Investigating Short Circuit Models for Wind Turbine Generators
- New Mexico State Univ., Las Cruces, NM (United States); New Mexico State University
Short circuit study needs to be performed at the planning stage before connecting large gen erators with the electric grid. This study will reveal the impact of new generation on the fault levels of the grid. The fault levels will determine the rating of nearby equipment and setting of relays. However, a short circuit model of the new generation is required for this study. This report investigates and quantifies the short circuit models of different wind turbine generators (WTGs) and of wind farms, and using these models, explores the effects of the wind farms on the nearby switchgear. WTGs, commonly classified as Type 1, Type 2, Type 3, and Type 4, exhibit different short circuit behavior depending upon their architecture and controls. The short circuit model (synonymous to Thevenin model) of Type 1 WTG is well known; it comprises of a voltage source behind a transient reactance, X'. The transient reactance X' is derived using the reactance of stator, rotor and the magnetizing reactance. In case of Type 2 WTG, an external rotor resistance is used to increase the range of operating slips. The value of the external rotor resistance increases at higher operating slips. The effect of the external rotor resistance on the fault current response is analyzed by simulating a validated model of Type 2 WTG. The validation is performed using the field data from an actual 67-machine Type 2 wind farm located in Wyoming, USA. The results shows that the external rotor resistance should be considered in its Thevenin impedance for better accuracy. A wind farm contains a large number of WTGs, pad mounted transformers, and cables typically connected in a ‘daisy chain” configuration. The effect of these cable impedances on the short circuit model of Type 2 wind farm is examined using the validated models. It is found that the collector circuit cables inside the farm do not have a significant effect on the short circuit model of the farm. Based on this finding, a simple equivalencing procedure for the wind farm is proposed and validated. Faults inside and outside the wind farm are analyzed and recommendations to select main circuit breakers and generator circuit breakers are made. Type 3 WTG, also known as Doubly Fed Asynchronous Generator (DFAG), uses back to back converters in the rotor circuit to inject slip frequency voltage into the rotor circuit. Until recently, these machines used crowbar to short the rotor circuit during faults to protect the converter com ponents form high rotor currents resulting from faults. This type of WTG is simulated and the model is validated using field data from a 66-machine Type 3 wind farm in Wyoming, USA. The simulation study reveals that the crowbar feature is activated almost immediately after the fault takes place, and remains activated throughout the fault-period. This confirms that the short circuit model for Type 3 WTG with crowbar is similar to the model for Type 2 WTG. In the past, wind farms were allowed to disconnect during faults. However, recent grid codes around the world and in the United States demand that the wind farms be kept connected to the grid during faults, and provide certain reactive power quantified based on the voltage dip at the point of interconnection. This requirement, also called low voltage ride through (LVRT), can be accomplished by using Type 4 WTGs that employ full back to back converter interface between generators (permanent magnet synchronous generators are typically used) and the grid. However, the controls are proprietary, and hence the short circuit models are not well defined. This report proposes controls that enable the WTG to comply with the strictest grid codes, and based on this model, proposes a short circuit model of the Type 4 WTG. This model is not validated due to lack of validation data, but the use of grid code requirements as control objectives lends good credibility to the model. PSCAD/EMTDC®, a powerful and time-tested time-domain simulation software is used to implement all models. This software is capable of modeling in detail all components of wind farms, and all components and controls of converters in time domain.
- Research Organization:
- New Mexico State Univ., Las Cruces, NM (United States)
- Sponsoring Organization:
- USDOE Office of Energy Efficiency and Renewable Energy (EERE)
- DOE Contract Number:
- EE0001381
- OSTI ID:
- 1111345
- Report Number(s):
- DOE-NMSU--0001381
- Country of Publication:
- United States
- Language:
- English
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