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Comparison of Methods to Examine Sub-Synchronous Oscillations Caused by Grid-Connected Wind Turbine Generators

This paper compares three methods, the impedance model based analysis, the positive net damping analysis, and the open-loop modal resonance analysis. The differences and connections among these three methods are revealed, which helps to gain better understanding on the limitations of the methods for studying the mechanism of sub-synchronous oscillations (SSOs) caused by grid-connected wind turbine generators (WTGs) in a power system. Following useful conclusions are obtained, which are evaluated and demonstrated by study cases: First, both the negative resistance and negative net damping are highly related with strong dynamic interactions between the WTGs and ac grid under the condition of modal resonance. This explains why resistance and net damping may become negative from the standpoint of system modal condition. Second, it is possible that results of impendence model based analysis (IMA) and positive net damping analysis (PDA) may not reflect correctly the system SSO instability risk brought about by the WTGs. The reason is that the IMA and PDA are based on the sufficient condition of system stability. Assessment made by the IMA or PDA may be very conservative. In study cases, three methods are compared in the parameter space of converter controllers of the WTGs such that all the parameter conditions are covered in comparison.