44476804-500KV-Grid-station

PROJECT REPORT ON

500 KV GRID STATION N.T.D.C, MULTAN

SUBJECT TITLE: ELECTRICAL POWER TRANSMISSION SYSTEM PRESENTED TO: Engr. Tauheed Ur Rehman PRESENTED BY: Muhammad Arslan Yousaf

2008-EE-45

DEPARTMENT OF ELECTRICAL ENGINEERING, BAHAUDDIN ZAKARIYA UNIVERSITY, MULTAN

TABLE OF CONTENTS Content

Page

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Acknowledgements

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Executive Summary

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Introduction to N.T.D.C

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Grid Station

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National Grid System of Pakistan

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Functions of the Grid Station

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Incoming and Outgoing Circuits of the 500 KV Grid Station, Multan

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Essential Elements of the 500 KV Grid Station, Multan

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References

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Shunt Reactors

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Power Line Carrier Line Traps

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CCVT’s

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Surge Arresters

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Earthing System

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Circuit Breakers

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Paging System of The Grid Station

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Air Plant System

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Isolator Switch

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Current Transformer

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Power Transformers

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Fire Protection System

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Relays

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SF6 Plant System

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Bus Bars & Bus Couplers

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Insulators

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Switch Board & Control Room

ACKNOWLEDGEMENTS

First of all I will like to thank Allah, who blessed me with ability and wisdom to complete this project. I wish to express our heartiest gratitude to our lecturer, Engr. Tauheed Ur Rehman who was abundantly helpful and offered invaluable assistance, support and guidance. And especially thanks to N.T.D.C organization who provided me an opportunity of enhancing my professional experience and her members and especially workers so that they have cooperated with me. It needs volumes to write about the personality of Mr. Muhammad Hanif Memon, But to put into nutshell, he is a unique man who waged a unique struggle in the establishment of a unique project. I wish to express my love and gratitude to my family and friends; for their understanding & endless love, and help they provide while preparing this report.

EXECUTIVE SUMMARY

An electrical grid station is an interconnection point between two transmission ring circuits, often between two geographic regions. They might have a transformer, depending on the possibly different voltages, so that the voltage levels can be adjusted as needed. Grid station regulates and controls the power between interconnected transmission lines to increase the reliability of the power system. It receive power from the power station at extremely high voltage and then convert these voltage to some low levels and supplied electric power to the sub stations or to other grid stations at the same voltage level according to the requirements. National grid system of Pakistan contains an interconnected group of transmission lines in a ring system. It covers most of the power stations of the country in this single ring and supplied electric power to the different areas of the country. Main function of the grid station is switching between the connected line stations and the load centers. This report comprises on the basics of the 500KV grid station. It includes the functions and necessary information about the elements of the 500 KV grid station, NTDC, Multan.

National Transmission and Dispatch Company (NTDC): National Transmission & Dispatch Company (NTDC) Limited was incorporated on 6th November, 1998 and commenced commercial operation on 24th December, 1998. It was organized to take over all the properties, rights and assets obligations and liabilities of 220 KV and 500KV Grid Stations and Transmission Lines/Network owned by Pakistan Water and Power Development Authority (WAPDA).The NTDC operates and maintains nine 500 KV Grid Stations, 4160 km of 500 KV transmission line and 4000 km of 220 KV transmission line in Pakistan.

Grid and the Sub Station: An electrical power substation is a conversion point between transmission level voltages (such as 138Kv) and distribution level voltages (such as 11Kv). A substation has one or more step-down transformers and serves a regional area such as part of a city or neighborhood. Substations are connected to each other by the transmission ring circuit system by equipments. An electrical grid station is an interconnection point between two transmission ring circuits, often between two geographic regions. They might have a transformer, depending on the possibly different voltages, so that the voltage levels can be adjusted as needed. The interconnected network of sub stations is called the grid, and may ultimately represent an entire multi-state region. In this configuration, loss of a small section, such as loss of a power station, does not impact the grid as a whole, nor does it impact the more localized neighborhoods, as the grid simply shifts its power flow to compensate, giving the power station operator the opportunity to effect repairs without having a blackout.

National Grid System of Pakistan: Electricity is generated at a voltage level of 11 KV at the largest hydral power station (Tarbela) of the Pakistan and it steps up to the voltage level of up to 500 KV by using a unit transformer. A complex distributed network of 500 KV transmission lines are present in the Pakistan (from Peshawar to Karachi), the output of the unit transformer is given to these lines which then supplied this power to all of the country with the help of their interconnected network of transmission and distribution lines. In summer season, ice is reached in the Tarbela and Mangla’s reservoirs after melting from northern areas. So in this season there is enough water for the production of required electrical power and the generated electrical power is travel from Tarbela to Karachi side. But in winter season, situation is opposite to the above. Water is not

enough to produce a required power, so the capacity of Tarbela station is somewhat reduced and to compensate this reduced energy, the flow of electric power through the interconnected network is changes its direction toward Tarbela from Karachi instead towards Karachi. There is many station in our country but we consider only those have voltage level in between 220 and 500 KV. In National grid system of Pakistan, several power stations are connected in a ring system and they supplied electric power to different areas of the country under the supervision of WAPDA. All stations are transmitting their produced power to transmission line and from the ring main system; all regional grids supplied power to their own areas. By connecting several power stations into a single ring system, the system stability is increased.

Advantages of the Grid System  

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Any time electricity is available for the consumers at lower cost. Flow of electrical energy is continuous and sure. It is possible to fulfill the emergency demand of power. Better regulation of the voltages. Improved power factor It is possible to govern the generator according to the load. Safe transmission system. Reduced fault timings. Controlled frequency range.

Disadvantages of the Grid System Cost of the control system is increased and their maintenance is complicated. Power system is affected from the environmental factors. This system is unsafe during the war. Extended system is going to complexity. Due to the expensive equipments, additional load occurred on the consumers. During short circuit condition it is impossible to maintain the continuity of power. High initial and maintenance cost. During load shedding, capacity of industries connected with the grid is reduced which cause to industrial development problem.  For maintenance, qualified staff is required and for that reason our country has to spend more money to call expert engineers from other countries.        

Locations of the Interconnected 500 KV Grid Stations of Pakistan

Functions of a Grid Station: A Grid Station has the following functions… 1 - Supply of required electrical power. 2 - Maximum possible coverage of the supply network. 3 - Maximum security of supply. 4 - Shortest possible fault-duration. 5 - Optimum efficiency of plants and the network. 6 - Supply of electrical power within targeted frequency limits, (49.5 Hz and 50.5 Hz). 7 - Supply of electrical power within specified voltage limits. 8 - Supply of electrical energy to the consumers at the lowest cost. An important function performed by a grid station is switching, which is the connecting and disconnecting of transmission lines or other components to and from the system. Switching events may be "planned" or "unplanned". A transmission line or other component may need to be de energized for maintenance or for new construction; for example, adding or removing a transmission line or a transformer. To maintain reliability of supply, no company ever brings down its whole system for maintenance. All work to be performed, from routine testing to adding entirely new substations, must be done while keeping the whole system running. Perhaps more importantly, a fault may develop in a transmission line or any other component. Some examples of this: a line is hit by lightning and develops an arc, or a tower is blown down by a high wind. The function of the grid station is to isolate the faulted portion of the system in the shortest possible time. There are two main reasons: a fault tends to cause equipment damage; and it tends to destabilize the whole system. For example, a transmission line left in a faulted condition will eventually burn down, and similarly, a transformer left in a faulted condition will eventually blow up. While these are happening, the power drain makes the system more unstable. Disconnecting the faulted component, quickly, tends to minimize both of these problems

Incoming and outgoing circuits of the 500 KV grid station, NTDC, Multan: Grid input At the generating plants the energy is produced at a relatively low voltage between about 2.3 kV and 30 kV, depending on the size of the unit. The generator terminal voltage is then stepped up by the power station transformer to a higher voltage (115 kV to 765 kV AC, varying by country)

for transmission over long distances. Grid station receives power from the generating stations that are located too far from the grid at an extremely high voltage. 500KV grid station, NTDC Multan receives power at the voltage level of 500 KV from the following generating stations…     

Guddu Power station (2 Circuit) Muzaffar Garh power Station (1 Circuit) Rousch Power Station (1 Circuit) Gatti Grid Station (1 Circuit) Yousafwala Grid Station (1 Circuit)

Transmission Grid Exit In the grid station the voltages are somewhat decreased by using the step down transformers and then power is supplied to the sub stations. At the substations, transformers reduce the voltage to a lower level for distribution to commercial and residential users. This distribution is accomplished with a combination of subtransmission (33 kV to 132 kV) and distribution (3.3 to 25 kV). Finally, at the point of use, the energy is transformed to low voltage 220V or 440V. 500KV grid station, NTDC Multan has delivers/receive power at a voltage level of 220KV to or from the following stations… Kot Addu Power Station (4 Circuit) Muzaffar Garh power Station (2 Circuit) Vehari Substation (2 Circuit)  N.G.P.S Multan (2 Circuit)  Nishatabad (2 Circuit)  Additional (2 Circuit)   

Essential Elements of the 500 KV Grid Station

Shunt Reactor:

Transmission cables have much higher capacitance to earth than overhead lines. Long submarine cables for system voltages of 100 KV and more need shunt reactors. The same goes for large urban networks to prevent excessive voltage rise when a high load suddenly falls out due to a failure. Shunt reactors contain the same components as power transformers, like windings, core, tank, bushings and insulating oil and are suitable for manufacturing in transformer factories. The main difference is the reactor core limbs, which have non-magnetic gaps inserted between packets of core steel. To stabilize the line voltage the line inductance can be compensated by means of series capacitors and the line capacitance to earth by shunt reactors. Series capacitors are placed at different places along the line while shunt reactors are often installed in the stations at the ends of line. In this way, the voltage difference between the ends of the line is reduced both in amplitude and in phase angle. In this situation, the capacitance to earth draws a current through the line, which may be capacitive. When a capacitive current flows through the line inductance there will be a voltage rise along the line. 3-phase reactors can also be made. These may have 3- or -5-limbed cores. In a 3-limbed core there is strong magnetic coupling between the three phases, while in a 5-limbed core the phases are magnetically independent due to the enclosing magnetic frame formed by the two yokes and the two unwound side-limbs. The neutral of shunt reactor may be directly earthed, earthed through an Earthing-reactor or unearthed.

When the reactor neutral is directly earthed, the winding are normally designed with graded insulation in the earthed end. The main terminal is at the middle of the limb height, & the winding consists of two parallel-connected halves, one below & one above the main terminal. The insulation distance to the yokes can then be made relatively small. Sometimes a small extra winding for local electricity supply is inserted between the main winding & yoke. When energized the gaps are exposed to large pulsation compressive forced with a frequency of twice the frequency of the system voltage. The peak value of these forces may easily amount to 106 N/m2 (100 ton /m2). For this reason the design of the core must be very solid, & the modulus of elasticity of the non-magnetic (& non-metallic) material used in gaps must be high (small compression) in order to avoid large vibration amplitudes with high sound level consequently. The material in the gaps must also be stable to avoid escalating vibration amplitudes in the end. Testing of reactors requires capacitive power in the test field equal to the nominal power

In AC networks, shunt reactors and series reactors are widely used in the system to limit the overvoltage or to limit the short-circuit current. With more high-voltage overhead lines for long transmission distance and increasing network capacity, both types of reactors play an important role in the modern network system. For extra-high-voltage (EHV) transmission lines, due to the long distance, the space between the overhead line and the ground naturally forms a capacitor parallel to the transmission line, which causes an increase of voltage along the distance. Depending on the distance, the profile of the line and the power being transmitted, a shunt reactor is necessary either at the line terminals or in the middle. The advanced design and production technology will ensure the product has low loss and low noise level. The need for large shunt reactors appeared when long power transmission lines for system voltage 220 kV & higher were built. The characteristic parameters of a line are the series inductance (due to the magnetic field around the conductors) & the shunt capacitance (due to the electrostatic field to earth). An equivalent diagram for a line is show in the figure below Both the inductance & the capacitance are distributed along the length of the line. So are the series resistance and the admittance to earth. When the line is loaded, there is a voltage drop along the line due to the series inductance and the series resistance. When the line is energized but not loaded or only loaded with a small current, there is a voltage rise along the line (the Ferranti-effect) Shunt reactors carry out different types of tasks: They compensate the capacitive reactive power of the transmission cables, in particular in networks with only light loads or no load.  They reduce system-frequency overvoltage when a sudden load drop occurs or there is no load.  They improve the stability and efficiency of the energy transmission. 

Made for every requirement: Our oil-filled shunt reactors are manufactured in two versions: With an iron core divided by air gaps Without an iron core, with a magnetic return circuit. Shunt reactors offer individual solutions: They satisfy all the specified requirements regarding voltage, rating, type of operation, low-noise and low-loss levels, connection method and type of cooling, as well as transportation and installation. The windings, insulation, tank, monitoring devices and connection method are practically the same as those found in the construction of transformers. However, shunt reactors have some special features with regard to their design and their mastery of certain physical

properties .Oil-filled shunt reactors are generally made with ONAN cooling systems and, for high ratings also with ONAF cooling systems.

Power line carrier line Traps: Power line carrier communication (PLCC) is mainly used for telecommunication, teleprotection and tele-monitoring between electrical substations through power lines at high voltage, such as 110 kV, 220 kV, and 400 kV. PLCC integrates the transmission of communication signal and 50/60 Hz power signal through the same electric power cable. The major benefit is the union of two important applications in a single system. In a PLCC system the communication is established through the power line. The audio frequency is carried by a carrier frequency and the range of carrier frequency is from 50 kHz to 500 kHz. The modulation generally used in this system is amplitude modulation. The carrier frequency range is allocated to include the audio signal, protection and the pilot frequency. The pilot frequency is a signal in the audio range that is transmitted continuously for failure detection. The voice signal is converted/compressed into the 300 Hz to 4000 Hz range, and this audio frequency is mixed with the carrier frequency. The carrier frequency is again filtered, amplified and transmitted. The transmission of these HF carrier frequencies will be in the range of 0 to +32db. This range is set according to the distance between substations. PLCC can be used for interconnecting PBXs. The electricity board in India has an internal network PLCC between PBXs.

Transmitting information along high- voltage lines (PLC) has been one of the main and certainly the most economic means of communication in electric power systems for more than 50 years.

The purpose of PLC line traps    

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Provision of defined high voltage line impedances regardless of the configuration of the primary system switchgear. Prevention of signal losses due to propagation into other lines. Attenuation of RF signals from other parts of the power system, thus permitting multiple uses of the same frequency bands. PLC line traps are connected in series with the high-tension lines and must therefore be rated for the maximum continuous load current and be able to withstand the maxi- mum fault current at the place of Installation. DLTC line traps fulfill all the RF requirements as well as all the power system requirements of the latest IEC and ANSI recommendations.

Main advantages of PLC line traps  

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All versions available for either pedestal mounting or suspended installation. Wide choice of pedestals suitable for mounting on insulator posts, coupling capacitors and instrument transformers. Provision for nearly every type of conductor terminal. Solid construction permits high mechanical loads on terminals. High voltage withstand of tuning units ensures high reliability. Tuning units are tuned for either broadband blocking or damped single frequency blocking. Transient overvoltage protection by metal oxide arresters with better characteristics than arcgap arresters. Only arresters with a rating of 10 kA are used. Integrates optimally in the overall PLC network, because ABB not only supplies the line traps, but everything else for PLC systems. Quality assurance according to ISO 9001. Modern production techniques ensure consistent product quality. Backed by 50 years of experience in the design, manufacture, and operation of PLC line traps

Capacitance Coupled Voltage Transformer: A capacitor voltage transformer (CVT), or capacitance coupled voltage transformer (CCVT) is a transformer used in power systems to step down extra high voltage signals and provide a low voltage signal, for measurement or to operate a protective relay. 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 transformer to isolate and further step down the voltage for the instrumentation or protective relay. 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. CVTs are typically single-phase devices used for measuring voltages in excess of one hundred kilovolts where the use of voltage transformers would be uneconomical. 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. 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. Capacitance Coupled Voltage Transformer

Surge Arresters: Each piece of electrical equipment in an electrical system needs to be protected from voltage surges. To prevent damage to electrical equipment, surge protection considerations are paramount to a well- designed electrical system. Modern metal oxide arresters provide exceptional overvoltage protection of equipment connected to the power system. The proper selection and application of the arrester, however, involves decisions in several areas, which will be discussed in the paper. The original lightning arrester was nothing more than a spark air gap with one side connected to a line conductor and the other side connected to earth ground. When the line-to-ground voltage reached the spark-over level, the voltage surge would be discharged to earth ground. The modern metal oxide arrester provides both excellent protective characteristics and temporary overvoltage capability. The metal oxide disks maintain a stable characteristic and sufficient non-linearity and do not require series gaps. Due to the broad nature of this subject, this paper will concentrate on the application of the gapless metal oxide arrester to circuits and systems rated 1000 V and greater. 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.

Arrester Selection The objective of arrester application is to select the lowest rated surge arrester which will provide adequate overall protection of the equipment insulation and have a satisfactory service life when connected to the power system. The arrester with the minimum rating is preferred because it provides the greatest margin of protection for the insulation. A higher rated arrester increases the ability of the arrester to survive on the power system, but reduces the protective margin it provides for a specific insulation level. Both arrester survival and equipment protection must be considered in arrester selection. The proper selection and application of lightning arresters in a system involve decisions in three areas: 1. Selecting the arrester voltage rating. This decision is based on whether or not the system is grounded and the method of system grounding. 2. Selecting the class of arrester. In general there are three classes of arresters. In order of protection, capability and cost, the classes are: Station class, Intermediate class and Distribution class. The station class arrester has the best protection capability and is the most expensive. 3. Determine where the arrester should be physically locate Arrester Classes The class of lightning arrester to be applied depends upon the importance and value of the protected equipment, its impulse insulation level and the expected discharge currents the arrester must withstand. Station class arresters are designed for protection of equipment that may be exposed to significant energy due to line switching surges and at locations where significant fault current is available. They have superior electrical performance because their energy absorption capabilities are greater, the discharge voltages (protective levels) are lower and the pressure relief is greater. The value of the protected equipment and the importance of uninterrupted service generally warrant the use of station class arresters throughout their voltage range. Industry standards dictate the use of both station class and intermediate class arresters for equipment protection in the 5-to 20- MVA size ranges. Above 20 MVA, station class arresters are predominately used. Intermediate class arresters are designed to provide economic and reliable protection of medium voltage class power equipment. Intermediate arresters are an excellent

choice for the protection of dry-type transformers, for use in switching and sectionalizing equipment and for the protection of URD cables. Traditional applications include equipment protection in the range of 1 to 20 MVA for sub stations and rotating machines. Distribution class arresters are frequently used for smaller liquid-filled and dry-type transformers 1000 KVA and less. These arresters can also be used, if available in the proper voltage rating, for application at the terminals of rotating machines below 1000 KVA. The distribution arrester is often used out on exposed lines that are directly connected to rotating machines. All of the system parameters need to be considered while choosing an arrester classification. If the actual arrester energy duties are not known and a transient study cannot be performed, then it is suggested that Station class arresters be applied. This is a conservative approach that reduces the chances of misapplication

Earthing System: In electrical engineering, ground or earth may be the reference point in an electrical circuit from which other voltages are measured, or a common return path for electric current, or a direct physical connection to the Earth. Electrical circuits may be connected to ground (earth) for several reasons. In mains powered equipment, exposed metal parts are connected to ground to prevent contact with a dangerous voltage if electrical insulation fails. Connections to ground limit the build-up of static electricity when handling flammable products or when repairing electronic devices. In some telegraph and power transmission circuits, the earth itself can be used as one conductor of the circuit, saving the cost of installing a separate return conductor. For measurement purposes, the Earth serves as a (reasonably) constant potential reference against which other potentials can be measured. An electrical ground system should have an appropriate current-carrying capability in order to serve as an adequate zero-voltage reference level. In electronic circuit theory, a "ground" is usually idealized as an infinite source or sink for charge, which can absorb an unlimited amount of current without changing its potential. Where a real ground connection has a significant resistance, the approximation of zero potential is no longer valid. Stray voltages or earth potential rise effects will occur, which may create noise in signals or if large enough will produce an electric shock hazard. The use of the term ground (or earth) is so common in electrical and electronics applications that circuits in portable electronic devices such as cell phones and media players as well as circuits in vehicles such as ships, aircraft, and spacecraft may be spoken of as having a "ground" connection without any actual connection to the Earth. This is usually a large conductor attached to one side of the power supply (such as the "ground plane" on a printed circuit board) which serves as the common return path for current from many different components in the circuit.

Requirement of Good Earthing Good earth should have low resistance  It should stabilize circuit potential with respect to ground and limit overall potential rise.  It should protect men material from injury or damage due to over voltage.  It should provide low impedance path to fault currents to ensure prompt and consistent operation of protective relays, Surge arrester etc.  It should keep maximum potential gradient along the surface of the sub- station within safe limits during ground fault. Factors Influence the Condition of Earth The following factors in the earth should be maintained within the limit irrespective of seasons so that the earth should fulfill the above requirements. Factors     

Kind of Soil – Soil resistivity Moisture Content Salt Content Condition of Electrode Temperature Co-efficient

Classification of Earthing Earthing can be classified into the following categories based on the purpose for which the part of the equipment connected to the general mass of earth. System Earthing Equipment Earthing Reference Earthing Discharge Earthing

System Earthing Earthing associated with current carrying parts of the equipment is called system Earthing. The system security, reliability, performance, voltage stabilization, all relied only on the system Earthing. Earthing Neutral of Transformer, Surge arrester Earthing are its examples. System Earthing Methods  Solid Earthing  Resistance Earthing  Reactance Earthing  Through Grounding Transformer Earthing Practices Transmission lines E.H. T. lines: For 110 KV lines one aerial earth wire through the towers and for 230 KV lines and two earth wires are run. As per I.S. code, the aerial ground wire is to be connected to earth at least in 4 towers in every mile.  H. T. Lines: These towers (each) are earthed through earth pipes. The earth rods are driven at the base of the tower if the earth resistance is less than 15 Ohms. If it is not possible, two rods are driven within} a distance of 200feet, where in again the resistance is not to exceed 25 Ohms.  L.T. lines: All stay wires are provided with guy shackles at a height not less than3 meters from the ground. The cross arms are also earthed at specified intervals. 

Major sub-stations Earthing of equipment’s in the major sub-stations is taken much care. The various Earthing are discussed below. Power transformers The transformer body or tank is directly connected to earth grid. In addition, there should be direct connection from the tank to the earth side of the lightning arresters.  The transformer track rail should be earthed separately. 

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The neutral bushing is earthed by a separate connection to the earth grid. Clearer to the tank sell and collars.

Potential and current transformers The bases of the CTs and Pts. are to be earthed. All bolted cover plates of the bushing are also to be connected the earth grid. Lightning arresters The bases of the Lightning arresters are to be earthed with conductors as short and straight as Possible (for reducing impedance). The earth side of the Lightning arresters is tube connected directly the equipment to be protected. Each Lightning arrester should have individual earth rods, which are in turn connected to earth grid. Circuit breakers The supporting structures, C.T. chambers, P.T. tanks, Cable glands etc., are to be connected to earth

Circuit Breakers: 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. Circuit breakers are made in varying sizes, from small devices that protect an individual household appliance up to large switchgear designed to protect high voltage circuits feeding an entire city. Operation All circuit breakers have common features in their operation, although details vary substantially depending on the voltage class, current rating and type of the circuit breaker. The circuit breaker must detect a fault condition; in low-voltage circuit breakers this is usually done within the breaker enclosure. Circuit breakers for large currents or high voltages are usually arranged with pilot devices to sense a fault current and to operate the trip opening mechanism. The trip solenoid that releases the latch is usually energized by a separate battery, although some high-voltage circuit breakers are self-contained with current transformers, protection relays, and an internal control power source. Once a fault is detected, contacts within the circuit breaker must open to interrupt the circuit; some mechanically-stored energy (using something such as springs or compressed air) contained

within the breaker is used to separate the contacts, although some of the energy required may be obtained from the fault current itself. Small circuit breakers may be manually operated; larger units have 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 the circuit. Contacts are made of copper or copper alloys, silver alloys, and other materials. Service life of the contacts is limited by the erosion due to interrupting the arc. Miniature and molded case circuit breakers are usually discarded when the contacts are worn, but power circuit breakers and high-voltage circuit breakers have replaceable contacts. When a current is interrupted, an arc is generated. 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 in which the arc forms. Different techniques are used to extinguish the arc including:  Lengthening of 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 50Hz and 120 times per second for 60Hz 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. Arc interruption Miniature low-voltage circuit breakers use air alone to extinguish the arc. Larger ratings will have metal plates or non-metallic arc chutes to divide and cool the arc. Magnetic blowout coils deflect the arc into the arc chute. In larger ratings, oil circuit breakers rely upon vaporization of some of the oil to blast a jet of oil through the arc. Gas (usually sulfur hexafluoride) circuit breakers sometimes stretch the arc using a magnetic field, and then rely upon the strength of the sulfur hexafluoride (SF6) to quench the stretched arc. Vacuum circuit breakers have minimal arcing (as there is nothing to ionize other than the contact material), so the arc quenches when it is stretched a very small amount (