that it is CT saturation and therefore is an external (through) fault which doesn’t need tripping. In the GROUP
X 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 phase
comparison techniques used for unit protection of transmission lines using power line carrier.
3.8 OPTIONAL VOLTAGE CRITERIA
If a voltage transformer connection is available, you can use an undervoltage element to supervise the
current 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) to
Enabled or Disabled.
3.9 INSTANTANEOUS OR DELAYED TRIPPING
You can set a particular zone differential element to trip as soon as a fault is detected. For example, high
speed disconnection of a busbar from a transmission grid. Alternatively tripping can be delayed, for example
when disconnecting generator feeders.
In the GROUP X DIFF PROTECTION
column, set tDIFF to 0 for instantaneous tripping. Alternatively, set it to
the required time delay.
3.10 DEAD ZONE (BLIND SPOT) PROTECTION
The 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. This
is done using one stage of definite time-delayed overcurrent protection. To use this, in the
CONFIGURATION column, set Dead Zone OC to Enable, then in the GROUP X DEAD ZONE OC column
set I> Status
to Enable. The dead zone protection is only activated if the topology replica indicates a dead
zone condition.
MiCOM P747 Chapter 5 - Protection Functions
P747-TM-EN-1 109
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