 introduction to current source. This is a voltage source inverter. See the devices and see the filter, LCL, L C filter and here is the grid. This is the DC link voltage. Maximum power point tracker trying to operate somewhere in this point and I use pulse width modulation has large number of pulses per cycle voltage waveform and I get a fundamental component. This is a fundamental component. If I can have at the input whose voltage is being maintained by MPPT, I can take the current reference also. See here delta V is the variation to operate the solar panels in the maximum power point and the corresponding current is delta high. Now, if I choose delta high as a reference, delta V could I can choose instead of taking delta V, I can choose this current as well. When I am taking voltage as the reference or in a voltage source inverter, PV cell can be represented in this fashion. PV cell in parallel with the capacitor. Now, since I am taking current as the reference, what will happen here is something like this, an inductor. I will take this as the DC link voltage, DC link current. This is a point corresponds to this much of current I need to draw so that the solar panels are operated in maximum power point. So, this is a current source and see the devices now. Direction of this current because of this inductor current cannot change instantaneously. So, current is unidirectional. Voltage has to be bi-directional now. At this point voltage could be negative. So, these devices should be able to handle the negative voltage. See the devices now. Of course, these devices are, these diodes are not required. Since there is a built in diode that are there in IGBT, I am using the existing devices and I am connecting a diode to bypass these diodes. See the direction of the current direction. So, the current cannot flow in this fashion and now see the filters configuration. It is C and L. C and L. I cannot have, I cannot connect inductor in series with the current source. Remember, now I am doing switching here. So, current will have a large number of pulses and that will have a fundamental component. Voltage source inverter, at any given time, one device is on in each leg whereas, any one of the switches here and one of the switches in the bottom up. At any given time, only two devices are on during powering mode. During S1, S2, S3, upper switches, S4, S5, S6 are lower switch devices. At any instant, at least one switch from upper group and one switch from lower group should conduct. This is the powering mode. Now, when I am transferring the current from one device to another device, what do I need to do? Assume that S1 and S5 are on or say S1, sorry S1 and S6 are on. I want to turn on now S2. Since I have a current source here, this current has to be continuous, which we did not bother in voltage source because there are diodes there. This current has to be continuous. So, what I will do? I will turn on S2 as well or what we will do? In the sense S6, S1, S6 were on. Now, I want to turn off S1 and turn on S2. What will you do? What should we do? Current has to be continuous. You have to maintain the same current in the DC link. You turn on both the devices. As this device turns on, open the outgoing switch. You have to provide the path for the inductor current that you need to ensure. In the powering mode, overlap period during switch, overlap period is during switch transition. So, this is the switch transition is also known as the energy storage mode. It could be, this is the space vector representation of the current source. What is similar to the voltage source? Approximately, this looks approximately the same. These are the vectors that we can get. And there I said T1 and T2. Here, I am saying delta 1 and delta 2. It is okay. Do not try. It is not very difficult. You can go back and draw this figure. That is not a problem. This is how do I use the space vector modulation? See here, S1 and S5. This is the powering mode. This could be the zero voltage. This is the energy storage mode. So, if I do that, see what will happen is here, the DC link current is you are providing a path for the DC link current here by turning on S1 and S4, a short in the DC link. But then, since it is a current source, there are no issues. So, the same thing that I have explained here. Current control mode grid connection. This is what it is. Reference current, actual current, reference current, actual current, P i. Now, this is the reference power component. Reference power component iq corresponds. If you want to feed the power, if you want to feed the reactive power to the grid, have some q component and rest of the philosophy is the same. Rest of the philosophy is the same. It is not different. Here, there is no grid connection, but you want to have some reference voltage as well. Voltage control mode in stand alone. So, you want to feed the power at some particular value. Now, at what is the voltage at that point? V a, V b, V c are the voltages that you want to generate or that is a voltage that has to be maintained and this is the control loop almost the same. What are the advantages? I do not need to have a separate DC to DC converter. The additional DC to DC converter that is required in voltage source is eliminated here. DC to boost not required. Compact size, electrolytic capacitor is absent. By the way, electrolytic capacitor is determined by the ripple current that it has to supply, not the voltage ripple. It is the ripple current that has to supply. So, if I see the voltage ripple, if I calculate the capacitor side based on the voltage ripple, size may be very small, but then because of the current ripples, you may have to use a higher value of the capacitance and as it is if the inverter is mounted outdoor, life of the inverter, life of the capacitor reduces. So, one of the supposedly one of the weakest link in the inverter. So, we have replaced it by an inductor. Component count is the same. Actually, this diode instead of connecting here, I need to connect here. It is something like this reverse. It is if I have a reverse blocking IGBT, I do not need to have or if I have a device which can block voltages in both the directions. Yes, then I do not need to use this diode. It is GTO, conventional GTO, not the anode short GTO can block the voltage in both the directions, but then it is a low frequency device and this improved reliability. Why improved reliability? I have an inductor, not a capacitor there. Efficiency is an issue here. Efficiency is slightly lower here. Protection against open circuit in the DC link inductor. You need to ensure that DC link current has to be continuous, protection against phase to phase short circuit. In the sense, see here, if there is a phase to phase short circuit here, it is not going to be a powering mode. It is equaling to energy storage mode. So, you need to protect or you need to sense the phase to phase short as well and this is a common mode currents. There is something similar there. That is about the current source.