Siemens siprotec 7SA522 User Manual

2 Functions
318
7SA522 Manual
C53000-G1176-C155-3
the measured values. Phase rotation of measured voltages is checked by verifying the
phase sequences of the voltages
U
L1
beforeU
L2
beforeU
L3
This check takes place if each measured voltage has a minimum magnitude of
|U
L1
|, |U
L2
|, |U
L3
| > 40 V/√3
In case of negative phase rotation, the indication ´)DLO3K6HTµ (FNo. 171) is
output.
If the system has a negative phase rotation, this must have been set during the con-
figuration of the power system data (Subsection 2.1.3.1, address ). In such event,
the phase rotation monitoring applies to the corresponding opposite phase sequence.
Asymmetrical Mea-
suring Voltage
Failure "Fuse
Failure Monitor"
In the event of measured voltage failure due to a short circuit or broken conductor in
the voltage transformer secondary circuit certain measuring loops may mistakenly see
a voltage of zero, which due to the load current may result in an unwanted pick-up or
even trip.
If fuses are used instead of a secondary miniature circuit breaker (VT mcb) with con-
nected auxiliary contacts, then the “fuse failure monitoring” can detect problems in the
voltage transformer secondary circuit. Of course, the miniature circuit breaker and the
“fuse failure monitor” can be used at the same time.
The asymmetrical measured voltage failure is characterized by its voltage asymmetri-
cal with simultaneous current symmetry. Figure 2-141 depicts the logic diagram of the
“fuse failure monitor” during asymmetrical failure of the measured voltage.
If there is substantial voltage asymmetry of the measured values, without asymmetry
of the currents being registered at the same time, this indicates the presence of an
asymmetrical failure in the voltage transformer secondary circuit.
The asymmetry of the voltage is detected by the fact that either the zero sequence
voltage or the negative sequence voltage exceed a settable value ))08!PLQ. The
current is assumed to be sufficiently symmetrical if both the zero sequence as well as
the negative sequence current are below the settable threshold ))0,PD[.
In non-earthed systems, the zero-sequence system quantities are no reliable criterion
since a considerable zero-sequence voltage occurs also in case of a simple earth fault
where a significant zero sequence current does not necessarily flow. Therefore, the
zero-sequence voltage is not evaluated in such networks but only the negative-se-
quence voltage (parameter 6\VWHP6WDUSRLQW).
As soon as this state is recognized, the distance protection and all other functions that
operate on the basis of undervoltage (e.g. also weak infeed tripping) are blocked. The
immediate blocking demands current flow in at least one of the phases. The distance
protection may be switched over to definite time overcurrent emergency operation if
the overcurrent protection was configured accordingly (refer to Section 2.11).
The fast blocking may not occur as long as one phase is without voltage due to a
single-pole dead time condition, as the non-symmetry of the measured values arising
in this state is due to the switching state of the line and not due to a failure in the sec-
ondary circuits. Accordingly, the fast blocking is disabled when the line is tripped
single-pole (internal information “1pole open” in the logic diagram).
If a zero sequence or negative sequence current is detected within approximately 10 s
after recognition of this criterion, the protection assumes a short-circuit and removes
the blocking by the “fuse failure monitor” for the duration of the fault. If on the other
hand the voltage failure criterion is present for longer than approx. 10 s, the blocking
is permanently activated (latching of the voltage criterion after 10 s). Only 10 s after

Siemens siprotec 7SA522 Specifications

General

Brand	Siemens
Model	siprotec 7SA522
Category	Protection Device
Language	English