Impact of current transformer saturation on overcurrent protection operation

If current transformer (CT) characteristics are not properly selected for fault conditions, saturation will occur which can affect protection device operation. In some applications even if CTs are properly sized, it is necessary to locate protection devices far from the CTs. In these cases, the wire resistance, which adds to the CT burden, causes CT saturation at smaller currents than when located close to the protection devices. The impact of CT saturation on protection device operation is different for different protection device types and protection schemes. This paper investigates the influence of CT saturation on overcurrent protection when protection devices are located far from the CTs, causing their saturation during faults. A study was performed to verify that delayed overcurrent protection response will not cause mis-coordination with other protection devices. It is shown that CT saturation will not significantly delay overcurrent protection operation for most common settings.