Current Differential Intertripping; Figure 43: Permissive Intertripping Example - GE P4A Technical Manual

P54A/B/C/E
7

CURRENT DIFFERENTIAL INTERTRIPPING

Eight freely assignable intertripping signals are provided by a facility called InterMICOM64, which is described in
the chapter on Fibre Teleprotection. In addition there are two intertripping functions associated directly with the
Current Differential protection. Both are operational when the phase current differential is enabled:
The first is a differential intertripping function whereby if one terminal sees a differential fault, as well as issuing a
local trip command it will also send a command to trip to remote terminals to ensure that the fault is isolated at all
terminals.
The second is a permissive intertripping function. A Permissive Intertripping function (PIT) is associated with the
current differential protection and is visible and active if the phase current differential protection is enabled.
Note:
The term "permissive intertripping" associated with this current differential protection is not the same as that commonly used
in teleprotection schemes. It is specific to this type of Current Differential protection implementation.
A device can be configured to send a permissive intertrip command over the protection communication channel.
To use this function you need to map the Perm Intertrip DDB signal to one of the opto-inputs using the PSL.
IED 1
Permissive
Intertrip (PIT) 000

Figure 43: Permissive Intertripping example

Consider the above diagram. If a fault occurs as shown, it will be seen by the busbar protection which can trip its
local circuit breaker. The fault will not be seen by the differential protection, however, so the fault will continue to
be fed. An input signal from the busbar protection at the faulted end can be sent as a permissive intertrip (PIT)
command to a remote terminal to cause it to permissively trip the remote circuit breaker to clear the fault.
Tripping occurs if the current remains above the Phase Is1 setting of the phase current differential elements while
the PIT command is received. The condition must remain satisfied for a minimum time setting. The time is set in the
PIT Time setting in the CURRENT DIFF column. The permissive intertrip (PIT) timer can be set between 0 and 200
ms. This time should be set to provide discrimination with other protection devices. For example, if there is a
genuine busbar fault, the time delay should be set to allow busbar protection to clear the fault. A typical setting
may be 100 to 150 ms.
You can choose whether to use the local current value sent with the PIT command to make the decision at the
remote end, or whether to use the remote current value at the receiving end for the decision. This choice is made
using the PIT I selection setting in the CURRENT DIFF column.
Note:
The permissive intertripping function always trips three-phase.
P54xMED-TM-EN-1
Protected line
Digital communication link
PIT = 1 000
Data message
Chapter 6 - Current Differential Protection
IED 2
Busbar
protection
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