Decision Making in Protective Relays - Digital Signal Processing in Power System Protection and Control - page 201

6

Decision

Tripping

I pick-up

4

Blocking

2

0

Measurement with :
- full-cycle filters
- half-cycle filters

-2

-4

-6

0 20 40 60 80 100 120

Time [ ms ]

Fig. 10.2 Decision speed versus measurement data window

The decision threshold is to be set with care, i.e. taking into account the
maximum expected load current and minimum fault current in the protected
element:

I L

max \I pick up \I f

ð 10 : 2 Þ

min :

The speed of the overcurrent protection is closely related to the algorithm of the
current magnitude measurement. The shorter the data window of applied filters
(generally—data processing window of the algorithm), the faster the relay decision
can be made, Fig. 10.2 .
The measurement algorithm immunity to signal contamination, especially to
decaying DC component, is also important. Possible transient overestimation of
signal magnitude (overshoot) can lead to false decisions, if appropriate time delay
is not applied, which was already discussed in Chap. 8 .
Another situation exists when the applied criterion is not a single-dimension
variable but is a complex vector Z = R ? jX, as in case of fault distance estimation
in distance relays. In such a case the decision is not taken by simple checking of
( 10.2 ) but by comparing mutual location of the measured complex variable with
appropriately set characteristic.
With reference to Fig. 10.3 , the protection decision is made (fault confirmed)
when the measured impedance vector trajectory exceeds the Mho characteristic
and its locus is seen within the prescribed curve. The Mho curve should embrace
the region of fault loop impedance for in-zone fault cases, with possible fault
resistance, as shown in Fig. 10.3 . The trip decision is taken when

Z is within the Mho curve : ð 10 : 3 Þ