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S3 PROTECTION
Transformer thermal
protection
NOTE:
NOTE
6–80
Courtesy of NationalSwitchgear.com
The 345 relay provides one transformer thermal overload protection. Loads exceeding the
transformer load ratings over certain period of time, can degrade the insulation, and in
return may lead to short circuit conditions. The heating of the transformer is mostly of
resistive type (I2R), hence the generated heat is directly proportional to the square of the
flowing winding currents (I2 ). The relay uses the transformer thermal time characteristic
based on current squared and integrated over time. The relay will continuously calculate
the transformer thermal capacity as a percentage of the total thermal capacity as follows:
Where:
θ
(t) = Cable thermal capacity (%) at time t
θ
(t-1) = Cable thermal capacity (%) at time t-1
Δt/τ = Time step Δt divided by the heating/cooling time constant τ
2
2
I
= (I
/I
)
= Squared ratio between the actual load current and the pickup setting
phase
pickup
τ = Heating and cooling time constant (usually provided by the manufacturer).
The heating time constant is used when the squared load/pickup ratio is greater than the
thermal capacity θ(t-1) estimated in the previous time step. Otherwise the formula uses the
cooling time constant.
The time to trip is estimated when the load current exceeds the PKP setting, and the 49
element picks up. At the same time the thermal capacity will start to increase at a rate
depending on the current amplitude and the prior loading condition of the transformer.
When the thermal capacity exceeds the alarm level, the element will generate an alarm
signal. The thermal model alarm can be used as a warning for the start of dangerous
overloading conditions, and can prevent unnecessary tripping. When the thermal capacity
exceeds the trip level, the element will generate a trip signal. As per the formula below, the
operate time (time to trip) is determined from when the element picks up until it trips, and
depends on both the measured load over time, and the transformer heating and cooling
time constants.
Where:
T
= Time to trip in seconds
TRIP
θ
2
= 1 = Trip thermal state set to 100%
τ = Heating and cooling time constant, usually provided by the manufacturer.
2
I
= Squared ratio of the actual phase current and the pickup setting.
The time to trip will start timing out once the level of the computed thermal capacity (%)
becomes higher than 100% thermal capacity (
thermal capacity falls below 97% of the pickup level.
The measured Winding 1 phase currents are used as inputs by the Transformer
Thermal protection for detection of overload (overheating) conditions.
The following path is available using the keypad. For instructions on how to use the
keypad, please refer to Chapter 3 - Working with the Keypad.
PATH:
SETPOINTS > S3 PROTECTION > SETPOINT GROUP 1(2) > THERMAL MODEL
θ
=1 ). The trip flag will drop out when the
345 TRANSFORMER PROTECTION SYSTEM – INSTRUCTION MANUAL
CHAPTER 6: SETPOINTS
Eq. 12
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