As part of my PhD project, I worked on stability analysis of a class of online systems. Following that, in collaboration with other researchers, we came up with results on small-signal stability analysis of Power System Stabilisers (PSS) in a power grid. The Power System Stabiliseris a supplementary excitation controller used to damp generator electro-mechanical oscillations in order to protect the shaft line and stabilise the grid. In order to do so, PSS should read the frequency and voltage of the line. And with the ever-more widespread usage of wide-area network (WAN) measurements in power grids, a crucial question is: how does the time delay between when such measurements are done and when they are fed into the feedback loop of PSS will affect the stability of the synchronous generator? We performed a thorough assessment of the effect of delay in such cases. The results were published as a book chapter, called "Small-signal stability and time domain analysis of a power system with feedback delay" in a book titled: "Advances in Power System Modelling, Control and Stability Analysis". The first edition of the book was published in 2016 and the second edition in 2022. You can find the book here.