An Auxiliary Framework to Mitigate Measurement Inaccuracies Caused by Capacitive Voltage Transformers

The substantial cost of voltage transformers in high- and extra-high-voltage levels has persuaded power engineers to widely utilize capacitive voltage transformers (CVTs) in transmission systems. CVTs, however, demonstrate an inferior transient response in certain situations, which distorts their secondary voltages. Consequently, the voltage waveform produced by a CVT might not be an accurate stepped-down version of its primary voltage and can result in malfunction of power system components, such as overreaching of distance relays. This article proposes an auxiliary voltage-compensation-based framework to mitigate measurement inaccuracies caused by the weak transient response of CVTs. This framework enables power system components, such as relays, to work accurately when the secondary voltage of CVTs is distorted, without sacrificing their sensitivity and/or operation speed. The proposed method models a CVT with its state-space representation and uses unknown input observers to estimate the states of the CVT and its primary voltage, which is modeled as an unknown input. Thus, after implementing the proposed auxiliary framework, the estimated primary voltage of CVTs is used, instead of their distorted secondary voltages, in power system applications. The results of electromagnetic transient simulation in MATLAB/Simulink corroborate the effectiveness of the proposed method under various conditions and prove its independence from system and fault parameters.