4.161
Date Code 20170927 Instruction Manual SEL-751 Relay
Protection and Logic Functions
Logic Settings (SET L Command)
SELOGIC Control
Equation Operators
Use the Boolean operators to combine values with a resulting Boolean value.
Edge trigger operators provide a pulse output. Combine the operators and
operands to form statements that evaluate complex logic. SEL
OGIC control
equations are either Boolean type or math type. Because you have already
used the equals sign (=) as an equality comparison, both Boolean type and
math type of SEL
OGIC control equation settings begin with an “assignment”
operator (:=) instead of with an equals sign.
Boolean SEL
OGIC control equation settings use logic similar to Boolean alge-
bra logic, combining Relay Word bits together with one or more of the Bool-
ean operators listed in Table 4.60. Math SEL
OGIC control equation settings
operate on numerical values, using one or more of the Mathematical operators
listed in Table 4.60. These numerical values can be mathematical variables or
actual real numbers.
The relay converts variables from decimal to integer before performing math
operations, i.e., scales it by multiplying by 128 followed by rounding. After
the math operations, the relay converts the result back from integer to decimal
by scaling the value down by 128 before reporting the results. This effectively
means that math calculations are rounded. See Example 4.33 for an explana-
tion on improving the accuracy of the math operations by managing the pro-
cessing order.
EXAMPLE 4.33 Improving the Accuracy of Math Operations
If MV01 : = (60/4160) • 100,000, the relay performs the 60/4160 calculation
and scales it by 128, then rounds this up to a 2. The relay then multiplies it
by 100,000 and stores it as 200,000. When the number is reported it
divides out the scale factor (128) and reports 1562.5.
Alternately, If MV01 := (60 • 100,000) / 4160, the relay multiplies
(60 • 00,000) and then scales by 128 and then divides by 4160. This result is
then rounded and stored as 184,615. The relay then divides 184,615 by 128
and reports 1442.3.
Example 4.33 illustrates how important it is to avoid calculations where a
small number is divided by a large number followed by multiplication. It will
amplify the error significantly.
The executed result of a math SEL
OGIC control equation is stored in a math
variable. The smallest and largest values a math variable can represent are
–16777215.99 and +16777215.99, respectively. If the executed result exceeds
these limits, it is clipped at the limit value. For example, when the MV01:=
executed result is –16777219.00, MV01 is –16777215.99. Similarly, when the
MV02 := executed result is +16777238.00, MV02 is +16777215.99.
Comments can be added to both Boolean and math SEL
OGIC control equa-
tions by inserting a # symbol. Everything following the # symbol in a
SEL
OGIC control equation is treated as a comment. See Table 4.61 for this and
other Boolean and math operators and values.
Operator Precedence
When you combine several operators and operands within a single expression,
the SEL-751 evaluates the operators from left to right, starting with the high-
est precedence operators and working down to the lowest precedence. This
means that if you write an equation with three AND operators, for example
SV01 AND SV02 AND SV03, each AND is evaluated from the left to the
right. If you substitute NOT SV04 for SV03 to make SV01 AND SV02 AND
NOT SV04, the device evaluates the NOT operation of SV04 first and uses the
result in subsequent evaluation of the expression.
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