 We will start this lecture, before that I will quickly I recapitulate the point that I discussed in the last class. Yesterday, we discussed the grid connection, how to feed the power to the grid. Before that, output voltage of the pulse width modulator inverter has large number of pulses per cycle. So, if I take the harmonic spectrum, it has a fundamental component whose frequency is same as that of the modulating wave in a sinusoidal p-dolumni technique. I will tell you, I will repeat, the frequency of the fundamental component in the output voltage is same as that of the modulating wave. The magnitude of the fundamental component is proportional to m into V dc, where m is the modulation index, V dc is the DC link voltage. The second point that we discussed was, the equivalent circuit, when I am feeding the inverter, I am feeding the power to the grid, the equivalent circuit is something like this. This is the output voltage, the fundamental component is the output voltage of the inverter. This is the fundamental component of the output voltage of the inverter. I said V in or V inverter, this is V source, this is X. Our transfer is V inverter divided into V source divided by X into sin delta, where delta is the angle between V inverter and V source. If the inverter is supplying power to the grid, V inverter leads the V source by an angle delta. Now, this is a fundamental equivalent circuit. The output voltage of the inverter has harmonics as well. Now, I will assume that grid does not have any harmonics. The harmonic equivalent circuit is going to be something like this, V inverter again. This is the harmonic voltage. This is n into x, where x is the reactance at the fundamental frequency, n is the number of the harmonic. If the fifth harmonic, it is 5x. If it is seventh harmonic, it is 7x and the corresponding magnitude, corresponding magnitude from this equivalent circuit, I can determine the current supplied to the grid and therefore, it is possible to determine the THD. I have drawn this equivalent circuit. I can draw this equivalent circuit when the frequency of both the voltages are the same. In other words, I have to connect, I have to synchronize this inverter to the grid. Now, how do I synchronize? Before doing that, I told you that the frequency of the fundamental component of the invert output voltage is same as that of the modulating wave. Therefore, I have to generate a modulating wave whose frequency is same as that of the source and it is some phasor relationship with the source. In the sense, see here for any power transfer, the fundamental component of the inverter voltage should lead by an angle delta. This delta will change as the power changes. As the sense installation changes, power input to the inverter changes and therefore, the power output. So, as the power output changes, delta should change. I am assuming that V in and V source are held constant. So, the phasor relationship of the invert output voltage, fundamental component of the invert output voltage with the source is a function of P and I have to generate or the control circuit should generate a modulating wave whose phasor relationship I should be able to vary and it should be in synchronism with the source voltage. So, that is the requirement. So, yesterday I did discuss one, the so called the hardware method. There I have used a PLL as a multiplier. So, this is the theory behind it. See here, this is the square wave corresponds to the input grid voltage. So, the frequency of this is same as the grid voltage. So, as the frequency changes, the frequency of this square wave also will change. Now, I will divide this part into this square wave into n parts. n could be either 2, 5, 6 or 5, 1, 2 or 1, 0, 2, 4. I will digit as a sine wave and I have stored it in an EPROM. I will address this EPROM using a counter. The clock to the counter is generated using a PLL. PLL is a multiplier. So, as the frequency changes, the frequency of the sine wave also changes because at any given time at steady state, at steady state these two frequencies are the same. If these two frequencies are same, supply frequency is f 1. This also is going to be f 1. So, this will be n into f 1. This will be n into f 1. So, n into f 1 is being used as a clock to the counter. So, you can generate a sinusoid using this whose frequency is same as that of the supply. Now, how do I introduce delta? I said the inverter voltage should lead the source voltage by an angle delta. Now, depending upon the value of delta, if delta has 10 degrees at the 0 crossing of the source voltage, I need to address the EPROM wherein the sine 10 is stored and from there onwards I will go on increasing it. So, I need to have a control circuit which will tells me what should be the value of delta. I will use that value of delta to address the first location of the EPROM at the 0 crossing. I will address that particular location of the EPROM at the 0 crossing. That is all. So, that is the one way to incorporate or that is one of the ways to generate a reference a sinusoid. What is the second method? Second method I said is the software approach. PLL is as a multiplier there. Here I am using again a PLL and I gave you yesterday I discussed this harmonic oscillator. x dot is equal to omega into x, omega into y and y dot is equal to minus omega into x. So, the solution is x is equal to sine omega t and y is equal to cos omega t. Now, using this how do I generate? This is the implementation y dot integral of y. I will integrate I get y, y into omega is nothing but x dot. See here, y is equal to cos omega t. What is cos omega t? It is omega into x dot itself, omega into x dot itself. These are the two implementations. Omega is the instantaneous frequency. Now, software approach I will say the software PLL. I will just show you the show you the PLL simulation. These are the three grid voltages. These three grid voltages could be a could have harmonics. They could have a harmonics. They could be unbalanced as well. An ideal case is perfectly balanced and 120 degree apart. The frequencies of course, these are the grid voltages. Frequency will change over a narrow band. What I will do is I will convert these three voltages to alpha beta or x component and y axis component. The frequency of e alpha e beta is same as that of e a b b b c. Then I will transform then I will transform this alpha beta components to a synchronously rotating reference frame. The speed of the reference frame is same as that of the fundamental component of the grid voltage. I said grid voltage could have harmonics. So, if there are no harmonics, it is the fundamental the e a e c will be pure sinusoid. So, I am rotating this reference frame d q frame whose frequency is same as that of the fundamental component of the grid voltage. Under that condition, the fundamental component fundamental frequency component will appear as if it is a d c component. If there are harmonics there and if there are harmonics in the source voltage, they will appear as harmonics on a d c scale is something like this. In the synchronously rotating reference frame, the fundamental component will appears as d c and that d c and these are the high frequency component. I have to generate three modulating sinusoid whose frequency is same as that of the fundamental component of the grid voltage. That is the requirement. Having generated, I can incorporate delta later. That is not an issue. So, what do I need to do? I have at the synchronously rotating reference frame, I have a d c component and the d c component there could be high frequency components. I will eliminate this high frequency components by putting a small first order filter, low pass filter. Then I get only a constant d c. Yesterday I told you that somehow I have to generate a sinusoid which should be in phase or the d axis of the voltage space vector, d axis of the voltage space vector should be aligned with the d axis of the synchronously rotating reference frame. When that can happen, when the q component is 0, the q component is 0. So, what do I need to do? I will take, therefore I need to make the q component of q component of the voltage space vector in synchronously rotating reference frame. I will repeat, I have to make the q component of the voltage space vector in synchronously rotating reference frame. If I do that, voltage space vector v s is aligned along with the d axis of the synchronously rotating reference frame. So, that is what I am doing here. q, yet there is something known as a notch filter. Do not worry what exactly this notch filter does, I will explain to some time later. I will equate it to 0. At steady state, this component goes to 0. This is the P I regulator, because at steady state error has to be 0. So, output of the P I regulator is nothing but the frequency of the V a, V b, V c itself. I will just show you the MATLAB simulation. I have three phase sinusoid here, V a, V b, V c, I will just run this. V a, V b, V c, A V c, D q. Then, so the d or q component is equated to 0 here. P I regulator, this is nothing but the harmonic oscillator. This is nothing but the harmonic oscillator. Here, sine and cos, which is generated by the harmonic oscillator. Output of the harmonic oscillator is again the input to the d and q. See here, I can transform alpha and beta component to D q only when I know sine theta and cos theta. See here is the transformation matrix. Alpha beta, I can transform to D q if I know theta s. Theta is the angle, which is the synchronous rotating reference frame makes with alpha beta. I will just run this file. So, the first wave form is the grid voltage. Second is the PLL output. Output of the PLL, I will just expand it. I will show you. See the, during the initial time, initial period, initial period, this is the grid voltage. I have just taken only one wave form and this is the output of the PLL. They are not in phase. I will show you after some time, they are perfectly in synchronization. So, I am able to generate three sinusoids whose frequency is same as that of the grid voltage and they are in phase. Now, I have to incorporate delta. How to incorporate the delta? We will see later. So, I have generated a sinusoid whose frequency is same as that of the modulating grid. Now, what, see I will show you. This is the error. Initially, there is an error at steady state. This error becomes 0. This error becomes 0. This is nothing but the input to the PI regulator. This is the basic block. What if source has, source has the harmonics? What if the source has harmonics and what if source voltages are unbalanced? If the source voltage is unbalanced, I can divide the unbalanced voltages into positive sequence and negative sequence. The aim is to generate three phase sinusoids which are in phase with the positive sequence input voltage. Somehow, I have to eliminate the negative sequence component. How do I eliminate the negative sequence component? This negative sequence component, the direction of rotation of negative sequence component is opposite to that of the positive sequence component. So, if my reference frame is rotating at a speed corresponding to the supply frequency, grid frequency, fundamental component of the grid frequency and the direction is same as that of the positive sequence, the negative sequence component will appear as a AC component with double the supply frequency. See, I will repeat. If the speed of the d cube frame is same as that of the fundamental component of the grid voltage and the direction of rotation of d cube frame is same as that of positive sequence voltage, the negative sequence component will appear as a 100 hertz component in the synchronously rotating reference frame. The software approach to develop or to generate three phase sinusoids whose frequency is same as that of the grid voltage. I discuss the basic PLL. The basic PLL, there are the three phase grid voltages are purely sinusoids, they are displaced by 120 degrees. The second case I am discussing when the input voltages are unbalanced. When the input voltages are unbalanced, I can divide them into positive sequence and negative sequence. If the reference frame is rotating, the reference d cube frame is rotating in the same direction as that of the positive sequence voltages, these positive sequence voltages will appear as DC, while the negative sequence components will appear as if there are 100 hertz AC. So, on a DC component, this DC component corresponds to positive sequence voltage. The negative sequence of voltage will appear as 100 hertz on this DC component. Therefore, the aim is to generate three phase sinusoids which are phase with the positive sequence voltage. Therefore, I have to eliminate or I have to filter this 100 hertz component. So, that is the reason I am putting a notch filter. Notch filter has almost a zero gain or for infinite impedance for a 100 hertz component. So, it allows, it passes all the other frequency component and blocks 100 hertz. So, the input voltages are only unbalanced. I need to put a notch filter to eliminate the negative sequence component. I can put by putting in 100 hertz notch filter and rest is the same. So, I will show you the simulation. This is the basic block. I will run this file, fundamental component of supply and the PLL output. Just show you, one second I will show you. These are the input voltages. These are the unbalanced, sorry, this is the hormone. Wait, I run a different file, sorry about that. I will show the input voltages. The input voltages are unbalanced. So, that will be a positive sequence as well as a negative sequence voltage. Two phases of magnitude, this is the third phase. These are the input voltages A, B, C. A, B, C voltage, they are converted into DQ, synchronized rotatory irreversible, Q component, sorry, D, whatever the component is made zero. There is a low pass filter. See here. These are the, I have generated a sinusoid which is in phase with, phase with the input voltage, phase with the input positive sequence voltage. So, in other words, this is a positive sequence PLL. So, if we see in the beginning, they are not in phase. See, they are not in synchronization. After some time, it gets synchronized. The third case that I will discuss is input, could I have harmonics? Input voltage could be the harmonic. What do I need to do? Assume that input voltage has fifth and seventh harmonic. Either I will filter it at the input side itself. I will take A, B, C, give a AC filter the fifth harmonic and the seventh harmonic, convert into three phase to two phase e alpha beta, then do the transformation. Else, I will convert the three phase voltage as it is 3 to 2 alpha beta, alpha beta to D to DQ. Then, I will put a filter here to eliminate 100 hertz, sorry, I will put a low pass filter which blocks only DC and blocks which passes only DC and blocks the other frequency component. So, I need to put a DC low pass filter in the DQ frame, which is better that you need to decide. See here, I am filtering the AC components. In the second case, I am putting a filter in the DQ frame. So, the pros and cons of putting the filter on the AC side and putting a filter on the DC side, you find out which is better. I would prefer to put a filter in the DC side. What will happen if I filter it on the AC side? Go back to simple circuit theory and you find out. If you are not able to answer, you get back. So, I will show you that fifth and seventh. See fifth and seventh without filter, I will do it without filter. I will show you the input voltages, run this. The input voltage has harmonics. So, we will do all three cases. We will not put any filter. We will put a filter on the AC side. We will put filter on the DC side. Then, we will see. I will show you the results. Input voltages are unbalanced. Input voltage sorry input voltage has harmonics and see the wave form. It is not a sinusoid. It is not a perfect sinusoid. It is not a perfect sinusoid because it is not able to latch on to a component because in the DQ reference frame, it is not constant. It is continuously changing. So, somehow I am not able to it is not able to generate a sinusoid which is having a constant frequencies. So, in the second case, we will put a filter and we will see. I have put a low pass filter in the DQ frame. Input has a fifth and harmonic. See the sinusoid. I have a beautiful sinusoid in phase with the fundamental component. I have put a filter in the DC side. Now, we will put a filter on AC side. What will happen? Filter on the AC side. See there is no filter directly. D component is made equal to 0. D or Q does not matter. It depends on what is your D and what is your Q. Put a filter at the input side. See here. Low pass filters I have put here which blocks which passes only the 50 hertz component. 250 hertz and 350 hertz components are filtered out. I put a filter on the AC side. See output of the filter goes to ABC block and we will see what happens. What you can observe? See here. There is a lag between the fundamental component. Why? Because I am filtering on the AC side. So, that is the problem. So, if I the moment do AC filtering, there is going to be a see first order low pass filter. What it introduces? It introduces a lag whereas here I have put a I am filtering in the DC side. DC side. So, if you see here even if I see I will try to expand the lower lag is still there. The fundamental component is lagging. So, the fundamental component of the input voltage. So, it is better to filter on the DC side. Of course, if you want to put filter on the AC side, you can go ahead. But then you need to account for this delay and that is not a big issue with the DSP available. You can account for this delay introduced by the filter which is on the AC side. So, as of now we can or we are able to generate the reference sinusoid using a hardware method or wherein I am using a PLL as a multiplier or using a harmonic oscillator and with this block diagram I can or we are able to generate three phase pure sinusoid. Now, these sinusoids are in phase with the fundamental component of the grid voltage for power transfer. I need to shift this waveform or this waveform should lead by an angle delta. Now, this delta information should come from the so called the outer loop. What is that outer loop? The outer loop is yesterday and it did discuss outer loop is the DC voltage regulating loop. It looks something like this VDC reference and we are trying to regulate the DC link voltage. You measure the actual DC link voltage error p i delta this is delta. Why? At steady state error is 0 that can happen when the actual DC link voltage is equal to the reference that can happen only when there is a perfect power balance. Input power is equal to input power is equal to output power plus the system losses. So, output of the PR regulator is the delta is nothing but the power angle. So, in the overall interface there is a we have solar panels there is a DC to DC converter and there is an inverter, inverter feeding power to the grid and at that condition DC link voltage regulation is being done by the inverter control not the DC to DC controller. That is what I told someone ask me who is doing the voltage controller. I said when I am doing only the DC to DC controller part the duty cycle of the DC to DC converter is adjusted to control the output voltage. See I will repeat when I am doing only DC to DC converter output voltage of the DC to DC converter is regulated by varying the duty cycle. Now, the output voltage of the DC to DC converter forms an input voltage to the inverter and the power is being fed to the grid by the inverter. So, the overall DC link voltage control is being done by the inverter it just monitors or it just monitors the DC link voltage and tries to control and trying to maintain with the reference value. And what the DC to DC converter does DC to DC converter what is that is it tries to extract the maximum power assuming there is an MPPT tries to maximum power from the solar cell and dumps it to the DC link capacitor that is all. So, DC link voltage regulation is being done by the inverter not by the DC to DC converter remember that. Now, in addition if this inverter has a spare capacity it can supply Q. Now, I will take two cases one is when there is no sunlight at all in the night there is no active power transfer there will be only a reactive power transfer. So, during reactive power transfer since the inverter will draw a small amount of power from the grid and it will supply Q. Now, who will tell the inverter to supply how much Q to supply or how the inverter will know how much Q to supply if the inverter is doing the load compensation. I am telling if the inverter is doing the load compensation in other words the entire Q is consumed by the load is being supplied by the inverter the so called the war regulator or war control or war calculator will tell the inverter to supply the requisite amount of Q. Now, I will assume that information is coming to the inverter from other controller about the quantity of Q that is to be supplied how this inverter will try to supply the requisite amount of Q how it can do. So, it knows the reference value that comes from the outer controller this quantity the reference quantity is known the actual quantity is actual Q is supplied by what is the actual Q there is a source supplying load I have an inverter a three phase inverter Q all the Q has to supplied by this inverter to the load. So, the source is supplying only the active power current supplied by this in the current flowing through the inverter will determine the Q Q is given what is Q here Q is the voltage at this point multiplied by the current angle between them is 90 degrees if I assume I will assume that. So, what is the current flowing through the inductor it is nothing but V source minus V output of the inverter V in divided by X this current is the quadrature component of current in output voltage of the inverter is proportional to m into V d c if I keep V d c constant if I am trying to regular V d c constant just by changing the modulation index it controls Q it controls Q. Now, how do I change the modulation index in a sinusoidal control sinusoidal p w m technique I have to just change the model magnitude of the modulating wave. If the inverter is supplying Q to the grid how to determine the amount of Q that has to be supplied. So, that is being done by regulating the grid voltage to a reference value. So, what you will do is you will measure there is a reference value reference grid V a c reference actual grid voltage p i regulator this is Q this is Q sorry draw again the grid inverter the control loop is V a c reference V actual a c actual error p i this is Q why at steady state at steady state this is V a c reference V a c actual actual grid voltage error p i this is Q. Why it is Q that is because at steady state because of the p i regulator error has to be 0 that can happen only when actual grid voltage is equal to the reference sine wave sorry reference value at the grid. If it is more than the reference it implies that line is over compensated if it is less than the reference it is under compensated. So, therefore, output voltage output of the p i regulator is Q that Q to be supplied how the inverter changes this Q by by changing Q is nothing, but Q is proportional to m into V d c change m. Now, when the inverter is supplying both p and Q you need to ensure that you need to ensure that total overall rating of the inverter should be less than or equal to equal to the k v s supplied to the inverter sorry k v s supplied by the inverter. I will repeat if the inverter is supplying power as well as Q because when the when the sun's insulation is low power output of the solar solar panel would be low inverter has a spare capacity it can supply Q. Now, the sum of this p plus j Q should be equal to or should be less than or equal to the k v rating that you need to ensure that has to be incorporated in the control loop. So, that is about it I will take few questions on grid connection PLL feel free if you have any question. There is a question can we get the MDL file for the model that is that has been shown represent yes fine you will get it do not worry any other question PHU come with the come go ahead. Hello sir in three phase we are able to understand the A B C conversion and D Q component how the inverter is connected to the grid, but in single phase normally we do not have A B C conversion. So, what are the what kind of techniques that we can employ in single phase grid tie inverter over to you sir. Yeah single phase grid connection inverter I will discuss it do not worry I know I can understand your question I will I will see we will see I will discuss the single phase inverter feeding power to the grid I will discuss in the in the next half. And there is a very good question because single phase how do I synchronize how do I synchronize there is nothing like single phase PLL also we will see yeah Amrita Koyamathur go ahead. Good morning sir already the T N B E department having the grid they already have if the solar panel output that is directly connected to the grid existing one. You are saying output of the output of the solar solar panel directly connected to the grid. So, output of the solar panel is DC I guess there is a DC grid. No sir if we converted it if we converted it to the AC then we connect the output of the AC to the grid directly. That you need to answer ma'am in the sense I told you solar panel can be considered as a small AC power source that the source has to connected to the grid. Can I directly connect the connect that AC source to the grid you cannot you have to synchronize it you have to synchronize it. So, you have to use a PLL you have to all those things we have to do it if you are connecting inverter solar inverter to the grid grid synchronization is a must who will how does the inverter know how much power to supply. So, there has to be a controller see we have the same we have to go back to the circuit theory. Here is a source this could be solar V solar or V inverter there is nothing but V inverter this is grid this is x induct two sources are interconnected through an inductor. Now, the power transfer is V 1 V 2 divided by x into sin delta delta ok. So, to do that frequency of V 1 and V 2 should be the same. So, grid frequency continuously of course, does change. So, therefore, output voltage of the frequency of the fundamental component of the output voltage of the inverter should change and they should be in synchronism there is no difference between the circuit theory or whatever theory that we studied for machines and for inverter feeding power to the grid they are same in machines we use dark clamp method in the lab. Here we have to generate the modulating waves in a three phase inverter which are whose frequency is same as that of the grid voltage and use those modulating wave to generate the switching signals for the inverter. Same principle of operation of generator connected to the grid and inverter feeding power to the grid there is no difference at all. So, I cannot get the turn the output of the P I controller is Q it could be Q is not it in the previous case see here V D C reference V D C actual error P I it could be delta why because at steady state at steady state error has to be 0 in a PR regulator V actual V D C actual error P I delta why delta because at steady state error has to be 0 if error is when the error is 0 when V D C actual voltage is equal to reference when can that happen when there is a perfect power balance in other words inverter is supplying the required amount of power. Similarly, I have to control the or I have to regulate the grid voltage at a reference value when that can happen when the grid gets required amount of Q. If the line is under compensated voltage drops if it is over compensated voltage increases that we have studied if I am able to regulate the grid voltage at the required value. So, line is compensated fully. So, here is a control loop V A C reference V A C reference actual error P I now it could be either Q or M modulation index why output the inverter generates Q which is proportional to M into V D C. If V D C is regulated at a constant value the Q is proportional to M that is all because error has to be 0 that can happen only when the reference grid voltage is equal to the actual line voltage actual line voltage V line and reference line they are same that can happen when the line is gets the required amount of Q. So, therefore, output of the PR regulator could be either M or Q that is all some application of matrix converter show DC to AC conversion in that case you can use matrix converter for solar. No, do not get into all this some applications of matrix converter show for DC to AC conversion. Now, I have to for that I need to discuss what is the matrix converter you send me an email I will answer it. There is a question on matrix converter matrix converter nothing, but AC to AC conversion AC to AC direct AC to AC conversion this is a basic matrix converter. There are modifications people have made and they have created an so called a virtual DC link it could be possible it is possible literature says it is possible, but more on that we can communicate with each other rather than answering it in this forum. Suppose the inverter is providing real power exchange and reactive power how can we decide the value of coupling inductor. There is a good question suppose the inverter is providing real power exchange and reactive power compensation how can we decide the value of coupling inductor? There are two three issues one is the value of inductor is determined by the T H D because for a given switching frequency inverter has a fixed harmonic spectrum and based on the magnitude of the output voltage of the inverter the current the inductor will determine the current pumped in to the grid. So, X I can determine now higher the value of X higher the value of X lower will be the value of T H D because this is the equivalent circuit higher the value of X slower is the response higher the value of X slower is the response why this is V source this is V in V in minus phasor V source divided by X is the current higher the value of X slower because slower is going to be the response current cannot change instantaneously in an inductor. So, if you want to have a very fast response because see Q can change instantaneously Q of the load can change instantaneously because you have gone and put on the load which is high Q load that Q has to come from this from the inverter. So, that immediately the quadrature component of the current has to flow through this if you have a large inductor it may not allow that current to change your response deteriorates. So, the value of X depends on the how fast is your system should respond and what should be your T H D these are the two basic parameters then size cost and other things also will come. Sir the question is could you please give some reference material or books on the power grid connections which you are explaining now there is enough material may be I will try to give, but then remember there is absolutely no difference between the basic circuit theory that you have while doing synchronous machine power, synchronous machine feeding power to the grid and inverter supplying power to the grid only thing is that is rotating machine inverter is a static device that is all that is all. Now, only thing is I have to generate see I have to generate output voltage of inverter of the required frequency that I have to generate using a modulating wave in a sinusoidal P-dol-M technique or I need to use a space vector P-dol-M technique modulating wave how to generate I have discussed material may be I will try to give you do not worry what are the factors to cause loss of synchronism with the inverter with the grid what are the factors to which causes loss of synchronism of inverter with the grid I do not know I do not know what are the factors to cause loss of synchronism of the inverter with the grid in the means it see the loss of synchronism can happen when the PLL goes out of synchronism see here the loss of synchronism can happen when PLL is out of sync with the grid voltage when the PLL goes out of sync that can happen in many times suppose there is an you have designed a PLL you have designed a PLL assume that grid is perfectly balanced under that condition you do not put a notch filter a hundred notch filter in the DQ frame. Now, the grid is unbalanced what can happen now there is a DC there is a unbalanced negative sequence voltage which appears as a hundred as component in the DQ frame and the PLL may go out of sync if the PLL goes out of synchronism yes the inverter goes out of synchronism that could be that is one of the reasons there could be other reasons as well say which I have to find out. So, if the inverter if the PLL is in synchronism I do not see any other reasons of inverter going out of synchronism of course I am assuming that powers the total Q total S supplied by the inverter is less than or equal to it is rating. Sir, if the error is 0 the inverter is supplying S only when there is an error inverter supplies both P and Q what does that mean I do not know go to AVVC. Sir, previously told the error between the V actual and V reference. So, when the S is going to supply into Q. So, if it has some error it need to balance the grid supply and the inverter supply. So, it is going to compensate that voltage level by its own Q or it requires any special kind of attention. This is a grid that voltage has to be regulated how do you regulate that voltage why this voltage is falling or why the voltage is increasing voltage at this point can increase or decrease only based on Q here is compensation. Now you are trying to regulate when this line voltage is actual voltage is equal to the reference when it requires when it receives the required amount of Q whose supply is Q inverter I am assuming that inverter has the infinite capacity to supply this Q. If the required Q is higher than the rating yes you will not be able to regulate. Coimbatore if the KV rating of the inverter is less than Q required by the line then it will not be able to regulate the grid voltage. If the inverter rating is higher than the Q required by the line it can supply the Q demand and therefore, trying to maintain the voltage. Sir, how does a DC to DC converter help in maximum power transfer sir? DC to DC converter DC to DC converter what is the control variable there duty cycle how do you choose the duty cycle how will you choose the duty cycle you choose the duty cycle such that maximum power is being drawn from the solar cell that is all as the duty cycle changes current drawn from this also will change is not it. So, therefore, I can change the operating point in fact, D is determined by output of the MPPT output of the MPPT will determine D duty cycle of for this DC to DC a last question I will take can I do for angle is already time. Sir, what will be the effect of the transformer connected to the inverter sir? I think we will be connecting a transformer also after the inverter we will connect a transformer after the inverter and connected to the grid. So, what will be the effect of the transformer on the performance? Yeah, I do not there are see a question I will answer it there are in the in my next lecture there are inverter configuration with transformer and there are inverter configuration without transformer depending upon the output voltage of the inverter and the grid voltage. Yes if the if for high power application transformer is a mass because you need to isolate isolate high from the grid your inverter from the grid you need to have an inverter you need to have a transformer that transformer is nothing but a 50 hertz transformer it is a 50 hertz conventional transformer. So, it is a not a mass that all the topologies will have a transformer there are topologies where there is no transformer we will discuss it now. Sir, one last one questions for HVDC transmission can we directly connect this inverter and transmit using HVDC no I am asking sir inverters suppose using solar can we directly transmit it to for long distance long distance just like a HVDC how we do it sir? No I got your question in the sense yeah I think may be on my last lecture that is why I said different school of thought in my introductory lecture I told you that the conventional school of thought is solar panel DC to DC, DC to AC grid can we have a different school of thought that is what I had told you. Let us see can we have a DC grid and distribute it that is what I have told you we will discuss tomorrow by HVDC first let us talk about low voltage DC then we will go to HVDC I have almost finished my grid connection.