CHAPTER 6: SETPOINTS S3 PROTECTION350 FEEDER PROTECTION SYSTEM – INSTRUCTION MANUAL 6–256.4.1.1 TOC curvesDESCRIPTIONThe relay has a total of two phase, two neutral, and two ground/sensitive ground timeovercurrent elements. The programming of the time-current characteristics of theseelements is identical in all cases and will only be covered in this section. The required curveis established by programming a Pickup Current, Curve Shape, Curve Multiplier, and ResetTime. The Curve Shape can be either a standard shape or a user-defined shapeprogrammed with the FlexCurve™ feature.Accurate coordination may require changing the time overcurrent characteristics ofparticular elements under different conditions. For picking up a cold load, a different time-current characteristic can be produced by increasing the pickup current value. Thefollowing setpoints are used to program the time-current characteristics.• PICKUP: The pickup current is the threshold current at which thetime overcurrent element starts timing. There is no intentional ‘dead band’ when thecurrent is above the pickup level. However, accuracy is only guaranteed above a 1.5per unit pickup level. The dropout threshold is 98% of the pickup threshold. Enter thepickup current corresponding to 1 per unit on the time overcurrent curves as amultiple of the source CT. For example, if 100: 5 CTs are used and a pickup of 90 ampsis required for the time overcurrent element, enter “0.9 x CT”.• CURVE: Select the desired curve shape. If none of the standardcurve shapes is appropriate, a custom FlexCurve™ can be created by entering the triptimes at 80 different current values; see S2 SYSTEM SETUP > FLEXCURVE A. Curveformulas are given for use with computer based coordination programs. Calculatedtrip time values are only valid for I / Ipu > 1. Select the appropriate curve shape andmultiplier, thus matching the appropriate curve with the protection requirements. Theavailable curves are shown in the table below.• MULTIPLIER: A multiplier setpoint allows shifting of the selectedbase curve in the vertical time direction. Unlike the electromechanical time dialequivalent, trip times are directly proportional to the value of the time multipliersetpoint. For example, all trip times for a multiplier of 10 are 10 times the multiplier 1or base curve values.• RESET: Time overcurrent tripping time calculations are made withan internal ‘energy capacity’ memory variable. When this variable indicates that theenergy capacity has reached 100%, a time overcurrent trip is generated. If less than100% is accumulated in this variable and the current falls below the dropout thresholdof 97 to 99% of the pickup value, the variable must be reduced. Two methods of thisresetting operation are available, Instantaneous and Linear. The Instantaneousselection is intended for applications with other relays, such as most static units,which set the energy capacity directly to zero when the current falls below the resetthreshold. The Linear selection can be used where the relay must coordinate withelectromechanical units. With this setpoint, the energy capacity variable isdecremented according to the following equation.ANSI GE TYPE IAC IEC OTHERExtremely Inverse Extremely Inverse Curve A (BS142) Definite TimeVery Inverse Very Inverse Curve B (BS142) Flexcurve A TMNormally Inverse Inverse Curve C (BS142) Flexcurve BTMModerately Inverse Short Inverse IEC Short Inverse User Curve
