Fault Recovery Analysis of Grid-Forming Inverters With Priority-Based Current Limiters

Abstract

Grid-forming (GFM) inverters are required to operate robustly against grid faults. However, due to the limited over-current capability of inverters, current-limiting controls are usually applied to protect these semiconductor devices, which may prevent GFM inverters from a successful fault recovery. To understand this phenomenon, this study analyzes the fault recovery process of a GFM inverter with a priority-based current limiter. According to whether the GFM inverter can ensure transient stability and exit the current-limiting mode after fault clearance, three post-fault scenarios are identified, including normal operation, current limitation, and oscillations. Further, the impacts of the short-circuit ratio and control parameters on the post-fault behavior of GFM inverters are demonstrated. To illustrate the implications of these theoretical results, typical numerical examples are presented. Finally, the theoretical findings are validated through experimental tests.

A Review of Current-Limiting Control of Grid-Forming Inverters Under Symmetrical Disturbances

“…The instantaneous limiter can be implemented in different reference frames [17], [27], [35], [36], expressed as • Adjust power references [16] May fail to recover due to the windup of voltage controllers [17]- [19] Virtual impedance…”

“…The windup of these voltage controllers may further result in a current reference 𝑖 𝑟 𝑒 𝑓 whose magnitude is larger than 𝐼 𝑀 . Consequently, undesired current saturation occurs, which prevents the inverter from successful fault recovery [19]. Extra anti-windup designs are required for these voltage controllers to address this issue when current limiters are utilized.…”

“…Simulation tests of the magnitude limiter and the prioritybased limiter with 𝜙 𝐼 = 0 are conducted to demonstrate their performance during severe symmetrical disturbances. The control structures in [19], [51] are utilized. During the disturbances, the power references are adjusted according to (5).…”

“…FIGURE19. Fault current contribution with the virtual impedance when the grid voltage drops to 0.2 p.u.…”

Transient Stability Analysis of Virtual Synchronous Generator Equipped With Quadrature-Prioritized Current Limiter

“…Thus, the analysis of d-CLs can be significantly simplified by setting this angle difference to 0 o and eliminating variations of this angle difference. Similarly, this angle difference can also be set to a predefined negative constant by a modeswitching mechanism to increase the q-current injection of the VSG [1], [6]. Nevertheless, this simplification of the current angle does not apply to analyzing q-CLs, as the amount of qcurrent generated by q-CLs is not always pushed to the current limit, but depends on the grid and fault conditions as well as the dynamics of the primary control.…”