ABB REG670 - Page 311

quite high (i.e. in the order of kΩ). The equivalent resistance in the primary circuit can
be calculated as follows:
2
1
3 2
Eq N
R
U
R
U
=
æ ö
×
ç ÷
×
è ø
EQUATION2519 V1 EN
where:
U1/U2 is the turn (i.e. rated voltage) ratio of one phase of the power transformer, e.g.
8
3
500
3
kV
V
EQUATION2521 V1 EN
R
N
is the ohmic value of a resistor connected to the secondary open-delta windings
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 earth fault at generator terminals can be
calculated as follows:
EF_Max _
2
3
1
G Ph Ph
U U
U
U
-
= ××
EQUATION2520 V1 EN
where:
U2 / U1 is the turn (i.e. rated voltage) ratio of one phase of the power transformer, e.g.
500
3
8
3
V
kV
EQUATION2522 V1 EN
U
G_Ph-Ph
is the protected generator rated phase to phase voltage
Note that in case of an earth 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
1MRK502052-UEN B Section 7
Impedance protection
305
Technical manual