GE P24DM Technical Manual

Also see for MiCOM P40 Agile: Technical manual Technical manual Technical manual Technical manual Technical manual

12 BLOCKED OVERCURRENT PROTECTIONWith Blocked Overcurrent schemes, you connect the start contacts from downstream IEDs to the timer blockinginputs of upstream IEDs. This allows identical current and time settings to be used on each of the IEDs in thescheme, as the device nearest to the fault does not receive a blocking signal and so trips discriminatively. This typeof scheme therefore reduces the number of required grading stages, and consequently fault clearance times.The principle of Blocked Overcurrent protection may be extended by setting fast-acting overcurrent elements onthe incoming feeders to a substation, which are then arranged to be blocked by start contacts from the devicesprotecting the outgoing feeders. The fast-acting element is thus allowed to trip for a fault condition on the busbar,but is stable for external feeder faults due to the blocking signal.This type of scheme provides much reduced fault clearance times for busbar faults than would be the case withconventional time-graded overcurrent protection. The availability of multiple overcurrent and earth fault stages inthe General Electric IEDs allows additional time-graded overcurrent protection for back-up purposes.12.1 BLOCKED OVERCURRENT IMPLEMENTATIONBlocked Overcurrent schemes are implemented using the PSL. The start outputs, available from each stage of theovercurrent and earth fault elements (including the sensitive earth fault element) can be mapped to output relaycontacts. These outputs can then be connected to the relevant timer block inputs of the upstream IEDs via opto-inputs.12.2 BLOCKED OVERCURRENT LOGICTo facilitate the implementation of blocked overcurrent schemes, the device provides the following logic to providea Blocked Overcurrent Start signal I> BlockStart:Remove I > StartEnabled&CB Fail AlarmDisabled &1I>1 StartI>2 StartI>3 StartI>4 StartI>5 StartI>6 Start1 I> BlockStartV00648Figure 67: Blocked Overcurrent logicThe I> BlockStart signal is derived from the logical OR of the phase overcurrent start outputs. This output is thengated with the CB Fail Alarm DDB signal and the setting Remove I> Start setting.12.3 BLOCKED EARTH FAULT LOGICTo facilitate the implementation of blocked overcurrent schemes, the device provides the following logic to providethe Blocked Earth Fault signal IN/SEF>Blk Start:P24xM Chapter 6 - Current Protection FunctionsP24xM-TM-EN-2.1 143