Siemens SIPROTEC 4 7UT6 Series Manual page 118

Functions
2.2 Differential Protection
[diff-ausloesekennl-ohnefehlerk, 1, en_GB]
Figure 2-49
The tripping characteristic comprises two further branches. The base point of the first branch is determined by
address 1242 BASE POINT 1 and its slope by address 1241 SLOPE 1. This parameter can only be set with
DIGSI at Additional Settings. This branch covers current-proportional errors. These are mainly errors of the
main current transformers and, in case of power transformers with tap changers, differential currents which
occur due to the transformer regulating range.
The percentage of this differential current in this latter case is equal to the percentage of the regulating range
provided the rated voltage is corrected according Section
Transformers".
The second branch produces a higher restraint in the range of high currents which may lead to current trans-
former saturation. Its base point is set at address 1244 BASE POINT 2 and is referred to the rated object
current. The slope is set at address 1243 SLOPE 2. The restraint during current transformer saturation can be
influenced by this parameter branch. A higher gradient results in a higher restraint. This parameter can only be
set with DIGSI at Additional Settings.
Delay Times
In special cases it may be advantageous to delay the trip signal of the differential protection. For this, an addi-
tional delay can be set. The delay time 1226 T I-DIFF> is started if an internal fault in the protected object
has been detected by the I-DIFF> stage and the trip characteristic. 1236 T I-DIFF>> is the time delay for
the tripping stage I-DIFF>>. This parameter can only be set with DIGSI at Additional Settings. The dropout
time of all stages is determined by the minimum trip time duration of all protection functions.
All setting times are additional delay times which do not include the operating times (measuring time, dropout
time) of the protective function.
Increase of Pickup Value on Startup
The increase of the pickup value on startup serves as an additional safety against overfunctioning when a
nonenergised protection object is connected. At address 1205 INC.CHAR.START it can be switched to ON or
OFF. Especially for motors or motor/transformer units in block connection it should be set to ON.
The restraint current value I-REST. STARTUP (address 1251) is the value of the restraining current which is
likely to be undershot before startup of the protected object takes place. This parameter can only be set with
118
Tripping characteristic of the differential protection
2.1.4 Power System Data 1
C53000-G1176-C230-5, Edition 09.2016
under "Object Data with
SIPROTEC 4, 7UT6x, Manual