4.92
SEL-751 Relay Instruction Manual Date Code 20170927
Protection and Logic Functions
Group Settings (SET Command)
Equation 4.6
The relay calculates the equivalent heating current, THIEQ, according to
Equation 4.7.
Equation 4.7
Additionally, the relay calculates the maximum continuous current (IMC),
according to Equation 4.8:
Equation 4.8
where:
= The thermal level at time t
= The thermal level from the previous processing
interval
= The processing interval for the element, which is once
every power system cycle (i.e., 50 or 60 Hz). is
expressed in seconds for use in Equation 4.5 and
Equation 4.6.
= The equivalent heating current at time t, given in per
unit
= The equivalent heating current pick up threshold
setting, given in per unit
= The maximum continuous current, given in per unit
= User-selectable equipment hot time constant which
models the thermal characteristics of the equipment
when it is energized in state x. The TCONH[x] setting
is in minutes and expressed in seconds for use in
Equation 4.5, Equation 4.6, and Equation 4.11.
= User-selectable equipment cold time constant that
models the thermal characteristics of the equipment
when it is de-energized in state x. The TCONC[x]
setting is in minutes and expressed in seconds for use
in Equation 4.5, Equation 4.6, and Equation 4.12
= The ambient temperature factor
x = State of the thermal element that decides the set of
time constants to use; x can be 1 or 2
where:
= User-selectable thermal model operating current
= Nominal current rating of the input associated with
THRO operating current (i.e., 1 A or 5 A)
where:
= User-selectable maximum operating temperature of
the equipment
= Basic current value in per unit
If THIEQ IE QPU
THRL
t
THRL
t 1–
TCONC x
TCONC x t+
-----------------------------------------
=
THIEQ
THRO
I
NOM
----------------=
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