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A.2 OPERATING PRINCIPLE
APPENDIX A
A.2OPERATING PRINCIPLE
The thermal relays, based on the direct measure of the device/machine temperature present some difficulties when trying
A
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.
After an overload condition, the protection simulates the cooling process of the machine using a separate time constant.
A-2
MIFII Digital Feeder Relay
GEK-106237P
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