ABB Relion 670 series, Technical Manual

Page: 427 / 1432

The three-phase power transformer typically has rating of several tens of kVA (e.g. 129kVA) and
rated secondary winding voltage of up to 550V. Note that maximum voltage across secondary
resistor RN for an ground fault at generator terminals can be calculated as follows:
EF_Max _
2
3
1 G Ph Ph
U U
U
U
-
= ×
×
EQUATION2520 V1 EN-US
where:
U1
is the HV side rated voltage of the power transformer (for example 8kV / √3)
U2 is the LV side rated voltage of the power transformer (for example 500V / 3)
U
G_Ph-Ph
is the protected generator rated phase to phase voltage
Note that in case of an ground fault in the stator, the secondary current through the R
N
resistor
will be often in order of couple of hundred amperes. This maximum secondary current can be
calculated as follows:
_
_
EF Max
EF Sec
N
I
U
R
=
EQUATION2518 V1 EN-US
For all the alternatives the 100% stator earth fault protection can be applied.
7.14.8.3 100% Stator earth fault protection function
GUID-45579A6F-4731-4543-86C6-274D182FD898 v2
The injection to the stator is schematically shown in figure 225 . It should be observed that in this
figure injection equivalent circuit is also shown with all impedances and injection generator
related to the primary side of the neutral point voltage transformer. Points a & b indicate
connection terminals for the injection equipment. Similar equivalent circuit can be drawn for all
other types of generator stator grounding shown in latter figures.
1MRK 502 066-UUS B Section 7
Impedance protection
421
Technical manual