Siemens SIPROTEC Manual page 104

Functions
2.3 Differential Protection
Calculation Example:
110 kV single-conductor oil-filled cable
Cross section = 240 mm
Rated frequency f
Length s = 16 km
Capacitance C
Current transformer, transformer ratio 600 A/5 A
From that the steady-state charging current is calculated:
I = 3.63 · 10
C
For the setting with primary values at least double the value is to be set, i.e.:
Setting value I-DIFF> = 200
Setting value with charging current compensation I-DIFF> = 100 A
For the setting with secondary values this value has to be converted to secondary quantity:
If a power transformer with voltage regulation is installed within the protected zone consider that a differential
current may be present even during normal operation, dependent on the position of the tap changer. Please
also refer to the notes in chapter 2.1.2.1 Setting information, Margin heading „Power Transformer with Voltage
Regulation".
Pickup value during switch-on
When switching on long, unloaded cables, overhead lines and arc-compensated lines, pronounced higher-fre-
quency transient reactions may take place. These peaks are considerably damped by means of a digital filter
in the differential protection. A pickup value I-DIF>SWITCH ON (address 1213) can be set to reliably prevent
single-sided pickup of the protection. This pickup value is active whenever a device has recognized the con-
nection of a dead line at its end. For the duration of the seal-in time SI Time all Cl. which was set in the
general protection data at address 1132 (Section 2.1.4.1) all devices are then switched over to this particular
pickup sensitivity. A setting to three to four times the steady-state charging current usually ensures the stability
of the protection during switch-on of the line. For switch-on of a transformer or shunt reactor, an inrush restraint
is incorporated (see below under margin heading „Inrush Restraint").
The pickup thresholds are checked during commissioning. For further information please refer to the chapter
Mounting and Commissioning.
Delays
In special cases of application it may be useful to delay the tripping of the differential protection with an addi-
tional timer, e.g. in case of reverse interlocking. The delay time T-DELAY I-DIFF> (address 1217) is only
started upon detection of an internal fault. This parameter can only be altered with DIGSI at Additional Set-
tings.
If the differential protection is applied to an isolated or resonant-earthed network, it must be ensured that trip-
ping is avoided due to the transient oscillations of a single earth fault. With address 1218 T3I0 1PHAS pickup
on a single fault is therefore delayed for 0.04 s (default setting). For large resonant-earthed systems the time
delay should be increased. By setting the address to ∞, the single-phase pickup is suppressed entirely.
Please note that the parameter T3I0 1PHAS is also used by the distance protection function. The settings that
you make here also affect the distance protection (see Section 2.5.1.4 under margin heading „Earth Fault De-
tection").
104
2
= 50 Hz
N
' = 310 nF/km
B
–6
· U · f · C ' · s = 3.63 · 10
N N B
–6
· 110 · 50 · 310 · 16 = 99 A
C53000-G1176-C169-5, Release date 02.2011
SIPROTEC, 7SD5, Manual