Impact of Applied Voltage on the Frequency Response Measurements for Transient Modeling of Capacitive Voltage Transformers

The increase in renewable energy has led to new transients, increasing the importance of transient studies. Moreover, the broadband modeling of high-voltage equipment often employs black-box or grey-box techniques, using frequency response measurements. However, impedance analyzer limitations in applying voltages well below nominal levels compromise model accuracy, especially for low-frequency transients. This paper evaluates the impact of applied voltage on the frequency response measurements, focusing on modeling capacitive voltage transformers (CVTs) for transient studies. A CVT model, validated in prior research, serves as a reference. Measurements on a 145 kV CVT in a high-voltage laboratory with three distinct setups are analyzed. The results show a clear relationship: as the applied voltage gets closer to the nominal voltage, the deviation from the reference curve decreases. These findings highlight the need to be careful when using frequency response measurements for low-frequency modeling, especially regarding nonlinearities in magnetic core elements.