A method for setting low excitation limit in an excitation system

With the rapid development of ultra-high-voltage direct current (UHVDC), the role of large synchronous machines in providing transient and steady-state reactive power support to the power grid has become increasingly important. When the machine operates near the reactive power boundary in a steady-state leading phase, a sudden increase in AC busbar voltage may trigger the sub-transient adjustment of the synchronous machine and the voltage regulation of the excitation system, causing a rapid decline in reactive power. If there is a coordination issue between the low excitation limit setting and the demagnetization protection, it could lead to an unnecessary activation of the demagnetization protection or even tripping of the machine. The paper analyses the coordination principles of demagnetization protection and low excitation limit for various types of synchronous machine, and examines the coordination relationship between the two. It proposes a method for setting the low excitation limit in large synchronous machines and validates the proposed method through case analysis and simulation.