Functions; Current Differential Protection; High-Set Differential Overcurrent Protection; Restricted Earth Fault Protection - Toshiba GRT100 Manual

GRT100

FUNCTIONS

Current Differential Protection

GRT100 provides fast, selective protection for two-
and three-winding transformers. It has three phase-
segregated differential elements, each with a dual-
slope, percentage differential characteristic as shown
in Figure 1.
I
I
A
I
d
I I
B C
Operating zone
Slope 1
I Small current
set
characteristic
(Slope p1=10 - 100%)
0
I : Differential current (|I
d A
I : Restraining current ((|I
res A
I : Sensitivity setting of small current characteristic
set
Kp: Sensitivity setting of large current characteristic
Figure 1. Percentage differential element
Slope 1 provides sensitivity to low level faults. For
higher level faults, slope 2 with increased bias
compensates for the effects of CT saturation.
GRT100 includes internal vector group compensation
and CT ratio correction, so that the relay requires no
interposing CTs.
During periods of transformer energization, the use of
a second harmonic restraint method blocks the relay
operation.
When the transformer is overexcited due to a
transient power system disturbance, the use of a fifth
harmonic restraint method blocks the relay operation.
High-set Differential Overcurrent
Protection
GRT100 also includes
overcurrent element applied in the differential circuit
and ensures rapid clearance of heavy internal faults.
= 2I
d res
Slope 2
Large current
characteristic
(Slope p2=50 - 200%)
kp I
res
+ I + I |)
B C
| + |I | + |I |)/2)
B C
high-set unrestrained

Restricted Earth Fault Protection

Employing residual current of each winding and
neutral point current, Restricted Earth Fault (REF)
protection provides a highly sensitive differential
protection for earth faults in a transformer which has
a star point directly earthed or low impedance earthed.
The REF element has a dual slope, percentage
characteristic as shown in Figure 2 and the
independent elements can be applied for each
transformer winding.
3I
0
I
N
I
0d
Operating
zone
I
set Slope 1 (p1 = 10%)
kp
I : Residual differential current (|3I
od
I : Restraining current (Max.(I
res
I : Sensitivity setting of small current characteristic
set
Kp: Sensitivity setting of large current characteristic
Figure 2. Characteristic of REF
3
I = I
0d res
Slope 2
(p2 = 50 – 100%)
I
res
+ I |)
0 N
, I , I , I ))
A B C N