Siemens siprotec 7SA522 User Manual

2.15 Undervoltage and overvoltage protection (optional)
271
7SA522 Manual
C53000-G1176-C155-3
2.15 Undervoltage and overvoltage protection (optional)
Voltage protection has the function to protect electrical equipment against undervolt-
age and overvoltage. Both operational states are unfavourable as for example under-
voltage may cause stability problems or overvoltage may cause insulation problems.
The overvoltage protection in the 7SA522 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 displace-
ment voltage 3U
0
. Instead of the displacement voltage any other voltage that is con-
nected 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. Here com-
pounding is also possible which calculates the voltage at the remote line end.
For undervoltage protection 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
, and the positive-sequence system can be
used.
These voltage protection functions can be combined according to the user's require-
ments. They can be switched on or off separately, or used for alarm purposes only. In
the latter case the respective trip commands do not appear. Each voltage protection
function is two-stage, i.e. it is provided with two threshold setting stages, each one with
its respective time delay.
Abnormally high voltages often occur e.g. in low loaded, long distance transmission
lines, in islanded systems when generator voltage regulation fails, or after full load
shutdown of a generator from 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 sta-
bility and permissible torque thresholds.
2.15.1 Overvoltage Protection
Overvoltage
Phase–Earth
Figure 2-111 depicts the logic diagram of the phase voltage stages. The fundamental
frequency is numerically filtered from each of the three measuring voltages so that har-
monics 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
´8SKH!3LFNXSµ ´8SKH!!3LFNXSµ is given. The drop-out to pick-up ratio can
be set (8SKH!!5(6(7).
Every stage starts a time delay which is common to all phases. Expiry of the respective
time delay 78SKH! or 78SKH!! is signalled and usually results in the trip
command ´8SKH!!75,3µ.
The overvoltage protection phase–earth can be blocked via a binary input ´!8SK
H!!%/.µ.

Siemens siprotec 7SA522 Specifications

General

Brand	Siemens
Model	siprotec 7SA522
Category	Protection Device
Language	English