Impact of incipient faults on sensitive protection

Incipient faults first represent a challenge for the detection of such fault by the primary protection. These faults could last few milliseconds only, being intermittent in nature and possibly evolve eventually over time to a complete insulation breakdown, creating a permanent fault. However, these undetected “ghost” faults may have inadvertent impact on the sensitive protection for adjacent protection zones, compromising its security. This paper will focus on the impact of such faults on the sensitive protection, including transformer restricted ground fault (RGF) and sensitive ground fault protection. Incipient faults create DC offset in currents, which may drive CTs into a light saturation. This paper reviews a real field application case where a repetitive intermittent fault on a 22-kV underground cable was undetected for a long period of time, leading to the transformer neutral CT saturation and to the incorrect operation of two transformer RGF schemes in the substation. The gradual process of CT saturation is explained in detail. Referring to the transformer low impedance restricted ground fault, scheme security is accomplished by the specific algorithm with some additional supervisory features. Particular attention is dedicated to improving the security of the RGF scheme and other affected sensitive protections for this type of external incipient faults, without jeopardizing dependability and speed of operation.