4.33
Date Code 20170927 Instruction Manual SEL-751 Relay
Protection and Logic Functions
Group Settings (SET Command)
This I
N
to I
G
(or I
G
to I
N
) current switching discussed for Figure 4.25,
Figure 4.29, and Figure 4.31 also has an effect on zero-sequence impedance
settings Z0F and Z0R (see Figure 4.29 and Figure 4.31). Z0F and Z0R ( sec-
ondary) are set in reference to the phase current inputs (IA, IB, and IC; residual
current I
G
is derived internally from these phase currents). However, settings
Z0F and Z0R are applied to Figure 4.29 and Figure 4.31, where neutral cur-
rent I
N
(from neutral current channel IN) is also applied when GNDSW is
asserted. Settings Z0F and Z0R are adjusted internally (with CT ratio settings)
to operate on this I
N
current base:
Z0F • CTRN/CTR (I
N
base)
Z0R • CTRN/CTR (I
N
base)
If the logic in Figure 4.25, Figure 4.29, and Figure 4.31 operates on residual
current I
G
, as a result of current switching, then settings Z0F and Z0R are not
adjusted, and just operate as:
Z0F (I
G
base)
Z0R (I
G
base)
Zero-Sequence Voltage Sources
The directional elements that rely on zero-sequence voltage 3V
0
(ORDER set-
ting choices: V, S, P, and U, shown in Figure 4.27 and Figure 4.29 through
Figure 4.31) may use either a calculated 3V0 from the wye-connected volt-
ages VA, VB, and VC, or a measured 3V0 from the VS channel, which is typ-
ically connected to a broken-delta PT secondary. Setting VSCONN selects the
zero-sequence voltage source to be used by the affected directional elements.
When VSCONN := 3V0, the measured voltage on terminals VS-NS is scaled by
the ratio of Group settings PTRS/PTR to convert it to the same voltage base as
the VA, VB, and VC terminals, and the resulting signal is applied to the direc-
tional element “3V0” inputs.
When VSCONN := VS, the calculated zero-sequence voltage from terminals
VA, VB, and VC is applied to the directional element “3V0” inputs, provided that
the relay is connected to wye-connected PTs (DELTA_Y := WYE). If the
relay is connected to open-delta PTs (DELTA_Y := DELTA), 3V0 cannot be
calculated from the VA, VB, and VC terminals, and the directional elements that
require zero-sequence voltage are unavailable.
When testing the relay, it is important to note that the METER command VG
(3V0) quantity, when available, is always the calculated value from the wye-
connected PT inputs, even when VSCONN := 3V0. The METER command
VS quantity is always the measured value from the VS-NS terminals.
Refer to Figure 2.21: Voltage Connections (e) and (f) for Broken-Delta VS
Connection (setting VSCONN := 3V0).
Best Choice Ground Directional Element Logic
The Best Choice Ground Directional Element logic determines which direc-
tional element should be enabled to operate. The neutral ground and residual
ground overcurrent elements set for directional control are then controlled by
this enabled directional element.
Table 4.20 is the embodiment of the Best Choice Ground Directional Element
logic. Note in Table 4 .20 that any of the directional elements corresponding to
S, P, or U that operate on 0.2 A nominal neutral channel (IN) are listed last (or
by themselves) in any of the available setting combinations for the ORDER
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