Siemens SIPROTEC 4 7UT6 Series Manual page 119

DIGSI at Additional Settings. Please be aware of the fact that the restraint current is twice the traversing
operational current. The pre-set value of 0.1 represents 0.05 times the rated current of the protected object.
Address 1252 START-FACTOR determines by which factor the pickup value of the I-DIFF> stage is to be
increased on startup. The characteristic of this stage increases by the same factor. The I-DIFF>> stage is not
affected. For motors or motor/transformer in unit connection, a value of 2 is normally adequate. This param-
eter can only be set with DIGSI under Additional Settings.
The increase of the pickup value is set back to its original value after time period T START MAX (address
1253) has passed.
Add-on Restraint
In systems with very high traversing currents a dynamic add-on restraint is enabled for external faults. The
initial value is set at address 11261 I-ADD ON STAB.. The value is referred to the rated current of the
protected object. The slope is the same as for characteristic branch b (SLOPE 1, address 1241). This param-
eter can only be set with DIGSI at Additional Settings. Please note that the fact that the restraint current is
the arithmetical sum of the currents flowing into the protected object, i.e. it is twice the traversing current.
The additional stabilisation does not influence the stage I-DIFF>>.
The maximum duration of the add-on restraint after detection of an external fault is set to multiples of an
ACcycle (address 1262 T ADD ON-STAB.). This parameter can only be set with DIGSI at Additional Settings.
The add-on restraint is disabled automatically even before the set time period expires as soon as the device
has detected that the operation point Ι
near the fault characteristic (≥ 80 % of the fault characteristic slope).
Add-on restraint operates individually per phase, but blocking can be extended to all three phases (so-called
crossblock function). By means of address 1263 CROSSB. ADD ON it can be determined how long the cross-
block should be effective. This parameter can only be set with DIGSI at Additional Settings. Here, too, setting
is in multiple of one AC-cycle. If 0 Per. cycle is set, crossblock is ineffective, i.e. only the phase with detected
external fault will be blocked. Otherwise all phases will be blocked. in this case the same setting as for 1262 T
ADD ON-STAB. is advisable. When set to ∞, the crossblock function is always effective.
Harmonic Restraint
Restraint with harmonic content is available only when the device is used as transformer protection, i.e. the
PROT. OBJECT (address 105) is a 3 phase transf. or 1 phase transf. orAutotransf. or Autotr.
node. This function is also used for shunt reactors if current transformers are installed at both sides of the
connection points.
The inrush restraint function with 2nd harmonic can be switched in address 1206 INRUSH 2.HARM. OFF and
ON. It is based on evaluation of the 2nd harmonic present in the switch-on inrush current. The ratio of 2nd
harmonics to the fundamental (address 1271, 2. HARMONIC) is set to Ι
be used without being changed. To provide more restraint in exceptional cases, where energising conditions
are particularly unfavourable, a smaller value can be set at the afore-mentioned address. The restraint with
harmonics does not influence the stage I-DIFF>>.
The inrush restraint can be extended by the so-called crossblock function. This means that on harmonic
content overshoot in only one phase all three phases of the differential stage I-DIFF> stage are blocked. The
duration for which the crossblock function is active can be limited at address 1272 CROSSB. 2. HARM.
Setting is in multiple of the AC-cycle. This parameter can only be set with DIGSI at Additional Settings. If set
to 0 (presetting is 3) the protection can trip when the transformer is switched on a single-phase fault even
while the other phases carry inrush current. If set to ∞ the crossblock function remains effective for as long as
high-order harmonics are detected in any phase.
Apart from the second harmonic, the 7UT6x can provide restraint with a further harmonic. Address 1207
RESTR. n.HARM. is used to disable this harmonics restraint, or to select the harmonic for it. Available for
selection are the 3. Harmonic and the 5. Harmonic.
Steady-state overexcitation of transformers is characterised by odd harmonic content. The 3rd or 5th
harmonic is suitable to detect overexcitation. As the third harmonic is often eliminated in transformers (e.g. in
a delta winding), the fifth harmonic is more commonly used.
Converter transformers also produce odd harmonics which are practically absent in the case of an internal
short-circuit.
SIPROTEC 4, 7UT6x, Manual
C53000-G1176-C230-5, Edition 09.2016
/Ι stationary (i.e. via at least one cycle) within the tripping zone
diff stab
2.2 Differential Protection
/Ι = 15 % as default setting. It can
2fN fN
Functions
119