4.50
SEL-351A Relay Instruction Manual Date Code 20080213
Loss-of-Potential, Load Encroachment, and Directional Element Logic
Directional Control Settings (Not in SEL-351A-1)
Quantities needed to make the 32WFP and 32WRP 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 4.13 may come either from a calculation or from a
direct measurement, as described in Zero-Sequence Voltage Sources. When
using a broken-delta PT connection to terminals VS-NS as the zero-sequence
voltage source (global setting VSCONN = 3V0), there are some special
considerations in making the 32WFP and 32WRP settings that are related to
the scaling of the VS-NS input signal. The 32WFP and 32WRP 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 4.52
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 2.21). With such
a connection:
I
N
= 3I
0
In Figure 2.21, 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
create a high impedance circuit and thus a power system that is essentially
ungrounded (with much less current flow than a traditional ungrounded
system). In such an optimum tuned state, little current flows through the
Petersen Coil. Some Petersen Coils are continually adjusted automatically, as
load levels/system topology change, so that tuning remains optimum. The
“tuned circuit” resists sustaining an arc, so many ground faults are self-
extinguished by the circuit itself (no circuit breaker operation necessary).
Consider a permanent line-to-ground fault out on the feeder in Figure 2.21
(refer to the relay and feeder shown in Figure 2.21 as Relay 1 and Feeder 1,
respectively. Other feeders on the same bus, though not shown in Figure 2.21,
are then Relay 2/Feeder 2, etc.). In the zero-sequence network view in
Figure 4.27, Relay 2 (on unfaulted Feeder 2) sees mostly capacitance in front
of it. Assuming a “tuned circuit,” I
0
= 0 at the fault. Thus, the entire zero-
sequence capacitance shown in Figure 4.27 is canceled out by the inductance
of the Petersen Coil. So, with Feeder 1 capacitance C
1
in front of Relay 1, the
system behind Relay 1 appears net inductive.
Courtesy of NationalSwitchgear.com
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