Hardware and Functional Development of Protection Devices and Systems - Digital Signal Processing in Power System Protection and Control - page 13

Chapter 3
Hardware and Functional Development
of Protection Devices and Systems
3.1 Protection Generations
The age of protection began some 105 years ago, when control apparatus were for
the first time connected to current and voltage transformers, which reduced the
primary currents and voltages to secondary levels. The secondary currents and
voltages could be processed to generate TRIP or NO-TRIP decisions (Fig. 3.1 ).
The first relays were simple overcurrent and undervoltage apparatus (1905).
Both were electromagnetic or magnetoelectric in nature and resembled the mea-
suring apparatus. The relays were actuated either by r.m.s. values or mean values
of rectified signals. They had moving parts and their operation delay was the time
needed to close the output contacts.
Next step (around 1915) was introduction of inverse time overcurrent relays,
either thermal, or with rotating discs. Some 5 years later the differential principle
was adopted, which was a great step forward in protection of the power system
apparatus. The differential relays compared two currents from the protected plant
terminals.
From differential relays there was only one step to distance relays (1930), which
compared currents and voltages, their r.m.s. or mean values. They had the delay
being a function of the measured impedance Z = U / I . All those relays were
magnetoelectric or electromagnetic, all had moving parts and we consider them as
the first generation of protective devices.
The big change was caused by the invention of transistors (1947). They were
introduced to protective relays around 1955. It enabled to build the static devices
(without moving parts, except the output element) that began the era of second
generation of protective relays. The static relays had the operational criteria the
same as electromagnetic ones; however, their decisions (to trip or not to trip) were
based on the modified ways of signal processing. Because of that they offered a
number of advantages: