Inspection and Testing of various electrical equipment

Inspection and Testing of various electrical equipment in INSTALLATION AND MAINTENANCE OF ELECTRICAL EQUIPMENT

1. Inspection and Testing of various electrical equipment Unit-4

2. substation It is an assembly of apparatus which is installed to control transmission or distribution of electric power. The main function of substation is to receive energy, transmitted at high voltage from generating stations, reduce the voltage to suitable value for local distribution and provide facilities for switching.

3. Equipment in a substation Incoming lines Outgoing lines Control room, control and protection panels Transformers: Main power transformer, Auxiliary Transformers H.V and L.V switchgear, circuit breakers, Isolators etc Relay and metering panels, CTs, PTs, control panels Shunt reactors, shunt capacitors, SVS Drop out fuses Power cables Station service equipment such as lightning, auxiliary battery supply, transformer, oil purification set, compressed air system, battery system for dc source. Surge arrestors, O.H earth wires Station earthing system Galvanized steel structures Communication equipment such as carrier current equipment/telephone system, radio, internet, FAX

4. transformer Most convenient and economical apparatus for transfer of power from one voltage to another at same frequency. Most popular types of transformers which are being installed: Single phase or 3-phase transformers Single phase transformers are extensively used in case of power houses and substations which are located far away from rail heads and transport of heavy transformer is not feasible.

5. Three Phase Power transformer Rating about 250kVA Used in generating stations and substations at each end of power transmission line. They are put in operation at load period and may operate at low capacity at light load period. Therefore , power transformer should be designed to have maximum efficiency at or near full load condition . According to use, two types: For L.T Tr. Line i.e. 2 kVA to 15 kVA For H.T Tr. Line i.e. from 15 kVA and above It has been observed that 70 % of transformer troubles are due to bad workmanship during installation and repairs while the rest 30 % is due to some manufacturing defects or design defects. Mainly the trouble arise in the windings, the lead connected to them, in bushing and tap-changers. The power transformer does not require much maintenance work but annual preventive maintenance is essential.

6. Three Phase Distribution Transformer These are up to 200 kVA Used to step down the voltage to a standard service voltage. They are kept in operation all 24 hours a day with or without any load. They should be designed to have maximum efficiency at a load much lower than full load (about 50 %)

7. The following data is essential to specify a transformer: Number of phases, number of windings per phase Winding connections MVA or kVA rating Tap changing requirements Rated Insulations levels of windings Frequency Method of cooling Voltage ratio Rated voltage of each winding Reference standards

8. Main parts of a Transformer Core Windings Conservator tank Transformer oil Terminal and bushing Buchholz relay Breather Explosion valve Cooling tube and Radiator

9. CORE

10. Windings

11. Transformer Oil  The oil we use for the electrical  power transformer  insulation  and cooling purpose  is called the  transformer oil  or  insulating oil .

12. Characteristics of Insulating oil It should have a high dielectric strength not less than 30 kV in transformer tank for gap of 4mm of electrodes. It should be free from moisture contents and water vapour. Oil should be perfectly clear and pale in colour, transparent and free from suspended matter of sediments. Acidity contents should be very low in oil. It should have high flash points i.e. 104 ° C. Oil should be chemically stable. Should possess low viscosity. Density of oil at 20 ° C should be 0.89 gm/ Mineral oil grade B should be used.  

13. Testing of transformer oil To determine suitability of oil, following tests should be carried out before using insulating oil. Flash point or breakdown test- The point at which spark occurs and transformer trips is known as flash point. This test is performed to check dielectric strength of transformer oil. Crackle test- To check presence of moisture in oil. A sample of oil is taken in a beaker. One iron rod of 12.5 mm is made red hot and dipped in sample. If there is hissing sound, it indicates presence of moisture. Sight test- It is also for checking presence of moisture in oil. If bubble appears, it indicates moisture. Acidity test- If oil becomes acidic, the water content in the oil becomes more soluble in the oil. The acidity of oil deteriorates the insulation property of paper insulation of winding. Acidity accelerates the oxidation process in the oil. Acid also includes rusting of iron in the presence of moisture. The acidity test of transformer oil can be used to measure the acidic constituents of contaminants. The acidity of oil is expressed in mg of potassium hydroxide (KOH) required to neutralize the acid present in a gram of oil.

14. Transformer Terminals and Bushings The bushing of power transformer is a type of porcelain or ebonite post insulator put on the top or side of the transformer tank through which connections are made to the external circuit.

15. Conservator Tank The main function of conservator tank of transformer is to provide adequate space for expansion of oil inside the transformer. A level indicator is fixed to it, which gives alarm at low level. The oil expands and contract depending upon heat produced and so the oil level in conservator rises and falls.

16. Breather The breather controls the moisture level in the transformer. When the atmospheric air passes through the silica gel of the breather, the air's moisture is absorbed by the silica crystals. When silica gel crystals absorbs water/moisture from the air, it loses its colour and changes to pink. 

17. Buchholz Relay It is used to sense the faults occurring inside the transformer. It is a simple relay that is operated by the gases emitted during the decomposition of transformer oil during internal faults.  (Gas actuated relay) https://www.youtube.com/watch?v=Jo45OTMEID4

18. Faults in a transformer can cause localized heating and hotspots within the core and windings. These elevated temperatures can lead to the breakdown of the insulating materials and the oil . As the oil is subjected to thermal stress, it can start to decompose. Whenever there will be a minor internal fault in the transformer, the insulating transformer oil will be decomposed in different hydrocarbon gases, CO2 and CO. The gases produced due to the decomposition of transformer insulating oil will accumulate in the upper part of the Buchholz container which causes a fall of the oil level in it. Fall of oil level means lowering the position of the float and thereby tilting the mercury switch. The contacts of this mercury switch are closed and an alarm circuit energized. Sometimes due to oil leakage on the main tank air bubbles may be accumulated in the upper part of the Buchholz container which may also cause a fall of oil level in it and the alarm circuit will be energized. By collecting the accumulated gases from the gas release pockets on the top of the relay and by analyzing them one can predict the type of fault in the transformer .

19. Cooling Tube and Radiator Cooling tubes are used to cool the transformer oil. The radiator of transformer accelerates the cooling rate of transformer.

20. Explosion Vent An explosion vent or explosion relief vent in a transformer is a safety feature designed to mitigate the risk of an explosion or excessive pressure buildup within the transformer's enclosure. Transformers, especially those filled with oil, can potentially experience internal faults or electrical failures that generate gases and heat, leading to pressure buildup. If this pressure is not relieved, it can result in an explosion, causing significant damage to the transformer and posing a safety hazard.

21. Tap changing The voltage control of transmission and distribution system is obtained by tap-changing. Tap changers are either on-load or off-load tap changers. Tap changer is fitted with the transformer for adjusting secondary voltage. The voltage control of range ± 16 % can be achieved by tap changing transformers. Types- Off-load tap changer- Occasional adjustment in secondary voltage can be made by off circuit tap-changer. Such adjustment are for seasonal load variations. The tap is changed only after opening the C.B on secondary side. Typical rage ± 12% variation in 5 to 7 steps. Manually executed by sub-station operator .

22. On-load tap changer- The daily voltage variation due to changing load and short period voltage variations are controlled by on-load tap changer automatically. The tapping can be changed without interrupting in load current. It is fitted with power transformer. The voltage can be varied by ± 16 % by means of on-load tap changer.

23. dispatch of transformer Transformers are dispatched by manufacturer in 3 ways: Dried out, filled with oil, ready for service (small transformer) With oil covering, the core and coils only (medium transformer) Without oil in tank, filled with nitrogen at pressure (large transformers) Tank cover is always bolted into its position before final packing. The transformer may be packed in a strong wooden packing case for dispatch. Protective hooks may be specifically provided for long distance transport and for export orders. Special packing cases to protect cooling tubes and tank to prevent damage during transit. In case of extra H.V transformers. H.V bushings are packed separately and temporary connections are brought out for insulation resistance measurements.

24. Delivery of transformer at site Low power Trans. are transported to sites completely assembled. (Taken to sites on truck) Medium power Trans. Either completely assembled or with some parts dismantled and packed in boxes. (Taken to sites on truck) High power Trans. and all Trans. Above 110 kV are transported in partly dismantled condition with their radiators, high voltage bushing, oil conservators, vent pipe and air blast system packed separately. (Transported on rail wagons or road trailers)

25. Handling on arrival at site Use of cranes to unload Transformers at site. If crane is not available, The transformer should be unloaded from trailer by using wooden sleepers and rails for dragging the transformer to its plinth. Keep under base of main unit at least 300 – 400 mm above ground level by providing wooden slippers to facilitate jacking. Winches should be used for putting Trans. into position. Lugs are provided on Trans. Tank for handling purposes. https://www.slideshare.net/sanjaysingh299/transformer-unloading-236931702

26. Inspection of transformer The transformer should be unpacked and inspected for any damage that could have occurred during shipping. The bushings and the insulators should be inspected for cracks and chips in the porcelain. Oil leakage should be checked along valves and tank welds. The exterior finish should be inspected. If the paint has been worn or scraped off, it must be repaired. A unit that sits outside or in a corrosive environment will corrode, and leaks may develop. The cooling fins, if present, should be inspected for dents that may affect the ability of the cooling system to operate. To check dielectric strength of oil and insulation resistance of winding .

27. The following checks are performed. Checking oil- Samples of oil should be drawn from near top and bottom, and tested as per I.S : 1866-1961 ‘Code of Practice for maintenance of insulating oil’. A lower than 30 kV dielectric strength for a 4 mm gap would indicate presence of moisture. The oil should be filtered and dried at a suitable plant. Presence of water reduces the dielectric strength of oil. Checking Insulation resistance of winding- It should be checked with a 1000 V megger, the voltage should be applied for one minute. The insulation resistance is halved for every 10 degree rise in temperature. Therefore all reading should be taken at nearly same temperature.

28. storage Transformers which are not to be installed immediately need proper storage to avoid influx of moisture, effect of rains or dust etc. It is preferrable to store transformer indoors with proper covering and flooring. The oil should not be drained unless there is a provision for filling inert gas.

29. Testing of transformer Type tests- These are carried out on a single transformer of one type and are done to check the design characteristics. Temp. rise test, Impulse voltage test, Noise level test Routine tests- Carried out on every transformer before it leaves the factory to ensure that it is according to specifications. Ratio and polarity, Impedance measurement, Insulation resistance, Resistance of windings, No load current Special test- Conducted in presence of purchaser or his representative. Partial discharge, vibration test, short-circuit withstand test, star-delta test Commissioning tests- These are conducted at site before commissioning. General observations, Ratio test, Tripping test, Calibrated earthing resistance, Buchholz relay test, Alarm circuits, Fans and pumps, Phasing tests, Insulation tests, Oil level

30. Substations A sub-station is required to perform one or more following functions: To perform switching operation i.e. to switch on or off the power line To perform voltage transformation operation i.e. to step up or step down the voltage To perform power factor correction operation

31. Main equipment used in substation Lightning Arrestor Lightning Arrestor is the first member of the electrical substations. It protects the substation equipment from transient high voltage and also limits the duration and amplitude of the flow of current. The Lightning arrestor is connected between line and earth, i.e., in parallel with the equipment under protection at the substation. The lightning arrester diverts, the current of the surges to the earth and hence protects the insulation and conductor of the system from damage.

32. Power Transformer Power transformers are used for stepping up the voltage for transmission at generating station and for stepping down the voltage for further distribution at main step-down transformer substations. Usually naturally cooled, oil immersed type two winding, three phase transformers, are used for rating up to 10 MVA. The transformer for rating more than 10 MVA, are usually air blast cooled. For very high rating, the force oil, water cooling and air blast cooling may be used.

33. Instrument Transformer Instrument transformer is used to reduced high voltages and currents to a safe and practical value which can be measured by conventional instruments (normally range is 1A or 5A for current and 110 V for voltage). Two types Current Transformer  – A current transformer is a device for the transformation of current from a higher value to a lower value. It is used in parallel with AC instruments, meters or control apparatus so that the meter or instrument coil cannot conveniently be made of sufficient current carrying capacity. Voltage Transformer  –  A voltage transformer may be defined as an instrument transformer for the transformation of voltage from a higher value to the lower value.

34. Bus-Bar Bus-bar is a type of electrical junction in which the incoming and outgoing of electrical current take place. When the fault occurs in the bus- bar, then all the circuit equipment connected to that section must be tripped out to give complete isolation in the shortest possible time e.g. (60ms) so that the damage is avoided to the installation due to heating of conductors.

35. Power line carrier (PLC) communication Such equipment is installed in the substations for communication, relaying, telemetered or for supervisory control. The equipment is mounted in a room known as carrier room and connected to the high voltage power circuit. The main purpose of power line carrier communication is to convey messages from one substation to other substation through transmission lines at high frequencies . It also serves other purposes like tele-protection, telemetering etc. Equipment use d : Wave trap, Coupling capacitor, Switching equipment, PLC terminal, Line matching unit.

36. a) Wave trap Wave trap is used to create a high  impedance  to the carrier wave high-frequency communication entering into unwanted destinations typically substation. Carrier wave communication uses up to 150kHz to 800kHz frequency to send all the communication. These high-frequency damages the  power system  components which are designed to operate 50 or 60 Hz.  Wave traps are also called line traps.

37. b) Coupling capacitor - It acts as a conductor for high frequency carrier signal ranging 100 kHz to 500 kHz. It offers low impedance path for high frequency and high impedance path for low frequency . These are made of porcelain insulator shell. These are mounted on foundation and designed for voltages ranging 33 kV to 400 kV. c) Line matching unit - The carrier signal received from H.V transmission line through coupling capacitor and given to line matching unit. Line matching unit signal is transferred to power line communication terminals. d) Switching equipment - The signal is fed to switching system for different operation such as telemetering, tele-printing etc. ( managing and controlling the flow of data and signals over the power lines. ) e) PLC terminal - used for the transmission and reception of data signals over the power lines. PLC terminals often include modulation and demodulation capabilities. They modulate digital data into high-frequency carrier signals suitable for transmission over power lines and demodulate incoming signals back into digital data for processing by substation equipment.

38. Isolator Their primary purpose is to provide electrical isolation and mechanical disconnection between various parts of the power system, such as lines, equipment, and circuits. Isolators play a crucial role in ensuring the safety of personnel and equipment during maintenance, repair, and switching operations in substations. Isolator  switches are used  for opening an electrical circuit in the no-load condition . It is not proposed to be opened while current flows through the line . Generally, these are employed on circuit breaker both the ends thus the circuit breaker repair can be done easily without any risk.

39. Circuit Breaker The circuit breaker is a type of electrical switches which is used for opening or closing of electrical circuit whenever faults occur in the system. It consists of two moving contacts which are normally closed. Whenever the fault occurs in the system, the relay sends the tripping command to the circuit breaker and hence their contacts are moved apart. Thus, the fault occurs in the system becomes clear. Most commonly, circuit breakers are classified by their arc quenching medium, which includes: Oil circuit breaker Air circuit breaker SF 6  circuit breaker Vacuum circuit breaker

40. Relay It protects the power system component against abnormal conditions such as faults. The relay is a sensing device which senses the fault, then determines its location and finally, it sends tripping commands to the circuit. The circuit breaker after getting the command from the relay disconnects the faulted element. Relays protect the equipment from damage and hence subsequent hazards like fire, the risk to the life are reduced by removing the particularly faulted section. Control and relay panels

41. Insulator It is used in generating stations and substations to fix and insulate the bus bars systems. They may be subdivided into post and bushing type. A post insulator consists of porcelain body and their cap is made up of cast iron. It is directly bolted to the busbars by means of bus-bar clamps. A bushing or through insulator consists of porcelain-shell body, upper and lower locating washes used for fixing the position of bus-bar or rod in a shell.

42. Capacitor Bank Capacitor bank consists capacitors connected either in series or parallel. It stored the electrical energy in the form of electrical charges. Capacitor bank draws leading current which increases the power factor of the network and also the power transfer capability of the system increases.

43. Station Earthing System- To provide a low resistance earthing for- Discharging current from surge arresters, earthing switches. For equipment body earthing For providing path for neutral to ground currents for earth fault protection. Earth-mat and earth electrodes placed below ground level. Connected to equipment structures, neutral points for the purpose of equipment earthing and neutral point earthing.

44. Classification of substation

45. According to service requirement :  A sub-station may be called upon to change voltage level or improve power factor or convert a.c. power into d.c. power etc. According to the service requirement, sub-stations may be classified into: ( i ) Transformer sub-stations:  Those sub-stations which change the voltage level of electric supply are called transformer sub-stations. These sub-stations receive power at some voltage and deliver it at some other voltage. Most of the sub-stations in the power system are of this type.

46. (ii) Switching sub-stations:  These sub-stations do not change the voltage level i.e. incoming and outgoing lines have the same voltage. However; they simply perform the switching operations of power lines.

47. (iii) Power factor correction sub-stations:  Those sub-stations which improve the power factor of the system are called power factor correction sub-stations. Such sub-stations are generally located at the receiving end of transmission lines. These sub-stations generally use synchronous condensers as the power factor improvement equipment.

48. (iv) Frequency changer sub-stations:  Those sub-stations which change the supply frequency are known as frequency changer sub-stations. Such a frequency change may be required for indus­trial utilization.

49. (v) Converting sub-stations:  Those sub-stations which change a.c. power into d.c. power are called converting sub-stations. These sub-stations receive ac. power and convert it into d.c. power with suitable apparatus to supply for such purposes as traction, electroplating, electric welding etc.

50. (vi) Industrial sub-stations:  Those sub-stations which supply power to individual industrial concerns are known as industrial sub-stations.

51.  According to constructional features:   A sub-station has many components (e.g. circuit breakers, switches, fuses, instruments etc.) which must be housed properly to ensure continuous and reliable service According to constructional features, the sub-stations are classified as : Indoor sub-station Outdoor sub-station Underground sub-station Pole-mounted sub-station

52. 1. Indoor sub-stations:  For voltages upto 11 kV, the equipment of the sub-station is installed indoor because of economic considerations. However, when the atmosphere is contaminated with impurities, these sub-stations can be erected for voltages upto 66 kV.

53. 2. Outdoor sub-stations:  For voltages beyond 66 kV, equipment is invariably installed out.­ It is because for such voltages, the clearances between conductors and the space required for switches, circuit breakers and other equipment becomes so great that it is not economical to install the equipment indoor.

54. 3. Underground sub-stations:  In thickly populated areas, the space available for equipment and building is limited and the cost of land is high. Under such situations, the underground sub-station is created.

55. 4. Pole-mounted sub-stations:  This is an outdoor sub-station with equipment installed over­head on Single pole, H-pole or 4-pole structure. It is the cheapest form of sub-station for voltages not exceeding 11 kV (or 33 kV in some cases). Electric power is almost distributed in localities through such sub­station.

56. Substation based on Transformer mounting: Pole Mounted  : This configuration is used for small rating transformers. Basically there are two pole mounting structures used in Power system. Single Pole Mounting : In this configuration the transformer is mounted on single pole with the help of angles and other metallic structures collectively called transformer platform. H-Pole Mounting  : This arrangement is used for relatively high rating transformers. Two poles are installed so that to provide a base for the transformer. This arrangement is used because the weight of the transformer is more.

57. 2. Plinth Mounted Sub Station In this arrangement a foundation is prepared and the transformer is installed on that foundation. This is used for very heavy transformers. A fencing is provided around the transformer to protect it from animals and unauthorized access. The foundations serves two basic purposes : a) It provide the base to the transformer and b) It absorbs the vibrations, if any, in the transformer during the operation.

58. Location of substation Following points to be kept in mind while choosing location: Substation should be as close to load centres as possible Substations should be located at such point where all the prospective loads may be conveniently reached without any need for voltage regulation. Allow access to incoming transmission lines and outgoing distribution lines. Allow for reasonable amount of expansion of substation. Choose the site where substation building can be easily erected without any restriction. Keep load on substation within such limits so that in case of a shut down only a small number of consumers are affected. Cost of land.

59. Indoor substation Design, requirements, layout

60. Design and planning of indoor substation The type of supply required, load and earthing arrangement. Atmospheric conditions such as air temperature and moisture. Atmospheric pollution. Reliability of substation equipment so that there is minimum power interruption. Continuity of service through alternative supply if source of supply is interrupted. Possibility for modification or future extension. Noise reduction. Radio and telecommunication interference due to switching.

61. General requirements of layout of indoor substation Building construction- Provision should be made for adequate space required for placing transformers H.T and L.T switch gear and cable trenches for incoming and outgoing cables. The building of transformer room should be spacious and have necessary clearances. Sufficient passage and doorways should be provided so that large transformers can be moved in or out for repairs. Ventilation - There must be free circulation of air on all sides of transformers and within the building, entry of water and birds through the inlets and outlets for ventilation should be prevented by appropriate protection. For heavily loaded transformers exhaust fans should be provided. Earthing - The equipment installed in a medium voltage distribution substation should be solidly earthed. The transformer neutral should also be earthed. Plate and pipe earthing should be provided for indoor substation. Cable trenches- Cable trenches are provided for laying cables. The cables trenches should be protected against entry of water by blocking the opening with gravel or sand and covered with mild steel or RCC stabs.

62. Layout of indoor substation Layout with one transformer The transformer should be installed in a separate room. If this is not possible the transformer should be installed in one corner and protected by metal fencing. The position of 11 kV OCB and transformer should be such that minimum possible length of 11 kV cable is used. LT switchgear should be located in such a position so that operation of CB and switches is easy.

63. Layout with two transformers This type of substation is having two transformers of 50 kVA each. The usual primary voltage is 11 kV and secondary is 400/230 V. On primary side the switch gear consisting OCBs is only installed. The supply is given to primary side and secondary side is connected to low voltage bus bars. Several feeders connecting large consumer are taken out from substation through controlling equipment. The panel of each feeder consists of a switch, an isolator, and a circuit breaker. In addition, the panel is provided with three ammeters for measuring current in each phase, one voltmeter, one wattmeter, one p.f meter, and one energy meter . The auxiliaries : storage batteries, fire fighting equipment

64. Outdoor substation DESIGN, requirements, LAYOUT

65. Pole mounted Outdoor substation Such substations are constructed for mounting distribution transformers capacity upto 300 kVA. These are cheapest and smallest substations. Generally transformers upto 125 kVA are mounted on double pole structure . For transformers 125 kVA - 300 kVA 4 pole structure is used . The maintenance cost is low.

66. Layout of pole mounted substation 11 kV lightning arrestors three in number are erected at the top of H-pole structure for protection of transformer against lightning. An 11 kV gang operated (GO) switch is installed between 11 kV conductors for tapping 11 kV supply to transformer. 11 kV transformer fuses are provided between GO switch and 11 kV bushing terminals. 11 kV GO switch is provided to isolate transformer from 11kV supply whenever necessary. This can be done by operating handle located on one of the poles of structure at a distance of 1.25 m from ground. The transformer steps down the voltage to 400 V (3- phase), 230 V (1 phase).

67. FOUNDATION MOUNTED SUBSTATION These are built entirely in open and in such substations all equipment is assembled into one unit generally enclosed by a fence. Substations for primary and secondary transmission and for secondary distribution ( i.e above 300 kVA). Since equipment such as high kVA transformer, CB, control panels etc. required for such substations are heavy, therefore, site selected for setting up these substations must have good access for heavy transport. Installation of Outdoor Substation- The substations should be installed and commissioned to deliver power to distribution networks.

68. Selection of Site- The equipment is outdoor type installed on galvanised steel structures so the site selected should be away from city and should be located at centre of gravity of load. The site selection must have provision for future expansion. The following factors are considered while selecting the site for a substation: Type of Substation- Step up or step down

69. Availability of Suitable and sufficient land Land should be level and open from all sides Should not be water logged in rainy season Approach of Tr. Lines should be easily possible without obstruction Places near to aerodrome should be avoided. Type of Substation Area Required 400 kV Substation 50 Acres 220kV Sub station 25 Acres 132 kV Substation 10 Acres

70. 3. Communication facility- Suitable communication facility is desirable both during and after construction of substation. 4. Atmospheric pollution- Atmosphere around factories which produce metal corroding gases, air fumes, dust etc and near to sea coasts, where air may be more humid and may be salty, is detrimental to proper running of power system.

71. 5. Availability of essential amenities of Staff- The site should be such that where staff can be provided essential amenities like School, hospital, drinking water, housing etc. 6. Drainage facility- Proper drainage arrangement to avoid growth of micro-organisms detrimental to equipment and health.

72. Civil Activities Soil investigations Design and drawings of site Foundation work (excavation) Earthing system Roads, rails Drainage Cabling system Site offices Stores buildings Residence facilities

73. Drawings Layouts Drawing Stores Foundation Plan Drawing of structures Drawing for control room Drawing for earthing layout Drawing for cable layout

74. Dispatch to Site By ship By rail or road By air Depending upon the size, weight of consignment and transportation route. In substation equipment, power transformers. Shunt reactors. Long busbars, long bushings etc need special attention. The transport of these consignments should be planned before starting the detail design of such equipment. The arrangements of special wagon, road trailers, cranes etc. should be made in advance. The necessary permits from railway and road authorities should also be obtained.

75. Inspection on Arrival at Site The packing cases are inspected for transit damage. In case of any damage, the matter is reported to supplier and insurance company. The insurance claim is lodged. The insurance company conduct survey and assess the extent of transit damage.

76. Storage Three categories for substation equipment: Indoor closed store Outdoor fenced store Semi-enclosed fenced store Indoor equipment, C.B, kiosks should be stored indoor. Outdoor equipment are stored outdoor. The storage area should be above maximum flood level, and free from damp, mud etc. The approach roads from main road and to the substations should be firm and adequate. Indoor storage building should have sufficient large storage and sufficiently large doors on road side and substation side. It should be well ventilated and well lighted. Fire fighting arrangements must be adequate.

77. Foundations Building foundation, yard foundations, transformer foundations etc. Substation equipment needs firm and level foundation Foundation plan is recommended by the manufacturer Provision for cable trenches, earthing mat, ducting for ventilation, auxiliary supplies, drainage etc.

78. Erection of Yard Equipment The sequence is from top to bottom levels. The O.H earth wires are installed first. Thereafter, O.H buses are installed. Then CTs, VTs, surge arresters, isolators, C.B are erected. Installation of transformers- The tank is installed on foundation. The accessories are fitted. The oil is filled. The transformer is dried out by circulating oil through oil purifier set. The commissioning tests are carried out. SF6 C.B - The breaker is dried out by hot air circulation followed by evacuation. Thereafter SF6 gas is filled. The leakage tests are carried out by means of soap solution, SF6 leak detector and by monitoring pressure gauge. The commissioning tests are carried out. Porcelains, CTs, VTs- These are installed on respective structures. Commissioning tests are carried out. The porcelain are closely observed for cracks.

79. 4. Earthing system- Earth resistance is measured during dry season and wet season. The values should be within permitted limits. 5. Busbars - Special facility for Aluminium welding is necessary. The welded joints are checked by X-ray equipment for good weld. Bolted joints are tested by means of micro-ohmmeter. The resistance of joints should be below 15 micro-ohms. 6. Control cables- These are laid in trenches in accordance with the cable layout drawings.

80. Drying out The electrical equipment including transformers, C.Bs, bus-ducts etc may need drying prior to application of voltage. Power transformers are dried out by circulation of oil through oil vacuum chamber, heater, pumps etc. The oil is heated (90 degree Celsius), passed through filter press and subjected to vacuum. The oil is circulated through filtration plant for several hours, till the desired polarisation index and insulation resistance values are obtained. The breakdown value of oil is also tested in oil insulation testing set. Oil C.B are dried by circulating oil through filter plant.

81. Measurement of Insulation Resistance of Transformers The IR is measured by means of Megger. It gives clear indication about health, cleanliness and dryness of equipment insulation. A megger consists of a d.c generator and mega-ohm meter. The dc generator rotor can be driven by hand or by motor. The standard meggers are of 1000 V, 2.5 kV and 5 kV. 2.5 kV and 5kV Meggers with motor drive are used with testing equipment rated 33 kV and above. IR is the ratio of is applied across two conductors separated by insulation under test. For a healthy insulation, the megger reading is in mega-ohms to infinity. For defective or moist insulation, megger shows a very low IR value.  

82. In power transformer , IR is measured between Each winding and earth Between HV winding and LV winding In other apparatus , IRs are measured between terminal and earther frame. In CB , megger values are taken for- Upper terminal to lower terminal across open interrupter terminal to Earthed frame with breaker closed

83. Commissioning Tests Equipment functional checks and tests Sub-system checks and tests Complete system tests Performance test Equipment tests on each individual equipment are performed in accordance with site testing plan. IR measurements Phase sequence tests Polarity tests Ratio tests Continuity tests Applied voltage tests Mechanical operation tests Performance characteristic test

84. Pre-commissioning checks of Substation Before commissioning following points should be checked: The earth resistance of substation should not be more than 5 ohms. H.T and L.T fuses are of appropriate sizes. Air path of silica gel breather should be clean. If it is choked, it should be cleared. Operation of tap changing switch should be checked. Oil level in transformer should be up to mark indicated on transformer.

85. Grid Substation Several Generating Stations are connected to each other to form an interconnected grid system. The operation of such a grid is controlled from a load control center or load dispatch center. There are several advantages of interconnecting the large generating stations. Reserve plant capacity is reduced, Load Factor and Efficiency Increases Reliability of supply increases As load curves for different stations are different therefore, Diversity Factor is improved Generators of Large size can be used

86. Load Frequency Control of a Grid The Task of National control center is to maintain the power transfer between various regions and frequencies of various areas within set limits. Load Frequency Control (LFC) are  used to regulate and control the output frequency signal of the electric generated power within an area in response to changes in system loads and power in tie line changed with other area .

87. Load Shedding Load shedding is used to relieve stress on a primary energy source when demand for electricity is greater than the primary power source can supply.

88. Responsibilities of Individual regional grids Planning of Generation to meet demand Control its own load frequency Generate as per instructions of load control centers Co-operation with other regional grids The five regional power grids include Northern Region Eastern Region Western Region Northeastern Region Southern Region grids