Switch Gear
Content
16.0 Installation and Maintenance
16.1 Introduction : The correct operation of switchgear is necessary for protection of the system. The failure of the system in the event of a fault is likely to be result of disaster. The switchgear and relaying system should be alert during the year, moment to moment, through it might not be called upon to operate due to absence of fault. Now, a serious, unexpected fault may occur any time and the switchgear, which was in quiescent state, has to operate immediately. For such an operation, regular and detailed maintenance is necessary. The lack of maintenance may result in failure in operation. With rotating machines the fault or stoppage is noticeable, not so with the switchgear. Hence regular inspection is must. The switchgear manufacturer generally supplies "Instruction Manual of Installation, Operation and Maintenance". The maintenance staff should carefully study the manual. Certain training is necessary for maintenance men. In addition, the code of the practice booklets published by the standard institutions, regulations of electrical installations etc., should be strictly followed. Detailed program of the maintenance of switchgear should be prepared with pre-determined intervals between inspections. The period may be one to three months of switchgear operating frequently and six months to twelve months of switchgear operating rarely. After operation of the circuit breaker on abnormal condition, inspection should be carried out as early as possible. Further it is unwise to leave the circuit breaker closed for the period longer than six months without opening, because the mechanisms may become sluggish and contact may need cleaning. Hence during the periodic maintenance, the circuit breaker is purposely opened and closed. The maintenance schedule is usually in the form of log sheets on which weeks, months of the year are tabulated. Each equipment in the sub-station or a plant is provided with a column. The maintenance period is indicated against each equipment. Further, each major equipment is provided with history card. The details about inspection, operation and remarks are written on these cards. The "spares" are important for maintenance duty. Record of the spares required for each equipment, and the plant is kept ready. The spares are kept in stock with proper inventory control. The maintenance work is done according to the schedule. In case of difficult jobs, the manufacturer is consulted. The manufacturer provides trained personnel on request and necessary payment. 16.2 Maintenance of Circuit-Breakers : The discussion pertains to oil circuit breakers, but some features are common with other types. The description only gives a general guidance, not for specialists. Period of Inspection : (For Oil Circuit-Breakers) a. Under Normal Conditions :
•
Once in 6 months or 12 months for circuit-breaker operating in-frequently. • Once in one month or three months for circuit-breaker operating repeatedly or according to the manufacturing recommendations. b. On clearing a fault. As soon as the circuit breaker can be isolated from service. c. Overhaul. Once in three years or as recommended by the manufacturer. d. Replacement. When the life period expires. This is s matter of economics and technical considerations.
•
During the periodic check up the following checks should be
made :
a. Check the level and condition of oil. b. Clean the insulators with the fabric cloth that will not leave fibers. Do not use cotton waste in any case. For removing oil, grease, carbon deposit use Trichloroethylene or other chemical recommended by the manufacturer. c. Check contacts d. Check operating mechanism. e. Check indicating devices. f. Check auxiliary switches. g. Tighten nuts, bolts etc. h. Test insulation resistance by means of high voltage (1000V, D.C.) meager in case of high voltage circuit and by 500 V meager in 220 V control circuits. i. Carry out tests according to the specifications. j. Take the steps as mentioned in the subsequent paragraphs.
•
When the breaker operates on fault, the internal and external inspection should be carried out as soon the operating schedule permits. a. Examine the Oil. If badly deteriorated, change it. b. Check Arcing contacts. Clean with smooth file.If badly damaged, replaced them. c. Inspect the insulation, carefully check the surface. d. Check the arc control device. If damaged, replace the plates. e. Check the tripping circuit and operating mechanism. f. Be sure that no tools are left in the tank. Some further details are given below. Contacts : Contact pressure is important. In medium voltage circuit breakers it is about 5 kg. The pressures is tested as follows: A feeler gauge of 0.05 mm inserted between the contacts is pulled by the spring balance, until the feeler is freed. The pressure indicated on the balance is recorded. The method is, however, not very reliable. Springs should be adjusted or replaced if contact pressure is insufficient. Another way is to measure contact resistance. The contact resistance is of order of 20 micro ohms for 1200 a normal current rating, the resistance between the ends of pole gives the measures of the contacts resistance. If contacts are badly burned, they should be replaced. If lightly burned or pitted or metal globules are present or the surface is likely uneven, they are cleaned by the fine glass paper or fine file. Original contour be preserved. While cleaning the contacts, minimum material should be removed. On any account the contacts of circuit breaker should not be oiled or greased. Arc Control Devices : Slight blackening is not harmful. The condition of plates is important. If badly burned or deformed, the entire arc control device may need replacement. All vents and opening should be cleaned. Insulators : Porcelain insulators should be inspected for any sign of cracks or defects. They should be cleaned with trichloroethlylene. Carry out tests recommended by the specifications. Operating Mechanisms : Check opening and closing operations by manuals signal and by means of relay. Cleans all the moving parts. Lubricate the sliding parts and surfaces. Avoid excessive Lubrication. Check the tightness of nuts, bolts, pins, etc. Check the springs. Check the terminal block and the wiring. Check the auxiliary switch. Relays : It is advisable not to adjust the relay-mechanism. The faulty relay should be sent to manufacturer since relay repair is a specialized job.
Contacts of relays should be inspected for any sign of burning where necessary, glass paper should be used for cleaning. All the terminals of the relay should be checked for tightness. The wiring should be checked thoroughly for security. 16.3. Safety : The maintenance work should be carried out with written permission of responsible people. A scheme should be adopted to issue permit card authorizing the maintenance work to be done. Steps should be taken by concerned authorities to ensure safety. These steps includes: 1. Isolation of the part from the live parts during the period of maintenance. No switching on by mistake. 2. Danger notice such as the one given below should be placed. 3. The neighboring points should be locked to avoid switching by the third person. 4. Earthling - The work equipment and conductors should be earthen by means of earthling connections, from both ends. 5. Power tools, safety devices should be provided to the electricians. 6. The electricians should be well trained. 7. First-aid should be available. 8. Switching on should be allowed only after completion of work after cancellation of the permit by the authority. Death can be caused even on 400 V installations, because of negligence. 16.4 Installation of Switchgear : Preliminary Preparations : The preliminary preparation include study of drawings, acceptance, report checking certificates and test reports, of the equipment, completion of civil engineering work, arranging the tools, lifting gears etc., organizing the labor, prepare schedule of installation, preparing sequence cards for erection of major items etc. Such cards indicates the sequence of operation, item involved, procedure in brief etc. Sequence Card for Erection of Switchgear Equipment : Tools, Lifting gear, etc Drawing Number Technique & Procedure
S.No 1 2 3 The drawings include
Operation
1. 2. 3.
Circuit diagram of the plant Civil engineering plans, foundation plans etc. Dimension drawing of equipment.
Location of Switchgear : The switchgear may be Indoor and Outdoor. Generally up to 11kV, indoor is popular. Indoor switchgear should be located in clean, dry room free from vermin, snakes, moisture, dust etc. Floor should be dry and leveled. The floor should be withstanding load of about 1000 kg/m2 (200 1b/sq.ft). Enough space should be left in front and in the rear of the switchgear as recommended by the manufacturer. About 1.7 meters in front and 0.7 m in the rear of 11 kV switchgear. The following points are kept in mind: 1. 2. 3. 4. Fire proof doors, roof, ceilings etc. Sealing of cable ducts. Sub-division of switchgear. Installation of fire-fighting apparatus.
Unpacking : The equipment is packed in crates and is brought to sites by railway and motor-truck. Packing are lowered on the site by means of rope, hoist or crane carefully. Care is taken that they are always maintained in upright position throughout. On unpacking, the items are checked against the list. Further the items are carefully inspected visually. If any damage is found, the matter should be informed to the manufacturer and insurance company immediately, and the damaged equipment should be returned. Foundation : The foundation is prepared according to the foundation plan. Holes are provided for foundation bolt. Trenches and passages are provided for cables and other piping. The floor should be correctly leveled and marked according to the drawing. Erection : The equipment is installed according to the procedure mentioned in the instruction manual. Some type of lifting device, special tools etc. may be necessary. The assembly is erected vertically. The verticality is checked by means of spirit level. If necessary, packing is added in the base plate for obtaining proper level. After doing necessary adjustment and checking the level, the concrete mixture is poured into holes around foundation bolts and nuts are tightened. It should be remembered that porcelain insulator columns are weak in tension. During erection they should not be shifted under assembled state. The assembly is carried on site and the assembled units are not shifted. Circuit breaker should be dried out before filling gas/oil. Bus-bars Earthling Connections : The bus-bar contacts and making surface of connectors should be cleaned with emery paper or smooth file. The bus bars are assembled as soon as they are cleaned. Special grease is applied. Connections of Main Cable : The cable terminal box should be clean and moisture should be removed by blow lamps. The cable-filling compound should be warmed to about 157 to 1680C and allowing to cool pouring temperature of about 135-1400C. The bucket used for filling should be pre-heated to about 80C to avoid the splashing and inclusion of air bubbles, and should be trapped periodically to ensure the good bond between the body at the compound and the topping layer. The level of compound should be about 45 mm below the top cover to allow expansion during service conditions. The cable cover is bolted when the compound gets cold. The PVC hose is sleeved on the cable conductor, which is protected by varnish cambric tapes. The terminal lug in soldered to the cable conductor. Earthling : The earthling bars of the switchgear, the metallic non-current carrying parts are connected to station earthling system. 16.5 Erection of HV Air Blast Circuit-Breaker : The air receiver, switch cubicle insulators, and arc extinction chambers are usually dispatched in separate packing. The floor is prepared for all the three poles of the C.B, pipeline of compressed air and cables for controls are taken through trenches. The power connections are come from overhead conductors. The assembly is carried out for each pole. The insulators are mounted on air receiver. The arc extinction chambers are then fixed. Finally connections are made between the arc extinction chambers. Compressed air supply is given. Tests are carried out on site. Here also the trained staff carries out the assembly and commissioning. 16.6 Installation of Draw out Type Indoor Switchgear : Such switchgear is commonly used for indoor installations up to 11 kV. Each unit consist of a circuit breaker (of 11 kV, 350 MVA)mounted on a truck. The stationary unit consists bus bars, potential transformers, current transformers and other switchgear components. The unit has a cable box. The general scheme can be seen in Fig.16.6 While opening the circuit breaker, the operator can trip the circuit breaker by manual signal, later the circuit breaker is lowered and truck is drawn out. The following paragraph gives general idea about the maintenance and erection of draw out switchgear.
Fig.16.6 – Cascade Connection of HV Testing Transformers. 1. Prepare the floor and civil engineering work as described in 16.4. 2. Cut the necessary holes in the floor according to the marking. 3. Align the fixed housing on the unit according to their position in the drawing. 4. Check the vertical position of the units by the plumb line. 5. Connect the sub-bars, and earthing bus of the neighboring units. 6. Connect the earthing bars to station earthing bus. 7. Make the secondary connections, as per the wiring diagram. 8. Check the CT and PT polarity. 9. Check the circuit breaker, fill it with oil up to correct level. 10. Tighten all the bolts. 11. Check the circuit breaker rising mechanism, by rising it and lowering it. 12. Check the movement of draw out unit. 13. Carry out commissioning tests. 16.7 Dielectric Oil (Transformer Oil) : Liquid Dielectric Materials - In oil circuit breakers and minimum oil circuit breakers, oil serves two purposes. a) insulation and b) arc extinguishing medium. Insulating oil for transformers and switchgear (IS 335-1963), (BS 148 –1959) is recommended in these applications. Synthetic liquid resins are used as insulation for CTs and PTs. Properties of transformer oil (BS : 148-1959) are given in the Table given below. For more details about oil, refer chapter 11 of Transformers manual. Properties of Dielectric Oil : Characteristics Sludge(maximum) Flash Point(minimum) Viscosity at 700F Electric strength, 1 minute minimum Saponification value (maximum) Copper Discoloration Crackle Limiting Value 1.200% 2950F (1460C) 37 cS 40 kV rms. 1.00 mg KOH/gm Negative shall pass test
The fresh dielectric oil has pale clear yellow color. A dark color or cloudy appearance indicates deterioration. Impurities have bad effects on the properties of the oil. The oil sample is taken in a
standard oil test cell [33/8 inch x 21/2 inch cross-section area x 4 inch high]. Electrodes are polished brass sphere 12.7-mm dia. mounted horizontally. The gap spacing is 4mm ± 0.02 mm. Dielectric strength of oil is very important. A sample of insulation oil is taken from the bottom of the circuit breaker tank. Dark and brown, clouds indicate deterioration. Oil in good condition has pale yellowish uniform color. It is tested by the means portable oil-testing set, which consists of a auto-transformer, Voltmeter, tripping device etc. The voltage van is gradually increased. Oil in good condition should withstand 40 kV rm. for one minute in standard oil testing cup with 4-mm gap between electrodes. A gap of 2.5 mm should have breakdown value above 40 kV. The oil should be tested during the periodic check-up and whenever the circuit breaker clears a fault. Oil Maintenance : Oil Circuit-breakers used for repeated duty need frequent reconditioning of oil. SF6 and vacuum circuit-breakers require very modest maintenance. Oil maintenance is carried out in accordance with the standard titled "Code, Practices of Maintenance of Insulating Oils". The code refers to the contamination of oil and determination of weather the oil is suitable for further service. It also gives the procedure of treatment of oil at site. Contamination of oil due to moisture or solid can be dealt with satisfactorily at site. Centrifugal separators are effective in removing free water and fine solid impurities. The oil is heated to about 60C by means of electric heater. The purification set consists of purifier, heater, de-aerator, oil pumps, strainer and other accessories. The oil is circulated through the purifier till the desired dielectric strength is obtained. The dielectric oil should never contain suspended particles, Water-soluble acids and bases, active sulfur or colloidal carbon. These impurities accelerate deterioration of the insulating properties of the dielectric oil resulting in internal flashover. The dust particles, carbon particles and sludge in the oil which is in suspended form in the oil gathers along the conductor and insulator surface in presence of electric field. There by a thin conducting layer is formed gradually along the surface of internal insulation during service. The flashover cans occur along the surface of the insulation or trunk due to tracking. Dust and other particles are removed by means of filters in the oil-filtering unit. Moisture in oil lowers the dielectric strength. Thereby causing internal flashover. Moisture is introduced in the circuit breaker through defective seals. Sometimes, oil filled in the breakers may itself have higher moisture contents. Viscosity indicates fluidity. Oil with low viscosity has more fluidity and gives cooling and shorter opening time. At lower ambient temperatures the viscosity sharply rises and speed of contact travel reduces. Hence viscosity should be measured at various temperatures and characteristics of contact travel should be plotted at various temperatures. Higher, flash point (1450C) is preferred. Flashpoint indicates tendency to evaporate. For flash points lower than 1350C, the oil trends to evaporate rapidly, thereby the viscosity rises and total volume of oil reduces. SF6 Gas : During periodic maintenance, the gas sample from SF6 circuit-breaker is collected and tested for moisture and other impurities (IEC 376). The gas is circulated through filters containing activated alumina. The activated alumina absorbs the impurities like S2F2, SF4, moisture etc. the gas can be used again after regeneration. 16.8 Methods of Checking Contacts : The moving contacts of each phase should meet the fixed contacts practically simultaneously. An electrical method of checking the contacts is illustrated in Fig 16.8. A low voltage supply, lamps are needed. Simultaneous glowing of lamps indicates simultaneous making of contacts.
Fig.16.8 – Method of checking simultaneous contact meeting during slow closing.
16.1 Introduction : The correct operation of switchgear is necessary for protection of the system. The failure of the system in the event of a fault is likely to be result of disaster. The switchgear and relaying system should be alert during the year, moment to moment, through it might not be called upon to operate due to absence of fault. Now, a serious, unexpected fault may occur any time and the switchgear, which was in quiescent state, has to operate immediately. For such an operation, regular and detailed maintenance is necessary. The lack of maintenance may result in failure in operation. With rotating machines the fault or stoppage is noticeable, not so with the switchgear. Hence regular inspection is must. The switchgear manufacturer generally supplies "Instruction Manual of Installation, Operation and Maintenance". The maintenance staff should carefully study the manual. Certain training is necessary for maintenance men. In addition, the code of the practice booklets published by the standard institutions, regulations of electrical installations etc., should be strictly followed. Detailed program of the maintenance of switchgear should be prepared with pre-determined intervals between inspections. The period may be one to three months of switchgear operating frequently and six months to twelve months of switchgear operating rarely. After operation of the circuit breaker on abnormal condition, inspection should be carried out as early as possible. Further it is unwise to leave the circuit breaker closed for the period longer than six months without opening, because the mechanisms may become sluggish and contact may need cleaning. Hence during the periodic maintenance, the circuit breaker is purposely opened and closed. The maintenance schedule is usually in the form of log sheets on which weeks, months of the year are tabulated. Each equipment in the sub-station or a plant is provided with a column. The maintenance period is indicated against each equipment. Further, each major equipment is provided with history card. The details about inspection, operation and remarks are written on these cards. The "spares" are important for maintenance duty. Record of the spares required for each equipment, and the plant is kept ready. The spares are kept in stock with proper inventory control. The maintenance work is done according to the schedule. In case of difficult jobs, the manufacturer is consulted. The manufacturer provides trained personnel on request and necessary payment. 16.2 Maintenance of Circuit-Breakers : The discussion pertains to oil circuit breakers, but some features are common with other types. The description only gives a general guidance, not for specialists. Period of Inspection : (For Oil Circuit-Breakers) a. Under Normal Conditions :
•
Once in 6 months or 12 months for circuit-breaker operating in-frequently. • Once in one month or three months for circuit-breaker operating repeatedly or according to the manufacturing recommendations. b. On clearing a fault. As soon as the circuit breaker can be isolated from service. c. Overhaul. Once in three years or as recommended by the manufacturer. d. Replacement. When the life period expires. This is s matter of economics and technical considerations.
•
During the periodic check up the following checks should be
made :
a. Check the level and condition of oil. b. Clean the insulators with the fabric cloth that will not leave fibers. Do not use cotton waste in any case. For removing oil, grease, carbon deposit use Trichloroethylene or other chemical recommended by the manufacturer. c. Check contacts d. Check operating mechanism. e. Check indicating devices. f. Check auxiliary switches. g. Tighten nuts, bolts etc. h. Test insulation resistance by means of high voltage (1000V, D.C.) meager in case of high voltage circuit and by 500 V meager in 220 V control circuits. i. Carry out tests according to the specifications. j. Take the steps as mentioned in the subsequent paragraphs.
•
When the breaker operates on fault, the internal and external inspection should be carried out as soon the operating schedule permits. a. Examine the Oil. If badly deteriorated, change it. b. Check Arcing contacts. Clean with smooth file.If badly damaged, replaced them. c. Inspect the insulation, carefully check the surface. d. Check the arc control device. If damaged, replace the plates. e. Check the tripping circuit and operating mechanism. f. Be sure that no tools are left in the tank. Some further details are given below. Contacts : Contact pressure is important. In medium voltage circuit breakers it is about 5 kg. The pressures is tested as follows: A feeler gauge of 0.05 mm inserted between the contacts is pulled by the spring balance, until the feeler is freed. The pressure indicated on the balance is recorded. The method is, however, not very reliable. Springs should be adjusted or replaced if contact pressure is insufficient. Another way is to measure contact resistance. The contact resistance is of order of 20 micro ohms for 1200 a normal current rating, the resistance between the ends of pole gives the measures of the contacts resistance. If contacts are badly burned, they should be replaced. If lightly burned or pitted or metal globules are present or the surface is likely uneven, they are cleaned by the fine glass paper or fine file. Original contour be preserved. While cleaning the contacts, minimum material should be removed. On any account the contacts of circuit breaker should not be oiled or greased. Arc Control Devices : Slight blackening is not harmful. The condition of plates is important. If badly burned or deformed, the entire arc control device may need replacement. All vents and opening should be cleaned. Insulators : Porcelain insulators should be inspected for any sign of cracks or defects. They should be cleaned with trichloroethlylene. Carry out tests recommended by the specifications. Operating Mechanisms : Check opening and closing operations by manuals signal and by means of relay. Cleans all the moving parts. Lubricate the sliding parts and surfaces. Avoid excessive Lubrication. Check the tightness of nuts, bolts, pins, etc. Check the springs. Check the terminal block and the wiring. Check the auxiliary switch. Relays : It is advisable not to adjust the relay-mechanism. The faulty relay should be sent to manufacturer since relay repair is a specialized job.
Contacts of relays should be inspected for any sign of burning where necessary, glass paper should be used for cleaning. All the terminals of the relay should be checked for tightness. The wiring should be checked thoroughly for security. 16.3. Safety : The maintenance work should be carried out with written permission of responsible people. A scheme should be adopted to issue permit card authorizing the maintenance work to be done. Steps should be taken by concerned authorities to ensure safety. These steps includes: 1. Isolation of the part from the live parts during the period of maintenance. No switching on by mistake. 2. Danger notice such as the one given below should be placed. 3. The neighboring points should be locked to avoid switching by the third person. 4. Earthling - The work equipment and conductors should be earthen by means of earthling connections, from both ends. 5. Power tools, safety devices should be provided to the electricians. 6. The electricians should be well trained. 7. First-aid should be available. 8. Switching on should be allowed only after completion of work after cancellation of the permit by the authority. Death can be caused even on 400 V installations, because of negligence. 16.4 Installation of Switchgear : Preliminary Preparations : The preliminary preparation include study of drawings, acceptance, report checking certificates and test reports, of the equipment, completion of civil engineering work, arranging the tools, lifting gears etc., organizing the labor, prepare schedule of installation, preparing sequence cards for erection of major items etc. Such cards indicates the sequence of operation, item involved, procedure in brief etc. Sequence Card for Erection of Switchgear Equipment : Tools, Lifting gear, etc Drawing Number Technique & Procedure
S.No 1 2 3 The drawings include
Operation
1. 2. 3.
Circuit diagram of the plant Civil engineering plans, foundation plans etc. Dimension drawing of equipment.
Location of Switchgear : The switchgear may be Indoor and Outdoor. Generally up to 11kV, indoor is popular. Indoor switchgear should be located in clean, dry room free from vermin, snakes, moisture, dust etc. Floor should be dry and leveled. The floor should be withstanding load of about 1000 kg/m2 (200 1b/sq.ft). Enough space should be left in front and in the rear of the switchgear as recommended by the manufacturer. About 1.7 meters in front and 0.7 m in the rear of 11 kV switchgear. The following points are kept in mind: 1. 2. 3. 4. Fire proof doors, roof, ceilings etc. Sealing of cable ducts. Sub-division of switchgear. Installation of fire-fighting apparatus.
Unpacking : The equipment is packed in crates and is brought to sites by railway and motor-truck. Packing are lowered on the site by means of rope, hoist or crane carefully. Care is taken that they are always maintained in upright position throughout. On unpacking, the items are checked against the list. Further the items are carefully inspected visually. If any damage is found, the matter should be informed to the manufacturer and insurance company immediately, and the damaged equipment should be returned. Foundation : The foundation is prepared according to the foundation plan. Holes are provided for foundation bolt. Trenches and passages are provided for cables and other piping. The floor should be correctly leveled and marked according to the drawing. Erection : The equipment is installed according to the procedure mentioned in the instruction manual. Some type of lifting device, special tools etc. may be necessary. The assembly is erected vertically. The verticality is checked by means of spirit level. If necessary, packing is added in the base plate for obtaining proper level. After doing necessary adjustment and checking the level, the concrete mixture is poured into holes around foundation bolts and nuts are tightened. It should be remembered that porcelain insulator columns are weak in tension. During erection they should not be shifted under assembled state. The assembly is carried on site and the assembled units are not shifted. Circuit breaker should be dried out before filling gas/oil. Bus-bars Earthling Connections : The bus-bar contacts and making surface of connectors should be cleaned with emery paper or smooth file. The bus bars are assembled as soon as they are cleaned. Special grease is applied. Connections of Main Cable : The cable terminal box should be clean and moisture should be removed by blow lamps. The cable-filling compound should be warmed to about 157 to 1680C and allowing to cool pouring temperature of about 135-1400C. The bucket used for filling should be pre-heated to about 80C to avoid the splashing and inclusion of air bubbles, and should be trapped periodically to ensure the good bond between the body at the compound and the topping layer. The level of compound should be about 45 mm below the top cover to allow expansion during service conditions. The cable cover is bolted when the compound gets cold. The PVC hose is sleeved on the cable conductor, which is protected by varnish cambric tapes. The terminal lug in soldered to the cable conductor. Earthling : The earthling bars of the switchgear, the metallic non-current carrying parts are connected to station earthling system. 16.5 Erection of HV Air Blast Circuit-Breaker : The air receiver, switch cubicle insulators, and arc extinction chambers are usually dispatched in separate packing. The floor is prepared for all the three poles of the C.B, pipeline of compressed air and cables for controls are taken through trenches. The power connections are come from overhead conductors. The assembly is carried out for each pole. The insulators are mounted on air receiver. The arc extinction chambers are then fixed. Finally connections are made between the arc extinction chambers. Compressed air supply is given. Tests are carried out on site. Here also the trained staff carries out the assembly and commissioning. 16.6 Installation of Draw out Type Indoor Switchgear : Such switchgear is commonly used for indoor installations up to 11 kV. Each unit consist of a circuit breaker (of 11 kV, 350 MVA)mounted on a truck. The stationary unit consists bus bars, potential transformers, current transformers and other switchgear components. The unit has a cable box. The general scheme can be seen in Fig.16.6 While opening the circuit breaker, the operator can trip the circuit breaker by manual signal, later the circuit breaker is lowered and truck is drawn out. The following paragraph gives general idea about the maintenance and erection of draw out switchgear.
Fig.16.6 – Cascade Connection of HV Testing Transformers. 1. Prepare the floor and civil engineering work as described in 16.4. 2. Cut the necessary holes in the floor according to the marking. 3. Align the fixed housing on the unit according to their position in the drawing. 4. Check the vertical position of the units by the plumb line. 5. Connect the sub-bars, and earthing bus of the neighboring units. 6. Connect the earthing bars to station earthing bus. 7. Make the secondary connections, as per the wiring diagram. 8. Check the CT and PT polarity. 9. Check the circuit breaker, fill it with oil up to correct level. 10. Tighten all the bolts. 11. Check the circuit breaker rising mechanism, by rising it and lowering it. 12. Check the movement of draw out unit. 13. Carry out commissioning tests. 16.7 Dielectric Oil (Transformer Oil) : Liquid Dielectric Materials - In oil circuit breakers and minimum oil circuit breakers, oil serves two purposes. a) insulation and b) arc extinguishing medium. Insulating oil for transformers and switchgear (IS 335-1963), (BS 148 –1959) is recommended in these applications. Synthetic liquid resins are used as insulation for CTs and PTs. Properties of transformer oil (BS : 148-1959) are given in the Table given below. For more details about oil, refer chapter 11 of Transformers manual. Properties of Dielectric Oil : Characteristics Sludge(maximum) Flash Point(minimum) Viscosity at 700F Electric strength, 1 minute minimum Saponification value (maximum) Copper Discoloration Crackle Limiting Value 1.200% 2950F (1460C) 37 cS 40 kV rms. 1.00 mg KOH/gm Negative shall pass test
The fresh dielectric oil has pale clear yellow color. A dark color or cloudy appearance indicates deterioration. Impurities have bad effects on the properties of the oil. The oil sample is taken in a
standard oil test cell [33/8 inch x 21/2 inch cross-section area x 4 inch high]. Electrodes are polished brass sphere 12.7-mm dia. mounted horizontally. The gap spacing is 4mm ± 0.02 mm. Dielectric strength of oil is very important. A sample of insulation oil is taken from the bottom of the circuit breaker tank. Dark and brown, clouds indicate deterioration. Oil in good condition has pale yellowish uniform color. It is tested by the means portable oil-testing set, which consists of a auto-transformer, Voltmeter, tripping device etc. The voltage van is gradually increased. Oil in good condition should withstand 40 kV rm. for one minute in standard oil testing cup with 4-mm gap between electrodes. A gap of 2.5 mm should have breakdown value above 40 kV. The oil should be tested during the periodic check-up and whenever the circuit breaker clears a fault. Oil Maintenance : Oil Circuit-breakers used for repeated duty need frequent reconditioning of oil. SF6 and vacuum circuit-breakers require very modest maintenance. Oil maintenance is carried out in accordance with the standard titled "Code, Practices of Maintenance of Insulating Oils". The code refers to the contamination of oil and determination of weather the oil is suitable for further service. It also gives the procedure of treatment of oil at site. Contamination of oil due to moisture or solid can be dealt with satisfactorily at site. Centrifugal separators are effective in removing free water and fine solid impurities. The oil is heated to about 60C by means of electric heater. The purification set consists of purifier, heater, de-aerator, oil pumps, strainer and other accessories. The oil is circulated through the purifier till the desired dielectric strength is obtained. The dielectric oil should never contain suspended particles, Water-soluble acids and bases, active sulfur or colloidal carbon. These impurities accelerate deterioration of the insulating properties of the dielectric oil resulting in internal flashover. The dust particles, carbon particles and sludge in the oil which is in suspended form in the oil gathers along the conductor and insulator surface in presence of electric field. There by a thin conducting layer is formed gradually along the surface of internal insulation during service. The flashover cans occur along the surface of the insulation or trunk due to tracking. Dust and other particles are removed by means of filters in the oil-filtering unit. Moisture in oil lowers the dielectric strength. Thereby causing internal flashover. Moisture is introduced in the circuit breaker through defective seals. Sometimes, oil filled in the breakers may itself have higher moisture contents. Viscosity indicates fluidity. Oil with low viscosity has more fluidity and gives cooling and shorter opening time. At lower ambient temperatures the viscosity sharply rises and speed of contact travel reduces. Hence viscosity should be measured at various temperatures and characteristics of contact travel should be plotted at various temperatures. Higher, flash point (1450C) is preferred. Flashpoint indicates tendency to evaporate. For flash points lower than 1350C, the oil trends to evaporate rapidly, thereby the viscosity rises and total volume of oil reduces. SF6 Gas : During periodic maintenance, the gas sample from SF6 circuit-breaker is collected and tested for moisture and other impurities (IEC 376). The gas is circulated through filters containing activated alumina. The activated alumina absorbs the impurities like S2F2, SF4, moisture etc. the gas can be used again after regeneration. 16.8 Methods of Checking Contacts : The moving contacts of each phase should meet the fixed contacts practically simultaneously. An electrical method of checking the contacts is illustrated in Fig 16.8. A low voltage supply, lamps are needed. Simultaneous glowing of lamps indicates simultaneous making of contacts.
Fig.16.8 – Method of checking simultaneous contact meeting during slow closing.
