Schweitzer Engineering Laboratories SEL-351A - Page 141

4.15
Date Code 20080213 Instruction Manual SEL-351A Relay
Loss-of-Potential, Load Encroachment, and Directional Element Logic
Directional Control for Neutral-Ground and Residual-Ground Overcurrent Elements (Not in SEL-351A-1)
CTR/CTRN ≤ (5 A/0.05 A) = 100 (5 A nominal)
CTR/CTRN ≤ (5 A/0.01 A) = 500 (1 A nominal)
There is no effective overlap if:
CTR/CTRN > 100 (5 A nominal)
CTR/CTRN > 500 (1 A nominal)
With no effective overlap, as the neutral channel current I
N
exceeds the upper
detection threshold of neutral channel IN (5 A secondary), the unit still
operates on the neutral channel current I
N
until the lower detection threshold
of the residual-ground current I
G
(0.05 A secondary for 5 A nominal phase;
0.01 A secondary for 1 A nominal) is reached. It is better to have effective
overlap:
CTR/CTRN ≤ 100 (5 A nominal)
CTR/CTRN ≤ 500 (1 A nominal)
Relay Word bit GNDSW indicates whether the directional element for low-
impedance grounded or ungrounded/high-impedance grounded systems is
operating on neutral channel (IN) current I
N
(GNDSW = logical 1) or on
residual-ground current I
G
instead (GNDSW = logical 0).
This I
N
to I
G
(or I
G
to I
N
) current switching discussed for Figure 4.8 (and
Figure 4.12 and Figure 4.14) also has an effect on zero-sequence impedance
settings Z0F and Z0R (see Figure 4.12 and Figure 4.14). Z0F and Z0R
(Ω secondary) are set in reference to the phase current inputs (IA, IB, and IC;
residual current I
G
is derived internally from these phase currents), as are the
negative-sequence impedance settings Z2F and Z2R. However, settings Z0F
and Z0R are applied to Figure 4.12 and Figure 4.14, where neutral current I
N
(from neutral current channel IN) is also applied. 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 4.8 (and Figure 4.12, and Figure 4.14) 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
setting choices “V,” “S,” “P,” and “U,” shown in Figure 4.10 and Figure 4.12–
Figure 4.14) may use either a calculated 3V
0
from the wye-connected voltages
V
A
, V
B
, and V
C
, or a measured 3V
0
from the VS channel, typically connected
to a broken-delta PT secondary. The global 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
directional element “3V
0
” inputs.
When VSCONN = VS, the calculated zero-sequence voltage from terminals
VA, VB, and VC is applied to the directional element “3V
0
” inputs, provided that
the relay is connected to wye-connected PTs (global setting PTCONN =
Courtesy of NationalSwitchgear.com