Sel SEL-421 - Math Error Detection; EXAMPLE 3.13 Using Parentheses in Math Equations; Math Error Examples; Arithmetic

R.3.30
SEL-421 Relay Reference Manual Date Code 20111215
SELOGIC Control Equation Programming
SELOGIC Control Equation Operators
Parentheses
Use parentheses to control the order in which the relay evaluates math
operations within a math SEL
OGIC control equation. Also use parentheses to
group expressions that you use as arguments to function operators such as SIN
and COS. Include as many as 14 levels of nested parentheses in your math
SEL
OGIC control equation. Example 3.13 shows how parentheses affect the
operation and evaluation of math operations.
EXAMPLE 3.13 Using Parentheses in Math Equations
The free-form math SEL
OGIC control equations below show examples
of parentheses usage.
# Examples of parenthesis usage
AMV001 := AMV005 * (AMV004 + AMV003) # Calculate sum first, then
product
AMV002 := AMV010 * (AMV009 + (AMV016 / AMV015) ) # Nest
parentheses
AMV003 := SIN (AMV037 + PMV42) # Group terms for a function
Math Error Detection
If a math operation results in an error, the SEL-421 turns on the math error bit,
MATHERR, in the Relay Word. A settings change or the STATUS SC
command provides reset for this bit. For example, if you attempt to take the
square root of a negative number (SQRT –5), the math error bit will be
asserted until you clear the bit with a STATUS SC command or change
settings.
A math error is also declared when the ratio of settings B1KASP3/B1KASP1
is outside the range: 5
 B1KASP3/B1KASP1  100.
Arithmetic
Use arithmetic operators to perform basic mathematical operations on
numerical values. Arguments of an arithmetic operation can be either Boolean
or numerical values. In a numerical operation, the relay converts logical 0 or
logical 1 to the numerical value of 0 or 1. For example, multiply numerical
values by Boolean values to perform a selection operation. Use parentheses to
group terms in math SEL
OGIC control equations and control the evaluation
order and sequence of arithmetic operations.
Table 3.27 Math Error Examples
Example Value in PMV01 Typ e MATHERR
PMV01 := PMV02 / 0 Infinity Divide by zero Yes
PMV01 := LN ( 0 ) 0
a
a
Evaluation of expression results in an error and prevents storage of new result. In the
example, PMV01 remains 0. If the argument were a variable, PMV01 would contain the result
of the last evaluation when the argument is valid.
LN of 0 Yes
PMV01 := LN ( -1) 0
a
LN of negative number Yes
PMV01 := SQRT ( -1) 0
a
Square root of a negative
number
Yes