Siemens SIPROTEC 4 7UT6 Series Manual page 156

Functions
2.4 Time Overcurrent Protection for Phase and Residual Currents
High Set Current Stage 3I0>> Pickup
If stage 3I0>> (address 2211 or 2212) is combined with the 3I0> stage or the 3I0p stage, a two-stage charac-
teristic will be the result. If one stage is not required, the pickup value has to be set to ∞. Stage 3I0>> always
operates with a defined delay.
If the protected winding is not earthed, zero sequence current only emerges due to an inner earth fault or
double earth fault with one inner base point. Here, the 3I0>> stage is usually not required.
The 3I0>> stage can, for example, be applied for current grading. Please note that the zero sequence system
of currents is of importance. For transformers with separate windings, zero sequence systems are usually kept
separate (exception: bilateral starpoint earthing or earthed auto-transformer).
Inrush currents can only be created in zero sequence systems if the starpoint of the respective winding is
earthed. If the fundamental exceeds the setting value, the inrush currents are rendered harmless by delay
times (address 2213 T 3I0>>).
"Reverse Interlocking" is only sensible if the respective winding is earthed. In that case, the multi-stage func-
tion of the time overcurrent protection is beneficial: For example, stage 3I0>> is used as fast busbar protec-
tion with a short safety delay T 3I0>> (e.g. 50 ms). Stage 3I0>> is blocked for faults at the outgoing
feeders. Stages 3I0> or 3I0p serve as backup protection. The pickup values of both elements (3I0> or 3I0p
and 3I0>>) are set equal. Delay time T 3I0> or T 3I0p (IEC characteristic) or D 3I0p (ANSI characteristic)
is set in such manner that it overgrades the delay for the outgoing feeders. Here, the grading coordination
chart for earth faults, which mostly allows shorter setting times, is of primary importance.
The set time T 3I0>> is an additional delay time and does not include the operating time (measuring time,
etc.). The delay can be set to ∞. If set to infinity, the pickup of this function will be indicated but the stage will
not trip after pickup. If the pickup threshold is set to ∞, neither a pickup annunciation nor a trip is generated.
Definite Time Overcurrent Stage 3I0> Pickup
The minimum appearing earth fault current is relevant for the setting of the time overcurrent stage 3I0>
(address 2214 or 2215). Please note that, in case of various measuring locations, higher measuring tolerance
may occur due to summation errors.
The time delay to be set (parameter 2216 T 3I0>) is derived from the grading coordination chart created for
the network. For earth currents with earthed network, a separate grading coordination chart with shorter
delay times can be set up. If you set a very small pickup value, it must be taken into consideration that the
inrush restraint function cannot operate below 10 % nominal current (lower limit of harmonic filtering). An
adequate time delay could be reasonable if inrush restraint is used.
The set time is an additional delay time and does not include the operating time (measuring time, etc.). The
delay can also be set to ∞. If set to infinity, the pickup of this function will be indicated but the stage will not
trip after pickup. If the pickup threshold is set to ∞, neither a pickup annunciation nor a trip is generated.
Inverse Time Overcurrent Stage 3I0p Pickup with IEC Characteristics
The inverse time stage, depending on the configuration of the functional scope, address 122 (see
2.1.3.1 Setting
With the IEC characteristics (address 122 DMT/IDMT 3I0 = TOC IEC) the following options are available at
address 2226 IEC CURVE:
•
Normal Inverse (inverse, type A according to IEC 60255-3),
•
Very Inverse (very inverse, type B according to IEC 60255-3),
•
Extremely Inv. (extremely inverse, type C according to IEC 60255-3), and
•
Long Inverse (longtime, type B according to IEC 60255-3).
The characteristics and the equations on which they are based, are listed in the"Technical Data".
If the inverse time trip characteristic is selected, it must be noted that a safety factor of about 1.1 has already
been included between the pickup value and the setting value. This means that a pickup will only occur if a
current of about 1.1 times the setting value is present.
The current value is set under address 2221 or 2222 3I0p. The most relevant for this setting is the minimum
appearing earth fault current. Please note that, in case of various measuring locations, higher measuring toler-
ance may occur due to summation errors.
156
Notes), enables the user to select different characteristics.
SIPROTEC 4, 7UT6x, Manual
C53000-G1176-C230-5, Edition 09.2016