Improved protection scheme for selective high resistance earth fault clearing in Slovenian distribution networks

Normal earth-fault protection is based on the assumption that fault currents that flow to earth will be determined by the earth-fault impedance of the line and associated plant. In some cases, however, the nature of the ground fault may be such that it makes ineffective earth connection. In such a case a system fault to earth may result in a current which is too small to allow the protection to operate in accordance with the normal earth fault settings. If a line conductor should break and fall on to semi-insulating objects, such as trees or hedges, again the fault current may be below the normal primary earth-fault setting. The circuit will, therefore, not be isolated and the conductor, which will be maintained at normal phase voltage will constitute a serious danger to life. Various methods of selective high resistance fault clearing in medium voltage networks grounded via low resistance are discussed. The new method proposes the activation of: earth-fault protection with independent time delay characteristic; sensitive earth-fault protection with independent time delay characteristic; and directional sensitive earth-fault protection with independent time delay characteristic, in the integrated feeder protection and control device. The case study has shown that the new method is not selective enough. Therefore the method was improved by introducing the additional signal which enables the design of various tripping or blocking schemes.