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The protection functions applied for sensitive directional earth-fault protection in compensated MV-networks are based on measurement of residual current and voltage, and derivation of the resistive operation quantity thereof. In this paper, the importance of residual current measurement accuracy on earth-fault protection performance in compensated MV-networks is analysed. First the fundamental theory of directional earth-fault protection is recalled. Then the factors affecting the measuring errors of a Core Balance Current Transformer (CBCT) are described. Finally a selection guideline of CBCTs for residual current and neutral admittance-based directional earth-fault protection taking into account the measurement errors is given. INTRODUCTION Directional earth-fault protection in compensated networks A simplified three-phase equivalent circuit of a compensated network during a single phase earth fault is presented in Fig. 1. From it, general equations required for basic earth-fault protection analysis can be derived [1]: phasor of the residual voltage or the neutral point voltage of the network Uo = (UA + UB + UC)/3, and phasor of the residual current or the sum current I?̅? = (I?̅? + I?̅? + I?̅?) measured at the beginning of the protected feeder during an earth fault. In the following equations, ideal symmetry of the network is assumed. RF PE op a U U   (1a) RF Net EFFd OutsideEF op a j d I I ) 1 (      (1b)

Effect of core balance current transformer errors on sensitive earth-fault protection in compensated MV networks