Micom P721, Техническое руководство

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P72x/EN AP/C21 Application Guide
Page 10/52 MiCOM P721/P723
The relay is prevented from unwanted operation because most of the current will flow in the
R
CT
+ 2R
L
loop since R
ST
>> R
CT
+ 2R
L
.
A voltage will develop across the relay. This voltage is equal to I
F
×
(2R
L
+ R
CT
). If this voltage
is greater than the setting voltage, Threshold Vdiff , the relay will operate. This is
undesirable. Therefore, this becomes the setting criterion; the relay should be set to a
voltage greater than I
F
×
(2R
L
+ R
CT
).
R
Z
m1
R
CT1
R
L1
Z
m2
V
s
R
CT2
R
L3
I
F
I
S
R
L4
R
L2
Healthy CT
Saturated CT
A-G
Protected
circuit
I
R
ST
I = I
s
+ I
F
P4488ENa
FIGURE 8: HIGH IMPEDANCE DIFFERENTIAL PRINCIPLE
The voltage applied across the high impedance path is:
( )
L CT F
R 2 R I V
+ × =
Where:
I
F
= Maximum secondary external fault current (it is assumed that I
S
<<I
F
)
A stabilizing resistor R
ST
is used in series with the relay circuit to improve the stability of the
relay under external fault conditions. This resistor limits the spill current during the condition
described in FIGURE 8.
V
S
is the stability voltage. It depends on the relay current setting I
S
and the high impedance
path resistance:
( )
r ST S S
R R I V
+ × =
Where:
I
S
= relay current setting (corresponds to Threshold I Diff in the P72x)
V
S
= stability voltage setting (corresponds to Threshold V Diff in the P72x)
R
ST
= Stabilizing resistor
R
r
= Relay resistance, which is neglected in the P72x
The general stability conditions are obtained when:
( )
L CT F S
R 2 R I K V
+ × × >
Where:
K = Stability factor which is affected by the characteristic of the differential relay. In the
P72x the K factor depends on which Measurement filter is selected (Fourier, Sample or Fast
sample modes). In addition, K is also affected by the application; for example, high
impedance busbar protection, high impedance REF, etc. K is also influenced by the ratio