Current Differential Start Logic; Switched Communication Path Supervision; Figure 140: Current Differential Function Start Logic - GE P4A Technical Manual

P54A/B/C/E
2.1.2

CURRENT DIFFERENTIAL START LOGIC

The Permit Cdiff internal signal interacts with the Current Differential function to control the Current Differential
start signals. the following figure shows how this is achieved for the line differential currents. The same principle
applies to neutral differential current.
Permit Cdiff
Phase A Current Differential threshold reached
Phase B Current Differential threshold reached
Phase C Current Differential threshold reached
V02601

Figure 140: Current Differential function Start logic

2.2

SWITCHED COMMUNICATION PATH SUPERVISION

A feature is included in this product to supervise the operation of the Current Differential protection in the event of
communications paths being switched. The Ping-Pong method of time alignment requires symmetry of
communications paths. The absolute value of propagation delay is not important, but the symmetry is. Many
telecommunication systems feature self-healing mechanisms so that if a particular link fails, the integrity of the
system can be maintained by re-routing of traffic via unaffected links. Careful system management can ensure
that the reconfiguration of the system to bypass the failed link can respect the communications symmetry
requirements. Whilst the reconfiguration is in progress, however, there may be a temporary violation of the
symmetry requirements. The Switched Communications Paths Supervision function protects against incorrect
tripping during the period of transient symmetry violation.
The settings for the Switched Communications Paths Supervision reside in the CURRENT DIFF column.
The Current Differential protection continually monitors the communications propagation delay time. With
reference to the following figure, if the differences between 2 successive calculated propagation times exceed the
Comm Delay Tol setting in the PROT COMMS/ IM64 column, then the operating characteristic is modified from the
blue line to the red line.
P54xMED-TM-EN-1
SD
Q
1 R
Chapter 13 - Supervision
& IDiff>Start A
& IDiff>Start B
& IDiff>Start C
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