Fault Current Multi-Stages Calculation Method for DFIG-Based Wind Farms With Whole Fault Process Attributes Under Asymmetrical Grid Fault Conditions

The existing imprecise fault current calculation, due to incomplete stage division of whole fault process of DFIG-based wind farms (DBWFs), brings great hidden dangers to safety and stable operation of local grid. To tackle this challenge, a fault current multi-stages calculation (FCMSC) method is proposed to accurately calculate DFIG-based wind farm's output symmetrical and asymmetrical fault currents under grid whole fault process that including fault occurrence, fault ride-through and fault recovery. The main content of FCMSC method includes: 1) Whole fault process equivalent aggregation model, which contains protection response stage, first crowbar protection stage, demagnetization operation stage, reactive current injection stage, and second crowbar protection stage, is completely established while second crowbar protection stage is firstly dissected in detail. 2) Fault current contribution mechanism, considering wind speed, is revealed to have GSC injection and absorption modes under asymmetrical grid fault conditions. 3) Fault current universal expression, which covers whole fault process operation stages and scenarios, is conducted by replacing differential equations with simple algebraic operations for improving calculation accuracy, timeliness and universality. 4) Fault current characteristic components, which contain DC components, AC stable components, and AC attenuated components, is extracted for providing key data for fault identification and protection. Extensive test results under symmetrical and asymmetrical faults are illustrated for verifying the correctness of proposed FCMSC method.