that it is CT saturation and therefore is an external (through) fault which doesn’t need tripping. In the GROUPX DIFF PROTECTION column, you can set the threshold PhComp Thresh.Note:This is a single-phase device and the term ‘phase comparison’ does not have any association with the word ‘phase’normally used for single-phase and three-phase systems. However, it is closely associated with the phasecomparison techniques used for unit protection of transmission lines using power line carrier.3.8 OPTIONAL VOLTAGE CRITERIAIf a voltage transformer connection is available, you can use an undervoltage element to supervise thecurrent differential decision. In the GROUP X DIFF PROTECTION column,set Voltage Check to Enabled.The undervoltage settings then become visible. Set V< Status Zone x (where x is the zone number) toEnabled or Disabled.3.9 INSTANTANEOUS OR DELAYED TRIPPINGYou can set a particular zone differential element to trip as soon as a fault is detected. For example, highspeed disconnection of a busbar from a transmission grid. Alternatively tripping can be delayed, for examplewhen disconnecting generator feeders.In the GROUP X DIFF PROTECTION column, set tDIFF to 0 for instantaneous tripping. Alternatively, set it tothe required time delay.3.10 DEAD ZONE (BLIND SPOT) PROTECTIONThe CTs or open switches surrounding busbars normally define the limits of the main zones of protection.When a feeder switch is opened, a dead-zone or blind spot can be created between the switch and the CT.This product can detect the condition automatically from a topology replica and protect the dead zone. Thisis done using one stage of definite time-delayed overcurrent protection. To use this, in theCONFIGURATION column, set Dead Zone OC to Enable, then in the GROUP X DEAD ZONE OC columnset I> Status to Enable. The dead zone protection is only activated if the topology replica indicates a deadzone condition.MiCOM P747 Chapter 5 - Protection FunctionsP747-TM-EN-1 109
