Dynamics and Control of Electric Transmission and Microgrids

7Analysis and Control of Loss of Synchronism

7.1 Introduction

In the previous chapter we saw that a synchronous grid – a grid having synchronous machines connected by AC lines – may occasionally experience small‐signal angular instability, manifested as negatively damped power swings. Auxiliary controllers like power system stabilizers (PSSs) in generator excitation systems or power swing damping controllers in flexible AC transmission systems (FACTS) and high‐voltage direct current (HVDC) systems can add damping to these power swings. While small‐signal stability is essential for system operation, it does not guarantee that the system will be stable for any arbitrarily large deviation of the states from the equilibrium point. It is seen in synchronous grids that if the rotor angle and speed deviations (due to disturbances) lie outside the region of attraction of the equilibrium point, then it may lead to an unbounded increase in the relative rotor angles and speeds. The system then cannot settle back to a synchronous operating condition. This phenomenon is known as loss of synchronism (LoS).

A system is vulnerable to LoS when the region of attraction is small or when disturbances like faults cause large deviations from the equilibrium. If the system is small‐signal unstable or does not have an equilibrium, then the region of attraction is non‐existent. Under these circumstances too, the relative rotor angles and speeds may increase and eventually cause LoS. The terms pole‐slipping, ...