Decision Making in Protective Relays - Digital Signal Processing in Power System Protection and Control - page 214

P
o
w
e
r

Digital
signals

Adaptation block

Analogue
signals

S
y
s
t
e
m

x(n)

A decisi on

Analogue Digital signal

conditioning (filters)

Digital

Logic

D measur.

filtration

M e a s ur e m e nt - deci s i o n p ar t

Fig. 10.13 Block scheme of an adaptive protection

• decision making (change of thresholds and decision characteristics, selection of
new or additional criteria and logic signals).

Changes initiated by the adaptation block may include not only certain modi-
fication of selected algorithms but also their exchange for the ones better suited for
given power system operating conditions.
The typical and widely accepted examples of adaptive schemes include:

• time-dependent overcurrent relay (operation dependent on both time and current
amplitude, according to a standardized function),
• percentage differential scheme (decision threshold for differential current being
a function of the bias current),
• adaptive under-frequency load shedding (load shedding steps depending on
actual load measured at particular feeders),
• adaptive distance overreaching/underreaching schemes (first zone extended or
shortened according to the information gained from the opposite line terminals),
etc.

Below an example of adaptation in digital part of the protection relay is pre-
sented, that is related to the decision making performed in the differential pro-
tection of synchronous machines.

Differential relay adaptation
Even though the differential relays are considered quite reliable and robust,
there are situations when they may malfunction. Contradictory requirements of
high sensitivity and selectivity cannot reconcile in cases of faults with small
through currents which may evoke over-tripping if the CTs saturate due to content
of DC component with high value of decaying time constant. Such conditions may
arise e.g. during external faults close to generator-transformer units when this time
constant is as high as 200-300 ms. Contrary to high amplitude faults when the CTs
saturate within first full-cycle of the current signal after fault inception, the sat-
uration effects for DC-induced cases appear only few cycles later and have
somewhat different nature. In [ 9 ] the idea of adaptive differential relay is presented
with the aim of avoiding false tripping for such external faults.