Assessment of a non linear current control technique applied to MMC-HVDC during grid disturbances

Nowadays, the most advanced solution to connect offshore wind farms to the mainland grid is the use of High Voltage DC converters. Among the variety of High Voltage DC technologies available, the Modular Multi-Level Converter stands out. It is totally different to the traditional two or three level Voltage Source Converters, and it uses a voltage source control scheme presenting advantages such as a better harmonic content, but also some disadvantages such as a more complex control system. However, its dynamic performance can be improved by using a non linear current control technique, taking advantage of its faster response during electrical transients when compared to voltage source or linear current techniques. In this assessment, it has been compared with the voltage source control technique by using a model of an offshore wind farm connected to the mainland grid. In order to make a more comprehensive study, tests with one, two and three phase to ground faults have been carried out. Finally, a solution to improve electric protections operation by increasing the current injected by the Modular Multi-Level Converter in the Point of Common Coupling above the converter rated current value during voltage sags has been assessed. (C) 2016 Elsevier Ltd. All rights reserved.