EQUATION2515 V1 EN-US
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 ground fault at generator terminals
can be calculated as:
_
EF_Max
3
2
1
G Ph Ph
U
U
U
U
-
= ×
EQUATION2516 V1 EN-US
where:
U1
is the rated primary voltage of the VT
U2
is the rated secondary voltage of the VT
UG_Ph-Ph
is the protected generator rated phase to phase voltage
B: Effective high-resistance grounding 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 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:
EQUATION2517 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
1MRK 502 066-UUS B Section 7
Impedance protection
419
Technical manual
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