Application Scope - Siemens SIPROTEC 4 7UT6 Series Manual

1.2

Application Scope

The numerical differential protection SIPROTEC 4 7UT6x is a selective short-circuit protection for transformers
of all voltage levels, for rotating machines, for series and shunt reactors, or for short lines and mini-busbars
with 2 to 5 feeders (depending on the version). Being a single-phase device, it can also be used for small
busbars with up to 7, 9 or 12 feeders (depending on the version). The individual application can be config-
ured, which ensures optimum matching to the protected object.
The devices 7UT613, 7UT633 and 7UT635 can also be run with 2-phase connection for 16.7 Hz applications.
A major advantage of the differential protection principle is the instantaneous tripping in the event of a short-
circuit at any point within the entire protected zone. The current transformers limit the protected zone at the
ends towards the network. This rigid limit is the reason why the differential protection scheme shows such an
ideal selectivity.
For use as transformer protection, the device is normally connected to the current transformer sets which limit
the power transformer windings against the remainder of the system. The phase displacement and the inter-
linkage of the currents due to the winding connection of the transformer are matched in the device by calcula-
tion algorithms. The earthing conditions of the starpoint(s) can be adapted to the user's requirements and are
automatically considered in the matching algorithms. Also, the currents from multiple measuring points on
one side of the protected object can be combined.
For use as generator or motor protection, the device compares the currents in the starpoint leads of the
machine and at its terminals. Similar applies for series reactors.
Short lines or mini-busbars with 2 to 5 ends or feeders (depending on the version) can be protected as well.
Short means that the current transformer connections from the CTs to the device cause no impermissible
burden for the current transformers.
For transformers, generators, motors, or shunt reactors with earthed starpoint, the current between the star-
point and earth can be measured and used for highly sensitive earth fault protection.
The 7, 9 or 12 standard current inputs (depending on the version) of the device allow for a single-phase
protection for busbars with up to 7, 9 or 12 feeders. One 7UT6x is used per phase in this case. Alternatively,
(external) summation transformers can be installed in order to allow a busbar protection for up to 7, 9 or 12
feeders with one single 7UT6x relay.
Where not all analog measuring inputs are needed for the measured values of the protected object, the
remaining inputs can be used for other, independent measurement or protection tasks. If a 7UT635 (with 5
threephase measuring inputs) is used, for instance, on a three-winding transformer, the two remaining meas-
uring inputs c
One or two additional current inputs designed for very high sensitivity are also available. They may be used
e.g. for detection of small leakage currents between the tank of transformers or reactors an earth, thus recog-
nising even high-resistance faults. High-resistance voltage measurement is also possible using an external
series resistor.
For transformers (including auto-transformers), generators, and shunt reactors, a high-impedance unit protec-
tion system can be formed using high-impedance earth fault protection. In this case, the currents of all current
transformers (of equal design) at the ends of the protected zone feed a common (external) high-ohmic
resistor. The current of this resistor is measured using one of the high-sensitive current inputs of the device.
The device provides backup time overcurrent protection functions for all types of protected objects. The func-
tions can be enabled for any side or measuring location.
A thermal overload protection function is available for any type of machine. The functions can be enabled for
any side. External detectors account for the coolant temperature (by means of an external RTD-box). This
allows to calculate and output the hot-spot temperature and the relative ageing rate.
An unbalanced load protection function is provided for the detection of unsymmetrical currents. Phase failures
and negative sequence currents, which are especially dangerous for rotating machines, can thus be detected.
Performance functions allow devices with voltage measuring inputs to implement a reverse power protection
or monitor the forward power supply(in the power station sector). In the system they can be used for network
decoupling. Power results and their components can be emitted as measured values.
The versions with voltage inputs are provided with an integrated overexcitation protection for the detection of
excessive induction states in shunt reactions (transformers, shunt reactors). This protection function monitors
the ratio U/f, which is proportional to the induction B in the iron core. An imminent iron core saturation, which
SIPROTEC 4, 7UT6x, Manual
C53000-G1176-C230-5, Edition 09.2016
Introduction
1.2 Application Scope
19