Equivalent Circuit Model of Grid-Forming Converters With Circular Current Limiter for Transient Stability Analysis

Abstract

This letter develops an equivalent circuit model of a grid-forming (GFM) converter with a circular current limiter and analyzes its transient stability. It is revealed that the inner control loop can be simplified as a voltage source behind an equivalent resistor. Based on the developed model, theoretical analysis and experimental tests demonstrate that the transient stability of a P − f droop-controlled GFM converter with the circular current limiter can be assured whenever there exist stable equilibrium points.

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

“…During normal operation, the output of the voltage integrator is close to zero in the steady-state since it is merely used to remove the impact of modeling errors [30]. Therefore, once the current limiter is triggered, the voltage integrator is kept at zero to avoid its windup [23].…”

“…In [22], a theoretical approach is proposed to explain the post-fault behavior of GFM inverters with virtual impedance. Further, Fan et al [23] demonstrated the fault recovery process of GFM inverters with circular current limiters based on an equivalent circuit model. In [24], [25], transient instability induced by daxis priority-based current limiters when faults are cleared is revealed.…”

“…In [15], it is shown that the virtual impedance methods are inherently free of these phenomena since the voltage control loops are always utilized to track their references in either normal operating mode or current-limiting mode [21], [22]. Moreover, the results in [23] reveal that the GFM inverter with circular current limiters can also avoid these phenomena since the inverter behaves similarly to those with virtual resistors. Different from the virtual impedance methods and the circular current limiters, the GFM inverters with priority-based current limiters will behave as power-synchronized current sources after fault clearance [28].…”

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

“…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).…”

“…In addition to the commonly used methods, such as back-calculation, clamping, etc., novel anti-windup algorithms for different current-limiting controls may be developed to help GFM inverters recover from the undesired current saturation. For example, in [51], the integrator of the voltage vector controller is set to zero when the current magnitude limiter is triggered to improve the fault recovery capability of the GFM inverter. In [76], [82], the outer-loop controllers are re-designed for prioritybased current limiters to avoid the windup of the voltage vector controller.…”