“STUDY ELECTRICAL DESIGN OF A 220 KVSUBSTATION” PROJECT WORK UNDER THE ESTEEMED GUIDANCE OF Mr. S.D. Guatam Asst. Manager (technical) 220 KV P.P.K. I “Delhi Transco Limited” SUBMITTED BY: Ishank bounthiyal C.R.R.INSTITUTE OF TECNOLOGY KANJHAWALA DELHI 1 CERTIFICATE This is to certify that Mr. ISHANK BOUNTHIYAL (roll no. 305119), students of diploma in (instrumentation & control) of C.R.R.INSTITUTE OF TECNOLOGY has successfully completed their project work at DELHI TRANCO LIMITED on “Study Electrical Design Of a 220 KV Substation” under the guidance of Mr. S.D. Guatam.The students have performed all the related activities during 02/07/2009 to 18/07/2009 i.e of their project duration. Mr. S.D. Guatam Asst. Manager (technical) (DELHI TRANSCO LIMITED) 2 ACKNOWLEDGEMENT I express my gratitude to the management of “Delhi Transco Limited” for providing me with this opportunity to undergo training in this esteemed organization. I take the prerogative to express my gratitude to Mr.A.Guruswami , Asst. manager(technical), for his valuable suggestions and guidance throughout my training period. I also like to thank the entire staff of “Delhi Transco Limited” for making my brief stay in Substation a memorable one. Ishank bounthiya ( roll no 305119) Chotu ram rural Institute of Engineering 3 Delhi Transco Limited is the State Transmission Utility of the National Capital Territory of Delhi. It is responsible for transmission of power at 220KV and 400KV level, besides upgradation operation and maintenance of EHV Network as per system requirements. After the enactment of Electricity Act 2003, a new department under the name and style of State Load Dispatch Center (SLDC) under Delhi Transco Limited was created, as an Apex body to ensure integrated operation of the power system in Delhi. SLDC Delhi started its function on the First of January 2004. SLDC is responsible for the real time Load Dispatch function, O&M of SCADA System and Energy Accounting. 4 Electricity in India Electric power generation in India is done predominantly by government sector entities. These are controlled by various central public sector corporations, such as: National Hydroelectric Power Corporation, National Thermal Power Corporation and various state level corporations (state electricity boards - SEBs). The transmission and distribution is managed by the State Electricity Boards (SEBs) or private companies. The current per capita power consumption is about 612 KWH per year while the world average is 2596 KWH. Generation Grand Total Installed Capacity is 1,44,564.97 MW Thermal Power Current installed base of Thermal Power is 92,216.64 MW which comes to 64.6% of total installed base. • • • Current installed base of Coal Based Thermal Power is 76,298.88 MW which comes to 53.3% of total installed base. Current installed base of Gas Based Thermal Power is 14,716.01 MW which comes to 10.5% of total installed base. Current installed base of Oil Based Thermal Power is 1,201.75 MW which comes to 0.9% of total installed base. 5 Nuclear Power Currently.76 MW which comes to 24.9% of total installed base).00 MW (2.7% of total installed base.033. Renewable Power Current installed base of Renewable Power is 12194.Hydro Power India was one of the pioneering states in establishing hydro-electric power plants.57 MW which comes to 7.7% of total installed base. Current installed base of Hydro Power is 36. 6 . The power plant at Darjeeling and Shimsa was established in 1898 and 1902 respectively and is one of the first in Asia. seventeen nuclear power reactors produce 4.120. Today Hydro sector has turbines as large as 250 MW and single stage projects as big as 1500 MW . namely. certain pockets in the power system could not safely operate even under normal conditions. had traditionally been linked to generation projects as part of the evacuation system. The transmission system planning in the country. The entire country has been divided into five regions for transmission systems. Transmission planning has therefore moved away from the earlier generation evacuation system planning to integrated system planning. in the past. While the predominant technology for electricity transmission and distribution has been Alternating Current (AC) technology. Eastern Region. generally of 132kV and above.Transmission Transmission of electricity is defined as bulk transfer of power over a long distance at high voltage.000ckm today. due to various reasons such as spatial development of load in the network. However. The Interconnected transmission system within each region is also called the regional grid. High Voltage Direct Current (HVDC) technology has also been used for interconnection of all regional grids across the country and for bulk transmission of power over long distances. North Eastern Region. Southern Region and Western Region. non-commissioning of load centre generating units originally planned and deficit in reactive compensation. This had necessitated backing down of generation and operating at a lower load generation balance in the past. Ability of the power system to safely withstand a contingency without generation rescheduling or load-shedding was the main criteria for planning the transmission system. Northern Region. In India bulk transmission has increased form 3708ckm in 1950 to more than 265. 7 . due to lack of adequate investment on T&D works. the T&D losses have been consistently on higher side. Apart from an extensive transmission system network at 500kV HVDC. Distribution The total installed generating capacity in the country is over 135000MW and the total number of consumers is over 144 million. 400kV. AT&C loss captures technical as well as commercial losses in the network and is a true indicator of total losses in the system. However. 132kV and 66kV which has developed to transmit the power from generating station to the grid substations. and reached to the level of 32. which has resulted in unplanned extensions of the distribution lines. High technical losses in the system are primarily due to inadequate investments over the years for system improvement works. 220kV. 8 .. overloading of the system elements like transformers and conductors. a vast network of sub transmission in distribution system has also come up for utilization of the power by the ultimate consumers.The reduction of these losses was essential to bring economic viability to the State Utilities. and lack of adequate reactive power support. concept of Aggregate Technical and Commercial (AT&C) loss was introduced.86% in the year 2000-01. As the T&D loss was not able to capture all the losses in the net work. 220 KV Substation 9 . substation. The size of the busbar is important in determining the maximum amount of current that can be safely carried.BUS BAR A bus bar in electrical power distribution refers to thick strips of copper or aluminium that conduct electricity within a switchboard. so hollow or flat shapes are prevalent in higher current applications. Busbars can have a cross-sectional area of as little as 10 mm² but electrical substations may use metal tubes of 50 mm in diameter (1. Neutral busbars may also be insulated. A hollow section has higher stiffness than a solid rod. segregated-phase bus. A busbar may either be supported on insulators. Often joints between high-current bus sections have matching surfaces that are silver-plated to reduce the contact resistance. Busbars are typically either flat strips or hollow tubes as these shapes allow heat to dissipate more efficiently due to their high surface area to cross-sectional area ratio. corona around the connections becomes a source of 10 . or other electrical apparatus.000 mm²) or more as busbars. which allows a greater span between busbar supports in outdoor switchyards. distribution board. Busbars may be enclosed in a metal housing. in the form of bus duct or busway. or isolated-phase bus. At extra-high voltages (more than 300 kV) in outdoor buses. The skin effect makes 5060 Hz AC busbars more than about 8 mm (1/3 in) thick inefficient. Busbars may be connected to each other and to electrical apparatus by bolted or clamp connections. Busbars are protected from accidental contact either by a metal enclosure or by elevation out of normal reach. Earth busbars are typically bolted directly onto any metal chassis of their enclosure. or else insulation may completely surround it. called through faults.radio-frequency interference and power loss. selective and should be stable for external faults. 11 . A busbar must have its own protection although their high degrees of reliability bearing in mind the risk of unnecessary trips. so the protection should be dependable. so connection fittings designed for these voltages are used. Protection Bus bars are vital parts of a power system and so a fault should be cleared as fast as possible. BUS BAR SCHEMES Bus scheme selection criteria System reliability Possibility of major shut-down Continuity of supply in the event of bus fault Availability of bus in the event of stuck of circuit breaker Redundancy Bus switching scheme Operation flexibility Taking a line / equipment in or out Taking a bus bar in or out Taking a circuit breaker in or out Ease of maintenance Taking out of components for maintenance without loss of feeder and with ease of changeover Limitation of short circuit level Simplicity of protection arrangement Ease of extension Availability / requirement of land Cost 12 . Normally load will be distributed on both the buses and the bus coupler will kept closed. 13 . For maintenance & extension of any one of the buses the entire load will be transferred to the other bus. On load bypassing of any circuit for breaker maintenance is not possible.Double bus scheme Most commonly used bus scheme. On load transfer of a circuit from one bus to the other bus is possible through bus isolators provided the bus coupler is closed and thereby two buses are at the same potential. by electromagnetic induction. PARTS OF A POWER TRANSFORMER : ACTIVE PART: 1) core 2) windings (LV. transforms a system of alternating voltage and current into another system of voltage and current usually of different values and at the same frequency for the purpose of transmitting electrical power. tertiery) 3)tap changer 4)cleats and leads 5)tank ACCESSORIES: 14 .HV.regualting.TRANSFORMER Definition : Transformer is defined as A static piece of apparatus with two or more windings which. 1) radiators AUXILLARIES: 1)bushings 2)buchholz relay/oil surg relay 3)temprature indicators 4)oil level indicator 5)pressure relief device 6)marshalling box/control cubical 7)oil preservation system 8)conservators(gas sealed .bellows/membrane sealed) 9)silica gel breather 15 . POWER TRANSFORMER (100MVA) 16 . this ensures that the magnetic field lines produced by the primary current stay within the iron — instead of "leaking" out into the surrounding air — and pass intact through the secondary coil.BASIC PRINCIPLE The transformer is based on two principles: first. that a changing magnetic field within a coil of wire induces a voltage across the ends of the coil (electromagnetic induction). one changes the strength of its magnetic field. such as iron. By changing the current in the primary coil. The primary and secondary coils are wrapped around a core of very high magnetic permeability. second. a voltage is induced across the secondary A current passing through the primary coil creates an electromagnet. the current and its magnetic field are proportional to one another. since the secondary coil is wrapped around the same magnetic field. so that if the current changes. that an electric current can produce a magnetic field (electromagnetism) and. 17 . so does the magnetic field. sometimes termed iron loss. The losses vary with load current.• INDUCTION LAWS The amount of voltage induced across the secondary coil may be calculated from Faraday's law of induction. and may furthermore be expressed as "no- 18 . sometimes termed copper loss. If the turns of the coil are oriented perpendicular to the magnetic field lines. where VS is the instantaneous voltage. the flux is defined as the product of the magnetic field strength B and the area A through which it cuts Since the same magnetic flux passes through both the primary and secondary coils in an ideal transformer. NS is the number of turns in the secondary coil and Φ equals the total magnetic flux through one turn of the coil. the instantaneous voltage across the primary winding equals Taking the ratio of the two equations for VS and VP gives the basic equation for stepping up or stepping down the voltage • ENERGY LOSSES IN TRANSFORMER Transformer losses are attributable to several causes and may be differentiated between those originating in the windings. which states that. and those arising from the magnetic circuit. and lending impetus to development of low-loss transformers Losses in transformer arise due to: I) Winding Resistance(copper loss) ii) Hysteresis losses (core loss) iii) Eddy current loss iv)Magnetostriction V) Mechanical losses vi) Stray losses • CONSTRUCTION Laminated steel core Transformers for use at power or audio frequencies typically have cores made of high permeability silicon steel. Winding resistance dominates load losses. The universal transformer equation indicates a minimum crosssectional area for the core to avoid saturation 19 .Later designs constructed the core by stacking layers of thin steel laminations. or at an intermediate loading. The steel has a permeability many times that of free space. meaning that even an idle transformer constitutes a drain on an electrical supply. and confine the flux to a path which closely couples the windings. and the core thus serves to greatly reduce the magnetising current. Each lamination is insulated from its neighbors by a thin non-conducting layer of insulation.load" or "full-load" loss. whereas hysteresis and eddy currents losses contribute to over 99% of the no-load loss. The no-load loss can be significant.. a principle that has remained in use. using this technique the the magnetic core is equivalent to many small magnetic circuits.hence provides a more resistive path for eddy current.hence low core loss and also high permeability.each one reciving a small fraction of the magnetic flux. • For eddy current A small addition of silicon upto 3% increases the resistivity of iron core by 3-4 times. The effect of laminations is to confine eddy currents to highly elliptical paths that enclose little flux.furthermore these circuits have a resistance higher than the non magnetic core because of their reduced section.the main advantages of using CRGO is that it has a smaller hysterisis loop.the addition of silicon in iron increases its magnetic properties.The core loss in transformer caused due to eddy current and hysterisis can be delt with by utilizing the following: • For the hysterisis The transformer cores are made up of COLD ROLLED GRAIN OREINTED STEEL(CRGO) . but in all cases the individual turns must be electrically insulated from 20 . and so reduce their magnitud • WINDINGS The conducting material used for the windings depends upon the application. the magnetic cores are made of thin isulated iron sheets. also. in which currents are low and the potential difference between adjacent turns is small. the windings may be arranged in a way to minimise leakage inductance and stray capacitance to improve high-frequency response. This is known as a stacked type or interleaved winding. For small power and signal transformers. This can be done by splitting up each coil into sections.each other to ensure that the current travels throughout every turn. The primary and secondary windings are arraged in such a way as to reduce the flux lekage. the coils are often wound from enamelled magnet wire. and those sections placed in layers between the sections of the other winding. windings are arraged concentrically to minimize flux lekage. For signal transformers. Larger power transformers operating at high voltages may be wound with copper rectangular strip conductors insulated by oilimpregnated paper and blocks of pressboard. • TRANSFORMER OIL 21 . 22 . the application of a vacuum. To improve cooling of large power transformers. These are safety devices that can sense gas buildup inside the transformer (a side effect of corona or an electric arc inside the windings) and then switch off the transformer.Transformer oil is usually a highly-refined mineral oil that is stable at high temperatures and has excellent electrical insulating properties Following are the functions of transformer oil:i) to insulate the windings ii) to supress corona iii) to supress arching iv) to serve as coolant The oil helps cool the transformer. or both to ensure that the transformer is completely free of water vapor before the cooling oil is introduced. transformer oil must remain stable at high temperatures over an extended period. Very large or high-power transformers (with capacities of millions of watts) may also have cooling fans. oil pumps. which perform a similar function as the Buchholz relay. Transformers without conservators are usually equipped with sudden pressure relays. using electrical self-heating. the oilfilled tank may have external radiators through which the oil circulates by natural convection. Oil filled transformers with conservators (an oil tank above the transformer) tend to be equipped with Buchholz relays. Large. high-voltage transformers undergo prolonged drying processes. and even oil-to-water heat exchangers. Because it also provides part of the electrical insulation between internal live parts. This helps prevent corona formation and subsequent electrical breakdown under load. where the added expense of a fireresistant liquid offsets additional building cost for a transformer vault. nontoxic. polychlorinated biphenyl (PCB) was often used as a dielectric fluid since it was not flammable • TRANSFORMER OIL SAMPLING AND TESTING The life of a transformer is dependent upon three parametes : 1.oxygen 3. temprature 2.the test given below are : 1. to Naphthenic mineral oil too.dielectric strength of oil EFFECT OF MOISTURE ON TRANSFORMER OIL 23 . readily biodegradable. stable silicone-based or fluorinated hydrocarbons are used. and have higher flash points than mineral oil.BDV(break down voltage test) 2. Prior to about 1970.Oil alternatives : Today.DGA(dissolved gas analysis) 3. Esters are non toxic. Natural or synthetic Esters are becoming increasingly common as alternative.Generally we use minral oil for transfomer winding insulation and cooling .moisture it is necessary to remove the moisture form the transformer after regular interval of time there are a few tests to determine the quality of oil . its effects and the preventive measures. SOURCE OF MOISTURE Once in service the transformer is subjected to the following source of moisture External form the atmosphere Internal form manufacture Internal form cellulose ageing 1)BDV(breakdown voltage )TESTING: 24 .Most of the power transformwers use paper and oil as the main form of insulation and during manufacture stringent efforts are made to ensure that both are as dry as possible when the new plant is set up.once in service the moisture content begins to increase.excessive can put the life of the transformer at risk .it is important to understand the source of this moisture . it is based on magnetic effect.8kv.we test the property in the BDV.we use spherical electrode in this test as per IEC STANDARDS.the BDV should be approximate around 60 kv for transformer oil.there will be a spark flashing at this breakdown • Note down the reading of BDV . Value by this experiment for UAT(unit auxillary transformer) -2 is found -52.DIELECTRIC STRENGTH OF MINRAL OIL using as a coolant and insulator for transformer winding.take 5 more reading • This advanced kit is well programmed to give printed average value of all 6 observation.it is necessary to mixed up the oil suitably.CYLINDRICAL. 25 .it increases gradually. these electrode can be one of the type as:SPHERICAL.5mm for the special electrode . • At a perticular value of voltage oil break down. • After 5 min stat applying voltage between electrodes .the procedure of this experiment is given s followes:• This test is performed by advanced test kit specially design fopr this test • In this kit we use special bucket which contains electrode. • First of all wash every active component of the kit with the transformer oil for removing moisture and dust contents from the elctrodes and buckets . • In this kit we have special facility to steering the oil automatically. • Set the gap between the elctrode at exactly 2.OR MUSHROOM. • Set 5 min time duration for steering.after taking the sample of transformer oil with suitable equipment. such as a slow oil leak. This flow of oil operates a switch attached to a vane located in the path of the moving oil. Buchholz relays have a test port to allow accumulated gas to be withdrawn for testing. and oil flows rapidly into the conservator. On a slow accumulation of gas. A float operated switch in the relay is used to initiate an alarm signal. due perhaps to slight overload. This switch normally will operate a circuit breaker to shut down (isolate) the apparatus before the fault causes additional damage. Flammable gas found in the relay indicates some internal fault such as 26 . is a safety device mounted on some oil-filled power transformers and reactors.BUCHHOLZ RELAY In the field of electric power distribution and transmission. equipped with an external overhead oil reservoir called a conservator. This same switch will also operate on low oil level. gas is generated. The Buchholz Relay is used on conservator type oil preservation systems as a protective device sensitive to events which occurs during dielectric failure inside the equipment. gas accumulates in the top of the relay and forces the oil level down. gas accumulation is rapid. If an arc forms. a Buchholz relay. The relay has two different detection modes. When an electric arc or overheating develops inside the coils. also called a gas relay or a sudden pressure relay. whereas air found in the relay may only indicate low oil level or a leak. Buchholz relays have been applied to large power transformers at least since the 1940.overheating or arcing. 27 . hence it pushes the air inside the consevator outwards. 28 . when the volume of oil in the transformer increases the oil expansds and moves into the consevators .SILICA GEL BREATHER breather provide an economic and efficient means of controlling the level of moisture entering electrical equipment during the change in volume of the cooling medium and/or airspace caused by temperature gradients.therefore the excess of air is absorbed by the breather at the time of expansion and gives of the air when the oil contracts.as the level of moisture is to be controlled inside the transformer . Description The bushing is hollow. and may be coated with a semi-conducting glaze to assist in equalizing the electrical stress along the length of the bushing.BUSHINGS A bushing is a transformer component that insulates a high voltage conductor passing through a metal enclosure. Different types of bushings: i) oil impregnated paper(OIP) ii)epoxy resin impregnated paper(ERIP) Bushings with varity of external insulations are: i)porceline in brown and grey 29 . circuit breakers and other high voltage equipment. Bushings appear on switchgear. Bushings are often made of wet-process fired porcelain. The use of polymer bushings for high voltage applications is becoming more common.. The inside of the bushing may contain paper insulation and the bushing is often filled with oil to provide additional insulation. Bushings for medium-voltage and low-voltage apparatus may be made of resins reinforced with paper. allowing a conductor to pass along its centre and connect at both ends to other equipment. transformers. Their construction allows them to interrupt fault currents of many hundreds or thousands of amps. A piece of switchgear may be a simple open air isolator or it may be insulated by some other substance. and other communication equipment allowing for intelligent control of the substation. medium-voltage (~15kV) circuit breakers feeding the distribution system. fuses and/or circuit breakers used to isolate electrical equipment. An effective although more costly form of switchgear is "gas insulated switchgear" (GIS). Also contained inside these Power Control Centers are various relays. Circuit breakers are a special type of switchgear that are able to interrupt fault currents.SWITCHYARD EQUIPMENTS: The term switchgear. The switchgear located on the low voltage side of the transformers in distribution type substations. Another common type is oil insulated switchgear. • Lightning arrester 30 . meters. or grid. Typically switchgear in substations is located on both the high voltage and the low voltage side of large power transformers. refers to the combination of electrical disconnects. where the conductors and contacts are insulated by pressurized (SF6)sulfur hexafluoride gas. Inside this building are typically smaller. used in association with the electric power system. Switchgear is used both to de-energize equipment to allow work to be done and to clear faults downstream. now are typically located in what is called a Power Distribution Center (PDC). • • • • • • • • Wire end cables Capacitive voltage transformer(CVT) isolators current transformers(CT) circuit breakers bus-bar lines wave trap Battery Box LIGHTNING ARRESTER 31 . but a very low resistance to extremely high voltage. Inside the porcelain housing of an arrester.silicon carbide Silicon carbide has variable resistance at. After the surge is passed. Hence the LA provides a low resistance path in case of high voltage surge. Lightning arresters are generally located on both the high and low side of a substation transformer to protect it from strikes coming in either direction. 32 .Lightning arrester is a device which is provided in the switch yard to protect the equipments from lightning. When lightning strikes there is a sudden rapid rise in voltage. The silicon carbide resistance breaks down. It has a very high resistance to comparatively low-voltage. allowing the current to be conducted to ground. Material used: . Silicone carbide has an unusual electrical characteristic. It has very high resistance at low voltage and very low resistance at very high voltage. Lightning arresters are designed to safely channel a lightning strike to ground without damaging equipment. the resistance of the blocks increases back to normal levels. are a series of spark gaps plus one or more silicon carbide blocks. WIRE CABLES Wire cables are use to transport the high voltage transmission lines underground in case of some physical obstruction. CAPACITIVE VOLTAGE TRANSFORMER 33 .Outer insulation of LA is made up of porcelain. A capacitor voltage transformer (CVT) is a transformer used in power systems to step-down extra high voltage signals and provide low voltage signals either for measurement or to operate a protective relay. a high-voltage terminal for connection to the high voltage signal. a ground terminal and at least one set of secondary terminals for connection to the instrumentation or protective relay. 34 . In practice the first capacitor. CVTs are typically single-phase devices used for measuring voltages in excess of one hundred kilovolts where the use of voltage transducers would be uneconomical. DESIGN: In its most basic form the device consists of three parts: • two capacitors across which the voltage signal is split • an inductive element used to tune the device to the supply frequency • a transformer used to isolate and further step-down the voltage for the instrumentation or protective relay The device has at least four terminals. CVT SPECIFICATIONS: 35 . due to short circuit in transformer secondary. This results in a large voltage drop across the stack of capacitors that replaced the first capacitor and a comparatively small voltage drop across the second capacitor. C2. The capacitor units operate is a complete pressure free mode over a very wide ambient temperature range.a stainless steal diaphragm (expansion bellows) is installed to preserve the integrity of oil by maintaining the hermetic seal while allowing the thermal expansion and contraction of the oil. tuning reactor. HERMETIC SEALING SYSTEM: Each capacitor unit is hermetically sealed . is often replaced by a stack of capacitors connected in series. and hence the secondary terminals. RF choke. PROTECTION : A protective surge arrester/spark gap shall preferably be provided to prevent break down of insulation by incoming surges and to limit abnormal rise of terminal voltage of shunt capacitor. etc. The details of this arrangement (or alternative arrangement) shall be furnished by Contractor for Employer's review.C1. The base tank is filled with degassed mineral oil hermetically sealed from the environment and from the synthetic oil in the capacitor unit. Total burden - 100 MVA Thermal burden – 750 VA Rated voltage – 220/ 3^.5 kV Highest system voltage – 245 kV Insulation level – 460 kv/1050 kV Rated freq 50 Hz - Primary capacitance (c1) – 4881 pf Secondary capacitance (c2) – 47186 pf CURRENT TRANSFORMER 36 . The CT acts as a constant-current series device with an apparent power burden a fraction of that of the high voltage primary circuit. with a secondary of many hundreds of turns. a 4000:5 CT would provide an output current of 5 amperes when the primary was passing 4000 amperes. with five taps being common for multi ratio CTs Current transformers can be used to supply information for measuring power flows and the electrical inputs for the operation of protective relays associated with the transmission and distribution circuits or for power transformers. The current transformer safely isolates measurement and control circuitry from the high voltages typically present on the circuit being measured.A current transformer (CT) is a type of instrument transformer designed to provide a current in its secondary winding proportional to the current flowing in its primary. often in the presence of high voltages. The CT's primary circuit therefore consists of a single 'turn' of conductor. They are commonly used in metering and protective relaying in the electrical power industry where they facilitate the safe measurement of large currents. Hence the primary circuit is largely unaffected by the insertion of the CT. The secondary winding can be single ratio or multi ratio. These current transformers have the primary winding connected in series with the 37 . DESIGN: The most common design of CT consists of a length of wire wrapped many times around an annular silicon steel ring passed over the circuit being measured. Common secondaries are 1 or 5 amperes. For example. conductor carrying the current to be measured or controlled. The secondary winding is insulate from the high voltage and can then be connected to low-voltage metering circuits. CT SPECIFICATIONS: Highest system: Insulation level: Rated primary current: Rated STC: Freq: CT ratio: 245v 460/1050 kV 600 A 27 Ka for 1 sec 50 Hz 600-300/1/1/1/1 CIRCUIT BREAKERS 38 . Thermal or thermal-magnetic operation. a circuit breaker can be reset (either manually or automatically) to resume normal operation. which operates once and then has to be replaced. Examples of high-voltage AC circuit breakers are: 39 .commercial and light industrial applications at low voltage 1. MCCB (Moulded Case Circuit Breaker)—rated current up to 1000 A. TYPES OF CIRCUIT BREAKERS : There are many different technologies used in circuit breakers and they do not always fall into distinct categories. Trip current may be adjustable. Unlike a fuse. from small devices that protect an individual household appliance up to large switchgear designed to protect high voltage circuits feeding an entire city. Thermal or thermal-magnetic operation 2. Types that are common in domestic. Circuit breakers are made in varying sizes. MCB (Miniature Circuit Breaker)—rated current not more than 100 A. Trip characteristics normally not adjustable. Electric power systems require the breaking of higher currents at higher voltages.A circuit breaker is an automatically-operated electrical switch designed to protect an electrical circuit from damage caused by overload or short circuit. 1. Oil circuit breaker water type circuit breakers air circuit breakers vaccum circuit breakers 5. 4. SF6 circuit breakers 40 . 3. 2. Operating Range Of Different Circuit Breaker DIELECTRIC STERNGTH OF CIRCUIT BREAKERS VACUUM CIRCUIT BREAKERS: 41 . glass envelope fecilitates the examining of the breakers from outside after operation. vacume of the order of 10^-5 Hg is maintained . which corresponds roughly to the medium-voltage range of power systems.The main idea behind the VCB is to eliminate the medium of contact s. These can only be practically applied for voltages up to about 35.000 V. Vacuum circuit breakers tend to have longer life expectancies between overhaul than do air circuit breakers. these breakers interrupt the current by creating and extinguishing the arc in a vacuum container.If it becomes milky white from orignal finish of silver thenit is a sign of loosing vacuum.in such a low pressure the elctron crosses the gap without any collision .arc is formed by neutral atom.after the envelope there is a sputter sheild made up of stainless steel to 42 .ions and electron emitted from the electrode themselves. Vacuum circuit breaker—With rated current up to 3000 A. CONSTRUCTION: The outer envelope is made up of glass which is joined with metallic end caps . .prevent trhe metal vapour reaching the envelope .In side there is a moving and fixed contact the metalic bellows are made up of stainless steel. so that it is able to withstand the transient recovery voltage that is applied across the contacts after current interruption.000 K to less than 2000 K in a few hundred microseconds. such as sulfur hexafluoride (SF6). Sulfur hexafluoride is 43 . Gas blast applied on the arc must be able to cool it rapidly so that gas temperature between the contacts is reduced from 20. After contact separation. SF6 CIRCUIT BREAKERS: Current interruption in a high-voltage circuit-breaker is obtained by separating two contacts in a medium. having excellent dielectrical and arc quenching properties. current is carried through an arc and is interrupted when this arc is cooled by a gas blast of sufficient intensity. 15 kg/cm2 Rated duration of short circuit current. Characteristics of SF6 circuit breakers can explain their success: 1. Possible compact solutions when used for GIS or hybrid switchgear.40 kA Rated oprating pressure. allowing at least 25 years of operation without reconditioning. 8. 3. 4. 5. SF6 specifications: Rated lighting impulse-1050 kvp(peak voltage) Rated short circuit current.40kA. Low noise levels. The possibility to obtain the highest performance. 9. High electrical endurance. Integrated closing resistors or synchronised operations to reduce switching overvoltages. 7. with a reduced number of interrupting chambers.21 kg Rated voltage-245 kv 44 . 2.generally used in present high-voltage circuit-breakers (of rated voltage higher than 52 kV). Reliability and availability. 3 sec(fault level) Gas weight. Autonomy provided by the puffer technique.5 cycles. Simplicity of the interrupting chamber which does not need an auxiliary breaking chamber. 6. Short break time of 2 to 2. up to 63 kA. Rated freq-50 Hz Rated normal current – 3150 A Rated closing voltage – 220 V DC Rated opening voltage – 220 V DC Gas pressure – 6 kg/cm2 Relays: Automotive style miniature relay A relay is an electrical switch that opens and closes under the control of another electrical circuit. a form of an electrical amplifier. the switch is operated by an electromagnet to open or close one or many sets of contacts. The coil current can be on or off 45 . Current flowing through the coil of the relay creates a magnetic field which attracts a lever and changes the switch contacts. In the original form. it can be considered to be. It was invented by Joseph Henry in 1835. in a broad sense. A relay is an electrically operated switch. Because a relay is able to control an output circuit of higher power than the input circuit. Most ICs (chips) cannot provide this current and a transistor is usually used to amplify the small IC current to the larger value required for the relay coil. To prevent damage you must connect a protection diode across the relay coil. the link is magnetic and mechanical. but it can be as much as 100mA for relays designed to operate from lower voltages. The coil of a relay passes a relatively large current. Relay coils produce brief high voltage 'spikes' when they are switched off and this can destroy transistors and ICs in the circuit. For further information about switch contacts and the terms used to describe them please see the page on switches. Relays allow one circuit to switch a second circuit which can be completely separate from the first. for example relays with 4 sets of changeover contacts are readily available. The maximum output current for the popular 555 timer IC is 200mA so these devices can supply relay coils directly without amplification. The coil will be obvious and it may be connected either way round. Relays are usuallly SPDT or DPDT but they can have many more sets of switch contacts. Most relays are designed for PCB mounting but you can solder wires directly to the pins providing you take care to avoid melting the plastic case of the relay. 46 . There is no electrical connection inside the relay between the two circuits. The supplier's catalogue should show you the relay's connections. For example a low voltage battery circuit can use a relay to switch a 230V AC mains circuit.so relays have two switch positions and they are double throw (changeover) switches. typically 30mA for a 12V relay. 47 . this is to reduce noise. the armature is returned by a force approximately half as strong as the magnetic force to its relaxed position. Usually this is a spring. In a low voltage application. In a high voltage or high current application. the resulting magnetic field attracts an armature that is mechanically linked to a moving contact. When the current to the coil is switched off. The movement either makes or breaks a connection with a fixed contact. Most relays are manufactured to operate quickly. but gravity is also used commonly in industrial motor starters. this is to reduce arcing.Operation When a current flows through the coil. If the coil is designed to be energized with AC. This "shading ring" creates a small out-of-phase current. a diode is frequently installed across the coil. to dissipate the energy from the collapsing magnetic field at deactivation. to control a high-current circuit with a low-current signal. consisting of a capacitor and resistor in series. as in the starter solenoid of an automobile.[1] Applications Relays are used: • • • to control a high-voltage circuit with a low-voltage signal. a small copper ring can be crimped to the end of the solenoid. which increases the minimum pull on the armature during the AC cycle. Some automotive relays already include that diode inside the relay case.If the coil is energized with DC. which would otherwise generate a spike of voltage and might cause damage to circuit components. may absorb the surge. Alternatively a contact protection network. to detect and isolate faults on transmission and distribution lines by opening and closing circuit breakers (protection relays). as in some types of modems or audio amplifiers. 48 . They may also be controlled by room occupancy detectors in an effort to conserve energy. The time period can be varied by increasing or decreasing the flow rate. For a slightly longer (up to a minute) delay. for example when controlling a mains-powered device from a low-voltage switch. a dashpot is used. A very short (a fraction of a second) delay would use a copper disk between the armature and moving blade assembly. When the relay operates.A DPDT AC coil relay with "ice cube" packaging • to isolate the controlling circuit from the controlled circuit when the two are at different potentials. connected to a current transformer and calibrated to operate at or above a specific current level. Overcurrent relay An "Overcurrent Relay" is a type of protective relay which operates when the load current exceeds a preset value. In a typical application the overcurrent relay is used for overcurrent protection. Relays can be modified to delay opening or delay closing a set of contacts. which may be often moved as needs change. A dashpot is a piston filled with fluid that is allowed to escape slowly. Current flowing in the disk maintains magnetic field for a short time. lengthening release time. one or more contacts will operate and energize a trip coil in a Circuit Breaker and trip (open) the Circuit Breaker. a mechanical clockwork timer is installed. For longer time periods. 49 . The latter is often applied to control office lighting as the low voltage wires are easily installed in partitions. • to perform time delay functions. The ANSI Device Designation Number is 50 for an Instantaneous OverCurrent (IOC). 51 for a Time OverCurrent (TOC). and even distance relays that would trip for faults up to a certain distance away from a substation but not beyond that point. protective relays had well-established. Such relays were very elaborate. over.Protective relay A protective relay is a complex electromechanical apparatus. designed to calculate operating conditions on an electrical circuit and trip circuit breakers when a fault was found. over-voltage.frequency. with a score of individual electromechanical devices. selectable. For example. solenoid-type operators. and phase-shifting networks to allow the relay to respond to such conditions as over-current.and under. The various protective functions available on a given relay are denoted by standard ANSI Device Numbers. These protective relays provide various types of electrical protection by detecting abnormal conditions and isolating them from the rest of the electrical system by circuit breaker operation. 50 . Such relays may be located at the service entrance or at major load centers. operating and restraint coils. often with more than one coil. using arrays of induction disks. reverse power flow. shaded-pole magnets. a relay including function 51 would be a timed overcurrent protective relay. telephonerelay style contacts. time/current (or other operating parameter) curves. Unlike switching type relays with fixed and usually illdefined operating voltage thresholds and operating times. An important transmission line or generator unit would have had cubicles dedicated to protection. However. By combining several functions in one case. Zone 2 covers the last 20% of the feeder line length and provides backup to the next line by having a slight over reach. due to their very long life span. To prevent mal-operation the zone has a 0. Zone 1 is instantaneous in operation and has a purposely set under reach of 80% of the total line length to avoid operation for the next line. These errors can be up to ±20% of the line impedance. The main types of distance relay protection schemes are • • • • Three step distance protection Switched distance protection Accelerated or permissive intertrip protection Blocked distance protection In three step distance protection. errors in voltage and current transformers and relay tolerances. Distance relay The most common form of feeder protection on high voltage transmission systems is distance relay protection.Design and theory of these protective devices is an important part of the education of an electrical engineer who specializes in power systems. 51 . the relays are separated into three separate zones of impedance measurement to accommodate for over reach and under reach conditions. Power lines have set impedance per kilometre and using this value and comparing voltage and current the distance to a fault can be determined.5 second time delay. numerical relays also save capital cost and maintenance cost over electromechanical relays. This is due to measurements of impedance of lines not being entirely accurate. tens of thousands of these "silent sentinels" are still protecting transmission lines and electrical apparatus all over the world. hence the zones 80% reach. Today these devices are nearly entirely replaced (in new designs) with microprocessor-based instruments (numerical relays) that emulate their electromechanical ancestors with great precision and convenience in application. Zone 3 provides backup for the next line and has a time delay of 1 second to grade with zone 2 protection of the next line. Electrical circuits may be connected to ground (earth) for several reasons. the term ground or earth has several meanings depending on the specific application areas. protecting circuit insulation from damage due to excessive voltage.Ground (electricity) In electrical engineering.Note the green and yellow marked earth wire. In some types of telegraph and power transmission circuits. or a direct physical connection to the Earth. a common return path for electric current (earth return or ground return). the earth itself can be used as one conductor 52 . A connection to ground helps limit the voltage built up between power circuits and the earth. Ground is the reference point in an electrical circuit from which other voltages are measured. A typical earthing electrode (left of gray conduit) . Connections to ground may be used to limit the build-up of static electricity when handling flammable products or when repairing electronic devices. a connection to ground is done for safety purposes to protect people from the effects of faulty insulation on electrically powered equipment. In power circuits. of the circuit. By bonding (interconnecting) all exposed non-current carrying metal objects together. The use of the term ground (or earth) is so common in electrical and electronics applications that circuits in vehicles such as ships. 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 proper bonding to earth will result in the circuit overcurrent protection operating to de-energize the faulty circuit. the Earth serves as a (reasonably) constant potential reference against which other potentials can be measured. a 'ground' is usually idealized as an infinite source or sink for charge. and spacecraft may be spoken of having a "ground" connection without any actual connection to the Earth AC power wiring installations In a mains electricity (AC power) wiring installation. By connecting the cases of electrical equipment to earth. These may be located locally. saving the cost of installing a separate run of wire as a return conductor. The ground wire is also usually bonded to pipework to keep it at the same potential as the electrical ground during a fault. any fault currents in the system will not produce dangerous voltages which could cause electric shock. any insulation failure will result in current flowing to ground that would otherwise energize the case of the equipment. The power ground grounding wire is (directly or indirectly) connected to one or more earth electrodes. This grounding wire is usually but not always connected to the neutral wire at some point and they may even share a cable for part of the system under some conditions. which can absorb an unlimited amount of current without changing its potential. Water supply pipes often used to be used as ground electrodes but this was banned in some countries when plastic pipe such as PVC 53 . be far away in the suppliers network or in many cases both. aircraft. For measurement purposes. the ground is a wire with an electrical connection to earth. and pipes and cables entering the bathroom are sometimes crossbonded. costs are saved by using just a single high voltage conductor for the power grid. The size of power ground conductors is usually regulated by local or national wiring regulations. The power ground is also often bonded to the house's incoming pipework. In Single Wire Earth Return (SWER) AC electrical distribution systems. Buried grounding electrodes are used to make the connection to the earth. When very large fault currents are injected into the earth. This is done to try to reduce the potential difference between objects that can be touched simultaneously. rather than as a direct use of the power ground.became popular. A particular concern in design of electrical substations is earth potential rise. Permanently installed electrical equipment usually also has permanently connected grounding conductors. This is due to the limited finite conductivity of the layers of soil in the earth. This is especially common in schemes with submarine cables as sea water is a good conductor. A power ground serves to provide a return path for fault currents and therefore allows the fuse or breaker to disconnect the circuit. while routing the AC return current through the earth. Power transmission Some HVDC power transmission systems use the ground as second conductor. but this is mainly to stop the power ground carrying noise into the systems which the filters protect. the area around the point of injection may rise to a high potential with respect to distant points. This type of ground applies to radio antennas and to lightning protection systems. (see Domestic AC power plugs and sockets). The gradient of the 54 . The site of these electrodes must be chosen very carefully in order to prevent electrochemical corrosion on underground structures. Filters also connect to the power ground. This system is mostly used in rural areas where large earth currents will not otherwise cause hazards. Portable electrical devices with metal cases may have them connected to earth ground by a pin in the interconnecting plug. or communication wires entering a substation may see different ground potentials inside and outside the substation. Lightning protection systems Lightning protection systems form a very specialised application of grounding used in an attempt to lessen damage to man-made structures caused by lightning strikes. such as brick. This signal ground may or may not actually be connected to a power ground. This common reference point is called ground and considered to have zero voltage.voltage (changing voltage within a distance) may be so high that two points on the ground may be at significantly different potentials. The concept and goal of lightning protection systems is to mitigate the extreme fire hazard which lightning poses to some types of man-made structures. or electrically resistant materials. which appears between two points having some electrical potentials. or through porous materials which can contain water. especially those which are built of flammable materials. such as wood. in order to lessen the heating effects of lightning's current flowing through or around flammable structural materials. stone. a reference point must be selected to measure against. or concrete. such as brick. A system where the system ground is not actually connected to another circuit or to earth (though there may still be AC coupling) is often referred to as a floating ground. creating a dangerous touch voltage Circuit ground versus earth Voltage is a differential quantity. as the water contained in these rain-soaked masonry elements may explode when flashed to steam by lightning's heat. creating a hazard to anyone standing on the ground in the area. or concrete. To measure the voltage of a single point. 55 . low-resistance path for the lightning circuit to follow. stone. rails. Pipes. A lightning protection system is an attempt to provide a preferred. These conductors are connected to earth either through the metal structure of a pole or tower. an over head transmission line may have two overhead ground conductors. While lightning (as all electrical current) will tend to follow the path of least resistance. overhead power lines with voltages below 50 kV do not have a ground conductor.e.000 amperes).e. and all of the proper connectors and supports to complete the system.To appreciate the limitations of lightning protection systems. lightning will often follow many distinct paths. Because of the incredibly high electrical potential of lightning (oen exceeding 100 million volts and 40. lightning rods or strike termination devices). plates. or concrete. The air terminals are typically arranged at or along the upper points of a roof structure. and property insurers have accepted and relied upon the benefits of basic lightning protection systems for well over a century.[3] In overhead transmission lines. electrical engineers. and are electrically bonded together by bonding conductors (sometimes called "down conductors" or misleadingly called "downloads"). but most lines carrying more than 50 kV do. and secondary side-flashes can be enough to ignite a fire. electrodes. or its occupants. bonding conductors (usually heavy stranded copper or aluminum wires or thick braided or solid copper or aluminum straps).[2] The components of a basic lightning protection system are air terminals (i. As a general rule. or mesh). ground or earthing rods. which are connected by the most direct route possible to one or more grounding or earthing terminals installed into the earth or ground. or towers. a ground conductor may also be the top most wire on pylons. 56 . its contents. Depending on local conditions and reliability requirements. ground terminals (i. or by additional ground electrodes installed at regular intervals along the line. no lightning protection system can guarantee absolute safety from lightning to a structure. Nonetheless. This ground conductor is intended to protect the power conductors from lightning strikes. poles. stone. blow apart brick. it is important to understand the magnitude of lightning's energy. or injure occupants within a structure or building. scientists. This helps discharge any static which a worker has built up. so that the operator will not be exposed to a high differential voltage due to a fault in the substation. It is generally made of a conductive plastic or metal mesh covered substrate which is electrically attached to ground. 57 . obviously static discharge is undesirable during fuel-transfer operations. Ground mats are also found on fuel trucks. as well as any static on tools or exposed components laid on the mat. Similarly. a ground cable connects the tanker (truck or airplane) to the fuel-seeking craft to eliminate charge differences before fuel is transferred. It is used most commonly in computer repair. In an electrical substation a ground mat is a mesh of conductive material installed at places where a person would stand to operate a switch or other apparatus. in aircraft refueling. it is bonded to the local supporting metal structure and to the handle of the switchgear. BATTERY BOX Batteries are installed in substation to provide power to switching components and to power the substation control equipment in times of AC power loss. which are otherwise insulated from ground as they make physical contact only with their (rubber and air) tires.Ground mat A ground mat or grounding mat is a flat. They require regular maintenance for proper working . flexible pad used for working on electrostatic sensitive devices. Lead Acid Batteries are widely used for years. other alternative is Nickel-cadmium battery LEAD Acid Battery Electrochemistry Each cell contains (in the charged state) electrodes of lead metal (Pb) and lead (IV) dioxide (PbO2) in an electrolyte of about 33. The chemical reactions are (charged to discharged): Anode (oxidation): Cathode (reduction): 58 . as the battery discharges and the concentration of sulfuric acid decreases. the electrolyte is more likely to freeze. Due to the freezing-point depression of water.5% w/w (6 Molar) sulfuric acid (H2SO4). In the discharged state both electrodes turn into lead(II) sulfate (PbSO4) and the electrolyte loses its dissolved sulfuric acid and becomes primarily water. overcharging with excessive charging voltages will generate oxygen and hydrogen gas by electrolysis of water.Because of the open cells with liquid electrolyte in most lead-acid batteries. The acid electrolyte is also corrosive. forming an explosive mix. 59 .