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11. ANNEX 1 THERMAL IMAGE UNIT
11.1
INTRODUCTION
Protective relays are devices designed to detect and eliminate defects or faults in the Power System. The elimination
of the fault is carried out by the opening of the circuit breaker or breakers that supply power to the fault.
Faults in the Power System usually create very high current situations on lines, generators, transformers, etc. These
high currents are much greater than the rated currents for which these equipment were designed, introducing an
additional stress and possible direct damage, as a consequence of the thermal and dynamic effects of the high short-
circuit currents.
Due to this fact, the most common protective device is the overcurrent relay. Its operating principle is to detect if the
current in the system is under or above a set level, and depending on the current level, issue an instantaneous trip or
a fixed time delayed trip. There are overcurrent relays that include a current versus time tripping characteristic curve,
that makes the relay to trip faster for high currents and following an inverse I vs. t equation, trip slower for lower fault
currents.
Operating times range from tens of milliseconds to some seconds for slower operation curves.
However, for some applications, this type of protective relay has some limitations.
In the case of two transformers, operating in parallel, feeding a distribution bus bar, working both of them at 70% of
rated load: if an overcurrent relay is installed on each transformer, and due to any reason, one transformer is out of
service, the other one will work at 140% of rated load.
Under these circumstances, the relay on the transformer that is in service will trip after a relatively short time, taking
the transformer out of service, and leaving the bus bar without any supply.
However, transformers are designed to withstand an overload condition like the described one for some minutes,
without any deterioration, allowing during this time to the substation operator to take the appropriate actions to restore
the situation and take the transformer back in service, before the other one gets over heated.
The Thermal Image protection is especially applicable to this situations, due to its operating principle. In general, it is
a standard backup protection for many protection schemes, for almost any device, motors, generators, cables, etc.
11.2
OPERATING PRINCIPLE
The thermal relays, based on the direct measure of the device/machine temperature present some difficulties when
trying to measure the temperature of the sensitive elements of the device/machine to protect (i.e. windings in a
transformer). The temperature is measured on the surrounding zones (i.e. oil, isolators, etc) loosing effectiveness due
to the high thermal inertia.
Due to this reason, thermal image relays are used. These relays use mathematical algorithms (derive from physical
models and equations) to simulate the heating of the machine, taking electrical magnitudes (currents) as inputs to the
algorithm.
For regular overloading situations, heating is the main concern, leaving apart the dynamic effects.
Thermal Image relays operate when the computed temperature (Thermal Image of the machine) reaches a level
considered to be dangerous. Compared with an overcurrent relay, the thermal image relay does not start when a fault
is detected, but it is continuously operating, computing the thermal status of the protected piece of equipment. The
operating time depends on the thermal status and current flowing at a given point in time until the limit temperature is
reached. The starting temperature depends on the previous "history" of the machine, the current measured and the
amount of time it has been applied. In this sense, it can be said that Thermal Image relays have memory, as they
remember the previous status of the machine, and start computing temperature based on that status.
GEK-106273L
ANNEX 1 THERMAL IMAGE UNIT
MIF Digital Feeder Protection
11-1
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