5-220 D30 LINE DISTANCE PROTECTION SYSTEM – INSTRUCTION MANUALGROUPED ELEMENTS CHAPTER 5: SETTINGS55.7.7.2 Inverse TOC curve characteristicsThe inverse time overcurrent curves used by the time overcurrent elements are the IEEE, IEC, GE Type IAC, and I2 t standardcurve shapes. This allows for simplified coordination with downstream devices.If none of these curve shapes is adequate, FlexCurves can be used to customize the inverse time curve characteristics. Thedefinite time curve is also an option that can be appropriate if only simple protection is required.Table 5-24: Overcurrent curve typesA time dial multiplier setting allows selection of a multiple of the base curve shape (where the time dial multiplier = 1) withthe curve shape (CURVE) setting. Unlike the electromechanical time dial equivalent, operate times are directly proportionalto the time multiplier (TD MULTIPLIER) setting value. For example, all times for a multiplier of 10 are 10 times the multiplier 1or base curve values. Setting the multiplier to zero results in an instantaneous response to all current levels above pickup.Time overcurrent time calculations are made with an internal energy capacity memory variable. When this variableindicates that the energy capacity has reached 100%, a time overcurrent element operates. If less than 100% energycapacity is accumulated in this variable and the current falls below the dropout threshold of 97 to 98% of the pickup value,the variable must be reduced. Two methods of this resetting operation are available: “Instantaneous” and “Timed.” The“Instantaneous” selection is intended for applications with other relays, such as most static relays, which set the energycapacity directly to zero when the current falls below the reset threshold. The “Timed” selection can be used where therelay must coordinate with electromechanical relays.IEEE curvesThe IEEE time overcurrent curve shapes conform to industry standards and the IEEE C37.112-1996 curve classifications forextremely, very, and moderately inverse curves. The IEEE curves are derived from the operate and reset time equations.Eq. 5-7whereT = operate time (in seconds)TDM = Multiplier settingI = input currentIpickup = Pickup Current setting PHASE TOC 2 PHASE IOC 1See page 5-227 PHASE IOC 2 PHASE DIRECTIONAL 1See page 5-228 PHASE DIRECTIONAL 2IEEE IEC GE type IAC OtherIEEE Extremely Inverse IEC Curve A (BS142) IAC Extremely Inverse I 2 tIEEE Very Inverse IEC Curve B (BS142) IAC Very Inverse FlexCurves A, B, C, and DIEEE Moderately Inverse IEC Curve C (BS142) IAC Inverse Recloser CurvesIEC Short Inverse IAC Short Inverse Definite Time
