ABB REG670 - Page 309

These earthing alternatives are characterized by the following properties:
A: High-resistance earthing with a neutral point resistor
This earthing method utilizes a high resistance in the primary circuit by inserting
resistor R
N
between the generator neutral and the ground. The actual resistance value
of R
N
is generally in order of kΩ. Such high resistance is required in order to limit the
primary earth fault current to a quite small value (i.e. always < 20A and quite often <
10A primary). Actual primary earth fault current can be calculated as follows:
_
EF_Max
I
3
G Ph Ph
N
U
R
-
=
×
EQUATION2515 V1 EN
where:
R
N
is the ohmic value of the primary resistor
U
G_Ph-Ph
is the protected generator rated phase-to-phase voltage
Typically a VT is connected in parallel with this resistor in order to measure voltage
in the stator neutral point.
The VT must have a rating of at least 100VA and a rated secondary
winding voltage of up to 120V. It must adhere to IEC 61869-3:2011
section 5.5.301 Rated Output Values and the standard values
specified according to burden range II.
The maximum voltage on the secondary side of the VT for an
earth fault at generator
terminals can be calculated as:
_
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 VT
UG_Ph-Ph is the protected generator rated phase to phase voltage
B: Effective high-resistance earthing via a distribution transformer
This earthing method utilizes a distribution transformer that provides high resistance
in the primary circuit by utilizing a small resistor R
N
connected to the secondary
winding of the distribution transformer. The primary winding of the distribution
transformer is connected between the generator neutral and ground. The actual
1MRK502052-UEN B Section 7
Impedance protection
303
Technical manual