Numerical modelling of ground faults in MV networks for reliable testing of protection algorithms

The paper addresses the problems of digital modelling of MV networks with the focus on phenomena having a significant impact on the reliability of resulting data subsequently used for testing protection relays. In ATPDraw program the Ferranti current transformer (core-balance current transformer) model was developed that allows taking into account the composite error that is due to discrepancy of magnetic coupling between the cable cores and the secondary coil as well as to the magnetizing current of the transformer. The impact of higher harmonics in the supplying voltage on algorithm performance in pre- and fault conditions has been investigated as well. The test results obtained for data without mentioned disturbances have been compared with outcomes for faults in the system with current transformation errors and harmonics in the pre-fault MV network signals.