ABB Relion 670 series, Техническое руководство

Страница: 426 / 1432

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 ground fault at generator terminals can be calculated as follows:
_
EF_Max
3
2
1
G Ph Ph
U
U
U
U
-
= ×
EQUATION2516 V1 EN-US
where:
U1
is the HV side rated voltage of the distribution transformer
U2 is the LV side rated voltage of the distribution transformer
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
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
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 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-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)
R
N
is the ohmic value of a resistor connected to the secondary open-delta windings
Section 7 1MRK 502 066-UUS B
Impedance protection
420
Technical manual