Identification of Three-Phase Grid Impedance in the Presence of Parallel Converters

Grid impedance is an important parameter which affects the control performance of grid-connected power converters. Several methods already exist for optimizing the converter control system based on knowledge of grid impedance value. Grid impedance may change rapidly due to fault or disconnection of a transmission line. Therefore, online grid identification methods have been recently proposed to have up-to-date information about the grid impedance value. This is usually done by perturbing the converter output current and measuring the response in output voltage. However, any parallel converters connected to the same interface point will cause errors, since the measured current differs from the current that is flowing through the grid interface point. This paper points out challenges and errors in grid impedance identification, caused by parallel converters and their internal control functions, such as grid-voltage support. Experimental grid-impedance measurements are shown from the power hardware-in-the-loop setup developed at DNV-GL Flexible Power Grid Lab.

Structural Separation to Create Competition? The Case of Freight Railways

The freight railways sector has three attributes that have proved problematic for recent experiments with vertical separation: a) a relatively high share of network costs in total delivered service costs, b) an apparent persistence of economies of scale at the "competitive" train operations level, and, perhaps most important, c) strong economies of vertical integration that are focused on the interface point of wheel and rail – that is, exactly where vertical separation takes place. The third factor in particular seems a clear illustration of the rationales discussed by Coase and Williamson for the broad vertical scope of a single firm and the disadvantages of relying on market transactions under certain conditions.