The integrated damper winding behaves like a continuous classic damper winding during sudden short circuits in large, saturated hydro generators

However, due to the relatively low voltage and current applied in these tests, the derived ratio is only valid for unsaturated conditions.

This paper presents a method to evaluate 𝑋q″ and the ratio 𝑋q′′ ⁄𝑋d′′ from sudden three-phase short circuit tests. The method can be applied in the whole range up to rated pre-short circuit terminal voltage, and provides clear advantages compared to current methods described in IEC and IEEE [1], [2]. The method is thus a candidate for future incorporation in these standards.

Tests performed on five generators (M1-M5), each with rated power and speed in the ranges 50 − 170 MVA and 100 − 200 rpm, are being evaluated. As the pre-sudden short circuit terminal voltage increases for all four machines with integrated damper winding (M1-M4), the ratio 𝑋q′′ ⁄𝑋d′′ drops significantly and at rated voltage approaches that of a salient pole synchronous machine with a continuous classic damper winding. Based on 2D FE simulation, this work shows that the 𝑋q′′ ⁄𝑋d′′ behavior measured and discussed in this paper is caused by a change in pole-to-pole impedance. Analytical methods and 3D FE models are used to investigate the pole-to-pole impedance under unsaturated and saturated conditions. The most important implication of the lower 𝑋q′′ ⁄𝑋d′′ ratio at rated voltage is reduced peak torques in the event of a sudden two-phase short circuit and 120° out-of-phase synchronization.

For generator M1 the lower 𝑋q′′ ⁄𝑋d′′ ratio at rated voltage reduces the calculated peak torque for those two fault cases by around 7 %, compared to a calculation using the 𝑋q′′ ⁄𝑋d′′ ratio for the unsaturated case.