Effect of Current Observers in Dual-Loop State-Space Controllers for Grid-Forming Converters

Grid-forming (GFM) converters are required to achieve a good voltage—frequency response, an accurate voltage tracking and total cancellation of disturbances at the output voltage. A wide stability range and robustness are also desirable in order to permit their operation in grid-connected and standalone scenarios. State-space controllers implement observers with the aim of estimating output disturbances and the state variables of the system. This article is particularly focused on the effect that including or not a current observer has on the stability of dual-loop state-space controllers for grid-forming applications. Based on the analysis of the sensitivity function, the article evidences that the supression of the current observer significantly improves the disturbance rejection response, increases the robustness, and extends the stability margin of the controller, which is fundamental for grid-connected GFM converters. However, the current control loses efectiveness. On the other hand, the fact of including two observers, one for each control loop, yields zero steady-state error at the controlled variables.