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APPENDIX A
A.4 DIGITAL TECHNOLOGY AND THERMAL IMAGE RELAYS
A.4DIGITAL TECHNOLOGY AND THERMAL IMAGE RELAYS
It is clear that digital technology characteristics fit the thermal image applications.
A
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).
GEK-106237P
MIFII Digital Feeder Relay
A-5
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