What is the percentage differential mismatch for an electromechanical relay protecting a 138kV-69kV Delta-Wye transformer when the LTC is raised by 10%?

To determine the percentage differential mismatch for an electromechanical relay protecting a Delta-Wye transformer when the Load Tap Changer (LTC) is raised by 10%, it is essential to understand the operation of such transformers and the principles behind differential protection.

In a Delta-Wye transformer configuration, the line-to-neutral voltages on the Wye side are influenced by the tap changer settings and the transformation ratio. When the LTC is adjusted by raising it 10%, the voltage on the Wye side increases, thereby affecting the differential calculations.

The calculation for the percentage differential mismatch typically involves analyzing the ratio of the voltage change on one side of the transformer to the reference voltage of that side multiplied by 100%. In this scenario, if we assume the percentage differential mismatch is derived from the change in voltage readings between the Delta and Wye sides once the LTC is adjusted, a mismatch arises due to the altered secondary voltages.

This mismatch can be computed using transformer equations that reflect the change in turns ratio introduced by the LTC adjustments. Rising the LTC modifies the phase relationships and the voltages involved in the protective relay calculations. Therefore, a key factor affecting the computed mismatch is the inherent characteristics of the transformer and its windings under load conditions.

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