Definite-Time Overcurrent Protection (I>>, ANSI 50, 51, 67) with Direction Detection
45
7UM62 Manual
C53000-G1176-C149-3
I>>Time-
Overcurrent Stage
Address
is used to switch the definite time I>> stage for phase
currents
ON
or
OFF
, or to block only the trip command (
Block Relay)
. The high-
current stage
(parameter
and its associated delay time
) is
used for current grading with large impedances existing for example with
transformers, motors or generators. The definite time-overcurrent stage must be
specified in a way ensuring that it picks up for short circuits up to this impedance.
Current
Transformer in the
Starpoint
(without direction
detection)
Example: Unit connection
a) Short-circuit calculation
Three-pole short circuit
b) Setting value:
The setting value is achieved by means of a conversion on the secondary side. In
order to exclude an unwanted operation caused by overvoltages or transient
phenomena, an additional safety factor of about 1.2 to 1.3 is recommended.
A value of
= 0.1 s is recommended as tripping time delay, in order to enable
preferred tripping of the differential protection.
Current
Transformers at the
Output Side
(with direction
detection)
If at address
was configured as directional, the addresses
and
are accessible. The inclination
of the direction straight line (see figure 2-14) representing the separating line between
the tripping and the blocking zone can be adapted to the network conditions by way of
the
parameter. To do this, the line angle of the network is set. The
direction straight line is perpendicular to the set direction angle. Together with the
parameter
=
Forward
oder
Reverse
, this parameter
covers the entire impedance level. This is the
reverse
direction, provided that the
protective relay has been connected correctly according to one of the diagrams in
Appendix A.4. A small zone is located between the forward and the reverse zone. Due
to phase displacement angles of the transformers, a safe direction decision is not
possible in this small zone. Consequently, there is no tripping in the selected preferred
direction in this zone.
Rated apparent power - generator
S
N, Gen
= 5.27 MVA
Rated voltage - generator
U
N, Gen
= 6.3 kV
Direct-axis transient reactance
x’
d
= 29 %
Transient synchronous generated voltage
(Salient-pole generator)
U’
P
=1.2
⋅
U
N, Gen
Rated apparent power - transformer
S
N, T
= 5.3 MVA
Rated voltage, on the generator side
U
N, VT prim
= 6.3 kV
Short-circuit voltage
u
sc
= 7 %
Current transformer
I
N, CT, prim
=500 A
I
N, sec
=1 A
I
SC 3pol
U’
P
3
(
)
⁄
x’
d
100 %
-----------------
U
N, Gen
2
S
N, Gen
-------------------
⋅
0.5
u
sc
100 %
-----------------
U
N, VT prim
2
S
N, Gen
------------------------------
⋅
⋅
+
-----------------------------------------------------------------------------------------------------------------
1.2 6.3 kV
3
(
)
⁄
⋅
2.18
Ω
0.26
Ω
+
----------------------------------------------
1789 A
≈
≈
≈
1.2
I
SC 3pol
I
N, CT, prim
--------------------------
I
N, sec
⋅
⋅
1.2
1789 A
500 A
-------------------
1 A
⋅
⋅
4.3 A
=
=
I>>
=
