Decision Making in Protective Relays - Digital Signal Processing in Power System Protection and Control - page 215

I 1, 2>
I S >
I d >

i 1(n)
i 2(n)

Adaptive adjusting
of the differential curve

Pick - up ,
Start of the
procedure

t s 1

t s 2

Ye s

Calculation of
I 1 a , T 1 a
I 2 a , T 2 a

I S <3 I 1n

?

No

Determination of
adaptation
degree
I dmax , I d / I S

CT

Calculation of
B 1 max , B 1 s
B 2 max , B 2 s

d ata

Stop

Ye s

B 1 max > B s
or
B 2 max > B s

No

Calculation of
t 1 s1

,
t 2 s2 , t 1 s2 , t 2 s1 ,

Fig. 10.14 Block scheme of the adaptive differential protection proposed

The developed procedure of adaptive adjusting of the relay differential char-
acteristic consists of the following steps (Fig. 10.14 ):

• calculation of the initial value (amplitude) I a and time constant T a of the
decaying DC component as well as the amplitude of the current fundamental
frequency component I 1 for the time instant just after fault inception,
• determination (prediction) of the flux density value B s leading to CT saturation,
maximal expected flux density level B max in the CT core and comparison of the
values,
• calculation of the expected CT saturation period, i.e. time instants t s1 and t s2 of
saturation beginning and fizzling out,
• determination of the necessary level of adaptation for given fault case (shifting
up of the differential curve or slope changing of the stabilizing section),
• measurement of the through current amplitude values (differential curve
adjusting takes place for fault currents not higher than three times rated machine
current),
• execution of the on-line adaptation of the differential curve for the time period
[t s1 , t s2 ].
It is worth to notice that the calculation of above-mentioned values is performed
for the signals from CTs at both sides of the protected plant. The adaptation is
carried into effect slightly before the predicted time instant of CT saturation
(earlier of both calculated values is accepted for this purpose).