Grid Forming Inverters Part 2

Grid Forming Inverters (GFIs) are a new generation of inverters designed to address the challenges faced by Inverter-Based Resources (IBRs) in an inverter-dominated field. The traditional goal of existing inverters is to maximize current output to the grid, with minimal additional capabilities. However, as IBRs become more popular, there is a need for additional functions from these inverters, such as automatic voltage control, capability to provide frequency response, fast frequency response, and system stability maintenance. These features are typically provided by traditional synchronous generators.

In a system dominated by synchronous generators, when an IBR generator provides input into the system, the IBRs follow the grid frequency, making them Grid Following Inverters (GFL). However, in a future scenario with fewer synchronous generators and less inertia in the system, the additional GFL IBRs can cause the electric system to become unstable. This instability arises because all the GFL IBRs interact with each other, and since each is designed to follow the other, there is nothing to stabilize the system.

GFMs are voltage source inverters that provide fast controlled current injected into the system to balance the system, rather than operating at maximum power point tracking (MPPT) like GFL inverters. They are designed to mitigate the issues faced in an inverter-dominated field.

 A weak grid refers to a situation where the distribution or transmission system has a low short circuit ratio. In such scenarios, when the impedance value fluctuates, the sensitivity of the voltage fluctuates as well. In weak grid scenarios, GFM IBRs can help stabilize the system.

Types of GFM Control Methods: Several types of GFM control methods are being developed that can be largely classified into phaser domain or time domain approaches. These methods can be Virtual Sync machine, Matching control, Dropped Based control, and Virtual oscillator control.

However from the grid control level should be agnostic of the type of technology used for GFM inverters. Fundamentally, the grid control is concerned about two outputs: Real Power (P) and Reactive Power (Q). In traditional GFL inverters, both P and Q are constant. In basic GFM inverter, P can change based on the situation while keeping Q constant. An advanced GFM inverter can change both P and Q based on system conditions.

Real World Applications: In the near term, GFM inverters are being used in microgrid designs and transmission systems with low fault current and low inertia. In the future, it is anticipated that GFMs will be utilized in the distribution grid, necessitating stable and reliable coordination between these inverters. Examples of current GFM utilization include:

a. Microgrids:

·    Micanopy microgrid in FL, which has 8.5 MW of Battery Energy Storage System (BESS) to support the town of Micanopy and nearby neighbors during grid outages.

·    National Grid NY, with a 20 MW, 40 MWh BESS and a 75 MVA circuit. The system includes 5 substations, a 46 KV sub-transmission line, and 10 feeders that can separate to form an island.

·    Watertown, Canada, where a section of the medium-voltage (MV) feeder operates as a microgrid with 1.6 MW and 5.2 MWh BESS.

b. Grid Islands:

·    Dersalloch Wind Farm in Scotland, which is exploring black start capabilities with wind farms.

While GFM IBRs can help strengthen weak grids, they are not a universal solution for every situation. Other solutions to strengthen weak grids include strengthening the transmission system to increase short circuit strength, re-tuning fast control loops to recognize low short circuit conditions, re-imagining IBR controls to introduce additional flexibility in operation, and the addition of synchronous condensers.