Optimal Controller Design for Transient Stability Enhancement of Grid-Following Converters Under Weak-Grid Conditions

Abstract

Modeling and design-oriented control of transient stability of grid-following converters have attained an increasing interest in recent years. Despite novel nonlinear models enabling a design-oriented enhanced transient stability controller, the focus has so far been limited to study only the synchronization dynamics of the phase-locked loop. To expand upon the knowledge of large-signal performance and stability, this work proposes a systematic analysis procedure of a grid-following converter under weak-grid conditions and large-signal disturbances including the outer dc-link and ac-side voltage control loops. A reducedorder large-signal model is used to analyze the large-signal nonlinear behavior of the system using the area of the basin of attraction as a measure for large-signal robustness. Here, stabilizing and destabilizing trends for outer loop controller parameters are given. Through a surrogate-model expensive blackbox optimization algorithm, a computational-efficient optimal design of the outer loop controller parameters is proposed to maximize the large-signal robustness. Finally, a recommendation and a design guideline for converter constraints and outer-loop controller parameters are given. This can be used to identify the influencing parameters for grid-following converters under largesignal disturbances, and as a tool for fast controller optimization towards large-signal robustness.

Conditions of Existence and Uniqueness of Limit Cycle for Grid-Connected VSC With PLL

“…To determine the main factors that influence the studied system stability, one reduced second-order model is firstly proposed in [8] that the fast inner-current control of VSC and the network dynamics are reduced into the algebraic equations and only the dynamic equations of PLL are reserved. Phase portrait method that maps the VSC velocities and angles information during the transient process in the phase plane is widely utilized in the stability analysis of VSC embedded system [9]. The stability of the system can be clearly judged if the system trajectory is escaping from SEP.…”

Direct Method of Lyapunov Applied to Synchronization Stability of VSC with PLL

“…Moreover, the impacts of damping term of PLL dynamics can be identified, and it thus provides a clear insight into the controller design. In [10]- [12], it is revealed that high grid impedance, improper setting of PLL parameters, and heavy load conditions might deteriorate the system stability. To enhance the large disturbance stability of CIGs embedded system, an adaptive PLL is proposed in [13] by switching between the second-order and first-order PLL during the fault conditions.…”

Optimal Model Predictive Controller for Grid-Connected Voltage Source Converters

“…Currently, depending on the operational mode, the grid-tied VSCs are usually divided into grid following, grid supporting, and grid forming converters [2], [3]. For any mode of operation, inner current control is an inevitable control structure.…”