Handbook of Electrical Engineering

PROTECTIVE RELAY COORDINATION 337

Figure 12.15 Protection devices for high voltage motors.

12.7.1 Overloading or thermal image

As with most electrical power equipment the thermal characteristic is based on anI^2 t law. The
equation for the thermal image as given by IEC60255 Part 8 when the motor is cold is,

tc=Tthloge

[

I^2

I^2 −Io^2

]

seconds

Where I=Relay current as a multiple of the nominal current, pu.
Io=Reference current in pu that determines the position of the asymptote, e.g.
at tc→10,000 seconds.Iohas a typical value between 1.015 and 1.065.
Tth=Thermal time constant in seconds, usually given in minutes for a particular motor.

Note: The equation is only valid forI>Io.

A similar equation is used for the hot condition of the motor,

th=Tthloge

[

I^2 −Ip^2

I^2 −Io^2

]

seconds

WhereIp=motor load current before the overload, pu.