Ultra-Low Frequency Oscillation in a Thermal Power System Induced by Doubly-Fed Induction Generators with Inertia Control

Ultra-low frequency oscillation has been regarded as a typical instability issue in power systems consisting of hydro turbine synchronous generators due to the water hammer phenomenon. However, the increasing installation of renewable power generators gradually changes the stability mechanisms within multiple frequency bands. In this digest, a new kind of ultra-low frequency oscillation caused by doubly-fed induction generators (DFIGs) equipped with a df/dt controller in a thermal power generation system is introduced. To reveal the underlying mechanism, the motion equation model of the DFIG is constructed, and the simplified analytical model is proposed. The results show that when integrating a df/dt-controlled DFIG into a normal three-machine, nine-bus system, the damping ratio decreases to more than 0.2 when the virtual inertia parameter increases from 5 to 20, leading to a conflict between fast virtual inertial response and stability requirements. Other controllers related to active power regulation are also vital to stability. The frequency domain characteristics of the system are studied to illustrate the influence of key parameters on system stability. Finally, simulation verifications are conducted in MATLAB/Simulink.