Functions
2.15 Earth Current Differential Protection (ANSI 87GN,TN)
SIPROTEC, 7UM62, Manual
C53000-G1176-C149-7, Release date 03.2010
134
Note
When using the I
EE2
input, it must be kept in mind that this is a sensitive current input. The current amplitude
is limited to approx. √2 1.6 A. A secondary rated current of 1 A is to be used for the starpoint CT. If a 5-A trans-
former is used, the appropriate transformation ratio has to be set. When checking the stabilisation during an
external single-pole fault, a difference current will occur in the operational measured values at input I
EE2
due to
the current limitation. This fault will not initiate a trip because the current direction is evaluated as well.
Address 2101 REF PROT. serves to switch the function ON or OFF or to block only the trip command (Block
relay).
Note
When the device is delivered, the earth current differential protection is set to OFF. The reason is that the pro-
tection must not be in operation unless at least the assigned side and CT polarity have been properly set
before. Without proper settings, the device may show unexpected reactions (incl. tripping)!
Pickup Values
The sensitivity of the protection is determined by I-REF> setting (address 2110). This is the earth fault current
flowing in from the starpoint of the protected object (transformer, generator), and in some cases also from the
network. This value should be chosen on the basis of the most unfavourable case, i.e. fault currents entering
from one side only. The current set refers to the nominal current of the protected object or the protected side.
The sensitivity limit is as a rule set by the CTs. A setting between 0.1 and 0.15 I/InO is quite practical.
For the operating characteristic, the default settings can be used. If necessary, these settings can be changed
with the DIGSI communication software. The advanced parameters define the slope (2113 SLOPE) and the
base point (2114 BASE POINT) of the characteristic.
To stabilize the protection function, address 2102 can be set to blocking by the phase current (REF I> BLOCK).
As a rule of thumb, the pickup value should never be more than twice the nominal current. With low-ohmic star-
point earthing, the general formula is: nominal current + earth current resulting from the starpoint resistance.
The zero voltage enabling depends on the operating range of the protection function. 95 % of a generator stator
winding is a good value. Therefore, the secondary-side value has been set to 5.0 V (2103 REF U0>RELEASE).
Where the zero voltage enabling is not used, it must be set to 0.0 V.
Note
For the protection function, the zero voltage calculated from the phase-to-earth voltages has been multiplied
with √3, which corresponds to the voltage present in a broken delta winding.
No settings need to be made for the angle enabling and the additional evaluation of the directly measured star-
point current (where used).
For special applications, it may be advantageous to delay the trip command of the protection. This can be done
by setting an additional delay time (address 2112 T I-REF>). Normally, this delay time is set to 0. A minimum
command duration was set in common for all protection functions (see Section 2.5.1 under „Command Dura-
tion“).
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