Decision Making in Protective Relays - Digital Signal Processing in Power System Protection and Control - page 200

6

I ( t )

Decision

Tripping

I pick-up

4

Blocking

2

0

-2

i ( t )

-4

-6

0 20 40 60 80 100 120

Time [ms]

Fig. 10.1 Illustration of the decision making in overcurrent protection

10.1 Deterministic Decision Making

The simplest and, at the same time, the most frequently used approach to the
decision making is the deterministic one. Deterministic approach means that the
algorithm to issue a decision is precisely defined, whereas the decision thresholds
may be fixed or being changed according to the prescribed scenario.
Generally speaking, with the deterministic approach the decision concerning
state of the protected plant is taken by comparing the estimated/measured criteria
values with appropriately set thresholds or characteristics. For the single-dimen-
sion decision problem (single criterion taken into account) the discrimination is
made when the criterion signal crosses or exceeds the pre-defined border value
(threshold) separating two classes of events to be distinguished, e.g. normal
operation versus fault conditions. Depending on the problem the discrimination
may be of one of the two types:

• overreaching, when the criterion value is higher than the threshold, e.g. for
overcurrent protection, see illustration in Fig. 10.1 ,or
• underreaching, when the criterion value is lower than the threshold, e.g. for
undervoltage protection.

With reference to Fig. 10.1 , the decision to trip the protected element is taken
when the measured current (instantaneous value or signal magnitude) exceeds the
pick-up value:

I ð n Þ [ I pick up : ð 10 : 1 Þ