ABB REG670 - Page 544

3T 3N
3N
U U UBeta+ ³
EQUATION1712 V2 EN (Equation 134)
U
3N
, and U
3T
are third harmonic phasors with real and imaginary parts. The factor
Beta must be set not to risk operation under non-faulted conditions.
The voltage U
3N
is measured via a voltage transformer between the generator neutral
point and earth. The voltage U
3T
can be measured in different ways. The setting
TVoltType defines how the protection function is fed from voltage transformers at the
high voltage side of the generator. If U
3T
is lower than the set level UT3BlkLevel,
STEFPHIZ function is blocked. The choices of TVoltType are:
NoVoltage: There is no voltage measured from the generator terminal side. This can
be the case when there are only phase-to-phase voltage transformers available at the
generator terminal side. In this case the protection will operate as a simple neutral
point 3
rd
harmonic undervoltage protection, which must be blocked externally during
generator start-up and shut-down.
ResidualVoltage: The function is fed from an open delta connection of the phase to
earth connected voltage transformers at the generator terminal side,
U
3T
=(1/3)*U_Open_Delta.
AllThreePhases: The function is fed from the three phase to earth connected voltage
transformers at the generator terminal side. The 3
rd
harmonic voltage U
3T
is calculated
in the IED, U
3T
=(1/3)*(U3L1+U3L2+U3L3).
PhaseL1, PhaseL2, or PhaseL3: The function is fed from one phase voltage
transformer only. The 3
rd
harmonic zero sequence voltage is assumed to be equal to
any of the phase voltages, as the third harmonic voltage is of zero sequence type,
U
3T
=U3Lx.
A simplified block diagram describing the stator earth fault protection function shown
in figure
259.
Section 9 1MRK502052-UEN B
Voltage protection
538
Technical manual