Experimental Determination of Synchronous Generator Saturation Factors using AVR Open-Circuit Testing

Accurate modelling of power system components is essential for reliable power system analysis, particularly in excitation system and AVR studies. While IEEE Std. 421.5–2016 defines how generator saturation factors should be obtained, practical modelling often continues to use standard or factory data, with limited measurement-based updates for aged generators in actual service. This is a key research gap, since saturation characteristics may vary over time due to aging and operating stresses. Accordingly, this study presents the experimental determination of saturation factors of a 20 MW synchronous generator at Sapugaskanda Power Station, Sri Lanka. An open-circuit test was selected because it provides the direct terminal voltage–field current characteristic required for IEEE-based saturation-factor calculation. With the AVR in manual mode, the field current was varied within specified protection limits, and steady-state terminal voltage and field current were recorded using a Hioki data acquisition system and analysed in Excel to construct the open-circuit saturation curve and air-gap line. The saturation factors at 1.0 pu and 1.2 pu were then calculated using the IEEE method. The results provide updated field-based inputs for improved AVR and exciter model accuracy.