Section 4
1MAC050144-MB C
Protection functions
322
615 series ANSI
Technical Manual
Figure 153:
Functional module diagram
Differential and bias calculation
Differential calculation module calculates the differential current. The differential current
is the difference in current between the phase and neutral sides of the machine. The phase
currents
I
1
and
I
2
denote the fundamental frequency components on the phase and neutral
sides of the current. The amplitude of the differential current I
d
is obtained using the
equation (assuming that the positive direction of the current is towards the machine):
Equation 25
During normal conditions, there is no fault in the area protected by the function block, so
the currents
I
1
and
I
2
are equal and the differential current I
d
= 0.
However, in practice some differential current exists due to inaccuracies in the current
transformer on the phase and neutral sides, but it is very small during normal conditions.
The module calculates the differential current for all three phases.
The restrained differential protection is designed to be secured for an external fault with a
restraining current. Restraining means that the differential current required for tripping
increases according to the restraining current and the operation characteristics. When an
internal fault occurs, the currents on both sides of the protected object are flowing into it.
This causes the biasing current to be considerably smaller, which makes the operation
more sensitive during internal faults.
OPR_LS
Through
fault
detection
BLK_OPR_LS
BLOCK
BLK_OPR_HS
I_A1
I_B1
TRIP
INT_BLKD
DC
component
detection
Differential
and bias
calculation
OPR_HS
Restrained
Differential
(Low Stage)
CT
saturation
based
blocking
Unrestrained
Differential
(High Stage)
I_C1
I_C2
I_A2
I_B2
Содержание 615 Series ANSI
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Страница 707: ...Section 15 1MAC050144 MB C Applicable standards and regulations 700 615 series ANSI Technical Manual ...
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