Direct AC voltage control for grid-forming inverters

Abstract

Grid-forming inverters usually use inner cascaded controllers to regulate output AC voltage and converter output current. However, at the power transmission system level where the power inverter bandwidth is limited, i.e., low switching frequency, it is difficult to tune controller parameters to achieve the desired performances because of control loop interactions. In this paper, a direct AC voltage control-based state-feedback control is applied. Its control gains are tuned using a linear quadratic regulator. In addition, a sensitivity analysis is proposed to choose the right cost factors that allow the system to achieve the imposed specifications. Conventionally, a system based on direct AC voltage control has no restriction on the inverter current. Hence, in this paper, a threshold virtual impedance has been added to the state-feedback control in order to protect the inverter against overcurrent. The robustness of the proposed control is assessed for different short-circuit ratios using smallsignal stability analysis. Then, it is checked in different grid topologies using time domain simulations. An experimental test bench is developed in order to validate the proposed control.

Enhanced TVI for Grid Forming VSC under Unbalanced Faults

“…Finally, the angle of the ellipse (α1) is the angle of the semi-major axis (A), where the two sequences align (14). This information provides the positions of the positive and negative sequence current vectors, as shown in Figure 9.…”

“…Finally, the angle of the ellipse (α1) is the angle of the semi-major axis (A), where the two sequences align (14). The width and height parameters of an ellipse are called the semi-major (A) and semiminor (B) axes.…”

“…In order to improve the control performance, an inner control loop is used in many control structures [14]. So, the aim of the inner control loop is to enhance the AC voltage dynamics.…”

Fundamentals of power systems modelling in the presence of converter-interfaced generation

“…In case of overcurrent, a virtual impedance can also be adjusted to limit the current. Note that these schemes can be applied either on a Modular Multilevel Converter (MMC) or VSC connected to the grid with an LC filter provided that the internal current and voltage loop are correctly tuned [97][98][99]. This section has introduced three main grid-forming controls.…”

Improved Damping Control Method for Grid-Forming Converters Using LQR and Optimally Weighted Feedback Control Loops

“…These control techniques aim to design a robust controller to achieve a better performance in terms of settling time. However, these techniques are complex and require very high computations [21].…”