High Voltage Transmission Techniques Lecture Notes PDF

 INDEX S. No Topic Page No. Week 1 Advantages of HVAC/DC Transmission, Introduction to Grid 1 Management...

 INDEX S. No Topic Page No. Week 1 Advantages of HVAC/DC Transmission, Introduction to Grid 1 Management 1 Transmission system development , Important components of 2 transmission system 18 3 Insulation coordination, over voltage in power systems 33 4 Design/selection of insulators, Importance of grading/cc rings 49 5 Non ceramic insulators performance-service experience 64 Week 2 6 Failure of apparatus in the field, importance of reliability and testing 86 7 Pollution flashover phenomena, modeling etc 98 8 Planning of High Voltage laboratories 118 9 Importance of High Voltage testing and techniques employed 142 10 Basic philosophy of HV testing, tests for various HV apparatus 154 Week 3 11 HV testing techniques for various apparatus 169 12 HV testing on Composite Insulators 193 13 Surface degradation studies on composite insulators 205 14 Surface morphological techniques for composite insulators 227 15 Conductors used for EHV/UHV transmission 245 Week 4 16 Corona nad interference on transmission lines 257 17 Introduction of HTLS conductors and their advantages 267 18 Mechanical considerations for HV conductors 281 19 Introduction to Towers and importance of foundations 306 20 Selection/Design of clearances for HV towers 322 Week 5 21 Design Optimization for UHV towers 333 22 Introduction to 1100kV HVDC 353 23 Introduction to HV Substations 366 24 Types of Substations, comparison 381 25 Insulation coordination, Components in a typical substation 401 Week 6 26 Preventive maintenance of Substation 428 27 Electric and magnetic fields, mitigations techniques 447 28 Importance of Grounding, reducing Earthing resistance 463 29 Introduction to the use of Fiber optic cables, OPGW 484 30 Introduction to communication and SCADA 499 Week 7 31 Precautions and safety measures in substation 520 32 Electrical hazards, minimum clearances in substation 535 33 Importance of Generation of HVDC in the laboratory 547 Importance of Generation of HVAC, Impulse Voltage and Currents in 34 the laboratory 556 35 Measurements of High Voltages 571 Week 8 36 Measurements of High Voltages (cont) 586 37 Introduction to digital recorders, measurement 603 38 Upgradation/uprating of transmission lines- advantages 623 39 Upgradation/uprating of transmission lines- advantages(cont) 638 40 Summary of the course 656  Advances in UHV Transmission and Distribution Prof. B Subba Reddy Department of High Voltage Engg (Electrical Engineering) Indian Institute of Science, Bangalore Lecture – 01 Advantages of HVAC/DC Transmission, Introduction to Grid Management Welcome to the course. We were discussing about the syllabus and references to be looked in to the course. (Refer Slide Time: 00:38) So, I would I like to have a quick look at the various resources which are available for the energy conversion. So, we have categorized in to 2 divisions, renewable sources which are basically non-conventional. These can be generated in a short amount of time. And these are essentially unlimited; so abundantly available. So, the resources come under this category are hydro solar wood trash geothermal wind etcetera, all these come under the category of renewable energy sources. We have other category non-renewable or a conventional type of sources. These resources cannot be replaced in a short amount of time and they are basically available in unlimited way. 1 (Refer Slide Time: 01:39) So, these are fossil fuels, natural gas coal oil petroleum and the sources because of the fission. So, these are some of the energy sources which are being used for the generation of hydro electricity. So, further the available resources which in the country referring to the major energy resources, this map show the entire sources resources are available in our country. So, basically like coal hydro etcetera. These resources you can see over map of the country. So, we have abundant resources particularly in the Northern part of the country like Punjab- Northeastern areas. So, where we have hydro potential we have resources which are in the places like Jharkhand, Orissa, Chhattisgarh and parts of Madhya Pradesh where abundantly coals is available that is in some parts of central India. So, hydroelectric generation can be done in Northeastern and North Himalayan region. Coastal based generation is also available in the parts of Andhra Pradesh, Tamil nadu and Gujarat. So, this is the spread of the energy resource resources. We can look in to the map and see that all the resources are unevenly distributed across the country. So, the generation is limited to an area may be thermal may be hydroelectric may be nuclear. So, these generations of the voltages which is being taken is to be transmitted to a far of distances from one region or one place to the longer with to a longer distances places which are of very further away. 2 (Refer Slide Time: 03:23) So, for this we require the transmission which is a very important component after the generation. So, for long distance a transmission it is advantages to going for the high voltage AC or high voltage DC transmission. So, before going in to the advantages or benefits of high voltage AC or DC transmission, let us look in to the typical technical data which is available. So, these data is of current which is being available in the country. So, installed capacity is being somewhere around 250 giga watts, the peak demand met is 135 giga watts. We have a renewable installed capacity which consists of solar wind small hydro which is of 31 giga watts in case of wind solar being 2 giga watts, small hydro being around 3 giga watts. So, presently we have around more than 1230 transmission lines at 400 kV operating and more than 45 transmission lines which are operating it 765 kilo volts. So, we have number of generating units more than 1800 and which are of 500 Megawatts and above are of 130 numbers. So, apart from this AC we also have a high voltage DC links which are both type that is back to back and bipole in nature these are of 9 numbers. So, this is the data and the demand which is met from the grid operation related is the peak demand is met is around 135 Giga watts. We have maximum energy which is met per day is around 3131 million units per day. So, maximum wind generation is above 240 million units per day the short term open access which is available is around 240 3 million units per day. So, the integrated regional exchange is around 200 million units. That is the typical data which has been obtained from the central electricity authority or the power grid government of India. (Refer Slide Time: 05:45) So, we were looking into the benefits for long distance and we have a generation may be hydro nuclear thermal at particular location the power has to be transmitted to a far of distances. This is being done because we were looking in to the resources available. The resources are at a particular place, the transmission has to be carried out over a large long distances. So, the benefits for going in high voltage AC transmission is here we can see that the electric power transmitted by over head AC system is approximately given by formula- 2 V P= Z where P is the power, V is the operating voltage at particular voltage level and Z being the surge impedance of the transmission line. The Z the surge impedance, we will be discussing in the next slide- what is exactly the surge impedance of a transmission line. So, just consider typically normally in the transmission lines the surge impedance is taken anywhere between 250 to 400 ohms. So, considering for the present case for a long distance transmission for surge impedance of 250 ohms, the above formulae in case of a 4 voltage level for 220 KV, we get a power of 193 point approximately 194 mega watts in case for a voltage level of 400 KV, we get 640 mega watts for voltage level of 765 KV transmission, we get the power of 2341 mega watts. So, for 1200 KV 5760 is the power which is being transmitted by the high voltage lines. So, by looking in to the table we can very clearly see one 765 KV transmission line can nearly carry transmit 4 times the power which is being carried by a 400 KV transmission system and one 1200 KV line can carry more than twice that of 765 KV transmission line. So, it is very clear from these data that going in for a very high voltage AC transmission is much more economical. (Refer Slide Time: 08:24) So, surge impedance of a transmission line as I was mentioning, normally it is taken between 250 to 400 ohms. Basically the surge impedance is the ratio of amplitudes of voltage and current of a single wave propagating along the line in one direction. In the absence of reflections in the other direction, it is simple that if there is a sending end, we have a receiving end. So, the amplitude of voltage and current of a single wave which is propagating towards one direction, and reaching the receiving end without any reflections from the receiving end the absence of reflection. So, this surge impedance is known as surge impedance the surge impedance loading are the SIL of a line is the power which is load having a load at which the net the reactive power is 0. 5 (Refer Slide Time: 09:54) So, with this importance of high voltage transmission we have this is being high voltage transmission, we have this is being high voltage transmission is being employed to transfer the bulk power from the generating station the generating stations may be as mentioned earlier may be nuclear thermal hydroelectric renewables etcetera. From the generating station to the load centers, again the load centers may be industry may be domestic use or may be agricultural purpose. So, with lesser losses is the intension for going for high voltage transmission. Here the power is transferred in AC due to feasibility in stepping up the voltage and also stepping down. So, we have the advantage particularly when we going for AC transmission. Then with increasing in transmission voltage also the size of conductor that is a diameter of the conductor gets reduced that is the cross section of the conductors gets reduced and the current required to carry also reduces, because of the voltage is being transmitted in very high voltages. So, this implies that the smaller conductor, that conductor dia when it becomes smaller the cross sectional area and also the lower cost for the conductor and the current dependent which the copper losses are also reduced because of the size of the conductor. So, with reduction in current carrying the losses results, which are lesser, will result in a better efficiency for going in for high voltage transmission. So, in AC systems we all know that the steady state stability limit is proportional to the square of voltage. So, as 6 the higher the voltage therefore, this improves the power system stability margin by going in for higher voltage transmission systems. (Refer Slide Time: 11:54) However, with increase in voltage levels beyond 400 KV of the transmission that is particularly for AC extra high voltage EHV, extra high voltage transmission has certain limitations or disadvantages. So, they are basically the corona losses, we will be discussing about the corona losses. This corona loss happens because of the conductors or a transmission hardware insulator hardware connecting clamps many of these, things the insulation requirement for the conductor has to be properly designed then the radio interference, because of the conductors which are operating in the environment. This radio interference is the discharges which are seen at 0.5 to 2.5 megahertz range. The discharges which are emitted from the conductor at the voltage levels above 400 KV can disturb radio sets which are operated near by the transmission lines. So, these have to be content to a level. So, we will be discussing about the radio interference and corona losses. Apart from that going in for 400 KV AC above we have to have a heavy supporting structures, where the clearances are required between phase to phase conductor phase to ground conductor depending upon the increase in voltage. Therefore, for transmission with EHV that is extra high voltages and for long distances 7 HVDC line is becoming more economical high voltage this a lines are becoming more economical and or being adopted across the globe. (Refer Slide Time: 13:45) Just before going to the slide. I would like to mention the high voltages. when we talk about the high voltages. So, where exactly is the high voltage defined? So, when we see the international standard 60060, it very clearly mentions any voltage above 1000 volts, 1 0 0 volts is stated as high voltage in case of AC, and 1400 volts and above is stated as high voltage DC. So, this we have to bear in mind. The graph which is being displayed here gives the cost verses the distance is a very important graph, where it gives an indication of which when we are going in for HVDC transmission or HVAC transmission which is more economical. This is very clear in that we can see that the pink line displays the total cost for the AC transmission lines the blue line gives you the total cost for the DC transmission lines. So, you can very clearly see the distance at a typical joint this point is at a distance of 400 kilo meters. So, comparing at a distance 400 kilo meters for both AC and DC, we see that the AC terminal cost are very less in case comparison to the DC level which is much higher. The losses component you can see the DC losses comparison of DC and AC losses, but ones after the 400 KV transmission 400 kilo meters distance increases the DC line becomes much more economical you can see the AC line going in a very linear fashion. So, before 400 kilo meters AC is much cheaper, after 400 kilo meters DC becomes much 8 economical. So, it is advisable to go in for DC long distance lines in case the (Refer Time: 16:19) of power is to be done over a long distances. (Refer Slide Time: 16:17) So, HVDC is advisable the main advantages of high voltage DC transmission. The first one is the skin effect we know that in HVDC transmission current distributes uniformly over the cross section the conductor. So, hence there are no losses and there is no skin effect which is present in case of AC transmission. And in HVDC transmission requires only 2 conductors, the power loss hence will be lower in case of DC line compare to the AC transmission losses which are seen for AC lines. In case of voltage regulation for DC lines the voltage drop does not exist due to the inductive reactance. So, the voltage regulation will be much better in case of high voltage DC. Then surge impedance loading for long EHV AC lines you know that extra high voltage and ultra voltage AC lines are loaded less than 80 percent of the normal load because of the surge impedance loading. So, this condition will not be applicable in case of long distance or EHV and UHV HVDC transmission system. The corona and radio interference as mentioned, corona losses which are directly proportional to the frequency. Therefore, in DC corona losses are much lower compare to AC line this has been studied a lot and lot of literature has been available pertaining in to this view. 9 (Refer Slide Time: 18:05) So, the operating voltage is particularly for 400 kilo volts and above, we all know that apart from lightning and normal power frequency voltages switching surges or switching impulse voltages are more severe than the lightning surges. So, switching surge level is lesser when compare to DC with comparison to AC line. Hence the requirement of insulation for the DC line gets reduced. So, the next is the reactive power composition compensation unlike AC transmission line, DC line does not require any reactive power compensating devices. So, this is because of the absence of the charging currents particularly in DC and the power factor operation so much more economical or much more advantages than HVAC system. Then short circuit currents particularly during fault in DC line are seem to be lower compare to the AC line with the experience. So, for the last many years even in the country and many other places where HVDC lines have been adopted have been commissioned have been having the experience have shown that, it is much more economical and we also has a greater reliability in comparison to the HVAC particularly for long distance and bulk power and more power transmission. 10 (Refer Slide Time: 19:43) The graph shows the development how the transmission voltages particularly in case of AC power systems have happened since 1900. We can very clearly see over a century a 120 years the growth of power has started from very low voltage and across the globe reached 1200 KV and increase of about 3 percent per year approximately has been witness across the globe. So, this is across the globe. So, we would like to see that how the growth of the transmission voltages particularly from both AC and DC as seen the rise in our country. (Refer Slide Time: 20:29) 11  This graph shows the development of high voltage AC and high voltage DC transmission in our country. So, we are not lacking in terms of a transmission of at the higher voltages when comparison to the international. So, this graph very clearly indicates we shows the voltage level verses the year, which has been the development has taken place we can very clearly see somewhere in 1970s we had the first 200 KV transmission the ninety we had in 80s or mid 80s 400 KV transmission which has come later HVDC has been adopted in nineties or late 90s. Then further in end of the 95 to 98 a lot of experimental work have been carried out for 800 KV that is the 760 65 KV operating voltages. So, the line first line in the country from Kishanpur to Moga was commissioned somewhere in 1998 where 765 KV transmission network was established the country further 800 KV high voltage DC. So, earlier 500 KV HVDC was in operational from 1980 1995 onwards. So, the country graduated somewhere around 205 to 206, 2006 800 KV HVDC further recently we are having 1200 KV AC experimental line that is being presently experimentation in a large scale is being conducted at the power station which has been established by the government, and also with the help of the private and public sector companies. So, the experimental line is stay is been established at Bina in Madhya Pradesh. So, where experimentation for 1200 KV transmission system including the transformer, circuit breaker, transmission components, insulators, many of these things are being carried out. We hope at the very early 1200 KV transmission system or lines will be established in the country and we will be seeing the 1200 KV of transmission towers being erected across for the long distance at transmission soon. 12 (Refer Slide Time: 23:05) So, these show the standard transmission voltages. So, when the transmission voltages which have inception have started somewhere from a very low voltages and we have reached up to 1200 KV in case of AC and plus minus 800 KV in case of HVDC. So, there should be some standard which has to be followed, both nationally and internationally. So, that it is beneficial to the utilities, beneficial to the manufactures and the design engineers. So, keeping this in mind the voltages which are adopted for transmission particularly for bulk power transform have to confirm to the specifications formulated by many international standard groups in countries. So, that it is beneficial to every utility and manufacturer. So, necessary this is also necessary in view of the import export and domestic manufactures design and use. So, in India we as per is that is international standard 2026 these are the following line to line voltages, which are adopted for the high voltage transmission system. So, nominal system voltage for 132 KV the maximum operating voltage will be 145 they insulation level will be designed in case of 132 KV system the maximum insulation will be designed for 145 KV for the components which are being operated at that voltage. So, similarly for 220 KV 245 is the maximum operating voltage and for 400 kilo volts 400 and 20 KV is the maximum operating likewise 765 KV nominal AC voltage the operating maximum operating voltage will be 800 KV and similarly for 1200 KV maximum operating voltage 1150 is the nominal operating or nominal system voltage which is being standardized. 13 So, the maximum operating voltages are specified as shown above in no case these should be exceeded in any part of the system because the insulation levels of any equipment, the design of the equipment are based upon these voltage levels. So, therefore, the primary responsibility of any design engineer is to provide sufficient and proper type of reactive power at suitable places in the transmission system. (Refer Slide Time: 25:51) So, in case of voltage rise and inductive composition for voltage drops capacity compensation much suitably be used. So, that we have see in the voltage levels up to 1200 KV standardized. So, before going in for the country how it has been progress; how we have reached the 1200 KV transmission systems, how the evolution of grid inter connection India happened. So, way back in 1950s there were local generations the local generation again it may be of ideal it may be of local or thermal plants or other resources. So, this local grid formations happened in 1950s further several of such things were thought that at a state level are the resources should be pulled and a grid should be maintained well the distribution could be made for the entire state this concept was thought in 1960s, then the concept of regional grid inter connections was thought. So, in 1970s a regional grid concept was consider and India were made in to 5 regional grids and many states which are coming in that particular region got were made to at particular region and these are 1970s the regional grid concept was thought. Further in 1990s thought was made that entire nation should have a single grid this will be much 14 beneficial for the country in terms of operation and much to cater the needs for the entire country. (Refer Slide Time: 27:31) So, looking back at the evolution of the grid how it happened because this is required for the changing load profile and also the integration of renewables particularly with the other sources like hydro nuclear and thermal. So, there was need for more flexible in the system. So, as mentioned pre 1991 earlier 5 regional grids where formed and these 5 regional grids where operating at particular frequencies and somewhere in October 1991, 1991 East and North grids where synchronized and where seen that the 4 grids were formed out of 5 the regions the 4 regions were made. Further in 2003 a thought was made and 2003 is an important year particularly for the electricity act it is a very important act, where this was very clearly mentions about the inclusion of the private partnership in the transmission and distribution system and many regulations and rules were formed in the electricity act and subsequently lot of amendments have also been done. So, further in august 2006 the other 2 grids were synchronized with the central grid and India became a 2 grid system. So, lot of advantages than it was designed it was thought that a single grid which we much economical and much beneficial to the country. 15 (Refer Slide Time: 29:44) So, this happened in December 2013 the all India synchronized grid happened with the addition of 500 megawatts and above generating units of 760 KV transmission line from the Karnataka to Maharashtra where we had a single grid single operating grid with single one grid one nation concept was achieved. So, looking at the Indian power system presently we are amongst the largest in the world one among the largest in the world there are around 6 or 7 countries which have a large power network. So, India also becomes one in the group of the largest operating power system network group. So, this here the spread over 3000 kilo meters from the North to the South of the country for Jammu to the Kerala and East from Gujarat to North-East Bihar around 2900 kilo meters; so this is the spread of the transmission network system in the Indian power system very large complicated network system consisting of 132 kilo volt lines, 220 kilo volt lines, 400 kilo volts, 765 KV AC lines, then 500 plus minus HVDC and 800 plus minus HVDC transmission systems. So, this is the typical map which shows the spread. So, before going in for the details of the transmission system the important factors have to be considered before designing any insulation. So, the factors which are responsible particularly for the insulation design or the voltage levels up to 4 kilo volts the mechanical clearances are the important to be considered that is from the high voltage to the ground the design is made depending upon the mechanical clearances. 16 Further 4 kilo volts to 33 kilo volts or 34.5 kilo volts a corona is a factor because of the discharges the corona is basically ionization near the (Refer Time: 32:02) conductor this corona happens because of the hardware or a transmission conductors in case of the sharp edges so on. So, this has to be taken care before designing the insulation. Above 66 KV the lightning switching surges come in to the design criteria. So, where the transmission system above 66 KV to 220 KV lightning over voltages apart from the normal operating for frequency voltages, switching over voltages have to be considered. Above 220 that is above 400 KV lightning switching, apart from lightning switching we importance is the contamination or a pollution, which is a very important factor. So, lightning normal power frequency over voltages lightning over voltages because of the natural lightning switching surges this may happen because of the opening and closing of the circuit breakers, but contamination is a phenomena which is not because of over voltages it is because the phenomena of the flash over or the break down occurs at the normal working voltages. So, this is a very serious implication to the transmission system. So, above 400 kilo volts of the contamination or a pollution criteria has to be taken in to consideration before design of the transmission line insulation. So, above 765 KV particularly for 1000 or 1200 kilo volts’ part from the major important insulating criteria is being the contamination. So, contamination is a very important criteria to be considered for the design aspect particularly, at EHV that is extra high voltage of a 400 KV and ultra high voltage is for 765 KV in case of AC or plus minus 800 KV in case of DC is a very important factor for the insulation design. So, we will be looking in to importance of this contamination how the contamination or pollution affects the transmission system when we go further in the course. 17  Advances in UHV Transmission and Distribution Prof. B Subba Reddy Department of High Voltage Engg (Electrical Engineering) Indian Institute of Science, Bangalore Lecture – 02 Transmission system development, Important components of transmission system Good morning, welcome. The transmission system is very important for the development of the any country. So, what the high voltage or extra high voltage transmission system, there are some issues pertaining to the development. And these issues and how the advancements have taken place we will be looking into this aspect. (Refer Slide Time: 00:39) So, the main aim of the country is to develop a strong transmission system between generation complex or generating station and the bulk consumption centres. So, the bulk consumption centres maybe the industries or the domestic or agriculture requirement. However, for the strong transmission system there are few issues which have to be looked into the first being the minimization of right of way. This is an important aspect. We will be looking into this aspect ahead of the course. The right of way is the minimum clearance to be maintained from the mid of the tower to either side. So, later we look into the issues being protection of flora and fauna and wild life. So, when the transmission system is being constructed and long distance transmission, we 18 know that the transmission lines run over a long distance hundred of kilometres thousands of kilometres. So, it is likely to cross the forest where the flora and fauna are likely to be affected including the wild life. So, this has to be properly taken care before going in for other strong transmission network. Then further to this with a creation of high capacity transmission corridors, this will be helpful to enable to the minimum cost per megawatt transfer as well as optimal transmission losses. That is a main reason for going in for UHV transmission systems, apart from that as we looked into. So, protection of flora and fauna and also because of the transmission system, high voltage transmission system the impact to the environment should be minimal. These are requirement where utility or the government when they are going in for a long distance transmission have to be there in mind. Then further strengthening of the national grid, this will be very helpful to control the entire grid through a single point system. So, these are some of the issues which are to be addressed and what are the issues and how the advancements have been taken place by overcoming this we will be looking into. (Refer Slide Time: 03:09) So, this shows the most important components of a transmission as network transmission system. Example is shown here a transmission tower, where the tower consists of are the following components which are very important. Insulators, the first is the insulator. 19 Insulators has you can see here. There are various types of insulators which are being used for transmission. This transmission again the insulators are of kept in arranged in a different configuration like the suspension, attention, quarter pole tension, v suspension so on and so forth. So, what are the importance of this insulator. So, insulators in important component of a transmission system; it performs duel function, one is it supports the tower mechanically and other it electrically isolates the conductor from the tower. So, this performs a dual function. Insulators are of mainly 3 types. Initially for last 100 years we have been using the ceramic or porcelain insulators, Europeans use glass insulators, then of recent origin polymer composite or silicon rubber insulators are being employed for the transmission network. So, these are of recent origin. So, we will be looking into various types of insulators how they come into existence, what are the advancements in this insulator technology and what are the problems related to the insulator technology particularly using at extra high voltages and ultra-high voltages transmission systems. The second important component apart from the insulator is the conductor. The conductor is again a much more important component it transmits is the voltage from long distances hundreds of kilometres from generating to the transmission system. So, various type of conductors is an existence. So, we will be focussing into the types of conductors, various materials which are being employed for the manufacturing of the conductors and the recent advances in the conductor technology. So, we also look in to the various types and the recent employed conductors. The third being the much more important component that is the towers, so without towers the conductor’s insulators cannot be used. So, the tower is one of the important component of a UHV or EHV transmission systems and the apart from towers the foundation what type of foundation is being used for the towers this again depends on the area where the tower is being erected, either it is in a hilly area or in a normal plane conditions or a high altitude. So, the foundation also is a very important aspect which has to be taken care for the natural calamities. It may be because of higher winds it may be because of any natural calamities which are occurring. So, towers and foundations are also the important components of a transmission system. So, next comes the earth wire or the ground wire. The earth wire of earth wire is shown 20 here. This is the earth wire which is connected to the top most portion of the tower in a EHV or UHV transmission tower. In case of lightning this has to protect the equipments like the insulators strings and further the substation components. So, the earth wire or ground wires place an important role particularly during the lightning striking to the long high voltage transmission systems. Then we have the hardware fittings hardware. Fittings consists of different and various types. Hardware fittings are used for insulators to connect to the tower. So, hardware fittings this is the yoke plates and yoke plates which are being connected. Apart from yoke plates we have a corona control rings. Then we have a several other components which are being used for shrinking the insulators to the tower. So, we will be looking into the hardware fittings what is the developmental aspects which has undergone for a period of time, and how the hardware fittings are being changed when it comes to the 765 KV, 800 KV voltage levels that is the UHV voltage levels. And final the important component is also the accessories, so various types of accessories which most of the engineering students would have not known, so the accessories are very important part of any transmission system. So, some of the accessories are when you go to the near any transmission system, you see a small components here this are nothing but the vibration dampers. So, similar to the vibration damper there is a specific which it controls the damping or oscillations that we will be looking into that. The importance of vibration damper and few accessories like mid span compression joint repairs lives T connectors many of these things these all fall under the accessories. So, these are some of the important components of ultra high voltage transmission system. So, we will be looking into each component. And we will be looking into how the developments have been taken place over the period of time and what are the type of insulators conductors and different types of towers clearances which are being employed for the UHV and EHV transmission levels. 21 (Refer Slide Time: 08:50) The first component being the transmission insulator we will briefly look into the history of the transmission insulator. How an insulator functions how the insulation coordination what are the factors which determine the insulation design for the transmission insulators, how importance is that then mechanical and electrical design criterias, which are to be adopted before the insulators are being used in the transmission system. Then we will be looking into the design part or the selection of an insulator. Particularly for a normal condition or a contamination condition this again whether the transmission line is passing near the polluted zones or normal or heavy rain or heavy fog areas. So, depending upon the area where the transmission line is passing the selection of insulators is be done. We will have discussion on the selection and design aspect of the insulator of or various conditions. So, how important is the design of CC is the control corona control rings or a grading rings. We will be having a look into that this is a very important aspect for the insulator shrink where this will be having multi functions. So, we will be discussing about this aspects, importance of the design of the corona control rings. And the service experience which ceramic or glass which has been used for which are been used for long more than hundred years in the transmission system. So, what are the experiences which have come across using the ceramic and glass? And we will be looking mainly into the non ceramic 22 insulators or a polymer or silicon rubber insulators which are very important and which are being of recent origin, and are being employed for the EHV and UHV transmission. Finally, we will be comparing all the 3 technologies or of porcelain glass and ceramic insulator. And we will be looking in to some of the failure analysis of various type of this insulator, porcelain glass or non ceramic of few case studies which have been reported. The important aspect of this transmission insulator is the reliability. The how reliable as an insulator should perform in field is very important. So, we will be looking into the reliability aspect, and we will also be touching into the basic philosophy of testing of these insulators before there being used in the field. So, it is importance will be looked into. (Refer Slide Time: 11:32) So, how the history of transmission insulators: when we look basically the insulators grew out of the needs of telegraph industry. So, initially for power system the telegraph insulators were being employed somewhere in 1700s and early 1800s. So, early history centres around what today we consider very low DC voltages initially DC voltages where in existence initially. Gradually technical needs increased as high AC voltages grew with development of electric power industry across the globe. So, during 1840s to 50s much before this insulator technology grew. So, they use glass plates to insulate the telegraph lines, before the telegraph insulators were invented. So, 23 many trials with different materials like wood cement porcelain, beeswax, soaked rag, wrapped around the wire etcetera. So, several of these things we are tried out. Ultimately porcelain or ceramic and glass prevailed and they started using for the telegraphic needs. (Refer Slide Time: 12:41) So, somewhere in 1893 the wet process porcelain was developed particularly for high voltage applications. Initially the telegraphic insulators where used up to 11 kilovolts. So, when the voltage level was increased for transmission system. So, the utilities face the flash over across this telegraphic insulator that is where in the technology have to be developed and the porcelain tech was developed for high voltages. So, the porcelain industry started growing during this period for the requirement of very high voltage applications. So, applications of the voltages increase as I mentioned. So, insulators designs also became larger and much complex. So, ceramic that is a glass or porcelain are only choice of high voltages during that period. So, they were the technology was improved and voltage levels when has to when the voltage levels were higher. So, the designs have to be altered. So, it is in 1907 where mister Harold buck of Niagara Falls power corporation and Edward Hewlett of general electric invented the disc insulator which is of a day present technology insulator what we call. 24 (Refer Slide Time: 14:05) So, these are the earlier insulators which are used for telegraphic system, and what you see is the Edward Hewlett and Harold buck of Niagara Falls these are the people who are used the technology for other for developing the insulator technology. During 1960s are the first non ceramic insulators that are polymers silicon rubber composite what we called the trial started in 1960s, but they were not successful initially because of the problems which they were facing. So, many companies try to start and some companies have to close down and this new industry also or parallelly looking into the ceramic insulators how improvemental aspect of the ceramic insulators. So, during 1960s to 65 the Europeans develop modern non ceramic insulator with fibreglass rod technology which are considered of the first generation. 25 (Refer Slide Time: 15:13) So, during 70s to the present the non ceramic insulator industry really begin with much better information, much better field trials of the insulators, since then new manufacturers, new designs, new materials were incorporated new type of fillers were introduced into the compounds were in the non ceramic insulators, which are of present generation third have been available with various type of a filler materials embedded into the silicon rubber. So, the ceramic manufacturers parallelly, when the technology grew up for polymer or a silicon rubber, the ceramic manufactures have also not been idle. They also try to improve the development by using the high strength porcelain resistive glazes on the coating on the insulator surfaces and so on and so forth to sustain the technologies for the transmission system. So, these are the various disc insulators which have been shown here. So, these are the ceramic insulators this again a ceramic insulator which is used with a higher capacitive length for a certain particular area. Then this is the glass type of insulator this is the anti fog insulator particularly for the areas where the transmission lines are possible high fog density locations. So, these are various types of polymer or a new type of composite or silicon rubber insulators which have been in use. 26 (Refer Slide Time: 16:43) A very quick look into the flow chart of the manufacturing process, for both the ceramic and the polymer will show you how much difficulty is to manufacturer porcelain or a ceramic insulator on the time period which takes place for the manufacture of the ceramic or a porcelain insulator. So, for a ceramic insulator to completely come out from the raw material to the final stage it take around 21 to 24 days, whereas the polymer insulator is so quick it can be done in a matter of 5 to 6 hours. So, that is one of the advancements which has been happening. So, we look how the porcelain or ceramic insulators are manufactured in a company or a industry. So, initially the raw materials which consist of the clay feldspar alumina etcetera are got, and they are grinded using the ball millings. And further this grinded material or the ball milling material is passed on for the sieving, where further of the sieving the material is kept in a suspension, where the water which is comprising of 1 part and 3 parts of the clay material which was used has to be removed. So, this further after the multi stock sieving is sent after the water removal is sent for the shaping of the insulator. Here the shaping of insulator takes place depending upon the die sets which are being used either for transmission or for hollow post insulator or for a long rod insulator. This shaping of the material is being done, dewatering is being done again finally, the material which is kept in suspension mode is send it for this is again 27 demagnetization of sieving. Then it is in suspension dewatering then it goes to the shaping. So, after the shaping it comes back to the shaping machines where the insulators are with the help of a die set are manufactured either for disc or for the hollow type of insulator. Then further the insulators which have been made or dried they are glazed depending upon the pigmentation on the colour requirement. These either hollow insulators or transmission insulators are glazed and further after the glazing this insulators are send for firing again the firing depends more than a 24 hours with the certain temperature. So, after the firing the sorting or a weeding out of the defectives are done. Further the testing of each and every insulator is being carried out for the porosity and many of the other tests which are being done at the factory. So, after the testing the assembly of the insulator which connects both the pin and the cap: so initially it is only a ceramic or a porcelain shell. So, now, the assembly consist of a metal pin and a metal cap with the porcelain shell. So, further after the assembly, the insulator undergoes mechanical and electrical test in the factory. So, again there have a routine testing for various mechanical and electrical parameters. Finally, after the testing, the inspection physically inspection of the insulators is being done. It is a packed in a krait is and it is dispatched to the required utilities this is how the ceramic flow charts for manufacturing process. (Refer Slide Time: 20:32) 28 So, similarly when you look into the manufacturing process of a polymer that is the recent advanced material polymer material, which is being used for the high voltage transmission system. So, you can see the raw materials which are here. Raw materials may be of silicon rubber basically silicon rubber is added with filler. Then you have raw materials the fibber glass rods which are used what the core of the material then you have metal and fittings on both side of the insulator these are the basically raw materials. Then the metal fittings is done at the end of both the fibber glass rod. So, proper crimping has to be carried out after the crimping to the metal n fittings on the fibber glass rod a primer is coated on the fibber glass rod. Further with the help of injection moulding machine suitably under compression and curing this polymer material with the proper die set is being manufactured. It is later cured, after the curing de flashing of the insulator is done. So, after the de flashing where it undergoes the extra material which is being present after the curing or the compression is being removed and either it is neatly stacked. Then after stacking it is send for the regular testing both again for mechanical and electrical testing. And the testing includes apart from a electrical and mechanical a hardness specific gravity tensile strength and elongation for the rubber. So, these are again chemical related test which are been carried out using hardness tester or a specific gravity meter or a tensile strength machines in the factory. So, the entire process of a manufacturing of a polymer or a silicon rubber insulator happens anywhere between 4 to 5 hours. So, there it was for ceramic or porcelain used to take 24 a days. So, this is how the technology has improved and these are the recent insulators or of recent origin or organic in nature and are being used for the EHV and UHV transmission systems. 29 (Refer Slide Time: 23:01) So, we have looked into the different type of insulators or which are being used over a period time. And we know that power transmission from the generating station to the load centres is by overhead transmission lines. So, as mentioned earlier the string insulators which are shown here perform dual functions. One is the mechanically support the tower high voltage tower and second is electrically isolate the conductor. These are the conductor these are the different spacers. So, these have to be electrically isolated from the tower as the conductors are at very high voltages. So, during the transmission network in the system which it exist on a outdoor applications, on outdoor systems these insulator strings are subjected to prevailing ambient and overvoltage conditions. So, overvoltage we will be seeing that overvoltage is maybe because of the normal or frequency working or frequency voltages the overvoltages maybe because of the lightning aspects which are known as lightning surges or lightning impulses which are seen on the transmission system. Then switching surges or switching impulses which are seen on the transmission because of the opening and closing of the circuit breakers. So, these have to be subject this will be subject to this type of conditions the insulators have to be stand in the field. So, transmission we know the transmission lines, run over a hundreds or thousands of kilometres from the generating to the load centres; so failure at any point in the network that is from the generating to the load centres. So, any point can 30 bring down the entire system. So, such is an importance of the insulator technology which is being used and it has to be seen that the failure does not happen in the transmission network. (Refer Slide Time: 24:54) So, we looked into how the insulator functions. We saw that it is a important both for mechanical and electrical. Particularly it maintains the distance that is air gap between line and the ground. So, that voltage which is seen on the voltage on the conductor the minimum clearances has to be maintained from the ground to see that normal conditions or during the fog conditions or rain conditions or during the polluted conditions the flash work should not happen from the line to the ground. So, this again depends on the system voltage the safety margin which has been employed for the design the contamination levels are that area where the tower exists so on and so forth. So, the insulator has to withstand mechanical stresses. Again mechanical stresses maybe of static in nature or tension compression or due to dynamic loading because of wind air because of any natural calamities which may happen. So, they have to withstand the dynamic and compression loads. So, apart from this mechanical stresses insulators have to withstand electrical stresses. As mentioned the system voltages which is the normal working voltages and fields and over voltages. As I mentioned over voltages comprise of a lightning over voltages or 31 switching over voltages or sometimes for a short duration the power frequency over voltages. Apart from mechanical and electrical stresses these insulator strings are to be withstand the environmental stresses like the temperature, very high temperature areas where the transmission system is erected. The second thing the cold areas like very high altitude where the temperature comes down drastically. So, this has to function in that environment and because of the ultra violet radiations particularly from the sun and also because sometimes the UV radiations happening because of the corona effect the corona effect is because on the hardware you can see this is the phenomena which occurs near the vicinity of the conductor where the air break down takes place and there are discharges, Luminas discharges which are coming from the hardware if it is not properly designed. And these discharges will are continuously heat the insulators either ceramic glass or porcelain. In case of ceramic not much of the issues, but in case of polymer these corona discharges which continuously heat the surface of the polymer insulators particularly which is organic in nature is likely to degrade it is surface lose it hydrophobicity over a period of time, and likely to see that the flash over or the degration further degradation which happens and insulator may give away. So, this we will be discussing also when for the polymer insulator. So, these are all various environmental stresses the insulators have to withstand in service. 32  Advances in UHV Transmission and Distribution Prof. B Subba Reddy Department of High Voltage Engg (Electrical Engineering) Indian Institute of Science, Bangalore Lecture – 03 Insulation coordination, over voltage in power systems (Refer Slide Time: 00:19) So we see about the day design and criteria which is being used for the transmission insulators; very important is the insulation coordination which the high voltage engineers have to be careful while designing the insulation particularly for EHV and UHV transmission levels. So, insulation coordination mainly aims at selecting proper insulation level, for various voltage stresses in a particular a rational manner. So, that is it is main objective is to assure that insulation has enough strength to meet the stress on it. In case of the stresses which we discussed, may be mechanical may be electrical may be environmental insulation has to with stand doing emergencies conditions. So, to see all equipments which are to properly protected. So, it is desired that insulation of various protective devices must be properly coordinated. This is the main aim of insulation coordination. The graph which is shown here it gives the voltage verses the probability density your voltages and insulation flashover. Simple looking into the insulation aspects of any particular equipment, this is the insulation strength which is designed for the equipment or the component, about this shows the stress which is being 33 continuously on it. So, the level of the coordination or the insulation coordination will definitely be seen that it has more strength compare to the stress which has to undergoing the field because of several stress which is likely to face. (Refer Slide Time: 02:02) So, the maximum over voltages as we know occur rarely, and like wises insulation strength occurs very rarely. Because very insulation strength rarely decreases to it is lowest value. This has been in practice which has been seen. So, the likelihood of both the events that is the over voltages occurrence and also the insulation and decreasing simultaneously is very limited and very rare event. Therefore, considerable economy may be achieved by recognizing the probabilistic nature of both voltage stress and insulation strength by accepting a certain risk of a failure. So, this is where the design aspects looking in to the economic point of you have to be taken care so this leads to substantial decrease in the line insulation, if you properly design properly take care of the parameters. Particularly spark distances the dimensions of the tower the weight of the entire insulator saying consisting of the conductors’ accessories corona control ring so on so forth. Then write of a clearance of a write of a resulting decreased cost of line. So, this proper coordination proper planning proper insulation design will help in the decrease of cost of the line. And this decrease in line cost must be seen again is the increase risk of failure and the cost of such failures. 34 So, the design engineers have to bare in mind while designing proper insulation either it is not require to over designed insulator under designed the insulator insulation. So that cost must vivid against the risk of failure and the cost of failure which may happen in the system. (Refer Slide Time: 03:57) This is an important insulation coordination curves which shows of the peak kV peak of flashover voltage, with the volt time curves is verses the time. You can see that various curves are there. These are the level of protections which are being done for the insulation level. The initial you see the a, b, c, d are different curve here the insulation level example for the transformer the line insulation will be somewhere here to protect the transformer we have LA that is lightening arrester, which has lower insulation level then the transformer in case of over voltages in case of failure in case of abnormal conditions the initial sacrificing should be lightening arrester were the lightening arrester should conduct in the transmission system and protect the transformer. So, this is how line insulation is designed. Further, the transformer there may be some other insulation which has been designed like the bus bar and so on and so forth. So, the insulation coordination proper planning proper insulation coordination is to be done to protect the major equipment, particularly the transformer circuit breakers and controls in the substation and for the generating in the transmission network. 35 (Refer Slide Time: 05:23) So, earlier I mentioned about the over voltages which are likely to be seen in our system network. So, temporary over voltages again these temporary voltages are lightly damped oscillatory type of voltages at supply frequency. I was telling at normal 50 hertz frequency the over voltages are likely to happen and these are known as temporary power frequency voltages. So, switching over voltages or switching surges, these are damped oscillation at frequencies of less than 10 kilohertz. Again switching over voltages are likely to occurs because of the opening or closing of the circuit breakers in the system. The switching over voltages typically has a wave shape of 250 microseconds, 2500 microseconds. So, switching over voltages as I was mentioned are damped double exponential oscillations, which may at frequencies less than 10 kilohertz. So, switching over voltages are of in these nature where the front time will be somewhere on 250 microseconds on the tale time is at 2500 microseconds the time front to time tale is 250 to 2500 microseconds. This are normally happening because of the closing and opening circuit breakers in a transmission system. Then coming to lighting over voltages these are again damped oscillation at frequencies which are less than 100 kilohertz. These lighting over voltages may occur due to the national lighting which may strike the tower which may strike the tower which may strike the equipment. So, this national lightening will be of the front 1.2 microseconds and a tale time that is 50 percent of the magnitude is the tale time. This is the 50 36 microseconds. So, lightening impulse is 1.2 by 50 microseconds whereas switching is 250 by 2500 microseconds. So, these over voltages are occurred in the power system and proper protections for these over voltages have to be taken in to consideration. (Refer Slide Time: 08:23) So, this table shows various over voltages which what we discussed which may occur because of the external or the internal application, internal service condition. So, the external is because of the lighting as we are mention. So the natural lightening will create a direct induced or back flashover. These aspect will create a various type of impulse is various type of over voltages on the transmission network. Then the second externals being the NEMP that is nuclear electrum magnetic pulse, this is because of the nuclear accident which may happen which creates very sharp pulse and may create problem to the transmission network. So, non-nuclear electromagnetic third external over voltages being the non nuclear electrum magnetic pulse this may be because of the magnetic solar storms are so on so forth. So, second being the internal operations of the power systems can cause the transient over voltages. This transient what we were talking about is a surge or the switching over voltages which may happen because of the closing and reclosing, that is a opening and closing of the lines or interruption of capacitive or inductive currents or 37 switching on transformers or fault initiation or clearing of the fault. So, these aspects may create transient or switching surge voltages in a transmission network. An internal again a temporary over voltages this temporary voltages over voltages may occur because of the load rejection because of the Ferranti effect which the sending and receiving voltages defer. So, the Ferranti effect also will cause a temporary over voltages, then self excitation because of the saturation effect because of the ground faults and some asymmetric faults. So, all these are likely to cause the temporary over voltages in the system. The third internal over voltages may be because of the steady state operations also. So, this steady state internal over voltages may occur because of the contact with circuits at particularly of higher voltages may because of the neutral inversion the arcing which is happening in the ground side that is a arcing ground phenomena and the resonance phenomena. So, these are various classification of over voltages in the power network or power systems have to be content to see the proper functional of the equipment which are used in the transmission or the distribution network. (Refer Slide Time: 11:13) So, we have looked in to the slides. So, very important this gives us the idea what are the factors which determine the designs for insulation so the voltage levels particularly lesser than 4 kilo volts we have to look in to the mechanical clearances from the ground the 38 tower which are being used have to be looked at the mechanical clearances below 4 kilo volts not much of the lightening switching surges play a role. So, above 4 kV to 33 kV 33.5 kV the corona and lightening surges player role if the design of insulation; so the corona discharges which can create insulation has break down particularly at that voltage levels which may generate because of the transmission conducted a hardware, insulated accessories, and also the lightening surges which may dominate during this voltage insulation design engineer has to keep in mind are the various factors which are responsible for the design at particular voltage levels. Coming up above 66 kV to 220 kilo volts we have to look in to the consideration of lightning and switching surges which we previously look. So, lightening is again a natural phenomenon. Switching surge may be happening because of the transmission network closing and opening of the circuit breakers so on so forth. So, the design has to be a taken care for lightening and switching over voltages up to 220 kilo volt. Above 220 that is extra high voltage 400 kV to 7400 kV level, which we are normally call at extra high voltage levels and ultra high voltage levels above up to 765 kV or 800 kV apart from lightening the switching surges and contamination or a pollution dominates on the insulations design. So, the insulation engineer has to take care of the switching surges contamination or the pollution above up to 765 or 800 kV. Further ultra high voltage insulation design mainly depends on the contamination aspects. The contamination or the pollution is a very important aspect to be considered for the design above ultra high voltage insulation. So, for EHV UHV that is extra high voltage or ultra high voltage pollution or a contamination performance is a very important aspect for the design of line insulation. And we will be looking in to this contamination how serious how it happens the physics behind the contamination. 39 (Refer Slide Time: 14:04) So, we were looking in to the electrical aspects now the design criteria which are followed for the mechanical aspects. So, it is primary function is to support the line insulator string is to support the line mechanically. One of the important factors apart from the electrical isolation mechanical it has to support the power. So, we have to estimate the maximum load which an insulator string consists including the insulator the line conductor the accessories, the hardware and the design has to be done how the load the maximum load including the insulator string will see under normal and also overload factors overload again depending upon the dynamic overloads of because of the compression because of the tensile aspect or because of the heavy winds and so on. So this has to be kept in mind before designing the requirement for mechanical criteria. So, the mechanical properties of electric component of high voltage insulator typically when we look in to that mechanical strength of porcelain is somewhere around 30 to 100 and for glass it is 100 to 120 and polymer it is 20 to 35 and resin bonded material is 1300 to 1600, resin bonded glass fiber RBGF used for core of the polymeric insulator which we were talking about the (Refer Time: 15:48) which is employed for the polymer insulator core. So, resin bonded glass fiber with stands tensile strength of 1300 to 1600. And in case of compression the mechanical strength requirement for porcelain 240 to 820, the glass being 210 to 300, polymer is 80 to 170 and 700 to 750 in case of the polymer or resin bonded glass fiber. 40 (Refer Slide Time: 16:10) So, these mechanical criteria particularly porcelain or ceramic insulators when the porcelain begins crack it electrically punctures. So, that we have to keep in mind. So, the mechanical strength may deter ate the insulation, initially this will lead to the electrical punctures. So, we should not exceed electrically, should not exceed 50 percent of the mechanical and electrical rating of the insulators. So, this has to be checked for cantilever rating which d not exceeds more than 40 percent of the actual load. In case of polymer insulator non ceramic insulator the load should not be beyond specified mechanical load for each type of voltage level the specified mechanical load is being mentioned. Similarly specified tensile load, routine test load and the rated cantilever load, all these things which have been mention will be applicable for a particular voltage level particular insulator dimensions have to be strictly followed so that the failure does not occur and the insulation does not weaken in the system. 41 (Refer Slide Time: 17:21) The important component while designing the electrical criteria is the strike distance. What is a strike distance or a dry arcing distance which is known with a high voltage terminology? The strike distance or the arching distance is the shortest distance through the surrounding medium, near the insulator between the terminal electrodes. You can see here this is the insulator the insulator can be of porcelain ceramic glass or hollow insulator. Representation here very clearly it is the both here the end fitting or metallic end fittings, the distance between end fitting from here to the end fitting in between the there is a complete insulation. So this distance the direct distance clearance from the metal to the metal is known as the dry arcing distance. So, in case when one of that is connected to the high voltage other is connected to the ground. Because of the voltage stress increases the likely to flashover, and the nearest flash over will happen during this metal to metal a junction. So that shortest distance through the surrounding media that is air surrounding media in here, should be from the terminal electrodes. So, this known as a strike distance for a follow a porcelain bushing. Similarly, in case of a insulator string a representation of 3 insulator is given. You can very clearly see from the top cap the metal cap to the bottom most metal thin. This is the arcing or dry arc distance strike distance which being refered. So, the dry power frequency PF is the power frequency flashover and the impulse flashover based are 42 designed on the importance of the strike distance by the insulation or designing engineer. So, the power frequency voltages and impulse flash over voltages are likely to see depending upon the dry arc distance which is being provided and wet flash over also. See in case of insulator if there is rain because of the wetness of the wetting of insulator surface the flashover voltages is likely to come down. So, this flashover voltage also kept in mind while designing the arcing or a strike distance. So, the leakage distance mainly helps to maintain the surface resistance of the strike distance. So, this is important design criteria for the electrical. So, a very clear distension has to be made between the strike distance and why it is important in the design of insulation. (Refer Slide Time: 20:13) Next is the leakage distance what is the importance of a leakage distance. So, this is an example of insulator string which consist of several insulators double attention insulators several insulators are there. And here you can see the metallic ring type of arrangement is known as arcing hones. So, this distance between this is known as the strike distance what we discussed earlier, that tip of this metallic to the tip is a strike or the arcing distance which is being maintained for this insulator string. In case of over voltages the discharge should happen from metal to metal it should go to the ground; so where the protection for the insulators string is done. 43 So, similarly what is leakage distance, the leakage distance is the sum of shortest distances measured across along the insulating surface between the conductive parts as arranged in here. So, this shortest distance each insulator each ceramic portion minus the metal part that is the cap and the pin will constitute a leakage distance. The actual insulation provided for entire string minus the metal parts will give you the leakage or distance for the insulator string. Here we have to take from the first petticoat of the insulator on the surface here and again from here. So, this addition of this porcelain insulation will give you the leakage distance of in a insulator string. (Refer Slide Time: 21:55) So, as discussed we were looking in to we are talking about the over voltages which happened in the transmission. So here are few examples which have been given the first is the lightening impulse or a lightening surge which occur in the transmission, how it looks. The second is the lightening impulse chopped wave and third is the switching impulse. So, we will look one by one. The first is the full lightening impulse voltage which is being shown without oscillation or overshoot. Typically, this is a test book type of wave form which is been shown, but practically you may happen to see the oscillations here in this raising portion. So here it is not been shown, to see that how it is being constructed. So, the full lightening impulse without oscillations which is shown here is from the 0 to the 1, where shows the magnitude. And the time to front is calculated either from the 10 to 90 percent in some cases it is 30 to 90 44 percent depending upon the oscillations which are seen here. So, time to tale is taken as a total magnitude there is a 50 percent of that you see the time to tale. So, this is the typical lightning impulse for a lightning surge wave from which is used for the application of the various component to check their performance in the laboratory before it is being used in the transmission system. Similarly, you have a lightning impulse chopped. This again a similar wave from the chopping is intentionally done either on the tale or on the front. So, in the transmission network these type of surges are likely to see across the transformer are any other equipment were intention is to see simulate such condition in the laboratory and check the insulation could withstand in the laboratory. That is where chopped lightening impulse is being carried out again there are standard for this how many applications of lightening impulse voltages whether it is 10 positive 10 negative or 10 positive of this thing and 10 negative it depends for each component standards have been specified for this applications based on the standard performance is verified in the laboratory before it is being used the field. So, here again wave form shows a chopping in the tail section and similarly chopping could also we done in front portion. So varies type of a lightening impulses which will be intentionally chopped at a required level will be tested for the insulation. The third is the switching impulse over voltages or a switching surge what we call. Here as I mention earlier the time to front will be 250 microsecond and the time to tale from here to the magnitude 50 percent magnitude will be 2500 microseconds. The switching surges are likely to happen because of the closing and opening of the circuit breaker and in that transmission network here the front time will be 1.2 microsecond and tale time will be 50 microseconds for lighting surges for switching it is 250 by 2500 microseconds. 45 (Refer Slide Time: 25:32) So, now we come in to the selection of a suitable insulator, how to design or select an insulator for electrical for various levels in the before it is being put in a transmission. A thumb rule has been given here design aspect consist of several design consist of several aspects for it is being put in the service. As a thumb rule just for understanding have given you here the selection of how suitable insulator is made for the transmission system. A simple example here I have given for 2 voltage level that is 66 kV and 240 kV similarly the higher the voltages could be used with the factors which have been employed will be a slightly different. So, a simple example for 220 kV system voltages the 220 kV the highest system considered that second portion of the Column, the values in second portion. The 220 kilo volt transmission system is a normal operating voltage will be 186 and the maximum operating or the design of insulation for a 220 kV will be done to 245 k v. So, this is what I have shown here a system highest system voltage of a 220 kV to 245 kV and the system line to ground voltage that is whatever I indicate in the a the system line to ground voltage divide by √3 that is 1.73 into 1.05 is a 5 percent extra is taken in to consideration for design it gives 170 value is 170, this will be using it later. So, further peak of line to ground voltage is nothing, but the value which have got in 170, 170 into √ 2 that is 1.414 gives you 220.5 is a number which obtain for line to ground. 46 So, this how electrical parameter are calculated further the leakage distance how leakage distance it is calculated. So, for leakage distance we have to consider where the transmission line or a transmission tower is being erected. So, that is very important. I was mentioning you various type of pollution zones exist if it is pollution zones as per the standard a pollution level system classified into very non pollution level, very clearly clean level. Then there will be a light pollution a moderate pollution or a heavy pollution zones. So, the transmission line passing through different zones while calculating the leakage distance have to bear in mind about the factors to be considered. So, in case your transmission tower is running near a moderate pollution zone area the factor of 1.5 to 1.7 considered similarly if it is high 2 to 2.8 is considered. So, next after the leakage distance the insulator minimum leakage distance is calculated simply with the help of 170 is the b is 170 into the inches per kV that is simple for 220 kV it will be one hundred and 70 inches that is a leakage distance which have converted into inches per kV. Then for calculation of switching over voltages a factor of b that is again 170 is taken and multiplication of factor a particularly for 250 kV and lesser some where the factor will be 3 to 5, I have assumed 5 has a factor multiplication factor. So, one for 220 kV the b is 170, 170 in to 5 gives you 850, is a switching surge voltage level for a 220 kV system this all the thumb rule which a have been indicated here. Next for calculation or designing for lightening over voltages this simple formula is considered here where the current is a resistance and the value of p v is taken for the calculation so typical value of current in case of lightening strike it is taken anywhere between 2 to 50 kilo for a transmission system and during that period resistance is likely to be anywhere between 10 to 20 ohms. So, for a 220 kV system I have considered 44 kilo amps somewhere 90 ohms plus 220 kV approximately. This gives 1056 kV if u want to maintain the insulation level for lightening volt for switching it is 850 for lighting 1056 k V. So, when you summaries the system requirement, it approximate value for a 220 kV system. You see the leakage distance should minimum be 170 inches switching surge voltage should be 850 kilo volt and lighting impulse with stand voltage should 1056 kilo volts. So, these are the values which you obtain or a design engineer is likely to look in 47 to these aspects as a voltage level goes up in case of 400 765 800 kV. Similar excise will be done much before insulate also apart from these several other consideration will be taken into consideration. 48  Advances in UHV Transmission and Distribution Prof. B Subba Reddy Department of High Voltage Engg (Electrical Engineering) Indian Institute of Science, Bangalore Lecture – 04 Design/selection of insulators, Importance of grading/cc rings (Refer Slide Time: 00:16) So we are looking at the design and selecting of a suitable insulator, so for 220 KV a system. As mentioned we have the following parameters which we have got the leakage distance being 170 inches. The switching surge or a switching impulse voltage will be 850 KV and the lightning impulse a withstand value will be 1056 KV. These are all for the dry conditions that is for the normal conditions, in a insulator strings are in the outdoor environment. That is without considering the rain fog or pollution conditions. In case of pollution rain or fog the wet flash over voltages or the wet switching surge lightning impulse and the values gets changed; so that we will be discussing. So, this, whatever approximate values which we have are shown here for 220. Similarly, for 66 KV the table shows you in case a 66 KV the maximum system voltage will be 69 KV and the line to ground will be of around 41.8 and line to ground voltage into √2 will be 59.1. And here again for a 69 KV, if you take the insulator strings are operating at different environments we see leakage distance being at 41.8 and switching surge voltages 125 KV and lightning surge voltages being at 359 KV. 49 So, these are the 2 different values for different voltage level. Similarly, for a higher the voltages 400 KV, 765 and higher voltages: similar a pattern will be follow with other considerations for the design aspect. So, just have looking in to the approximate values, the approximate values, which we have obtained for both the voltage levels are shown here the 66 and 220 KV. The leakage distance being 41.6 in case of 66 KV and 220 KV it is 170. Switching surge voltages being 125 KV in case of 66 and 850 and lightning impulses 359 and 1056, as I mentioned these values are for dry conditions. (Refer Slide Time: 02:43) So, now the selection whatever we have a made or we have seen are from the data for the 66 KV. You know the leakage distance when you look the required is 42 inches. So, the available from the manufacture is 46 inches. So, which is higher than the required which we have try to estimate. Similarly, in case of a impulse a voltage levels, we have 125 KV and the available is 240 KV for a wet switching search and impulse withstand voltages the required is 359 as per our calculations the available is 374 KV. So, these voltage values by the manufacturers which are available are higher than the values which are been estimated. So, similarly for mechanical and electrical strengths, the mechanical strength requirement is 12,000 pounds whereas available mechanical strength with the insulator from the manufacture is 15000 pounds. 50 (Refer Slide Time: 03:52) So, various contaminated environments I was mentioning you in case of a wet or a polluted condition. So, polluted conditions are again divided into a various zones a like light, medium, heavy, very heavy, very light and so on. So, these areas where the insulator strings are to be used are being taken care particularly depending upon the area of the polluted zones. So, for this term known as ESDD. ESDD is equivalent salt deposit density. This is given in milligrams per a centimeter square. This equivalent salt deposit density depending upon the site severity, I was telling the location where the insulator strings are to be connected or the towers, which are to be erected depends upon the leakage distance which is to be consider that is I is the suspension string I was mentioning you before a configuration of suspension string v being the v type of a string. So, here it will be inches per KV, the values for line to ground voltages. So, you can see that ESDD is nothing but the equivalent salt deposit density on the surface, of the insulator how it is calculated. So, normally the insulator is allowed to be in the field for a certain period of time, based on the site severity whether the area is light moderate or heavy polluted zone. The insulator after period of time is taken out and the ceramic or a portion except the cap and pin the contents or contaminants which are spread on the only insulating surface are removed carefully with a known quantity of distilled water and the 51 connectivity is measured and this is how we see the equivalent salt as a deposit density on the surface of insulators. So, based on this data we can say that the line which is being erected comes under pollution of light medium or heavy polluted zones. So, this is the application there is a application guide a particularly, how to be used during the insulation design. There are standard and also the international bodies which have the data for this. (Refer Slide Time: 06:29) Similarly a important IEC international electro technical commission a standard 60815 which completely gives about the information on the polluted conditions the values which are to be used for the simulation in the laboratory for a particular KV voltage level is described in standard 60815 for various type of insulators depending upon the creep- age length of the insulator how the insulator can perform in this conditions. So, the criteria has been given and various values as per the standard specified values of ESDD that is equivalent salt deposit density are to be used in case the laboratory verification or laboratory testing of this insulator for various a types of a insulators to be a tested. 52 (Refer Slide Time: 07:34) So, again the recommended leakage distances various a groups. This includes a IEEE international study groups and IEC is a electrical or international electro technical commission. Apart from there are several other standard bodies which converge and they look in to these requirements for various leakage distance and they have recommended. So, these are the curves which show for the recommendation of the leakage distances in each per KV, it is a line to ground verses the equivalent salt deposit density that is in milligrams per centimeter square. The values have been defined for various pollution zones. So, both for ceramic glass and a polymeric a materials which have to be used for the laboratory evaluation for the verification or performance of a insulators. 53 (Refer Slide Time: 08:29) Then again this graph shows the improved contamination performance, mainly gives about the flash over verses the equivalent salt deposit density, the flash over voltage across the equivalent salt deposit density. The curves very clearly give you the idea which are been used for porcelain then EPDM and polymer insulator there is a new SRA silicon rubber insulator aged silicon rubber insulator, ethylene poly die monomer is the EPDM material which is being used for a the insulation purpose. So, this gives you the improved contamination performance, the measure of a equivalent salts deposit density. How it is being used in the field with the help of the pollution monitors using a dummy insulators in service or removing a in service insulators a evaluation of the equivalent salt deposit density and selecting of the appropriate leakage distances to be used in that the areas so very important a graph which gives you the idea for the improved contamination performance. 54 (Refer Slide Time: 09:43) So, the contamination or the pollution performance of an insulator is a very important issue when you go for extra high voltage and ultra high voltage transmission I was mentioning. Now for last 100 years we were using the polymer porcelain or the ceramic insulators and glass insulators. Polymer insulators as I mentioning it was it is of the recent origin and a particularly they are organic in nature. So, the polymer insulators presently what we have see after better contamination flashover performance then porcelain yes, but again the a performance has to be judged over a period of time these insulators being of a recent origin the field data available a for the longer performance or longer a time performance is not yet available. So, we have to see the performance over a period of time and then conclude that a polymer insulator can offer better performance for a very long period of time. Yes short term performance they are performing good and are giving the better contamination flashover performance in comparison to the porcelain or glass type of a insulators. The polymer insulators or silicon rubber or a composite insulator have a smaller core there is for the glass core and a weather shed diameter there is a petty coats which the molded on the farther glass or rod. This can increase the leakage current a density. So, it is a important comparison compared to the a porcelain, looks aesthetically much better than the earlier porcelain or a glass type of insulators a higher leakage. Current density what we were looking here a means a more ohmic heating. Again more ohmic heating 55 helps to dry the contaminate layer and reduce the leakage current. So, this is a one of the very important aspect when using in for a polymer a type of insulator, where hydrophobicity the property of the surface of the polymer insulator try to repel the water and sees that it has better leakage current density and sees that the contamination layer and reducing of the leakage current which helps the better performance of the flashover particularly in pollution of a contaminated areas. In addition the polymer insulators hydrophobicity helps to minimize the filming. Hydrophobicity is property of the surface of the material where it is tries to repel the water droplet us or the polluted droplet us which are being on the surface, particularly the water droplet us and sees that the filming formation on the surface is a reduced, and where the contaminants or the pollutants goes on accumulating it is reduced. That is one more advantage with the polymer type of insulator the contamination performance of a composite or a polymer or a silicon rubber insulator exceeds that of a their porcelain or a glass a counterparts which are being used for a long period of a time this is an important a point to be noted. The next being the contamination flashover performance whether it exceeds that of the EPDM units again depending upon the number of years the insulators in service in comparison to the electric ethylene porcelain die monomer type of units which for the earlier version of the silicon rubber insulators are presently being used. The presently I have mentioned that being used or of a third generation. So, EPDM were earlier used a how much is better the time comparison is very important. This has to be properly verified in the laboratory from the field insulators and can be judged the performance of the silicon the information where the flashover performance could be better when comparison to the earlier EPDM units. 56 (Refer Slide Time: 14:02) Next the grading rings as I was mentioning corona control rings, the grading rings, arching horns these are all the similar a names or a same hardware which are being used for proper grading that is a voltage distribution in case of a various insulator strings. So, this grading rings corona control rings are very important. This simulate a larger or more spherical object in case as the voltage level goes up the corona control rings of an spherical nature or a rectangular type which are being used in a long transmission insulators long transmission lines and high voltage and extra high voltage and ultra high voltage transmission for insulator strings. These help in a reducing the gradients particularly associated with the shielded object. So, the insulator string is having hardware as I mentioned the yoke plate along with several accessories which are connected to the tower side and also to the line side. So, this have to be equally reduced so that the insulator never sees stress in a particular region. So, the equal uniform distribution of voltage is to be maintained. And the higher gradients have to be reduced for this the grading rings or the corona control rings are employed for the transmission line insulator strings. So, reduction in gradients helps to minimize the RIV and TVI. RIV is a radio interference voltage and a TVI is television voltage interference. So, we will be discussing about the radio interference voltage and TVI in future, when we come to the importance of these parameters. So, the gradients which in case if it is 57 not reduced can lead to a discharges on from the particularly hardware on corona control or grading rings. And these discharges which are of impulsive in nature will communicate through the conductors and to the neighboring areas where radio sides or a television could be affected because these are high impulse in nature and operated that voltage levels. So, radio interference voltage measurements which are carried in the laboratory see that the gradients are quite a lower and the discharges which are being from the hardware and grading rings do not interfere in radio interference as radio a circuitry or the television interferences causing interference to the radio on television sets. So, mentioning about the porcelain or glass these are inorganic and break down very slowly which are being used for last hundred years or so. Mechanically they are good on performance it has been judged over a period of time. Now, the recent non ceramic insulators or polymer silicon rubber or composite insulators, are more susceptible to seasoning due to corona. So, corona is again a phenomena which happens near the hardware, corona control rings particularly in vicinity of high voltage where the discharges on the year background takes place and the corona maybe initially audible then the visible discharges coming out from the hardware or a corona control a rings or a grading rings continuously impinge on the surface of the particularly non ceramic insulator polymer insulators over a period of time, which in the intensity is more may cause the insulator surface to lose it hydrophobicity and likely over a period can reduce the surface property insulating properties. This how it happens due to the corona we will be also discussing about the effect of corona and the polymer insulators and how it could affect the insulation in future. So, you next the UV ultra violet again these are short wavelength range this it may attack the polymer bonds on the silicon rubber or a polymer insulators. So, this ultra violet again maybe from the natural sun, during the day time, or due to the corona the effect of corona also causes the ultra violet radiations which may impinge on the surface and attack the polymer bonds making the surface of the insulator less hydrophobic and where the surface may degrade over a period of time, and the insulation level could be reduced. So, the most short wavelength ultra violet is filtered by the environment, but the ultra violet us which is coming from the corona is not filtered and it may damage the surface. So, ultra violet due to corona is not filtered, this ultra violet and due to corona is from the hardware corona control rings or grading rings. 58 (Refer Slide Time: 19:35) So, non ceramic insulators have a different corona control rings, which are designed not like the porcelain or a glass type of insulator. So, here the design of the grading rings or the corona control rings depends on a how the hardware end fittings of the non ceramic insulators is designed and for the particular voltage level. So, due to corona cutting, what we call and also water droplet corona which this water droplet corona may happen because of the rain droplet us which settle on the surface of a polymer insulator. So, this are likely to develop a small discharges and later on corona discharges which slowly degrade the surface on the insulator and the hydrophobicity effect could be over a period of time it may lose the hydrophobicity characteristics later on the degradation of the insulator could happen. So, the non ceramic insulators may require the application of the corona control or grading rings particularly to grade the field on the polymer material on the weathershed housing. So, this is very important in case proper design of corona control rings for the non ceramic or a polymer insulator is not done the material could the weathering on the particularly on the weather sheds petty coats or the weather shed of the material could lose the hydrophobicity over a period of time. With these corona rings must be or grading rings must be properly positioned. The positioning of the corona control rings is equally important with the design. So, not only the design place a role the positioning of the corona control ring alignment in the string is very important. So, that the end fitting on which they are mounted has to be properly mounted, else there could be shift in the 59 position and this where lead to higher corona discharges which may damage the surface of the polymer sheds. So, the orientation positioning is to provide grading to the polymer material or a polymer insulator string. As a general rule the corona control rings or the grading ring which should be over the polymer brackets which should be on the hardware; so hard this brackets or accessories are provided on the insulator metal end fittings which are connected to the tower. So, they should be properly positioned properly mounted and properly oriented with the help of brackets on the hardware that is on the yoke or the connecting hardware to the tower and to the conductor side. (Refer Slide Time: 22:28) So, what is the service experience which has been observed over a period of time? So, we will discuss about for both the ceramic and the polymer insulators which have served the transmission system for a period of time. What are the likely problems or issues which could happen in case of a polymer insulator or porcelain insulator or a glass insulator we will be discussing the particular issues in the field a related field use and so on. 60 (Refer Slide Time: 23:00) The ceramic or porcelain I was mentioning there more than hundred years old. Initially started off as telegraphic insulators for employed for the transmission very early stages, and presently the porcelain or a ceramic insulators are used for all voltages from a very low voltage to the ultra high voltage range up to 800 KV or even for the 1200 KV experimental lines in the polymer porcelain insulators are being used for the line insulation. These provide, porcelain insulator provide greater flexibility because have a cap and a pin arrangement this is the cap. This is a pin of the insulator this is the porcelain shell and these are the petty coats or sheds what we call. So, the cap and pin are connected are fixed to the porcelain shell with the help of a Portland cement and properly the gin coating is carried out to see the corrosion never happens in the field. So, a schematic of the design of a porcelain suspension insulators is also shown here, where the porcelain shell is this one and the cap wherever the cap which you see is the malleable or a ductile iron material or which is fixed on the as a cap of the insulator an