Hardware and Functional Development of Protection Devices and Systems - Digital Signal Processing in Power System Protection and Control - page 14

1905 1955 1985 2008

I generation
(electromagnetic)

II gener.
(static)

III gen.
(digital)

IV g.

B C D

A E F G

Fig. 3.1 History of protection technology. A overcurrent and undervoltage relays with CTs and
VTs, B inverse time overcurrent relays, C differential relays and directional relays, D distance
relays with time graded characteristics, E static relays with filters and comparators, F digital
relays measuring phasors, G first wide-area measurement protective systems

• improvement of cooperation with CTs and VTs,
• reduced dimensions and modularity,
• facilitation of testing, maintenance and repairs,
• complex operational characteristics,
• increased speed of operation.
The third generation of protective devices started with the widespread appli-
cation of inexpensive digital processors and memories (around 1985). Within the
digital relay processing of relay input signals are realized in five basic steps:

• input of analog signals, which usually are currents and voltages derived from
CTs and VTs, enter the antialiasing low-pass filters that remove components of
high frequency, which could irreversibly corrupt the digital signals;
• from the antialiasing filter the signals—still in the analog form—enter the A/D
converter and at the output of that block they have forms of trains of samples;
• the digital signals enter the block of initial processing, where they are filtered,
orthogonalized, etc.;
• the results of initial processing enter the block of digital measurements, in which
the criteria signals, their specific parameters and mutual relations are calculated;
• the final block generates the protection decisions, which are based on comparing
of calculated criteria values with the pre-set thresholds or other characteristics.

One has to remark that the relays of previous generations did not actually
measure particular signals. Their operations were based on comparators, which
only decided, if the given quantity was smaller or greater than the operational
threshold. In cases of digital relays the signals are actually measured and the
comparison with the thresholds come afterwards [ 5 - 9 ].
Starting from around 1985 the digital relays became dominant in the offer of
manufacturers. It is so, because they offer a number of advantages:

• integration of functions,
• further reduction of power derived from secondary terminals of CTs and VTs,
• reduction of secondary cabling,
• complex algorithms, which process digital signals using values of the samples,
• increased speed of operation,