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2 CURRENT DIFFERENTIAL SUPERVISION
Current Differential protection of transmission lines or distribution feeders requires communication of measured
values of currents between terminals so that a comparison of current entering and leaving the protected zone can
be made. To determine the health of the protected zone, the current values received from a remote terminal must
be synchronised to locally acquired values. One method of achieving this is the so called ‘Ping-Pong’ method. For
this method to work, there is a requirement that the time taken for signals communicated from one terminal to
another (the propagation delay time) is equal to the time taken for signals to be communicated between the same
devices in the opposite direction. This is an assumption that is valid for many applications and which has been
widely applied. In some applications however, the communications symmetry may be violated either temporarily
or permanently. If the requirement is violated, the Ping-Pong method cannot work correctly and maloperation may
occur. Some products in this range can use a Global Positioning Satellite (GPS) input signal to provide a precise
timing signal that can be used to synchronise the alignment of the current values. This allows the protection to
work in applications where the communications paths may not be symmetrical. If there is no GPS synchronisation
signal available or its performance becomes severely degraded, a technique based on the Ping-Pong method is
employed to maintain the Current Differential protection. Unfortunately, with this compromised GPS
synchronisation technique, as well as with the standard Ping-Pong technique, switching of communications paths
can lead to miscalculation of bias and differential currents which could lead to maloperation. To help prevent this a
number of supervision checks can be applied to Current Differential protection that is not GPS synchronised. These
are:
● Current Differential Starter Supervision (Permit Current Differential)
●
Switched Communications Paths Supervision
● Communications Asymmetry Supervision.
2.1 CURRENT DIFFERENTIAL STARTER SUPERVISION
An unexpected communications asymmetry condition can cause an apparent rise in the differential current. This
causes a condition that could be interpreted as a three-phase (balanced) fault condition. Positive sequence current
components are apparent in normally operating systems. A balanced fault will increase the magnitude of these
positive sequence components. Any increase resulting from a communications asymmetry will be modest.
However, a real three-phase fault will generally cause the magnitude of the positive sequence components to
increase far more than an asymmetric communication condition would. The measurement of positive sequence
current components can therefore be used to supervise Current Differential protection for balanced fault
conditions.
Unbalanced faults always produce negative sequence current components. Negative sequence current
components can therefore be used to supervise Current Differential protection for unbalanced fault conditions.
The components that implement this supervision are called Starter Elements. Both positive-sequence current (I1)
and negative-sequence current (I2) are derived. For both I1 and I2, there are two types of starter element for each
sequence component; fixed threshold and rate-of-change threshold (delta). There are four Starter Elements in all:
● Start I1 low
●
Delta I1 low
● Start I2 low
●
Delta I2 low
Which starter elements are used and in which combination depend on the specific application and customer
requirements. The device provides complete flexibility, allowing you to use each of them individually or in
combination with one another using PSL.
The starter element settings are located in the CURRENT DIFF column. You can choose to enable, or disable them,
or select Idiff Permit (discussed later). They are disabled by default.
Chapter 13 - Supervision P54A/B/C/E
278 P54xMED-TM-EN-1