B90 LOW IMPEDANCE BUS DIFFERENTIAL SYSTEM – INSTRUCTION MANUAL 8-1B90 Low Impedance Bus Differential SystemChapter 8: Application of settingsApplication of settingsThis chapter provides example(s) of use.8.1 Overview8.1.1 IntroductionThis chapter provides an example of setting calculations for a sample bus. The selected example includes various busconfigurations to clarify a number of typical situations. Both the bus configuration and numerical data used are not meantto reflect any specific utility practice or design standards.It is also assumed that the CTs have been selected without considering a B90 application, but the UR settings are to becalculated for proper relay application. The CT data used in this example are kept to a minimum and in a generic form. TheCT data does not reflect any particular notation or national standards.The analysis provided in this chapter has been performed with the following goals:• The limits of linear operation of the CTs considering zero remanent flux have been determined in order to select thehigh breakpoint settings of the biased differential characteristic• The limits of linear operation of the CTs considering a remanent flux of 80% have been determined in order to selectthe low breakpoint settings of the biased differential characteristic• Saturation of the CTs has been analyzed in order to select the higher slope of the biased differential characteristic andthe high set differential overcurrent settingThe analysis tools and safety margins applied are examples only and do not reflect any particular protection philosophy.Typically, for the CT saturation related calculations, it is sufficient to consider the weakest (most prone to saturation) CTconnected to the bus and the total bus fault current combined with the longest time constant among all the circuitsconnected to the bus. This chapter provides more detailed analysis (see the Slopes and High Set Threshold section) in orderto illustrate the idea of using setting groups to enhance the B90 performance when the bus configuration changes (seethe Enhancing Relay Performance section).8.1.2 Sample busbar and dataThe figure shows a double bus arrangement with North and South buses. This station has five circuits (C-1 through C-5)and a tiebreaker (B-7). Circuit C-1 is connected to the North bus; circuits C-2, C-3 and C-4 can be routed to either bus viaswitches S-1 through S-6; circuit C-5 can be connected to either bus via breakers B-5 and B-6.
