Thermal Overload Protection 49; Functional Description - Siemens SIPROTEC 4 Manual

2.10

Thermal Overload Protection 49

The thermal overload protection is designed to prevent thermal overloads from damaging the protected
equipment. The protection function represents a thermal replica of the equipment to be protected (overload
protection with memory capability). Both the previous history of an overload and the heat loss to the environ-
ment are taken into account.
Applications
•
The thermal condition particularly of generators and transformers can be monitored in this way.
2.10.1

Functional Description

Thermal Replica
The device calculates the overtemperature in accordance with a single-body thermal replica, based on the
following differential equation:
[thermdifferentialgleichung-260602-kn, 1, en_US]
with
Θ
τ
th
Ι
k
Ι
Nom Obj.
The protection function provides a thermal replica of the protected object (overload protection with memory
capability). The history of an overload is taken into consideration.
When the calculated overtemperature reaches the first settable threshold 49 Θ ALARM, an alarm annuncia-
tion is issued, e.g. to allow time for the load reduction measures to take place. When the calculated overtem-
perature reaches the second threshold, the protected equipment may be disconnected from the system. The
highest overtemperature calculated from the three phase currents is used as the criterion.
The maximum thermally permissible continuous current Ι
currentΙ :
Nom Obj.
= k · Ι
Ι
max Nom Obj.
In addition to the k factor (parameter 49 K-FACTOR) the TIME CONSTANT τ
Θ ALARM (in percent of the trip temperature Θ
Since the 7SJ80 does not offer a connection option for an RTD box, the current temperature Θ is always equal
to zero.
Overload protection also features a current warning element (I ALARM) in addition to the temperature
warning element. The current warning element may report an overload current prematurely, even if the calcu-
lated operating temperature has not yet attained the warning or tripping levels.
Extension of Time Constants
When using the device to protect motors, the varying thermal response at standstill or during rotation may be
correctly evaluated. When running down or at standstill, a motor without external cooling looses heat more
slowly, and a longer thermal time constant must be used for calculation. For a motor that is switched off, the
7SJ80 increases the time constant τ
SIPROTEC 4, 7SJ80, Manual
E50417-G1140-C343-A8, Edition 12.2017
Present overtemperature related to the final overtemperature at maximum allowed phase
current k · Ι
Nom Obj.
Thermal time constant of the protected object's heating
Present true r.m.s value of phase current
k–factor indicating the maximum permissible constant phase current referred to the
nominal current of the protected object
Nominal current of protected object
by a programmable factor (kτ factor). The motor is considered to be off
th
is described as a multiple of the object nominal
max
) must be specified.
TRIP
Functions
2.10 Thermal Overload Protection 49
and the alarm temperature 49
th
147