Decision Making in Protective Relays - Digital Signal Processing in Power System Protection and Control - page 210

Calculation of
criterion 1

Partial
decision 1

ν

11

Calculation of
criterion 2

Partial
decision 2

Final
de cision

Aggregation
of partial
decisions

ν

21

Calculation of
criterion n

Partial
decision n

ν

n

Fig. 10.10 Decision making with multiple criteria

plants where the number of expected phenomena and operating conditions is not
very big; thus with single criterion discrimination of the plant state is quite pos-
sible. It is so for instance for transmission and distribution lines that are usually
protected with application of differential, distance or overcurrent functions (either
as main or backup protection), i.e. by analyzing the course and values of single
criterion signal (current amplitude, impedance locus). For the plants with com-
plicated structure or installed at a point where the operating conditions might be
tough and/or difficult to be analyzed, the second option with multiple criteria is
usually applied. Since additional information is delivered and processed by the
relay, the multi-criteria decision making brings about improvement of protection
reliability in terms of:

• increased decision confidence,
• elimination of wrong decisions,
• higher speed of decision making.
A very basic scheme of multi-criteria decision making is shown in Fig. 10.10 .
The final decision is worked out in several steps including:

• calculation of particular criteria values,
• generation of partial decisions for all criteria (e.g. by comparing their values
with some thresholds or characteristics, as described above),
• aggregation of partial decisions.

For particular criteria any of the decision making approaches i.e. deterministic
or probabilistic one can be applied.
Aggregation of partial decisions can be done with use of:

• simple Boolean logic operators (AND, OR),
• the strategy some-out-of-all (the decision is taken when not less than the minimum
number of partial decisions support one of the possible protection options),
• weighting factor method (confidence coefficients for particular criteria are used
to express their relative strength or quality and to calculate the weighted support
for final decision),