Dynamics and Control of Electric Transmission and Microgrids

6Damping of Power Swings

6.1 Introduction

The stability of the relative motion between the rotors of synchronous machines which are interconnected by AC transmission lines (a synchronous grid) is an important concern for power system engineers. This is also known as the angular stability problem. The relative motion is oscillatory when the system is subjected to small disturbances. The frequencies of these oscillations typically lie between 0.2 to 2 Hz. Usually these oscillations are stable, that is, they die out and the system settles down. Under certain operating conditions, the oscillations may become unstable, that is, they grow in amplitude instead of dying out. The growing oscillations may eventually become large and sustained, or may increase to such an extent that synchronism between the generators is irrevocably lost. In either case, this leads to large fluctuations in voltage, power flows, generator torques and speeds, which cannot be tolerated for a long duration; manual or automatic control/protective actions have to be initiated under these circumstances.

Figure 6.1 Stable and unstable swings observed in the Indian grid. (a) Stable swings (b) Unstable swings

Real‐life measurements of both stable and unstable oscillations, as seen in the frequency at various locations in the Indian grid, are shown in Figure 6.1. These were measured using a wide‐area frequency measurement ...