Schweitzer Engineering Laboratories SEL-351 - SEL OGIC Control Equation Falling Edge Operator; SEL OGIC Control Equation Operation Example - Tripping

Date Code 20001006 Setting SELOGIC
®
Control Equations G-7
SEL-351 Instruction Manual
If the rising edge operators / were not applied and setting ER was:
ER = 51P + 51G + OUT3
the ER setting would not see the assertion of OUT3, because 51G and 51P would continue to be
asserted at logical 1, as shown in Table G.1.
SELOGIC Control Equation Falling Edge Operator \
The falling edge operator \ is applied to individual Relay Word bits only – not to groups of
elements within parentheses. The falling edge operator \ operates similar to the rising edge
operator, but looks for Relay Word bit deassertion (element going from logical 1 to logical 0). The
falling edge operator \ in front of a Relay Word bit sees this logical 1 to logical 0 transition as a
“falling edge” and asserts to logical 1 for one processing interval.
For example, suppose the SELOGIC Control Equation event report generation setting is set with the
detection of the falling edge of an underfrequency element:
ER = ... + \81D1T
When frequency goes above the corresponding pickup level 81D1P, Relay Word bit 81D1T
deasserts and an event report is generated (if the relay is not already generating a report that
encompasses the new transition). This allows a recovery from an underfrequency condition to be
observed. See Figure 3.30 and Table 3.11 in Section 3: Overcurrent, Voltage, Synchronism
Check, Frequency, and Power Elements. Figure G.2 demonstrates the action of the falling edge
operator \ on the underfrequency element in setting ER.
Figure G.2: Result of Falling Edge Operator on a Deasserting Underfrequency Element
SELOGIC Control Equation Operation Example – Tripping
If tripping does not involve communications-assisted or switch-onto-fault trip logic, the SELOGIC
Control Equation trip setting TR is the only trip setting needed. Refer to Trip Logic in Section 5:
Trip and Target Logic.
Note that Figure 5.1 in Section 5: Trip and Target Logic appears quite complex. But since
tripping does not involve communications-assisted or switch-onto-fault trip logic in this example,
respective SELOGIC Control Equation trip settings TRCOMM and TRSOTF are not used. The
only effective input into logic gate OR-1 in Figure 5.1 is SELOGIC Control Equation trip setting
TR.
Courtesy of NationalSwitchgear.com