Computer-aided design of a new nonunit protection scheme for EHV teed circuits

The paper describes a new technique for protecting EHV teed transmission lines that is based on nonunit principles but exhibits the properties of a unit protection scheme. Fault generated high frequency (HF) signals are extracted from the coupling capacitor of a conventional capacitor voltage transformer (CVT) with a specially designed measurement circuit. Power line carrier line traps are used to confine the HF signals to the protected zone and their bandstop characteristics are used as a basis for discriminating between internal and external faults, A digital signal processing unit is used to process the signals and produce a trip output. Some simulated results are presented to show the ability of the scheme to rapidly detect faults on different teed circuits under a variety of system and fault conditions encountered in practice.