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DIGITAL TECHNOLOGY AND THERMAL IMAGE RELAYS
It is clear that digital technology characteristics fit the thermal image applications.
The use of relatively simple algorithms, together with the ability to show relevant information (Thermal Image value,
currents metering, fault information) and the integration of additional protection functions in the same relay (inverse or
definite time overcurrent) co-ordinated with the thermal image function, allows to design high performance protective
devices.
Besides that, thanks to the digital technology possibilities, more accurate models can be used, taking into account
radiation effects, and other heating sources different that Joule effect.
Equation [9] evaluation requires extensive computing resources, and thus, is not directly implemented in the
protection relay. Instead, an iterative algorithm that mimics very closely the real equation is used.
For some applications, the use of separate time constants can be useful. For motors, it can be useful to use a time
constant for normal conditions, and a different one, much lower, for locked rotor conditions (as the heat transmission
capability gets reduced when the machine is not spinning).
11.4
THERMAL CURVE
The time needed for an element to rise its temperature (from an initial temperature θ
temperature difference between the initial and the final temperature) is called "Time Constant", and it is represented
by the letter τ. Using an equation, it is the time it takes to reach an intermediate temperature θi where:
If θ
is the temperature origin, at a given time, the temperature can be written as:
0
Where:
θ
:
Temperature increase at a given time
θN
:
Rated temperature (temperature reached if I = IN)
IN
:
Protected element rated current
I
:
Current flowing through the protected element
11-4
ANNEX 1 THERMAL IMAGE UNIT
θ
θ
θ
θ
=
+
−
(
* )
. 0
∞
i
0
0
1
(
θ
θ
τ
=
−
)
N
*
1 (
e
)
( *
MIF Digital Feeder Protection
63
2
I
/
In
)
) the 63% of θ (θ is the
0
GEK-106273L
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