Future Grids and Grid Forming Inverters

As more and more inverter based Resouces, like Solar PV, Wind, and BESS, are being added onto the grids globally, dynamics of the grids are changing. Traditional synchronous machines with Inertia, Short Circuit contributing capabilities under faults are reducing in percentage as compared to IBRs. Most installations of the inverter based resources are Grid Following sources. These require a grid source as reference to produce power, and are unable to run in island mode. Grid Forming inverters present a solution wherein with special electronics and controls, the future grids would become independent, reliable, and stable.

GFM inverters have been widely researched in battery energy storage systems (BESS), wind power plants, solar photovoltaic (PV) plants, and hybrid plants. Furthermore, there are several installed projects where GFM functions have been successfully tested, including extremely fast power injection in the inertial time frame in response to frequency events, islanded operation capability without synchronous generation, blackstart capability, and operation in parallel with grid-following (GFL) resources and synchronous machines. Widespread understanding of GFM controls and their impact on BPS performance is still in the early stages; however, the technology shows significant promise. Study findings from system conditions with high IBR penetrations show the benefits for GFM controls, and equipment vendors have commercially available products that can provide GFM capability. While GFM inverters still need to be studied and tuned to specific system conditions (similarly to GFL controls), they do have advantages compared to the GFL control schemes applied in nearly all existing IBRs today. GFM IBRs are expected to be beneficial for increasing IBR penetration levels and will likely play an important role in contributing to the stability and reliability of the BPS (Bulk Power Sources) under future high IBR penetration conditions.

Grid Forming Control for BPS-Connected Inverter-Based Resources?are controls with the primary objective of maintaining an internal voltage phasor that is constant or nearly constant in the sub-transient to transient time frame. This allows the IBR to immediately respond to changes in the external system and maintain IBR control stability during challenging network conditions. The voltage phasor must be controlled to maintain synchronism with other devices in the grid and must also regulate active and reactive power appropriately to support the grid.

GFM IBRs could be expected to have many of the following functions and characteristics:

• GFM IBRs create open circuit voltage sources. A GFM IBR facility can be capable of operating in islanded mode so that the IBR can serve its own auxiliary load and the connected loads in the absence of a synchronous resource or other GFM IBR support for the isolated grid conditions. With this characteristic, IBR can operate in a stable manner without the need for synchronous machines.
• A GFM IBR can be controlled to synchronize and stably operate with other resources in the grid and different types of loads. These other resources include conventional synchronous machines and other GFM or GFL IBRs.
• Upon the occurrence of a large load step or generation trip event, a GFM IBR could contribute towards arresting the decline, increasing the frequency, and recovering frequency to the nominal value, assuming that energy and power margins are available.
• A GFM IBR would contribute towards provision of reactive power support and voltage regulation within the continuous operation region and outside the continuous operating region to some degree, thus aiding fast and stable voltage recovery after a fault.
• GFM IBRs also reduce adverse converter control interactions among GFM IBRs, GFL IBRs, other power electronic devices, and rotating machines on the grid.
• GFM IBRs provide the prescribed level of oscillation damping within the grid. As the IBR characteristics and penetration level change the grid, interactions or oscillation modes could change. Frequent studies and analysis may be required to verify the damping levels and adjust controls accordingly.
• GRM IBRs provide active low-order harmonics cancellation.
• GFM IBRs provide blackstart capability if needed and designed for this purpose.

Explanaiton of GFM Controls:

There are multiple types of GFM control strategies?as illustrated below in fig 1.1. These include, but are not limited to, the following:

• Droop-Based GFM Control:?GFM droop control is realized by active and reactive power droop control, which control the IBR voltage phasor frequency in proportion to the active power extracted from it. GFM reactive power droop control has similar logic for the Q-V relationship.
• Virtual Synchronous Machine (VSM):?VSM programs the IBR’s control to emulate an SG’s response so that the IBR can act similarly to an SG to provide an active power response that mimics a SG’s expected contribution to a sudden generation loss, load change, or system fault.
• Virtual Oscillator Control (VOC):?VOC controls are inspired by the phenomenon of self-synchronization in networks of non-linear oscillators. VOC controls cause the IBR to act as a non-linear oscillator with a dead zone.

Figure 1.2?and?Figure 1.3?are high-level examples of block diagrams for GFM and GFL control, respectively, and illustrate some of the similarities and differences between the different types of controls. Some of the main differentiations between GFM and GFL control are summarized in?Table 1.1.