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T
:
Time
τ
:
Time constant
11.5
MIF THERMAL CURVES
The equation for the temperature given before was:
The MIF uses an equation, in which the tripping time is a function of the current flowing through the protected
element, thus eliminating all references to the temperatures. The heating time constant
τ1.
By means of the keypad (or ENERVISTA MII SETUP program), a tap/pickup current must be set in the relay.
If the current is greater than the programmed tap current, the thermal protection will trip after a period of time given by
the following equation:
Where:
t = Tripping time.
τ1= Heating time constant.
I' = I / I
tap
I =Current through the element.
I
= Programmed tap/pickup current in the relay.
tap
This equation can only be applied if the relay starts from a thermal zero status, that is, from a condition at
which a current I = 0 was flowing through it. If the relay had stabilised at a condition at which a given current was
flowing through it, the value of which is smaller than the rated current, and at a given moment the current increases
up to a value greater than the rated current, the tripping time from the moment the increase takes place is given by
the equation:
Where:
I
=
Ime / I
e
tap
Ime =
Current at which the protected element had stabilised.
I
=
Programmed tap current.
tap
and the rest of the symbols have the same meaning as in the previous equation.
In the curves, the "Thermal Equivalent Current" (the greater current value from the current in the three phases)
is represented by the letter Ieq, and this is the value that the relay shows corresponding to the thermal image of the
protected element. The relay is running the thermal image for the three phases, but only the greater value is shown
in the display, as it is the one that will cause the trip.
When the protected element cools down, the time constant (Cooling Time Constant) may be different than the
heating time constant. For motors and generators applications, the heat transfer between the machine and the
GEK-106273L
ANNEX 1 THERMAL IMAGE UNIT
θ
θ
τ
−
=
−
1 (
)
N
*
1 (
e
)
( *
τ
=
2
2
t
*
ln(
I
'
/
I
'
1
⎡
2
I
τ
=
⎢
t
*
ln
1
⎣
I
MIF Digital Feeder Protection
2
I
/
IN
)
.........(
) 1
−
1
))
⎤
−
2
I
e
⎥
−
2
⎦
1
τ
is the MIF is designated as
11-5
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