Decision Making in Protective Relays - Digital Signal Processing in Power System Protection and Control - page 199

Chapter 10
Decision Making in Protective Relays
Having in mind that protective relays are devices that are supposed to evaluate the
state of the protected plant and to react properly in case of abnormal operating
conditions, the relay final output is usually the command to trip the protected
element or to raise an alarm, whenever it is necessary. Before a tripping signal to
the associated circuit breaker or an alarm is issued the decision is to be taken that
is based on the locally measured criteria values and additional information from
other protective relays and/or control centers. While in preceding chapters the
measurement techniques and algorithms were presented, here the possible
approaches to the decision making are outlined and the detailed decision proce-
dures are described.
The very basic classification of the decision making philosophies that lead to
particular decision procedures are the following:
• deterministic decision making (according to the specified recipe, with fixed or
adaptive decision boundaries),
• statistic decision making (that mostly reduces to testing of statistical hypotheses
related to the state of protected plant).
In many cases, especially for the protected plants of higher complexity or
significant importance for the power system operation (e.g. synchronous machines
or power transformers), the final relay output is worked out taking into consid-
eration multiple criteria that bring information about respective types of
phenomena or events, which may occur in or around the protected object.
Aggregation of particular sub-decisions in the final output is not trivial and may be
done in many ways.
Below the selected decision making procedures are described, with an example
of the advanced decision scheme for generator protection, closing the chapter.