Analysis and control of a medium voltage DC link interface between DFIG wind turbine system and weak power grid

摘要

Direct current (DC) power technology, is well-established as a fully controlled power link between different ac grids. Nowadays, distributed generation (DG) has substantially changed the traditional power grid structure, with the high penetration of the renewable energy systems (RES) playing a key role on the system performance. The stochastic and intermittent nature of the power injected by the RES into the local and the main grid create a challenging issue in controlling effectively the power flow of the DG system under some critical quality standards. Voltage source converters (VSCs), used as a power interface in DG systems, offer a great opportunity for implementing the aforementioned demands. This is exactly the gap that the medium voltage DC transmission can cover in a much more systematic way. Hence, in this paper, a DG scheme is considered that involves a doubly-fed induction generator-(DFIG) based wind system, which feeds a local load through the DC link, which in turn, is connected to the main grid via an ac line. A detailed analysis for this scheme is provided based on the frequency domain representation. The analysis provides the range wherein the gains of the controllers have to lie in order to ensure a stable and robust operation. The aim is to effectively regulate the system power and voltages, in a manner that adverse impact responses of the different grid parts are avoided.