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Protection Relays Chapter | 18 201

18.4.3.3 Substation Control System
In the 1980s, a substation automation system was introduced to the transmis-
sion and distribution sector in substations, making the engineering easier and
also reducing the cost. The need for a communication system which can con-
nect the different types of relays from different manufacturers led to inte-
grate of the IEDs in local area network (LAN) architecture using a protocol
called IEC 61850, which is the standard mostly common used today.
Substation automation architecture is divided into the following levels (as
shown in Fig. 18.4.11):

Master station level;
Station level;
Bay level;
Process level.

Subchapter 18.5

Overcurrent Relays

18.5.1 INTRODUCTION
Overcurrent relays can be set as DT characteristic or inverse definite mini-
mum time characteristic (IDMT), and can be applied to protect overhead
lines and to be coordinated with an optimal coordination time of 0.4 second
as shown in Figs. 18.5.1 and 18.5.2 to achieve selectivity and coordination
by time grading with the two philosophies available, namely:

1. Definite time; or
2. Inverse definite minimum time.
For the first option, the relays are graded using a DT interval of approxi-
mately 0.5 second, an optimal 0.4 second at the relay R 3 , at the extremity of
the network, is set to operate in the fastest possible time, whilst its upstream
relay R 2 is set 0.5 second higher. Relay operating times increase sequentially
at 0.5-second intervals on each section, moving back toward the source as
shown in Fig. 18.5.1.
The problem with this philosophy is that the closer the fault is to the
source, the higher the fault current and the slower the clearing time—which
is exactly the opposite to what we should be trying to achieve. On the other
hand, inverse curves, as shown in Fig. 18.5.2, operate faster at higher fault
currents and slower at lower fault currents, thereby offering the features that
are desired. This explains why the IDMT philosophy has become standard
practice throughout many countries over the years.