switchgear designed to protect high voltage circuits feeding an entire city. The generic function of a circuit breaker, RCD or a fuse, as an automatic means of removing power from a faulty system is often abbreviated to ADS (Automatic Disconnection of Supply).

All circuit breaker systems have common features in their operation, but details vary substantially depending on the voltage class, current rating and type of the circuit breaker.

Electrical circuit breaker is a switching device which can be operated manually and automatically for controlling and protection of electrical power system respectively. As the modern power system deals with huge currents, the special attention should be given during designing of circuit breaker for safe interruption of arc produced during the operation of circuit breaker. This was the basic definition of circuit breaker.

The circuit breaker must firstly detect a fault condition. In small mains and low voltage circuit breakers, this is usually done within the device itself. Typically, the heating and/or magnetic effects of electric current are employed. Circuit breakers for large currents or high voltages are usually arranged with protective relay pilot devices to sense a fault condition and to operate the opening mechanism. These typically require a separate power source, such as a battery, although some high-voltage circuit breakers are self-contained with current transformers, protective relays, and an internal control power source.

Once a fault is detected, the circuit breaker contacts must open to interrupt the circuit; This is commonly done using mechanically stored energy contained within the breaker, such as a spring

or compressed air to separate the contacts. Circuit breakers may also use the higher current caused by the fault to separate the contacts, such as thermal expansion or a magnetic field. Small circuit breakers typically have a manual control lever to switch off the load or reset a tripped breaker, while larger units use solenoids to trip the mechanism, and electric motors to restore energy to the springs.

The circuit breaker contacts must carry the load current without excessive heating, and must also withstand the heat of the arc produced when interrupting (opening) the circuit. Contacts are made of copper or copper alloys, silver alloys and other highly conductive materials. Service life of the contacts is limited by the erosion of contact material due to arcing while interrupting the current. Miniature and molded-case circuit breakers are usually discarded when the contacts have worn, but power circuit breakers and high-voltage circuit breakers have replaceable contacts.

When a high current or voltage is interrupted, an arc is generated. The length of the arc is generally proportional to the voltage while the intensity (or heat) is proportional to the current. This arc must be contained, cooled and extinguished in a controlled way, so that the gap between the contacts can again withstand the voltage in the circuit. Different circuit breakers use vacuum, air, insulating gas, or oil as the medium the arc forms in. Different techniques are used to extinguish the arc including:

Lengthening or deflecting the arc Intensive cooling (in jet chambers) Division into partial arcs Zero point quenching (contacts open at the zero current time crossing of the AC waveform,

effectively breaking no load current at the time of opening. The zero crossing occurs at twice the line frequency; i.e., 100 times per second for 50 Hz and 120 times per second for 60 Hz AC.)

Connecting capacitors in parallel with contacts in DC circuits.

Finally, once the fault condition has been cleared, the contacts must again be closed to restore power to the interrupted circuit.

A substation is a part of an electrical generation, transmission, and distribution system. Substations transform voltage from high to low, or the reverse, or perform any of several other important functions. Between the generating station and consumer, electric power may flow through several substations at different voltage levels. A substation may include transformers to change voltage levels between high transmission voltages and lower distribution voltages, or at the interconnection of two different transmission voltages.

A circuit breaker is an automatically operated electrical switch designed to protect an electrical circuit from damage caused by overload or short circuit. Its basic function is to detect a fault condition and, by interrupting continuity, to immediately discontinue electrical flow. Unlike a fuse, which operates once and then has to be replaced, a circuit breaker can be reset (either manually or automatically) to resume normal operation

Fig: Circuit breaker

A Circuit breaker is used to protect an individual feeder or incoming line and normally trips when rated current is exceeded and may be reset.

It is an automatic device capable of making and breaking an Electrical Circuit under normal and abnormal conditions such as short circuits. SF6 is the arc quenching media for all the 400 KV and 220 KV breakers installed in the switchyard. Pneumatic operating system is provided in AEG, ABB, AREVA and NGEF make breakers and Hydraulic operating system is

provided in BHEL make breakers. 132KV breakers provided in 132 KV lines are of Minimum oil type operating on spring charge mechanism.

Working Principle of Circuit Breaker:-

The circuit breaker mainly consists of fixed contacts and moving contacts. In normal "ON" condition of circuit breaker, these two contacts are physically connected to each other due to applied mechanical pressure on the moving contacts. There is an arrangement stored potential energy in the operating mechanism of circuit breaker which is released if switching signal is given to the breaker. The potential energy can be stored in the circuit breaker by different ways like by deforming metal spring, by compressed air, or by hydraulic pressure. But whatever the source of potential energy, it must be released during operation. Release of potential energy makes sliding of the moving contact at extremely fast manner.

All circuit breaker have operating coils (tripping coils and close coil), whenever these coils are energized by switching pulse, and the plunger inside them displaced. This operating coil plunger is typically attached to the operating mechanism of circuit breaker, as a result the mechanically stored potential energy in the breaker mechanism is released in forms of kinetic energy, which makes the moving contact to move as these moving contacts mechanically attached through a gear lever arrangement with the operating mechanism. After a cycle of operation of circuit breaker the total stored energy is released and hence the potential energy again stored in the operating mechanism of circuit breaker by means of spring charging motor or air compressor or by any other means. Till now we have discussed about mechanical working principle of circuit breaker. But there are electrical characteristics of a circuit breaker which also should be considered in this discussion of operation of circuit breaker.

Let's have a discussion on electrical principle of circuit breaker. The circuit breaker has to carry large rated or fault power. Due to this large power there is always dangerously high arcing between moving contacts and fixed contact during operation of circuit breaker. Again as we discussed earlier the arc in circuit breaker can be quenching safely if the dielectric strength between the current carrying contacts of circuit breaker increases rapidly during every current zero crossing of the alternating current. The dielectric strength of the media in between contacts can be increased in numbers of ways, like by compressing the ionized arcing media since compressing accelerates the deionization process of the media, by cooling the arcing media since cooling increase the resistance of arcing path or by replacing the ionized arcing media by fresh gasses. Hence a numbers of arc quenching processes should be involved in operation of circuit breaker.

Types of Circuit Breaker

According different criteria there are different types of circuit breaker. According to their arc quenching media the circuit breaker can be divided as- According to their arc quenching media the circuit breaker can be divided as-

Oil circuit breaker.

Air circuit breaker.

SF6  circuit breaker .

Vacuum circuit breaker.

According to their services the circuit breaker can be divided as-

Outdoor circuit breaker

Indoor breaker.

According to the operating mechanism of circuit breaker they can be divided as-

Spring operated circuit breaker.

Pneumatic circuit breaker.

Hydrolic circuit breaker.

According to the voltage level of installation types of circuit breaker are referred as-

High voltage circuit breaker.

Medium voltage circuit breaker.

Low voltage circuit breaker.

7. WAVE TRAP :-

Wave traps are used for communication purpose during fault conditions.

It traps high frequency communication signals sent on the line from the remote substation and diverting to the telecom/telephone protection panel in the substation control room.

Wave Traps are used at sub-stations using Power Line Carrier Communication (PLCC). PLCC is used to transmit communication and control information at a high frequency over the power lines. This reduces need for a separate infra for

communication between sub-stations.

The Wave Traps extract the high frequency information from the power lines and route it to the telecomm panels. They also block any surges from passing through.

Wave Traps are simply resonant circuits that produce a high impedance against PLCC carrier frequencies (24kHz - Wave trap is used for communication purpose in substations..Why wave trap is located at entrance of substation?as communication waves are high frequency (and not power frequency) they will act as harmonics towards electrical equipment's like transformer,breaker etc...,so in order to protect them we should connect at entrance (after the CVT) so that wave trap will trap the communication waves.500kHz) while allowing power frequency (50Hz - 60Hz).

Since telephone communication system cannot be directly connected to a high voltage lines,suitably designed coupling devices have therefore to be employed. There usually consists of high voltage capacitors with potential devices used in conjunction with suitable line matching units (LMU's) for matching the impedance of line to that of coaxial cable connecting the unit to PLC to transmit -receive equipment.

Also the carrier currents used for communication have to be prevented from entering the power equipment used in G.S.S as this would result in high attenuation or even complete loss of communication signals when earthed at isolator. To prevent this loss, wave trap or line traps are employed. These consists of suitably designed choke coils connected in series with line, which offers negligible impedence to RF carrier current.

How it works:-Signals from main centers far away from the substations are sent which are captured by the routers placed in substations, these are high frequency signals much higher above the power frequency(in kHz), these signals are then trapped by a coupling circuit and are routed to the fields via underground cables.

They work at higher frequencies and  allows power frequency signals to pass through  and traps the communication signals and hence are named wave traps. The communication signals

captured and decoded contain signals like trip the CB, or change the tap changer or something of the kind.

Fig: Wave trap

Wave Traps are used at sub-stations using Power Line Carrier Communication (PLCC). PLCC is used to transmit communication and control information at a high frequency over the power lines. ... Wave trap is used for communication purpose insubstations.

Wave Trap is a parallel resonant circuit tuned to the carrier frequency connected in series with the line conductor at each end of the protected transmission line section. Wave trap offers high impedance path for high frequency signals and low impedance path for power frequency current. This keeps carrier signal confined to the protected line section and does not allow the carrier signals to flow into the neighboring sections.

Working of wave traps:-

It is connected series with transmission line. its used for communication purpose. It will block the high frequency signal (communication signal) and allow the low frequency (power).

Wave trap in electrical systems are used for power plant communication through transmission wires.It is a combination of low inductance in series and high capacitance in parallel with transmission wires in power frequencies. But communication signals are very low amplitude and very high frequency signals. So, these signals are nearly open circuited by the inductance and short circuited by the capacitance.That is how wave trap power and communication waves and used in PPC.

As communication waves are high frequency (and not power frequency) they will act as harmonics towards electrical equipment's like transformer,breaker etc...,so in order to protect them we should connect at entrance (after the CVT) so that wave trap will trap the communication waves.OrSignals from main centers far away from the substations are sent which are captured by the routers placed in substations, these are high frequency signals much higher above the power frequency(in kHz), these signals are then trapped by a coupling circuit and are routed to the fields via underground cables.

They work at higher frequencies and  allows power frequency signals to pass through  and traps the communication signals and hence are named wave traps. The communication signals captured and decoded contain signals like trip the CB, or change the tap changer or something of the kind.

Wave trap is used for trapping the high frequency communication signals sent on the line from the remote substation and diverting to the telecom/tele protection panel in the substation control room (through coupling capacitor and LMU). This is relevant in Power Line Carrier Communication (PLCC) systems for communication among various substations without dependence on the telecom company network. The signals are primarily tele protection signals and in addition, voice and data communication signals. The Line trap offers high impedance to the high frequency communication signals thus obstructs the flow of these signals in to the substation bus bars.

A line trap (high-frequency stopper) is a maintenance-free parallel resonant circuit, mounted inline on high-voltage (HV) AC transmission power lines to prevent the transmission of high frequency (40 kHz to 1000 kHz) carrier signals of power line communication to unwanted destinations. Line traps are cylinder-like structures connected in series with HV transmission lines. A line trap is also called a wave trap.

The line trap acts as a barrier or filter to prevent signal losses. The inductive reactance of the line trap presents a high reactance to high-frequency signals but a low reactance to mains frequency.

This prevents carrier signals from being dissipated in the substation or in a tap line or branch of the main transmission path and grounds in the case of anything happening outside of the carrier transmission path. The line trap is also used to attenuate the shunting effects of high-voltage lines.

8. EARTHING SWITCH:-

Earth switch is used to discharge the voltage on the circuit to the earth for safety. Earth switch is mounted on the frame of the isolators. Earth switch is located for each incomer transmission line and each side of the bus –bar

section.

In electricity supply systems, an earthing system defines the electrical potential of the conductors relative to that of the Earth's conductive surface. The choice of earthing system has implications for the safety and electromagnetic compatibility of the power supply.

Earth switch is mounted on the isolator base on the line side or breaker side depending upon the position of the isolator. The earth switch usually comprises of a vertical break switch arm with the contact, which engages with the isolator contact on the line side. Earth switch is required to discharge the trapped charges on the line or equipment (under shut own) to earth for maintaining safety. Earth switch can be operated only from local either by electrical operation or manually.

Fig: Earth switch

Earthing switch is used to discharge the charges that are trapped in line after opening of line by circuit breaker.

This charges are discharge to earth by the earthing switch.

 Once a circuit has been isolated it is common to physically earth the dead end to ensure that any cable or transmission line capacitance does not charge the line due to possibly nearby feeders or transmission lines or in case anyone inadvertently closes the CB and or IS. Like an IS it has no protection function and cannot trip If someone erroneously switches back the IS and CB it will create a dead short through the earth switch and trip the CB and protect people working on the line. n an electrical installation or an electricity supply system an earthing system or grounding system connects specific parts of that installation with the Earth's conductive surface for safety and functional purposes. The point of reference is the Earth's conductive surface, or on ships, the surface of the sea. The choice of earthing system can affect the safety and electromagnetic compatibility of the installation. Regulations for earthing systems vary considerably among countries and among different parts of electrical systems, though many follow the recommendations of the International Electrotechnical Commission which are described below.

This article only concerns grounding for electrical power. Examples of other earthing systems are listed below with links to articles:

To protect a structure from lightning strike, directing the lightning through the earthing system and into the ground rod rather than passing through the structure.

As part of a single-wire earth return power and signal lines, such as were used for low wattage power delivery and for telegraph lines.

In radio, as a ground plane for large monopole antenna. As ancillary voltage balance for other kinds of radio antennas, such as dipoles. As the feed-point of a ground dipole antenna for VLF and ELF radio.

Functional earthing:A functional earth connection serves a purpose other than electrical safety, and may carry current as part of normal operation.[1] The most important example of a functional earth is the neutral in an electrical supply system when it is a current-carrying conductor connected to the earth electrode at the source of electrical power. Other examples of devices that use functional earth connections include surge suppressors and electromagnetic interference filters.

 The use of earth switches is to send any trapped charge in the line into ground after the line is switched off.Mainly what happens, when the line is switched off, some charges remain trapped in line due to their own capacity and also due to influence of side by X-line. This trapped charge is dangerous if someone goes for maintenance of that very line. So it is very much required to neutralise that much charge and here comes the use of earth switches.

 

9. POWER TRANSFORMER:-

Power transformers  are used for transmission as a step up devices so that the I2r loss can be minimized for a given power flow. These transformers are designed to utilize the core to maximum and will operate very much near to the knee point of B-H curve (slightly above the knee point value).This brings down the mass of the core enormously.

Power transformers are used in transmission network of higher voltages for step-up and step down application (400 kV, 200 kV, 110 kV, 66 kV, 33kV) and are generally rated above 200MVA.

Transformer Size / Insulation Level:

 Power transformer is used for the  transmission purpose at heavy load, high voltage greater than 33 KV & 100% efficiency. It also having a big in size as compare to distribution transformer, it used in generating station and Transmission substation .high insulation level.

Iron Losses and Copper Losses:

 Power transformerare used in Transmission network so they do not directly connect to the consumers, so load fluctuations are very less. These are loaded fully during 24 hr’s a day, so Cu losses & Fe losses takes place throughout day the specific weight i.e. (iron weight)/(cu weight) is very less.

The average loads are nearer to full loaded or full load and these are designed in such a way that maximum efficiency at full load condition. These are independent of time so in calculating the efficiency only power basis is enough

.Power transformer are used in Distribution Network so directly connected to the consumer so load fluctuations are very high. these are not loaded fully at all time so iron losses takes place 24hr a day and cu losses takes place based on load cycle. the specific weight is more i.e. (iron weight)/(cu weight).average loads are about only 75% of full load and these are designed in such a way that max efficiency occurs at 75% of full load.

Power transformers   have the matched iron losses and copper losses at peak load (i.e. the maximum efficiency point where both the losses match) are used for transmission as a step up devices so that the I2r loss can be minimized for a given power flow. These transformers are designed to utilize the core to maximum and will operate very much near to the knee point of B-H curve (slightly above the knee point value).This brings down the mass of the core enormously.

Naturally these transformers have the matched iron losses and copper losses at peak This brings down the mass of the core enormously.Naturally these transformers

Naturally these transformers have the matched iron losses and copper losses at peak load (i.e. the maximum efficiency point where both the losses match). Power transformer generally operated at full load. Hence, it is designed such that copper losses are minimal. 

Fig: Power transfoemer

A transformer is an electrical device that transfers electrical energy between two or more circuits through electromagnetic induction. A varying current in one coil of the transformer produces a varying magnetic field, which in turn induces a voltage in a second coil. Power can be transferred between the two coils through the magnetic field, without a metallic connection between the two circuits. Faraday's law of induction discovered in 1831 described this effect. Transformers are used to increase or decrease the alternating voltages in electric power applications.

Since the invention of the first constant-potential transformer in 1885, transformers have become essential for the transmission, distribution, and utilization of alternating current electrical energy. A wide range of transformer designs is encountered in electronic and electric power

applications. Transformers range in size from RF transformers less than a cubic centimeter in volume to units interconnecting the power grid weighing hundreds of tons.

Use of Power Transformer:-

Generation of electrical power in low voltage level is very much cost effective. Theoretically, this low voltage level power can be transmitted to the receiving end. This low voltage power if transmitted results in greater line current which indeed causes more line lossesBut if the voltage level of a power is increased, the current of the power is reduced which causes reduction in ohmic or I2R losses in the system, reduction in cross sectional area of the conductor i.e. reduction in capital cost of the system and it also improves the voltage regulation of the system. Because of these, low level power must be stepped up for efficient electrical power transmission. This is done by step up transformer at the sending side of the power system network. As this high voltage power may not be distributed to the consumers directly, this must be stepped down to the desired level at the receiving end with the help of step down transformer. Electrical power transformer thus plays a vital role in power transmission.Two winding transformers are generally used where ratio of high voltage and low voltage is greater than 2. It is cost effective to use auto transformer where the ratio between high voltage and low voltage is less than 2. Again a single unit three phase transformer is more cost effective than a bank of three single phase transformers unit in a three phase system. But a single three phase transformer unit is a bit difficult to transport and have to be removed from service entirely if one of the phase winding breaks down.

Types of Transformer:-

Transformers can be categorized in different ways, depending upon their purpose, use, construction etc. The types of transformer are as follows,

Step Up Transformer and Step Down Transformer - Generally used for stepping up and down the voltage level of power in transmission and distribution power system network.

Three Phase Transformer and Single Phase Transformer - Former is generally used in three phase power system as it is cost effective than later. But when size matters, it is preferable to use a bank of three single phase transformer as it is easier to transport than one single three phase transformer unit.

Electrical Power Transformer, Distribution Transformer and Instrument Transformer - Power transformers are generally used in transmission network for stepping up or down the voltage level. It operates mainly during high or peak loads and has maximum efficiency at or near full load. Distribution transformer steps down the voltage for distribution purpose to domestic or commercial users. It has good voltage regulation and operates 24 hrs a day with

maximum efficiency at 50% of full load. Instrument transformers include C.T and P.T which are used to reduce high voltages and current to lesser values which can be measured by conventional instruments.

Two Winding Transformer and Auto Transformer - Former is generally used where ratio between high voltage and low voltage is greater than 2. It is cost effective to use later where the ratio between high voltage and low voltage is less than 2.

Outdoor Transformer and Indoor Transformer - Transformers that are designed for installing at outdoor are outdoor transformers and transformers designed for installing at indoor are indoor transformers.

Oil Cooled and Dry Type Transformer - In oil cooled transformer the cooling medium is transformer oil whereas the dry type transformer is air cooled.

Core type, Shell type and Berry type transformer - In core type transformer it has two vertical legs or limbs with two horizontal sections named yoke. Core is rectangular in shape with a common magnetic circuit. Cylindrical coils (HV and LV) are placed on both the limbs.Shell type transformer: It has a central limb and two outer limbs. Both HV, LV coils are placed on the central limb. Double magnetic circuit is present.Berry type transformer: The core looks like spokes of wheels. Tightly fitted metal sheet tanks are used for housing this type of transformer with transformer oil filled inside.