Abnormal States in Power Systems and Criteria for Their Recognition - Digital Signal Processing in Power System Protection and Control - page 10

• Trade-offs in relay setting, e.g.:

- the 2nd harmonic restraint may be set low to increase the security during
inrush conditions, but this would increase delay or result in missing operations
during internal faults,
- the bias characteristic of transformer differential relay may be set sensitive but
this may result in false tripping during thorough faults and saturation of the
CTs;

• Limited speed of measurement:

- any relay asserts the tripping signal based on the symptoms of abnormalities,
- those symptoms need certain time to occur,
- extra time is needed for accurate enough measurement of those symptoms,
- there is no perfect measuring algorithm that solves the conflict between the
speed and the accuracy.

• New complex devices in today's power systems create new problems or even
make some of the traditional relaying principles to fail (HVDC, series com-
pensation, non-linear loads, FACTS).

In all above mentioned cases the normal operation and overload/fault regions
may overlap and thus it is difficult or even impossible to set a threshold that would
separate the operation and blocking areas completely. Besides, even if the steady-
state loci of measured criteria values are situated within proper decision areas, they
may be seen in wrong part of the decision space during measurement transients.
Therefore, new intelligent ideas are needed to improve operation of protection
devices for such situations.
The protection improvement possibilities may include:

• application of adaptive protection schemes—adjusting of the protection func-
tions in real time (criterion values measurement and decision-making algorithms
can be adequately adapted),
• processing larger amount of information (multi-criteria approach, decision-
making with probabilistic algorithms),
• application of Artificial Intelligence techniques (Artificial Neural Networks,
Fuzzy Logic, Expert Systems, etc.).

Both the traditional and intelligent approaches and algorithms of signal pro-
cessing and reasoning in protective relays are presented in the following chapters
of this topic.

References

1. Clemens C, Rothe K (1991) Schutztechnik in Elektroenergiesystemen. VDE Verlag, Berlin
2. Elmore WA (1994) Protective relaying—theory and applications. Marcel Dekker Inc.,

New York