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Protection Relays Chapter | 18 197

microprocessor relays. The burden of these relays is very low and in recent
digital relays, the relay can perform multiple functions based on the operat-
ing algorithm. Also, by using these relays we can generate relay characteris-
tics which cannot be generated by an electromechanical one. The cost is also
reduced, and the relay can monitor its internal circuits and give an alarm if
something is wrong and can block its operation if there is an internal fault
which cannot be solved by the relay itself (the relay failure monitoring fea-
ture which is not available in the old electromechanical type), the mainte-
nance is also reduced as there are no moving parts in these relays.
On the other hand, these analog relays perform with many failures at the
beginning of its used when used at site some faults in relay cards and in its DC
supply converter but it is solved later and this is very rare in digital relays, refer
to Figs. 18.4.8 and 18.4.9 for static analog relay and digital relay, respectively.

FIGURE 18.4.8 Static analog relays.

FIGURE 18.4.9 Static digital relays.

18.4.3 INTELLIGENT ELECTRONIC DEVICE RELAYS
IEDs refer to intelligent electronic devices, which are multifunctional
digital relays with a built-in algorithm and can communicate with other
IEDs in the substation and with the substation control system. These IEDs
can carry out control functions and replace the hardwire interlocking
system, circuit breaker (CB) local control, and monitoring of SF 6 in a gas