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Training at 132 kv substation

B.E. - EEE
ROLL NO.- UE 114069
Training has an important role in exposing the real life situation in
an industry. It was a great experien...
1. Introduction
1.1 Types of substation
2. Steps in designing a substation
2.1 Earthing & Bonding
2.2 Substaion E...
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Training at 132 kv substation

  1. 1. INDUSTRIAL TRAINING AT 132 KV BISSAU SUBSTATION (RVPNL) TEJVEER CHOUDHARY B.E. - EEE ROLL NO.- UE 114069 4TH YEAR University Institute of Engineering & Technology Panjab University, Chandigarh
  2. 2. ACKNOWLEDGEMENT Training has an important role in exposing the real life situation in an industry. It was a great experience for me to work on training at 132 kV BISSAU (RVPNL) Substation, BISSAU through which I could learn how to work in a professional environment. Now, I would like to thank the people who guided me and have been a constant source of inspiration throughout the tenure of my summer training. I am sincerely grateful to Mr. Dilip Singh (Assistant Engineer) at 132 kV substation, BISSAU who rendered me his valuable assistance, precious time, constant encouragement and able guidance which made this training actually possible. -TEJVEER CHOUDHARY
  3. 3. Contents 1. Introduction 1.1 Types of substation 2. Steps in designing a substation 2.1 Earthing & Bonding 2.2 Substaion Earthing Calculation Methodology 2.3 Earthing Material 2.4 Layout of Substaion 2.5 Design of Busbar 3. Single line diagram of Bissau Substaion 4. Transformer 4.1 Current Transformer 4.2 Capacitor Voltage Transformer 4.3 CVT Frequency Response 4.4 Bus Voltage Representation of CVT 5. Circuit Breaker 5.1 SF6 Circuit Breaker 5.2 Vacuum Circuit Breaker 6. Protective Relay 6.1 Differential Relay 6.2 Overcurrent Relay 6.3 Directional Relay 6.4 Buchholz Relay
  4. 4. 7. Miscellaneous 7.1 Isolator 7.2 Wavetrap 7.3 Busbars 7.4 Substation Grounding System 7.5 Insulators 7.6 Earth Wires & OPGW 7.7 Conductors 7.8 Support Structure (POLE) 7.9 Types of Conductor for Power Transmission 7.10 DC Supply Room (Battery) 7.11 Lightening Arrester 8. Conclusion
  5. 5. 1. Introduction FIG- View of Substation The present day electrical power is generated, transmitted and distributed in the form of the alternating current. The electric power is produced at power plant stations which are located at favorable places generally quite away from the consumers. It is delivered to the consumers through a large network of transmission and distribution. At many places in the power system, it may be desirable and necessary to change some characteristics e.g. voltage, ac to dc, frequency, power factor etc. of electric supply. This accomplished by suitable apparatus called substation. For example; generation voltage (11 KV or 33 KV) at the power station is set up to high voltage (say 220 KV or 132 KV) for transmission of electric power. The assembly of apparatus (e.g. transformer etc.) used for this purpose in the substation. Similarly near the consumer’s localities, the voltage may have to be step down to utilization level. This job is again accomplished by suitable apparatus called substation. The assembly of apparatus to change some characteristic of electric power supply is called substation.
  6. 6. 1.1 Types of Substation: (A) According to the service requirement: 1) Transformer substation 2) Switch substation 3) Power factor correction substation 4) Frequency change substation 5) Converting substation (B) According to the constructional features: 1) Indoor substation 2) Outdoor substation 3) Underground substation 4) Pole mounted substation
  7. 7. 2. Steps in Designing a Substation 2.1 Earthing and Bonding The function of an earthing and bonding system is to provide an earthing system connection to which transformer neutrals or earthingimpedances may be connected in order to pass the maximum fault current. The earthing system also ensures that no thermal or mechanical damage occurs on the equipment within the substation, thereby resulting in safety to operation and maintenance personnel. The earthing system also guarantees equipotential bonding such that there are no dangerous potential gradients developed in the substation. In designing the substation, three voltage have to be considered. 1. Touch Voltage: This is the difference in potential between the surface potential and the potential at an earthed equipment whilst a man is standing and touching the earthed structure. 2. Step Voltage: This is the potential difference developed when a man bridges a distance of 1m with his feet while not touching any other earthed equipment. 3. Mesh Voltage: This is the maximum touch voltage that is developed in the mesh of the earthing grid. 2.2 Substation Earthing Calculation Methodology Calculations for earth impedances and touch and step potentials are based on site measurements of ground resistivity and system fault levels. A grid layout with particular conductors is then analysed to determine the effective substation earthing resistance, from which the earthing voltage is calculated. In practice, it is normal to take the highest fault level for substation earth grid calculation purposes. Additionally, it is necessary to ensure a sufficient margin such that expansion of the system is catered for. To determine the earth resistivity, probe tests are carried out on the site. These tests are best performed in dry weather such that conservative resistivity readings are obtained.
  8. 8. 2.3 Earthing Materials 1). Conductors: Bare copper conductor is usually used for the substation earthing grid. The copper bars themselves usually have a cross- sectional area of 95 square millimetres, and they are laid at a shallow depth of 0.25-0.5m, in 3-7m squares. In addition to the buried potential earth grid, a separate above ground earthing ring is usually provided, to which all metallic substation plant is bonded. . 2.) Connections: Connections to the grid and other earthing joints should not be soldered because the heat generated during fault conditions could cause a soldered joint to fail. Joints are usually bolted, and in this case, the face of the joints should be tinned. 3). Earthing Rods: The earthing grid must be supplemented by earthingrods to assist in the dissipation of earth fault currents and further re- duce the overall substation earthing resistance. These rods are usually made of solid copper, or copper clad steel. 4). Switchyard Fence Earthing: The switchyard fence earthing practices are possible and are used by different utilities. These are: (a) Extend the substation earth grid 0.5m-1.5m beyond the fence perimeter. The fence is then bonded to the grid at regular intervals. (b) Place the fence beyond the perimeter of the switchyard earthinggrid and bond the fence to its own earthing rod system. This earthing rod system is not coupled to the main substation earthinggrid. 2.4 Layout of Substation The layout of the substation is very important since there should be a Security of Supply. In an ideal substation all circuits and equipment would be duplicated such that following a fault, or during maintenance, a connection remains available. Practically this is not feasible since the cost of implementing such a design is very high. Methods have been adopted to achieve a compromise between complete security of supply and capital investment. There are four categories of substation that give varying securities of supply: • Category 1: No outage is necessary within the substation for either maintenance or fault conditions. • Category 2: Short outage is necessary to transfer the load to an alter- native circuit for maintenance or fault conditions.
  9. 9. • Category 3: Loss of a circuit or section of the substation due to fault or maintenance. • Category 4: Loss of the entire substation due to fault or maintenance. 2.5 Design of Bus Bars Bus bars are Cu/Al rods of thin walled tubes and operate at constant voltage. The bus-bars are designed to carry normal current continuously. The cross section of conductors is designed on the basis of rated normal current and the following factors: System voltage, position of substation. Flexibility, reliability of supply and cost. Our design must ensure easy and uninterrupted maintenance, avoiding any danger to the operating of operating personnel. It must be simple in design and must possess provision for future extension. Any fluctuation of load must not hinder its mechanical characters. The sub-station bus bars are broadly classified in the following three categories: 1. Outdoor rigid tubular bus-bars. 2. Outdoor flexible ACSR or Al alloy bus-bars. 3. Indoor bus bars.
  10. 10. 3. Single Line Diagram of BISSAU Substation FIG- Single line diagram
  11. 11. 4. Transformer Transformer is a static machine, which transforms the potential of alternating current at same frequency. It means the transformer transforms the low voltage into high voltage and high voltage to low voltage at same frequency. It works on the principle of static induction principle. When the energy is transformed into a higher voltage, the transformer is called step up transformer but in case of other is known as step down transformer. 4.1 Current Trnsformer:- A current transformer (CT) is used for measurement of alternating electric currents. Current transformers, together with voltage (or potential) transformers (VT or PT), are known as instrument transformers. When current in a circuit is too high to apply directly to measuring instruments, a current transformer produces a reduced current accurately proportional to the current in the circuit, which can be conveniently connected to measuring and recording instruments. A current transformer isolates the measuring instruments from what may be very high voltage in the monitored circuit. Current transformers are commonly used in metering and protective relays in the electrical power industry. An essential objective of current transformer design is to ensure the primary and secondary circuits are efficiently coupled, so the secondary current is linearly proportional to the primary current. The CT's primary circuit consists of a single 'turn' of conductor, with a secondary of many tens or hundreds of turns. Usage:- Current transformers are used extensively for measuring current and monitoring the operation of the power grid. 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 primary circuit is largely unaffected by the insertion of the CT. Load, or burden, of the CT should be a low resistance. If the voltage time integral area is higher than the core's design rating, the core goes into saturation toward the end of each cycle, distorting the waveform and affecting accuracy.
  12. 12. Safety precautions Care must be taken that the secondary of a current transformer is not disconnected from its load while current is in the primary, as the transformer secondary will attempt to continue driving current across the effectively infinite impedance up to its core saturation voltage. This may produce a high voltage across the open secondary into the range of several kilovolts, causing arcing, compromising operator and equipment safety, or permanently affect the accuracy of the transformer. Accuracy The accuracy of a CT is directly related to a number of factors including:  Burden  Burden class/saturation class  Rating factor  Load  External electromagnetic fields  Temperature and Physical configuration.  The selected tap, for multi ratio CTs  Phase change 4.2 CAPACITOR VOLTAGE TRANSFORMER (PT):- A capacitor voltage transformer (CVT), or capacitance coupled voltage transformer (CCVT), is a transformer used in power systems to step down extra high voltage signals and provide a low voltage signal, for metering or operating a protective relay. Components In its most basic form, the device consists of three parts: two capacitors across which the transmission line signal is split, an inductive element to tune the device to the line frequency, and a voltage transformer to isolate and further step down the voltage for the metering devices or protective relay. The tuning of the divider to the line frequency makes the overall division ratio less sensitive to changes in the burden of the connected metering or protection devices. In practice, capacitor C1 is often constructed as a stack of smaller capacitors connected in series. This provides a large voltage drop across C1 and a relatively small voltage drop across C2. As the majority of the voltage drop is on C1, this reduces the isolation 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.
  13. 13. 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 800Hz. Then it decreases drastically and remains almost level out after 2000Hz. 4.3 CVT Current Frequency Response The C2 current is linear with frequency. The frequency response for voltage transformer current has a resonance peak at around 800Hz. C2 current is substantially larger than voltage transformer current. 4.4 Bus Voltage Representation of CVT 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 C2 current, 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. Other Applications :- The CVT is also useful in communication systems. CVTs in combination with wave traps are used for filtering high frequency communication signals from power frequency. This forms a carrier communication network throughout the transmission network. FIG- CVT
  14. 14. 5. Circuit Breaker There are two types of C.B.: 1) Sulphur hexafluoride circuit breaker (SF6) 2) Vacuum circuit breaker SF6 circuit breaker are being used in 132KV and Vacuum circuit breaker are being used in 33KV side. 5.1 SF6 Circuit Breaker In such circuit breaker, sulphar hexafluoride (SF6) gas is used as the arc quenching medium. The SF6 is an electronegative gas and has a strong tendency to absorb free electrons. The SF6 circuit breaker have been found to a very effective for high power and high voltage service. SF6 circuit breakers have been developed for voltage 115 KV to 230 KV, power rating 10 MVA. Ratings: • Make- Crompton Greaves • Rated voltage- 132 Kv • Rated normal current- 3150A • C.T. ratio- 800-400-200/1 • Gas weight- 8kg • Gas Pressure- 7 kg/sq. cm • Weight of oil- 150 liters
  15. 15. FIG- SF6 Circuit Breaker It consists of fixed and moving contacts. It has chamber, contains SF6 gas. When the contacts are opened, the mechanism permits a high pressure SF6 gas from reservoir to flow towards the arc interruption chamber. The moving contact permits the SF6 gas to let through these holes.
  16. 16. 5.2 Vacuum Circuit Breaker Vacuum circuit breakers are circuit breakers which are used to protect medium and high voltage circuits from dangerous electrical situations. Like other types of circuit breakers, vacuum circuit breakers literally break the circuit so that energy cannot continue flowing through it, thereby preventing fires, power surges, and other problems which may emerge. These devices have been utilized since the 1920s, and several companies have introduced refinements to make them even safer and more effective. FIG- Vacuum Circuit Breaker
  17. 17. 6. Protective Relay FIG- Relay Panel In a power system it is inevitable that immediately or later some failure does occur somewhere in the system. When a failure occurs on any part of the system, it must be quickly detected and disconnected from the system. Rapid disconnection of faulted apparatus limits the amount of damage to it and prevents the effects of fault from spreading into the system. For high voltage circuits relays are employed to serve the desired function of automatic protective gear. The relays detect the fault and supply the information to the circuit breaker. The electrical quantities which may change under fault condition are voltage, frequency, current, phase angle. When a short circuit occurs at any point on the transmission line the current flowing in the line increases to the enormous value.This result in a heavy current flow through the relay coil, causing the relay to operate by closing its contacts. This in turn closes the trip circuit of the breaker making the circuit breaker open and isolating the faulty section from the rest of the system. In this way, the relay ensures the safety of the circuit equipment from the damage and normal working of the healthy.
  18. 18. 6.1 Differential Relay FIG- Differential Relay A differential relay is one that operates when vector difference of the two or more electrical quantities exceeds a predetermined value. If this differential quantity is equal or greater than the pickup value, the relay will operate and open the circuit breaker to isolate the faulty section.
  19. 19. 6.2 Over Current Relay This type of relay works when current in the circuit exceeds the predetermined value. The actuating source is the current in the circuit supplied to the relay from a current transformer. These relay are used on A.C. circuit only and can operate for fault flow in the either direction. This relay operates when phase to phase fault occurs. FIG- Over Current Relay
  20. 20. 6.3 Directional Relay This type of relay is in the conjunction with main relay. When main relay sense any fault in the system, it immediately operates the trip relay to disconnect the faulty section from the section. FIG- Directional Relay 6.4 BUCHHOLZ RELAY Buchholz relay is a safety device mounted on some oil filled powertransformers and reactors, equipped with an external overhead oil reservoir. The Buchholz Relay is used as aprotective device sensitive to the effects of dielectric failure insidethe equipment. The relay hasmultiple methods to detect afailing transformer. On a slow accumulation of gas, due perhaps toslight overload, gas produced by decomposition of insulating oilaccumulates in the top of the relay and forces the oil level down. Afloat switch in the relay is used to initiate an alarm signal.
  21. 21. Depending on design, a second float may also serve to detect slowoil leaks.If an arc forms, gas accumulation is rapid, and oil flows rapidly intothe conservator. This flow of oil operates a switch attached to a vanelocated in the path of the moving oil. This switch normally willoperate a circuit breaker to isolate the apparatus before the faultcauses additional damage. Buchholz relays have a test port to allowthe accumulated gas to be withdrawn for testing. Flammable gasfound in the relay indicates some internal fault such as overheatingor arcing, whereas air found in the relay may only indicate low oil level or a leak. FIG- BUCHHOLZ RELAY
  22. 22. 7. MISCELLANEOUS 7.1 Isolator Circuit breaker always trip the circuit but open contacts of breaker cannot be visible physically from outside of the breaker and that is why it is recommended not to touch any electrical circuit just by switching off the circuit breaker. So for better safety there must be some arrangement so that one can see open condition of the section of the circuit before touching it. Isolator is a mechanical switch which isolates a part of circuit from system as when required. Electrical isolators separate a part of the system from rest for safe maintenance works. So definition of isolator can be rewritten as Isolator is a manually operated mechanical switch which separates a part of the electrical power system normally at off load condition. FIG- Isolator
  23. 23. Types of Electrical Isolators : There are different types of isolators available depending upon system requirement such as : 1) Double Break Isolator 2) Single Break Isolator 3) Pantograph type Isolator. Depending upon the position in power system, the isolators can be categorized as 1) Bus side isolator – the isolator is directly connected with main bus. 2) Line side isolator – the isolator is situated at line side of any feeder. 3) Transfer bus side isolator – the isolator is directly connected with transfer bus. 7.2 Wavetrap A line trap (high-frequency stopper) is a maintenance-free parallel resonant circuit, mounted inline on high voltage AC transmissionpower lines to prevent the transmission of high frequency (40 kHz to 1000 kHz) carrier signals of power line communication to unwanted destinations. Line traps are cylinder like structures connected in series with HV transmission lines. A line trap is also called a wave trap. The line trap acts as a barrier or filter to prevent signal losses. The inductive reactance of the line trap presents a high reactance to high-frequency signals but a low to mains frequency. What this does is prevent carrier signals from being dissipated in the substation or in a tap line/branch of the main transmission path and grounds in the case of anything happening outside of the carrier transmission path. The line trap is also used to attenuate the shunting effects of high voltage lines.
  24. 24. FIG- WAVETRAP 7.3 Busbars When numbers of generators or feeders operating at the same voltage have to be directly connected electrically, bus bar is used as the common electrical component. Bus bars are made up of copper rods operate at constant voltage. The following are the important bus bars arrangements used at substations: Single bus bar system Single bus bar system with section alisation.
  25. 25. FIG- Busbars Auxiliary bus bar system In large stations it is important that break downs and maintenance should interfere as little as possible with continuity of supply to achieve this, dupli- cate bus bar system is used. Such a system consists of two bus bars, a main bus bar and a spare bus bar with the help of bus coupler, which consist of the circuit breaker and isolator. In substations, it is often desired to disconnect a part of the system for general maintenance and repairs. An isolating switch or isolator accomplishes this. Isolator operates under no load condition. It does not have any specified current breaking capacity or current making capacity. In some cases isolators are used to breaking charging currents or transmission lines. While opening a circuit, the circuit breaker is opened first then isolator while closing a circuit the isolator is closed first, then circuit breakers. Isolators are necessary on supply side of circuit breakers, in order to ensure isolation of the circuit breaker from live parts for the purpose of maintenance. A transfer isolator is used to transfer main supply from main bus to transfer bus by using bus coupler (combination of a circuit breaker with two isolators), if repairing or maintenance of any section is required.
  26. 26. 7.4 Substation Grounding System There are many parameters that have an effect on the voltages in and around the substation area. Since voltages are site-dependent, it is impossible to design one grounding system that is acceptable for all locations. The grid current, fault duration, soil resistivity, surface material, and the size and shape of the grid all have a substantial effect on the voltages in and around the substation area. If the geometry, location of ground electrodes, local soil characteristics, and other factors contribute to an excessive potential gradient at the earth surface, the grounding system may be inadequate from a safety aspect despite its capacity to carry the fault current in magnitudes and durations permitted by protective relays. During typical ground fault conditions, unless proper precautions are taken in design, the maximum potential gradients along the earth surface may be of sufficient magnitude to endanger a person in the area. Moreover, hazardous voltages may develop between grounded structures or equipment frames and the nearby earth. FIG- GROUNDING OF SUBSTATION  Relatively high fault current to ground in relation to the area of the grounding system and its resistance to remote earth  Soil resistivity and distribution of ground currents such that high potential gradients may occur at points at the earth surface
  27. 27.  Presence of a person at such a point, time, and position that the body is bridging two points of high potential difference  Absence of sufficient contact resistance or other series resistance to limit current through the body to a safe value under the above circumstances  Duration of the fault and body contact and, hence, of the flow of current through a human body for a sufficient time to cause harm at the given current intensity The relative infrequency of accidents is due largely to the low probability of coincidence of the above unfavorable conditions. To provide a safe condition for personnel within and around the substation area, the grounding system design limits the potential difference a person can come in contact with to safe levels.
  28. 28. 7.5 INSULATORS Insulators proposed for use on this Project will be polymeric-type post insulators. They provide a connection between conductors and structures and ensure electrical insulation between the high voltage of the conductors and the (earthed) pole. The length of the insulators depends on line voltage, clearance requirements and environmental considerations. For example, additional length is required for acidic or salty air, neither of these conditions occurs within the Project Area. Special galvanised steel of aluminium fittings connect both the line end of the insulator to the conductors and the pole end to the structure. Typical post insulators are shown on the pole structure in Photo. FIG- Typical post insulators 7.6 Earthwires and OPGW Overhead earthwires provide protection to the conductors from direct strike by lightning and also support optical fibre cables used for communication purposes. Two earthwires, one of which will be the optical fibre pilot ground wire (OPGW) are fitted to each structure. OPGW construction will typically be of the ‘slotted core’ type with 48 optical fibres supported by a combination of aluminium and steel wires. The earthwire will generally be of the AAC type, which consists of aluminium wires. For this Project, the proposed earthwire and OPGW will each have overall diameters of approximately 14 millimetres (mm), although this may be subject to change to suit the final design.
  29. 29. 7.7 Conductors Conductors (wires) are the part of the power-line which transports high voltage electricity. Conductors proposed to be used for the power-line would consist of a strandedaluminium alloy with an overall diameter of around 33.8 mm. Each pole structure would support six conductors, plus two earth wires to protect the line from lighting strikes. For a typical doublecircuit 132 kV pole, the distance between pairs of conductors on steel poles will be approximately 1.9 m on the vertical plane and 4.6 m on the horizontal plane (Figure 3.1). Electrical continuity on angle structures is provided by ‘bridging conductors’ which are suspended beneath the main conductors or around the outside of the pole connecting the terminated conductors on either side of the pole. Where necessary, these conductors are restrained by ‘bridging insulators’ to maintain electrical clearance. The minimum electrical clearance distance from the ground to the lowest point of the conductors is 6.7 m; however as noted above, larger clearances are often required by the DTMR over road and rail crossings. 7.8 Support structures( POLE) :- Poles are self-supporting structures used to keep the high voltage conductors separate from each other, clear from vegetation, the ground and other obstacles. Minimum clearance requirements between energized conductors and various types of obstacles are specified by the Electrical Safety Regulations 2002. For this Project a mixture self-supporting concrete or steel poles are envisaged. The distance between support structures (span) and their height is determined by topography and clearance requirements. Poles are made in a range of heights, generally in 1 m increments, to allow optimum height of structure to be provided at each site. The standard overall length of a typical double-circuit RVPNL concrete pole is around 25 to 40 m long. When installed, the ‘out of ground’ height to the top of the pole will typically be in the range of 20 to 35 m, with taller and shorter structures used as required to suit the terrain and other constraints. Typically, shorter poles are found on elevated areas such as hills, with taller poles in gullies or where additional clearance is required over a mid-span obstacle. The ‘duty’ of a pole structure is related to the method by which it supports the conductors and this in turn influences the type of structure used. The two means of conductor support are intermediate and tension.
  30. 30. Intermediate structures are used where the line follows a straight line or has a very small deviation angle, generally less than four degrees. The structures are designed to carry the weight (vertical load) of the conductors, and transverse (horizontal) load from wind on the conductors, earthwires and on the structure itself. A typical intermediate pole is shown. Features of this type of pole are their relatively light construction and include three post type insulators on each side of the upper pole (superstructure) directly supporting the conductors. Tension structures are used for line deviation angles greater than about four degrees, and at line terminations. A typical tension pole is shown. Requirements for staying of poles or the need to install double poles to address torsional moments on the structures will depend upon the change of direction of the powerline that the pole needs to accommodate. A suite of poles may be designed for a particular project to cover a range of angle duties and these remain to be progressed by RVPNL. It is expected that these tension poles will look similar in silhouette to the intermediate poles. FIG- Typical intermediate pole FIG- Typical tension pole
  31. 31. FIG- Dimensions of a typical RVPNL concrete pole
  32. 32. Various designs of concrete and steel poles have been used in Queensland over the last 30 years. These poles have evolved into a standard form of support structure for 132 kV and 110 kV high-voltage powerline construction in non-cyclonic areas. Each pole is manufactured off site and transported to the required location in one or two more sections, depending on the pole’s overall length. Additional components, such as insulators or earthwire attachment fittings, are fabricated from galvanized steel and are attached to each pole prior to erection. The standard foundation for each pole is formed by backfilling (with concrete) around the pole erected in an augured (bored) hole, in a similar manner to that used for traditional timber poles. The typical diameter of a pole at ground level is approximately 700 mm. The double-circuit powerline will be constructed using typical RVPNL concrete or steel poles spaced approximately 250 m apart, with two sets of three conductors, and twin earthwires strung between the tops of the poles above the power conductors. Single-circuit powerlines have only one set of three conductors and a single earthwire. Ground clearance The conductors will be strung with a minimum statutory vertical clearance of 6.7 m between the bottom conductor and the road or ground under all operating conditions. The height and spacing of individual supporting structures will vary depending on the terrain, vegetation and other factors. When installed, a typical double-circuit pole will be 20–35 m high, with taller and shorter structures being used as required to suit the terrain and other constraints. Where required by various regulations, additional clearance, usually necessitating taller supporting poles, will be provided to cross flood-prone areas, sensitive vegetation or facilities such as roads, railway lines and other infrastructure. The line will be designed to meet these clearances when the conductors are operating at their highest temperature and hence have sagged by the greatest amount. Under normal operating conditions, ground clearances will be significantly greater than the statutory minimum.
  33. 33. 7.9 TYPES OF CONDUCTOR FOR POWER TRANSMISSION In the early days conductor used on transmission lines were usually Copper, but Aluminium Conductors have completely replaced Copper because of the much lower cost and lighter weight of Aluminium conductor compared with a Copper conductor of the same resistance. The fact that Aluminium conductor has a larger diameter than a Copper conductor of the same resistance is also an advantage. With a larger diameter the lines of electric flux originating on the conductor will be farther apart at the conductor surface for the same voltage. This means a lower voltage gradient at the conductor surface and less tendency to ionize the air around the conductor. Ionization produces the undesirable effect called corona. The following sizes have now been standardized for transmission lines of different voltages: (i) For 132 KV lines: 'Panther' ACSR having 7strands of steel of dia 3.00 mm and 30Strands of Aluminum of dia 3.00 mm (ii) For 220 KV lines: 'Zebra' ACSR having 7strands of steel of dia 3.18 mm and 54Strands of Aluminium of dia 3.18 mm. (iii) For 400 KV lines: Twin 'Moose' ACSR having 7Strands of steel of dia 3.53 mm and 54Strands of Aluminium of dia 3.53 mm. 7.10 DC SUPPLY ROOM (BATTERY) It is used to calibrate relays & other measuring meters. It is also called heart of a power system. The room where these batteries are kept is painted black & is also fully/nearly closed so that batteries can’t get exposed to sun light so they don’t leak or their efficiency remain as rated. 7.11 LIGHTENING ARRESTER (132 kv) A lightning arrester is a device used on electrical power systems and telecommunications systems to protect the insulation and conductors of the system from the damaging effects of lightning. The typical lightning arrester has a high voltage terminal and a ground terminal. When a lightning surge (or switching surge, which is very similar) travels along the power line to the arrester, the current from the surge is diverted through the arrestor, in most cases to earth. In telegraphy and telephony, a lightning arrestor is placed where wires enter a structure,
  34. 34. preventing damage to electronic instruments within and ensuring the safety of individuals near them. Smaller versions of lightning arresters, also called surge protectors, are devices that are connected between each electrical conductor in power and communications systems and the Earth. Their purpose is to limit the rise in voltage when a communications or power line is struck by lightning or is near to a lightning strike. If protection fails or is absent, lightening that strikes the electrical system introduces thousands of kilovolts that may damage the transmission lines, and can also cause severe damage to transformers and other electrical or electronic devices. Lightning produced extreme voltage spikes in incoming power lines can damage electrical home appliances. FIG- Lightning Arresters Types of Lightning Arresters for outdoor application There are several types of lightening arresterin general use. They differ only in constructional details but operate on the same principle, providing low resistance path for the surges to the round. 1. Rod arrester 2. Expulsion type lightning arrester 3. Valve type lightning arrester 1.Rod arrester It is a very simple type of diverter and consists of two rods, which are bent at right angles with a gap in between. One rod is connected to the line circuit and the other rod is connected to earth. The distance between gap and insulator (i.e. distance P) must not be less than one third of the gap length so that the arc may not reach the insulator and damage it.
  35. 35. Generally, the gap length is so adjusted that breakdown should occur at 80% of spark-voltage in order to avoid cascading of very steep wave fronts across the insulators. Under normal operating conditions, the gap remains non-conducting. On the occurrence of a high voltage surge on the line, the gap sparks over and the surge current is conducted to earth. In this way excess charge on the line due to the surge is harmlessly conducted to earth FIG- ROD ARRESTER 2. Expulsion type arrester This type of arrester is also called ‘protector tube’ and is commonly used on system operating at voltages up to 33kV. It essentially consists of a rod gap AA’ in series with a second gap enclosed within the fiber tube. The gap in the fiber tube is formed by two electrodes. The upper electrode is connected to rod gap and the lower electrode to the earth. One expulsion arrester is placed under each line conductor. On the occurrence of an over voltage on the line, the series gap AA’ spanned and an arc is stuck between the electrodes in the tube. The heat of the arc vaporizes some of the fiber of tube walls resulting in the production of neutral gas. In an extremely short time, the gas builds up high pressure and is expelled through the lower electrode, which is hollow. As the gas leaves the tube violently it carries away ionized air around the arc. This de-ionizing effect is generally so strong that the arc goes out at a current zero and will not be reestablished.
  36. 36. FIG- Expulsion type arrester 3. Valve type arrester Valve type arresters incorporate non linear resistors and are extensively used on systems, operating at high voltages. It consists of two assemblies (i) series spark gaps and (ii) non- linear resistor discs in series. The non-linear elements are connected in series with the spark gaps. Both the assemblies are accommodated in tight porcelain container. The spark gap is a multiple assembly consisting of a number of identical spark gaps in series. Each gap consists of two electrodes with fixed gap spacing. The voltage distribution across the gap is line raised by means of additional resistance elements called grading resistors across the gap. The spacing of the series gaps is such that it will withstand the normal circuit voltage. However an over voltage will cause the gap to break down causing the surge current to ground via the non-linear resistors. The non-linear resistor discs are made of inorganic compound such as thyrite or metrosil. These discs are connected in series. The non-linear resistors have the property of offering a high resistance to current flow when normal system voltage is applied, but a low resistance to the flow of high surge currents. In other words, the resistance of these non-linear elements decreases with the increase in current through them and vice-versa.
  37. 37. Non-linear resistor discs Under normal conditions, the normal system voltage is insufficient to cause the breakdown of air gap assembly. On the occurrence of an over voltage, the breakdown of the series spark gap takes place and the surge current is conducted to earth via the non-linear resistors. Since the magnitude of surge current is very large, the non-linear elements will offer a very low resistance to the passage of surge. The result is that the surge will rapidly go to earth instead of being sent back over the line. When the surge is over, the non-linear resistors assume high resistance to stop the flow of current. FIG- Valve type arrester POWER LINE CARRIER COMMUNICATION:- Power line communication (PLC) carries data on a conductor that is also used simultaneously for AC electric power transmission or electric power distribution to consumers. It is also known as power line carrier, power line digital subscriber line (PDSL), mains communication, power line telecommunications, or power line networking (PLN). A wide range of power line communication technologies are needed for different applications, ranging from home automation to Internet access which is often called broadband over power lines (BPL). Most PLC technologies limit themselves to one type of wires (such as premises wiring within a single building), but some can cross between two levels (for example, both the distribution network and premises wiring). Typically transformers prevent propagating the signal, which requires multiple technologies to form very large networks. Various data rates and frequencies are used in different situations. A number of difficult technical problems are common between wireless and power line communication, notably those of spread spectrum radio signals operating in a crowded environment. Radio interference, for example, has long been a concern of amateur radio groups
  38. 38. 8. CONCLUSION The inplant training was completed successfully.I learnt things like how actually substation works. I also learnt maintenance and tests carried out in substation.I understood the operation and real time working of different equipments in the substation.The training will definitely help me in my future.