Page 193 - Practical Power System and Protective Relays Commissioning

194 Practical Power System and Protective Relays Commissioning

Subchapter 18.4

History of Relays

Relays began as electromechanical relays, which employ the magnetic coils, then
developed to static relays, which employ transistors, then digital relays, which
employ microprocessors, and finally numerical relays, which use digital relays,
which can communicate with each other with different control protocols.
Different types of relays include the following:

1. Electromechanical relays;
2. Static relays—analog relays;
3. Static relays—digital relays;
4. Static relays—numerical relays.

18.4.1 PRINCIPLES OF THE CONSTRUCTION AND
OPERATION OF THE ELECTROMECHANICAL IDMT RELAY
These relays, for example, an induction disk, have an inverse time character-
istic with respect to the current input to the relay from the secondary of the
CT, and these relays were used in the past and are still sometimes used in
11-kV switchgear, generator protection, and 66-kV line protections. The
same principles are used in static and digital recent relays (see Fig. 18.4.3).
The above relay is electrically represented as shown in Fig. 18.4.4.
The force that rotates the disk is proportional to the product of I1 3 I2
sinα, where α is the angle between I1 and I2, and this relay has an inverse
characteristic as shown in Fig. 18.4.5.
It can be seen that the operating time of an Inverse Definite Minimum
Time (IDMT) relay is inversely proportional to a function of the current, that
is, it has a long operating time at low multiples of setting current and a rela-
tively short operating time at high multiples of setting current.

FIGURE 18.4.3 Electromechanical relay—overcurrent type.