How is a line commutated inverter different from a grid-following inverter?

The U.S. Department of Energy provides this definition of a line commutated inverter:

An inverter that is tied to a power grid or line. The commutation of power (conversion from direct current to alternating current) is controlled by the power line, so that, if there is a failure in the power grid, the photovoltaic system cannot feed power into the line.

In my mind, a commutator is a mechanical device for converting ac to dc using a rotor -- the power electronics corollary would be a rectifier, which is the opposite of an inverter.

The DOE definition goes the opposite direction (dc to ac) for commutation, suggesting that a synonym for "line commutated inverter" would be "line inverted inverter."

However, all of this to me sounds just like a grid following inverter, that requires an ac grid connection in order to convert dc to ac. Do these two terms mean the same thing, or is there more nuance that I'm missing? Why the use of "commutated" in this odd way?

• The commutator in a DC motor converts DC to AC by periodically reversing the direction of current in the armature coil as it rotates, so its use is consistent. You wouldn't really describe an inverter as "inverting" power. Commented Jul 28, 2021 at 16:40
• @Finbarr that makes sense, but the use of the term "commutated" (typically used for motors) when applied to an inverter (power electronics) seems inconsistent, no? Commented Jul 28, 2021 at 16:45
• Commutation can also refer to an exchange or trade, which is what the inverter is doing. Commented Jul 28, 2021 at 16:47

This use of the term "line commutated" does seem a bit odd in the context of prior use in power electronics. "Line commutated" has been used to distinguish the use of the reversal of the AC line voltage and current to commutate a thyristor rather than using a forced commutation method. Commutation "controlled by the power line" seems more like commutation synchronized with the power line rather than using the power line to reverse bias the switching device to turn it off.

A grid-following inverter is synchronized with the grid and accepting the grid as the voltage regulator. A grid-forming inverter would attempt to regulate the grid voltage expecting other sources to accept that.

The following is an example of the use of the term "line-commutated" from Bimal K. Bose, Power Electronics and Motor Drives Advances and Trends, Copyright © 2006, Elsevier Inc:

Thyristor converters are characterized by line (or natural), load, or forced commutation. Line-commutated converters are used extensively in utility systems, and these will be discussed in this chapter. Force-commutated thyristor converters that require auxiliary transient circuits are practically obsolete. Converters that use devices such as power MOSFETs, GTOs, IGBTs, and IGCTs are characterized by self-commutation.

Another example comes from Philip T. Krein, Elements of Power Electronics, Copyright © 1998, Oxford University Press (emphasis in original):

The term commutation refers to the general process of changing the current flow path from once circuit configuration to another. Diodes and SCRs exhibit what is termed line commutation, in which ac sources and line parameters such as inductance define the way in which current switches among paths. Line commutation typically means that a natural "make before break" action occurs in which a new current path is established before the initial path is cut off. Transistors and GTOs [gate turn-off SCRs] exhibit forced commutation, in which the switch can be turned off actively even before another path has been established.

• Would you suggest an alternate definition for "line commutated inverter" then? And is this different from a grid-following inverter? Commented Jul 28, 2021 at 19:55
• I would say that "grid tied inverter" or "grid following inverter" would be better terms. However a grid tied inverter could be either a grid forming inverter or a grid following inverter, so the more specific terms should probably be used in most contexts. A grid following inverter could be either line commutated or self commutated and synchronized. See also addition to my answer.
– user80875
Commented Jul 29, 2021 at 0:26

Commutation implies switching polarity of current. It can be a switch with DC or a sine wave controlled by PWM or a Hall sensor to detect rotor magnetic position.

Commutation is a generalized term to indicate current polarity must controlled without saying how it is done.

Line Commutation implies the Line supplies the phase and frequency to follow as opposed to an islanded inverter..

Power must be sinusoidal and in sync with the grid supplied by current regulation.

SO yes it is a GTI , the commercial name. Tied has other industrial implications which I guess they wanted to avoid.

Grid Following or Tied are called GTI’s and supply current in phase with voltage such that p.f. =>99%. With <1 % harmonics desireable and there is a mandatory limit. There are also stringent lightning protections. Phase lock loop and active PFC are necessary.

Std. Inverters are isolated or “island” inverters and far less stringent requirements to track the grid voltage variations with load and frequency.

Consumer grade invertors can be pseudo-sine, which could create havoc with GTI’s with grid harmonics in causing circulating currents from the GTI to compensate for the noisy grid. I have heard of this happening from Duke Power consultant on Huawei GTI’s which are exceptionally good. When competitor GTI’s went online, they had to raise the fuse current ratings.

• But what about "line commutated inverter"? Commented Jul 28, 2021 at 16:44
• That”s Ok but “ line inverted inverter." is ambiguous because although flow of current is inverted , the voltage is in phase. Commented Jul 28, 2021 at 16:51