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4 - 37
Siemens AG
⋅
May 1998
4.7.8.7 Differential current supervision for lineaĆ
rized current transformers
Linearized current transformers may have angle erĆ
rors. The secondary current then lags the primary curĆ
rent. If a feeder short-circuit is interrupted by the cirĆ
cuit-breaker (CB interrupts at current zero) then the
secondary current continues to flow and decays acĆ
cording to an e-function. The angle error and the time
constant depend mainly on the protection burden.
The angle error increases and the time constant deĆ
creases with increasing burden.
The current which continues to flow in the CT circuit
after disconnection is seen by the busbar protection
as a differential current. At first, erraneous tripping is
prevented by the stabilizing current which decays acĆ
cording to an e-function with a time constant of 64
ms. In order to eliminate erroneous tripping after the
stabilizing current has decayed, the 7SS52 supervises
the zero-crossings of the differential current. The difĆ
ferential current limit can be parameterized
> ZERO
CR (
DA 6310/ZE
). The release of zero crossing suĆ
pervision is settable ZERO CR SUP (
DA 6309/ZE
). If
the zero-crossing does not appear after at least 32
ms f
N
= 50 Hz resp. 27 ms f
N
= 60 Hz, then a DC curĆ
rent is present and the protection is blocked selecĆ
tively per busbar zone. The blocking is maintained
until the measured value drops below the limit.
4.7.8.8
Cyclic test
The "cyclic test" as part of the autodiagnosis checks
cyclicly all measuring systems and connected bay
units from the digitized measured values up to the
command relays. This test is treated like a real fault
and uses the same transmission links for measured
values and trip commands. Thus it yields utmost seĆ
curity and reliability.
This test does not influence the protection function.
The test is performed only, if no real fault is detected
on the busbar or the connected feeders.
In the event of an internal failure detection, the identiĆ
cal test routine is repeated twice before failure alarm
is issued. Failure information contains the affected
phase, the measuring channel (ZPS-BSZ1, 2 or 3) and
the corresponding feeder "TetsfltLz-y:Axx" (
FNo 196
- 204/ZE
).
The failure reaction can be programmed via the paraĆ
meter TESTFLT (
DA 6312/ZE
) to "annunciate only" or
"protection block" (incl. annunciation).
Table 4.3 shows an overview of the various monitoĆ
ring functions.
Table 4.3
Summary of the monitoring functions
Supervision
Possible cause and response
1. Current sum (measured value supervision)-FE
4
I
i
n
I >
>MV-SUPER
(DA 5106/ZE)
n=1
4
4
I
i
n
I > 0,125 x
I i
n
I
n=1
n=1
Internal unit failure during measured value acquisition
(Blocking of protection and annunciation)
MV-Super.:Axx (
FNo 192/ZE
)
Meas.val-super
(
FNo 161/FE
)
2. Auxiliary voltage supervision
FE: (O V)
Digit
>OFFSET SUPV
(DA 5107/ZE)
(15 V)
Digit
>15 VOLT SUPV
(DA 5108/ZE)
ZE: 15 V
24 V
Failure of the DC-DC converter output voltages
(Blocking of protection and annunciation)
0 V-Super.:Axx (
FNo 195/ZE
)
15 V-Super.:Axx (
FNo 193/ZE
)
15 V-Super. CU (
FNo 206/ZE
)
24 V-Super. CU (
FNo 207/ZE
)
Failure of the 5V voltage causes reset of the annunciation device
failure and thus blocks the device completely.
3. Cyclic monitoring of the memories (FE, ZE)
After three unsuccessful restart attempts, the protection is blokĆ
ked.
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Summary of Contents for SIPROTEC 7SS52
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