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S150-34-1
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q The signal from CT saturation detectors shunts the measurement for a time proportional to the saturation.
This strongly contributes to stability during through faults while maintaining high sensitivity to internal faults.
The operation logic in each phase is functionally explained in the following diagram.
Operation of the main zone relay takes place if the following conditions exist:
I
d
> [I
dS
]
I
d
> I
S
I
S
= [I
dS
] + [R] • I
R
Operation of the Circuit Breakers connected to each busbar zone takes place if the zone relay and the check
zone relay both operate.
6.4 OPERATION OF THE BIASED DIFFERENTIAL ELEMENTS
In each phase the relay measures the differential current existing in the busbar zone (vector
summation of all the incoming and outgoing currents).
When no fault inside the zone is experienced should be I
d
= 0.
Due to differences among the CTs and mostly to CT saturation when faults outside the zone make large
through current flow, the actual summation of the zone currents is not zero.
The larger is the through current the larger is the resultant I
d
.
The bias effect is than proportional to the through current
The operation is based on the following programmable variables :
q Minimum operation setting threshold : I
dS
= (0.2-2)In
q Per Unit bias coefficient in the area [1I
R
]<I
R
<[2I
R
] : 1R = (0.4-1) pu
q Action starting point of 1R : 1I
R
= (0.5-2)In
q Per Unit bias coefficient in the area I
R
>[I
R2
] : 2R =(0.4-1) pu
q Action starting point of 2R : 2I
R
= (3-8)In
∑
=
id
II
r
∑
=
i
R
II
A/D
1
A/D
2
I
d1
I
d2
2I
R
1I
R
I
R
I
d
I>[I]
SVd1
I>[I]
SVd2
x 0.9
x 0.9
ENABLE
ENABLE
I>I
Sd
I>[I]
dSd2
I>I
Sd1
I>I
Sd2
XOR
OR
XOR
OR
&
trf
tsv
ZONE
TRIP
RELAY
FAULT
CTs
FAULT
&
I>[I]
dSd1
I>[I]
dSd