A method of operating a compressor of a compressor system uses three models. Each model of the three models describes a surge line of the compressor as a function of any two of three operating parameters of the compressor. The three operating parameters include head (H), flow (Q), and speed (N). The method includes measuring operating characteristics of the compressor system using sensors, and determining a current value of the three operating parameters based on at least some of the measured operating characteristics. The method also includes locating operating points of the compressor on each of the three models based on the current value of the operating parameters, and identifying a sensor fault that affects the determination of at least one of the operating parameters. The method further includes avoiding surge of the compressor using one model of the three models. The one model being a model that is a function of two operating parameters unaffected by the sensor fault.