Relaying method for protecting a transformer using a difference of current

A relaying method for protecting a transformer comprising the steps of:

• 1) selecting a decision function based on one of a primary current of the transformer, a secondary current of the transformer, and a differential current therebetween:
• 2) determining a value of the decision function by measuring the primary current and/or the secondary current, and determining whether the value of the decision function is greater than a predetermined critical value at predetermined intervals;
• 3) memorizing a certain time point as a first detection time when the value of the decision function is initially determined as a value greater than the critical value at a certain time point or after a previous secondary detection time point;
• 4) determining as an inrush the case where the value of the decision function is detected as a value greater than the critical value at a certain time point before a predetermined time and after the first detection time point;
• and 5) determining as an internal fault the case where the value of the decision function is not detected as a value greater than the critical value at a certain time point after a predetermined time and the first detection time point, and generating a signal for driving a circuit breaker to protect the transformer.