I think the basic difference between the two converters are:

1) The task of Grid forming converter is to maintain the voltage and frequency of the microgrid. It is essentially like a slack bus in power system.

2) The task of Grid supporting converter is to simply dispatch some set active and reactive power or dispatch based on some command signal. So, it is more like the PQ bus in power system.

I am not an expert in microgrid controller but, I think a converter can have both controllers and can easily select the control it desires based on some algorithm. Please also see the attached paper and sorry I cannot answer your question on power quality.

Samundra Gurung It is a good paper. I have read it. Thank you for your explanation.

Grid forming converters can be represented as an ideal AC voltage source with a low output impedance. As voltage sources present a low output impedance so, they need an extremely accurate synchronization system to operate in parallel with other grid forming converters. e.g. UPS

AC voltage generator by the grid forming power converter will be used as a reference, for the rest of the grid feeding power converter connected to it. It is very much suitable for islanded mode of operations.

Grid supporting converter can be represented either as an ideal AC control current source, in parallel with the shunt impedance or as an ideal AC voltage source in series with a link impedance. Its main aim is to participate in regulating the amplitude and frequency of the AC grid voltage by controlling the active and reactive power supplied to the grid.

Ravinder Kumar Thank you for your time. If grid supporting converter is being operated as an ideal voltage source then can we call it grid forming as well ? As it is supporting the grid voltage and frequency through droop control. Similarly we can call it grid feeding converter if operated the former in AC current source mode.

I think your understanding is right as long as the converter has the capability to transition from one state to another.

Samundra Gurung Thank you.

See this you will get better idea Control of Power Converters in AC Microgrids

Thank you Zeeshan Akhtar I have read this.

A grid forming control can be technically converted to a grid supporting control by adding a droop control interface and virtual impedance. The essence of this is to allow the parallel operation of several inverters in parallel, performing the same frequency and voltage regulations simultaneously based on their respective capacities and droop gains. Each inverter in the grid supporting arrangement emulates a typical voltage source, and they are connected to the rest of the grid using a link impedance similar to a typical synchronous generator. However, this link impedance can be emulated in control. The virtual impedance helps solve the drawback of the conventional droop control

Elutunji Buraimoh Thank you for the informative insight. I really appreciate it. I have a question. You wrote that, "A grid forming control can be technically converted to a grid supporting control by adding a droop control interface and virtual impedance".

However, i do not agree with this statement. Because grid forming converter is itself based on droop control while the difference between grid forming and grid supporting converter is that the former one act as a grid while the later supports the grid by following it. The virtual impedance is used to regulate the voltage of converter in order to coup with high resistance lines.

Correct me if i am wrong.

Shah Fahad . Thanks for the observation.

I am talking from the implementation point of view. In my unpublished works, I have carefully implemented a grid forming control, grid feeding, and grid supporting controls (both voltage source and current source). Please find attached the diagrams. It can only be used for islanded operation in the absence of any energized grid, and grid forming inverter will never work in parallel with any other inverter, not even a grid feeding. Only the voltage magnitude and frequency references are specified for the grid forming control. And the implementation only allows for a single inverter in an autonomous mode.

But let me make this clarification: please note that there are two types of grid supporting systems. Grid supporting system could behave like a voltage source (Grid supporting-Grid forming) or a current source (Grid supporting- Grid feeding). The grid supporting system that I explained in my previous comment is the grid supporting operating as a voltage source (grid supporting-grid forming) because the discussion is within the purview of a grid forming system.

In summary,

A grid feeding control is used for a single inverter to deliver power to an already energized (main) grid at a predefined power (active and reactive) references

A grid forming control is used for a single inverter to set the voltage amplitude and frequency in an islanded microgrid. Please note that it does not operate in parallel with any inverter and doesn’t share the voltage and frequency regulation responsibility with any inverter.

A grid supporting operating as a current source (grid supporting –grid feeding) has the main objective of delivering power (active and reactive). This is technically an improved grid feeding control that uses the droop control to contribute to the main frequency and voltage regulation and may operate in parallel with other “like-minded” inverters

A grid supporting operating as a voltage source (grid supporting-grid forming) doest operate synchronously with a traditional grid system. It operates within a large autonomous microgrid just as a grid forming controlled inverter. In this case, it is called grid supporting, not because it is connected to a traditional main grid. It is called grid supporting because it operates in parallel with “like-minded” inverters with the same grid supporting control strategy. With the help of the droop, these inverters are able to collectively regulate the frequency and voltage amplitude of the autonomous microgrid where they operate. In fact, I am currently working on an autonomous inverter-based microgrid and I applied grid supporting controls for all the constituent inverters. This is the more reason why contributed to the discussion.

Thanks for your time Shah Fahad

Dear Elutunji Buraimoh ,

Thank you so much for the detailed answer. I really appreciate it. However, i may not agree with the last paragraph in which you claimed that, " A grid supporting operating as a voltage source (grid supporting-grid forming) doest operate synchronously with a traditional grid system". I do not agree with this statement as i have worked on grid supporting converter working synchronously with the grid using coordination strategy. The paper is in last stage, if accepted, i will share it with you.

The inverter is connected at a bus in the network. The grid is "seen" by inverter at that bus, and this perception is in the form of voltage and frequency. Question is, who is responsible for maintaining that voltage/frequency? Whether V/f will still be maintained if inverter under consideration is disconnected? Every type of inverter injects power into the bus it's connected, however the key deciding factor of its role as grid forming/feeding/supporting is maintaining V/f.

Technically, if inverter control structure directly incorporates voltage tracker/regulator and its frequency is auto-generated (not sensed from bus), then it's a grid forming inverter.

On contrary, if voltage tracker is not employed and frequency reference is obtained from sensed/measured/estimated frequency locked to the corresponding bus by mechanism like PLL or FLL, then it's grid following.

Grid support is an "additional" feature, a grid supporting inverter can be of both types: grid forming or feeding. If the references/set-points of inverter (set-points includes voltage, frequency, active/reactive power, virtual flux, virtual torque, injected currents etc.) are adjusted based on "additional" inputs from information of other buses/loads in the network, for serving extra ancillary features (apart from local v/f regulation and power injection), then the inverter is said to "support the grid". The additional support include: power quality enhancement, stability support, reserve provision, event ride-through, economic dispatch etc.

See below examples:

1) Single inverter feeding standalone network: It is alone responsible for producing and maintaining V/f at all buses in network, thus "Grid Forming". If it adjusts its V/f setpoints for other purposes (like brown-out, current limiting for overloads/faults, improving voltage profile of a remote bus, loss reduction etc.), then it is also "Grid Supporting", it is supporting the grid formed by itself! Otherwise, if V/f are regulated at constant values, it is not grid supporting.

2) Parallel connected droop controlled inverters: All inverters contribute or participate in process of grid formation by means of droop coefficients. They serve additional purpose of power sharing, hence they are both grid forming and grid supporting.

3) PV inverters always operated at MPP: Irrespective of V/f of the bus it's connected, it only injects available peak power into grid, hence only grid feeding.

4) PV inverters with derated MPP: They inject certain fraction of available peak power to grid, and power reference is adjusted as per reserve requirement of the grid. Hence, it's an example of both grid feeding and grid supporting.

Pankaj Dilip Achlerkar

Perfectly explained. Thank you so much for your feedback.

A new perspective of coordination of grid supporting converters is presented in:

Article Demand Management of Active Distribution Network Using Coord...

@ Shah Fahad Pankaj Dilip Achlerkar Elutunji Buraimoh

Thank you all for this discussion, help me a lot to understand about Power converters in Microgrid. However I have a doubt about the controlling method of grid-forming and grid feeding converters in DC microgrid. Here the power converters are assumed to be DC/DC type.

Can you all please tell me what kinds of control methods are being used for controlling grid-forming and grid-feeding converters in DC microgrid?

Dear Elutunji Buraimoh thank you very much for your kind and detailed response. I have a query that I would like you to clarify if possible:

- Concerning Grid forming converters, you state as follows:

"A grid forming control is used for a single inverter to set the voltage amplitude and frequency in an islanded microgrid. Please note that it does not operate in parallel with any inverter and doesn’t share the voltage and frequency regulation responsibility with any inverter. "

I do not know if this means that in your opinion we could not operate an islanded grid with a grid forming converter regulating voltage and frequency and one or several grid following converters delivering power.

Furthermore, could said grid following converters be actually droop-controlled, grid supporting units?

Thanks in advance and BR!

Grid forming converter operates in islanded mode only and cannot work in parallel operation with other converters. However, grid supporting converters can operate in both grid-connected mode and islanded mode. Moreover, this converter can operate in parallel operation with other converters.

The following link contains all types of converters in AC microgrid:

Thank you Beshoy Abdou Aziz.

"Grid forming converter operates in islanded mode only and cannot work in parallel operation with other converters" ........True/False.

Please read Chapter 5 (section 5.5 ) of "Microgrids Design and Implementation" by Antonio Carlos Zambroni de Souza  Miguel Castilla Editors https://doi.org/10.1007/978-3-319-98687-6

Shamsher Ansari Indeed you are right. Thank you for sharing the reference. I will surely look into it.

The attached reference, co-authored by Prof. Tim Green, may help.

Nabil Mohammed Thank you for adding an important answer to this question.

This link will take you to a similar ResearchGate discussion thread question:

https://www.researchgate.net/post/What_is_your_scientific_explanation_of_why_the_sun_is_becoming_stormier_and_how_does_this_affect_a_nations_power_grid

A grid forming inverter generate an AC signal (voltage) at its output load which is conventionally a resistive one. The control task in this system is to track a time periodic sinusoidal reference voltage while a grid following inverter is connected to the main grid and the main task is to control the current injected into or drawn from the grid.

Thank you Prof. Abdelali El Aroudi for giving us your insight.

Grid forming regulates its output voltage and frequency usually standalone inverter. On the other hand, grid spurting inverter support the grid with active and reactive power (grid connected inverter).

I would like to add the following paper to this thread.

https://ieeexplore.ieee.org/abstract/document/9079647

You can find a complete explanation of each mode in "Chapter 18 - AC and DC Microgrid Control" of the book "Control of Power Electronic Converters and Systems: Volume 2".

https://www.elsevier.com/books/control-of-power-electronic-converters-and-systems/blaabjerg/978-0-12-816136-4

Naser Padar Thank you for sharing this information.

Another addition to this thread.

Article A coordination control strategy for power quality enhancemen...

• Grid-forming and grid-supporting inverters are two types of inverters used in power systems to connect renewable energy sources or energy storage systems to the grid. The main technical difference between them is their ability to control the voltage and frequency of the grid.
• Grid-forming inverters are capable of generating the voltage and frequency of the grid, which means that they can operate in stand-alone mode without any external voltage or frequency reference. They are typically used in microgrids or in areas with weak or no grid connection, where they can form a stable and reliable grid. Grid-forming inverters provide active control of the voltage and frequency of the grid and can supply reactive power to the grid, which helps to stabilize the system.
• Grid-supporting inverters, on the other hand, require an external voltage and frequency reference to operate. They are typically used in grid-connected systems, where they are required to synchronize with the grid and follow the grid voltage and frequency. Grid-supporting inverters provide a passive control of the voltage and frequency of the grid and can supply or absorb reactive power to regulate the voltage level.
• In summary, the main technical difference between grid-forming and grid-supporting inverters is their ability to control the voltage and frequency of the grid. Grid-forming inverters can generate the voltage and frequency of the grid while grid-supporting inverters require an external voltage and frequency reference to synchronize with the grid.

Rana Hamza Shakil Thank you so much for the detailed response.

I have one concern though.

You mentioned that in islanded mode, the grid forming inverter does not require any reference. So in islanded mode, what reference does the grid forming converter follow ? There should be a reference frequency and voltage to follow such as w_n or V_n.

Shah Fahad In my opinion, you can follow this reason to clear your understanding regarding the mentioned concern.

• In islanded mode, the grid-forming inverter operates independently without being connected to the grid. In this mode, the inverter generates a voltage and frequency reference based on the load demand and the operating conditions of the microgrid. The inverter does not require any external reference from the grid, since it is responsible for regulating its own voltage and frequency to maintain the desired power balance in the microgrid.
• Typically, the grid-forming inverter in islanded mode generates a frequency reference based on the rotational speed of the generator, which is proportional to the frequency of the AC power output. The inverter can use feedback control to regulate the frequency of the output voltage based on the deviation of the actual frequency from the desired frequency reference.
• Similarly, the inverter can regulate the output voltage magnitude by using feedback control based on the deviation of the actual voltage from the desired voltage reference. The reference voltage can be set based on the load demand and the operating conditions of the microgrid.
• Therefore, in islanded mode, the grid-forming inverter generates its own voltage and frequency references based on the microgrid requirements and uses feedback control to regulate its output voltage and frequency to maintain the desired power balance.

Rana Hamza Shakil

Thank you for your detailed response. However, to simplify this discussion,

let's suppose we have this equation for a VSG controlled grid forming converter:

J*dw/dt = Pm - Pe - D(w_n - w)

In islanded mode, I believe the term w_n is the reference chosen as 2*pi*f=314 (for a 50Hz system). However, according to your explanation, there is no external reference required for this VSG in islanded mode.

Where do you think this reference should come from?

Shah Fahad

In islanded mode, the frequency of the system is not constant and can vary depending on the load and generation balance. The VSG controller uses the feedback signals from the grid voltage and frequency to adjust the converter's output and maintain a stable grid frequency.

So, to answer your question, the reference frequency for the VSG-controlled grid forming converter in islanded mode is not a fixed value that comes from an external source. Instead, it is dynamically adjusted by the VSG controller based on the measured frequency of the system.

Dear Rana Hamza Shakil I apologise for getting into your discussion, however, I wonder if your statement is correct how is supposed that all home appliances will operate without a threshold (reference frequency as mentioned by Shah Fahad ) I believe the set point of the standalone microgrid must be set and committed because the gadgets/home-appliances will not work properly and as consequences there will appear failures maybe not in microgrid but in the houses since the PCC is not steady or even steady it does not meet the frequency requested by the manufacturer (50 or 60 Hz).

Dear Tony Castillo-Calzadilla Thank you for your question.

You are correct that most home appliances and electronic devices are designed to operate within a specific frequency range, typically 50Hz or 60Hz. However, in a standalone microgrid, the frequency is not necessarily fixed at a specific value but rather allowed to fluctuate within a certain range. This is because in a standalone microgrid, the generation and consumption of power are balanced locally, and the frequency is allowed to vary within a certain range to maintain this balance.

While it is true that some appliances and electronic devices may not operate optimally or may even fail if the frequency deviates significantly from the nominal value, most modern electronic devices are designed to tolerate some degree of frequency variation. In addition, some types of devices, such as those with variable speed drives, can actually benefit from frequency variations, as they can adjust their operation to optimize energy efficiency.

That being said, you are correct that it is important to set and maintain a reference frequency within the acceptable range for the appliances and electronic devices in a standalone microgrid. This reference frequency can be set and controlled by various means, such as using a frequency controller, a droop controller, or an adaptive controller. The specific control scheme used will depend on the characteristics of the microgrid and the desired operating parameters.

Dear Rana Hamza Shakil , thank you for taking the time to reply to my concerns. To add to your exciting analysis, I fully agree that some modern domestic appliances can operate within this range of frequency variation - this range can be set arbitrarily. Moreover, it's worth pointing out that this lack of regulation may be allowed for domestic or regular cases. Still, it's not allowed if your off-grid system is powering a sanitary installation such as a hospital due to the high impact that a minimal deviation in frequency has on the hospital's pieces of equipment, even if it may sound an extreme situation, there are plenty of places all over the world that lack a connection to the utility grid and are forced to connect small ambulatories to standalone systems.

I take advantage to share with you a survey paper that deals with some of the issues dealt with on this site but with a social perspective feel free to share and comment on it with me:

https://www.sciencedirect.com/science/article/pii/S2352484722017917?via%3Dihub.

Rana Hamza Shakil

Thank you for sharing your analysis. However, about islanded mode VSG being operated without a reference point, yes the VSG operates in V/f mode and tries to balance the power imbalance. In this scenario, the frequency is not exactly a constant value. Up to this point, we can agree.

However, imagine a synchronous generator being operated without dampers, it would destabilize the generator and the power imbalance feature would not be enough. Same is the case with virtual damping component of the swing equation in VSG control in islanded mode.

I have read your theoretical analysis and If you could comment on the equation itself, then your argument might be justified. For instance, what happens to w_n in D*(w_n - w) in the swing equation in islanded mode.

Please comment on the equation.

Thank you.