Page 350 - Handbook of Electrical Engineering
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PROTECTIVE RELAY COORDINATION      337



































                               Figure 12.15 Protection devices for high voltage motors.


           12.7.1 Overloading or Thermal Image

                                                                                     2
           As with most electrical power equipment the thermal characteristic is based on an I t law. The
           equation for the thermal image as given by IEC60255 Part 8 when the motor is cold is,
                                                       2
                                                      I
                                       t c = T th log e  2  seconds
                                                    2
                                                   I − I o
           Where I = Relay current as a multiple of the nominal current, pu.
                  I o = Reference current in pu that determines the position of the asymptote, e.g.
           at     t c → 10,000 seconds. I o has a typical value between 1.015 and 1.065.
                 T th = Thermal time constant in seconds, usually given in minutes for a particular motor.

           Note: The equation is only valid for I> I o .


           A similar equation is used for the hot condition of the motor,
                                                     2   2
                                                   I − I p
                                       t h = T th log        seconds
                                                 e  2    2
                                                   I − I o
           Where I p = motor load current before the overload, pu.
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