Schweitzer Engineering Laboratories SEL-351A - Page 142

4.16
SEL-351A Relay Instruction Manual Date Code 20080213
Loss-of-Potential, Load Encroachment, and Directional Element Logic
Directional Control for Neutral-Ground and Residual-Ground Overcurrent Elements (Not in SEL-351A-1)
WYE). If the relay is connected to open-delta PTs (global setting PTCONN =
DELTA), 3V
0
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 3V0
quantity, when available, is always the calculated value from the wye-
connected PT inputs. The METER command VS quantity is always the
measured value from the VS-NS terminals.
See Broken-Delta VS Connection (Global setting VSCONN = 3V0) on
page 2.15 and Settings for Voltage Input Configuration on page 9.36.
Best Choice Ground
Directional Element
Logic
The Best Choice Ground Directional Element logic determines which
directional 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 4.2 is the embodiment of the Best Choice Ground Directional Element
logic. Note in Table 4.2 that any of the directional elements 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 setting (directional elements
corresponding to S, P, or U). This is because preference is given to selected
directional elements that operate off of bigger signals (i.e., directional
elements corresponding to Q and V: setting choice “I” cannot be listed with S,
P, o r U ) .
Figure 4.4 shows no control emanating from the Best Choice Ground
Directional Element logic to the directional elements corresponding to S or P
(Figure 4.12, and Figure 4.13, respectively). This Best Choice Ground
Directional Element logic for the directional elements corresponding to S or P
is effectively handled with the “disable inputs” (internal enables 32QGE and
32VE) running into the internal enable logic of Figure 4.8. If neither 32QGE
nor 32VE is asserted (and thus their corresponding directional element is not
enabled), then the internal enable logic of Figure 4.8 is free to run for the last
directional element selected in setting ORDER (if S or P is the last element
listed in setting ORDER).
Setting choice U (ungrounded/high-impedance grounded) can only be listed
by itself (ORDER = U), so Best Choice Ground Directional Element logic is
irrelevant in this case (as it is also essentially irrelevant when Q, V, I, or P are
listed by themselves in setting ORDER).
Directional Elements
Refer to Figure 4.4, Figure 4.5, and Figure 4.9–Figure 4.14.
The Best Choice Ground Directional Element logic in Tab le 4.2 determines
which directional element will run.
Note in Figure 4.13 that the incremental conductance directional element
outputs F32C/R32C do not propagate to directional outputs F32N/R32N,
respectively, as do the wattmetric directional element outputs F32W/R32W.
Incremental conductance elements are used more for alarming purposes, than
for controlling overcurrent elements for tripping. Incremental conductance
elements provide more sensitivity for detecting high-resistance faults on
Petersen coil grounded systems (as compared to the wattmetric elements). For
more information on the operation and application of incremental
conductance elements for Petersen Coil (resonant) grounded systems, see the
paper: Review of Ground Fault Protection Methods for Grounded,
Ungrounded, and Compensated Distribution System, by Jeff Roberts, Hector
Altuve, Daqing Hou, presented at the 28th Annual Western Protective Relay
Conference, Spokane, Washington, October 22–24, 2001.
Courtesy of NationalSwitchgear.com