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

Abstract

Grid-forming (GFM) inverters are recognized as a viable solution to increase the penetration of renewable energy in bulk power systems. However, they are physically different from synchronous generators in terms of overcurrent capability. To protect the power semiconductor devices and support the power grid under severe symmetrical disturbances, the GFM control systems should be able to achieve the following requirements: current magnitude limitation, fault current contribution, and fault recovery capability. Various current-limiting control methods are reported in the literature to fulfill these goals, including current limiters, virtual impedance, and voltage limiters. This paper presents an overview of those methods. Emerging challenges that need to be addressed, including temporary overcurrent, unspecified output current vector angle, undesired current saturation, and transient overvoltage, are pointed out. Comparative simulations are conducted to demonstrate the performance of different methods under grid voltage drops and phase jumps. Finally, open issues of current-limiting control methods for GFM inverters, including transient stability assessment, voltage source behavior under overcurrent conditions, and windup of voltage controllers, are shared.INDEX TERMS Grid-forming (GFM) inverter, current-limiting control, current magnitude limitation, fault current contribution, fault recovery capability. NOMENCLATURE

Control of Grid-Forming VSCs: A Perspective of Adaptive Fast/Slow Internal Voltage Source

“…It is known from (9) that Iomag is monotonously increased with the increase of δ before triggering the current limitation, which yields Iomag < Iomag_th ↔ δ < δth. Yet, it is clear from (10) that Iomag_th is not a constant value for specific δth, but is affected by two other variables, i.e., grid impedance Xg and grid voltage Vg. Define Iomag_th_min is the minimum value of Iomag_th considering different Xg and Vg, the sufficient condition for δ < δth becomes Iomag < Iomag_th_min.…”

“…To calculate Iomag_th_min, the largest possible value of Xg, i.e., Xg=1p.u. (corresponding SCR=1) is considered, as Xg appears in the denominator of (10). On the other hand, the value of Vg for calculating Iomag_th_min can be obtained by solving…”

“…On top of the small-signal stability, control of GFM-VSC under large grid disturbances, including voltage sags, phaseangle jumps, high rate of change of frequency (RoCoF), and the combination thereof, is also of concern. Except for current limitation [8]- [10], GFM-VSCs should also remain synchronism with the grid (i.e., transient stability) during and H. Wu is with the AAU Energy, Aalborg University, 9220 Aalborg, Denmark (e-mail:, hew@energy.aau.dk).…”

Nonlinear Stability of Complex Droop Control in Converter-Based Power Systems

“…Remark 4 (Current limiting issue). The transient stability of GFM converters is also related to their current limiting strategies [34]. When a grid fault occurs, the grid enters into an abnormal operation stage, during which converters should maintain GFM operation with auxiliary control/protection strategies.…”

Voltage-Based Current Limitation Strategy to Preserve Grid-Forming Properties Under Severe Grid Disturbances

“…As power-electronic converters have limited overcurrent capability, the ability to limit the current not only during normal operation but also during grid disturbances is a necessity for all converter control schemes. In contrast to grid-following control, GFM-based converters are controlled to emulate a voltage source behind an impedance; as a consequence, the current exchanged between the converter and the grid is indirectly controlled and thereby its limitation is challenging [10], [11]. Current limitation is typically reached during large grid disturbances, such as faults or large frequency excursions, during which the grid is most vulnerable and in need of support.…”