Possibilities of increasing the sensitivity of current backup protection in symmetrical modes

Abstract

This paper examines the problem of increasing the sensitivity of backup protection systems, specifically long-range backup protection systems (LRPS), at 110-220 kV branch substations (BSS) with relatively low-power power trans-formers (6.3-16 MVA). Short-circuit currents (SC) during faults on the low-voltage (LV) side are comparable to maxi-mum-load currents fed from overhead power lines (OPLs). The objective of the study was to evaluate the current measuring units of emergency components (CMUs) of the backup relay protection system, which provides LRPS func-tions for transformers in overhead power lines (OPLs) during symmetrical short circuits, from the standpoint of recogniz-ing emergency conditions against permissible conditions. The study utilized calculation methods in the Mathcad math-ematical package for emergency and alternative permissible modes: short circuits in the presence of transient resistance, maximum load conditions, and high-voltage electric motor starts and self-starts. The degree of recognition was deter-mined by assessing the distance between emergency and permissible mode zones, including automatic reclosing of overhead power lines and short circuits in transit. The Euclidean metric was used as a measure of proximity between these zones. Based on the conducted research, it was shown that the most effective in terms of detecting symmetrical three-phase faults through contact resistance under maximum load conditions and during high-voltage electric motor starting and self-starting modes are emergency reactive component measuring devices whose adaptive response pa-rameters are constructed in the following planes: «emergency reactive current components - increment of the arguments of monitored line currents» and «emergency reactive current components - monitored line currents». This ensures the required sensitivity of multiparameter backup protection for at least one side of the overhead line, followed by fault localization in a cascade mode. The materials of the article are of interest both to researchers developing modern relay protection systems and to specialists of electric grid companies for assessing the technical perfection of existing ZDR solutions.