Siemens siprotec 7SA6 User Manual

Functions
6-2077SA6 Manual
C53000-G1176-C133-1
6.14 Voltage Protection
General The overvoltage protection avoids stress of electrical equipment by extremely high
voltages and the resultant insulation problems.
Abnormally high voltages often occur in weak-loaded, long distance transmission
lines, in islanded systems when generator voltage regulation fails, or after full load
shutdown of a generator and external generators (not connected to the system). Even
if compensation reactors are used to avoid line overvoltages by compensation of the
line capacitance and thus reduction of the overvoltage, the overvoltage will endanger
the insulation if the reactors fail (e.g. due to fault clearance). The line must be
deenergized within very short time.
The undervoltage protection can be applied, for example, for disconnection or load
shedding tasks in a system. Furthermore, this protection scheme can detect menacing
stability problems. With induction machines undervoltages have an effect on the
stability and permissible torque thresholds.
The overvoltage protection in the 7SA6 detects the phase voltages U
L1–E
, U
L2–E
and
U
L3–E
, the phase-to-phase voltages U
L1–L2
, U
L2–L3
and U
L3–L1
, as well as the
displacement voltage 3U
0
. Instead of the displacement voltage any other voltage that
is connected to the fourth voltage input U
4
of the device can be detected. Furthermore
the device calculates the positive sequence system voltage and the negative
sequence system voltage so that the symmetrical components are also monitored.
The phase voltages U
L1–E
, U
L2–E
and U
L3–E
, the phase-to-phase voltages U
L1–L2
,
U
L2–L3
and U
L3–L1
, as well as the positive sequence system can also be used for the
undervoltage protection.
These voltage protection functions can be combined according to the user’s
requirements. They can be switched on or off separately, or used for alarm purposes.
In the latter case the respective trip commands do not appear. Each voltage protection
function is dual-stage, i. e. it is provided with two threshold settings each with the
appropriate times delay.
6.14.1 Method of Operation
6.14.1.1 Overvoltage Protection
Overvoltage
Phase–Earth
Figure 6-105 depicts the logic diagram of the phase voltage stages. The fundamental
frequency is numerically filtered from each of the three measuring voltages so that
harmonics or transient voltage peaks are largely eliminated. Two threshold stages
8SKH! and 8SKH!! are compared with the voltages. If a phase voltage exceeds
these thresholds it is indicated phase-segregated. Furthermore, a general pick-up
indication “8SK²H!3LFNXS” and “8SK²H!!3LFNXS” is given. The drop-off to pick-
up ratio can be set (8SKH!!5(6(7).
Every stage starts a time delay which is common to all phases. The expiry of the
respective time delay 78SKH! or 78SKH!!is indicated and issues the trip
command
“8SK²H!!75,3”.
The overvoltage protection phase–earth can be blocked via a binary input
“!8SK²H!!%/.”.

Siemens siprotec 7SA6 Specifications

General

Brand	Siemens
Model	siprotec 7SA6
Category	Relays
Language	English