Concerning Short-Circuit Current Contribution Challenges of Large-Scale Full-Converter Based Wind Power Plants

The calculation of short-circuit current contributions from full-converter based type IV wind turbines and other inverter-based power plants are often simplified. They are represented as simple fixed current sources for the steady-state stage of the fault during short-circuit evaluations. As wind power penetration increases, it becomes increasingly important to factor the details of short-circuit contribution from such inverter-based power plants for long-term planning as well as equipment and protection system design. The challenge lies in the fact that this type of inverter-based power plants is fully connected through power electronic devices, and their short-circuit contributions cannot be easily represented and calculated based on traditional physical laws developed for synchronous generators. This paper addresses the main challenges and limitations of the current accepted practices in the industry. The challenges are discussed for all three stages during a fault referring to the stages of more traditional power plants, namely sub-transient, transient and steady-state. An electromagnetic transient (EMT) model validated through field tests on a full-scale prototype wind turbine was used to simulate over 20,000 scenarios and examine the three stages in detail. Results show that during the sub-transient stage, in the first milliseconds of the fault, the converter exhibits a so-called quasi-voltage source behavior with high currents limited by transient hardware limitations and high negative sequence current injection for unbalanced faults, regardless of the current priority chosen. During the transient stage, the converter starts transitioning to a grid code/voltage-dependent current source until reaching the steady-state stage of the fault. The paper also includes a theoretical analysis, a comparison with the conventional ac-dc fault representation, and thorough time-domain and statistical analyses of the relevant variables during a short-circuit. Finally, it outlines future challenges and identifies areas of improvement for enhancing the modeling of fault currents in Type IV wind turbine generators.