Ashida ADR233B - Thermal Overload Protection Function

ADR233B
Doc ID : ADR233B/IM/01
Ref ID : ADR233B/IM/PS
Rev No. : 05
Page No. : 150 of 479
4.17 Thermal Overload Protection Function
ADR233B is provided with the thermal overload function which can be selected either Winding1
side or winding 2 sides by using the following settings “THOL Enable” can be set as W1 / W2 /
Disabled.
The thermal withstand capability of the power system equipment is affected by overheating prior
to a fault. The temperature should not exceed the thermal withstands capability of power system
equipment. The RMS currents are measured and analyzed to monitor the thermal state. In case
of thermal overload function the calculation of the trip time is given by:
t = τ x ln ((K²- A)/(K²- Th. Trip))
Where
t : Time to trip (in seconds)
τ : (T) Time constant 1 of the equipment to be protected
K : Thermal overload capacity (Irms / k* Ith>trip)
Where:
Irms : RMS current corresponding to the largest phase current
Ith>trip : Thermal Trip or Full load current rating (settable)
Service Factor : constant is settable.
A : Initial thermal state. If the initial thermal state is 50% then A =0.5
The calculation of the thermal state is given by the following formula:
Θi+1 = (Irms /k* Ith>trip) ² · [1 – exp (-t/Te)] + Θi · exp (-t/Te)
When thermal state reached the Thermal Alarm “Ith> Alarm” settings then relay generates
Thermal ALARM signal and Start LED will Glow, simultaneously relay assigned to alarm relay
will be operated. If Thermal State is reached 100% of thermal capacity, then Relay generates
Thermal Trip and the trip LED will Glow. The minimum current required to operate Thermal Trip
is 0.05A.
Thermal trip will be reset when thermal state is reduced below 100%, General trip will be reset
based on any of these following conditions,
• If Thermal state is reduced below 100%
• Current is less than 5% of rated current.