Network Reduction and Time-Domain Simulation Approaches for Increased System Security

Recent blackouts have shown that system swings have a critical effect on the operation of protective relays. The common method of investigating system swings and their effect on relays is to use a stability program and determine the effective impedance seen by the relay. Unfortunately, it is difficult to test a relay scheme with the resulting impedance trajectory. The inputs to relays are voltage and current waveforms, not voltage and current phasors or impedances. This paper describes a method for generating the voltage and current waveforms associated with system swings. A transients simulation program, ATP-EMTP, is used with support from a stability program, PSS/E, to produce a reduced system model for simulation of the voltage and current waveforms while retaining the machine dynamics of interest. The first part of the paper will describe the differences in the modeling assumptions of the two programs and how to translate the modeling assumptions of one program to those of the other. The second part presents an example of a four machine, two-area system, reduced to a two machine, two-area system.