Thermal Overload Protection (49-1, 49-2); Method Of Operation - Siemens 7UT51 Instruction Manual

7UT51 v3
9 Thermal Overload Protection (49
The thermal overload (49) protection elements
respond to excessive load currents that will raise the
temperature of the protected object to a damaging
level. Each 7UT51 relay has two independent 49
protection elements, which are called 49-1 and 49-2.
Each can protect one winding (or side) of the
protected object. When a 7UT513 is protecting a
two-sided object, one of its 49 elements can protect a
separate, additional object.
Each 49 element has both a warning-alarm level and a
trip level. The alarm level can control signal contacts
to start cooling devices or shed part of the load. Also,
binary inputs controlled by external devices can
change which set of relay settings is in use (see
Section 13.2 on page 126); therefore, each 49
element can adjust to changing conditions.
The 49 elements are used for protection against
excessive load currents. For protection against
overheating caused by a short-circuit current, use the
backup overcurrent element (see Chapter 8).
9.1

Method of Operation

The load current passing though each winding of the
protected object causes ohmic heating of the winding
due to resistance. This heat flows out into the ambient
environment at a rate that increases as the difference
between the winding temperature and the ambient
temperature increases; therefore, if the load current
increases to a new steady value, the temperature of
the winding will rise to a new equilibrium level. If the
overload is too large, the temperature will begin to rise
to a level that the winding will be damaged. But if the
overload ends before a damaging temperature is
reached, the winding will safely cool down. The value
of importance is the amount of time that an overload
can be tolerated before the thermal protection element
should trip.
PRIM-2330C

Thermal Overload Protection (49-1, 49-2)

1, 49 2)
- -
The element does not measure the temperature of the
winding directly, but instead continuously calculates
the temperature change using a single-body thermal
model expressed as the following differential equation:
Θ
d
------ -
+
t d
where:
I is the load current (the true rms thermal
load
current, including harmonics)
t is a constant corresponding to the thermal
response of the protected object to changes
in the current flowing through it, see
Section 9.3.2 on page 108
• Q is the temperature rise after time t as a fraction
of Q , (the temperature rise that would result
max
from the maximum allowed continuous overload
current, I ; calculated by assuming an initial
max
temperature of 40°C and applying Equation 9.1
for five periods of t).
The amount of time to allow an overload before a trip
event occurs is given by the solution to Equation 9.1:
( ⁄
⎛
I I
load
τ ln ×
⎜
---------------------------------------------------------------------------
t =
⎝
(
I
where
t is the time until trip.
t is a constant describing the thermal behavior
of the object (a setting, see Section 9.3.2 on
page 108).
I is the present measured load current.
load
I is the previous measured load current.
preload
I
is the maximum allowed continuous overload
max
current (a setting, see Section 9.3.1 on
page 108)
ln means "the natural logarithm of..."
This equation is plotted in Figure 9.1. The time to
pickup is recalculated for each measurement period.
Hence, the time will shorten if the object is heating
more rapidly due to an increasing overload current,
and lengthen if the overload current drops.
2
I
Θ
load
--- - ------------- -
=
τ τ
2 2
) ( ⁄ )
⎞
– I I
max preload max
⎟
⎠
2
⁄ )
I – 1
preload max
9
(9.1)
(9.2)
107