Investigation of the effect of current and voltage transformers´ errors on a differential-like impedance relaying algorithm — Protection from a probabilistic view

Dependable and secure operation of protection system is a basic requirement. In the era of smart grids, this requirement is indeed much higher. Errors due to current and voltage transformers form the main source of errors in the measured current and voltage of the protection relays. These errors affect the used protection algorithm. For instance, in percentage differential relays, measurement errors are alleviated by increasing the slope of the differential relay characteristics. This in turn reduces the sensitivity of the relay. If the slope of the characteristics is reduced to achieve high sensitivity, the security is much reduced. This paper proposes a new protection function that can achieve a high dependable and secure operation of protection relays. The new function incorporates the effect of errors of current and voltage transformers in a way that enables detection of faults with very high fault path impedance while the function remains secure against non-fault phenomena like load encroachment, voltage instability and power swing conditions. Comparison between the proposed function and conventional percentage differential relays shows superiority of the proposed approach. Other advantages are discussed throughout the text.