ABB Relion REG670, Technical Manual, Page: 310 / 1298

resistance value R
N
is generally extremely small (i.e. typically < 1Ω); however, the
imposed ohmic value to the primary circuit becomes quite high (i.e. in the order of
kΩ). The equivalent resistance in the primary circuit can be calculated as follows:
2
1
2
Eq N
R
U
R
U
=
æ ö ×
ç ÷
è ø
EQUATION2517 V1 EN
where:
U1/U2
is the turn (i.e. rated voltage) ratio of the distribution transformer
R
N
is the ohmic value of the resistor connected to the secondary winding
The distribution transformer typically has rating of several kVA (e.g. 33kVA) and
rated secondary winding voltage of up to 240V. Note that maximum voltage on the
secondary side of the distribution transformer for an earth fault at generator terminals
can be calculated as follows:
_
EF_Max
3
2
1
G Ph Ph
U
U
U
U
- = ×
EQUATION2516 V1 EN
where:
U2/U1
is the turn (i.e. rated voltage) ratio of the distribution transformer
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
C: High-resistance earthing via a delta, grounded-wye transformer
This earthing method utilizes a specially constructed three-phase, five-limb power
transformer that provides high resistance in the primary circuit by utilizing a relatively
small resistor R
N
connected to the secondary open-delta connected windings. The
primary windings of this transformer is star (i.e. wye) connected and this neutral point
is directly connected to ground. The actual resistance value RN is relatively small (i.e.
typically < 5Ω); however, the imposed ohmic value to the primary circuit becomes
Section 7
1MRK502052-UEN B
Impedance protection
304
Technical manual