Abstract: As the key indicator of frequency monitoring and decision basis of frequency control, the frequency nadir in the process of frequency response with high power deficit including the maximum frequency deviation and the time of frequency nadir should be estimated accurately. A low-order simulation model for frequency nadir estimation with a high power deficit is developed. The impulse response characteristics of governor are described by polynomial fitting the differential response with valve disturbance. The Laplace transformation of the governor response characteristics of each unit is adopted to establish the equivalent governor model of the system. The effectiveness of the low-order simulation model is presented based on the analysis of frequency deviation process with a high power deficit. Case studies show that the proposed model could reduce the order of frequency response model, improve the computation efficiency and achieve a satisfactory computation accuracy of frequency nadir.