Behaviour of Doubly-Fed Induction Generators during Grid Faults and the Impact on a Distance Relay

Since the evolution to big scale wind power plants, wind turbines are also coupled to the transmission grid instead of the distribution grid, causing new protection problems. Problems arise as the short-circuit behavior of wind turbines equipped with doubly-fed induction generators (DFIG) differs fundamentally from traditional power plants equipped with synchronous generators. When applying a three phase short-circuit at the terminals of the DFIG, the magnetic flux stops rotating and causes a DC-component in the stator. Additional, a DC-component in the rotor arises, causing an AC-component in the stator. In contrast with the three phase fault, the single-line-to-ground fault short-circuits only one phase and keeps the two other phases excited. Due to this excitation, the current amplitudes do not drop to zero in contrast with the three phase fault. To analyze the influence of DFIGs on the transmission protection, a study case of a wind farm, equipped with DFIGs, in the Belgian grid is considered and analyzed in two steps. The first step includes the simulating of short-circuits in a Belgian grid topology with a wind farm. The second step injects the simulations’ results in a real distance relay using COMTRADE data exchange. At full load, three phase faults are initially detected but not removed by a distance relay. Single line-to-ground faults are detected and removed correctly when the wind turbines are operating at full load. The impact of the following parameters is analyzed: fault resistance, crowbar resistance, load and number of generators. From this analysis, one can conclude that three phase faults are never removed correctly. A single line-to-ground fault is detected and removed correctly unless the crowbar resistance or the load is lowered. These results confirm that the distance relay with actual settings is not suitable for this grid topology including wind turbines equipped with DFIG.

ISBN:9781617823107

Jaar van publicatie:2010

Toegankelijkheid:Open