Section 4 1MAC309294-MB FProtection functions142 RER620Technical ManualThe setting parameter Minimum trip time defines the minimum desired trip time for IDMT.The setting is applicable only when the IDMT curves are used.The timer calculates the pickup duration (PICKUP_DUR) value which indicates thepercentage ratio of the pickup situation and the set trip time. The value is available throughthe Monitored data view.Blocking logicThere are three operation modes in the blocking functionality. The operation modes arecontrolled by the BLOCK input and the global setting “Configuration/System/Blockingmode” which selects the blocking mode. The BLOCK input can be controlled by a binaryinput, a horizontal communication input or an internal signal of the relay program. Theinfluence of the BLOCK signal activation is preselected with the global setting Blockingmode.The Blocking mode setting has three blocking methods. In the “Freeze timers” mode, thetrip timer is frozen to the prevailing value. In the “Block all” mode, the whole function isblocked and the timers are reset. In the “Block TRIP output” mode, the function operatesnormally but the TRIP output is not activated.4.1.4.5 Directional ground-fault principlesIn many cases it is difficult to achieve selective ground-fault protection based on themagnitude of zero sequence current only. To obtain a selective ground-fault protectionscheme, it is necessary to take the phase angle of I0 into account. This is done bycomparing the phase angle of I0 to that of the zero sequence voltage (-V0).Relay characteristic angleThe Characteristic angle, also known as Relay Characteristic Angle (RCA), Relay BaseAngle or Maximum Torque Angle (MTA), is used in the “Phase angle” mode to turn thedirectional characteristic, if the expected fault current angle does not coincide with thepolarizing quantity to produce the maximum torque. That is, RCA is the angle between themaximum torque line and polarizing quantity. If the polarizing quantity is in phase withthe maximum torque line, RCA is 0 degrees. The angle is positive if operating current lagsthe polarizing quantity and negative if it leads the polarizing quantity.Example 1.The “Phase angle” mode is selected, compensated network (φRCA = 0 deg)=> Characteristic angle = 0 degThe Minimum trip time setting should be used with great care because theoperation time is according to the IDMT curve, but always at least the valueof the Minimum operate time setting. For more information, see theGeneral function block features section in this manual.
