Large-scale integration of wind farms in transmission networks has led to several challenges; one of which is the need for increased transmission capacity to transport a bulk amount of wind power. Series compensation is an established means of enhancing the power transfer capability of existing transmission lines and is being increasingly considered for integrating large wind power plants. However, series compensated transmission lines may cause subsynchronous resonance (SSR) in turbine-generators, which can lead to electrical instability at subsynchronous frequencies and potential turbine-generator shaft failures.
This thesis deals primarily with the potential of subsynchronous resonance in induction generator based wind farms connected to series compensated lines. Two types of Induction Generators - single cage and double cage, are considered to develop a state space model of the overall wind farm system. Eigenvalue analyses followed by participation factor analysis and sensitivity studies are performed over a wide range of operating conditions. These analyses include variations in the size of wind farm, wind power output, and series compensation levels. The potential for SSR in a wind farm is examined through a comprehensive small signal analysis. A novel equivalent circuit analysis is also presented in this thesis for investigating the impact of fault at wind farm terminals based on the resonant speeds of wind turbine generators. Both eigenvalue analysis and equivalent circuit studies are validated through electromagnetic transient simulations carried out using PSCAD/EMTDC software. These studies are conducted for both modified IEEE First SSR Benchmark systems and Second SSR Benchmark systems, and with three different commercially available wind turbine generators. It is found that induction generator effect based SSR may be experienced with large wind farms even at realistic levels of series compensation.
This thesis proposes a STATCOM to alleviate SSR in such series compensated wind farms. Two STATCOM controllers are proposed and their performances compared. A detailed SSR study is also conducted to examine the impact of HVDC lines on Induction Generator Based wind farms. However, it is concluded that HVDC converter controller may not have the potential for torsional interaction with IG based wind farms.
This thesis also presents a study of turbine trippings in a large commercially operated wind farm in Ontario. Detailed analysis has led to the conclusion that the trippings were caused due to harmonic resonances resulting from large transmission capacitor and wind park capacitors. Several recommendations are made in this thesis to avoid the future wind turbine trippings.
Moharana, Akshaya Kumar, "Subsynchronous Resonance in Wind Farms" (2012). Electronic Thesis and Dissertation Repository. 912.
Farmer, Richard G. Department of Electrical Engineering, Arizona State University, Tempe, Arizona.
Agrawal, Bajarang Arizona Public Service Company, Phoenix, Arizona.
- Physical principles
- Links to Primary Literature
The resonance between a series-capacitor-compensated electric system and the mechanical spring-mass system of a turbine-generator at subsynchronous frequencies, that is, at frequencies that are less than the synchronous frequency. Beginning about 1950, series capacitors were installed in long alternating-current transmission lines [250 km (150 mi) or more] to cancel part of the inherent inductive reactance of the line. Until 1971, up to 70% of the 60-Hz inductive reactance was canceled by series capacitors in some long lines with little concern for side effects. (If 70% of a line's inductive reactance is canceled, the line is said to have 70% series compensation.) In 1970, and again in 1971, a turbine-generator at the Mohave Power Plant in southern Nevada experienced shaft damage that required several months of repairs on each occasion. This followed switching events that placed the turbine-generator so that it was radial on a series-compensated transmission line. The shaft damage was due to torsional oscillations between the two ends of the generator-exciter shaft. Shortly after the second event, it was determined that the torsional oscillations were caused by torsional interaction, which is a type of subsynchronous resonance (SSR). There are two other types of subsynchronous resonance, the induction generator effect and torque amplification. There has been one reported incidence of the induction generator effect type of SSR related to a wind farm in Texas in 2009, but there has been no reported incidence of torque amplification.
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