Transformer and bus protection misoperations and how to avoid them

The first section of the paper analyzes a transformer differential misoperation that dropped a paralleled pair of large distribution transformers serving a sizeable block of contiguous load on the eastern seaboard. This section presents the unfiltered currents measured by the numerical transformer protection relay during the unwanted operation. The misoperation occurred when the faulted distribution feeder CT saturated due to the high magnitude fault current. A recommendation is made how to prevent these types of misoperations. The second section of the paper analyzes two outages that occurred at another distribution substation in less than one month due to the misoperation of the fast bus trip scheme and a defect in the numerical overcurrent relay protecting one of the 23 kV feeders. The paper outlines the modification of the fast bus trip scheme logic to prevent this type of outage from re-occurring.