Definition Problems…

The Australian Renewable Energy Agency (ARENA) released its large-scale battery energy storage system report this week which is available here: https://arena.gov.au/assets/2019/09/large-scale-battery-storage-knowledge-sharing-report.pdf

It provides a summary of ARENA funded work in the area of energy storage systems across Australia, specifically the following 4, to a total funding of $100M.

And although the learnings are very interesting at a first reading, I cannot go past some terminology issues that I find quite unclear and will likely create further confusion in the industry.

LSBS instead of BESS or large-scale BESS

This is more of a personal pet peeve but the literature prefers the terms ESS for energy storage systems and BESS for battery energy storage systems. So the use if the term LSBS (Large-Scale Battery Storage) to refer to large-scale BESS (those >5MW due to AEMO registration purposes) makes me irrationally angry every time I come across it.

Large-Scale Battery Storage is where you store your big batteries, not a large battery system that stores energy!!!

In fact, here is the search for Large-Scale Battery Storage in Google Scholar:

Here is one for Battery Energy Storage Systems:

And here is a shameless plug of our article from 2016 on Power  Electronics for BESS:

Wang, G., Konstantinou, G., Townsend, C. D., Pou, J., Vazquez, S., Demetriades, G. D., & Agelidis, V. G. (2016). A review of power electronics for grid connection of utility-scale battery energy storage systems. IEEE Transactions on Sustainable Energy, 7(4), 1778-1790.

https://ieeexplore.ieee.org/abstract/document/7506096

Freely available here: https://www.researchgate.net/publication/305037268_A_Review_of_Power_Electronics_for_Grid_Connection_of_Utility-Scale_Battery_Energy_Storage_Systems

Grid-forming inverters

On a serious note, here are some instances where the report mentions the term “grid forming” in relation to inverters:

1. “The ESCRI-SA project is also the first LSBS project in Australia to operate in voltage source mode as a virtual synchronous generator while grid connected (this capability is sometimes known as grid-forming).”
2. “The project provides seamless islanding of the local network in the event of a network outage. In this case, the LSBS system provides the grid forming functionality and has primary responsibility for maintaining the system voltage and frequency within required limits.”
3. “Some advanced grid forming inverters now also offer a simulated inertia function (and also some current source inverters with a similar parallel operating mode),”
4. “In addition to the potential inertia and system strength benefits of grid forming inverters, they can be of particular benefit in a large power system”
5. ” As noted above, grid forming batteries (and some current source batteries with similar parallel operating modes) have the potential to provide simulated inertia.”

Some questions that immediately pop up based on the above definitions:

1. Does a grid forming inverter provide virtual inertia?
2. Can a grid forming inverter be connected to a grid?
3. Can an inverter provide more than one functionalities?

I find that it is important to define the terminology appropriately into three rather than two categories to make appropriate sense of the functionalities. In fact, this was done in the following article back in 2012:

Rocabert, J., Luna, A., Blaabjerg, F., & Rodriguez, P. (2012). Control of power converters in AC microgrids. IEEE transactions on power electronics, 27(11), 4734-4749.

Available freely here: https://www.researchgate.net/file.PostFileLoader.html?id=5635cddb60614bc5488b4585&assetKey=AS%3A290897104261121%401446366682854

The terminology introduced is that of :

• Grid forming converter. Reference is frequency and voltage and the converter controls frequency and voltage. They serve as the grid  (usually in islanded operation) and their goal is to maintain a constant voltage and a constant frequency. A grid forming inverter would have no capacity to emulate inertia or act as a virtual synchronous machine as its reference will remain that of a constant voltage and frequency.

• Grid feeding inverters. These is the common operation of an inverter in a grid, following a reference for active and reactive power. In a similar manner, these inverters can also regulate the dc side voltage while power flow is controlled by the dc-side, e.g. solar inverters or wind turbine inverters via MPPT.

• Grid supporting inverters. These inverters are connected to grid but have the capacity to support grid functionalities by emulating inertia (VSM) which is a form of droop control for generating the active and reactive power references and allowing power sharing between different inverters.

In short:

1. Grid forming -> Islanded inverter setting voltage and frequency
2. Grid feeding -> Grid connected supplying P and Q
3. Grid supporting -> Grid connected with tertiary (droop, VSM, VISMA, VSG etc) functionalities.

I hope that sometime soon, a clarification on the definitions and the terminology used around grid forming inverters will be provided officially.