Generalised locus charts for the evaluation of differential feeder protection performance

The paper describes a method by which generalised fault locus charts may be derived for the fundamental differential feeder protection relaying quantities, and applied to the evaluation of protection performance. The charts are plotted for the basic system parameters, and for all types of fault condition on single-circuit and double-circuit lines. Each chart, plotted in terms of the ratio of the effective currents at each end of the protected feeder and the prefault source voltages, takes the form of orthogonal circles whose orientation and scaling are defined by appropriate constants for each particular fault and summation device. Tables are provided which give the values of the constants for most common types of fault in terms of generalised system parameters, and examples show how particular charts can be derived from these tables. By means of the charts, the performance of any differential protection scheme may be determined, and two typical, although idealised, characteristics are used to demonstrate the method. The examples illustrate certain shortcomings in differential protection performance, from which the limiting conditions for the general circuit parameters can be examined.