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7UT51 v3
2.2.4
Thermal Overload
Protection (49
Two independent thermal overload functions can be
assigned to any two desired windings or bus-branches
of the protected device (or to a separate additional
protection zone with a 7UT513). The thermal overload
protection function calculates the temperature
increase for transformer or machine windings. During
an overload condition, the thermal tripping time takes
preloading into consideration. An adjustable, warning
signal is available to correct the condition, such as for
load shedding.
2.2.5
Ground Differential
Protection (87N)
Most phase-current differential protection schemes for
transformers are not sensitive enough to detect an
internal phase-to-ground-fault, if the fault is located
near the neutral point of the transformer. Also, it is
difficult to detect a ground-fault if the transformer is
resistance or reactance grounded, since the current
will be limited.
The Siemens 7UT51 ground differential protection
function (7UT513) offers a superior advantage of
detecting internal ground-faults over existing
electromechanical relays. Electromechanical relays
are impaired in the event of CT saturation. They might
trip when the fault is external (or not trip when the fault
is internal). In addition, inrush effects can also cause
undesirable protection behavior.
Ground differential protection applies the principle of
differential protection to a winding with a grounded
common point. The three phase currents are used to
compute the expected zero sequence current, which is
then compared to the measured ground current. An
excessive ground current indicates that a ground-fault
exists. The function can be configured to distinguish a
ground-fault inside the protected zone from one
outside the protected zone (see Section 4.3.6 on
page 50).
PRIM-2330C
1, 49
2)
-
-
2.2.6
Tank Leakage Protection
(64T)
Tank leakage protection monitors the current flowing
between the tank and earth, based either on the
fundamental wave or the rms value. It can be applied
to transformers with tanks that are installed with
isolated or high-resistive against ground. It can be
connected either to a normal relay current input or to a
special highly sensitive measured current input that
can sense a ground current as small as 10 mA. This
function requires a 7UT513.
2.2.7
Metering
The 7UT51 relay provides access to measured data,
which can be displayed on the front display or via the
communications port to a PC running WinDIGSI
values can also be sent to a remote indicator via
IEC-870-5.
•
Load Currents
-
Primary winding
-
Secondary winding
-
Tertiary winding
•
Ground Current
-
Calculated from Phase Currents
-
Measured from Ground CT (87N application)
•
Overload values
-
Calculated Temperature Rise (T/T
Description of Relay
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max
2
15
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