Woodward highprotec MRI4, User Manual

Page: 487 / 727

Protective Elements
ThR-Protection Module: Thermal Replica [49]
ThR
The maximal permissible thermal loading capacity, and consequently the tripping delay of a component, depends
on the amount of the flowing current at a specific time, the
»
previously existing load (current)
«
as well as on a
constant specified by the component.
The thermal overload protection is in compliance with IEC255-8 (VDE 435 T301). A complete thermal replica
function is implemented in the device as Homogeneous-Body Replica of the equipment to be protected and by
taking the previously existing load into account. The protection function is of one step design, provided with a
warning limit.
For this the device calculates the thermal load of the equipment by using the existing measured values and the
parameter settings. When knowing the thermal constants, the temperature of the equipment can be established
(simulated).
The general tripping times of the overload protection can be gathered from the following equation according to IEC
255-8:
Legend:
487 MRI4 DOK-HB-MRI4-2E
t = Tripping delay
τ-cool = Cooling time constant
K = Overload Factor: The maximum thermal limit is defined as k *IB, the product of the overload factor and the base current .
I = measured current (x In)
Ip = Preload Current
Ib = Base current: Maximum permissible thermal continuous current.
τ-warm = Warming-up time constant
( )
t = τ-warm ln
I
2
- Ip
2
I
2
- (K*Ib)
2
.