Practical Experience in Setting Transformer Differential Inrush Restraint

The second harmonic inrush restraint function of transformer differential relays maintains security of the differential protection during transformer inrush events. The typical setpoint for the second harmonic restraint is the relay manufacturer's default or recommended setting of 20% of fundamental current, with some adjustment based on operating experience. However, some operating situations may result in levels of second harmonic current lower than 20% during inrush, and levels may be as low as 5%. This lower level of second harmonic current requires a lower inrush restraint setting that may impact the tripping time of the differential element for fault conditions. In addition, inrush restraint is typically performed on a per-phase basis, so a loss of security is possible if inrush restraint performs incorrectly on only one phase of the protected transformer. This paper provides several examples of actual events where loss of security occurred due to incorrect settings of the second harmonic restraint function, or due to mis-application of cross-phase blocking. Based on the information from these events, the paper directly discusses considerations and recommendations for setting the second harmonic restraint to maintain security during transformer inrush including a discussion of traditional and adaptive inrush restraint techniques. The paper also includes recommendations on when to apply cross-phase blocking techniques such as 1-out-of-3 blocking, 2-out-of-3 blocking and Averaging blocking methods. A key message from this paper is the use of the actual inrush characteristic of the protected transformer to determine optimum setpoints for the differential relay.