Advances in UHV Transmission and Distribution Lecture 25 PDF

(Refer Slide Time: 29:50) So, substation capital costs or in comparison of this three technologies, so the common costs for both the air insulated and the gas insulated options are mainly the high voltage power transformers very important components. The protection systems this could be several CTs...

(Refer Slide Time: 29:50) So, substation capital costs or in comparison of this three technologies, so the common costs for both the air insulated and the gas insulated options are mainly the high voltage power transformers very important components. The protection systems this could be several CTs current transformer, potential transformer, the surge arresters, the isolator switches several relays and the SCADA arrangements, the professional fees, design fees and other miscellaneous items; all this come under the capital cost when the planning is being carried out for the any substation. These cost under comparison are civil high voltage equipment provision and also installation. The material cost are based on either the internal data base of the term cost contract materials or the manufacture cost for a specific project and a specific voltage level and going in for what type of substation to be adopted. So, the cost comparison between the air insulated, the gas insulated type substations is again a very difficult comparison; this depends on the area where this is to be compared. So, comparing the air insulated, gas insulated in general maybe of a difficult scenario, it could be compared only where the; in case of the metro or the thickly populated the areas. In such cases where land is of a prime cost, so in such cases the comparison could be made. In case the land availability is at a less economical or less cost; the comparison may not be the correct thing to be done. So, this comparison cannot be directly compared with the GIS and AIS and it can be done based on the that particular area which is to be carried out. 398 (Refer Slide Time: 31:58) This generally this graph as you can see it shows, a general comparison diagram for three technologies; gas insulated, air insulated and the hybrid insulated or a going in for that hybrid substation. This shows the cost ratio in the Y axis and the surface ratio; for various busbar, the comparison made for 145 to 245 and a 400kV busbar with single and double bus. You can very clearly see the comparison going in for a complete gas insulated substation and the cost will be definitely high in comparison with the air insulated cost and the surface ratio also is being plotted and going in for the hybrid could be anywhere between the gas insulated and reduction; substation reduction comparison to the gas insulated could be seen after the air insulated. So, again the utilities and the planners have to decide to going for which type of technology has to be adopted either fully gas insulated or the air insulated substation which is to be planned. 399 (Refer Slide Time: 33:19) So, apart from this substations; and some of the cases is the requirement could be a mobile type of substation and emergency and some related requirements. So, in such cases few of the countries are also adopted, the mobile substations for the emergency catering, you could not see one typical 245kV substation which is available in Abu Dhabi, where from mobile it could be used in case of the emergency needs. Similarly, for railways in Switzerland; a mobile type of substation is available; these are few examples in case of emergency requirements. 400  Advances in UHV Transmission and Distribution Prof. B Subba Reddy Department of High Voltage Engg (Electrical Engineering) Indian Institute of Science, Bangalore Lecture – 25 Insulation coordination, Components in a typical substation So we have compared various substations; the air insulated, the gas insulated and the hybrid type of substations. The equipments or the main components which are housed in any substations are of prime importance and this maintenance of these components also is equally important. So, we will be seeing the main components which are housed in any air insulated or a gas insulated type of substation the general principle of the operation of these equipments which are installed. (Refer Slide Time: 00:59) The first component being the isolators which are a very important component in any substation mainly helps in the proper maintenance and the isolating of the circuit during the maintenance aspect. So, 132 kV and above will have set of 3 individual poles which have a double breaking arrangement with one vertical break earthing blade per pole that is very important and this are suitable for fixing on either scale of the poles. So, this isolators is similar to a circuit breaking arrangement have a earthing blades and have some short time current rating further operation and these are both thermal and could act as a dynamic short time current rating as that of the main blades interlocks are also provided to 401 prevent the operation of isolator particularly when the corresponding circuit breaker is on. So, the very important point without interlock interlocking facility is also been provided. So, this earth switches shall also be manually operated by a separate a motor mechanism operating mechanism for a typical in which is being used in any substation for the isolation of the circuits connected with it. So, the operating mechanism shall be suitable and it could hold the isolator either in a closed or in open position wherever the requirement is there and this will prevent the operation by gravity or by the wind or are those because we have short circuit of forces and could be because of the seismic forces there could be wind or vibration shock and the accidental touching etcetera. So, several of this aspects could influence the operating mechanism of isolator. (Refer Slide Time: 03:01) The circuit breaker is a very important switching device which is basically a mechanical switching device and capable of either making and carrying and breaking the currents under normal condition and also under specified abnormal conditions. So, types of circuit breakers which are in use for various voltage levels a 12 kV and 36 kV generally used or minimum oil or a bulk oil type or vacuum circuit breakers or a SF6 circuit breakers sulfur hexafluoride insulation media. So, for more than 36 kV again a minimum bulk oil minimum oil bulk oil or SF6 are available for voltage levels of 145 kV and above a minimum oil MCB or a minimum oil bulk oil or the air blast or the SF6 a insulating a breakers are generally use in a substation. 402 Similarly, for a 220 kV and above 420 and above either minimum oil or a bulk oil type air blast or SF6 these 4 types of circuit breakers are generally apply used for a higher the voltages; however, both the minimum oil the bulk oil and the air blast are faced out with the advancement of technology. So, very the recent advancement because of the SF6 tech technology and having better insulating properties compared to the earlier oil type of or a minimum oil type of breakers. So, voltage levels above 145 kV and above employ SF6 breakers and vacuum circuit breakers are generally used for 11 and 33 kV or a lower voltage levels. So, these circuit breakers maybe of live tank or a dead tank design, so live tank breakers are generally used for outdoor substations which have an interrupters housed in a porcelain a weathering shields. So, the circuit breakers pertaining to the dead tank type have interrupters outs housed in a earthed metallic container with their connections which are normally brought out through the porcelain bushings and these bushings maybe used to house the current transformers. So, this is the use of the porcelain housings for the circuit breakers either if it is a dead tank or it is of the live tank arrangement. (Refer Slide Time: 05:50) So, these circuit breakers are very important component as said and are most critical switching elements in a power system and circuit breakers are only a means of a interrupting fault currents in the extra high voltage or ultra high voltage a system are very important component. So, we will just focus about the importance of the circuit breakers the type of 403 circuit breakers which are used in the EHV and UHV substations the circuit breakers are very fast and secure fault interruption is required not only for the protection of the transmission system equipment, but also for a overall operational system stability is a at most important point. The interrupted rated current under a no load and the loaded condition and also under the fault conditions up to rated symmetrical fault currents which should be able to operate circuit breakers are placed on a variably considerable with nature of a circuit involving the being a switched on to the required time and interruption of a fault currents at very high voltage levels could lead to high thermal and the dielectric stress on this breaker. (Refer Slide Time: 07:35) So, this has to be considered in forgoing infer the proper design of the requirement of the breaker even low level fault currents which are of capacity or inductive in nature can also place a very high stresses on the equipment that is in a circuit breaker. So, what are the main the important functions or the circuit breaker has to perform are classified as a follows. So, the circuit breaker should primarily focus in to the 4 important functions it should carry the rated a current at the rated voltage a very important point it should carry the rated current at rated voltage and power frequency whatever the power frequency voltage which it is intend to operate continuously when in closed position. So, when the circuit breaker is closed it should able to carry the rated current of the bus bar or the rated voltage which is to be operated and the power frequency 50 hertz or which is a 404 designed in the closed position and it should interrupt the rated current at the rated voltage and power frequency on trip commands. So, once the command pertaining to the opening of the circuit breaker is given it should be able to interrupt at that supply current or rated current which it is being operated. So, these interrupt fault current in case of fault in the transmission system also interrupt line charging currents and induction currents. So, it should be able to interrupt any fault currents which are generated because of the transmission system and the final point to ways in his it has to be maintain the rated a dielectrics that is the insulation properties both for the power frequency and the impulse or the search withstand levels when it is in open position. (Refer Slide Time: 09:33) So, the insulation properties of the circuit breaker are also equally important for power frequency and surges which it withstand during the opening of the circuit breaker with the transient recovery voltage is a very important term in the characteristics in the circuit breaker, so voltage which appears across the terminals of the current interruption. So, the characteristics of the transient recovery voltage that is both the amplitude and the rate of a rise this could lead to either successful current interruption or to the failure because of a re strike or re-ignition a re-strike or re ignition. So, for successful current interruption there should be neither re-ignition nor a re-strike. So, this has to be done. So, that the characteristics have to be successfully quenched without the re-ignition or the re-strike. So, the transient recovery voltage waveform is generally 405 represented by the 2 parameters which are senior the current waveform which is indicated in the here and the transient a recovery voltage or waveform you can see the transient and the recovery voltage the supply voltage is somewhere here the which is the dotted line and this is the transient recovery voltage over a period of time. So, very important it should be able to see the successfully interrupts the re-strike or the re ignition which is happened. (Refer Slide Time: 10:59) So, how to select the rating of a circuit breaker is again an important planning which has to be carried out by the substation designers for various voltage levels. So, some of the typical technical details are given here as per the international standards which are available you can see the rated voltage levels from a very low 36 kV to the 800 kV a voltage levels the requirement for the circuit breaker the rated a short circuit a breaking capability or the current in terms of kilo amps for a 36 kV it should be able to break the current at 25 on 31.5 kilo amps as the voltage level increases you can see for a 800 kV or a EHV that is a 420 and above the rated short circuit breaking current should be able to break 40 to 263 amps; kilo amps sorry and for a 800 rated voltage minimum of 40 kilo amps should be the rated short circuit breaking capability of circuit breaker and the normal rated current in terms of amperes you can see for a 800 kV system the normal rated normal current could be anywhere between 2000; 2000-3150 amps for a voltage, it becomes for a lesser voltage it could be a 1600 amps the rated normal current which it sees. 406 So, the total break time as per the international standard that is 62271 clearly mentions about in case 145 kV voltage level the circuit breaker should be able to operate within a 60 minutes milliseconds to 100 milliseconds and in case of 245 kV, the total break time which is mentioned as per standard should not exceed more than a sixty milliseconds and for UHV transmission substation or EHV transmission levels more than 420 kV, it should not exceed 40 milliseconds and for 800 kV it should not exceed 40 milliseconds. (Refer Slide Time: 13:32) So, these are the a total break time as per the international commission which has been specified for the selection of the circuit breaker a ratings. So, this is a typical example for a high voltage or extra high voltage circuit breaker looks you can see it has to perform a various functions has mentioned earlier a huge equipment which is to be in the circuit and it has to properly function either making or the breaking of the circuit whenever it is required. 407 (Refer Slide Time: 13:59) So, this again is one of the typical example for the extra high voltage circuit breaker which is being employed for a very high voltage or ultra high voltage levels. (Refer Slide Time: 14:13) Further with the development of the SF6 technology a sulfur hexafluoride insulating media is used as a insulating a property. So, here again design of the circuit breakers with SF6 technology as a gained a importance in the recent us and several of the breakers with SF6 technology are being employed for the a higher the voltage EHV and UHV air transmission levels consisting of above the interrupting units with several interrupting units are connected 408 in series it is an advantage and also high speed a switch for a closing the resisters. So, several options are been available and designs are available from various manufacturers as the globe. (Refer Slide Time: 15:16) So, the weight of the circuit breaker has also drastically reduced by going infer a SF6 technology. So, for thousand the 100 kV various type of circuit breaker from different voltage levels are just highlighted here you can see for 300 kV with single interrupter for 550 kV with 2 interrupters and 550 kV with PIR, what is PIR is very important pre insertion resisters. So, why this pre insertion registers are used for circuit breaker we will discuss about this and this circuit breaker again is used for 1100 kV with the PIR. So, the technology as advanced and the compactness of the breaker as also been achieved by going in with the SF6 technology. So, this a PIR; a pre insertion resister is a very important which is being used in the circuit breakers generally it is 200 to 400 ohms and which this gets temporarily closed before closing the circuit breaker the sequence is operation of the circuit breaker use the closing of the pre insertion resister after closing of the pre insertion resister with a gap of 10 to 12 milliseconds the closing of the main circuit takes place a circuit breaker contact takes place. So, this is the important function how it is important see when again in opening the pre insertion a resister is first disconnected by the breaker operating mechanism and after ten milliseconds the main a contact of the breaker are generally opened. So, this is the mechanism of the use of the pre insertion resister in the circuit breaker the main purpose the main important purpose of using the pre insertion resister used to limit the initial 409 charging current of the transmission line. So, as the charging current of long line will be much more. So, it is advisable to use the breaking breaker with a pre insertion resister. So, few of the examples are shown here for the very high voltage with for a 550 kV with the pre insertion resisters similarly for 1100 kV. So, the PIR serves to limit the initial charging current of a long line and it is advisable to use the breaker for better protection above EHV levels with the PIR inserted with the breaker. (Refer Slide Time: 17:57) So, that was about the in some of the important components which are in the substation continuing that a insulation a coordination is a very important aspect for the proper design of the substation or the transmission or any of the equipment electrical equipment which is a being used for the very high voltage levels. 410 (Refer Slide Time: 18:12) So, the electrical equipment we know that the insulation or dielectrics must be specified a designed and constructed in an optimized way this we have try to discuss with the comparison and the main intension of going in for insulation withstand not only to see that it has to be in optimized way and also to see that it withstands the electrical stresses for a long period of time which can be expected in the electrical network the main goal is also to see that it essential for the reliable operation of any electrical system and this has to be achieved by applying the principles and practices of a insulation coordination as the voltage level goes high higher and higher at extra high voltage and ultra high voltage the planning of insulation coordination gains are very important or is considered to be an important aspect for a very high voltage levels. So, the insulation coordination is in fact, the methodology which will help to a certain the electrical stress that could occur in maybe electrical network and this will coordinate with the stress with the withstand characteristics of the electrical equipment in a techno economic manner that is important point to be considered techno economic manner. So, it should be also economical it should be technically viable. So, that the result in selection of the insulation level that is optimized form reliability and an economic perspective so, the insulation co-ordinate coordination a place a very vital role above a 400 kV or EHV or UHV transmission substitution or insulation aspect. 411 (Refer Slide Time: 20:19) So, various voltage stresses in service we have discussed earlier the main is the system voltage and the overvoltages again the overvoltages are categorized into a temporary overvoltages which could be for a few cycles a few tens of milliseconds and light switching surges again it switching could be of the operation of the a circuit breakers closing and opening of the circuit breakers or any other switching activity and lightning is mainly caused by the natural lightning the lightning surges which impinge on the transmission or the distribution network could create the voltage stresses in the insulation of the equipment in service. (Refer Slide Time: 21:05) 412 So, this overvoltages in any network when you look in to this graph which shows the amplitude that is a voltage amplitude in per unit of the peak value was the y axis and x axis become is shown as the voltage duration that is of time period in a seconds you can see here for the lightning aspects in case of the lightning the surges which are developed because of the lightning activity could be a very fast fronted type of a overvoltage surges. So, earlier we have discussed. So, lightning a overvoltages could be of the fast fronted nature that is a very stiff fronted or could be of in microseconds that is 1.2 microseconds could be a front and the tail could be anywhere between 50 microseconds. So, this lightning surges could be of a very high magnitude and fast fronted the second surges are of switching in nature the switching again because of the closing or the opening of the circuit breakers this switching surges are comparatively slow fronted surges are overvoltages this low front again it could be 250 milliseconds there it is a microseconds 1.2 microseconds this 250 milliseconds sorry microseconds and 2500 microseconds. So, the surges of switching have a typical level shape of 250 by 2500 microseconds apart from lightning and switching surges we have a temporary a overvoltages which could last for few cycles this again are of low frequency could be of the power frequency or related overvoltages slightly above the normal operating voltages and finally, the entire power frequency voltages which are in continuous in nature where the electrical stress on the insulation lies this will be for a longer period of time. So, this gives the clear differentiation between a various surges or the transients overvoltages light switching lightning and temporary overvoltages which are of low frequency and the power frequency which could be for a very long period of time these are various overvoltages which occur in a transmission or a distribution network. 413 (Refer Slide Time: 23:23) So, how the strength of a typical insulation is estimated you can see here this graph gives the voltage amplitude per unit of peak value again versus the voltage at duration this gives you the basic insulation a level in case of the lightning impulse a withstand of the equipment or the switching impulse or switching surge withstand impulse level including the surge impedance a loading of any equipment. So, the power frequency withstand is given for the voltage amplitude which is shown as the per unit. So, you can see the basic lightning impulse withstand level could be anywhere between 4 per unit, but the basic insulation level is 2.823 per 2 point less than 2.8 per unit for switching impulse or switching surges withstand it could be 3.5 with surge impedance loading on lesser than that it could be 2 to 2.2 per unit and for power frequency it could be around 2 per unit of the peak value where this surges or the typical insulation strength of the equipment is considered what are the insulation levels and the clearances this also have been discussed earlier. 414 (Refer Slide Time: 24:54) So, insulation levels and clearances are very important and this will be based on the proper a design or insulation coordination studies which are to be conducted a typical example for one particular substation in the country that is a Seoni substation of 765 kV is given here how the parameters are been identified and how the insulation coordination has been done for the actual system of 765 kV substation very clearly you can see electrical clearances which are designed the main parameters and the value are given here the electrical clearances from a phase to earth that is a conductor to the structure the required coordinate value is 4900 millimeter for a phase to earth that is a rod to structure it is 6400 millimeter phase to phase that is a conductor to conductor clearances is 7600 mm for 765 kV substation and the phase to phase conductor to rod it is given estimated to be 9400 mm in the circuit breaker requirements that is a insulation level requirements for a circuit breaker the lightning impulse withstand level LIWL is 2100 kV switching impulse withstand level is 1550 kilovolts for the transformer and the shunt reactor the insulation level required for 765 kV substation is lightning impulse withstand level is 1950 kV and for switching impulse withstand level is 1550 kilovolts. So, some of the practical information which is being used in the country for a 765 kV ultra high voltage substation these are some of the clearances for the transformer circuit breaker in the shunt reactor. 415 (Refer Slide Time: 27:04) And the values prescribed based on the insulation level and the clearances required similarly insulation levels for a very high ultra high voltage systems a comparison is generally done here we can see for a equipment which are used at a 1100 kV test station in Japan and the equipment which is in service in china and the equipment under construction in India is been given here we can see the first row corresponds to the equipment under test at Japan for 1100; the power grid in India for the values at 1200 kV and in China 1100 kV comparison is been made you can see the lightning impulse withstand values the rated voltages are being 1100 in case of Japan 1200 in case of India and 1100 in case of china the lightning impulse withstand voltage level for a circuit breaker in case of Japan 1100 kV the requirement has been estimated to be 2250 kilovolts in India for 1200 kV it is 2400 kilovolts and the similar voltage level 1100 for in china it is follow 2400 kilovolts. For transformer it is 1950 in case of the comparison Japan 1100 kV system it is 2250; 2250 both in china and India for 1200 and 1100 kV respectively. It is about the lightning impulse withstand for switching impulse withstand for the circuit breaker and the transformer the values are given here you can see both for 1200 kV and 1100 kV the values are identical that is 1800 kilovolts both for circuit breaker and the transformer are being estimated and being planned to be used in the substation for ultra high voltage substation. 416 (Refer Slide Time: 28:58) So, comparison of the insulation levels for a different system voltages are given here based on the estimation. So, you can see here a various a voltage levels from a voltage levels of 220 kV to the EHV level ultra high voltage a levels 800 and 1200 kV the rated voltage a peak one per unit per kV is very clearly given here for 240, it is a 200 for 420 kV it is 340; 3800, it is 653 and for 1200 it is 980 kilovolts. So, 1 minute power frequency withstand voltage for this insulating system for 245 kV the requirement is 460 kilovolts and for 400 it is 630, 800 kV it is 960 and for 1200 it is 1200 kV. So, it has to withstand for one minute operating voltage power frequency withstand and switching impulse withstand lightning impulse withstand values are also given here for 220 kV system the lightning impulse will be 1050 and for 2400 kV, it is a 1050 in case of switching and 1425, in case of lightning impulse for 800 it is 1550; 2100 kV and for 1200 it is 11; 1800 and 2400 kV is the specified limits; for the lightning and switching impulse high voltages in case of per unit these gives the values for switching impulse voltage lightning impulse and lightning impulse withstand a level for rated a voltage these are the a per unit values which are being given you can see for a 420 kV, it is 3.06 and for 1200 it is 1.86 and in case of 800; it is 2.3 for switching impulse and for lightning impulse it is 4.15; 3.22 and 2.55, this is a very important aspect to be considered for the various insulation level this gives an idea of the insulation coordination which is being done for the various voltage levels which are being adopted. 417 (Refer Slide Time: 31:11) So, the important component of the substation is the surge arrester which is a plays a major role to control or to contain the overvoltages which have been discuss it could be lightning it could be switching or either overvoltages. So, surge arresters plays a major role and proper planning coordination of this is very important the parameters of various equipment are decided considering the presence of this surge arrester in the system and these surge arresters are connected to each phase and ground. So, the system to control the voltage stress that this equipments will experience under normal and also any contingency conditions. So, the surge arresters require very high energy discharge capability very important as the voltage level goes the current carrying capability the power requirement will be high. So, the substations which the surge arresters are housed have to see the energy discharge capability; so, very important. So, again the energy it could be a distribution it could be a class 1, 2, 3, 4, 5. So, various class of surge arresters are being used for a different a voltage levels to see the energy which is discharge and the surge arresters should be capable of discharging surges are to the ground in a to see the equipment is being a properly protected. 418 (Refer Slide Time: 32:57) So, the discharge capability is considered with stringent duties and sustained overvoltages particularly in the ultra high voltage system and extra high voltage systems. So, a single phase to earth fault gives rise to overvoltages in healthy phases up to 1.4 per unit. So, the fault is generally cleared in hundred to 140 milliseconds by opening local end circuit breaker at one twenty milliseconds and the remote end of the circuit breaker at 140 milliseconds. So, the energy which is to be handled is very important the energy which is to be handle by metal oxide surge arrester during these conditions during the fault clearance conditions or the fault is to be cleared with the opening and closing conditions such a huge energy is to be handle by this metal oxide surge arresters and due to the temporary overvoltages could be about 35 mega joules so, high energy up to 35 mega joules. So, the above duty followed by the discharge class that is a class 5 of the surge arresters the 2 shots of 5 mega joules each and a margin for non uniformity accounts usually for about 55 mega joules per metal oxide surge arresters are used for such a high discharge class. 419 (Refer Slide Time: 34:21) So, when we look into the VI characteristics the volt current characteristics of any a typical surge arrester the y axis shows the kilo volt per peak in magnitude and the current kilo amps in a x axis you can see the characteristics of the surge arrester is typically looks like this where initially rises and tries to stabilize at a current rating after say 8 kilo amps. (Refer Slide Time: 34:58) And you can see that disability is emended what are the surge arrester a parameters the surge arrester rating in case if it is 800 and 50 kVrms the continuous operating voltage of the 420 arrester will be 693 kV. So, for a rating of 850 kilovolts the continuous operating voltage of a surge arrester will be 693 kilovolt rms. The minimum continuous operating voltage MCOV of arrester is 723 kilovolts and lightning impulse protection level at 20 kilo amps is the 1700 a kilovolts per peak and switching impulse a protection level at 2 kilo amps is 1500 kilovolts per peak the energy of the arrester which is to be diverted will be 55 mega joules and pressure relief class is 40 kilo amps the line discharge class is class 5 as per the international electro technical commission. So, this technical parameters are for UHV ultra high voltage range more than 765 kV level protection system which is being used in the substation so, or higher the 1100 kV or 1200 kV systems. (Refer Slide Time: 36:23) So, similarly some of the technical requirements or is a metal oxide gaplessed arresters again arresters are basically a classified into gapped and gapless liked to mention here the old technology the technology which was earlier used was gap type of arrester containing the silicon carbide elements stack one above the other with gaps in between each blocks the recent technology is using the metal oxide a zinc oxide based arrester elements which are stacked one above the other without a gap. So, that is a reason which is known as a gapless a type of arresters which is of a recent technology being used for the extra high voltage and a ultra high voltage a transmission a system. So, this table gives information about various voltage levels of the arrester which is being used rated arrester voltage 322 116 120 33 kV class and the maximum continuous operating 421 voltage level is given here and typically installation is outdoor a class could be heavy duty station class that is a gapless type of arresters are employed. So, type of construction for ten kilo amps rated arrester it should be single column it will be single column if the voltage level goes higher and higher the surge arresters have to be imparallelly connected and going for 2 or a multiple columns and normal discharge current corresponding to the lightning impulse of the surge arrester at eight by twenty microsecond wave shape will generally be a 10 kilo amp rms for all the type the type of mounting will be pedestal these are some of the technical aspects of the lightning arrester which are used for lower high voltage class less than 400 kV and the long duration class is class 3 and there are class 2, 3 and 4 and 5. (Refer Slide Time: 38:27) So, ratio of switching impulses these are some of the technical parameters for switching and a symmetrical fault current in case of pressure relief has to be operated. So, these are the values particular amps rms and the corona extension it depends on the rated voltage of the arrester the corona extension voltage as to be defined by the rating of the arrester and what is the maximum interference in a radio interference with the energized minimum continuous operating voltage it should not exceed 1000 micro volts or 500 micro volts if the voltage is very less and the minimum creepage distance of the arrester housing a requirement again it depends on the voltage level here. 422 (Refer Slide Time: 39:13) This is the creepage length which has been technically given. So, this shows the typical surge arrester which is being used for a EHV or a UHV transmission the column of the ceramic bushings containing the number of elements in blocks zinc oxide blocks which are housed in this a ceramic housing a similar surge arrester is shown here. (Refer Slide Time: 39:45) So, these are typical examples which are used for the EHV and UHV transmission systems again this figure shows the surge arrester or a zinc oxide blocks these are the zinc oxide blocks which are a stacked inside the porcelain housing with suitable pressure relief 423 arrangement and pressure valves in case of emergency. So, this is how the blocks are rated for a various classes from the distribution class 1, class 2, class 3, 4 or 5 which are being used for ultra high voltage systems a very important component and the insulation coordination is mainly to see that the equipment like the transformer the switch gears are protected. (Refer Slide Time: 40:37) So, surge arresters plays a very important role in the substation for the coordination of the insulation this is one of the example which of a first prototype of a 850 kV surge arrester which is being used for 1200 kV system is shown here it consists of a multiple a stacks 4 stacks parallel stacks combined with several zinc oxide elements housed in this ceramic shells. So, this is one of the first type of a design when required for a 1200 kV experimental line in the country was developed. 424 (Refer Slide Time: 41:15) Some information about the parameters and some arrester details are based on the values how the rating of the element is calculated the information is available in the a standards pertaining to the surge arresters, but anyhow could i would like to give some information on how the number of a blocks are chosen for a particular a rating. So, you can see here depending upon the discs number of disc in case it is used for the discharge class 5 the rating of the disc continuous the rating of the disc could each be 3.3 kilovolts. So, in case of the 3 point rating of the zinc oxide element that is a block is 3.3 kV rms used for 20 kilo amp level and the discharge class is 5. So, how many blocks in one column it is estimated that 850 kV is the voltage level divided by the 3.3 will give 258 arrester blocks to be used. So, the residual voltage or the protection level lightning impulse protection level with considering 4 parallel columns at 20 kilo amps is 5 kilo amps per column. So, it comes down to as 4 columns are there. So, 258 blocks into 6.65 kilovolts will give you 1715.7 kV peak is the total residual voltage of the surge arrester of 850 kV rating. So, similarly residual voltage protection level for switching impulse with 4 columns comes to 1496 kilovolts again this is calculated based on the number blocks and the voltage level switching voltage level of one single arrester. So, it comes to 1496 kV then minimum continuous operating voltage of the arrester is 2.84 it will be 3.3 kV disc. So, 2.84 is the minimum continuous operating into 258 blocks will give you 732 kV rms the energy handling capability for 4 columns as shown here this 4 columns will be the operating duty 425 test into number of discs into number of columns this will be typically 55.78 mega joules which is very high. So, the energy in long duration withstand test again it depends on number of discs into number of columns say 54 kilojoules into 258 into 4 it is 58; 55 mega joules of energy has to be handle by the arrester elements. (Refer Slide Time: 44:04) So, this is again the matching of electrical characteristics which are used for 1200 kV a system operating voltage is 850 kV for a voltage current rating of 20 kilo amps in discharge class as mentioned earlier class 5. So, the maximum system voltage is 1200 kilovolts the rating of arrester which is already decide is 850 that is 71 percent of the maximum voltage which is operated 20 kilo amps discharge class. So, maximum continuous operating voltage 1200 will be =3693 kilovolts; the desired minimum continuous operating is 727 kilo volt √3 rms. So, the tested value of the minimum continuous operating of arrester both for duty class and for transient conditions is seven twenty seven. So, the equipment which is tested is lightning impulse withstand and switching impulse withstand high voltage levels is 2200 and 800 kV respectively and in case of programmed case lightning arrester lightning impulse protection level is 1725 and a switching is 1500 kV, but actually it is 1715 and 1496. So, the protective margin is higher for both lightning impulse and switching impulse. So, other detail of 426 estimation like the total creepage distance of the entire arrester housing is 32300 mm which actual requirement is 32800. So, height from a mounting plane is nine thousand seven hundred mm and dry arcing distance 8150 mm the grading ring to arrester bottom and the routine bending load specified long term load is 6 kilonewton. So, these are some of the technical parameters which are required to be estimated for the 1200 kV lightning arrester which is being housed in the substation. Thank you, we will continue. 427  Advances in UHV Transmission and Distribution Prof. B Subba Reddy Department of High Voltage Engg (Electrical Engineering) Indian Institute of Science, Bangalore Lecture – 26 Preventive maintenance of Substation So good morning we were discussing about the various substations. Particularly gas insulated substation the air insulated in the hybrid type of substations. (Refer Slide Time: 00:24) Which are recently being used for the EHV and UHV voltage levels. Now we will discuss about the important components of the switchyard in a substation. So, we have discussed earlier various components important components like the busbar, disconnector, the circuit breakers, current transformers, voltage transformers, the earthing switches and surge arrestors. These are the various components in any typical switchyard of a EHV or any high voltage HV EC or HV DC substation. We will look into the various functions of the components which are being installed in any substations. So, the main important component being the busbar. So, this busbar main function of the busbar is to connect and deliver the loads or transmission the loads to the required centers. Then we have a disconnector switch the disconnector switch the main function of the disconnector switch is to connect and disconnect the circuit, disconnect the circuit for the maintenance or for any up gradation of the equipments in 428 the substation or any important changing over of the equipment or changing of the relays and so on. The third bring the circuit breaker circuit breaker is an important component as mentioned earlier. So, the important function of a circuit breaker is to connect the circuit of a normal operating conditions. Then disconnect in case of the over voltages or during the fault conditions. And the main other function of the circuit breaker is also to detect the faults and operate accordingly. Then we have 2 important in information components one is a current transformer and the other being the voltage or a potential transformer. So, these mainly detect the information and try to transform the information either in a current or a voltage to the control unit. Or in case of a SCADA the information is being fed to the control center. So, similarly we have a earthing switch earthing switch again this is being used mainly for the protection and also for the safety aspects, in case of any changing of the equipments in the substation the earthing switch is kept open and the equipment is being attended. So, then the finally, we have discussed in the last class about the a lighting arrestor or a surge arrestor. The surge arrestor is an very important component. And it quenches the surges or a lightning or a switching surges and protects the major equipments like the transformer circuit breakers and other release and controls in the substation. So, these are the important components in any switchyard of a high voltage or UHV substation. So, the preventive maintenance of these components is essential at a regular intervals so that the performance of these components or performance of this equipments over a long period of time has to be seen to be considered in the field. 429 (Refer Slide Time: 04:10) So, the busbars, the busbars are long tubular structures in any substation. So, these busbars or a overhead ground wires these are the busbars in any a substations. So, these busbars at least once a year visible inspection is to be carried out regular intervals, in examining of all the wiring connections which are connected to the busbar the end connections of the busbar. To it has to be properly checked whether the following insulator support the insulators may be a (Refer Time: 04:43) or long rod type of insulators. So, these have to be verified and the contamination or the spread or the insulator surface have to be cleaned at a regular intervals. Or if necessary a suitable high voltage insulating coating if required has to be applied on these insulators in case of any necessity. So, we have the physical condition of these cables or the busbars have to be attended at a regular time period, to check there physical conditions particularly as these are operating in the open environmental condition. The effect of corrosion or any other because of oxidation. So, a proper check is essential for the busbars. Similarly for the ground wires which are being connected a proper a check or the testing of the grounding system. So, we will be discussing about the grounding system in detail in a later lecture, but they very important is to see the ground wires are also being attended to. 430 (Refer Slide Time: 06:01) Then comes the disconnector or the earthing switch. So, this disconnector switch comprises of a moving contact with a flexible a fingers as shown. Again these disconnectors switches very with different voltage levels various types of connector switches are being employed at different voltage levels. So, these disconnecting switches have to be attended at least once a month a proper visible inspection has to be made and necessary check has to be maintained for the heating resistor particularly the near the contacts at it is control panel or whether there is a proper functioning or the contacts have being corroded or it needs inspection and also some remedial action to be done. So, a regular maintenance apart from a regular maintenance of monthly once. At least once in a year the following have to be attended, that is the contacts of the disconnector switches and earthing switches have to be properly cleaned up and the electrical contact grease if necessary has to be applied for the smooth contact and to see the corrosion of formation near the contact or on the contact is happening. Then the disconnector switches or to be checked a particularly at the joints near the joints where the bearings of the opening and closing of the units required. So, this will help to check the flexible connections of earthing switches also. Then all the important joints or particularly the joints for the unit have to be properly a tightened, and the insulators which are supporting these disconnector switches have also be have also to be cleaned at a regular intervals. And any excess amount of a pollutant or a contaminant has to be 431 which is been accumulated, has to be cleaned so that the surface discharges on these support insulator in our happens. So, the regular maintenance has to be carried out for the smooth functioning of the disconnector switches. (Refer Slide Time: 08:29) Then we have the 2 important components, the voltage transformer or the potential transformer and the current transformer. So, the voltage transformer at least once a month we have the inspection is essential, to see that the voltage divider to be sure that there is no oil leak, particularly which is a serious because this oil leak or a serious accumulation of soot dust or salt, composite could be present to it has to be removed so that is a reason for to see the maintenance has to be carried out at least a month. So, inspect the intermediate voltage transformer and it has be checked for the minimum permissible oil level has to be maintained in the voltage and also the current transformer. And at least once in a year what you check all joints and the contact points for that there is no loosening of the contacts and the all the insulators which are again associated with this cities have to be properly cleaned, so that the dirt accumulation is a not being and then on the surface of the insulator which will again, during the a monsoon condition there is likely would of the discharges because of the surface contamination this has to be avoided. 432 (Refer Slide Time: 10:04) And the voltage or transformer and the current transformer both have to be at a regular intervals have to be monitored and properly calibrated, that is very important. So, that the readings which these equipment or the information is the this current transformer or voltage transformer has going to be communicated have to be accurate, that is again one of the important aspects for the proper calibration has to be carried at least once in a year. So, here current transformer visual inspection is important. To check oil level and also the defects of possible oil leaks in the current transformer. So, again here the insulating supports have to be properly checked for the dirt accumulation, and necessary primary or secondary connections and the conducting parts have to be properly tightened, and nowhere and the loose contacts are to be seen. The important point to be noted is that never open a secondary winding of a ct current transformer particularly while on service. So, this is a not advisable to see the current transformer secondary should not be open. That is one of the instruction to it should be carried out by the personal over attending to these maintenance job. So, both the equipments have to be calibrated nearly once that has to be done for proper information. 433 (Refer Slide Time: 11:29) Then we have discuss lot about the surge arrestor or a lightning arrestor and the used for the transmission and distribution networks, mainly for protection of the transformers and other equipments. So, this surge arrestors has mentioned earlier consist of various a blocks of zinc oxide elements, which are stacked inside with the pressure relief or the surge counting device near the ground side and you have a surge counter and also leakage current monitor which gives the information about, the for performance of this lightning or a surge arrestor. So, this is a typical surge arrestor for various voltage levels, you can see the grounded portion here and this is a high voltage portion with the corona control rings. So, near the grounded before the grounded connection, you have counter surge counter and also a leakage current monitor to give the information about the number of a lightning surges which you happened over a period of time. Here again the surge arrestors require a regular maintenance. So, visual inspection and examining of all the wiring connections which are being done to the arrestor. And again here the hollow insulator bushing which accommodates the surge arrestor elements have to be properly clean, including the corona control rings the metal rings, adjacent rings which are being used for the surge arrestor blow housing, have to be properly cleaned again using the high voltage insulating coating. If necessary and a physical check for the connections that is the busbar connection to the surge arrestor is essential at a regular intervals. 434 (Refer Slide Time: 13:28) And ground wire or the earthing wire place a very important role in the substation, where in case of the lightning surges or a lightning flashes which occur or the proper shielding has to be done so that this surges are diverted to the ground without the voltages or a rise potential seen to the other equipments which are connected. So, very important the earth wire of the ground wire. This regular check on the grounding system is essential and the arrestors which are connected, it is shown here each phase of the transmissions conductors have been connected with a surge arrestors. So, these surge arrestors should never be touched unless completely disconnected, while maintaining the ground wire connections or verifying checking the ground wire or shielding wires. So, surge arrestors have to be properly disconnected from all the live lines and the equipment effectively which are connected to ground at the line side of the arrestor. So, very important everything has to be earth then the maintenance has to be carried out. This shows a typical a lightning or flash which is occurring to the grounding or a shielding wire, which is connected in the substation. And also you see the importance of the tower footing resistance the high footing impedance, we will be discussing about tower footing impendence’s or the what is the minimum resistance which is required for the earthing or a grounding in any high voltage substations. 435 (Refer Slide Time: 15:10) So, apart from this we have a capacitor units, several units’ capacitor banks. And these are very important components in any substation very essential components. In a substation the capacitor banks will improve the power factor of the system, if the load is leading it is useful introduction particularly in the power system losses, and also these are used to see the voltage regulation is improved. So, capacitor banks that is a certain capacitor banks are normally used to improve the quality of electrical supply, and also for the efficient operation of the electrical system network. And capacitor banks are typically used to reduce the overvoltage’s in the substation. (Refer Slide Time: 16:02) 436 So, various types of capacitor banks are being used, some maybe fixed or automatic again and depends on different types and also the capacitor bank vary with type of voltage levels whether it is distribution or medium voltage or a high voltage or extra high voltage or ultra high voltage. There is a capacitor banks are suitably designed to be kept in the substation. So, the types of capacitor bank distribution pole, this could again be a fixed or an automatic type. Distribution pad type which could also be a fixed or automatic substation, sometimes is a metal enclosed capacitor banks are installed whether in any required substation where it is again a fixed or an automatic substation. And some of the cases are also similar to these substation of rack mounted capacitor units, and this is one of the example which is used for the improvement of power factor and also reduction in the power system losses. So, one of the example is assembly of the rack mounted units of the capacitor bank. (Refer Slide Time: 17:16) Similar examples are connected with the capacitor banks. So, several types again before the distributed pole mounted you have arrangement of the distribution system. This is metal enclosed type of a capacitor banks which are arranged in a typical substation, along with the capacitor bank you have the harmonic filters in a EHV or a UHV substation. So, these are various types which are capacitor banks which are being used for the proper correction of the power factor. 437 (Refer Slide Time: 17:58) So, that was about the main equipment which are housed in any substation, very important. So, further to this in any substation, very important, because of the various components and the operation at very high voltage levels. So, the important aspect to be or considered is the electric and magnetic fields. So, electric and magnetic fields are play a vital importance in the proper design of the substation, and also helps to see the personal who are working in substation have to be considered, because of the clearances and the effect of the electric and magnetic fields pertaining to the health. So, this is one of the concern as the voltage level in any typical substation goes above EHV or UHV level the electric and magnetic fields are very important. So, proper planning and proper estimation of this fields is necessary, and also the measurements is necessary to see the humans or not affected by the fields which are generated, because of the high voltage operations at the substations. 438 (Refer Slide Time: 19:22) So, several mitigation techniques are also being standardized, for the power frequency magnetic fields which are normally originated by the electric power systems. So, there is CIGRE working group again this is CIGRE working group is the international council, for the large high voltage systems which work towards the standardization of the equipments and related to the electrical very high voltage aspects. So, this working group CIGRE has suggested some of the mitigation techniques particularly for the personal over working in the utilities and particular to the substations, and the specifications which are the regulations framed, the magnetic field and the electric fields have to be contained, so that there is no harm full effect to the humans over working in the substations. 439 (Refer Slide Time: 20:22) So, where do the sources of this electric and magnetic fields or seen? Particularly when the power frequency levels. So, we know that again going back we have a generating plants various type of generation we have step up transformer to transmit the voltage levels. At a very high voltage then we step down at the required levels, and the from the step in down we will through the substation we try to distributed to the local requirement it could be industries could be domestic purpose. Here during this process of the entire process the flow of energy as earlier mentioned and from the generating to the customer level. At the power frequency operating voltages could be of distribution or extra high voltage or ultra high voltage. The reason here the power frequency electric or a magnetic fields, and the sources from where it is being generated and ah, the techniques which are to be classified and how to mitigate is very, very important for the utility engineers and also for the personal over working in the electricity organizations. So, here the power frequency magnetic field sources could be classified according to their origins. So, we are do the originate that is very important, it could be from the power lines which are transmitting from the generating to the load centers. It could be from the underground cables. Or it could be from the complex sources maybe from the substations, any of these substations. 440 (Refer Slide Time: 22:27) So, the sources could be any of these transmission or distribution or through the substation side, because of the equipment because of. So, when you compare the differences are between the electric and magnetic fields, very important aspect, how it could affect the humans particularly. You can very clearly see here indication diagram of human which is a shown here with the to the effect of electric field and to the magnetic field, it will be explained here in the following ways. So, this gives electric field lines in case you consider residential house, where you can see how the electric field lines in case if the transmission conductor is very near to the residential line residence residential area. How the electric field lines could affect the residence, and it is very clear that the electric field lines normally do not penetrate inside the house. But you can see this B being the magnetic fields magnetic fields of which are originating from the substation or through the transmission because of high power could a penetrate the residential house also. So, very important factor. Similar explanation is being given here; the electric field does not penetrate the house that is to be noted. Electric fields generated as a consequences are normally diverted to the earth through the grounding arrangement. Even in case the lightning strikes the transmission line which is the going near the residential area. The suitable lightning rods which are available say, again if the house residential house of at all structure has a lightning rod this, normally is connected to the earthing ground where the surge which is a comes in the contract will be diverted to the ground. 441 So, the lightning rods connected to the ground will do the diversion successfully. So, no much of harm with the electric fields because of the transmission or because of the lightning aspect. In case of magnetic fields you can see that magnetic field could penetrate the house if it is near the vicinity of the site where the effect of magnetic field is very high. Here only certain materials particularly which specific geometries or dedicated circuit could oppose this action. Else if the materials are not capable of particularly oppose in the magnetic field, then there is a likely chance of the magnetic field entering the or penetrating the residential apartments, in case if it is then vicinity of the magnetic field zone. So, the purpose important aspect, the purpose of a designing the mitigation techniques for the magnetic field the is to see and find out what are the most appropriate materials which could be used, and the what are the type of a geometries to be employed to see that the effect of shielding for the magnetic fields could be effectively carried out, the very important aspect. So, magnetic fields are much more dangerous in compare to the electric fields. So, it has to be properly contained and it has to be suitably mitigated with the help of the available techniques. (Refer Slide Time: 26:11) So, electrics and magnetic fields both are invisible areas of energy. These are often referred as the radiations, which is known as electric electro field magnetic field radiations. And these are all associated with the use of electrical power at very high 442 current ratings and sometimes could be natural and also, maybe due to the manmade lightning aspects also. So, EMF or electromagnetic fields are typically characterized by the wave lengths or the frequency, into one of the 2 radioactive categories which are internationally categorized. 2 important categorization where the electromagnetic fields are classified. One is the non ionizing a type. So, non ionizing type is a low level radiation which is generally perceived as harmless to the humans. So, the magnitude of the radiation which is emitting from this category, which is classified in this cat as a non ionizing level is not much harmful to the humans whereas, ionizing type, the second category, which is having a high level of radiation could be a factor which has to be considered, and where it could have the potential particularly for damaging the cellular and also the DNA that is very concerns. So, the ionizing type of a magnetic fields are the important consideration to see that it has to be contained. So, these categories have been classified as follows which is that shown in this table. The radiation type, what is the non ionizing type, that is a low level radiations, then the ionizing type which is high level radiations. So, we see the definition for non ionizing is low to mid frequency radiation. Which is generally perceived as harmless due to it is lack of potency, it could not cause much of harmful to the humans as the magnitude of this radiations are very, very low. The forms of radiation could be extremely low of frequency. It could be in the region of radiofrequency levels, or it is because of the microwaves or because of the general visual lighting which is artificially which is a manmade arrangements. So, these all come under non ionizing magnetic fields. Again the sources could be from the microwave ovens. So, lot exposure of microwave oven is also not advisable the computers, again the computers could also be a source of low magnetic radiations. Then house energy smart meters the wireless Wi-Fi networks, the cell phones which are operating at a gigahertz or very high megahertz range. The Bluetooth devices gigahertz range, the power lines and MRIs, magnetic resonance imaging. So, so these could be the source of the non ionizing type of radiations which are generally not harmful to the humans. To second category which is to be contained could be operating or could be do anywhere between mid to the high frequency radiation, this is cause could be because of the high 443 frequency radiation, which can under certain circumstances may lead to the cellular or DNA damage with particularly for a long exposure to this fields. So, the magnetic fields which are following in this category the personal or not supposed to be exposed for a long period of time, and how to be taken proper percussions so that the penetration could be avoided. So, what are the sources normally forms of radiations could be in this category are ultraviolet rays could be x rays or a gamma rays. So, these are some of the categories or forms of a radiations could be categorized in this. And the source examples from where the ionizing type of type of electromagnetic radiations could occur could be from the ultraviolet exposure to the ultraviolet light, for a long period of time. The x rays which are typically range from 30∗10 16 hertz to 30∗10 19 hertz. So, anywhere these frequency range where the x rays are being taken, if it is exposed for a long duration time the radiations could be higher. And also some gamma rays, this could again be because of the gamma rays. So, the personal should not be exposed for a long duration, and there are been internationally specified values for the non ionizing and ionizing type of magnetic fields. You could also see in the x ray personal who are operating in the generally using the x ray machines in the hospitals. There could be indication which is being put on their prone where it is says and indicates the exposure level, so that a suitable action has to be seen or is exposure, limit should not cross of the specified levels, there is a very important aspects. Particularly for people who are working in the x ray units and over expose to the ultraviolet radiations. 444 (Refer Slide Time: 32:33) Could electromagnetic fields be harmful to the human health? This is a question. It is to be known that we have to understand whether this is of a concern or a serious concern, or it could be not issue. So, here during somewhere 1990s that is, decades, 2 decades back most of the electromagnetic field research particularly focussed on extremely low frequency exposures, from conventional power sources, power lines the electrical substations and also home appliances. So, when a people who had a concern. So, can electromagnetic be harmfully yes in some studies I have shown that a possible link could be established between the electromagnetic field strength and an increased risk of childhood leukaemia. But their findings indicated that such an association was very weak. So, there is no proper support that the electromagnetic fields could be harmful. Now in the age of cellular telephone wireless networks routers portable GPS devices, all sources of electromagnetic field radiations. The concerns regarding possible connections between the electromagnetic fields and the adverse health effect still persists among the humans, though current research continues to point to same weak association, there is no concrete proof that the field which are generated by these sources could be harmful to the humans. But few studies that have also been conducted to on adults, show no evidence of link between the electromagnetic field exposure. And the adult cancers or leukaemia, brain cancer or breast cancer. So, there are no solid proofs which support the exposure of 445 electromagnetic fields have let to this disease. So, nevertheless very important the health standards have to be followed. And international health standards have been recommended, and which are continually and also to be educated on practical ways of reducing the exposures, in particular to the electromagnetic fields. So, this is very important point to be noted, for the personal who are working in the areas where the electromagnetic sources have to be seen that long exposure to this fields have to be contained, this is very important aspect, right. Thank you, we will stop here. 446  Advances in UHV Transmission and Distribution Prof. B Subba Reddy Department of High Voltage Engg (Electrical Engineering) Indian Institute of Science, Bangalore Lecture – 27 Electric and magnetic fields, mitigations techniques Good morning we were discussing about the electromagnetic fields. Whether this electromagnetic fields which are caused by the transmission lines could be harmful for the human and also the animals. So, this is a interesting topic and concerned topic for the research as well as for the utilities. So, during 1990s most of the EMF that is a electromagnetic field research was focused on low frequency exposures. (Refer Slide Time: 00:38) Particularly from the conventional power sources from the power lines or generating from the electrical substations, or could be of the home appliances like the microwave ovens, refrigerators, air conditioner units so on. So, some of the studies have a showed a possible link could be between the electromagnetic field a strength and a increased risk of a childhood leukemia. But this findings indicated that such an association was very weak. So, there were no concrete proof that the magnetic fields generated by the appliances and the electrical power lines do support the a theory of a causing harmfulness to the human life. So, with the age of cellular technology so, several of the wireless routers being used in the cellular networks 447 a likewise the portable GPS devices. So, the concerns regarding the possible connections between the electromagnetic fields and the adverse health effects still persists among the people, though current research continuous to point to the same a weak association. Some of the studies have been conducted on the humans show no evidence of any link between the electromagnetic field exposure and the cancer related or a leukemia pertaining to the human disease like brain cancer breast cancer so on nevertheless the international health standards recommend and also the continued education on the practical ways of reducing the exposures particularly to the electromagnetic fields is very important. And people should be told about the importance and also to see that a minimum exposure to the electromagnetic field is necessary for the health issues. (Refer Slide Time: 02:58) So, typically when you see the transmission towers you have both the electric field and the magnetic field in the vicinity of the power transmission lines. The here are few of the examples for various voltage levels 115, 230 and the 500 kilovolts, where you see that these are the towers and the distance from the towers, you can see that both the electric and magnetic field electric field in kV per meter and magnetic field in a milligauss the values are very clearly given here say in example of a 500 kV transmission system. You can see the electric field the 7 kV per a meter being at the midpoint of the tower, or very near to the conductor, as the distance increases from the tower you can see the values of the electric field and the magnetic field getting reduced initially 7 kV per meter 448 being the electric field and 86.7 milligauss being a magnetic field near the high voltage tower. So, it reduces as the distance decreases or you go further away from the tower, say 91 meters approximately around 300 feet you see the electric field reducing to 0.1 meter and the magnetic field reducing to 1.4 milligauss. So, this is information for various electric and magnetic field pertaining to the transmission higher transmission levels. So, the electric fields from the power lines are relatively stable because, the voltage does not change. So, all the transmission a lines which are operating at that particular voltage see at 765 kV level the voltage level fluctuation will be very less. So, the line because of the voltage level being maintained the electric fields, will be relatively stable. But the magnetic fields on the lines may fluctuate greatly, because this is due to the current changes in the response to the charging load. So, as the load which is being supplied by the transmission which is being supplied to the load, when the loads change the current magnitudes change drastically, because of a this the magnetic fields which are there should be of concern and must be described statistically because in terms of averages or other maximum or a minimum values which the fluctuation depends upon the load aspects. So, the magnetic fields above the mean normally calculated for a various transmission a lines and it could be monitored and a measurements could be done. So, various values for the different transmission systems are being given here. 449 (Refer Slide Time: 06:26) As mentioned earlier, the international commission on non ionizing a radiation protection which is a organization on comprising of more than 15000 scientist from more than forty nations, specialized in radiation protection have come to a common platform, and they have frame the guidelines a pertaining to the electromagnetic fields exposure where the severity of the magnetic field and the electric field near the people particularly were working in the transmission or distribution or in the substation utilities, should not be exposed with the specified values. So, here are some of the guidelines which have been framed by the association of ICNIRP you can see here the exposure say for a 60 hertz power frequency supply 60 or 50 hertz you in the country it is 50 hertz. So, in some of the countries they follow 60 hertz supply. So, the guidelines mention that is occupational the people who are working in the utilities say in the substations, or say people who are exposed for the transmission or the substation electric and magnetic field were working in the utilities or people who are involved in that, they should not be exposed or more than 8.3 kV per meter in case of electric field, and in case of a magnetic field the maximum exposure should not be more than 4.2 or 4 4200 a milligauss this is a value. And in case of a general public it the values of electric field it should not increase by 4.2 kilovolt per meter or 0.833 gauss or 833 milligauss. These are the some of the guidelines which have been framed and it is a necessary to be adopted for the safe a healthiness of 450 the humans over a working in the utilities or in the transmission companies. So, electric field and above the 1 gauss or a 1 tesla is equal to 10000 gauss. (Refer Slide Time: 09:19) This is one point to be noted. So, which is a electromagnetic fields are measured in terms of a gauss. So, 1 tesla is equal to 10000 gauss. So, the mitigation of the electric and magnetic fields is very, very important. And particularly concerning the transmission and distribution systems could be over volt overhead lines or substations the proper mitigation techniques have to be employed ah. So that a proper shielding factors or shielding has to be taken care. So, that it could be reduced for the people who are working in the substation or in the a transmission sector. So, various techniques are being employed for the EHV and a UHV transmission and the substation aspects. So, one of the technique is to increase the height of the mast. Height of the mast is the tower mast where it is increased and the clearances are more. So, with the clearances the electric field kV per meter come down and also the magnetic fields are will get reduced on the people who are working on the earth ground level. Second being the conductor a management again we will be discussing about these aspects in a detail. So, with the help of the conductor proper conductor management, the mitigation could be done or reduced electric field and magnetic fields could be reduced with the proper a conductor management. And third being the compensation techniques. 451 So, again various a compensation techniques could be used to see how these could be reduced. The shielding factor is one of the important aspects to see that a proper shielding will help to reduce the fields. (Refer Slide Time: 11:01) This graph shows you the effect of the mast a like increasing the mast increasing the height of the mast. You see the reduction for various heights, you can see like when the conductor is consider the tower with the conductor here a 3 conductor bundle here. So, this height whatever the clearances from the ground, say this clearances from the ground, typically transmission system of 380 kilovolts is considered here, with the current carrying capability of 1500 amps for the conductor and the simulation of electric field is carried out a for various a heights. And you can see as the clearance increases and the electric field were reduces that is a distance from the center is considered here. You can see for various height and that is one meter above the ground. The micro tesla that is a magnetic fields which are could be generated, because of the 1500 A carrying capability or the conductors. So, the height from 11.34 meters that is H is considered at 11.34 meters to 12 14 and up to 24 meters, you can very clearly see as the height decreases, the magnetic field increases. You can see the values typically for a 11 meter clearance height with a conductor being charge at 1500 amps for a voltage level of 380 kV a system; the magnetic field could be somewhere 23 to 24 micro tesla. As the distance increases the micro the magnetic fields gets reduced you can see for a 14 meter 452 16 20 so on at a height of 24 meter you can see the reduction from the 23 micro tesla to approximately around 6 to 7 micro tesla. So, this shows that increasing mass or the clearances could bring down the magnetic fields particularly the magnetic fields near the ground surface on the near the tower a high voltage or the extra high voltage tower which are carrying more power. (Refer Slide Time: 13:35) And also it should be noted that magnetic fields as mentioned earlier, mainly depend on the current carrying capability of the conductors, your magnetic field will be more only when the current or is very high. So, when the voltage is very high current is less so magnetic fields are not much of a concern, electric field ar of a concern. So, the magnetic fields as a power transfer goes high and the current load is higher and then the magnetic fields are the magnitude of the magnetic fields also go high. So, this is again a typical 765 kV transmission a line a tower configuration for the vertical a super bundle. You can see 2 circuit is here A B C, and again here A B C. And you have a 2 earth wires and lightning on the tower schematic here, this is a ground plane intentionally the earth wires are connected here. And you see the A B C and the phases are change and it is a proper transpose is done to see that the advantages for transmission is better and also reduction of the electric and magnetic fields to a certain extent in case of the conductor carrying more current. 453 (Refer Slide Time: 14:59) So, is there some of the things we some of the magnetic field and electric field values which have been simulated in the using developed software. So, for a 765 kV the curves have been shown both for the untransposed, this is for the untransposed normal tower 765 and this is for the transposed tower you can see height above the ground verses the distance from the center of the tower. These are the equipotential plots which given indication, and this gives the electrical a field strength from the center of the tower. You can very clearly see here we have try to plot both for untransposed and also the transposed. You can see the colour red color which is been shown here is for the untransposed part, and this is for the ground he is the transposed apart. So, compared to the transposed and untransposed, the electric field strength reduces this is one of the management of the conductors placement and going in for your transposed conditions. So, very important one of the mitigation very important mitigation technique which is being flowed in the transmission system. 454 (Refer Slide Time: 16:25) Again comparing for the ground field strengths electric field strengths, for different configuration again for the 765 towers, you can see that for various towers we have carried out the simulations. This simulations results very clearly show that for the different towers and also for the transposition and a untransposed, conditions that is a vertical super bundle type untranposed and vertical tower with a transport system, and also for a flat tower and a delta type of configuration. So, 4 type of a configuration studies have been done, and here you can see the electric field kV per meter verses the distance from the center of the tower. Very clearly you can see when the line is transposed when the conductors have transposed the levels of electric field stress reduces this is again in case of delta the pink color shows for the vertical super bundle whereas, for a untransposed you can see that the electric field strength is very, very high near to 11 or a 12 kV per meter, for the same tower with the transposition you come it comes down to 4. So, it reduces by one third value. So, the very clear conclusion that the transposition going in for transposition will be the better option for a reducing the electric fields. 455 (Refer Slide Time: 17:54) This is a similar study conducted for the tower configuration for a bipolar HDVC transmission lines for a various voltage levels, a 500 kV 800 and 1000 kV lines from the data which is being obtained in the literature for the tower details have been the data for the tower details have been obtained from the utility and also from the literature. You can this is a again a schematic of the HDVC a transmission tower, you can see the ground clearances if the conductors are connected is bipolar to line DC line this GW1 and GW2 are the ground wire connections 1 and 2 for the tower. This as is a ground plane and the distance between the 2 conductors will give the pole to pole spacing that as for the standards for 500, 800 and 1100 kV, have been taken into consideration while estimating the electric fields. So, for the DC we have try to estimate the electric fields for a EHV and a UHV DC lines. So, here you can see for a 500, 800 and 1100 kV estimations were carried out for a bipolar HDVC line. You can see the 500 kV HDVC line given in the red line, a shows red curve shows a approximately 11 kV per meter electric field similarly, for 800 kV it is a 13 kV and for a 1100 kV system it is a 12 kV per meter again it depends on the height of the clearances which have been given and the tower pole to pole spacing and the ground clearance very important. So, the values which are estimated have been compared with the actual dimensions of the available data from the literature and the simulations have been carried out. 456 So, for the 500 800 and 1000 kV you see the fields could lie anywhere between a 11 to a 13 kV per a meter considering the distances. (Refer Slide Time: 20:19) So, similarly comparison for the ground field strength, for both ultra high voltage AC and DC transmission lines was estimated, and a comparison is made here. You can very clearly see the comparison conducted for both ultra high voltage AC and DC lines, for the electric field strength this y axis shows the electric field strength verses the distance from the center of the tower. So, for high voltage AC and the DC you can see the comparison, the red color gives the high voltage DC 500 kV line which gives approximately the 11 kV per a meter which we have seen in the previous case, again the 800 kV being 13 kV per meter. For HVAC you can see the fields are substantially less here around 4 to 5 kV per a meter. And in case of a 1200 it is hardly around 6 kV per meter. These are done with the proper going in for the transposition of the conductors, again this would reduce drastically. For comparing the 1100 kV HDVC and 1200 kV HVAC or 800 kV HDVC and 765 kV HDVC, you can see the electric fields will be definitely higher in case of HDVC, slightly higher and for HVAC with the proper transposing fields will be lesser. 457 (Refer Slide Time: 21:59) So, again a comparison of the ground magnetic sorry, strength for high voltage AC and HDVC, this gives the summary of the system details that from 500 kV DC 765 AC up to 1200 kV AC both for AC and DC. The for various towers the flat type of towers what was the maximum stress which was obtained kV per meter are shown here in case of a 500 it is summarized is 11.30, 765 kV it is 4.77 one third reduction in comparison with the 800 kV DC lines. Similarly, for 1100 kV a HDVC it is 12 and 1100 kV a 1200 kV HVAC it is 6. So, almost a 50 percent of the fields which have been or the maximum stress which have been seen in case of DC comparatively lesser in case of AC. 458 (Refer Slide Time: 23:03) The second point the one was the clearances a height, the as mentioned earlier the conductor management a very important, this we have also seen that is a conductor going in for the different configuration. And also the conductors being going in for a transposition this will help in the reduction of fields electric fields. So, here or a magnetic fields you can see a small example here example of the design of a vertical type of tower or a delta configuration type of all the conductors are managed and conductors are connected this will be helpful. So, a various options have been shown here see, example this is a one type of arrangement where the conductors have been connected and this is the clearances B is the vertical conductors placement like as shown here. Third being the delta configuration in a rectangular sorry equilateral triangle type, and the D being the inverted equilateral triangle. The placement of the conductors is being modified and the studies have been carried out to see the fields the behavior. You can see here for the magnetic fields that is a microtesla per kiloamp. So, it depends what is the current rating of the or the load which is being carried by this conductors. So, for various current rating in terms of kiloamp the microtesla is been estimated that is a magnetic field is estimated, and you see a particularly of this configuration the magnetic fields are higher. And going in for a either vertical or a delta or a inverted equilateral triangle type of a arrangement of the conductors could lead to the better 459 performance of the line or also the magnetic fields getting a reduced. So, this is how it is being shown here. And this gives the per unit verses the distance from the center of the values. (Refer Slide Time: 25:10) So, the further the conductor management again, there is a one more option which is being done like the phase splitting. So, phase splitting a typical example is shown here, for a 300 kV a transmission line carrying a current of 1500 amps, and you can see for various configuration how the performance of the magnetic fields from the center is shown for various configuration A B and C, you can see how the configuration is given here. So, that this will improve the performance and gives a better information to the utility engineers who could plan to see how properly the conductor management could be made for a reducing the magnetic fields or the electric fields. This is a one more method of going in for passive compensation. Here you can see that the distance from the center again here the magnetic field reduction is seen with a loop or particularly for a using a series capacitor of 12 microfarad, this is without loop the magnetic field could be slightly higher. So, various types of a mitigation techniques are being employed in the by the utilities for the shielding by a metallic materials. 460 (Refer Slide Time: 26:40) So, the now the materials play a very important role. So, either shielding of this magnetic fields could be made by using a magneto static shielding or using a flux a shunting mechanism where typically using a proper metallic a materials where the magnetic flux which comes in the vicinity could be diverted because of a using a proper shielding mechanism. So, either shielding by eddy currents or by induced current mechanism or going in for magneto static shielding will help in a reducing the magnetic fields. (Refer Slide Time: 27:23) 461 Going in for a magnetic or a pure a magnetic shielding aspects. Here again the various materials are available with a different relative permeability that is a initial and high permeability maximum permeability values are also specify. So, various materials like iron steel low carbon steel ultra carbon hot rolled ultra low carbon silicon steel and permalloy which is also known as micro material, some of this magnetic shielding using this materials are also been tried out to reduce the Fields. (Refer Slide Time: 28:04) How to mitigate the fields particularly for the underground cables? For the overhead conductors we have seen, for the underground cabling we again it depends on laying geometry in the cable, how it is laid and what is the depth of the cable trench which is being adopted for the voltage and the dimension of the cable. So, here also introducing passive loops could help in reducing the magnetic fields also allowing currents to flowing the metallic sheets on the cables. Mean shielding by conductive metallic materials will again further help introducing the magnetic fields of the underground cables, and shielding by the for a magnetic metallic materials will also help to reduce the fields. 462  Advances in UHV Transmission and Distribution Prof. B Subba Reddy Department of High Voltage Engg (Electrical Engineering) Indian Institute of Science, Bangalore Lecture – 28 Importance of Grounding, reducing Earthing resistance (Refer Slide Time: 00:16) So, coming to the magnetic fields which are being generated in the typical substation. Substations again could be of extra high voltage any high voltage substation, depending upon the voltage level depending upon the power which is being distributed. So, this forms very important. The main characteristics of the sources and the ones that differentiate from the power lines and underground cables are basically in substation is the complexity of the arrangement in a substation. The local concentration and also the proximity effect several of these things come into the causing the effect of magnetic fields in the substations. The list of possible contributing magnetic fields which are emitted because of the power frequency magnetic field could be the busbars, that is high voltage conductors, the transformers the important component in the substation. The third being the low voltage cables which are being connected in the substation then low voltage connections could be of control cabling or for the metering panels. So, these also would like to contribute, 463 then high voltage cables the neutral and stray currents which are normally witnessed in the substation. So, these all the these things could be possible sources which contribute to the power frequency magnetic fields in the substation. (Refer Slide Time: 01:59) So, typically low voltage in house substations, which could be located in a cellar of a building could also be emitting the magnetic fields because of the substations being in the ground underground the ground floor, and the office or a scada center or a control room could be of the first floor. So, here again the electric field may not be of consequence magnetic field are of concern. So, proper management and proper planning is essential and the measurements have to be carried out for the emission of magnetic field by the equipment which are stationed in the substation. So, this could be again busbars inside or could be the transformers or could be the high voltage cables. So, this each of these equipment contribute to the magnetic field and typical example is shown here, the magnetic field caused by the cables or high voltage transformer busbars, could be a 33 micro tesla an example here. So, as the distance increases it gets reduced you can see and it goes on reducing, but still you will be expose to the magnetic fields in this area. 464 So, that clearance measurements have to be seen and safe planning for the people who are working have to be done in any in house located substations or a cellular buildings which are being adopted the high voltage substations are put up. (Refer Slide Time: 03:42) To the field mitigation techniques employed for the medium voltage and low voltage substations. So, these are the various sources that is assort. Could be a busbars near the residentials and long busbars in case of industries, and the transformers and cables being the major equipments again the transformers depends upon the rating and the load which is it is being connected and the cables are also to be mitigated. So, strategy mitigation at the source level could be done, again mitigation at the source may not be cost efficient because busbars. Thus mitigation at the affected area maybe needed that is where ever near the industry near the area has to be taken care. In case of transformers the mitigation for the electric and magnetic field at the sources that is by optimizing the connections at the secondary side could be reduced. Again for cables mitigation should be done at the source level. So, what are the techniques adopted in case of busbars in for the residential purpose. So, conductive shielding example aluminium or copper or proper shielding could be done and a passive compensation going in for loop type of arrangement could reduce the fields in the medium voltage or low voltage. For long busbars particularly for the 465 industrial sections a conductive or ferromagnetic shielding is advised, and again going in for active compensation could reduce the fields here. For equipment like transformers the phase cancellation technique is normally adopted. And also the distance management the clearances will reduce the fields magnetic or electric fields. So, cables the shielding with metal plates are going in for passive composition with a loop arrangement will reduce the magnetic or electric fields in case of medium voltage or low voltage substations. (Refer Slide Time: 05:43) These are some examples from were the magnetic field could be generated from the transformer. It could be because of the core, it could be because of the transformer fields which could be emitted. So, some of the possible mitigation technique is optimize the phase mixing. So, phase mixing will try to help in case of the transformer, 3 core transformer to reduce the magnetic fields which are the generating from the transformer. So, various types of transformer. So, here a proper techniques mitigation techniques should be followed before the equipment is being installed in the substation. 466 (Refer Slide Time: 06:33) So, mitigation for frequency magnetic fields which are could be generating from high voltage or medium voltage substation we have seen for low voltage substation. So, as the voltage level goes up the current carrying capability of the conductors will be very high and also the switchyard which is of a bigger area in comparison is of importance and were long busbars transformers and many switch gear equipments which are placed are also could generate the magnetic or the electric fields. So, the places particular in high voltage substations, the highest magnetic fields are normally registered at the secondary side of the equipment. So, a possible mitigation technique is again the distance or clearance management that is moving towards the affected area or extending the fence that is clearance So that this fields could be reduced as the distance goes up you can see the magnetic field, as a distance goes up the field gets reduced. So, this is very important to be followed for the higher voltages substations both for a magnetic and electric fields. 467 (Refer Slide Time: 08:02) So, some of the simulations which have been carried out for various voltage levels in the substation are shown here, you can see the magnetic fields depending upon the distance for 230 kv substation, the magnitude varies. (Refer Slide Time: 08:22) So, the all electromagnetic radiation has been available from the literature could be harmful if it exceeds a certain field strength specified limit is and exposure for long period of time. 468 So, some effects of radiofrequency and microwave radiation are also listed in the following lecture notes particularly on safety aspect. So, very important points to be considered. The radiated energy which when exposed for a long period of time could cause damage to areas of low blood supply in the body. The microwave energy above 3000 micro hertz, mega hertz is reflected or observed by the skin. So, a warning of excessive exposure is provided when the body feels a warm sensation. So, proper measuring gadgets to be worn by the people who are working in the substations or near the vicinity of the electromagnetic fields, where the sensing devices should be such that it should be able give the information the fields which are being exposed. So, the electromagnetic radiation energy, it is said that below 3000 megahertz is observed below the skin without a significant temperature increase. So, if it is below 3000 megahertz not much of issue. If energy is in the range of 1000 to 10000 megahertz, could cause eye cataract with the problems for the eye, with critical frequency being above 3000 megahertz. So, thi