Bus Protection

Abstract

Differential protection relays are commonly used for bus protection. The transmission line currents injected to a busbar are measured by CTs as inputs of the relays. CT saturation can cause mal-operation of a protection relay or even prevent tripping. The waveform of the secondary current is severely distorted as the CT is forced into deep saturation when the residual flux in its core adds to the flux change caused by faults. In this chapter, a morphological lifting scheme (MLS) is introduced to extract features contained in the waveform of signal [91]. The detection of the CT saturation is accurately achieved and the points of the inflection, where the saturation begins and ends, are found by the scheme. This chapter also presents a compensation algorithm, based on the detection results, to reconstruct healthy secondary currents. MLS and the compensation algorithm are evaluated on a sample power system. The simulation results clearly indicate that they can successfully detect and compensate the distorted secondary current of a saturated CT containing residual flux.