Impact of the q-axis sub-transient reactance on the rotor oscillations of a hydro generator

Abstract

In the process of refurbishing a hydro generator, the owner has requested manufacturers to supply a machine that exhibits the same electrical parameters than the old one. The selected supplier has informed that such requirement can be met with the exception of the value of the qaxis sub-transient reactance. Fulfilling the requested value of the q-axis sub-transient reactance would result in a significant cost increase. Transient and sub-transient reactances and time constants determine the dynamic response of synchronous machines. Precisely, transient and sub-transient reactances in the d-axis explain the response of the response of the synchronous machine in case of a sudden short circuit at the machine terminals at no load. In contrast, there is not intuitive explanation of the physical meaning of the q-axis sub-transient reactance. Transient and sub-transient reactances and time constants of synchronous generators affect to the damping of rotor oscillations. Rotor oscillations result in oscillations of the electrical variables of the generator such as voltage, power and frequency. Rotor oscillations are in the range between 0.1 and 2 Hz depending of the generators involved in. The usual approach to damp rotor oscillations is to add a Power System Stabilizer (PSS) to the excitation system. A PSS modulates the electrical torque in opposite phase of the speed deviation by modulating the voltage set point of the automatic voltage regulator. This paper investigates the impact of the variation of the q-axis sub-transient reactance of an actual hydro generator on the damping of the rotor oscillations. Moreover, it is shown that a properly tuned PSS can overcome the impact of variations of the q-axis sub-transient reactance on the damping of rotor oscillations. The PSS is designed using a robust approach based on eigenvalue sensitivities. The design of the power system stabilizer is addressed considering a range of connecting impedances reflecting the changes of grid topology.