Ground Differential Protection (87N) - Siemens 7UT51 Instruction Manual

7UT51 v3
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Ground Differential Protection (87N)

The 7UT513 relay can be ordered with a ground
differential protection function (87N) to protect power
transformers, shunt reactors, neutral grounding
transformers, or rotating machines with a common
point that is connected to an earth ground.
The ground differential protection function is highly
sensitive, regardless of the phase angle between the
ground current and 3Io. Even internal faults causing
heavy CT saturation problems can be detected by the
relay.
Ground differential protection protects one winding by
comparing the calculated zero sequence current to the
measured ground current of the winding. The
application settings are simplified so that the many
time consuming commissioning test and fine
adjustments, necessary for the classical
ground-differential solutions (using directional
overcurrent relays), are avoided.
The low-impedance ground differential protection
algorithm is base on Kirchoff's law. The information
provided to the algorithm is sampled values of the
phase current and the ground current.
Using the known phase and ground CT ratio
information, specified in the relay settings, the
sampled current values are normalized relative to the
nominal current of the protected transformer
winding, I .
n
Note that when protecting a 60Hz object, the relay can
provide ground differential protection if the protected
object has one or two windings (or sides), but not
three. If desired, the ground differential protection
function can protect a separate, additional object
instead of the main protected object.
PRIM-2330C
Ground Differential Protection (87N)
Figure 7.1 Ground Differential Protection of a
Wye-Winding
Ground Differential Protection of a Delta
Figure 7.2
Winding with an Artificial Common Point
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