Technical data

Siemens SIP · 2008
12 Motor Protection / 7SK80
12
12/13
Protection functions
Directional overcurrent protection, ground
(ANSI 67N)
Directional ground protection is a separate
function. It operates in parallel to the non-
directional ground overcurrent elements.
Their pickup values and delay times can be
set separately. Definite-time and inverse-
time characteristics are offered. The trip
-
ping characteristic can be rotated by
± 180 degrees.
For ground protection, users can choose
whether the direction is to be calculated
using the zero-sequence or negative-
sequence system quantities (selectable).
If the zero-sequence voltage tends to be
very low due to the zero-sequence imped
-
ance it will be better to use the negative-
sequence quantities.
(Sensitive) directional ground-fault detec
-
tion (ANSI 64, 67Ns, 67N)
For isolated-neutral and compensated net-
works, the direction of power flow in the
zero sequence is calculated from the zero-
sequence current I
0
and zero-sequence
voltage V
0
.
For networks with an isolated neutral, the
reactive current component is evaluated;
for compensated networks, the active cur-
rent component or residual resistive cur-
rent is evaluated. For special network
conditions, e.g. high-resistance grounded
networks with ohmic-capacitive
ground-fault current or low-resistance
grounded networks with ohmic-inductive
current, the tripping characteristics can be
rotated approximately ± 45 degrees.
Two modes of ground-fault direction
detection can be implemented: tripping
or “signalling only mode”.
It has the following functions:
•
TRIP via the displacement voltage V
E
.
•
Two instantaneous elements or one
instantaneous plus one user-defined
characteristic.
•
Each element can be set to forward,
reverse or non-directional.
•
The function can also be operated in
the insensitive mode as an additional
short-circuit protection.
(Sensitive) ground-fault detection
(ANSI 50Ns, 51Ns / 50N, 51N)
For high-resistance grounded networks, a
sensitive input transformer is connected
to a phase-balance neutral current trans-
former (also called core-balance CT).
The function can also be operated in the
normal mode as an additional short-
circuit protection for neutral or residual
ground protection.
Phase-balance current protection (ANSI 46)
(Negative-sequence protection)
By measuring current on the high side of
the transformer, the two-element phase-
balance current/negative-sequence protec
-
tion detects high-resistance phase-to-phase
faults and phase-to-ground faults on the
low side of a transformer (e.g. Dy 5 or
Delta/Star 150 deg.). This function provides
backup protection for high-resistance faults
through the transformer.
Breaker failure protection (ANSI 50BF)
If a faulted portion of the electrical circuit
is not disconnected when a trip command
is issued to a circuit-breaker, another trip
command can be initiated using the breaker
failure protection which trips the circuit-
breaker of an upstream feeder. Breaker fail-
ure is detected if, after a trip command is
issued and the current keeps on flowing into
the faulted circuit. It is also possible to make
use of the circuit-breaker position contacts
(52a or 52b) for indication as opposed to
the current flowing through the circuit-
breaker.
Fig. 12/9
Directional determination
using cosine measurements
for compensated networks