ABB Relion 670 Series Applications Manual page 214

Section 8
Current protection
For a resonant grounded system, the correct directional measurement is ensured regardless
of how many Petersen coils are used throughout the interconnected power network. The
function is not sensitive to the actual compensation degree of the coils. It operates equally
well in under- or over-compensated system. A parallel neutral resistor to the Petersen coil is
not required to correctly determine earth fault direction. However, the neutral resistors can
still be used if these are already installed in the network.
The function is suitable to be used in distribution and in meshed HV sub-transmission
networks using high-impedance grounding. In meshed systems a directional permissive
scheme is required in order to ensure selective tripping of a faulty line. Alternatively, the
function can be used solely for signaling of the earth-fault location to the SCADA system when
the power network is allowed to operate for a longer time with an earth-fault being present
8.17.2.1 CT and VT inputs
The APPTEF function requires the measurement of the residual current (that is 3Io) and
residual voltage (that is 3Uo) of the protected line.
The residual current measurement can be obtained in three different ways:
1. Measured by a dedicated CT input on the IED which is connected to a Holmgreen
connection of individual phase CTs of the protected line
2. Internally calculated by the IED from individually connected three-phase CT inputs
3. Measured by a dedicated CT input on the IED which is connected to a line core balance CT
(that is cable CT). Such a solution is typically only used on distribution voltage levels. In
such installations it shall be ensured that any present cable shield(s) for all three phases
are always passed-back through the CT window and only then connected to ground to
prevent any eventual shield current to disturb the 3Io current measurement.
The residual voltage measurement can be obtained in three different ways:
1. Measured by a dedicated VT input on the IED which is connected to an open delta
connection of individual phase VTs of the protected line. In this case the 3Uo voltage will
be directly measured by the IED and the VT ratio for this input shall be set accordingly. For
example if the open delta VT winding has ratio of (132kV/sqrt(3)) / (110V/3) (equal to the
value of 2078.5 for overall ratio) then set the following values in the IED: VTPrim=3*(132kV/
sqrt(3)) = 228.6kV and VTSec=3*(110V/3)=110V which also correspond to the same overall
value for the VT ratio.
2. Internally calculated by the IED from individually connected three-phase VT inputs.
3. Measured by a dedicated VT input on the IED which is connected to a single-phase VT
connected between power transformer neutral point and ground (in parallel with the
grounding impedance). In this case only Uo voltage will be measured by the IED. To
compensate for that, the VT ratio in the IED shall be set to the three times higher value.
For example, if the used neutral point VT has a ratio of (10kV/sqrt(3)) / (100V) (equal to
value of 57.75 for overall ratio) the following values shall be set in the IED:
VTPrim=3*(10kV/sqrt(3)=17.32kV and VTSec=100V which will provide a three times higher
voltage value inside the IED.
More details about CT and VT connections and settings are available in the other part of the
Application Manual.
Connections when internal calculations for 3Io and/or 3Uo are used are not given in this figure.
208
Figure 100 shows the possible external CT and VT connections.
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