in the previous lecture we completed introduction to power system and now in this presentation we will try to understand working of AC power supply system I will explain how a C power is transmitted or supplied from the generating end to the load end this end here is the generating and you can see a thermal power plant at this end and power generated at this hand is transmitted following this path to the load end so this end here is the load and and in this particular case I am taking a society or locality as the load there are various houses acting as the load loads are of different types you can have industry as a load or college or Hospital or market but for this case we have taken a society or locality as the load generating end is this source and we can simply call generating and as the source end because this hand is the source of electrical energy we are burning coal to produce electricity so this is the source and this houses are the load now the first question comes in our mind is why we need to transmit the AC power over this long distance why can't we construct the power station near the load this used to happen in the early days we were having two options the first option was to construct the power station near the load where the electricity is demanded or we can construct the power station near the raw material the raw material for thermal power plant is coal and it is not important that all the loads are near the coal mine let's say this is the coal mine here and for example the power plant is generating 2000 mega watt of power and let's say 200 mega watt is consumed near this coal mine there are people living near the coal mine so 200 megawatt is consumed near the coal mines and we are left with 1,800 mega watt so we need to transmit this 1800 mega watt to the regions where we don't have coal mines or we don't have rivers for hydro power plant so this is the practical scenario when we are bound to transmit the power over a long distance for example the electricity generated in Bihar or Jharkhand we transmit to Delhi Dell is the capital it is the Metro City and the requirement of electricity is very high so we need to transmit the power to Delhi not all will be transmitted to Delhi for example out of this 1,800 megawatt and let's say 800 megawatt is consumed in this state itself Johar cannot be higher and remaining 1000 megawatt is transmitted to Delhi this is simply a hypothetical scenario I am NOT talking about the exact figures this is a simple example to explain you why we need to transmit the AC power over a long distance so we are constructing the power station near the coal what if we construct the power station near the load for example we construct the power station in Delhi itself there are few power plants in Delhi for example in Dadri we have power plant it is not politically in Delhi but it is for Delhi NCR loads so let's try to understand what will happen if we construct the power plant near the load in that scenario you need to transport the coal over this long distance now you are transmitting the AC power over a long distance but in case the power station is constructed near the load you need to transport coal over this long distance now how you can transport the coal you can transport the coal using the Indian Railways of course and the cost of transportation will be very high as compared to the cost of transmitting the AC power the cost of transportation of coal will be higher because we need a huge amount of coal to run the power plant to meet the one-day demand to meet the one-day demand of the load we need thousands of tons of Cole and to supply this much coal we need to invest huge amount of money so to cut the cost to reduce the prices of electricity we need to construct the power plant near the source that is coal in case of thermal power plant and this is the reason we need to construct the transmission lines very long distance transmission lines to transmit the electrical power now what about nuclear power plant and hydro power plant can we construct them near the load we cannot construct a nuclear power plant near the load because we are producing the electricity from the nuclear fission process and to perform the nuclear fusion we require radioactive material so we are dealing with radioactive material and from safety point of view the nuclear power plant must be constructed away from the human population so again we are required to transmit the AC power over a long distance in case of hydro power plant we need water we need river or any other water source and the dam must be constructed in hilly regions only we need to hold the water and to avoid the flooding situations we need hilly regions so again we are having one constrain and it is not important that all the loads are there in the hilly region so again we are required to transmit the AC power over a long distance so this is all for the need of transmission line and now we will talk about other aspects of AC power supply system we are producing electrical power using the generator and let's say the voltage rating of generator is 11 kV we can have generators having the voltage rating equal to 15 kV also but for this particular case let's say the voltage is 11 kV now we will step up this voltage to a higher value of 400 kV again there is one question why we are stepping up the voltage from 11 kV to 400 kV we are stepping up voltage because we need to avoid the line says we are using these lines to transmit the power and these lines we call as transmission lines and they are made of aluminium and steel aluminum and steel will definitely have some resistance and as current is flowing through this wire there will be I square R losses now if you increase the voltage from 11 kV to 400 kV if you increase the voltage the current will decrease the current is decreasing because power is constant and power is equal to VI so if you increase V I will decrease to maintain the same power so this is the reason we are stepping up the voltage from 11 kV to 400 kV in India we have few lines having the voltage rating more than 400 kV we have lines having the voltage rating equal to 700 kV but for this we will consider the voltage rating to be 400 kV so one thing we can point along with generator we require a transformer which is acting as step-up transformer this particular transformer we call generator transformer the generating transformer is same as the other power transformers but the operation is different in case of generator transformer it will always work as step up but the power transformers can work as step up or step down so there is a transformer known as generator transformer or generating transformer associated with generator every time to step up the voltage now we have stepped up the voltage we will transmit it over a long distance and as we require lower voltages at the load end we need to step down the voltage as well for this purpose we use substations substations are multi-purpose stations we have transformers and other electrical equipments in substation there are different roles performed by the substation but for this example we will only assume that substation is stepping down the voltage so from 400 kV from 400 kV we will step down the voltage to 220 kV the primary side of the transformer is getting 400 kV and the secondary side of the transformer is giving us 220 kV this 220 kV is again transmitted and we will step down 220 kV further to 132 kV 132 kV is transmitted and stepped down at other substation to a voltage level of 33 kV 33 kV 33 kV is stepped down to 11 kV and 11 kV 11 kilo volt is stepped down to 400 volt so this is how the standard transmission voltages look like this voltages are not random voltages I have not taken them by my own but these voltages we call as standard standard transmission voltages okay now there is one important thing you must see in this diagram here we are having three phase three wire system three phase three wire system means we have three phases are Y B red yellow blue and three wires are there so I will write three phase three wire system here also we are having three phase three wire system three phase three wire system again three phase three wire system three phase three wire system three phase three wire system and this one is the last three phase last three phase 3 wire system here you can see one extra line the yellow line so this system is three-phase four-wire system this is three-phase four-wire system the fourth wire the new wire is neutral the initial three ones are red yellow blue and this one is the neutral so we are having three-phase four-wire system and different houses are connected between R and yn or BN for example this is your house and this is your friend's house and your house is connected between R and n your house is connected between R and neutral and your friend's house is connected between yellow and neutral it is connected between yellow and neutral and what is the voltage you are getting at your house what is the voltage you are getting here this transformer is stepping down the voltage to 400 volt but you will say we get 230 volt in our house this is true you will not get 400 volt because 400 volt is the voltage between two different lines I will try to explain this concept because it is important we are having three lines we are having three lines red yellow and blue and in the last system we are having three-phase four-wire line so other line is there which is the neutral and if you calculate the voltage between r and y you will get 400 volt if you calculate the voltage between yellow and blue you will again get 400 volts and between red and blue between red and blue you will get 400 volt so all these voltages these voltages are line voltages line voltages and you already know what is voltage the voltage between two different lines is line voltage now we will find out the phase voltage there is new term phase voltage phase voltage is the voltage between any of these three lines and the neutral for example if you calculate the voltage between red and neutral it will be equal to 400 volt divided by root 3 which is equal to 230 volt because phase voltage let's call it V pH and line voltage V L is having the relation V pH equal to VL by root 3 VL is equal to 400 so V pH will be 400 over root 3 which is equal to 230 volt now look at here your house your house is connected between root red and neutral and between red and neutral what is the voltage it is equal to 230 volt so your house will get 230 volt of voltage and what about your friends house he will also get 230 volt because his house is connected between yellow and neutral yellow and the neutral is also the phase voltage the phase voltage which is equal to 230 volt so both of you will get 230 volt in your house and most of the appliances in our household is single-phase load I will write this down single-phase load by single-phase load I mean the loads like AC television fridge washing machine etcetera are connected between 1 phase and neutral the one phase may be red yellow or blue and this is neutral so the load is connected between red and neutral wire neutral blue or neutral for example in your house your television is connected between your television is connected between red and neutral so it is getting the voltage equal to 230 volt but in industries we may have three phase load we may have three phase load and these three phase loads are connected to the three phases of the line the load will be connected to red as well as yellow as well as blue this is the three phase load connected to red yellow and blue three phase load I hope the explanation is clear to you sometimes you will find there is no power at your home but there is power at your friend's home this is because your house is connected between red and neutral and his house is connected between yellow and neutral we are having fuses we are having fuses like this and let's say this fuse the red fuse is blown up so there will be no power at your house because the fuse is gone but at your friend's house there will be power because this fuse is working these are few things you must know now let us talk about transmission distribution and generation in this diagram if I divide it I will get the generation part the generation part before this line this part is the generation part and from here to here we have transmission part I will write this down this part is generation part from here to here we have transmission part transmission part and from here to the load we have distribution part you can also divide transmission into two different paths primary transmission and secondary transmission the same thing you can do with distribution also primaries to and secondary distribution but for now it is important to understand which part is generation but which part is transmission part and which part is distribution part because power system is all about generation transmission and distribution so this is all for this lecture if you have any doubt you may ask in the comment section we will try to cover other basics of power system in the coming presentations [Applause] [Music]