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132 kv gss summer training report from CPWD vidhyadar nagar jaipur

Training report
In partial fulfilment
For the award of the Degree of
Bachelor ...
I would like to express my deep gratitude to Er. Mukesh Kumar (Assistant
Engineer) for giving me honour ...
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132 kv gss summer training report from CPWD vidhyadar nagar jaipur

  1. 1. 1 GRID SUB STATION, CPWD JAIPUR A Training report Submitted In partial fulfilment For the award of the Degree of Bachelor of Technology In Department of Electrical Engineering Supervisor Submitted By: Er. Mukesh Kumar RAMESH KUMAWAT Designation Roll No.: 15EEAEE036 GOVT. ENGINEERING COLLEGE, AJMER (Run under society act) Badliya circle, NH-08, Ajmer www.ecajmer.comTelephone: 0145-2671800, 2671801 Rajasthan Technical University
  3. 3. 3 ACKNOWLEDGEMENT I would like to express my deep gratitude to Er. Mukesh Kumar (Assistant Engineer) for giving me honour to work with this esteem organization. First of all I would like to convey my sincere thanks to Dr. K.G SHARMA (Head of Department , Electrical Engineering) and Ms. INDIRA RAWAT(Associate Professor) of Electrical Department for giving me permission for training program at R.R.V.P.N.L.,132 KV GSS CPWD JAIPUR, Ajmer I would also like to thanks to Mr. AJAY AGGRAWAL(Assistant Professor) and all faculty members of the Department, who had helped and gave guidance to me for preparing the report. RAMESH KUMAWAT 15EEAEE036
  4. 4. 4 CONTENTS CHAPTER NO. PAGE NO CERTIFICATE ACKNOWLEDEGEMENT LIST OF CONTENT INDEX OF FIGURES ABSTRACT i ii iii viii x Chapter 1: THE GLANCE OF GSS 1.1 Introduction 1.2 The Grid Sub Station (G.S.S.) 1.3 The Equipments In A Substation 1.4 132kv Grid Substation, CPWD, Jaipur 1.4.1 Incoming Feeder (132 KV) 1.4.2 Outgoing Feeders (33 KV) 1-6 1 2 3 4 5 5
  5. 5. 5 Chapter 2: LIGHTNING ARRESTER 2.1 Introduction 2.2 Description 2.3 Installation Location 2.4 Grounding 2.5 Clearances 2.6 Arrester Voltage 2.7 Working 2.8 Types Of Arrestors 2.8.1 Rod/Sphere Gap 2.8.2 Expulsion Type La 2.8.3 Valve Type La 6-8 6 6 6 7 7 8 8 8 8 8 8 Chapter 3: BUS BARS AND ISOLATORS 3.1 Introduction 3.2 Bus Bar Arrengement Adopted By R.R.V.P.N.L. 3.2.1 Single Bus Bar Arrangement 3.2.2 Double Bus Bar Arrangement 3.2.3 Double Bus Bar Arrangements Contains Main Bus With Auxilary Bus 3.3 Isolators 3.4 Applications Of Isolators Chapter 4: INSULATOR 9-11 9 9 9 9 10 10 11 12-14
  6. 6. 6 4.1 Introduction 4.2 Type Of Insulators 4.2.1 Pin Type 4.2.2 Suspension Type 4.3.3 Strain Insulator 4.2.4 Shackle Insulator 12 12 12 13 14 14 Chapter 5: CONTROL ROOM 5.1 Introduction 5.2 Synchronizing 5.3 Measuring Instrument Used 5.3.1 Energy Meter 5.3.2 Wattmeters 5.3.3 Frequency Meter 5.3.4 Voltmeter 5.3.5 Ammeter 5.3.6 Maximum Demand Indicator 5.3.7 Mvar Meter 5.4 Control Panels 15-17 15 15 16 16 17 17 17 17 17 17 17 Chapter 6: POWER LINE CARRIER COMMUNICATION 6.1 Introduction 6.2 Basic Principle Of Plc 18-26 18 18
  7. 7. 7 6.3 Wave Trap 6.4 Coupling Capacitors 6.5 Drainage Coils 6.6 Block Diagram 6.7 Applications 6.8 Advantages 6.9 Disadvantages 6.10 Future Scope 19 19 21 21 22 23 24 26 Chapter 7: CURRENT TRANSFORMER 7.1 Introduction 7.2 Funtion Of Current Transformer 7.3 Use Of Current Transformer 7.4 Safety Of Current Transformer 27-31 27 28 30 31 Chapter 8: CAPACITIVE VOLTAGE TRANSFORMER 8.1 Introduction 8.2 Components Of Capacitive Voltage Transformer 8.3 Frequency Response 32-33 32 32 33 Chapter 9: CIRCUIT BREAKER 9.1 Introduction 9.2 Operation 9.3 Arc Interruption 34-40 34 34 36
  8. 8. 8 9.4 Short Circuit 9.5 Types Of Circuit Breaker 9.5.1 High Voltage Circuit Breaker 9.5.2 Sf6 Circuit Breaker 37 37 37 39 Chapter 10: RELAYS 10.1 Introduction 10.2 Basic Design And Operation 10.3 Types Of Relays 10.3.1 Latching Relay 10.3.2 Coaxial Relay 10.3.3 Time Delay Relay 10.3.4 Buchholz Relay 10.4 Pole And Throw 41-46 41 41 43 43 44 44 45 45 CONCLUSION 47 BIBLIOGRAPHY 48
  9. 9. 9 INDEX OF FIGURES Figure No. Figure Name Page no. Figure-1.1 132 KV GSS CPWD, jaipur 3 Figure-2.1 L.A. At 132kv GSS CPWD 7 Figure-3.1 Isolators 10 Figure-4.1 Pin Type Insulators 13 Figure-4.2 Suspension Type Insulator 13 Figure-4.3 Strain Type Insulator 14 Figure-4.4 Shackle Type Insulator 14 Figure-5.1 Control Room 16 Figure-6.1 Wave Trap 20 Figure-6.2 Block Diagram Of Plc 21 Figure-7.1 Current transformer 28 Figure-7.2 Function of current transformer 30 Figure-8.1 A capacitor voltage transformer 32 Figure-9.1 High Voltage Circuit Breaker 39 Figure-9.2 SF6 Circuit Breaker 40
  10. 10. 10 ABSTRACT A substation is an assembly of apparatus, which transform the characteristics of electrical energy from one form to another say from one voltage level to another level. Hence a substation is an intermediate link between the generating station and the load units. There is one bus bars in a 132 KV yard and two bus-bars in 33kv yard. The incoming feeders are connected to bus-bar through lighting arrestors, capacitive voltage transformer, line isolator, circuit breakers, current-transformers, line isolator etc. The bus-bars are to have an arrangement of auxiliary bus. In the 132KV GSS the income 132 KV supply is stepped down to 33Kv with the help of transformers which is furthers supplied to different sub-station according to the load. 132 Kv G.S.S. has a large layout consisting of 2 Nos. of (40/50MVA and 20/25MVA) transformers, with their voltage ratio respectively 132/33Kv. At "132kv CPWD,Jaipur" the separate control room switches and fuses are available. There are meters for reading purpose. A circuit concerning the panel is shown on the panel with standard color, provided for remote protection of 132KV switch yards transformer incoming feeder, outing feeders. The control panel carries the appropriate relays. The training at grid substation was very helpful. It has improved my theoretical concepts of electrical power transmission and distribution. Protection of various apparatus was a great thing. Maintenance of transformer, circuit breaker, isolator, insulator, bus bar etc was observable. I had a chance to see the remote control of the equipments from control room itself, which was very interesting.
  11. 11. 11 Chapter:-1 THE GLANCE OF GSS 1.1 INTRODUCTION “Rajasthan State Electricity Board” started working form 1 July, 1957. When India becomes independent its overall installed capacity was hardly 1900 MW. During first year plan (1951-1956) this capacity was only 2300 MW. The contribution of Rajasthan state was negligible during 1 & 2 year plans the emphasis was on industrialization for that end it was considered to make the system of the country reliable. Therefore Rajasthan state electricity board came into existence in July 1957.The aim of RSEB is to supply electricity to entire Rajasthan state in the most economical way. Government of Rajasthan on 19th July 2000, issued a gazette notification unbundling Rajasthan State Electricity Board into 1. Rajasthan Rajya Vidyut Utpadan Nigam Ltd (RRVUNL), the generation Company; 2. Rajasthan Rajya Vidyut Prasaran Nigam Ltd, (RRVPNL), the transmission Company And the three regional distribution companies namely 1. Jaipur Vidyut Vitran Nigam Ltd, (JVVNL) 2. Ajmer Vidyut Vitran Nigam Ltd (AVVNL) and 3. Jodhpur Vidyut Vitran Nigam Ltd (JdVVNL). The Generation Company owns and operates the thermal power stations at Kota and Suratgarh, Gas based power station at Ramgarh , Hydel power station at Mahi and mini hydel stations in the State. The Transmission Company operates all the 400KV, 220 KV, 132 KV and 33KV electricity lines and system in the State. The three distribution Companies operate and maintain the electricity system below 66KV in the State in their respective areas. Power obtain from these stations is transmitted all over Rajasthan with the help of grid stations. [1]. Depending on the purpose, substations may be classified as 1. Step up substation
  12. 12. 12 2. Primary grid substation 3. Secondary substation 4. Distribution substation 5. Bulky supply and industrial substation 6. Mining substation 7. Mobile substation 8. Cinematograph substation Depending on constructional feature substations are classified as 1. Outdoor type 2. Indoor type 3. Basement or Underground type 4. Pole mounting open or kilos type 1.2 THE GRID SUB STATION (G.S.S.) The assembly of apparatus used to change some characteristics (e.g. voltage ac / dc, freq., p.f. etc) of electric supply is called sub-station.Electrical power is generated, transmitted in the form of alternating current. The electric power produced at the power stations is delivered to the consumers through a large network of transmission & distribution. The transmission network is inevitable long and high power lines are necessary to maintain a huge block of power source of generation to the load centers to inter connected. An electrical substation is a subsidiary station of an electricity generation, transmission and distribution system where voltage is transformed from high to low or the reverse using transformers. Electric power may flow through several substations between generating plant and consumer, and may be changed in voltage in several steps. Substations have switching, protection and control equipment and one or more transformers. In a large substation, circuit breaker are used to interrupt any short-circuits or overload currents that may occur on the network.
  13. 13. 13 Figure-1.1 132KV CPWD,GSS Depending on the constructional feature, the high voltage substations may be further subdivided 1. Outdoor substation 2. Indoor substation 3. Base or Underground substation Following are the feeders established in a substation. 1. Tie Feeders 2. Radial Feeders 1.3 THE EQUIPMENTS IN A SUBSTATION 1. Structures; 2. Power Transformers; 3. Bus-Bars; 4. Circuit Breakers ( 132KV AND 33 KV);
  14. 14. 14 5. Isolators or Isolating Switches ( 132KV AND 33 KV); 6. Earthing Switches; 7. Insulators; 8. Power and Control Cables; 9. Control Panel; 10. Lightning Protection − Surge Arrestors; 11. Instrument Transformers (Current and Power Transformers, I.E., CTs and PTs); 12. Earthing Arrangements; 13. Reactive Compensation; 14. DC Supply Arrangement; 15. Auxiliary Supply Transformer; and 16. Fire-Fighting System. 1.4 132KV GRID SUBSTATION, MADAR It is part of RRVPNL. It is situated at JAIPUR. The power mainly comes from 220KV GSS Chomu and 220 KV GSS Kalwar. The substation is equipped with various equipments and there are various arrangements for the protection purpose. The equipments in the GSS are listed previously. 132 KV GSS CPWD is an outdoor type primary substation and distribution as well it has not only step down but the distribution work. The electrical work in a substation comprises to 1. Choice of bus bar arrangement layout. 2. Selection of rating of isolator. 3. Selection of rating of instrument transformer. 4. Selection of rating of C.B.
  15. 15. 15 5. Selection of lighting arrester (LA) 6. Selection of rating of power transformer 7. Selection of protective relaying scheme, control and relay boards. 8. Selection of voltage regulator equipment. 9. Design a layout of earthing grids and protection against lightening strokes. 1.4.1 Incoming Feeder (220 KV) 1. Chomu 2. Kalwar 1.4.2 Outgoing Feeders (33 KV) 1. Vidyadhar nagar 2. Ambabari 3. panipech
  16. 16. 16 Chapter:-2 LIGHTNING ARRESTER 2.1INTRODUCTION A lightning arrester is a device connected between line and earth i.e. in parallel with the over headline, HV equipments and substation to be protected. It is a safety valve which limits the magnitude of lightning and switching over voltages at the substations and provides a low resistance path for the surge current to flow to the ground. The practice is also to install lightning arresters at the incoming terminals of the line. All the electrical equipments must be protected from the severe damages of lightning strokes. The techniques can be studied under:- 1. Protection of transmission line from direct stroke. 2. Protection of power station and sub-station from direct stroke. 3. Protection of electrical equipments from travelling waves. 2.2 DESCRIPTION The ThyriteAlugard Lightning arrester consists of a stack of one or more units connected in series depending on the voltage and the operating condition of the circuit three single pole arresters are required for 3-phase installation. The arresters are single pole design and they are suitable for indoor and out-door service.Each arrester unit consists essentially of permanently sealed Porcelain housing equipped with pressure relief and containing a number of thyrite value-element discs and exclusive locate gaps shunted by Thyrite resistors metal fitting cemented of the housing provide means for bolting arrester units into a stack. Each arrester unit is shipped assembled. No charging or testing operation is required before placing them in service. 2.3 INSTALLATION LOCATION Install arrester electrically as close as possible to the apparatus being protected Line and ground connections should be short and direct.
  17. 17. 17 Figure 2.1: L.A. At 220 KV GSS Madar 2.4 GROUNDING The arrester ground should be connected to the apparatus grounds and the main station ground utilizing a reliable common ground network of low resistance. The efficient operation of the Lightning arrester requires permanent low resistance grounds Station class arresters should be provided with a ground of a value not exceeding five ohms. 2.5 CLEARANCES These are given on the drawings. These are the maximum recommended. The term ‘clearance’ means the actual distance between any part of the arrester or disconnecting device at line potential, and any object at ground potential or other phase potential.
  18. 18. 18 2.6 ARRESTER VOLTAGE The Thyrite station-class arrester is designed to limit the surge voltages to a safe value by discharging the surge current to ground; and to interrupt the small power frequency follow current before the first current zero. The arrester rating is a define limit of its ability to interrupt power follow current. Therefore it is important to assure that the system power frequency voltage from line to ground under any condition, switching, fault, overvoltage never exceeds the arrester’s rating. 2.7 WORKING Lightning, is a form of visible discharge of electricity between rain clouds or between a rain cloud and the earth The electric discharge is seen in the form of a brilliant arc, sometimes several kilometers long, stretching between the discharge points How thunderclouds become charged is not fully understood, but most thunderclouds are negatively charged at the base and positively charged at the top However formed, the negative charge at the base of the cloud induces a positive charge on the earth beneath it, which acts as the second plate of a huge capacitor. When the electrical potential between two clouds or between a cloud and the earth reaches a sufficiently high value (about 10,000 V per cm or about 25,000 V per in), the air becomes ionized along a narrow path and a Lightning flash results [3]. 2.8 TYPES OF ARRESTORS 2.8.1 Rod/Sphere Gap It is a very simple protective device i.e. gap is provided across the stack of Insulators to permit flash-over when undesirable voltages are impressed of the system. 2.8.2 Expulsion Type La It have two electrodes at each end and consists of a fiber tube capable of producing a gas when is produced. The gas so evolved blows the arc through the bottom electrode. 2.8.3 Valve Type La It consists of a divided spark-gap in series will a non linear resistor. The divided spark gap consists of a no. of similar elements.
  19. 19. 19 Chapter:-3 BUS BARS AND ISOLATORS 3.1 INTRODUCTION Bus Bars are the common electrical component through which a large no of feeders operating at same voltage have to be connected.If the bus bars are of rigid type (Aluminum types) the structure height are low and minimum clearance is required. While in case of strain type of bus bars suitable ACSR conductor are strung/tensioned by tension insulators discs according to system voltages. In the widely used strain type bus bars stringing tension is about 500-900 Kg depending upon the size of conductor used.Here proper clearance would be achieved only if require tension is achieved. Loose bus bars would affect the clearances when it swings while over tensioning may damage insulators. Clamps or even affect the supporting structures in low temperature conditions.The clamping should be proper, as loose clamp would spark under in full load condition damaging the bus bars itself. 3.2BUS BAR ARRENGEMENT ADOPTED BY R.R.V.P.N.L. 3.2.1 Single Bus Bar Arrangement This arrangement is simplest and cheapest. It suffers, however, from major defects. 1. Maintenance without interruption is not possible. 2. Extension of the substation without a shutdown is not possible 3.2.2 Double Bus Bar Arrangement 1. Each load may be fed from either bus. 2. The load circuit may be divided in to two separate groups if needed from operational consideration. Two supplies from different sources can be put on each bus separately. 3. Either bus bar may be taken out from maintenance of insulators. The normal bus selection insulators cannot be used for breaking load currents. The arrangement does not permit breaker maintenance without causing stoppage of supply.
  20. 20. 20 3.2.3 Double Bus Bar Arrangements Contains Main Bus With Auxilary Bus The double bus bar arrangement provides facility to change over to either bus to carry out maintenance on the other but provide no facility to carry over breaker maintenance. The main and transfer bus works the other way round. It provides facility for carrying out breaker maintenance but does not permit bus maintenance. Whenever maintenance is required on any breaker the circuit is changed over to the transfer bus and is controlled through bus coupler breaker. 3.3 ISOLATORS “Isolator" is one, which can break and make an electric circuit in no load condition. These are normally used in various circuits for the purposes of Isolation of a certain portion when required for maintenance etc. Switching Isolators are capable of 1. Interrupting transformer magnetized currents 2. Interrupting line charging current 3. Load transfer switching Figure-3.1 Isolators At 220 KV GSS Madar
  21. 21. 21 3.4 APPLICATIONS OF ISOLATORS Its main application is in connection with transformer feeder as this unit makes it possible to switch out one transformer, while the other is still on load. The most common type of isolators is the rotating centre pots type in which each phase has three insulator post, with the outer posts carrying fixed contacts and connections while the centre post having contact arm which is arranged to move through 90` on its axis. The following interlocks are provided with isolator 1. Bus 1 and2 isolators cannot be closed simultaneously. 2. Isolator cannot operate unless the breaker is open. 3. Only one bay can be taken on bypass bus. 4. No isolator can operate when corresponding earth switch is on breaker.
  22. 22. 22 Chapter:-4 INSULATOR 4.1 INTRODUCTION The insulator for the overhead lines provides insulation to the power conductors from the ground so that currents from conductors do not flow to earth through supports. The insulators are connected to the cross arm of supporting structure and the power conductor passes through the clamp of the insulator. The insulators provide necessary insulation between line conductors and supports and thus prevent any leakage current from conductors to earth. In general, the insulator should have the following desirable properties: 1. High mechanical strength in order to withstand conductor load, wind load etc. 2. High electrical resistance of insulator material in order to avoid leakage currents to earth. 3. High relative permittivity of insulator material in order that dielectric strength is high. 4. High ratio of puncture strength to flash over. These insulators are generally made of glazed porcelain or toughened glass. Poly come type insulator [solid core] are also being supplied in place of hast insulators if available indigenously. The design of the insulator is such that the stress due to contraction and expansion in any part of the insulator does not lead to any defect. It is desirable not to allow porcelain to come in direct contact with a hard metal screw thread. 4.2 TYPE OF INSULATORS 4.2.1 Pin Type Pin type insulator consist of a single or multiple shells adapted to be mounted on a spindle to be fixed to the cross arm of the supporting structure. When the upper most shell is wet due to rain the lower shells are dry and provide sufficient leakage resistance these are used for transmission and distribution of electric power at voltage up to voltage 33 KV. Beyond operating voltage of 33 KV the pin type insulators thus become too bulky and hence uneconomical.
  23. 23. 23 Figure-4.1 Pin Type Insulators 4.2.2 Suspension Type Suspension type insulators consist of a number of porcelain disc connected in series by metal links in the form of a string. Its working voltage is 66KV. Each disc is designed for low voltage for 11KV. Figure-4.2 Suspension Type Insulator
  24. 24. 24 4.2.3 Strain Insulator The strain insulators are exactly identical in shape with the suspension insulators. These strings are placed in the horizontal plane rather than the vertical plane. These insulators are used where line is subjected to greater tension. For low voltage lines (< 11KV) shackle insulator are used as strain insulator. Figure-4.3 Strain Type Insulator 4.2.4 Shackle Insulator In early days, the shackle insulators were used as strain insulators. But now a day, they are frequently used for low voltage distribution lines. Such insulators can be used either in a horizontal position or in a vertical position. They can be directly fixed to the pole with a bolt or to the cross arm. Figure-4.4 Shackle Type Insulator
  25. 25. 25 Chapter:-5 CONTROL ROOM 5.1 INTRODUCTION Control panel contain meters, control switches and recorders located in the control building, also called the dog house. These are used to control the substation equipment to send power from one circuit to another or to open or to shut down circuits when needed. To remote control of power switch gear requires the provision of suitable control plates located at a suitable point remote from immediate vicinity of CB’s and other equipments. In GSS the separate control room provided for remote protection of 132KV switch yards transformer incoming feeder, outing feeders. Bus bar has their own control plant in their control rooms. The control panel carrier the appropriate relays. Necessary meters indicating lamp control switches and fuses. There are meters for reading purpose. A circuit concerning the panel is shown on the panel with standard color. On each panel a control switch is provided for remote operation of circuit breaker. There are two indicators which show that weather circuit breaker is closed or open. A control switch for each insulator is also provided. The position indicator of isolator is also done with the help of single lamp and indicator. The color of signal lamps are as follows:- 1. Red:- for circuit breaker or isolator is close option 2. Green - for circuit breaker is in open position. 3. Amber - indicates abnormal condition requiring action. In addition to used indication, an alarm is also providing for indicating abnormal condition when any protective relay or tripping relay has operated. Its constants energies on auxiliary alarm, Relay which on operation completes the alarm belt circuit. 5.2 SYNCHRONIZING There is a hinged Synchronizing panel mounted at the end of control panel, before coupling any incoming feeders to the bus bar. It is just be synchronized with switches. When the
  26. 26. 26 synchronous copy shows zero we close the circuit breaker. Synchronous scope is used to determine the correct instant of closing the switch which connects the new supply to bus bar. The correct instant of synchronizing when bus bars incoming voltage: 1. Are in phase 2. Are equal in magnitude 3. Are in some phase sequence 4. Having same frequency 5. The voltage can be checked by voltmeter the function of synchronoscope is to indicate the difference in phase and frequency. Figure-5.1 Control Room At 220 KV GSS MADAR 5.3MEASURING INSTRUMENTUSED 5.3.1 Energy Meter To measure the energy transmitted energy meters are fitted to the panel to different feeders the energy transmitted is recorded after one hour regularly for it MWHr, meter is provided.
  27. 27. 27 5.3.2 Wattmeters It is attached to each feeder to record the power exported from GSS 5.3.3 Frequency Meter To measure the frequency at each feeder there is the provision of analogor digital frequency meter. 5.3.4 Voltmeter It is provided to measure the phase to phase voltage .It is also available in both the analog and digital frequency meter. 5.3.5 Ammeter It is provided to measure the line current. It is also available in both the forms analog as well as digital. 5.3.6 Maximum Demand Indicator There are also mounted the control panel to record the average power over successive predetermined period. 5.3.7 Mvar Meter It is to measure the reactive power of the circuit. 5.4 CONTROL PANELS Control panels installed within the control building of a switchyard provide mounting for mimic bus, relays, meter, indicating instruments, indicating lights, control switches, test switches and other control devices. The panel contains compartments for incoming lines, outgoing lines, bus-bars with provision for sectionalizing, relays, measuring instruments, etc. The panel is provided with 1. Suitable over-current and earth fault relays to protect the equipment against short circuit and earth faults; and
  28. 28. 28 Chapter:-6 POWER LINE CARRIER COMMUNICATION 6.1 INTRODUCTION Power line communication or power line carrier (PLC), also known as Power line Digital Subscriber Line (PDSL), mains communication, power line telecom (PLT), or power line networking (PLN), is a system for carrying data on a conductor also used for electric power transmission. Broadband over Power Lines (BPL) uses PLC by sending and receiving information bearing signals over power lines.Electrical power is transmitted over high voltage transmission lines, distributed over medium voltage, and used inside buildings at lower voltages. Power line communications can be applied at each stage. Most PLC technologies limit themselves to one set of wires (for example, premises wiring), but some can cross between two levels (for example, both the distribution network and premises wiring). Typically the transformer prevents propagating the signal so multiple PLC technologies are bridged to form very large networks. All power line communications systems operate by impressing a modulated carrier signal on the wiring system. Different types of power line communications use different frequency bands, depending on the signal transmission characteristics of the power wiring used. Since the power wiring system was originally intended for transmission of AC power, in conventional use, the power wire circuits have only a limited ability to carry higher frequencies. The propagation problem is a limiting factor for each type of power line communications. A new discovery called E-Line that allows a single power conductor on an overhead power line to operate as a waveguide to provide low attenuation propagation of RF through microwave energy lines while providing information rate of multiple GBPS is an exception to this limitation [5]. 6.2 BASIC PRINCIPLE OF PLCC In PLCC the higher mechanical strength and insulation level of high voltage power lines result in increased reliability of communication and lower attenuation over long distances.
  29. 29. 29 Since telephone communication system cannot be directly connected to the high voltage lines, suitably designed coupling devices have therefore to be employed. These usually consist of high voltage capacitors or capacitor with potential devices used in conjunction with suitable line matching units (LMU’s) for matching the impedance of line to that of the coaxial cable connecting the unit to the PLC transmit-receive equipment. Also the carrier currents used for communication have to be prevented from entering the power equipment used in G.S.S as this would result in high attenuation or even complete loss of communication signals when earthed at isolator. Wave traps usually have one or more suitably designed capacitors connected in parallel with the choke coils so as to resonate at carrier frequencies and thus offers even high impedance to the flow of RF currents. In PLCC system the following Equipments are used 1. PLCC Station 2. Line matching Unit 3. CVT/CC 4. Earth Switch 5. Lightening Arrestor 6. Wave Trap 7. Co axial cable 6.3 WAVE TRAP Line trap also is known as Wave trap. What it does is trapping the high frequency communication signals sent on the line from the remote substation and diverting them to the telecom/teleprotection panel in the substation control room (through coupling capacitor and LMU).
  30. 30. 30 Figure-6.1 Wave Trap This is relevant in Power Line Carrier Communication (PLCC) systems for communication among various substations without dependence on the telecom company network. The signals are primarily teleprotection signals and in addition, voice and data communication signals. The Line trap OFFERS HIGH IMPEDANCE TO THE HIGH FREQUENCY COMMUNICATION SIGNALSthus obstructs the flow of these signals in to the substation bus bars. If there were not to be there, then signal loss is more and communication will be ineffective/probably impossible. 6.4 COUPLING CAPACITORS The coupling capacitor is used as part of the tuning circuit. The coupling capacitor is the device which provides a low.Impedance path for the carrier energy to the high voltage line and at the same time, it blocks the power frequency current by being a high impedance path at those frequencies. It can perform its function of dropping line voltage across its capacitance if the low voltage end is at ground potential. Since it is desirable to connect the line tuner output to this low voltage point a device must be used to provide a high impedance path to ground for the carrier signal and a low impedance path for the power frequency current. This device is an inductor and is called a drain coil. It is desirable to have the coupling capacitor value as large as possible in order to lower the loss of carrier energy and keep the bandwidth of the coupling system as wide as possible. However, due to the high voltage that must be handled and financial budget limitations, the
  31. 31. 31 coupling capacitor values are not as high as one might desire. Technology has enabled suppliers to continually increase the capacitance of the coupling capacitor for the same price thus improving performance. 6.5 DRAINAGE COILS The drainage coil has a pondered iron core that serves to ground the power frequency charging to appear in the output of the unit. The coarse voltage arrester consists of an air gap, which sparks over at about 2 KV and protects the matching unit against line surges. The grounding switch is kept open during normal operation and is closed if anything is to be done on the communication equipment without interruption to power flow on the line. The matching transformer is isolated for 7 to 10 KV between the two winding and former two functions. Firstly it isolates the communication equipment for the power line. Secondly it serves to match the characteristic impedance of the power line 400-600 ohms to that of the co-axial vacuum arrester (which sparks) is over at about 250 V is provided for giving additional protection to the communication equipment. 6.6 BLOCK DIAGRAM Figure-6.2 Block Diagram Of Plc
  32. 32. 32 This block diagram explanation will give the basic idea about the flow of data fromone user to another. It will give the general idea about the processing that the dataunderwent from one section to another. Following is the end to end jest of the project. Wefirst generated a sequence of random bits with the use of Lab view. This will act as thedata given by the user which is to be transmitted to the receiver. Random bits aregenerated for illustration purpose. The functioning of the receiver side blocks is the exact opposite as that of transmitterside. The coupling circuitry on the receiver side not just provides the isolation but alsoacts as a tuned circuit will allows only the high frequency in a selected band i.e. between800Hz to 3000Hz to enter the receiver side. 6.7 APPLICATIONS 1.Transmission & Distribution Network PLCC was first adopted in the electricaltransmission and distribution system to transmit information at a fast rate. 2.Home control and Automation PLCC technology is used in home control andautomation. This technology can reduce the resources as well as efforts for activities likepower management, energy conservation, etc.Home automation or also known as SmartHome technology is a collection of systems and devices in a home that have an ability tointeract with each other or function individually in order to be optimized in best way. 3.Entertainment PLCC is used to distribute the multimedia content throughout thehome. 4.Telecommunication Data transmission for different types of communications liketelephonic communication, audio, video communication can be made with the use ofPLCC technology. 5.Security Systems
  33. 33. 33 In monitoring houses or businesses through surveillance cameras,PLCC technology is far useful. The surveillance cameras connected over Power Systemin a light bulb is unique; simply screw it into any light socket. Hidden inside the "bulb" isa sophisticated Low-Light Monochrome Camera, coupled with PLCC circuitry. Once the Decoder is plugged in and connected, live video is delivered to the TV or VCR.It's simple and easy. There are no wires to run, no holes to drill, and no antennas orcomplicated "tuning" is required. 6.Automatic Meter Reading Automatic Meter reading applications use the PLCCtechnology to send the data from home meters to Host Central Station. Automatic MeterReading using PLCC technology is quite useful as it saves a lot of human efforts and alsomakes the whole system more efficient. The automatic meter reading system consists ofthree components, namely, Multifunction Node (MFN), Concentrator & CommunicationNode (CCN) and Operation & Management System (OMS). 6.8 ADVANTAGES 1. No separate wires are needed for communication purposes as the power lines themselves carry power as well as the communication signals. Hence the cost of constructing separate telephone lines is saved. 2. When compared with ordinary lines the power lines have appreciably higher mechanical strength. They would normally remain unaffected under the condition which might seriously damage telephone lines. 3. Power lines usually provide the shortest route between the power stations. 4. Power lines have large cross-sectional area resulting in very low resistances per unit length. Consequently the carrier signal suffers lesser attenuation than when travel on usual telephone lines of equal lengths. 5. Power lines are well insulated to provide negligible leakage between conductors and ground even in adverse weather conditions. 6. Largest spacing between conductors reduces capacitance which results in smaller attenuation at high frequencies. The large spacing also reduces the cross talk to a considerable extent.
  34. 34. 34 7. PLCC uses existing power line for communication so it provides many advantages overtraditionally used telephone models and other communication systems. The mainadvantage of power line carrier communication is cost on infrastructure is reduced to alarge extent. Consider any building or company office as an example. To supplyelectricity for the whole structure first Mains cables are dropped throughout the buildingand it bears all the load thrusters by fridge , fans and all the other equipments or devicesrun on electricity. If PLCC modem is used in such an area, no additional wiring orcabling is required as PLCC uses this power line only. Data is sent on this withequipotential coupling circuitry using modulator and it's retrieved at the receiver's sideusing demodulator. Thus elimination of wiring or cabling is the biggest bliss incommunication system. This is also a flexible type of service which can have differentformulations as per the need or application. Half duplex PLCC or Full Duplex PLCCmodems are available. Thus they are now greatly used in houses and small officenetworks. It is also considered as a replacement to intercom as it doesn't need any extracabling.The biggest relief to the customers is when you talk about installation charges andservice tax. Apart from initial installation charges, user doesn't need to pay service taxand government taxes for it as he's using existing power line only for communication. 8. With built-in Error Checking and direct interface with uc as an ad-on , it's the biggestsource of Research and Development since last 50 years. With other applications likeAutomatic meter reading , Fire & Security Alarm Systems and Lighting Control thisserves as the major Integration for all the tasks that can be computed easily. With theinvention of new modems such as PLC Modem from Sunrom Technologies, care has been taken that though using a same platform for transmitter and receiver side , highpower side and low power side are isolated so chances of shock hazards are reduced tolarge extent and device is made more user friendly. 6.9 DISADVANTAGES 1. Proper care has to be taken to guard carrier equipment and persons using them against high voltage and currents on the line.
  35. 35. 35 2. Reflections are produced on spur lines connected to high voltage lines. This increases attenuation and create other problems. 3. High voltage lines have transformer connections, which attenuate carrier currents. Sub- station equipments adversely affect the carrier currents. 4. Noise introduced by power lines is much more than in case of telephone lines. This due to the noise generated by discharge across insulators, corona and switching processes. 6.10 FUTURE SCOPE 1.Telecommunication Data transmission for different types of communications liketelephonic communication, audio, video communication can be made with the use ofPLCC technology. The user will be free to choose the necessary mode of communication. 2.Industrial Automation In an industrial environment the PLC communicationnetworks can be used to give electric energy related services, such as meter reading,demand management and remote billing but also to give value added services like remotecontrol and security, automation or even, education, information and e businessopportunities. On the other hand it can also offer telecommunication services such astraditional telephony and Internet. 3.System protection The communication link can be used to transit control signals thatmay be used to protect the system. For example, PLC can be successfully used in order todetect is landing operation of DER units. 4.Telecommunication services Current PLC networks are able to reach speeds of200Mbps. Telephony and Internet services can be delivered at high speed throughbroadband PLC networks. Traditional telephony uses Plesiochronous Digital Hierarchy ,PDH. PDH uses Time Division Multiplexing, TDM. One possibility is to send the TDMframe over IP, and the voice over TDM, VoTDM. However, this service shouldaccomplish the quality and reliability criteria, like Bit Error Rate, timing
  36. 36. 36 and latency, andunfortunately the delay in Vo TDM transmissions exceeds 25ms. Nevertheless, it ispossible to give a good telephony service over IP. Over TCP/IP, VoIP.
  37. 37. 37 Chapter-7 CURRENT TRANSFORMER 7.1 INTRODUCTION A current transformer (CT) is a type of transformer that is used to measure AC Current. It produces an alternating current (AC) in its secondary which is proportional to the AC current in its primary. Current transformers, together with voltage transformers (VTs) or potential transformers (PTs), which are designed for measurement, are known as an Instrument transformer. The main tasks of instrument transformers are: i. To transform currents or voltages from a usually high value to a value easy to handle for relays and instruments. ii. To insulate the metering circuit from the primary high voltage system. iii. To provide possibilities of standardizing the instruments and relays to a few rated currents and voltages. When the current to be measured is too high to measure directly or the system voltage of the circuit is too high, a current transformer can be used to provide an isolated lower current in its secondary which is proportional to the current in the primary circuit. The induced secondary current is then suitable for measuring instruments or processing in electronic equipment. Current transformers have very little effect on the primary circuit. Current transformers are the current sensing units of the power system. The output of the current transformers are used in electronic equipment and are widely used for metering and protective relays in the electrical power industry.
  38. 38. 38 Fig. 7.1 Current transformer 7.2 FUNTION OF CURRENT TRANSFORMER Like any transformer, a current transformer has a primary winding, a core and a secondary winding, although some transformers, including current transformers, use an air core. In principle, the only difference between a current transformer and a voltage transformer
  39. 39. 39 (normal type) is that the former is fed with a 'constant' current while the latter is fed with a 'constant' voltage, where 'constant' has the strict circuit theory meaning. The alternating current in the primary produces an alternating magnetic field in the core, which then induces an alternating current in the secondary. The primary circuit is largely unaffected by the insertion of the CT. Accurate current transformers need close coupling between the primary and secondary to ensure that the secondary current is proportional to the primary current over a wide current range. The current in the secondary is the current in the primary (assuming a single turn primary) divided by the number of turns of the secondary. In the illustration on the right, 'I' is the current in the primary, 'B' is the magnetic field, 'N' is the number of turns on the secondary, and 'A' is an AC ammeter. Current transformers typically consist of a silicon steel ring core wound with many turns of copper wire as shown in the illustration to the right. The conductor carrying the primary current is passed through the ring. The CT's primary therefore consists of a single 'turn'. The primary 'winding' may be a permanent part of the current transformer, i.e. a heavy copper bar to carry current through the core. Window-type current transformers (aka zero sequence current transformers, or ZSCT) are also common, which can have circuit cables run through the middle of an opening in the core to provide a single-turn primary winding. To assist accuracy, the primary conductor should be centered in the aperture. CTs are specified by their current ratio from primary to secondary. The rated secondary current is normally standardized at 1 or 5 amperes. For example, a 4000:5 CT secondary winding will supply an output current of 5 amperes when the primary winding current is 4000 amperes. This ratio can also be used to find the impedance or voltage on one side of the transformer, given the appropriate value at the other side. For the 4000:5 CT, the secondary impedance can be found as ZS = NZP = 800ZP, and the secondary voltage can be found as VS = NVP = 800VP. In some cases, the secondary impedance is referred to the primary side, and is found as ZS’ = N2ZP. Referring the impedance is done simply by multiplying initial secondary impedance value by the current ratio. The secondary winding of a CT can have taps to provide a range of ratios, five taps being common. Shapes and sizes vary depending on the end user or switch gear manufacturer. Low-voltage single ratio metering current transformers are either a ring type or plastic molded case.
  40. 40. 40 Split-core current transformers either have a two-part core or a core with a removable section. This allows the transformer to be placed around a conductor with minimum disturbance. Split-core current transformers are typically used in low current measuring instruments, often portable, battery-operated, and hand-held (see illustration lower right). Fig.7.2 Function of current transformer 7.3 USE OF CURRENT TRANSFORMER Current transformers are used extensively for measuring current and monitoring the operation of the power grid. Along with voltage leads, revenue-grade CTs drive the electrical utility's watt-hour meter on virtually every building with three-phase service and single-phase services greater than 200 amperes. High-voltage current transformers are mounted on porcelain or polymer insulators to isolate them from ground. Some CT configurations slip around the bushing of a high-voltage transformer or circuit breaker, which automatically centers the conductor inside the CT window. Current transformers can be mounted on the low voltage or high voltage leads of a power transformer. Sometimes a section of a bus bar can be removed to replace a current transformer.
  41. 41. 41 Often, multiple CTs are installed as a "stack" for various uses. For example, protection devices and revenue metering may use separate CTs to provide isolation between metering and protection circuits, and allows current transformers with different characteristics (accuracy, overload performance) to be used for the devices. The burden (load) impedance should not exceed the specified maximum value to avoid the secondary voltage exceeding the limits for the current transformer. The primary current rating of a current transformer should not be exceeded or the core may enter its non linear region and ultimately saturate. This would occur near the end of the first half of each half (positive and negative) of the AC sine wave in the primary and would compromise the accuracy. 7.4 SAFETY OF CURRENT TRANSFORMER Current transformers are often used to monitor hazardously high currents or currents at hazardously high voltages, so great care must be taken in the design and use of CTs in these situations. The secondary of a current transformer should not be disconnected from its burden while current is in the primary, as the secondary will attempt to continue driving current into an effective infinite impedance up to its insulation break-down voltage and thus compromise operator safety. For certain current transformers this voltage may reach several kilovolts and may cause arcing. Exceeding the secondary voltage may also degrade the accuracy of the transformer or destroy it. Energizing a current transformer with an open circuit secondary is the dual of energizing a voltage transformer (normal type) with a short circuit secondary. In the first case the secondary tries to produce an infinite voltage and in the second case the secondary tries to produce an infinite current. Both scenarios can be dangerous and damage the transformer.
  42. 42. 42 Chapter-8 CAPACITIVE VOLTAGE TRANSFORMER 8.1 INTRODUCTION A capacitor voltage transformer (CVT or CCVT), is a transformer used in power systems to step down extra high voltage signals and provide a low voltage signal, for metering or operating a protective relay. Fig. 8.1 A capacitor voltage transformer 8.2 COMPONENTS OF CAPACITIVE VOLTAGE TRANSFORMER line signal is split, an inductive element to tune the device to the line frequency, and a voltage transformer to isolate and further step down the voltage for metering devices or protective relay. In its most basic form, the device consists of three parts: two capacitors across which the transmission The tuning of the divider to the line frequency makes the overall division ratio less sensitive to changes in the burden of the connected metering or protection devices.[1] The device has at least four terminals: a terminal for connection to the high voltage signal, a ground terminal, and two secondary terminals which connect to the instrumentation or protective relay. In practice, capacitor C1 is often constructed as a stack of smaller capacitors connected in series. This provides a large voltage drop across C1 and a relatively small voltage drop across
  43. 43. 43 C2. As the majority of the voltage drop is on C1, this reduces the required insulation level of the voltage transformer. This makes CVTs more economical than the wound voltage transformers under high voltage (over 100kV), as the latter one requires more winding and materials. "Capacitive voltage transformers exist and are used by utilities for high-voltage (greater than 66 kV) metering. They have a capacitive voltage divider but also have a dual-winding transformer to couple the divided voltage to the metering circuit. They tend to have lower allowable burdens than a wound transformer but can be made economically at higher voltage ratings. Another difference is that even though they decrease voltage, they do not increase current as found in wound electromagnetic transformers - an ampere drawn by the load is an ampere drawn from the primary circuit. And of course they can only reduce voltage, not increase". The above is a part of a Wikipedia write-up. I might sum up the definition of a capacitor voltage transformer as a step down transformer with a convenient node of a series-connected capacitor network connected in series with the primary winding. The free end of the capacitor and the free end of the transformer primary constitute the primary terminals. This device is presently used as a potential transformer to monitor high voltages. Of course ordinary step down transformer does not employ series capacitor in the primary. 8.3 FREQUENCY RESPONSE With the rated load at the voltage transformer secondary side, The output voltage of CVT initially decrease a little bit, then reaches the resonance peak at around 800 Hz. Then it decreases drastically and remains almost level out after 2000hz. The C2 current is linear with frequency. The frequency response for voltage transformer current has a resonance peak at around 800 Hz. C2 current is substantially larger than voltage transformer current. The bus voltage in frequency domain can be calculated by summing the voltages on C1 and C2. From the calculation result it can be seen that the bus voltage only relates to C2current, voltage transformer current and their ratios.This result is helpful to reconstruct the bus voltage with the C2 current, voltage transformer current. For the ratio, it can be achieved by using a summing amplifier.
  44. 44. 44 Chapter-9 CIRCUIT BREAKER 9.1 INTRODUCTION A circuit breaker is an automatically operated electrical switch designed to protect an electrical circuit from damage caused by excess current, typically resulting from an overload or short circuit. Its basic function is to interrupt current flow after a fault is detected. Unlike a fuse, which operates once and then must be replaced, a circuit breaker can be reset (either manually or automatically) to resume normal operation. Circuit breakers are made in varying sizes, from small devices that protect low-current circuits or individual household appliance, up to large switchgear designed to protect high voltage circuits feeding an entire city. The generic function of a circuit breaker, RCD or a fuse, as an automatic means of removing power from a faulty system is often abbreviated as ADS (Automatic Disconnection of Supply). 9.2 OPERATION All circuit breaker systems have common features in their operation, but details vary substantially depending on the voltage class, current rating and type of the circuit breaker. The circuit breaker must firstly detect a fault condition. In small mains and low voltage circuit breakers, this is usually done within the device itself. Typically, the heating and/or magnetic effects of electric current are employed. Circuit breakers for large currents or high voltages are usually arranged with protective relay pilot devices to sense a fault condition and to operate the opening mechanism. These typically require a separate power source, such as a battery, although some high-voltage circuit breakers are self-contained with current transformers, protective relays, and an internal control power source. Once a fault is detected, the circuit breaker contacts must open to interrupt the circuit; This is commonly done using mechanically stored energy contained within the breaker, such as a spring or compressed air to separate the contacts. Circuit breakers may also use the higher current caused by the fault to separate the contacts, such as thermal expansion or a magnetic field. Small circuit breakers typically have a manual control lever to switch off the load or
  45. 45. 45 reset a tripped breaker, while larger units use solenoids to trip the mechanism, and electric motors to restore energy to the springs. The circuit breaker contacts must carry the load current without excessive heating, and must also withstand the heat of the arc produced when interrupting (opening) the circuit. Contacts are made of copper or copper alloys, silver alloys and other highly conductive materials. Service life of the contacts is limited by the erosion of contact material due to arcing while interrupting the current. Miniature and molded-case circuit breakers are usually discarded when the contacts have worn, but power circuit breakers and high-voltage circuit breakers have replaceable contacts. When a high current or voltage is interrupted, an arc is generated. The length of the arc is generally proportional to the voltage while the intensity (or heat) is proportional to the current. This arc must be contained, cooled and extinguished in a controlled way, so that the gap between the contacts can again withstand the voltage in the circuit. Different circuit breakers use vacuum, air, insulating gas, or oil as the medium the arc forms in. Different techniques are used to extinguish the arc including: i. Lengthening or deflecting the arc ii. Intensive cooling (in jet chambers) iii. Division into partial arcs iv.Zero point quenching (contacts open at the zero current time crossing of the AC waveform, effectively breaking no load current at the time of opening. The zero crossing occurs at twice the line frequency; i.e., 100 times per second for 50 Hz and 120 times per second for 60 Hz AC.) v. 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.
  46. 46. 46 9.3 ARC INTERRUPTION Low-voltage miniature circuit breakers (MCB) use air alone to extinguish the arc. These circuit breakers contain so-called arc chutes, a stack of mutually insulated parallel metal plates which divide and cool the arc. By splitting the arc into smaller arcs the arc is cooled down while the arc voltage is increased and serves as an additional impedance which limits the current through the circuit breaker. The current-carrying parts near the contacts provide easy deflection of the arc into the arc chutes by a magnetic force of a current path, although magnetic blowout coils or permanent magnets could also deflect the arc into the arc chute (used on circuit breakers for higher ratings). The number of plates in the arc chute is dependent on the short-circuit rating and nominal voltage of the circuit breaker. In larger ratings, oil circuit breakers rely upon vaporization of some of the oil to blast a jet of oil through the arc.[4] Gas (usually sulfur hexafluoride) circuit breakers sometimes stretch the arc using a magnetic field, and then rely upon the dielectric strengthof 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 (less than 2– 3 mm (0.079–0.118 in)). Vacuum circuit breakers are frequently used in modern medium- voltage switchgear to 38,000 volts. Air circuit breakers may use compressed air to blow out the arc, or alternatively, the contacts are rapidly swung into a small sealed chamber, the escaping of the displaced air thus blowing out the arc. Circuit breakers are usually able to terminate all current very quickly: typically the arc is extinguished between 30 ms and 150 ms after the mechanism has been tripped, depending upon age and construction of the device. The maximum current value and let-through energy determine the quality of the circuit breakers.
  47. 47. 47 9.4 SHORT CIRCUIT Circuit breakers are rated both by the normal current that they are expected to carry, and the maximum short-circuit current that they can safely interrupt. This latter figure is the ampere interrupting capacity (AIC) of the breaker. Under short-circuit conditions, the calculated maximum prospective short circuit current may be many times the normal, rated current of the circuit. When electrical contacts open to interrupt a large current, there is a tendency for an arc to form between the opened contacts, which would allow the current to continue. This condition can create conductive ionized gases and molten or vaporized metal, which can cause further continuation of the arc, or creation of additional short circuits, potentially resulting in the explosion of the circuit breaker and the equipment that it is installed in. Therefore, circuit breakers must incorporate various features to divide and extinguish the arc. The maximum short-circuit current that a breaker can interrupt is determined by testing. Application of a breaker in a circuit with a prospective short-circuit current higher than the breaker's interrupting capacity rating may result in failure of the breaker to safely interrupt a fault. In a worst-case scenario the breaker may successfully interrupt the fault, only to explode when reset. Typical domestic panel circuit breakers are rated to interrupt 10 kA (10000 A) short-circuit current. Miniature circuit breakers used to protect control circuits or small appliances may not have sufficient interrupting capacity to use at a panel board; these circuit breakers are called "supplemental circuit protectors" to distinguish them from distribution-type circuit breakers. 9.5 TYPES OF CIRCUIT BREAKER 9.5.1 High Voltage Circuit Breaker Electrical power transmission networks are protected and controlled by high-voltage breakers. The definition of high voltage varies but in power transmission work is usually thought to be 72.5 kV or higher, according to a recent definition by the International Electrotechnical Commission (IEC). High-voltage breakers are nearly always solenoid-
  48. 48. 48 operated, with current sensing protective relays operated through current transformers. In substations the protective relay scheme can be complex, protecting equipment and buses from various types of overload or ground/earth fault. High-voltage breakers are broadly classified by the medium used to extinguish the arc: i. Bulk oil ii. Minimum oil iii. Air blast iv. Vacuum v. SF6 vi, CO2 Due to environmental and cost concerns over insulating oil spills, most new breakers use SF6 gas to quench the arc. Circuit breakers can be classified as live tank, where the enclosure that contains the breaking mechanism is at line potential, or dead tank with the enclosure at earth potential. High- voltage AC circuit breakers are routinely available with ratings up to 765 kV. 1,200 kV breakers were launched by Siemens in November 2011, followed by ABB in April the following year. High-voltage circuit breakers used on transmission systems may be arranged to allow a single pole of a three-phase line to trip, instead of tripping all three poles; for some classes of faults this improves the system stability and availability. High-voltage direct current circuit breakers are still a field of research as of 2015. Such breakers would be useful to interconnect HVDC transmission systems.
  49. 49. 49 Fig.9.1 High Voltage Circuit Breaker 9.5.2 Sf6 Circuit Breaker A sulfur hexafluoride circuit breaker uses contacts surrounded by sulfur hexafluoride gas to quench the arc. They are most often used for transmission-level voltages and may be incorporated into compact gas-insulated switchgear. In cold climates, supplemental heating or de-rating of the circuit breakers may be required due to liquefaction of the SF6 gas.
  50. 50. 50 Fig, 9.2 SF6 Circuit Breaker
  51. 51. 51 Chapter-10 RELAYS 10.1 INTRODUCTION A relay is an electrically operated switch. Many relays use an electromagnet to mechanically operate a switch, but other operating principles are also used, such as solid-state relays. Relays are used where it is necessary to control a circuit by a separate low-power signal, or where several circuits must be controlled by one signal. The first relays were used in long distance telegraph circuits as amplifiers: they repeated the signal coming in from one circuit and re-transmitted it on another circuit. Relays were used extensively in telephone exchanges and early computers to perform logical operations. A type of relay that can handle the high power required to directly control an electric motor or other loads is called a contactor. Solid-state relayscontrol power circuits with no moving parts, instead using a semiconductor device to perform switching. Relays with calibrated operating characteristics and sometimes multiple operating coils are used to protect electrical circuits from overload or faults; in modern electric power systems these functions are performed by digital instruments still called "protective relays". Magnetic latching relays require one pulse of coil power to move their contacts in one direction, and another, redirected pulse to move them back. Repeated pulses from the same input have no effect. Magnetic latching relays are useful in applications where interrupted power should not be able to transition the contacts. Magnetic latching relays can have either single or dual coils. On a single coil device, the relay will operate in one direction when power is applied with one polarity, and will reset when the polarity is reversed. On a dual coil device, when polarized voltage is applied to the reset coil the contacts will transition. AC controlled magnetic latch relays have single coils that employ steering diodes to differentiate between operate and reset commands. 10.2 BASIC DESIGN AND OPERATION A simple electromagnetic relay consists of a coil of wire wrapped around a soft iron core (a solenoid), an iron yoke which provides a low reluctance path for magnetic flux, a movable
  52. 52. 52 iron armature, and one or more sets of contacts (there are two contacts in the relay pictured). The armature is hinged to the yoke and mechanically linked to one or more sets of moving contacts. The armature is held in place by a spring so that when the relay is de-energized there is an air gap in the magnetic circuit. In this condition, one of the two sets of contacts in the relay pictured is closed, and the other set is open. Other relays may have more or fewer sets of contacts depending on their function. The relay in the picture also has a wire connecting the armature to the yoke. This ensures continuity of the circuit between the moving contacts on the armature, and the circuit track on the printed circuit board (PCB) via the yoke, which is soldered to the PCB. When an electric current is passed through the coil it generates a magnetic field that activates the armature, and the consequent movement of the movable contact(s) either makes or breaks (depending upon construction) a connection with a fixed contact. If the set of contacts was closed when the relay was de-energized, then the movement opens the contacts and breaks the connection, and vice versa if the contacts were open. When the current to the coil is switched off, the armature is returned by a force, approximately half as strong as the magnetic force, to its relaxed position. Usually this force is provided by a spring, but gravity is also used commonly in industrial motor starters. Most relays are manufactured to operate quickly. In a low-voltage application this reduces noise; in a high voltage or current application it reduces arcing. When the coil is energized with direct current, a diode is often placed across the coil to dissipate the energy from the collapsing magnetic field at deactivation, which would otherwise generate a voltage spike dangerous to semiconductor circuit components. Such diodes were not widely used before the application of transistors as relay drivers, but soon became ubiquitous as early germanium transistors were easily destroyed by this surge. Some automotive relays include a diode inside the relay case. If the relay is driving a large, or especially a reactive load, there may be a similar problem of surge currents around the relay output contacts. In this case a snubber circuit (a capacitor and resistor in series) across the contacts may absorb the surge. Suitably rated capacitors and the associated resistor are sold as a single packaged component for this commonplace use.
  53. 53. 53 If the coil is designed to be energized with alternating current (AC), some method is used to split the flux into two out-of-phase components which add together, increasing the minimum pull on the armature during the AC cycle. Typically this is done with a small copper "shading ring" crimped around a portion of the core that creates the delayed, out-of-phase component, which holds the contacts during the zero crossings of the control voltage. 10.3 TYPES OF RELAYS 10.3.1 Latching Relay A latching relay (also called "impulse", "keep", or "stay" relays) maintains either contact position indefinitely without power applied to the coil. The advantage is that one coil consumes power only for an instant while the relay is being switched, and the relay contacts retain this setting across a power outage. A latching relay allows remote control of building lighting without the hum that may be produced from a continuously (AC) energized coil. In one mechanism, two opposing coils with an over-center spring or permanent magnet hold the contacts in position after the coil is de-energized. A pulse to one coil turns the relay on and a pulse to the opposite coil turns the relay off. This type is widely used where control is from simple switches or single-ended outputs of a control system, and such relays are found in avionics and numerous industrial applications. Another latching type has a remanent core that retains the contacts in the operated position by the remanent magnetism in the core. This type requires a current pulse of opposite polarity to release the contacts. A variation uses a permanent magnet that produces part of the force required to close the contact; the coil supplies sufficient force to move the contact open or closed by aiding or opposing the field of the permanent magnet. A polarity controlled relay needs changeover switches or an H bridge drive circuit to control it. The relay may be less expensive than other types, but this is partly offset by the increased costs in the external circuit. In another type, a ratchet relay has a ratchet mechanism that holds the contacts closed after the coil is momentarily energized. A second impulse, in the same or a separate coil, releases the contacts.[10] This type may be found in certain cars, for headlamp dipping and other functions where alternating operation on each switch actuation is needed.
  54. 54. 54 A stepping relay is a specialized kind of multi-way latching relay designed for early automatic telephone exchanges. An earth leakage circuit breaker includes a specialized latching relay. Very early computers often stored bits in a magnetically latching relay, such as ferreed or the later remreed in the 1ESS switch. Some early computers used ordinary relays as a kind of latch—they store bits in ordinary wire spring relays or reed relays by feeding an output wire back as an input, resulting in a feedback loop or sequential circuit. Such an electrically latching relay requires continuous power to maintain state, unlike magnetically latching relays or mechanically racheting relays. In computer memories, latching relays and other relays were replaced by delay line memory, which in turn was replaced by a series of ever-faster and ever-smaller memory technologies. 10.3.2 Coaxial Relay Where radio transmitters and receivers share one antenna, often a coaxial relay is used as a TR (transmit-receive) relay, which switches the antenna from the receiver to the transmitter. This protects the receiver from the high power of the transmitter. Such relays are often used in transceivers which combine transmitter and receiver in one unit. The relay contacts are designed not to reflect any radio frequency power back toward the source, and to provide very high isolation between receiver and transmitter terminals. The characteristic impedance of the relay is matched to the transmission line impedance of the system, for example, 50 ohms. 10.3.3 Time Delay Relay Timing relays are arranged for an intentional delay in operating their contacts. A very short (a fraction of a second) delay would use a copper disk between the armature and moving blade assembly. Current flowing in the disk maintains magnetic field for a short time, lengthening release time. For a slightly longer (up to a minute) delay, a dashpot is used. A dashpot is a piston filled with fluid that is allowed to escape slowly; both air-filled and oil-filled dashpots are used. The time period can be varied by increasing or decreasing the flow rate. For longer time periods, a mechanical clockwork timer is installed. Relays may be arranged for a fixed
  55. 55. 55 timing period, or may be field adjustable, or remotely set from a control panel. Modern microprocessor-based timing relays provide precision timing over a great range. Some relays are constructed with a kind of "shock absorber" mechanism attached to the armature which prevents immediate, full motion when the coil is either energized or de- energized. This addition gives the relay the property of time-delay actuation. Time-delay relays can be constructed to delay armature motion on coil energization, de-energization, or both. Time-delay relay contacts must be specified not only as either normally open or normally closed, but whether the delay operates in the direction of closing or in the direction of opening. The following is a description of the four basic types of time-delay relay contacts. First we have the normally open, timed-closed (NOTC) contact. This type of contact is normally open when the coil is unpowered (de-energized). The contact is closed by the application of power to the relay coil, but only after the coil has been continuously powered for the specified amount of time. In other words, the direction of the contact's motion (either to close or to open) is identical to a regular NO contact, but there is a delay in closing direction. Because the delay occurs in the direction of coil energization, this type of contact is alternatively known as a normally open, on-delay: 10.3.4 Buchholz Relay A Buchholz relay is a safety device sensing the accumulation of gas in large oil- filled transformers, which will alarm on slow accumulation of gas or shut down the transformer if gas is produced rapidly in the transformer oil. The contacts are not operated by an electric current but by the pressure of accumulated gas or oil flow. 10.4 POLE AND THROW Since relays are switches, the terminology applied to switches is also applied to relays; a relay switches one or more poles, each of whose contacts can be thrown by energizing the coil. Normally open (NO) contacts connect the circuit when the relay is activated; the circuit is disconnected when the relay is inactive. Normally closed (NC) contacts disconnect the circuit when the relay is activated; the circuit is connected when the relay is inactive. All of the contact forms involve combinations of NO and NC connections.
  56. 56. 56 The National Association of Relay Manufacturers and its successor, the Relay and Switch Industry Association define 23 distinct electrical contact forms found in relays and switches. Of these, the following are commonly encountered: (i) SPST-NO (Single-Pole Single-Throw, Normally-Open) relays have a single Form A contact or make contact. These have two terminals which can be connected or disconnected. Including two for the coil, such a relay has four terminals in total. (ii) SPST-NC (Single-Pole Single-Throw, Normally-Closed) relays have a single Form B or break contact. As with an SPST-NO relay, such a relay has four terminals in total. (iii) SPDT (Single-Pole Double-Throw) relays have a single set of Form C, break before make or transfer contacts. That is, a common terminal connects to either of two others, never connecting to both at the same time. Including two for the coil, such a relay has a total of five terminals. (iv) DPST – Double-Pole Single-Throw relays are equivalent to a pair of SPST switches or relays actuated by a single coil. Including two for the coil, such a relay has a total of six terminals. The poles may be Form A or Form B (or one of each; the designations NO and NC should be used to resolve the ambiguity). (v) DPDT – Double-Pole Double-Throw relays have two sets of Form C contacts. These are equivalent to two SPDT switches or relays actuated by a single coil. Such a relay has eight terminals, including the coil (vi) The S (single) or D (double) designator for the pole count may be replaced with a number, indicating multiple contacts connected to a single actuator. For example, 4PDT indicates a four-pole double-throw relay that has 12 switching terminals.
  57. 57. 57 CONCLUSION Training at 132KV GSS CPWD,JAIPUR gives the insight of the real instruments used. There are many instruments like transformer, CT, PT, CVT, LA, relay, PLCC, bus bars, capacitor bank, insulator, isolators, Battery etc. What is the various problem seen in substation while handling this instruments. There are various occasion when relay operate and circuit breaker open, load shedding, shut down, which has been heard previously.To get insight of the substation, how things operate, how things manage all is learned there. Practical training as a whole proved to be extremely informative and experience building and the things learnt at it would definitely help a lot in snapping the future ahead a better way.Transmission and distribution stations exist at various scales throughout a power system. In general, they represent an interface between different levels or sections of the power system, with the capability to switch or reconfigure the connections among various transmission and distribution lines. The major stations include a control room from which operations are coordinated. Smaller distribution substations follow the same principle of receiving power at higher voltage on one side and sending out a number of distribution feeders at lower voltage on the other, but they serve a more limited local area and are generally unstaffed. The central component of the substation is the transformer, as it provides the effective in enface between the high- and low-voltage parts of the system. Other crucial components are circuit breakers and switches. Breakers serve as protective devices that open automatically in the event of a fault, that is, when a protective relay indicates excessive current due to some abnormal condition. Switches are control devices that can be opened or closed deliberately to establish or break a connection. An important difference between circuit breakers and switches is that breakers are designed to interrupt abnormally high currents (as they occur only in those very situations for which circuit protection is needed), whereas regular switches are designed to be operable under normal currents. Breakers are placed on both the high- and low-voltage side of transformers. Finally, substations may also include capacitor banks to provide voltage support.
  58. 58. 58 BIBLIOGRAPHY 1. “Electronic Communication System”, George Kennedy, Brendan Devis, S.R.M.Prasanna, TataMcGraw Hills, Third Edition, 2010. 2.“Carrier Commnucation Over Power Limes”, Heinrich,Podszeck,SpringerPublication,Second Edition,2008 3. “Principle of Electrical Transmission Line in Power and Communication”,J.H.Gridley, P.Hammond , Elsevier Science Publication, Fourt Edition,2011 4. Wikipedia 5. Manuals present at RRVPNL, JAIPUR 6. According training notes