DFIG Wind Turbine Modeling and Fault Protection

Problem Definition
Currently in the power industry, "green" energy generation has a higher priority over traditional generation from fossil fuels such as coal or natural gas. One of the most common "green" production methods is to harness the wind. Today there are numerous wind farms with hundreds of wind turbine generators producing green energy. These wind farms are a huge monetary investment. Therefore protecting these wind turbines and keeping these green technologies in production under varying conditions has become a concern. The newer types of wind turnines (Type III and IV) can be modeled as a Doubly Fed Induction Generator(DFIG). Currently, the effects of faults on type III/IV wind turbines and the responses of the DFIGs to faults are not completely understood.

There are limited protection designs available on the market today for this problem. Our team plans to model and simulate a doubly-fed induction generator (DFIG), which is similar to a Type III wind turbine, and study the effects of a fault on a model power system. We plan to develop a protection scheme that can quickly detect a fault thereby preventing damage to the DFIG.

Background
According to the paper [Addressing Protection Challenges Associated With Type 3 and Type 4 Wind Turbine Generators] (Bing Chen, CG Power Solutions USA Inc. et al)  changes in governmental policies intended to expand rapidly growing renewable energy technologies have led to the increased penetration of wind turbine generators (WTGs) in the power system. Almost all new megawatt-scale wind power plants that are being developed use either variable- speed doubly fed asynchronous (Type 3) or full converter-based (Type 4) WTGs. These Type 3 and Type 4 WTGs can produce energy over a wide range of wind speeds, allow for fast and independent control of active and reactive power, limit fault current, and comply with low-voltage ride-through (LVRT) requirements set forth by industry regulatory agencies. Due to the interconnection of these wind power plants to the power grid, it is important to understand their short-circuit behavior in order to develop adequate protection systems that will make the system safer and easy to operate. Short-circuit studies allow protection engineers to selectively determine which circuit breaker ratings, relay settings, and protection methods to adopt for a particular section of the power system. The short-circuit behavior of fixed-speed squirrel cage induction (Type 1) and variable-slip wound-rotor induction (Type 2) WTGs depends upon the physical characteristics (transient and subtransient impedances) of the WTGs and is therefore well understood.

For faults near the WTG terminals, the fault current can be several times the rated full-load current and is only limited by the system and the WTG impedances. The fault current characteristics for Type 1 and Type 2 WTGs are accurately represented in most commercially available short-circuit analysis tools used by protection engineers. On the other hand, Type 3 and Type 4 WTGs have much more complex fault current characteristics and are governed by the proprietary controls of the converters used in these generators. As such, they are subject to arbitrary design choices and can change with each revision of the control algorithm. For the Type 4 WTG, the fault current contribution is usually limited to 1.1 to 1.2 times the rated full-load current, following any initial transients. Short-circuit characteristics of the Type 3 WTG are similar to those of the Type 4 WTG, except during severe faults when the crowbar circuit connected at the rotor is activated. This crowbar circuit is used to help force the field current to zero to avoid overvoltage. With the crowbar circuit activated, the fault current characteristics of the Type 3 WTG transition from a controlled current source to that of an induction generator.

The relatively nascent nature of the Type 3 and Type 4 WTG model development makes it very difficult to use a number of the widely available short-circuit analysis tools employed by protection engineers. This is because the existing tools do not accurately model the dynamics of the WTG control system. From a system protection perspective, it is important to have models that reflect true fault behavior. Unfortunately, such models are mostly manufacturer-specific and proprietary in nature. Even for the same manufacturer, the control technique employed in one design class can be significantly different from that of another. In order to protect the power electronic switching devices used in the converters, these WTGs limit fault current and modify the current waveform during a fault. This unconventional behavior of the Type 3 and Type 4 WTGs presents a host of challenges to the protection engineer responsible for protecting a power system with WTGs.

Deliverables
Comparison and verification of RSCAD DFIG model. Protection scheme including detection and DFIG protection from fault.

Project Learning
Through this project, we hope to attain a better understanding of the processes associated with protection engineering and the real world challenges inherent in protecting Type III and Type IV wind turbine generators.