Grid-Tied Power Electronics. So far, we've spent a lot of time in DC-DC conversion. Now, it's time to move to AC-DC or DC-AC conversion, and that's the situation we have when we are interfacing to the grid. We will be talking about low-harmonic rectifiers, and finally in the last segment, I'll be talking about low-harmonic inverters. What do we mean by this, really? To get started, I give you here a block diagram of a complete off-line power supply system. On the input side, we are receiving power from the grid, which can be in a single-phase form or a three-phase form. Most of the time during the class, we're going to spend on single-phase cases. Let's suppose we have a single-phase AC line input, and ultimately, the entire system is suppose to deliver tightly regulated DC power to a variety of loads. Many electronic systems will have many different loads with many different requirements in terms of voltage regulation, current regulation and so on. Those would be called the loads with point of load converters performing those tight regulations. But where are point of load converters receiving their power from? Well, typically, they are receiving their power from one or a sequence of DC-DC converters that ultimately supplied from a front-end converter that performs rectification function. We would take the AC input, and let's say, again, in the single-phase form, we will rectify that into a DC bus voltage at the output of the rectifier stage, and from the DC bus voltage then we would have an array of DC-DC converters to supply individual loads. That's a typical off-line power system. What we do now in this part of the class is really focus on this front-end interface. How do we actually get power from the AC line input as opposed to a DC input? How do we rectify that into a DC bus voltage? What are the converters used for that purpose? What are the control loops involved in that function? To get started, I want to bring up here one issue immediately that we will be concerned about. When you're designing these days, systems that are supposed to operate from an AC line, you almost invariably design that for what is called universal input. You would like to be able to plug that into anywhere in the world, and that worldwide operation means that the root mean square value of the input voltage can be anything between a very wide range. Universal input typically implies the capability of operating with AC line voltage with RMS value for a single-phase case, anywhere between as low as 85 volts and as high as 260 volts. That includes not just different nominal voltages across the world, it also includes relatively wide tolerances the power systems allow on the AC line RMS values. For any given voltage, and on top of that, any frequency, 50 hertz or 60 hertz, the rectifier stage must be able to successfully consume, take power from the AC grid, and deliver that power in DC form to the rest of the system. When we say successfully take the power from the AC grid, the picture that we are typically looking at is going to have this form right here. If this is our v_ac of t, although there's going to be some distortion, for a moment, assume that's essentially sinusoidal voltage, the best possible case of taking the i_ac current from that v_ac voltage is to follow the voltage in shape, and basically, take the current from the grid also in a sinusoidal manner with no harmonics present in that current. Going from here to here, you see that we have an ideal situation where i_ac of t is equal to v_ac of t over some equivalent emulated resistance. The input port of this power electronic system that we plug in to the AC grid, should look like a resistor. You can say, "Well, that's probably the simplest possible case." It's not. This is actually not so easy to do. Rectifier that actually behaves as a resistive load to the AC grid is what does require attention. That's what we're going to be doing in class. We're going to figure out exactly how to control a switching converter to present a resistive input port with respect to the AC grid. You may ask, well, why is this ideal? Why do we actually want the input port to look resistive here? When the load is presenting resistive load to the AC grid, that means that we're actually consuming power from the AC grid with what is called a unity power factor. This is the most effective, and I will say efficient and most effective way of utilizing AC power is with unity power factor, or interpreted in terms of the harmonics of the AC current, we are going to be talking about the low-harmonic rectification, meaning that our current should really be sinusoidal given a sinusoidal input voltage and should also be in phase with that sinusoidal input voltage. We generally refer to rectifiers and inverters tied to the grid as being low-harmonic components, precisely for the reason that we're outlining in the waveform right here. We want to have essentially just a fundamental current present and what is taken from the grid or delivered to the grid. We will have opportunities to discuss converter design, and control loop design, and a lot of interesting stuff actually there. One point that I want to bring up immediately is when you see that the AC current is supposed to be following the AC voltage, what type of control technique comes to mind? Yeah, we want the input current or the converter to pay a certain shape. How do we do that? Current control, and actually in most cases, average current mode control. The control variable of interest is going to be the current. There are other techniques to do that, but average current mode control is going to be one mainstream, most commonly used technique for achieving low-harmonic rectification or low-harmonic inversion.
