This paper presents an overview of the key issues in the use of frequency rate change (df/dt) in power system emergency control schemes. The role of df/dt in designing effective under frequency load shedding (UFLS) plan is discussed. The impact of new variable renewable sources (such as wind and solar units) on system frequency gradient is analyzed, and the need for the revising of frequency performance standards, re-tuning of automatic UFLS relays, and use of ∆f/∆t rather than df/dt are emphasized.
