CCVT Failure due to Improper Design of Auxiliary Voltage Transformers

In-situ tests of coupling capacitor voltage transformers (CCVTs) in some of the high-voltage substations indicate that the ratio error of a considerable number of CCVTs has increased to more than 5%. Experimental analyses reveal that the CCVTs excessive ratio error is mainly due to the short-circuited capacitor elements of CCVTs. Comprehensive tests and investigations indicate that the occurrence of ferroresonance due to saturation of the auxiliary voltage transformers (AVTs) is one of the main reasons for this problem. These small transformers are usually utilized inside the protective relays and control and measuring equipment or are connected to the secondary side of CCVTs as an interface between the CCVT and the equipment. Imposed stress on CCVT components and the voltage oscillations´ effect on transmission-line distance relay are also scrutinized in this paper. Finally, appropriate specifications for AVTs are recommended to prevent undesirable CCVT overvoltages and distance relay maloperation.