Voltage Protection; Method Of Operation; Overvoltage Protection - Siemens siprotec 7SA6 User Manual

6.14

Voltage Protection

General

6.14.1 Method of Operation

6.14.1.1 Overvoltage Protection

Overvoltage
Phase–Earth
7SA6 Manual
C53000-G1176-C133-1
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
U , the phase-to-phase voltages U
L3–E
displacement voltage 3U
is connected to the fourth voltage input U
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 and U , as well as the positive sequence system can also be used for the
L2–L3 L3–L1
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.
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
8SKH! and 8SKH!! 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 (8SKH! ! 5(6(7).
Every stage starts a time delay which is common to all phases. The expiry of the
respective time delay 7 8SKH! or 7 8SKH!! is indicated and issues the trip
command
"8SK²H! ! 75,3".
The overvoltage protection phase–earth can be blocked via a binary input
"!8SK²H! ! %/.".
, U
L1–L2 L2–L3
. Instead of the displacement voltage any other voltage that
0
of the device can be detected. Furthermore
4
, U and U , the phase-to-phase voltages U
L2–E L3–E
, U
L1–E
and U , as well as the
L3–L1
Functions
and
L2–E
,
L1–L2
6-207