Ct And Vt Positions - ABB RELION RET670 Applications Manual

Section 8
Impedance protection
8.21.2.5
8.21.2.6
8.21.2.7
506
Zone 3 provides protection for phase-to-ground, phase-to-phase and three phase faults
on the HV side of the system. Hence, all these faults can be detected using three phase-
to-phase loops or three phase-to-ground loops similar to zone 2. These options can be
selected in the function and their operation is quite similar to the operation of zone 2.

CT and VT positions

Voltage transformer is located at the terminals of the generator, but current transformer
can be located either at neutral side of the stator winding or at the terminals of the
generator.
If the current transformer is located at the neutral side of the generator winding, the
forward reach will be of the generator, transformer and connected power system
impedance. If the current transformers are located at the terminal of the generator
always the forward reach is only generator impedance and reverse reach comprises of
transformer impedance and the connected transmission lines impedance.
Undervoltage seal-in function
For faults close to generating terminals the CTs might go in to saturation. The problem
is due to very long DC constant of the generators. The persistent DC component of
primary currents even if relatively small has a tendency to drive current transformers
into saturation. The ZGVPDIS under this condition might reset for some duration. A
reliable backup protection is provided under these conditions by providing an
undervoltage seal-in feature.
The undervoltage function is enabled from zone 2 or zone 3 pickup.
Load encroachment for zone 2 and zone 3
As zone 2 and zone 3 have larger reaches, there is a possibility of load impedance
encroaching into mho characteristics during heavy load conditions. Hence zone 2 and
zone 3 are provided with load encroachment blinder feature which is to be enabled by
the user. This feature measures the impedance based on positive sequence voltage and
current. As the load from the generator corresponds to the positive sequence signals.
Positive sequence voltage and current will be used for load encroachment blocking
logic,
Figure 254 shows the implemented load encroachment characteristic.
1MRK 504 163-UUS A
GUID-AA8734FD-AE94-4B29-A319-7F3FB69BB24F v3
GUID-C4A86119-25C6-407B-87DB-632D04098044 v3
GUID-183E429E-B490-4CEF-B1EA-D380EE3EE6B3 v3
Transformer protection RET670 2.2 ANSI
Application manual