Alternative method of determining zero-sequence voltage for fault current passage indicators in overhead medium voltage networks

Zero-sequence voltage is one of the main quantities that serve as a criterion for detecting earth faults in medium voltage networks. This paper presents a novel but simple method of determining zero-sequence voltage for smart grid protection devices, especially for use in fault current passage indicators. The presented method can also be used to measure zero-sequence voltage in any point of the network, where such values are required and application of other classical measurement methods is not possible. The method is based on continuous measurement of only one phase-to-earth voltage performed by an overhead voltage sensor. Both laboratory and field tests of the proposed method were carried out and their results indicate that the maximum expected relative error in determining the zero-sequence voltage is delta U-0 = 10, 15%. Taking into account the simplification applied in the method, the obtained measurement results are sufficient to be used in the earth fault detection criteria in fault current passage indicators. The use of the proposed method allows to reduce the installation cost of this type of devices, while maintaining their high efficiency.