4.71
Date Code 20170927 Instruction Manual SEL-751 Relay
Protection and Logic Functions
Group Settings (SET Command)
59RES—Wattmetric 3V
0
Overvoltage Pickup (Petersen Coil-Grounded
System)
If setting ORDER does not contain P (Petersen coil directional element is not
enabled) or the model does not have a 0.2 A nominal neutral channel (IN), then
setting 59RES is not made or displayed.
Setting 59RES should be set greater than the value of 3V
0
zero-sequence volt-
age present for normal system unbalance. It is part of the enabling logic for
the wattmetric element part of the Petersen coil directional element (see
Figure 4.30).
The 3V
0
input to Figure 4.30 may come either from a calculation or from a
direct measurement, as described in Zero-Sequence Voltage Sources on
page 4.33. When using a broken-delta PT connection to terminals VS-NS as the
zero-sequence voltage source (VSCONN := 3V0), there are some special con-
siderations in making the 59RES setting that are related to the scaling of the
VS-NS input signal. The 59RES setting must be entered on the same secondary
base as the voltage terminals VA, VB, and VC. See Settings Considerations for
Petersen Coil-Grounded Systems on page 4.73 for an example.
DIRWFP and DIRWRP—Wattmetric Forward and Reverse Pickups
(Petersen Coil-Grounded System)
If setting ORDER does not contain P (Petersen coil directional element is not
enabled) or the model does not have a 0.2 A nominal neutral channel (IN), then
settings DIRWFP and DIRWRP are not made or displayed.
Quantities needed to make the DIRWFP and DIRWRP wattmetric pickups
calculations are:
3V
0
zero-sequence voltage in secondary (from inputs, VA, VB, VC; or input
VS when VSCONN := 3V0)
I
N
current in secondary (from 0.2 A nominal neutral channel input, IN)
The 3V
0
input to Figure 4.30 may come either from a calculation or from a
direct measurement, as described in Zero-Sequence Voltage Sources on
page 4.33. When using a broken-delta PT connection to terminals VS-NS as the
zero-sequence voltage source (VSCONN := 3V0), there are some special con-
siderations in making the DIRWFP and DIRWRP settings that are related to
the scaling of the VS-NS input signal. The DIRWFP and DIRWRP settings
must be entered on the same secondary base as the voltage terminals VA, VB,
and VC. See Settings Considerations for Petersen Coil-Grounded Systems on
page 4.73 for an example.
I
N
is the current measured by current channel IN. Channel IN is connected in
such a manner that it monitors the system zero-sequence current (e.g., channel
IN is connected to a window CT through which the three phase conductors
pass and thus monitors 3I
0
zero-sequence current, see Figure 2.30). With such
a connection:
I
N
= 3I
0
In Figure 2.30, only one feeder position is shown, but one can imagine the bus
extending to the right, with other feeder positions. The Petersen coil in the
transformer neutral is tuned to cancel out the cumulative zero-sequence line
capacitance of all the connected feeders. The Petersen coil and the zero-
sequence line capacitance are a parallel LC circuit. In a “tuned state,” they cre-
ate a high impedance circuit and thus a power system that is essentially
ungrounded (with much less current flow than a traditional ungrounded sys-
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