Evaluation of Rotor Speed Stability Margin of a Constant Speed Wind Turbine Generator

Many developing countries like India have installed large number of constant speed wind turbine generators (WTGs) as they are robust and economical. Most of the countries have their own grid codes (rules and regulations) to integrate WTGs into the utility grid. One of the primary grid code for WTGs is low voltage ride through (LVRT) capability. The regulations of grid integration are likely to make LVRT requirements mandatory for WTGs with high penetration level. To demonstrate LVRT capability, a WTG has to remain connected to the grid at a specific low voltage and for a specific duration. Generally, a constant speed WTG does not satisfy LVRT requirements. This is because during a nearby fault, the rotor accelerates to a very high speed, and hence the WTG becomes unstable. This phenomenon is referred to as rotor speed instability. To satisfy LVRT requirements, the rotor speed stability margin of a constant speed WTG has to be improved. One of the methods to meet the LVRT requirements is by providing additional reactive power support which can improve the terminal voltage during a disturbance. This, in turn, will increase the electromagnetic torque and hence, the rotor acceleration can be reduced. This paper presents an evaluation of rotor speed stability margin to obtain critical slip and critical clearing time of a constant speed wind turbine generator. For this analysis, analytical formulae have been presented to determine the exact amount of additional reactive power support required to meet the LVRT capabilities.