ABB RELION 670 Series Technical Manual page 426

Section 7
Impedance protection
R
N
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 ground fault at generator terminals can be calculated as follows:
U
EF_Max
EQUATION2516 V1 EN-US
where:
U1
U2
U
G_Ph-Ph
Note that in case of an ground fault in the stator, the secondary current through the R
will be often in order of couple of hundred amperes. This maximum secondary current can be
calculated as follows:
I
EF Sec _
EQUATION2518 V1 EN-US
C: High-resistance grounding via a grounded wye-broken delta transformer
This grounding 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
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 quite high (i.e. in the order of kΩ). The equivalent resistance in the
primary circuit can be calculated as follows:
æ
=
R ç
Eq
è
EQUATION2519 V1 EN-US
where:
U1
U2
R
N
420
is the ohmic value of the resistor connected to the secondary winding
U 2
U
-
= G Ph Ph _ ×
U 1
3
is the HV side rated voltage of the distribution transformer
is the LV side rated voltage of the distribution transformer
is the protected generator rated phase to phase voltage
U
EF Max _
=
R
N
2
ö
U 1
×
R
÷
N
×
3 U 2 ø
is the HV side rated voltage of the power transformer (for example 8kV / √3)
is the LV side rated voltage of the power transformer (for example 500V / 3)
is the ohmic value of a resistor connected to the secondary open-delta windings
1MRK 502 066-UUS B
resistor
N
N
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