Active Front-end or Grid Tie Inverter?
We have the habit here at Oztek of using the term "active front-end" (AFE) and grid tie inverter (GTI) interchangeably. On more than one occasion discussing our grid tie inverter controller, I’ve caught myself somewhere mid-conversation inadvertently switch to using the term active front-end. It's of no concern internally, but quite frequently its cause for confusion when speaking with customers. Earlier this week I received a call from a customer that was pretty certain an AFE would not work in his GTI application. This got me thinking, "What exactly is the difference between an active front-end and a grid tie inverter?"
At Oztek, we loosely define AFEs as inverters that develop a regulated DC link, and GTI’s as inverters that transfer power from a DC link to an AC line, but this is really only a generalization. I see the real difference being the application, not the inverter itself. The application may dictate agency approval requirements or current harmonic requirements. It may dictate how power is allowed to flow, or how the system is to respond under certain fault conditions, but even with these and other application differences, nowhere will you find a clear, differentiating factor in the inverter itself.
Both GTIs and AFEs (also known as active rectifiers) actively convert between AC and DC power. Common topologies are fully capable of transferring power in both directions. Limiting power flow to one direction, as is typically required for grid tie inverter applications, is ensured through control, and is not an inherent inverter characteristic. Both GTIs and AFEs must present a high impedance to the AC line, since the voltage is determined by other sources. This is the key difference from voltage source inverters, which are designed to have a low source impedance for driving AC loads. While not the only implementation method, we employ current loop control to provide a suitably high AC impedance at the line frequency. A voltage source inverter, on the other hand, will employ voltage feedback. This affects not only the control loop, but also the output filter design.
A simple Active Rectifier definition is “a non-isolated AC-DC converter with two key benefits over passive rectifier systems; output voltage regulation, and AC input harmonic reduction.” The term “Active Front-end” describes the same thing, while eliminating the term “rectifier”, which I think incorrectly implies a unidirectional converter. Rather than “output voltage”, I like to use the term “DC link voltage”, since as we’ve noted, the converter is inherently bi-directional, and thus the DC side can be the output or input. You can see where this is going. Call it what you like, an active front-end or active rectifier has the ability to transfer power from the DC link to the AC line, which is essentilly the same function that a grid tie inverter performs.
It seems then, that DC link regulation is really the key differentiating factor. Grid tie inverters don’t need to regulate their DC link. Rather, their function is to simply transfer power from the DC link to the AC line, but is that always the case? Feeding a grid tie inverter directly from a solar panel output may be a typical configuration, but it's not the only one. Often better system efficiency can be achieved by regulating the source with a DC-DC converter (think of the variability in a solar panel or wind turbine output), allowing the inverter to operate under much more tightly defined conditions. A common implementation is to configure the DC-DC converter to regulate its output voltage. This still has the grid tie inverter operating from a voltage source, but this is not always the best option.
We must also consider maximum power point tracking (MPPT) for many systems, and with a two-stage system, this is often best implimented in the DC-DC converter. In the purest sense, this leads to a DC-DC converter with a varying current output, not output voltage, but we can’t leave the DC link voltage uncontrolled. One solution is to add an additional voltage regulation loop, but by itself, this interferes with MPPT. An alternative solution is to put DC link voltage regulation back into the grid tie inverter, turning it into an AFE from the nomenclature point of view. In effect, this results in the grid tie inverter outputting as much power to the AC line as the DC-DC converter will support – exactly the goal of the system. And the difference between a GTI and an AFE in this case? Almost nothing.