Thermal Overload Protection - Toshiba GRE110 Instruction Manual

2.5 Thermal Overload Protection

The temperature of electrical plant rises according to an I
protection in GRE110 provides a good protection against damage caused by sustained overloading.
The protection simulates the changing thermal state in the plant using a thermal model.
The thermal state of the electrical system can be shown by equation (1).
2
I
θ =
2
I
AOL
where:
θ = thermal state of the system as a percentage of allowable thermal capacity,
I = applied load current,
I = allowable overload current of the system,
AOL
τ = thermal time constant of the system.
The thermal state 0% represents the cold state and 100% represents the thermal limit, which is the
point at which no further temperature rise can be safely tolerated and the system should be
disconnected. The thermal limit for any given system is fixed by the thermal setting I
gives a trip output when θ= 100%.
The thermal overload protection measures the largest of the three phase currents and operates
according to the characteristics defined in IEC60255-8. (Refer to Appendix A for the
implementation of the thermal model for IEC60255-8.)
Time to trip depends not only on the level of overload, but also on the level of load current prior to
the overload - that is, on whether the overload was applied from 'cold' or from 'hot'.
Independent thresholds for trip and alarm are available.
The characteristic of thermal overload element is defined by equation (2) and equation (3) for 'cold'
and 'hot'. The cold curve is a special case for the hot curve where prior load current Ip is zero,
catering to the situation where a cold system is switched on to an immediate overload.

Ln

t =τ·


Ln

t =τ·


where:
t = time to trip for constant overload current I (seconds)
I = overload current (largest phase current) (amps)
I = allowable overload current (amps)
AOL
I = previous load current (amps)
P
τ= thermal time constant (seconds)
Ln = natural logarithm
Figure 2.5.1 illustrates the IEC60255-8 curves for a range of time constant settings. The left-hand
chart shows the 'cold' condition where an overload has been switched onto a previously un-loaded
system. The right-hand chart shows the 'hot' condition where an overload is switched onto a system
t
 − 
1 e 100
− τ  ×

%
 
2
I


(2)
2 2
I I
−

AOL
2 2
I I

−
P

(3)
2 2
I I
−


AOL
 36 
2
t function and the thermal overload
(1)
6 F 2 T 0 1 7 2
. The relay
AOL